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Research on multiply charged anions (MCAs) in the gas phase has been intensively performed during the past decades, mainly to understand fundamental molecular physics phenomena, for example, intramolecular Coulomb repulsion and existence of the repulsive Coulomb barrier. However, the relevance of these investigations with respect to understanding MCAs' chemistry appears often vague. Here, we discuss how insights into the electronic structure obtained from negative ion photoelectron spectroscopy (NIPES) combined with theoretical calculations and collision-induced dissociation can provide a fundamental understanding of the intrinsic chemical reactivity of MCAs and their fragments. This is exemplified in our studies on polyhedral closo-borate dianions [BnXn]2- (n = 6, 10, 11, 12; X = H, F-I, CN) and their fragment ions. For example, the rational design of closo-borate dianions with specific electronic properties is described, which leads to generating highly reactive fragments. Depending on the dianionic precursor, these fragments are tuned to either bind noble gases effectively or activate small molecules like CO and N2. The intrinsic electronic properties of closo-borate dianions are further compared to their electrochemistry in solutions, revealing solvent effects on the redox potentials. Neutral host molecules such as cyclodextrins are found to bind strongly to [BnXn]2-, and gas phase NIPES provides insights into the intrinsic host-guest interactions. Finally, outlooks including the direct NIPES of molecular fragment ions that cannot be generated in the condensed phase and their utilization in preparative mass spectrometry are discussed.

Six monosolvated cyanate analogue clusters ECX-·Sol (ECX- = NCSe-, AsCSe-, and AsCS-; Sol = H2O and CH3CN) were investigated using negative ion photoelectron spectroscopy (NIPES). NIPES experiments show that these clusters possess similar spectra overall compared to their respective isolated ECX- anions but shift to higher electron binding energy with CH3CN solvent, stabilizing the excess electrons slightly more than H2O. For the ECX-·H2O series, vertical detachment energies and their increments relative to the bare species are measured to be 3.700/0.370, 3.085/0.415, and 3.085/0.430 eV for NCSe-, AsCSe- and AsCS-, respectively, while the corresponding values in the ECX-·CH3CN series are 3.835/0.505, 3.145/0.475, and 3.135/0.480 eV. Ab initio electronic structure calculations indicate that the excess charges were located at the terminal N and Se atoms in NCSe- and migrated to the central C atom in AsCSe- and AsCS-. For NCSe-, the solvation is driven by the interactions with the two negatively charged terminal ends, while for AsCSe- and AsCS-, the solvation revolves around the interactions with the central C atom, where all the excess negative charge is concentrated. Two nearly degenerate isomers for NCSe-·H2O are identified, one forming a single strong N···H-O hydrogen bond (HB) and the other featuring a bidentate HB with two hydroxyl H atoms pointing to N and Se ends. In contrast, the negative central C atom in AsCSe-/AsCS- allows the formation of a bifurcated HB with H2O. Similar effects are observed for the acetonitrile case, in which the three H atoms of the methyl group interact with the two negatively charged terminal ends in NCSe-, while preferring to bind to the central negative carbon atom in AsCSe-/AsCS-. The different binding motifs derived in this work may suggest different solvation properties in NCSe- versus AsCSe-/AsCS- with the former anion leading to asymmetric solvation at the N end of the solute, while the latter species creates more "isotropic" solvation around the central C equatorial plane.

We present a transition state spectroscopic study of the OH + H2O reaction using the experimental technique of cryogenic negative ion photoelectron spectroscopy (NIPES). The recorded NIPE spectrum ...

