We report infrared photodissociation spectra of manganese–CO2 cluster anions, [Mn(CO2)n]− (n = 2–10) to probe structural motifs characterizing the interaction between Mn and CO2 in the presence of an excess electron. We interpret the experimental spectra through comparison with infrared spectra predicted from density functional theory calculations. The cluster anions consist of core ions combining a Mn atom with a variety of ligands, solvated by additional CO2 molecules. Structural motifs of ligands evolve with increasing cluster size from simple monodentate and bidentate CO2 ligands to oxalate ligands and combinations of these structural themes.

We measured the electronic spectra of mass-selected [M(bpy)3]2+ (M = Fe and Os, bpy = 2,2′-bipyridine) ions in vacuo by photodissociation spectroscopy of their N2 adducts, [M(bpy)3]2+·N2. Extensive band systems in the visible (predominantly charge transfer) and near-ultraviolet (ππ*) spectral regions are reported. The [M(bpy)3]2+·N2 target ions were prepared by condensing N2 onto electrosprayed ions in a cryogenic ion trap at ca. 25 K and then mass-selected by time-of-flight mass spectrometry. The electronic photodissociation spectra of the cold, gas-phase ions closely reflect their intrinsic properties, i.e., without perturbation by solvent effects. The spectra are interpreted using time-dependent density functional theory calculations both with and without accounting for relativistic effects.

We present IR spectra and quantum chemical calculations for anionic iron–CO2 clusters of the form [Fe(CO2)n]− (n = 3–7). All observed clusters have at least two CO2 units strongly bound to the metal atom. These strongly bound iron–CO2 complexes form the core ions of the clusters and are solvated by additional, weakly bound CO2 molecules. Larger clusters show clear infrared signatures of core ion isomers with three CO2 moieties as well. Dominant structural motifs are based on bidentate CO2 ligands with Fe–O/Fe–C bonds, oxalate ligands, and metal insertion into a CO bond.

•IR photodissociation spectra of CO2− and MCOO− ions are presented (M = H, Ag, Bi).•Intensity ratio of symm. and asymm. CO stretching modes are examined.•Fixed charge approximation does not reflect experimentally observed behavior.•Charge oscillation along normal modes enhances intensities of CO stretching modes.•Enhancement of symmetric stretching mode is strongly dependent on M.In this work, we compare the intensity ratio of symmetric to antisymmetric CO stretching vibrational transitions in CO2− and MCOO− (M = H, Ag and Bi) using photodissociation spectroscopy. We observe that this ratio depends strongly on the bonding partner M. We trace this behavior to a dynamic change in the charge distribution of the molecules during vibrational motion by comparing the derivatives of the dipole moment during vibrational motion calculated from a fixed-charge approximation, a charge distribution analysis, and the prediction of the infrared spectra of the ions under study. The calculations are based on density functional theory.Download high-res image (63KB)Download full-size image

We present electronic spectra in the π–π* region of a series of tris(bpy)–M(II) complex ions (bpy = 2,2′-bipyridine; M = Mn, Fe, Co, Ni, Cu, Zn) in vacuo for the first time. By applying photodissociation spectroscopy to cryogenically cooled and mass selected [MII(bpy)3]2+ ions, we obtain the intrinsic spectra of these ions at low temperature without perturbation by solvent interaction or crystal lattice shifts. This allows spectroscopic analysis of these complex ions in greater detail than possible in the condensed phase. We interpret our experimental data by comparison with time-dependent density functional theory.

We report the electronic spectra of mass selected [(bpy)(tpy)Ru–OH2]2+·(H2O)n clusters (bpy = 2,2′-bipyridine, tpy =2,2′:6′2″-terpyridine, n = 0–4) in the spectral region of their metal-to-ligand charge transfer bands. The spectra of the mono- and dihydrate clusters exhibit partially resolved individual electronic transitions. The water network forming at the aqua ligand leads to a rapid solvatochromic shift of the peak of the band envelope: addition of only four solvent water molecules can recover 78% of the solvatochromic shift in bulk solution. The sequential shift of the band shows a clear change in behavior with the closing of the first hydration shell. We compare our experimental data to density function theory (DFT) calculations for the ground and excited states.

