The stability and electron loss process of numerous multiply charged anions (MCAs) have been traditionally explained in terms of the classical Coulomb interaction between spatially separated charged groups. An understanding of these processes in MCAs with not well-separated excess charges is still lacking. We report the surprising properties and physical behavior of [B12X12]2–, X = F, Cl, Br, I, At, which are MCAs with not well-separated excess charges and cannot be described by the prevailing classical picture. In this series of MCAs, comprising a “boron core” surrounded by a “halogen shell”, the sign of the total charge in these two regions changes along the halogen series from X = F–At. With the aid of experimental photoelectron spectroscopy and highly correlated ab initio electronic structure calculations, we demonstrate that the trend in the electronic stability of these MCAs is determined by the interplay between the Coulomb (de)stabilization originating from the “boron core” and “halogen shell” and the extension of the overlap between the orbitals from both regions. The second excess electron is always taken from the most positively charged region, viz., the “boron core” for X = F, Cl, and the surrounding “halogen shell” for X = I, At. This change in the physical behavior is attributed to the position of the highest occupied molecular orbital, which dwells in a region that is spatially separated from the one containing the excess negative charge. The unusual intrinsic electronic structure of the [B12X12]2– MCAs provides the basis for a molecular level understanding of their observed unique physical and chemical properties and a new paradigm for understanding the properties of these MCAs with not well-separated charges that departs from the prevailing model used for spatially separated charges that is based on their classical Coulomb interaction.

Electron-withdrawing trifluoromethyl groups were characterized in combination with hydrogen-bond interactions in three polyols (i.e., CF3CH(OH)CH2CH(OH)CF3, 1; (CF3)2C(OH)C(OH)(CF3)2, 2; ((CF3)2C(OH)CH2)2CHOH, 3) by pKa measurements in DMSO and H2O, negative ion photoelectron spectroscopy and binding constant determinations with Cl–. Their catalytic behavior in several reactions were also examined and compared to a Brønsted acid (HOAc) and a commonly employed thiourea ((3,5-(CF3)2C6H3NH)2CS). The combination of inductive stabilization and hydrogen bonds was found to afford potent acids which are effective catalysts. It also appears that hydrogen bonds can transmit the inductive effect over distance even in an aqueous environment, and this has far reaching implications.

Three newly synthesized [Na+(221-Kryptofix)] salts containing AsCO–, PCO–, and PCS– anions were successfully electrosprayed into a vacuum, and these three ECX– anions were investigated by negative ion photoelectron spectroscopy (NIPES) along with high-resolution photoelectron imaging spectroscopy. For each ECX– anion, a well-resolved NIPE spectrum was obtained, in which every major peak is split into a doublet. The splittings are attributed to spin–orbit coupling (SOC) in the ECX• radicals. Vibrational progressions in the NIPE spectra of ECX– were assigned to the symmetric and the antisymmetric stretching modes in ECX• radicals. The electron affinities (EAs) and SO splittings of ECX• are determined from the NIPE spectra to be AsCO•: EA = 2.414 ± 0.002 eV, SO splitting = 988 cm–1; PCO•: EA = 2.670 ± 0.005 eV, SO splitting = 175 cm–1; PCS•: EA = 2.850 ± 0.005 eV, SO splitting = 300 cm–1. Calculations using the B3LYP, CASPT2, and CCSD(T) methods all predict linear geometries for both the anions and the neutral radicals. The calculated EAs and SO splittings for ECX• are in excellent agreement with the experimentally measured values. The simulated NIPE spectra, which are based on the calculated Franck–Condon factors, and the SO splittings nicely reproduce all of the observed spectral peaks, thus allowing unambiguous spectral assignments. The finding that PCS• has the greatest EA of the three triatomic molecules considered here is counterintuitive based upon simple electronegativity considerations, but this finding is understandable in terms of the movement of electron density from phosphorus in the HOMO of PCO– to sulfur in the HOMO of PCS–. Comparisons of the EAs of PCO• and PCS• with the previously measured EA values for NCO• and NCS• are made and discussed.

Sulfuric acid is commonly known to be a strong acid and, by all counts, should readily donate its proton to formate, which has much higher proton affinity. This conventional wisdom is challenged in this work, where temperature-dependent negative ion photoelectron spectroscopy and theoretical studies demonstrate the existence of the (HCOO–)(H2SO4) pair at an energy slightly below the conventional (HCOOH)(HSO4–) structure. Analysis of quantum-mechanical calculations indicates that a large proton affinity difference (∼36 kcal/mol), favoring proton transfer to formate, is offset by the gain in intermolecular interaction energy between HCOO– and H2SO4 through the electron delocalization and formation of two strong hydrogen bonds. However, this stabilization comes with a severe entropic penalty, requiring the two species in the precise alignment. As a result, the population of (HCOO–)(H2SO4) drops significantly at higher temperatures, rendering (HCOOH)(HSO4–) to be the dominant species. This phenomenon is consistent with the photoelectron data, which shows depletion in the spectra assigned to (HCOO–)(H2SO4), and has also been verified by ab initio molecular dynamics (AIMD) simulations.

Structures and energetics of o-, m-, and p-quinonimide anions (OC6H4N–) and quinoniminyl radicals have been investigated by using negative ion photoelectron spectroscopy. Modeling of the photoelectron spectrum of the ortho isomer shows that the ground state of the anion is a triplet, while the quinoniminyl radical has a doublet ground state with a doublet-quartet splitting of 35.5 kcal/mol. The para radical has doublet ground state, but a band for a quartet state is missing from the photoelectron spectrum indicating that the anion has a singlet ground state, in contrast to previously reported calculations. The theoretical modeling is revisited here, and it is shown that accurate predictions for the electronic structure of the para-quinonimide anion require both an accurate account of electron correlation and a sufficiently diffuse basis set. Electron affinities of o- and p-quinoniminyl radicals are measured to be 1.715 ± 0.010 and 1.675 ± 0.010 eV, respectively. The photoelectron spectrum of the m-quinonimide anion shows that the ion undergoes several different rearrangements, including a rearrangement to the energetically favorable para isomer. Such rearrangements preclude a meaningful analysis of the experimental spectrum.

Pinonic acid, a C10-monocarboxylic acid with a hydrophilic –CO2H group and a hydrophobic hydrocarbon backbone, is a key intermediate oxidation product of α-pinene – an important monoterpene compound in biogenic emission processes that influences the atmosphere. Molecular interaction between cis-pinonic acid and water is essential for understanding its role in the formation and growth of pinene-derived secondary organic aerosols. In this work, we studied the structures, energetics, and optical properties of hydrated clusters of the cis-pinonate anion (cPA−), the deprotonated form of cis-pinonic acid, by negative ion photoelectron spectroscopy and ab initio theoretical calculations. Our results show that cPA− can adopt two different structural configurations – open and folded. In the absence of waters, the open configuration has the lowest energy and provides the best agreement with the experiment. The added waters, which mainly interact with the negatively charged –CO2− group, gradually stabilize the folded configuration and lower its energy difference relative to the most stable open-configured structure. Thermochemical and equilibrium hydrate distribution analyses suggest that the mono- and di-hydrates are likely to exist in humid atmospheric environments with high populations. The detailed molecular description of cPA− hydrated clusters unraveled in this study provides a valuable reference for understanding the initial nucleation process and aerosol formation involving organics containing both hydrophilic and hydrophobic groups, as well as for analyzing the optical properties of those organic aerosols.