A great deal of effort has been focused on developing a metal-free catalytic system for epoxidation of unreactive alkenes. Fluoroketones are thought as remarkably promising catalysts for epoxidation reactions. The combination of fluorinated alcohols and catalytic amounts of hexafluoroacetone (HFA) gives a versatile and effective medium for epoxidation of various olefins with hydrogen peroxide. However, the fundamental physicochemical properties of HFA remained largely unclear, although they were very important to understand the related interactions. Here, we performed a joint study on the electron affinity and electronic structure of HFA employing negative ion photoelectron (NIPE) spectroscopy and quantum chemistry calculations. Two distinct bands with complicated vibrational progressions were observed in the 193 nm NIPE spectrum. The adiabatic/vertical detachment energies (ADE/VDE) were derived to be 1.42/2.06 and 4.43/4.86 eV for the ground singlet state and excited triplet state, respectively. Using the optimized geometries and vibrational frequencies of the anion and the neutral, the Franck-Condon factors were calculated for electron detachments to produce HFA in its lowest singlet and triplet states. Good agreements are obtained hereby for both bands between the experimental and calculated NIPE spectra, when taking into account combination vibrational excitations, unequivocally revealing that HFA possesses a singlet ground state with a giant singlet-triplet energy difference (ΔEST). The electron affinity (EA) and ΔEST of HFA were therefore determined to be EA = 1.42 ± 0.02 eV and ΔEST = -3.01 eV.

The recently synthesized acetylide compound KSeCCH containing the main group element selenium within the novel and in crystalline form unprecedented [SeCCH]− anion was successfully investigated in ...

Four new Zeise’s family ions with mixed-halide ligands, i.e., PtClnX3–n(C2H4)− (X = Br, I; n = 1, 2), were synthesized via ligand-exchange reactions of KX salts with KPtCl3(C2H4) in aqueous solutions, and were detected in vacuum via electrospray ionization mass spectrometry. Their photoelectron spectra reveal a series of well-resolved spectral peaks with their electron binding energies (EBEs) decreasing with increasing halide size, with I having a much stronger effect than Br, i.e., 4.57 (−Cl3) > 4.56 (−Cl2Br) > 4.53 (−ClBr2) > 4.34 (−Cl2I) > 4.30 eV (−ClI2). Ab initio electronic structure calculations including spin–orbit coupling (SOC) predict that the cis- and trans-isomers are nearly isoenergetic with the cis-isomer for −Cl2X and the trans-isomer for −ClX2 slightly favored, respectively. Excited-state spectra calculated with time-dependent density functional theory (TDDFT), and their comparison with the observed ones, suggest that for each species both the cis- and trans-configurations coexist in the ...

The transition-state (TS) region of the simplest heavy-light-heavy type of reaction, F• + H–F → F–H + F•, is investigated in this work by a joint experimental and theoretical approach. Photodetaching the bifluoride anion, [F···H···F]−, generates a negative ion photoelectron (NIPE) spectrum with three partially resolved bands in the electron binding energy (eBE) range of 5.4–7.0 eV. These bands correspond to the transition from the ground state of the anion to the electronic ground state of [F–H–F]• neutral, with associated vibrational excitations. The significant increase of eBE of the bifluoride anion, relative to that of F–, reflects a hydrogen bond energy between F– and HF of ∼46 kcal/mol. Theoretical modeling reveals that the antisymmetric motion of H between the two F atoms, near the TS on the neutral [F–H–F]• surface, dominates the observed three bands, while the F–H–F bending, F–F symmetric stretching modes, and the couplings between them are calculated to account for the breadth of the observed sp...

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.

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.

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(-).

Aspartate (Asp(2-)) and glutamate (Glu(2-)), 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(-), DCn(2-)), succinate (DC2(2-)) and propionate (DC3(2-)). Experimentally, the addition of the amine group for the n = 2 case (DC2(2-), Asp(2-)) significantly improves the stability of the resultant Asp(2-) dianionic species, albeit that NIPES shows only a small increase in adiabatic electron detachment energy (ADE) (+0.05 eV). In contrast, for n = 3 (DC3(2-), Glu(2-)), 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 Asp(2-) 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.

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, (-)O(2)C(CH(2))(n)CO(2)(-) (D(n)(2-)) in the gas phase by combined photoelectron spectroscopy (PES) and ab initio electronic structure calculations on nine M(+)-D(n)(2-) 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(+)-D(2)(2-), whereas the order is Li(+)Na(+) ≈ K(+) when M(+) interacts with a more flexible D(6)(2-) 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.