We report electronic spectra of a series of ruthenium polypyridine complexes of the form [(trpy)(bipy)RuII–L]2+ (bipy = 2,2′-bipyridine and trpy = 2,2′:6′,2″-terpyridine), where L represents a small molecular ligand that occupies the last coordination site. Species with L = H2O, CO2, CH3CN, and N2 were investigated in vacuo using photodissociation spectroscopy. All species exhibit bright metal-to-ligand charger transfer (MLCT) bands in the visible and near UV, but with different spectral envelopes and peak energies, encoding the influence of the ligand L on the electronic structure of the complex. Several individual electronic bands can be resolved for L = H2O and CO2, while the spectra for L = N2 and CH3CN are more congested, even at low ion temperatures. The experimental results are discussed in the framework of time-dependent density functional theory.

We present photodissociation spectroscopy and computational analysis of three monocationic Cu–bipyridine complexes with one additional ligand of different interaction strength (N2, H2O and Cl) in the visible and UV. All three complexes show similar ππ* bands with origins slightly above 4 eV and vibrational band contours that are due to bipyridine ring deformation modes. Experiments at low temperature show that excited-state lifetime is the limiting factor for the width of the vibrational features. In the case of Cl as a ligand, there is a lower lying bright ligand-to-ligand charge-transfer state around 2.75 eV. The assignment of the transitions was made based on equation-of-motion coupled-cluster calculations. While the nature of the ligand does not significantly change the position of the bright ππ* state, it drastically changes the excited-state dynamics.

Ruthenium(II) complexes are of great interest as homogeneous catalysts and as photosensitizers; however, their absorption spectra are typically very broad and offer only little insight into their electronic structure. We present the electronic spectrum of the aquo complex [(trpy)(bipy)RuII–OH2]2+ measured by photodissociation spectroscopy of mass-selected ions in vacuo (bipy = 2,2′-bipyridine and trpy = 2,2′:6′,2″-terpyridine). In the visible and near-UV, [(trpy)(bipy)RuII–OH2]2+ has several electronic bands that are not resolved in absorption spectra of this complex in solution but are partially resolved in vacuo. The experimental results are compared with results from time-dependent density functional theory calculations.

We present infrared spectra of [CoO(CO2)n]− and [NiO(CO2)n]− clusters and interpret them in the framework of computational results employing density functional theory. We find that both [CoO(CO2)n]− and [NiO(CO2)n]− clusters are generally composed of the same core isomers. The dominant isomers consist of an η2 CO2 ligand and a CO3 moiety that can be bound to the metal atom with monodentate (η1) or bidentate (η2) connectivity. Minor structural isomers observed are composed of a C2O4 moiety with a lone oxygen atom or a CO3 unit.

We  present infrared spectra of  [Cu(CO2)n]− (n = 2–9) clusters in the wavenumber range 1600–2400 cm–1. The CO stretching modes in this region encode the structural nature of the cluster core and are interpreted with the aid of density functional theory. We find a variety of core species in [Cu(CO2)n]− clusters, but the dominant core structure is a [Cu(CO2)2]− core where the two CO2 ligands are bound to the Cu atom in a bidentate fashion. We compare the results of [Cu(CO2)n]− clusters to those of other [M(CO2)n]− clusters (M = Au, Ag, Co, Ni) to establish trends of how the metal–CO2 interaction depends on the metal partner.

We present infrared photodissociation spectra of [Co(CO2)n]− ions (n = 3–11) in the wavenumber region 1000–2400 cm–1, interpreted with the aid of density functional theory calculations. The spectra are dominated by the signatures of a core ion showing bidentate interaction of two CO2 ligands with the Co atom, each forming C–Co and O–Co bonds. This structural motif is very robust and is only weakly affected by solvation with additional CO2 solvent molecules. The Co atom is in oxidation state +1, and both CO2 ligands carry a negative charge.