Ion specificity, a widely observed macroscopic phenomenon in condensed phases and at interfaces, is a fundamental chemical physics issue. Herein we report our recent studies of such effects using cluster models in an “atom-by-atom” and “molecule-by-molecule” fashion not possible with the condensed-phase methods. We use electrospray ionization (ESI) to generate molecular and ionic clusters to simulate key molecular entities involved in local binding regions and characterize them by employing negative ion photoelectron spectroscopy (NIPES). Inter- and intramolecular interactions and binding configurations are directly obtained as functions of the cluster size and composition, providing molecular-level descriptions and characterization over the local active sites that play crucial roles in determining the solution chemistry and condensed-phase phenomena. The topics covered in this article are relevant to a wide range of research fields from ion specific effects in electrolyte solutions, ion selectivity/recognition in normal functioning of life, to molecular specificity in aerosol particle formation, as well as in rational material design and synthesis.

The CO3 radical anion (CO3˙−) has been formed by electrospraying carbonate dianion (CO32−) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO3˙− shows that, unlike the isoelectronic trimethylenemethane [C(CH2)3], D3h carbon trioxide (CO3) has a singlet ground state. From the NIPE spectrum, the electron affinity of D3h singlet CO3 was, for the first time, directly determined to be EA = 4.06 ± 0.03 eV, and the energy difference between the D3h singlet and the lowest triplet was measured as ΔEST = − 17.8 ± 0.9 kcal mol−1. B3LYP, CCSD(T), and CASPT2 calculations all find that the two lowest triplet states of CO3 are very close in energy, a prediction that is confirmed by the relative intensities of the bands in the NIPE spectrum of CO3˙−. The 560 cm−1 vibrational progression, seen in the low energy region of the triplet band, enables the identification of the lowest, Jahn–Teller-distorted, triplet state as 3A1, in which both unpaired electrons reside in σ MOs, rather than 3A2, in which one unpaired electron occupies the b2 σ MO, and the other occupies the b1 π MO.

We report here a negative ion photoelectron spectroscopy (NIPES) and ab initio study of the recently synthesized planar aromatic inorganic ion P2N3−, to investigate the electronic structures of P2N3− and its neutral P2N3˙ radical. The adiabatic detachment energy of P2N3− (electron affinity of P2N3˙) was determined to be 3.765 ± 0.010 eV, indicating high stability for the P2N3− anion. Ab initio electronic structure calculations reveal the existence of five, low-lying, electronic states in the neutral P2N3˙ radical. Calculation of the Franck–Condon factors (FCFs) for each anion-to-neutral electronic transition and comparison of the resulting simulated NIPE spectrum with the vibrational structure in the observed spectrum allows the first four excited states of P2N3˙ to be determined to lie 6.2, 6.7, 11.5, and 22.8 kcal mol−1 above the ground state of the radical, which is found to be a 6π-electron, 2A1, σ state.

Negative ion photoelectron spectroscopy shows interesting regioisomer-specific electron affinities (EAs) of 2,5- and 7,23-para-adducts of C70 [(ArCH2)2C70] (Ar = Ph, o-, m-, and p-BrC6H4). Their EA values are larger than that of C70 by 5–150 meV with the 2,5-polar adducts' EAs being higher than their corresponding 7,23-equatorial counterparts, exhibiting appreciable EA tunable ranges and regioisomeric specificity. Density functional theory (DFT) calculations reproduce both the experimental EA values and EA trends very well.

cis-Pinic acid is one of the most important oxidation products of α-pinene – a key monoterpene compound in biogenic emission processes. Molecular level understanding of its interaction with water in cluster formation is an important and necessary prerequisite for ascertaining its role in the aerosol formation processes. In this work, we studied the structures and energetics of the solvated clusters of cis-pinate (cis-PA2−), the doubly deprotonated dicarboxylate of cis-pinic acid, with H2O, CH3OH, and CH3CN by negative ion photoelectron spectroscopy and ab initio theoretical calculations. We found that cis-PA2− prefers being solvated alternately on the two –CO2− groups with increase of solvent coverage, a well-known solvation pattern that has been observed in microhydrated linear dicarboxylate dianion (DCn2−) clusters. Experiments and calculations further reveal an intriguing feature for the existence of the asymmetric type isomers for cis-PA2−(H2O)2 and cis-PA2−(CH3OH)2, in which both solvent molecules interact with only one of the –CO2− groups, a phenomenon that has not been observed in DCn2− water clusters and exhibits that the subtle effect of the rigid four-membered carbon ring brought on the cis-PA2− solvation. The dominant interactions between cis-PA2− and solvent molecules form bidentate O−⋯H–O H-bonds for H2O, O−⋯H–O and O−⋯H–C H-bonds for CH3OH, and tridentate O−⋯H–C H-bonds for CH3CN. The formation of inter-solvent H-bonds between H2O and CH3CN is found to be favorable in mixed solvent clusters, different from that between H2O and CH3OH. These findings have important implications for understanding the mechanism of cluster growth and the formation of atmospheric organic aerosols, as well as for rationalizing the nature of structure–function relationship of proteins containing carboxylate groups in various solvent environments.

Bicarbonate plays a crucial biochemical role in the physiological pH buffering system and also has important atmospheric implications. In the current study, HCO3−(H2O)n (n = 0–13) clusters were successfully produced via electrospray ionization of the corresponding bulk salt solution, and were characterized by negative ion photoelectron spectroscopy and theoretical calculations. Photoelectron spectra reveal that the electron binding energy monotonically increases with the cluster size up to n = 10 and remains largely the same after n > 10. The photo-detaching feature of the solute HCO3− itself, which dominates in the small clusters, diminishes with the increase of water coverage. Based on the charge distribution and molecular orbital analyses, the universal high electron binding energy tail that dominates in the larger clusters can be attributed to the ionization of water. Thus, the transition of ionization from the solute to the solvent at a size larger than n = 10 has been observed. Extensive theoretical structural search based on the basin-hopping unbiased method was carried out, and a plethora of low energy isomers have been obtained for each medium and large-sized cluster. By comparing the simulated photoelectron spectra and calculated electron binding energies with the experiments, as well as by comparing the simulated infrared spectra with previously reported IR spectra, the best fit structures and the structural evolutionary routes are presented. The nature of bicarbonate–water interactions is mainly electrostatic as implied by electron localization function (ELF) analysis.

Benzene rings substituted with 1–3 thiourea containing arms (1–3) were examined by photoelectron spectroscopy and density functional theory computations. Their conjugate bases and chloride, acetate, and dihydrogen phosphate anion clusters are reported. The resulting vertical and adiabatic detachment energies span 3.93–5.82 eV (VDE) and 3.65–5.10 (ADE) for the deprotonated species and 4.88–5.97 eV (VDE) and 4.45–5.60 eV (ADE) for the anion complexes. These results reveal the stabilizing effects of multiple hydrogen bonds and anionic host–guest interactions in the gas phase. Previously measured equilibrium binding constants in aqueous dimethyl sulfoxide for all three thioureas are compared to the present results, and cooperative binding is uniformly observed in the gas phase but only for one case (i.e., 3·H2PO4–) in solution.