Hydroxyl substituted phenoxides, o-, m-, p-HO(C(6)H(4))O(-), and the corresponding neutral radicals are important species; in particular, the p-isomer pair, i.e., p-HO(C(6)H(4))O(-) and p-HO(C(6)H(4))O*, is directly involved in the proton-coupled electron transfer in biological photosynthetic centers. Here we report the first spectroscopic study of these species in the gas phase by means of low-temperature photoelectron spectroscopy (PES) and ab initio calculations. Vibrationally resolved PES spectra were obtained at 70 K and at several photon energies for each anion, directly yielding electron affinity (EA) and electronic structure information for the corresponding hydroxyphenoxyl radical. The EAs are found to vary with OH positions, from 1.990 +/- 0.010 (p) to 2.315 +/- 0.010 (o) and 2.330 +/- 0.010 (m) eV. Theoretical calculations were carried out to identify the optimized molecular structures for both anions and neutral radicals. The electron binding energies and excited state energies were also calculated to compare with experimental data. Excellent agreement is found between calculations and experiments. Molecular orbital analyses indicate a strong OH antibonding interaction with the phenoxide moiety for the o- as well as the p-isomer, whereas such an interaction is largely missing for the m-anion. The variance of EAs among three isomers is interpreted primarily due to the interplay between two competing factors: the OH antibonding interaction and the H-bonding stabilization (existed only in the o-anion).

We report a combined experimental and theoretical investigation of MI(2)(-) (M = Cs, Cu, Ag, Au) to explore the chemical bonding in the group IA and IB diiodide complexes. Both photoelectron imaging and low-temperature photoelectron spectroscopy are applied to MI(2)(-) (M = Cs, Cu, Au), yielding vibrationally resolved spectra for CuI(2)(-) and AuI(2)(-) and accurate electron affinities, 4.52 ± 0.02, 4.256 ± 0.010, and 4.226 ± 0.010 eV for CsI(2), CuI(2), and AuI(2), respectively. Spin-orbit coupling is found to be important in all the diiodide complexes and ab initio calculations including spin-orbit effects allow quantitative assignments of the observed photoelectron spectra. A variety of chemical bonding analyses (charge population, bond order, and electron localization functions) have been carried out, revealing a gradual transition from the expected ionic behavior in CsI(2)(-) to relatively strong covalent bonding in AuI(2)(-). Both relativistic effects and electron correlation are shown to enhance the covalency in the gold diiodide complex.

A photoelectron spectroscopy investigation of the fluorofullerene anions C60Fn− (n = 17, 33, 35, 43, 45, 47) and the doubly charged anions C60F342− and C60F462− is reported. The first electron affinities for the corresponding neutral molecules, C60Fn, were directly measured and were found to increase as n increased, reaching the extremely high value of 5.66 ± 0.10 eV for C60F47. Density functional calculations suggest that the experimentally observed species C60F17−, C60F35−, and C60F47− were each formed by reductive defluorination of the parent fluorofullerene, C3v-C60F18, C60F36 (a mixture of isomers), and D3-C60F48, respectively, without rearrangement of the remaining fluorine atoms. The DFT-predicted stability of C60F47− was verified by its generation by chemical reduction from D3-C60F48 in chloroform solution at 25 °C and its characterization by mass spectrometry and 19F NMR spectroscopy. Further reductive defluorination of C60F47− in solution resulted in the selective generation of a new fluorofulle...

Sulfate is an important inorganic anion and its interactions with water are essential to understand its chemistry in aqueous solution. Studies of sulfate with well-controlled solvent numbers provide molecular-level information about the solute-solvent interactions and critical data to test theoretical methods for weakly bounded species. Here we report a low-temperature photoelectron spectroscopy study of hydrated sulfate clusters SO(4)(2-)(H(2)O)(n) (n = 4-7) at 12 K and ab initio studies to understand the structures and dynamics of these unique solvated systems. A significant increase of electron binding energies was observed for the 12 K spectra relative to those at room temperature, suggesting different structural isomers were populated as a function of temperature. Theoretical calculations revealed a competition between isomers with optimal water-solute and water-water interactions. The global minimum isomers all possess higher electron binding energies due to their optimal water-solute interactions, giving rise to the binding energy shift in the 12 K spectra, whereas many additional low-lying isomers with less optimal solvent-solute interactions were populated at room temperature, resulting in a shift to lower electron binding energies in the observed spectra. The current work demonstrates and confirms the complexity of the water-sulfate potential energy landscape and the importance of temperature control in studying the solvent-solute systems and in comparing calculations with experiment.

