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.

A series of methylated and octylated pyridinium and quinolinium containing thiourea salts with a chiral 2-indanol substituent are reported. These organocatalysts are positively charged analogues of privileged bis(3,5-trifluoromethyl)phenyl substituted thioureas, and are found to be much more active catalysts despite the absence of an additional hydrogen bond donor or acceptor site (i.e., the presence of a heteroatom–hydrogen or heteroatom). Friedel–Crafts reactions of trans-β-nitorostyrenes with indoles are examined, and good yields and enantioselectivities are obtained. Mechanistic studies indicate that this is a second-order transformation under the employed conditions, and is consistent with the dimer of the thiourea being the active catalyst. Charged organocatalysts, consequently, represent an attractive design strategy for catalyst development.

•A dicarboxylate dianion clusters with alcohols.•Sequential solvation is observed.•Surprisingly, the weakest base undergoes proton transfer.•Solvation of the conjugate acid results in a kinetically enhanced process.•Coulomb barriers can be mitigated by cluster formation.Electrospray ionization of 2,6-naphthalenedicarboxylic acid readily affords its doubly deprotonated dicarboxylate dianion (12−). This species clusters with background water and added alcohols in an ion trap at ∼10−3 Torr. Sequential solvation is observed to afford mono and dicoordinated ions. Surprisingly, the latter cluster (12−• 2TFE) is protonated by 2,2,2-trifluoroethanol (TFE) whereas 12−and 12−• TFE are not even though ΔH°acid(TFE) = 361.7 ± 2.5 kcal mol−1 (as given in the NIST website at http://webbook.nist.gov) and the B3LYP/6-31+G(d,p) proton affinities are 384.7 (12−), 377.6 (12−• TFE), and 362.7 (12−• 2TFE) kcal mol−1. That is, only the weakest base in this series, and the dianion with an equal number of solvent molecules and charged sites, undergoes proton transfer. In a FTMS instrument at lower pressures (∼10−8 Torr) inefficient proton abstraction is observed with the monosolvated dianion. This difference, and the observed reactivities of 12−, 12−• TFE and 12−• 2TFE are rationalized with the aid of computed potential energy surfaces. The chemical structures of these cluster ions were also probed via collision-induced dissociations, infrared photodissociation from 2700 to 3200 cm−1, and extensive calculations. All of the TFE species are found to be solvated dianions, but incipient proton transfer to afford electrostatically defying anion-anion clusters is noted in two cases. In proton transfer reactions, formation of the conjugate acid as a solvated ion lowers the energy of the system and reduces the Coulomb repulsion barrier facilitating the overall process.Download high-res image (135KB)Download full-size image

The equilibrium acidity of styrene was measured (ΔH°acid(PhCHCH2) = 390.6 ± 0.5 kcal mol−1) and its deprotonation site was revised from the ortho position on the aromatic ring to the α-hydrogen atom based upon deuterium-labeling studies and extensive computations. Somewhat surprisingly, the nature of the anionic base plays a critical role in properly determining the ionization site and avoiding misleading results due to extraordinary hydrogen–deuterium exchange. Bracketing the electron affinity of α-styryl radical (PhCCH2, 23.1 ± 3.4 kcal mol−1) enabled the α-CH bond dissociation energy (100.1 ± 3.4 kcal mol−1) of styrene and the effect of a phenyl substituent at an sp2-hybridized carbon to be determined. These results were compared to B3LYP, M06-2X, G3 and G4 computations.

A novel type of phosphoric acid catalyst with enhanced reactivity is reported. These compounds possess one or two positively charged centers which electrostatically activate them. This is illustrated in several bond-forming transformations including Friedel–Crafts and Diels–Alder reactions as well as a ring-opening polymerization. Rate accelerations corresponding to orders of magnitude are observed.

A new class of readily prepared thiourea catalysts with one or more positively charged centers and no new hydrogen-bonding sites are exploited in several bond-forming reactions and are orders of magnitude more reactive than Schreiner’s thiourea. These findings provide the basis for a new strategy for activating hydrogen-bond catalysts.