We report here the results of a combined experimental and computational study of the negative ion photoelectron spectroscopy (NIPES) of the recently synthesized, planar, aromatic, HCPN3– ion. The adiabatic electron detachment energy of HCPN3– (electron affinity of HCPN3•) was measured to be 3.555 ± 0.010 eV, a value that is intermediate between the electron detachment energies of the closely related (CH)2N3– and P2N3– ions. High level electronic structure calculations and Franck–Condon factor (FCF) simulations reveal that transitions from the ground state of the anion to two nearly degenerate, low-lying, electronic states, of the neutral HCPN3• radical are responsible for the congested peaks at low binding energies in the NIPE spectrum. The best fit of the simulated NIPE spectrum to the experimental spectrum indicates that the ground state of HCPN3• is a 5π-electron 2A″ π radical state, with a 6π-electron, 2A′, σ radical state being at most 1.0 kcal/mol higher in energy.

The behavior of charged solute molecules in aqueous solutions is often classified using the concept of kosmotropes (“structure makers”) and chaotropes (“structure breakers”). There is a growing consensus that the key to kosmotropic/chaotropic behaviors lies in the local solvent region, but the exact microscopic basis for such differentiation is not well-understood. This issue is examined in this work by analyzing size selective solvation of a well-known chaotrope, a negatively charged SCN– molecule. Combining experimental photoelectron spectroscopy measurements with theoretical modeling, we examine evolution of solvation structure up to eight waters. We observe that SCN– indeed fits the description of weakly hydrated ion, and its solvation is heavily driven by stabilization of water–water interaction network. However, the impact on water structure is more subtle than that associated with “structure breaker”. In particular, we observe that the solvation structure of SCN– preserves the “packing” structure of the water network but changes local directionality of hydrogen bonds in the local solvent region. The resulting effect is closer to that of “structure weakener”, where solute can be readily accommodated into the native water network, at the cost of compromising its stability due to constraints on hydrogen bonding directionality.

Flexible acyclic alcohols with one to five hydroxyl groups were bound to a chloride anion and these complexes were interrogated by negative ion photoelectron spectroscopy and companion density functional theory computations. The resulting vertical detachment energies are reproduced on average to 0.10 eV by M06-2X/aug-cc-pVTZ predictions and range from 4.45–5.96 eV. These values are 0.84–2.35 eV larger than the adiabatic detachment energy of Cl– as a result of the larger hydrogen bond networks in the bigger polyols. Adiabatic detachment energies of the alcohol–Cl– clusters are more difficult to determine both experimentally and computationally. This is due to the large geometry changes that occur upon photodetachment and the large bond dissociation energy of H–Cl which enables the resulting chlorine atom to abstract a hydrogen from any of the methylene (CH2) or methine (CH) positions. Both ionic and nonionic hydrogen bonds (i.e., OH···Cl– and OH···OH···Cl–) form in the larger polyols complexes and are found to be energetically comparable. Subtle structural differences, consequently can lead to the formation of different types of hydrogen bonds, and maximizing the ionic ones is not always preferred. Solution equilibrium binding constants between the alcohols and tetrabutylammonium chloride (TBACl) in acetonitrile at −24.2, +22.0, and +53.6 °C were also determined. The free energies of association are nearly identical for all of the substrates (i.e., ΔG° = −2.8 ± 0.7 kcal mol–1). Compensating enthalpy and entropy values reveal, contrary to expectation and the intrinsic gas-phase preferences, that the bigger systems with more hydroxyl groups are entropically favored and enthalpically disfavored relative to the smaller species. This suggests that more solvent molecules are released upon binding TBACl to alcohols with more hydroxyl groups and is consistent with the measured negative heat capacities. These quantities increase with molecular complexity of the substrate, however, contrary to common interpretation of these values.

Dicarboxylic acids represent an important class of water-soluble organic compounds found in the atmosphere. In this work we are studying properties of dicarboxylic acid homodimer complexes (HO2C(CH2)nCO2–[HO2C(CH2)nCO2H], n = 0–12), as potentially important intermediates in aerosol formation processes. Our approach is based on experimental data from negative ion photoelectron spectra of the dimer complexes combined with updated measurements of the corresponding monomer species. These results are analyzed with quantum-mechanical calculations, which provide further information about equilibrium structures, thermochemical parameters associated with the complex formation, and evaporation rates. We find that upon formation of the dimer complexes the electron binding energies increase by 1.3–1.7 eV (30.0–39.2 kcal/mol), indicating increased stability of the dimerized complexes. Calculations indicate that these dimer complexes are characterized by the presence of strong intermolecular hydrogen bonds with high binding energies and are thermodynamically favorable to form with low evaporation rates. Comparison with the previously studied HSO4–[HO2C(CH2)2CO2H] complex (J. Phys. Chem. Lett. 2013, 4, 779–785) shows that HO2C(CH2)2CO2–[HO2C(CH2)2CO2H] has very similar thermochemical properties. These results imply that dicarboxylic acids not only can contribute to the heterogeneous complexes formation involving sulfuric acid and dicarboxylic acids but also can promote the formation of homogeneous complexes by involving dicarboxylic acids themselves.

[Ni(dddt)2]− (dddt = 5,6-dihydro-1,4-dithiine-2,3-dithiolate) and [Ni(edo)2]− (edo = 5,6-dihydro-1,4-dioxine-2,3-dithiolate) are two donor-type nickel bis(dithiolene) complexes, with the tendency of donating low binding energy electrons. These two structurally similar complexes differ only with respect to the outer atoms in the ligand framework where the former has four S atoms while the latter has four O atoms. Herein, we report a negative ion photoelectron spectroscopy (NIPES) study on these two complexes to probe the electronic structures of the anions and their corresponding neutrals. The NIPE spectra exhibit the adiabatic electron detachment energy (ADE) or, equivalently, the electron affinity (EA) of the neutral [Ni(L)2]0 to be relatively low for this type of complexes, 2.780 and 2.375 eV for L = dddt and edo, respectively. The 0.4 eV difference in ADEs shows a significant substitution effect for sulfur in dddt by oxygen in edo, i.e., noninnocence of the ligands, which has decreased the electronic stability of [Ni(edo)2]− by lowering its electron binding energy by ∼0.4 eV. The observed substitution effect on gas-phase EA values correlates well with the measured redox potentials for [Ni(dddt)2]−/0 and [Ni(edo)2]−/0 in solutions. The singlet–triplet splitting (ΔEST) of [Ni(dddt)2]0 and [Ni(edo)2]0 is also determined from the spectra to be 0.57 and 0.53 eV, respectively. Accompanying DFT calculations and molecular orbital (MO) composition analyses show significant ligand contributions to the redox MOs and allow the components of the orbitals involved in each electronic transition and spectral assignments to be identified.

The negative ion photoelectron (NIPE) spectrum of 1,2,4,5-tetraoxatetramethylenebenzene radical anion (TOTMB•–) shows that, like the hydrocarbon, 1,2,4,5-tetramethylenebenzene (TMB), the TOTMB diradical has a singlet ground state and thus violates Hund’s rule. The NIPE spectrum of TOTMB•– gives a value of −ΔEST = 3.5 ± 0.2 kcal/mol for the energy difference between the singlet and triplet states of TOTMB and a value of EA = 4.025 ± 0.010 eV for the electron affinity of TOTMB. (10/10)CASPT2 calculations are successful in predicting the singlet–triplet energy difference in TOTMB almost exactly, giving a computed value of −ΔEST = 3.6 kcal/mol. The same type of calculations predict −ΔEST = 6.1–6.3 kcal/mol in TMB. Thus, the calculated effect of the substitution of the four oxygens in TOTMB for the four methylene groups in TMB is very unusual, since the singlet state is selectively destabilized relative to the triplet state. The reason why TMB → TOTMB is predicted to result in a decrease in the size of −ΔEST is discussed.