Photoelectrons emitted from multiply charged anions (MCAs) carry information of the intramolecular Coulomb repulsion (ICR), which is dependent on molecular structures. Using photoelectron imaging, we observed the effects of ICR on photoelectron angular distributions (PAD) of the three isomers of benzene dicarboxylate dianions C6H4(CO2)2(2-) (o-, m- and p-BDC(2-)). Photoelectrons were observed to peak along the laser polarization due to the ICR, but the anisotropy was the largest for p-BDC(2-), followed by the m- and o-isomer. The observed anisotropy is related to the direction of the ICR or the detailed molecular structures, suggesting that photoelectron imaging may allow structural information to be obtained for complex multiply charged anions.

The cysteine anion was produced in the gas phase by electrospray ionization and investigated by photoelectron spectroscopy at low temperature (70 K). The cysteine anion was found to exhibit the thiolate form [ - SCH2CH(NH2)CO2H], rather than the expected carboxylate form [HSCH2CH(NH2)CO2 - ]. This observation was confirmed by two control experiments, that is, methyl cysteine [CH3SCH2CH(NH2)CO2 - ] and cysteine methyl ester [ - SCH2CH(NH2)CO2CH3]. The electron binding energy of [ - SCH2CH(NH2)CO2H] was measured to be about 0.7 eV blue-shifted relative to [ - SCH2CH(NH2)CO2CH3] due to the formation of an intramolecular -S - ‚‚‚HO2C- hydrogen bond in the cysteine thiolate. Theoretical calculations at the CCSD(T)/6-311++G(2df,p) and B3LYP/6-311++G(2df,p) levels were carried out to estimate the strength of this intramolecular -S - ‚‚‚HO2C- hydrogen bond. Combining experimental measurements and theoretical calculations yielded an estimated value of 16.4 ( 2.0 kcal/mol for the -S - ‚‚‚HO2C- intramolecular hydrogen-bond strength.

Two polyoxometalate Keggin-type anions, alpha-PM12O40(3-) (M = Mo, W), were transferred to the gas phase by electrospray; their electronic structure and stability were probed by photoelectron spectroscopy. These triply charged anions were found to be highly stable in the gas phase with large adiabatic electron detachment energies of 1.7 and 2.1 eV for M = Mo and W, respectively. The magnitude of the repulsive Coulomb barrier was measured as approximately 3.4 eV for both anions, providing an experimental estimate for the intramolecular Coulomb repulsion present in these highly charged anions. Density functional theory calculations were carried out and compared with the experimental data, providing insight into the electronic structure and valence molecular orbitals of the two Keggin anions. The calculations indicated that the highest occupied molecular orbital and other frontier orbitals for PM12O40(3-) are localized primarily on the mu2-oxo bridging ligands of the polyoxometalate framework, consistent with the reactivity on the mu2-oxo sites observed in solution. It was shown that the HOMO of PW12O40(3-) is stabilized relative to that of PMo12O40(3-) by approximately 0.35 eV. The experimental adiabatic electron detachment energies of PM12O40(3-) (i.e., the electron affinities of PM12O40(2-)) are combined with recent calculations on the proton affinity of PM12O40(3-) to yield O-H bond dissociation energies in PM12O39(OH)2- as approximately 5.1 eV.

The electronic structure of the mononuclear hydroxo MO3(OH)- and methoxo MO3(OCH3)- Group 6 oxometalate anions (M = Cr, Mo, and W) were examined by photodetachment photoelectron spectroscopy and electronic structure calculations at the density functional and CCSD(T) levels of theory. All of the anions exhibited high electron binding energies (4.9 eV), with the lowest-energy detachment features arising from oxygen 2p-based orbitals. The combined experimental and theoretical results allowed the change in molecular orbital energy levels to be investigated as a function of metal (Cr, Mo, or W) and ligand (-OH, -OCH3). A number of fundamental thermodynamic properties of the anions and corresponding neutrals were predicted on the basis of the theoretical calculations. The calculations indicate high O-H bond dissociation energies for MO2(OR)(O-H) (R = H, CH3) and MO3(O-H), consistent with their high Brønsted acidities (just below that of H2SO4 in the gas phase) and the high ionization energies of their conjugate base anions. This suggests that the corresponding radicals should readily abstract H atoms from organic molecules.