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.

The α-C–H bond dissociation energy (BDE) of phenylcyclopropane (1) was experimentally determined using Hess’ law. An equilibrium acidity determination of 1 afforded ΔH°acid = 389.1 ± 0.8 kcal mol–1, and isotopic labeling established that the α-position of the three-membered ring is the favored deprotonation site. Interestingly, the structure of the base proved to be a key factor in correctly determining the proper ionization site (i.e., secondary amide ions are needed, and primary ones and OH– lead to incorrect conclusions since they scramble the deuterium label). An experimental measurement of the electron affinity of 1-phenylcyclopropyl radical (EA = 17.5 ± 2.8 kcal mol–1) was combined with the ionization energy of hydrogen (313.6 kcal mol–1) to afford BDE = 93.0 ± 2.9 kcal mol–1. This enabled the effect of the phenyl substituent to be evaluated and compared to other situations where it is attached to an sp3- or sp2-hybridized carbon center. M06-2X, CCSD(T), G4, and W1BD computations were also carried out, and a revised C–H BDE for cyclopropane of 108.9 ± 1.0 kcal mol–1 is recommended.

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.

Acidities are commonly measured in polar solvents but catalytic reactions are typically carried out in nonpolar media. IR spectra of a series of phenols in CCl4 and 1% CD3CN/CCl4 provide relative acidities. Nonprotonated charged substituents with an appropriate counterion are found to enhance their Brønsted acidities and improve catalyst performance by orders of magnitude.

A series of neutral anion receptors with one to three thiourea arms were synthesized and their binding to tetrabutylammonium chloride, acetate, and dihydrogen phosphate salts in aqueous DMSO mixtures was examined. The three-armed thiourea host was found to strongly and selectively bind H2PO4− even in DMSO solutions containing up to 30% water. This enabled the dihydrogen phosphate salt to be extracted from water into chloroform in its dibasic form despite the high heat of the hydration of HPO42−.

Different strategies are employed in designing strong and selective anion receptors but stereoelectronic effects have been largely ignored. In this work, the stereo configuration of a non-interacting ether is found to have a large impact of more than two orders of magnitude on the binding of a rigid diol with tetrabutylammonium chloride in acetonitrile-d3. A favorable carbon–oxygen dipole and an intramolecular C–H⋯OH hydrogen bond in an equatorially substituted ether is found to be energetically more important than a stabilizing hydrogen bond in the corresponding axially oriented alcohol. IR spectroscopy is also used to probe the structures of the bound complexes and several binding motifs are identified.

•H/D exchange mechanisms of 16 small rigid anions were examined.•Four deuterated reagents were used to probe relay and flip-flop processes.•Distortion energies were found to account for relay mechanism barriers.Hydrogen–deuterium exchange reactions of 16 small rigid anions were examined using D2O, CH3OD, CH3CO2D, and (CF3)2C(CH3)OD. Bimolecular rate constants and extensive computations are reported. These deuterated reagents were used because D2O and CH3OD are weak acids that cannot react via sequential deuteron/proton transfers (i.e., the conventional H/D exchange mechanism of DePuy et al.) with the selected anions, and they provide a means to probe alternative pathways. The relative acidity of CH3CO2D and (CF3)2C(CH3)OD was measured (ΔΔHacid° = 1.1 ± 0.8 kcal mol−1) and their thermodynamic similarity but structural differences (i.e., the presence of a CO in the carboxylic acid but not the alcohol) were exploited to systematically investigate flip-flop and relay mechanisms. This has led to a number of generalizations relating to the occurrence of these pathways.