We present infrared photodissociation spectra of [Ni(CO2)n]− clusters (n = 2–8) in the wavenumber region of 1000–2400 cm–1 using the antisymmetric stretching vibrational modes of the CO2 units in the clusters as structural probes. We use density functional theory to aid in the interpretation of our experimental results. The dominant spectral signatures arise from a core ion composed of a nickel atom and two CO2 ligands bound to the Ni atom in a bidentate fashion, while the rest of the CO2 molecules in the cluster play the role of solvent. Other core structures are observed as well but as minor contributors. The results for [Ni(CO2)n]− clusters are discussed in the context of other anionic transition- metal complexes with CO2.

We report the UV photodissociation spectrum of mass-selected Cu(NO3)3– ions at photon energies between 3.0 and 5.6 eV. Upon photon absorption, Cu(NO3)3– undergoes reductive dissociation losing neutral NO3 and resulting in the formation of Cu(NO3)2–. The experimental results are discussed and interpreted with the aid of quantum-chemical calculations. The parent ion is calculated to have C2 symmetry with a strongly distorted octahedral coordination around the Cu ion. Time-dependent density functional theory is used to describe the accessible electronic transitions, which can be characterized as ligand-to-metal charge transfer transitions from the nitrate ligands to the copper ion.

In the present study, we investigate the spectroscopy and photochemical behavior of chromate ester cluster ions in vacuo in the visible and near-UV. Chromate ester cluster ions Nan[CrO3(OCH3)]n+1– (n = 1, 2) are generated by electrospray ionization of sodium dichromate in water and methanol. Upon irradiation with photon energies between 2 and 5.6 eV, dissociation occurs. The photodissociation spectra of these ions are very similar to the UV/vis absorption spectra of the sample solution with solvatochromic shifts less than 0.1 eV. The electronic excitations in this photon energy range are assigned to ligand-to-metal charge transfer from the oxygen ligands to the chromium within the chromate ester. Fragment channels corresponding to intracluster reactions involving reduction of the metal centers as well as evaporative processes leading to the loss of a neutral Na[CrO3(OCH3)] salt unit are observed. The results are discussed in the framework of organometallic redox mechanisms and density functional theory.

The development of efficient routes toward sustainable fuel sources by electrochemical reduction of CO2 is an important goal for catalysis research. While these processes usually occur in the presence of solvent, solvation effects in catalysis are largely not understood or even characterized. In this work, mass-selected clusters of silver anions with CO2 serve as a model system for reductive activation of CO2 by a catalyst in the presence of a well-controlled number of solvent molecules. Vibrational spectroscopy and electronic structure calculations are used to obtain molecular-level information on the interaction of solvent with the catalyst–CO2 complex and the effects of solvation on one-electron reductive activation of CO2. Charge transfer from the silver catalyst to CO2 increases with increasing cluster size. We observe the coexistence of catalyst–ligand complexes with CO2 monomer and dimer anions, indicating that CO2-based charge carriers can exist in the presence of a silver atom.

Catalytic activation and electrochemical reduction of CO2 for the formation of chemically usable feedstock and fuel are central goals for establishing a carbon neutral fuel cycle. The role of solvent molecules in catalytic processes is little understood, although solvent–solute interactions can strongly influence activated intermediate species. We use vibrational spectroscopy of mass-selected Au(CO2)n– cluster ions to probe the solvation of AuCO2– as a model for a reactive intermediate in the reductive activation of a CO2 ligand by a single-atom catalyst. For the first few solvent molecules, solvation of the complex preferentially occurs at the CO2 moiety, enhancing reductive activation through polarization of the excess charge onto the partially reduced ligand. At higher levels of solvation, direct interaction of additional solvent molecules with the Au atom diminishes reduction. The results show how the solvation environment can enhance or diminish the effects of a catalyst, offering design criteria for single-atom catalyst engineering.