Reaction of C60, C6F5CF2I, and SnH(n-Bu)3 produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C60(CF2C6F5)H (1) and 1,9-C60(cyclo-CF2(2-C6F4)) (2). The highest isolated yield of 1 was 35% based on C60. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C60. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn2(n-Bu)6 at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C6D6 by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp2). DFT calculations indicate that when 1 is deprotonated, the anion C60(CF2C6F5)− can undergo facile intramolecular SNAr annulation to form 2 with concomitant loss of F−. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an SNAr nucleophile towards an aromatic C–F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C60 and higher by 0.18(1) eV than the EA of phenyl-C61-butyric acid methyl ester (PCBM). In contrast, the relative E1/2(0/−) values of 2 and C60, −0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E1/2(0/−) of PCBM is −0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene–fullerene interactions with centroid⋯centroid (⊙⋯⊙) distances of ≤10.34 Å is significantly greater, and the average ⊙⋯⊙ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙⋯⊙ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙⋯⊙ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM.

Systematic theoretical and experimental investigations have been performed to understand the periodicity, electronic structures, and bonding of gold halides using tetrahalide [AuX4]− anions (X = F, Cl, Br, I, At, Uus). The [AuX4]− (X = Cl, Br, I) anions were experimentally produced in the gas phase, and their negative-ion photoelectron spectra were obtained, exhibiting rich and well-resolved spectral peaks. As expected, Au–X bonds in such series contain generally increasing covalency when halogen ligands become heavier. We calculated the adiabatic electron detachment energies as well as vertical electron detachment energies using density functional theory methods with scalar relativistic and spin–orbit coupling effects. The computationally simulated photoelectron spectra are in good agreement with the experimental ones. Our results show that the trivalent AuIII oxidation state becomes progressively less stable while AuI tends to be preferred when the halides become heavier along the Periodic Table. This series of molecules provides an example for manipulating the oxidation state of metals in complexes through ligand design.

Proposed in theory and then their existence confirmed, anion–π interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, anion–π interaction strengths that are free from complications of condensed-phase environments have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic, was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl−, Br−, I−, linear thiocyanate SCN−, trigonal planar nitrate NO3−, pyramidic iodate IO3−, and tetrahedral sulfate SO42−). The binding energies of the resultant gaseous 1:1 complexes (1·Cl−, 1·Br−, 1·I−, 1·SCN−, 1·NO3−, 1·IO3− and 1·SO42−) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion-specific effects. The binding strengths of Cl−, NO3−, IO3− with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal mol−1, but only about 40% of that between 1 and SO42−. Quantum chemical calculations reveal that all the anions reside in the center of the cavity of 1 with an anion–π binding motif in the complexes' optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and charge distribution analyses further support anion–π binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represents a powerful technique to probe anion–π interactions and has potential to provide quantitative guest–host molecular binding strengths and unravel fundamental insights in specific anion recognitions.

The oxidation power of permanganates (MnO4–) is known to be strongly dependent on pH values, and is greatly enhanced in acidic solutions, in which, for example, MnO4– can even oxidize Cl– ions to produce Cl2 molecules. Although such dependence has been ascribed due to the different reduced states of Mn affordable in different pH media, a molecular level understanding and characterization of initial redox pair complexes available in different pH solutions is very limited. Herein, we report a comparative study of [MnO4]− and [MnO4·Sol]− (Sol = H2O, KCl, and HCl) anion clusters by negative ion photoelectron spectroscopy (NIPES) and theoretical computations to probe chemical bonding and electronic structures of [MnO4·Sol]− clusters, aimed to obtain a microscopic understanding of how MnO4– interacts with surrounding molecules. Our study shows that H2O behaves as a solvent molecule, KCl is a spectator bound by pure electrostatic interactions, both of which do not influence the MnO4– identity in their respective clusters. In contrast, in [MnO4·HCl]−, the proton is found to interact with both MnO4– and Cl– with appreciable covalent characters, and the frontier MOs of the cluster are comprised of contributions from both MnO4– and Cl– moieties. Therefore, the proton serves as a chemical bridge, bringing two negatively charged redox species together to form an intimate redox pair. By adding more H+ to MnO4–, the oxygen atom can be taken away in the form of a water molecule, leaving MnO4– as an electron deficient MnO3+ species, which can subsequently oxidize Cl– ions.

We report the first low-temperature photoelectron spectra of isolated gas-phase complexes of the platinum II cyanide dianion bound to nucleobases. These systems are models for understanding platinum-complex photodynamic therapies, and a knowledge of the intrinsic photodetachment properties is crucial for characterizing their broader photophysical properties. Well-resolved, distinct peaks are observed in the spectra, consistent with complexes where the Pt(CN)42– moiety is largely intact. Adiabatic electron detachment energies for the dianion-nucleobase complexes are measured to be 2.39–2.46 eV. The magnitudes of the repulsive Coulomb barriers of the complexes are estimated to be between 1.9 and 2.1 eV, values that are lower than for the bare Pt(CN)42– dianion as a result of charge solvation by the nucleobases. In addition to the resolved spectral features, broad featureless bands indicative of delayed electron detachment are observed in the 193 nm photoelectron spectra of the four dianion-nucleobase complexes and also in the 266 nm spectra of the Pt(CN)42–·thymine and Pt(CN)42–·adenine complexes. The selective excitation of these features in the 266 nm spectra is attributed to one-photon excitation of [Pt(CN)42–·thymine]* and [Pt(CN)42–·adenine]* long-lived excited states that can effectively couple to the electron detachment continuum, producing strong electron detachment signals. We attribute the delayed electron detachment bands observed here for Pt(CN)42–·thymine and Pt(CN)42–·adenine but not for Pt(CN)42–·uracil and Pt(CN)42–·cytosine to fundamental differences in the individual nucleobase photophysics following 266 nm excitation. This indicates that the Pt(CN)42– dianion in the clusters can be viewed as a “dynamic tag” which has the propensity to emit electrons when the attached nucleobase displays a long-lived excited state.

Negative ion photoelectron (NIPE) spectra of the radical anion of meta-benzoquinone (MBQ, m-OC6H4O) have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show well-resolved peaks and complex spectral patterns. The electron affinity of MBQ is determined from the first resolved peak to be 2.875 ± 0.010 eV. Single-point, CASPT2/aug-cc-pVTZ//CASPT2/aug-cc-pVDZ calculations predict accurately the positions of the 0–0 bands in the NIPE spectrum for formation of the four lowest electronic states of neutral MBQ from the 2A2 state of MBQ•–. In addition, the Franck–Condon factors that are computed from the CASPT2/aug-cc-pVDZ optimized geometries, vibrational frequencies, and normal mode vectors, successfully simulate the intensities and frequencies of the vibrational peaks in the NIPE spectrum that are associated with each of these electronic states. The successful simulation of the NIPE spectrum of MBQ•– allows the assignment of 3B2 as the ground state of MBQ, followed by the 1B2 and 1A1 electronic states, respectively 9.0 ± 0.2 and 16.6 ± 0.2 kcal/mol higher in energy than the triplet. These experimental energy differences are in good agreement with the calculated values of 9.7 and 15.7 kcal/mol. The relative energies of these two singlet states in MBQ confirm the previous prediction that their relative energies would be reversed from those in meta-benzoquinodimethane (MBQDM, m-CH2C6H4CH2).