•Indenone's heat of hydrogenation was measured via two ion thermodynamic cycles.•High level G3 computations afford ΔHf° for cyclopentadienone (1) and indenone (2).•Both 1 and 2 are 4n π electron systems but are nonaromatic not antiaromatic.The heat of hydrogenation of indenone was measured via two partially independent thermodynamic cycles by carrying out energetic measurements (i.e., electron affinities, proton affinities and ionization potentials) on both negative and positive ions (ΔH°H2 = 17.8 ± 5.5 and 17.5 ± 5.7 kcal mol−1, respectively). High level G3 computations were also carried out to provide the heats of formation of indenone (16.8 kcal mol−1) and cyclopentadienone (14.0 kcal mol−1). These 4n π electron systems are found to be nonaromatic in contrast to previous views. A recent report on cyclopropenyl anion (J. Org. Chem. 2013, 78, 7370–7372) indicates that this ion is also nonaromatic, and suggests that NMR ring currents and nucleus independent chemical shift (NICS) calculations do not correlate with the energetic criterion for antiaromatic compounds.

Measured DMSO pKa values for a series of rigid tricyclic adamantane-like triols containing 0–3 trifluoromethyl groups (i.e., 3(0)–3(3)) are reported. The three compounds with CF3 substituents are similar or more acidic than acetic acid (pKa = 13.5 (3(1)), 9.5 (3(2)), 7.3 (3(3)) vs 12.6 (HOAc)), and the resulting hydrogen bond network enables a remote γ-trifluoromethyl group to enhance the acidity as well as one located at the α-position. Catalytic abilities of 3(0)–3(3) were also examined. In a nonpolar environment a rate enhancement of up to 100-fold over flexible acyclic analogs was observed presumably due to an entropic advantage of the locked-in structure. Gas-phase acidities are found to correlate with the catalytic activity better than DMSO pKa values and appear to be a better measure of acidities in low dielectric constant media. These trends are reduced or reversed in polar solvents highlighting the importance of the reaction environment.

Natural and synthetic anion receptors are extensively employed, but the structures of their bound complexes are difficult to determine in the liquid phase. Infrared spectroscopy is used in this work to characterize the solution structures of bound anion receptors for the first time, and surprisingly only two of three hydroxyl groups of the neutral aliphatic triols are found to directly interact with Cl–. The binding constants of these triols with zero to three CF3 groups were measured in a polar environment, and KCD3CN(Cl–) = 1.1 × 106 M–1 for the tris(trifluoromethyl) derivative. This is a remarkably large value, and high selectivity with respect to interfering anions such as, Br–, NO3– and NCS– is also displayed. The effects of the third “noninteracting” hydroxyl groups on the structures and binding constants were also explored, and surprisingly they are as large or larger than the OH substituents that hydrogen bond to Cl–. That is, a remote hydroxyl group can play a larger role in binding than two OH substituents that directly interact with an anionic center.

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.

The gas phase structure of deprotonated cysteine (Cys–H+)− has recently gained attention because of its counterintuitive calculated minimum energy structure in which it appears that deprotonation occurs at the –SH moiety rather than at the nominally more acidic carboxylic acid group. Because previous experimental efforts have not yielded to a consensus regarding the structure of the anion, we report the cryogenic ion vibrational predissociation (CIVP) spectra of its cryogenically cooled H/D isotopologues in an effort to clarify the situation. The unexpected isotope dependence of key features in the spectrum and the similarity of the band pattern to that displayed by the intramolecular H-bonded linkage in a deprotonated diacid (HCO2(CH2)10CO2−) indicate that the dominant form of the anion occurs with a strongly shared proton between the thiolate (–S−) and carboxylate (–CO2−) groups. An interesting aspect of this (–S−⋯H+⋯−O2C–) linkage is that, although the global minimum places the shared proton closer to the oxygen atom, the soft potential energy curve calculated for displacement of the bridging proton would likely support sufficient zero-point motion both to blur the distinction between thiolate- and carboxylate-based structures and to account for the unusual isotope effects.