We present an experimental study on the fluorescence behavior of the red fluorescent proteins TagRFP-S, TagRFP-T, mCherry, mOrange2, mStrawberry, and mKO as a function of pressure up to several GPa. TagRFP-S, TagRFP-T, mOrange2, and mStrawberry show an initial increase in fluorescence intensity upon application of pressure above ambient conditions. At higher pressures, the fluorescence intensity decreases dramatically for all proteins under study, probably due to denaturing of the proteins. Small blue shifts in the fluorescence spectra with increasing pressure were seen in all proteins under study, hinting at increased rigidity of the chromophore environment. In addition, mOrange2 and mStrawberry exhibit strong and abrupt changes in their fluorescence spectra at certain pressures. These changes are likely due to structural modifications of the hydrogen bonding environment of the chromophore. The strong differences in behavior between proteins with identical or very similar chromophores highlight how the chromophore environment contributes to pressure-induced behavior of the fluorescence performance.

Irradiation of nucleotides in the gas phase with ultraviolet light can lead to their fragmentation. We present a comparison of UV photofragmentation data on ribo-, deoxyribo- and cyclic nucleotides with guanine and adenine as nucleobases. The envelope of the UV photofragment spectra does not depend significantly on the detailed structure of the sugar–phosphate backbone. The fragment channels observed in the photofragmentation of ribonucleotides are very similar to those of deoxyribonucleotides with small differences in the relative abundances of the product ions. In fragmentation of cyclic nucleotides, the deprotonated base anions are the most abundant fragments, in contrast to the non-cyclic nucleotides, where this channel is significantly weaker. We discuss the abundances in the context of possible fragmentation mechanisms and structural differences of the parent molecules.Graphical abstractResearch highlights► Photodissociation of cyclic mononucleotides after UV excitation occurs predominantly by loss of anionic base. ► Decay of RNA and DNA mononucleotides is dominated by loss of neutral base and loss of phosphate-based anions. ► Differences between cyclic and non-cyclic fragmentation pathways can be attributed to different tethering of the phosphate group. ► The presence of different functional groups on the phosphate–sugar “backbone” of cyclic and non-cyclic nucleotides does not affect the envelope of the electronic spectra of adenosine and guanosine-based nucleotides.

Irradiation of AuCl4− and AuCl2(OH)2− in the gas-phase using ultraviolet light (220−415 nm) leads to their dissociation. Observed fragment ions for AuCl4− are AuCl3− and AuCl2− and for AuCl2(OH)2− are AuCl2− and AuClOH−. All fragment channels correspond to photoreduction of the gold atom to either Au(II) or Au(I) depending on the number of neutral ligands lost. Fragment branching ratios of AuCl4− are observed to be highly energy dependent and can be explained by comparison of the experimental data to calculated threshold energies obtained using density functional theory. The main observed spectral features are attributed to ligand-to-metal charge transfer transitions. These results are discussed in the context of the molecular-level mechanisms of Au(III) photochemistry.

We report gas-phase electronic photodissociation spectra of the undercoordinated bromoiridate complexes IrBr4– and IrBr5– at photon energies from 1 to 5.6 eV. Both ions have open-shell ground states with low-symmetry structures. The fragmentation is characterized by thresholds for the loss of one Br atom for IrBr4– and one or two Br atoms for IrBr5–. The experimental spectra consist of ligand-to-metal charge transfer transitions and reveal a large density of electronic states that can be recovered by time-dependent density functional theory.

If a negative ion has vibrational energy in excess of the binding energy of its most weakly bound electron, the anion can undergo vibrational autodetachment, similar to thermionic emission. When this effect occurs after targeted infrared excitation of a specific vibrational mode in the anion, it encodes information on the intramolecular vibrational relaxation processes that take place between excitation and electron emission. We present examples on how vibrational autodetachment can be used to obtain infrared spectra of molecular anions, and we discuss how a vibrational autodetachment photoelectron spectrum can be modeled, using vibrational autodetachment after excitation of CH stretching modes of nitromethane anions as a case study.