Negative ion photoelectron (NIPE) spectra of the radical anion of cyclopropane-1,2,3-trione, (CO)3•–, have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show broadened bands, due to the short lifetimes of both the singlet and triplet states of neutral (CO)3 and, to a lesser extent, to the vibrational progressions that accompany the photodetachment process. The smaller intensity of the band with the lower electron binding energy suggests that the singlet is the ground state of (CO)3. From the NIPE spectra, the electron affinity (EA) and the singlet–triplet energy gap of (CO)3 are estimated to be, respectively, EA = 3.1 ± 0.1 eV and ΔEST = −14 ± 3 kcal/mol. High-level, (U)CCSD(T)/aug-cc-pVQZ//(U)CCSD(T)/aug-cc-pVTZ, calculations give EA = 3.04 eV for the 1A1′ ground state of (CO)3 and ΔEST = −13.8 kcal/mol for the energy gap between the 1A1′ and 3A2 states, in excellent agreement with values from the NIPE spectra. In addition, simulations of the vibrational structures for formation of these states of (CO)3 from the 2A2″ state of (CO)3•– provide a good fit to the shapes of broad bands in the 266 nm NIPE spectrum. The NIPE spectrum of (CO)3•– and the analysis of the spectrum by high-quality electronic structure calculations demonstrate that NIPES can not only access and provide information about transition structures but NIPES can also access and provide information about hilltops on potential energy surfaces.

Clustering an anion with one or more neutral molecules is a stabilizing process that enhances the oxidation potential of the complex relative to the free ion. Several hydrogen bond clusters (i.e., A— • HX, where A— = H2PO4— and CF3CO2— and HX = MeOH, PhOH, and Me2NOH or Et2NOH) are examined by photoelectron spectroscopy and M06-2X and CCSD(T) computations. Remarkably, these species are experimentally found to have adiabatic detachment energies that are smaller than those for the free ion and reductions of 0.47 to 1.87 eV are predicted computationally. Hydrogen atom and proton transfers upon vertical photodetachment are two limiting extremes on the neutral surface in a continuum of mechanistic pathways that account for these results, and the whole gamut of possibilities are predicted to occur.

Rigid tricyclic locked in all axial 1,3,5-cyclohexanetriol derivatives with 0–3 trifluoromethyl groups were synthesized and photoelectron spectra of their conjugate bases and chloride anion clusters are reported along with density functional computations. The resulting vertical and adiabatic detachment energies span 4.07–5.50 eV (VDE) and 3.75–5.00 (ADE) for the former ions and 5.60–6.23 eV (VDE) and 5.36–6.00 eV (ADE) for the latter species. These results provide measures of the anion stabilization due to the hydrogen bond network and inductive effects. The latter mechanism is found to be transmitted through space via hydrogen bonds, and the presence of three ring skeleton oxygen atoms and up to three trifluoromethyl groups enhance the ADEs by 1.61–2.88 eV for the conjugate bases and 1.01–1.60 eV for the chloride anion clusters. Computations indicate that the most favorable structures of the latter complexes have two hydrogen bonds to the chloride anion and one bifurcated interaction between the remote OH substituent and the two hydroxyl groups that directly bind to Cl–.

Molecular species with electron affinities (EAs) larger than that of the chlorine atom (3.6131 eV) are superhalogens. The corresponding negative ions, namely, superhalogen anions, are intrinsically very stable with high electron binding energies (EBEs) and widely exist as building blocks of bulk materials and ionic liquids. The most common superhalogen anions proposed and experimentally confirmed to date are either ionic salts or compact inorganic species. Herein, we report a new class of superhalogen species, a series of tetracoordinated organoboron anions [BL4]− (L = phenyl (1), 4-fluorophenyl (2), 1-imidazolyl (3), L4 = H(pyrazolyl)3 (4)) with bulky organic ligands covalently bound to the central B atom. Negative ion photoelectron spectroscopy (NIPES) reveals that all of these anions possess EBEs higher than that of Cl– with the adiabatic/vertical detachment energy (ADE/VDE) of 4.44/4.8 (1), 4.78/5.2 (2), 5.08/5.4 (3), and 4.59/4.9 eV (4), respectively. First-principles calculations confirmed high EBEs of [BL4]− and predicted that these anions are thermodynamically stable against fragmentation. The unraveled superhalogen nature of these species provides a molecular basis to explain the wide-ranging applications of tetraphenylborate (TPB) (1) and trispyrazolylborate (Tp) (4) in many areas spanning from industrial waste treatment to soft material synthesis and organometallic chemistry.

Aspartate (Asp2–) and glutamate (Glu2–), two doubly charged conjugate bases of the corresponding amino acids, were investigated using low-temperature negative ion photoelectron spectroscopy (NIPES) and ab initio calculations. The effect of amine functionalization was studied by a direct comparison to the parent dicarboxylate species (−CO2–(CH2)n–CO2–, DCn2–), succinate (DC22–) and propionate (DC32–). Experimentally, the addition of the amine group for the n = 2 case (DC22–, Asp2–) significantly improves the stability of the resultant Asp2– dianionic species, albeit that NIPES shows only a small increase in adiabatic electron detachment energy (ADE) (+0.05 eV). In contrast, for n = 3 (DC32–, Glu2–), a much larger ADE increase is observed (+0.15 eV). Similar results are obtained through ab initio calculations. The latter indicates that increased stability of Asp2– can be attributed to the lowering of the energy of the singlet dianion state due to hydrogen bonding effects. The effect of the amino group on the doublet monoanion state is more complicated and results in the weakening of the binding of the adjacent carboxylate group due to electronic structure resonance effects. This conclusion is confirmed by the analysis of NIPES results that show enhanced production of near-zero kinetic energy electrons observed experimentally for amine-functionalized species.

The first excited state of the model green fluorescence protein (GFP) chromophore anion (S1) and its energy level against the electron-detached neutral radical D0 state are crucial in determining the photophysics and the photoinduced dynamics of GFP. Extensive experimental and theoretical studies, particularly several very recent gas-phase investigations, concluded that S1 is a bound state in the Franck–Condon vertical region with respect to D0. However, what remains unknown and challenging is if S1 is bound adiabatically, primarily due to lack of accurate experimental measurements as well as due to the close proximity in energy for these two states that even sophisticated high-level ab initio calculations cannot reliably predict. Here, we report a negative ion photoelectron spectroscopy study on the model GFP chromophore anion, the deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone anion (HBDI–) taken under low-temperature conditions with improved energy resolution. Despite the considerable size and low symmetry of the molecule, resolved vibrational structures were obtained with the 0–0 transition being the most intense peak. The adiabatic (ADE) and vertical detachment (VDE) energies therefore are determined both to be 2.73 ± 0.01 eV, indicating that the detached D0 state is 0.16 eV higher in energy than the photon excited S1 state. The accurate ADE and VDE values and the well-resolved photoelectron spectra reported here provide much needed robust benchmarks for future theoretical investigations.Keywords: bright excited state; green fluorescence protein (GFP); photodetached continuum; photoelectron spectroscopy;