•Indenone’s heat of hydrogenation was measured via two ion thermodynamic cycles.•High level G3 computations afford ΔHf° for cyclopentadienone (1) and indenone (2).•Both 1 and 2 are 4n π electron systems but are nonaromatic not antiaromatic.The heat of hydrogenation of indenone was measured via two partially independent thermodynamic cycles by carrying out energetic measurements (i.e., electron affinities, proton affinities and ionization potentials) on both negative and positive ions (ΔH°H2 = 17.8 ± 5.5 and 17.5 ± 5.7 kcal mol−1, respectively). High level G3 computations were also carried out to provide the heats of formation of indenone (16.8 kcal mol−1) and cyclopentadienone (14.0 kcal mol−1). These 4n π electron systems are found to be nonaromatic in contrast to previous views. A recent report on cyclopropenyl anion (J. Org. Chem. 2013, 78, 7370–7372) indicates that this ion is also nonaromatic, and suggests that NMR ring currents and nucleus independent chemical shift (NICS) calculations do not correlate with the energetic criterion for antiaromatic compounds.

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–.

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.

Anion recognition of two flexible diols in different solvents and binary mixtures were examined. Binding constants (K) in CD3CN and CDCl3 are surprisingly similar, and CD3CN–solvent mixtures led to reduced values of K that are smaller than in either pure solvent. A surprising U-shaped dependence is observed.

Infrared photodissociation (IRPD) spectra are reported for a proline–chloride anion cluster along with its d2- and d7-isotopomers. The spectral data indicate that proline is in its neutral form as opposed to a zwitterion, and computations are in agreement in that some neutral conformers are energetically low-lying and reproduce the observed spectra. Zwitterionic conformers are predicted to be essentially as stable as the neutral ones and should be significantly populated; however, there is no evidence for these structures in the IRPD spectra. An exploration of the potential energy surface for the loss of chloride anion, the observed fragmentation channel, reveals that it is 8.4 kcal mol–1 more difficult to break apart the zwitterionic cluster ion. This is a reflection of the 15.8 kcal mol–1 estimate for the gaseous proline zwitterion–neutral energy difference. Kinetic results suggest the presence of two photolabile populations in similar amounts (i.e., 56 vs 44%). The more abundant structure is also the more labile species, and the neutral form of proline is assigned to this cluster ion. The less abundant and slower fragmenting structure consequently is zwitterionic. As originally suggested by Evans et al.(33) in general, it appears that in this instance both spectral and kinetic data are needed to determine the structure of the proline–chloride anion cluster.

A central idea in organic chemistry for the past 50 years is that cyclopropenyl anion is antiaromatic. A correlation between cycloalkene acidities and allylic bond angles reveals that energetically this is not case, cyclopropenyl anion is nonaromatic.

The ionization energy (IE) of the 3-cyclopropenyl radical (6.00 ± 0.17 eV) was measured in the gas phase by reacting 3-cyclopropenium cation (c-C3H3+) with a series of reference reagents of known IEs. This result was combined in a thermodynamic cycle to obtain the heat of formation of c–C3H3• (118.9 ± 4.0 kcal mol–1) and the allylic C–H bond dissociation energy (BDE) of cyclopropene (104.4 ± 4.0 kcal mol–1). These experimental values are well reproduced by high level G3 and W1 computations and reveal that the BDE is similar to that for cyclopropane and the vinyl position of cyclopropene. This is unprecedented and is a reflection of the unusual nature of cyclopropene.

Like-charge ion pairing is commonly observed in protein structures and plays a significant role in biochemical processes. Density functional calculations combined with the conductor-like polarizable continuum model were employed to study the formation possibilities of doubly charged noncovalently linked complexes of a series of model compounds and amino acids in the gas phase and in solution. Hydrogen bond interactions were found to offset the Coulombic repulsion such that cation–cation clusters are minima on the potential energy surfaces and neither counterions nor solvent molecules are needed to hold them together. In the gas phase the dissociation energies are exothermic, and the separation barriers span from 1.7 to 15.6 kcal mol–1. Liquid-phase computations indicate that the separation enthalpies of the cation–cation complexes become endothermic in water and nonpolar solvents with dielectric constants of ≥7 (i.e., the value for THF). These results reveal that electrostatically defying noncovalent complexes of like-charged ions can overcome their Coulombic repulsion even in low-polarity environments.