We present experimental infrared spectroscopic data on mass-selected, hydrated nitromethane anion clusters with up to four water ligands. The vibrational bands in the OH stretching region encode the solvent structure, while the CH stretching bands contain information on the influence of the hydration shell on the solute ion. We interpret our findings using density functional theory calculations. The first water molecule binds symmetrically to the two oxygen atoms of the nitro group but couples to low-frequency vibrational modes that impart a very complicated structure on the OH stretching region. Competition between water−ion and water−water interaction makes the dihydrate very floppy and precludes unambiguous structural assignment. The tri- and tetrahydrate spectra can be interpreted on the basis of H-bonded ring structures. The excess electron is polarized by hydration, which can clearly be seen by shifting CH stretching frequencies.

UV excitation of isolated singly-charged deprotonated mononucleotide anions in the gas phase can lead to their dissociation. We present mass spectrometry results, photodepletion and photofragment action spectra on the UV-photodissociation of deprotonated 2′-deoxyribonucleobase-5′-monophosphates with adenine, cytosine, guanine and thymine as nucleobases. We observe the same anionic fragments as in earlier experiments on collision-induced dissociation, although their relative intensities are quite different, especially with respect to the abundance of the deprotonated base anions. The fragment channels correspond to loss of genetic information by cleavage of the CN glycosidic bond and to strand breaking by severing the phosphate–sugar link. We compare the photodissociation spectra with UV absorption spectra of aqueous solutions of the same species and discuss the photodissociation behavior in the context of possible mechanisms and ergodic versus non-ergodic fragmentation.

If the binding energy of an excess electron is lower than some of the vibrational levels of its host anion, vibrational excitation can lead to autodetachment. We use excitation of CH stretching modes in nitroalkane anions (2700−3000 cm−1), where the excess electron is localized predominantly on the NO2 group. We present data on nitroalkane anions of various chain lengths, showing that this technique is a valid approach to the vibrational spectroscopy of such systems extending to nitroalkane anions at least the size of nitropentane. We compare spectra taken by using vibrational autodetachment with spectra obtained by monitoring Ar evaporation from Ar solvated nitroalkane anions. The spectra of nitromethane and nitroethane are assigned on the basis of ab initio calculations with a detailed analysis of Fermi resonances of CH stretching fundamentals with overtones and combination bands of HCH bending modes.

UV excitation of isolated singly-charged deprotonated mononucleotide anions in the gas phase can lead to their dissociation. We present mass spectrometry results, photodepletion and photofragment action spectra on the UV-photodissociation of deprotonated 2′-deoxyribonucleobase-5′-monophosphates with adenine, cytosine, guanine and thymine as nucleobases. We observe the same anionic fragments as in earlier experiments on collision-induced dissociation, although their relative intensities are quite different, especially with respect to the abundance of the deprotonated base anions. The fragment channels correspond to loss of genetic information by cleavage of the CN glycosidic bond and to strand breaking by severing the phosphate–sugar link. We compare the photodissociation spectra with UV absorption spectra of aqueous solutions of the same species and discuss the photodissociation behavior in the context of possible mechanisms and ergodic versus non-ergodic fragmentation.

We present photodissociation spectroscopy and computational analysis of three monocationic Cu–bipyridine complexes with one additional ligand of different interaction strength (N2, H2O and Cl) in the visible and UV. All three complexes show similar ππ* bands with origins slightly above 4 eV and vibrational band contours that are due to bipyridine ring deformation modes. Experiments at low temperature show that excited-state lifetime is the limiting factor for the width of the vibrational features. In the case of Cl as a ligand, there is a lower lying bright ligand-to-ligand charge-transfer state around 2.75 eV. The assignment of the transitions was made based on equation-of-motion coupled-cluster calculations. While the nature of the ligand does not significantly change the position of the bright ππ* state, it drastically changes the excited-state dynamics.