We report quantifying the strengths of different types of hydrogen bonds in hydrogen-bond networks (HBNs) via measurement of the adiabatic electron detachment energy of the conjugate base of a small covalent polyol model compound (i.e., (HOCH2CH2CH(OH)CH2)2CHOH) in the gas phase and the pKa of the corresponding acid in DMSO. The latter result reveals that the hydrogen bonds to the charged center and those that are one solvation shell further away (i.e., primary and secondary) provide 5.3 and 2.5 pKa units of stabilization per hydrogen bond in DMSO. Computations indicate that these energies increase to 8.4 and 3.9 pKa units in benzene and that the total stabilizations are 16 (DMSO) and 25 (benzene) pKa units. Calculations on a larger linear heptaol (i.e., (HOCH2CH2CH(OH)CH2CH(OH)CH2)2CHOH) reveal that the terminal hydroxyl groups each contribute 0.6 pKa units of stabilization in DMSO and 1.1 pKa units in benzene. All of these results taken together indicate that the presence of a charged center can provide a powerful energetic driving force for enzyme catalysis and conformational changes such as in protein folding due to multiple hydrogen bonds in a HBN.

Cyclobutane-1,2,3,4-tetraone has been both predicted and found to have a triplet ground state, in which a b2g σ molecular orbital (MO) and an a2u π MO are each singly occupied. In contrast, (CO)5 and (CO)6 have each been predicted to have a singlet ground state. These predictions have been tested by generating the (CO)5•– and (CO)6•– radical anions in the gas phase, using electrospray vaporization of solutions of, respectively, the croconate (CO)52– and rhodizonate (CO)62– dianions. The negative ion photoelectron (NIPE) spectrum of the (CO)5•– radical anion gives an electron affinity of EA = 3.830 eV for formation of the singlet ground state of (CO)5. The triplet is found to be higher in energy by 0.850 eV (19.6 kcal/mol). The NIPE spectrum of the (CO)6•– radical anion gives EA = 3.785 eV for forming the singlet ground state of (CO)6, with the triplet state higher in energy by 0.915 eV (21.1 kcal/mol). (RO)CCSD(T)/aug-cc-pVTZ//(U)B3LYP/6-311+G(2df) calculations give EA values that are only approximately 1 kcal/mol lower than those measured and ΔEST values that are 2–3 kcal/mol higher than those obtained from the NIPE spectra. Calculations of the Franck–Condon factors for transitions from the ground state of each radical anion, (CO)n•– to the lowest singlet and triplet states of the n = 4–6 neutrals, nicely reproduce all of the observed vibrational features in the low-binding energy regions of all three NIPE spectra. Thus, the calculations of both the energies and vibrational structures of the two lowest energy bands in each of the NIPE spectra support the interpretation of the spectra in terms of a singlet ground state for (CO)5 and (CO)6 but a triplet ground state for (CO)4.

Despite the importance of group 6 metal-centered 17-electron radicals CpM(CO)3• (M = Cr, Mo, W) in establishing many of the fundamental reactions now known for metal-centered radicals, spectroscopic characterization of their electronic properties and structures has been very challenging, due to their high reactivity. Here we report a gas-phase study of these species by photodetachment photoelectron spectroscopy (PES) of their corresponding 18-electron anions and by theoretical electronic structure calculations. Three well-separated spectral features are observed by PES for each anionic species. Electron affinities (EAs) of CpM(CO)3• were experimentally measured from the threshold of each spectrum and were found to be 2.38 ± 0.02 (M = Cr), 2.63 ± 0.02 (Mo), and 2.63 ± 0.01 eV (W). These experimental values correlate well with the reported redox potentials measured in solution. Theoretical calculations for all anionic and neutral (radical) species gave calculated EAs and band gaps that are in good agreement with the experimental data. Molecular orbital (MO) analyses for each anion indicate that the top three occupied MOs are mainly metal-based and contribute to the first spectral feature, whereas the next two MOs are associated with Cp–M π bonding and contribute to the second spectral feature. The calculations further exhibit appreciable anion-to-neutral structural changes for all three species, with the change for the W species being the smallest.

Cyclobutane-1,2,3,4-tetrathione, (CS)4, has recently been calculated to have a singlet ground state, 1A1g, in which the highest b2g σ MO is doubly occupied and the lowest a2u π MO is empty. Thus, (CS)4 is predicted to have a different ground state than its lighter congener, (CO)4, which has a triplet ground state, 3B1u, in which these two MOs are each singly occupied. Here, we report the results of a negative ion photoelectron spectroscopy (NIPES) study of the radical anion (CS)4•–, designed to test the prediction that (CS)4 has a singlet ground state. The NIPE spectrum reveals that (CS)4 does, indeed, have a singlet ground state with electron affinity (EA) = 3.75 eV. The lowest triplet state is found to lie 0.31 eV higher in energy than the ground state, and the open-shell singlet is 0.14 eV higher in energy than the triplet state. Calculations at the (U)CCSD(T)/aug-cc-pVTZ//(U)B3LYP/6-311+G(2df) level support the spectral assignments, giving EA = 3.71 eV and ΔEST = 0.44 eV. These calculated values are, respectively, 0.04 eV (0.9 kcal/mol) smaller and 0.13 eV (3.0 kcal/mol) larger than the corresponding experimental values. In addition, RASPT2 calculations with various active spaces and basis sets converge on a 1B1u–3B1u energy gap of 0.137 eV, in excellent agreement with the 0.14 eV energy difference obtained from the NIPE spectrum. Finally, calculations of the Franck–Condon factors for transitions from the ground state of (CS)4•– to the ground (1A1g) and two excited states (3B1u, 1B1u) of (CS)4 account for all of the major spectral peaks and nicely reproduce the vibrational structure observed in each electronic transition. The close correspondence between the calculated and the observed features in the NIPE spectrum of (CS)4•– provides unequivocal proof that (CS)4, unlike (CO)4, has a singlet ground state.

The gas-phase electron affinity (EA) of phenyl–C61–butyric acid methyl ester (PCBM), one of the best-performing electron acceptors in organic photovoltaic devices, was measured by low-temperature photoelectron spectroscopy for the first time. The obtained value of 2.63(1) eV is only ca. 0.05 eV lower than that of C60 (2.683(8) eV), compared to a 0.09 V difference in their E1/2 values measured in this work by cyclic voltammetry. Literature E(LUMO) values for PCBM that are typically estimated from cyclic voltammetry and commonly used as a quantitative measure of acceptor properties are dispersed over a wide range between −4.38 and −3.62 eV; the reasons for such a huge discrepancy are analyzed here, and a protocol for reliable and consistent estimations of relative fullerene-based acceptor strength in solution is proposed.