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.

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.

The pKa of an acyclic aliphatic heptaol ((HOCH2CH2CH(OH)CH2)3COH) was measured in DMSO, and its gas-phase acidity is reported as well. This tertiary alcohol was found to be 1021 times more acidic than tert-butyl alcohol in DMSO and an order of magnitude more acidic than acetic acid (i.e., pKa = 11.4 vs 12.3). This can be attributed to a 21.9 kcal mol–1 stabilization of the charged oxygen center in the conjugate base by three hydrogen bonds and another 6.3 kcal mol–1 stabilization resulting from an additional three hydrogen bonds between the uncharged primary and secondary hydroxyl groups. Charge delocalization by both the first and second solvation shells may be used to facilitate enzymatic reactions. Acidity constants of a series of polyols were also computed, and the combination of hydrogen-bonding and electron-withdrawing substituents was found to afford acids that are predicted to be extremely acidic in DMSO (i.e., pKa < 0). These hydrogen bond enhanced acids represent an attractive class of Brønsted acid catalysts.

Bridgehead C–H bond dissociation enthalpies of 105.7 ± 2.0, 102.9 ± 1.7, and 102.4 ± 1.9 kcal mol–1 for bicyclo[2.2.1]heptane, bicyclo[2.2.2]octane, and adamantane, respectively, were determined in the gas phase by making use of a thermodynamic cycle (i.e., BDE(R–H) = ΔH°acid(H–X) – IE(H·) + EA(X·)). These results are in good accord with high-level G3 theory calculations, and the experimental values along with G3 predictions for bicyclo[1.1.1]pentane, bicyclo[2.1.1]hexane, bicyclo[3.1.1]heptane, and bicyclo[4.2.1]nonane were found to correlate with the flexibility of the ring system. Rare examples of alkyl anions in the gas phase are also provided.

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.

Infrared multiphoton dissociation spectra of protonated p-aminobenzoic acid generated by electrospray ionization (ESI) from aqueous methanol and acetonitrile solutions were recorded in the gas phase from 2800–4000 cm–1. The O-protonated ion is more stable than the N-protonated structure in the gas phase, whereas the opposite is true in both solutions. When CH3OH/H2O was used as the ESI solvent, only the O-protonated ion was observed. In contrast, a 70:30 mixture of the O- and N-protonated species were produced from CH3CN/H2O. These structural assignments are based on an assortment of experimental data (action spectra, photofragments, photofragmentation kinetics, and H/D exchange) and are fully supported by extensive computations. This work shows that ESI can lead to isomerization and that the ionization site may be varied by changing the solvent from which the substrate is analyzed.

Gas-phase deprotonation enthalpies were measured for chloric and perchloric acids and found to be 313.2 ± 3.3 and 299.9 ± 5.7 kcal mol−1, respectively. These values were combined with the previously reported electron affinities of ClO3 and ClO4 to obtain BDE(H−OClO2) = 97.6 ± 4.0 kcal mol−1 and BDE(H−OClO3) = 107.4 ± 6.1 kcal mol−1. These energetic determinations represent the first measurements of these quantities or extensive revisions of the currently available values. B3LYP, M06, M06−2X, G3, and G3B3 computations also were carried out to provide acidities, electron affinities, bond dissociation energies, and heats of formation via atomization energies for ClOx and HClOx, where x = 1−4. All of the methods do a reasonable job for the first three thermodynamic quantities but only M06 does a satisfactory job for the heats of formation, and its performance is similar to the highly accurate but extremely time intensive W4 method.