We report a photoelectron spectroscopy (PES) study on a series of fullerene oxides, C60Ox- (x = 1-3). The PES spectra reveal one isomer for C60O-, two isomers for C60O2, and multiple isomers for C60O3-. Compared to C60, the electronic structures of C60Ox are only slightly perturbed, resulting in similar anion photoelectron spectra. The electron affinity of C60Ox was observed to increase only marginally with the number of oxygen atoms, x, from 2.683 eV for C60, to 2.745 eV for C60O, and 2.785 eV/2.820 eV for C60O2 (two isomers). We also carried out theoretical calculations, which confirmed the observed isomers and showed that all the fullerene oxides are in the form of epoxide. The PES and theoretical calculations, as well as molecular orbital analysis, indicate that addition of oxygen atoms to the C60 cage only modifies the local carbon network and leave the rest of the fullerene cage largely intact geometrically and electronically.

Two doubly charged polyoxoanions, Mo6O192- and W6O192-, were observed in the gas phase using electrospray ionization. Their electronic structures were investigated using photoelectron spectroscopy and density functional calculations with relativistic effective core potentials. Each dianion was found to be highly stable despite the presence of strong intramolecular Coulomb repulsion, estimated to be about 2 eV for each system. The valence detachment features were all shown to originate from electronic excitations involving oxygen lone-pair type orbitals. Their observed energies were in excellent agreement with the theoretical vertical detachment energies calculated using time-dependent density functional theory. Despite being multiply charged, polyoxometalate oxide clusters can be studied in the gas phase, providing the opportunity for detailed benchmark theoretical studies on the electronic structures of these important transition-metal oxide systems.

Assembly and disassembly of protein-bound iron-sulfur clusters are involved in a wide variety of vital biological processes, ranging from biosynthesis to regulation of biological function. The study of the fission of analogue clusters can provide valuable insight into the various reaction mechanisms that can occur in proteins as well as tests of the ability of theoretical studies to interpret experiment. Previously, we observed symmetric fission of doubly charged Fe-S cluster anions, [Fe4S4X4] 2- f 2[Fe2S2X2] - (X ) SC2H5, Cl, Br) in the gas phase, which is surprising because four strong Fe-S bonds are being broken. Here, we report a study of the detailed fission mechanism using density functional theory in conjunction with photoelectron spectroscopic results for X ) Cl. Both the experimental and theoretical results suggest that the fission daughter products are low-spin [Fe2S2Cl2] - (S ) 1 /2) species. However, the layered structure of the cubane would seem to indicate a mechanism that would give high-spin daughter products. Thus, we investigate the symmetric fission of [Fe4S4Cl4] 2- along two possible reaction pathways, involving high-spin and low-spin [Fe2S2Cl2] fragments, respectively. Though the high-spin channel is endothermic by 1.34 eV with a high barrier of 2.65 eV, the reaction along the low-spin fission channel is more favorable with an exothermicity of 0.53 eV and a lower barrier of 1.51 eV. Two intermediates are observed along the low-spin fission channel, a spin-localized cubane [Fe4S4Cl4] 2- cluster, which contains two valence-localized Fe 3+ centers and two valence-localized Fe 2+ centers, and a half-opened [Fe4S4Cl4] 2- cluster. The spin-localized cluster is crucial to breaking the four strong Fe-S bonds in the symmetric fission.

We report a photoelectron spectroscopy and computational study of hydrated N3- anion clusters, N3-(H2O)n (n = 0−16), in the gas phase. Photoelectron spectra of the solvated azide anions were observed to consist of a single peak, similar to that of the bare N3-, but the spectral width was observed to broaden as a function of cluster size due to solvent relaxation upon electron detachment. The adiabatic and vertical electron detachment energies were measured as a function of solvent number. The measured electron binding energies indicate that the first four solvent molecules have much stronger interactions with the solute anion, forming the first solvation shell. The spectral width levels off at n = 7, suggesting that three waters in the second solvation shell are sufficient to capture the second shell effect in the solvent relaxation. Density functional calculations were carried out for N3- solvated by one to five waters and showed that the first four waters interact directly with N3- and form the first so...