Recent lab and field measurements have indicated critical roles of organic acids in enhancing new atmospheric aerosol formation. Such findings have stimulated theoretical studies with the aim of understanding the interaction of organic acids with common aerosol nucleation precursors like bisulfate (HSO4–). We report a combined negative ion photoelectron spectroscopic and theoretical investigation of molecular clusters formed by HSO4– with succinic acid (SUA, HO2C(CH2)2CO2H), HSO4–(SUA)n (n = 0–2), along with HSO4–(H2O)n and HSO4–(H2SO4)n. It is found that one SUA molecule can stabilize HSO4– by ca. 39 kcal/mol, three times the corresponding value that one water molecule is capable of (ca. 13 kcal/mol). Molecular dynamics simulations and quantum chemical calculations reveal the most plausible structures of these clusters and attribute the stability of these clusters to the formation of strong hydrogen bonds. This work provides direct experimental evidence showing significant thermodynamic advantage by involving organic acid molecules to promote formation and growth in bisulfate clusters and aerosols.Keywords: atmospheric aerosol; bisulfate clusters; H-bonding; photoelectron spectroscopy;

Nature employs flexible molecules to bind anions in a variety of physiologically important processes whereas supramolecular chemists have been designing rigid substrates that minimize or eliminate intramolecular hydrogen bond interactions to carry out anion recognition. Herein, the association of a flexible polyhydroxy alkane with chloride ion is described and the bound receptor is characterized by infrared and photoelectron spectroscopy in the gas phase, computations, and its binding constant as a function of temperature in acetonitrile.

Enzymes and their mimics use hydrogen bonds to catalyze chemical transformations. Small-molecule transition state analogues of oxyanion holes have been characterized by computations, gas-phase IR and photoelectron spectroscopy, and determination of their binding constants in acetonitrile. A new class of hydrogen bond catalysts is proposed (donors that can contribute three hydrogen bonds to a single functional group) and demonstrated in a Friedel–Crafts reaction. The employed catalyst was observed to react 100 times faster than its rotamer that can employ only two hydrogen bonds. The former compound also binds anions more tightly and was found to have a thermodynamic advantage.

Alkali metal cations often show pronounced ion-specific interactions and selectivity with macromolecules in biological processes, colloids, and interfacial sciences, but a fundamental understanding about the underlying microscopic mechanism is still very limited. Here we report a direct probe of interactions between alkali metal cations (M+) and dicarboxylate dianions, –O2C(CH2)nCO2– (Dn2–) in the gas phase by combined photoelectron spectroscopy (PES) and ab initio electronic structure calculations on nine M+–Dn2– complexes (M = Li, Na, K; n = 2, 4, 6). PES spectra show that the electron binding energy (EBE) decreases from Li+ to Na+ to K+ for complexes of M+–D22–, whereas the order is Li+ < Na+ ≈ K+ when M+ interacts with a more flexible D62– dianion. Theoretical modeling suggests that M+ prefers to interact with both ends of the carboxylate −COO– groups by bending the flexible aliphatic backbone, and the local binding environments are found to depend upon backbone length n, carboxylate orientation, and the specific cation M+. The observed variance of EBEs reflects how well each specific dicarboxylate dianion accommodates each M+. This work demonstrates the delicate interplay among several factors (electrostatic interaction, size matching, and strain energy) that play critical roles in determining the structures and energetics of gaseous clusters as well as ion specificity and selectivity in solutions and biological systems.

Despite a seemingly simple appearance, cyclobutanetetraone (C4O4) has four low-lying electronic states. Determining the energetic ordering of these states and the ground state of C4O4– theoretically has been proven to be considerably challenging and remains largely unresolved to date. Here, we report a low-temperature negative ion photoelectron spectroscopic approach. Well-resolved spectra were obtained at both 193 and 266 nm. Combined with recent theoretical studies and our own Franck–Condon factors simulations, the ground state of C4O4– and the ground and two low-lying excited states of C4O4 are determined to be 2A2u, 3B2u, 1A1g (8π), and 1B2u, respectively. The frequency of the ring breathing mode (1810 ± 20 cm–1), the electron affinity (3.475 ± 0.005 eV), and the term values of 1A1g (8π) (6.27 ± 0.5 kJ/mol) and 1B2u (13.50 ± 0.5 kJ/mol) are also directly obtained from the experiments.Keywords: diradical species; electronic structures of C4O4 and C4O4−; negative ion photoelectron spectroscopy;

Hydrogen bond interactions in small covalent model compounds (i.e., deprotonated polyhydroxy alcohols) were measured by negative ion photoelectron spectroscopy. The experimentally determined vertical and adiabatic electron detachment energies for (HOCH2CH2)2CHO–(2a), (HOCH2CH2)3CO– (3a), and (HOCH2CH2CH(OH)CH2)3CO– (4a)reveal that hydrogen-bonded networks can provide enormous stabilizations and that a single charge center not only can be stabilized by up to three hydrogen bonds but also can increase the interaction energy between noncharged OH groups by 5.8 kcal mol–1 or more per hydrogen bond. This can lead to pKa values that are very different from those in water and can provide some of the impetus for catalytic processes.

Electron-withdrawing trifluoromethyl groups were characterized in combination with hydrogen-bond interactions in three polyols (i.e., CF3CH(OH)CH2CH(OH)CF3, 1; (CF3)2C(OH)C(OH)(CF3)2, 2; ((CF3)2C(OH)CH2)2CHOH, 3) by pKa measurements in DMSO and H2O, negative ion photoelectron spectroscopy and binding constant determinations with Cl–. Their catalytic behavior in several reactions were also examined and compared to a Brønsted acid (HOAc) and a commonly employed thiourea ((3,5-(CF3)2C6H3NH)2CS). The combination of inductive stabilization and hydrogen bonds was found to afford potent acids which are effective catalysts. It also appears that hydrogen bonds can transmit the inductive effect over distance even in an aqueous environment, and this has far reaching implications.

Reaction of C60, C6F5CF2I, and SnH(n-Bu)3 produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C60(CF2C6F5)H (1) and 1,9-C60(cyclo-CF2(2-C6F4)) (2). The highest isolated yield of 1 was 35% based on C60. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C60. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn2(n-Bu)6 at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C6D6 by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp2). DFT calculations indicate that when 1 is deprotonated, the anion C60(CF2C6F5)− can undergo facile intramolecular SNAr annulation to form 2 with concomitant loss of F−. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an SNAr nucleophile towards an aromatic C–F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C60 and higher by 0.18(1) eV than the EA of phenyl-C61-butyric acid methyl ester (PCBM). In contrast, the relative E1/2(0/−) values of 2 and C60, −0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E1/2(0/−) of PCBM is −0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene–fullerene interactions with centroid⋯centroid (⊙⋯⊙) distances of ≤10.34 Å is significantly greater, and the average ⊙⋯⊙ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙⋯⊙ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙⋯⊙ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM.