Five CHB11X6Y5− carborane anions from the series X = Br, Cl, I and Y = H, Cl, CH3 were generated by electrospray ionization, and their reactivity with a series of Brønsted acids and electron transfer reagents were examined in the gas phase. The undecachlorocarborane acid, H(CHB11Cl11), was found to be far more acidic than the former record holder, (1-C4F9SO2)2NH (i.e., ΔH°acid = 241 ± 29 vs 291.1 ± 2.2 kcal mol−1) and bridges the gas-phase acidity and basicity scales for the first time. Its conjugate base, CHB11Cl11−, was found by photoelectron spectroscopy to have a remarkably large electron binding energy (6.35 ± 0.02 eV) but the value for the (1-C4F9SO2)2N− anion is even larger (6.5 ± 0.1 eV). Consequently, it is the weak H-(CHB11Cl11) BDE (70.0 kcal mol−1, G3(MP2)) compared to the strong BDE of (1-C4F9SO2)2N−H (127.4 ± 3.2 kcal mol−1) that accounts for the greater acidity of carborane acids.

Lithium monoxide anion (LiO−) has been generated in the gas phase and is found to be a stronger base than methyl anion (CH3−). This makes LiO− the strongest base currently known, and it will be a challenge to produce a singly charged or multiply charged anion that is more basic. The experimental acidity of lithium hydroxide is ΔH°acid = 425.7 ± 6.1 kcal·mol−1 (1 kcal = 4.184 kJ) and, when combined with results of high-level computations, leads to our best estimate for the acidity of 426 ± 2 kcal·mol−1.

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.

A series of neutral anion receptors with one to three thiourea arms were synthesized and their binding to tetrabutylammonium chloride, acetate, and dihydrogen phosphate salts in aqueous DMSO mixtures was examined. The three-armed thiourea host was found to strongly and selectively bind H2PO4− even in DMSO solutions containing up to 30% water. This enabled the dihydrogen phosphate salt to be extracted from water into chloroform in its dibasic form despite the high heat of the hydration of HPO42−.

The gas phase structure of deprotonated cysteine (Cys–H+)− has recently gained attention because of its counterintuitive calculated minimum energy structure in which it appears that deprotonation occurs at the –SH moiety rather than at the nominally more acidic carboxylic acid group. Because previous experimental efforts have not yielded to a consensus regarding the structure of the anion, we report the cryogenic ion vibrational predissociation (CIVP) spectra of its cryogenically cooled H/D isotopologues in an effort to clarify the situation. The unexpected isotope dependence of key features in the spectrum and the similarity of the band pattern to that displayed by the intramolecular H-bonded linkage in a deprotonated diacid (HCO2(CH2)10CO2−) indicate that the dominant form of the anion occurs with a strongly shared proton between the thiolate (–S−) and carboxylate (–CO2−) groups. An interesting aspect of this (–S−⋯H+⋯−O2C–) linkage is that, although the global minimum places the shared proton closer to the oxygen atom, the soft potential energy curve calculated for displacement of the bridging proton would likely support sufficient zero-point motion both to blur the distinction between thiolate- and carboxylate-based structures and to account for the unusual isotope effects.

Different strategies are employed in designing strong and selective anion receptors but stereoelectronic effects have been largely ignored. In this work, the stereo configuration of a non-interacting ether is found to have a large impact of more than two orders of magnitude on the binding of a rigid diol with tetrabutylammonium chloride in acetonitrile-d3. A favorable carbon–oxygen dipole and an intramolecular C–H⋯OH hydrogen bond in an equatorially substituted ether is found to be energetically more important than a stabilizing hydrogen bond in the corresponding axially oriented alcohol. IR spectroscopy is also used to probe the structures of the bound complexes and several binding motifs are identified.

Anion recognition of two flexible diols in different solvents and binary mixtures were examined. Binding constants (K) in CD3CN and CDCl3 are surprisingly similar, and CD3CN–solvent mixtures led to reduced values of K that are smaller than in either pure solvent. A surprising U-shaped dependence is observed.

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.