Negative ion photoelectron spectroscopy shows interesting regioisomer-specific electron affinities (EAs) of 2,5- and 7,23-para-adducts of C70 [(ArCH2)2C70] (Ar = Ph, o-, m-, and p-BrC6H4). Their EA values are larger than that of C70 by 5–150 meV with the 2,5-polar adducts' EAs being higher than their corresponding 7,23-equatorial counterparts, exhibiting appreciable EA tunable ranges and regioisomeric specificity. Density functional theory (DFT) calculations reproduce both the experimental EA values and EA trends very well.

cis-Pinic acid is one of the most important oxidation products of α-pinene – a key monoterpene compound in biogenic emission processes. Molecular level understanding of its interaction with water in cluster formation is an important and necessary prerequisite for ascertaining its role in the aerosol formation processes. In this work, we studied the structures and energetics of the solvated clusters of cis-pinate (cis-PA2−), the doubly deprotonated dicarboxylate of cis-pinic acid, with H2O, CH3OH, and CH3CN by negative ion photoelectron spectroscopy and ab initio theoretical calculations. We found that cis-PA2− prefers being solvated alternately on the two –CO2− groups with increase of solvent coverage, a well-known solvation pattern that has been observed in microhydrated linear dicarboxylate dianion (DCn2−) clusters. Experiments and calculations further reveal an intriguing feature for the existence of the asymmetric type isomers for cis-PA2−(H2O)2 and cis-PA2−(CH3OH)2, in which both solvent molecules interact with only one of the –CO2− groups, a phenomenon that has not been observed in DCn2− water clusters and exhibits that the subtle effect of the rigid four-membered carbon ring brought on the cis-PA2− solvation. The dominant interactions between cis-PA2− and solvent molecules form bidentate O−⋯H–O H-bonds for H2O, O−⋯H–O and O−⋯H–C H-bonds for CH3OH, and tridentate O−⋯H–C H-bonds for CH3CN. The formation of inter-solvent H-bonds between H2O and CH3CN is found to be favorable in mixed solvent clusters, different from that between H2O and CH3OH. These findings have important implications for understanding the mechanism of cluster growth and the formation of atmospheric organic aerosols, as well as for rationalizing the nature of structure–function relationship of proteins containing carboxylate groups in various solvent environments.

We report here a negative ion photoelectron spectroscopy (NIPES) and ab initio study of the recently synthesized planar aromatic inorganic ion P2N3−, to investigate the electronic structures of P2N3− and its neutral P2N3˙ radical. The adiabatic detachment energy of P2N3− (electron affinity of P2N3˙) was determined to be 3.765 ± 0.010 eV, indicating high stability for the P2N3− anion. Ab initio electronic structure calculations reveal the existence of five, low-lying, electronic states in the neutral P2N3˙ radical. Calculation of the Franck–Condon factors (FCFs) for each anion-to-neutral electronic transition and comparison of the resulting simulated NIPE spectrum with the vibrational structure in the observed spectrum allows the first four excited states of P2N3˙ to be determined to lie 6.2, 6.7, 11.5, and 22.8 kcal mol−1 above the ground state of the radical, which is found to be a 6π-electron, 2A1, σ state.

Proposed in theory and then their existence confirmed, anion–π interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, anion–π interaction strengths that are free from complications of condensed-phase environments have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic, was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl−, Br−, I−, linear thiocyanate SCN−, trigonal planar nitrate NO3−, pyramidic iodate IO3−, and tetrahedral sulfate SO42−). The binding energies of the resultant gaseous 1:1 complexes (1·Cl−, 1·Br−, 1·I−, 1·SCN−, 1·NO3−, 1·IO3− and 1·SO42−) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion-specific effects. The binding strengths of Cl−, NO3−, IO3− with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal mol−1, but only about 40% of that between 1 and SO42−. Quantum chemical calculations reveal that all the anions reside in the center of the cavity of 1 with an anion–π binding motif in the complexes' optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and charge distribution analyses further support anion–π binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represents a powerful technique to probe anion–π interactions and has potential to provide quantitative guest–host molecular binding strengths and unravel fundamental insights in specific anion recognitions.

The CO3 radical anion (CO3˙−) has been formed by electrospraying carbonate dianion (CO32−) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO3˙− shows that, unlike the isoelectronic trimethylenemethane [C(CH2)3], D3h carbon trioxide (CO3) has a singlet ground state. From the NIPE spectrum, the electron affinity of D3h singlet CO3 was, for the first time, directly determined to be EA = 4.06 ± 0.03 eV, and the energy difference between the D3h singlet and the lowest triplet was measured as ΔEST = − 17.8 ± 0.9 kcal mol−1. B3LYP, CCSD(T), and CASPT2 calculations all find that the two lowest triplet states of CO3 are very close in energy, a prediction that is confirmed by the relative intensities of the bands in the NIPE spectrum of CO3˙−. The 560 cm−1 vibrational progression, seen in the low energy region of the triplet band, enables the identification of the lowest, Jahn–Teller-distorted, triplet state as 3A1, in which both unpaired electrons reside in σ MOs, rather than 3A2, in which one unpaired electron occupies the b2 σ MO, and the other occupies the b1 π MO.

Pinonic acid, a C10-monocarboxylic acid with a hydrophilic –CO2H group and a hydrophobic hydrocarbon backbone, is a key intermediate oxidation product of α-pinene – an important monoterpene compound in biogenic emission processes that influences the atmosphere. Molecular interaction between cis-pinonic acid and water is essential for understanding its role in the formation and growth of pinene-derived secondary organic aerosols. In this work, we studied the structures, energetics, and optical properties of hydrated clusters of the cis-pinonate anion (cPA−), the deprotonated form of cis-pinonic acid, by negative ion photoelectron spectroscopy and ab initio theoretical calculations. Our results show that cPA− can adopt two different structural configurations – open and folded. In the absence of waters, the open configuration has the lowest energy and provides the best agreement with the experiment. The added waters, which mainly interact with the negatively charged –CO2− group, gradually stabilize the folded configuration and lower its energy difference relative to the most stable open-configured structure. Thermochemical and equilibrium hydrate distribution analyses suggest that the mono- and di-hydrates are likely to exist in humid atmospheric environments with high populations. The detailed molecular description of cPA− hydrated clusters unraveled in this study provides a valuable reference for understanding the initial nucleation process and aerosol formation involving organics containing both hydrophilic and hydrophobic groups, as well as for analyzing the optical properties of those organic aerosols.

Two new tripodal hydroxyl-based anion receptors (1 and 2) are reported and their 1:1 molecular complexes with Cl−, H2PO4−, and OAc− along with the (M − 1)− ion of 1 were characterized by negative ion photoelectron spectroscopy in the gas phase and by binding constant determinations in four solvents (i.e., CDCl3, CD2Cl2, CD3COCD3, and CD3CN). An intramolecular hydrogen bond network (HBN) in hexaol 1 was found to diminish its binding whereas the triol 2 is the strongest aliphatic hydroxyl-based receptor to date.

Bicarbonate plays a crucial biochemical role in the physiological pH buffering system and also has important atmospheric implications. In the current study, HCO3−(H2O)n (n = 0–13) clusters were successfully produced via electrospray ionization of the corresponding bulk salt solution, and were characterized by negative ion photoelectron spectroscopy and theoretical calculations. Photoelectron spectra reveal that the electron binding energy monotonically increases with the cluster size up to n = 10 and remains largely the same after n > 10. The photo-detaching feature of the solute HCO3− itself, which dominates in the small clusters, diminishes with the increase of water coverage. Based on the charge distribution and molecular orbital analyses, the universal high electron binding energy tail that dominates in the larger clusters can be attributed to the ionization of water. Thus, the transition of ionization from the solute to the solvent at a size larger than n = 10 has been observed. Extensive theoretical structural search based on the basin-hopping unbiased method was carried out, and a plethora of low energy isomers have been obtained for each medium and large-sized cluster. By comparing the simulated photoelectron spectra and calculated electron binding energies with the experiments, as well as by comparing the simulated infrared spectra with previously reported IR spectra, the best fit structures and the structural evolutionary routes are presented. The nature of bicarbonate–water interactions is mainly electrostatic as implied by electron localization function (ELF) analysis.