The electronic structures of a series of Fe(I) -Fe(I) model complexes of the di-iron subsite of [Fe]-hydrogenase, [(I-PDT)Fe2(CO)4(CN)2] 2- (I), [Fe2(CO)4{MeSCH2C(Me)(CH2S)2}(CN)2] 2- (II), [Fe2(CO)4{PhCH2SCH2C(Me)(CH2S)2}(CN)2] 2- (III), [Fe2(CO)4{PhCH2SCH2C(Me) (CH2S)2}(CN)] - (IV), and [Fe2(CO)4{MeSCH2C(Me)(CH2S)2}(CN)] - (V), were investigated in the gas phase using photodetachment photoelectron spectroscopy. The adiabatic electron detachment energy (ADE) of each species and the intramolecular Coulomb repulsion for the doubly charged species were obtained. The ADEs correspond to the intrinsic redox potentials (in vacuo) of reactions involving the Fe(I)-Fe(I)/Fe(I)-Fe(II) couples in these compounds. The photoelectron spectra were understood and qualitatively assigned by comparing with that of Fe2(CO)6S2, which has been well studied previously and exhibits similar valence spectral features as I-V. A “normal level scheme” is suggested for the electronic structure of these low spin di-iron compounds, in which all occupied 3d levels lie above all occupied ligand levels. We also observed subtle differences in the electronic structures of the five di-iron complexes due to the slightly different ligand environments.

Iron−sulfur proteins are an important class of electron carriers in a wide variety of biological reactions. Determining the intrinsic contribution of the metal site to the redox potential is crucia...

The electronic structure of a series of Fe−S complexes, Fe(SCN)3-, Fe(SCN)4-, Na+[Fe(SCN)42-], Fe(SCH3)3-, Fe(SCH3)4-, Na+[Fe(S2-o-xyl)22-], and Fe(S2-o-xyl)2- (where S2-o-xyl = o-xylene-α,α‘-dithiolate), was investigated in the gas phase using photodetachment photoelectron spectroscopy. Spectral features due to detachment from metal d orbitals or ligand orbitals were distinguished by comparing with the spectrum of a d0 complex, Sc(SCN)4-. A weak threshold feature was observed in the spectra of all ferrous complexes due to detachment of the minority spin Fe 3d electron [FeII (3d6) → FeIII (3d5)]. The spin majority Fe 3d electron signals were observed at much higher binding energies, revealing directly the “inverted level scheme” for these [1Fe] Fe−S complexes based on previous spin polarized calculations. The “inverted level scheme” is shown to be a general feature of the electronic structures of the [1Fe] Fe−S complexes, independent of the oxidation state and coordination number. Information about the ga...

We produced hydrated clusters of the sulfate dianion, SO42-(H2O)n (n = 4−40), using electrospray ionization and investigated their energetics and stabilities using photodetachment photoelectron spectroscopy (PES) at four photon energies. The adiabatic electron detachment energies of the hydrated cluster dianions were observed to increase with the number of water, from ∼0.4 eV for n = 3, the smallest number of water to stabilize SO42- in the gas phase, to ∼5.7 eV for n = 40. The PES features of the smaller clusters are similar, all due to the SO42- dianion, which was found to be in the center of the solvated clusters. The intensity of the SO42- solute PES features was observed to decrease with solvent coverage beyond the first solvation shell (n ≈ 12), whereas a strong high binding energy feature due to ionization of the solvent emerged. For n > 30, the solute PES features almost completely disappeared, and the water feature became dominant. The photon energy dependent studies allowed us to examine the rep...

The stability, energetics, and structures of three common inorganic phosphate species, H2PO4-, H2P2O72-, H3P3O102-, and their corresponding neutral radical and monoanions, were investigated in the gas phase using photodetachment photoelectron spectroscopy and theoretical calculations. We found that H2P2O72- and H3P3O102- are stable in the gas phase with adiabatic electron binding energies of 1.16 and 2.45 eV, respectively. A very high adiabatic electron binding energy of 4.57 eV was measured for H2PO4-. The intramolecular Coulomb repulsion energies in H2P2O72- (∼2.7 eV) and H3P3O102- (∼2.3 eV) were estimated from photon-energy-dependent photoelectron spectra. Density-functional theory calculations were used to search the optimal geometries for both the doubly and singly charged species. We found only one minimum energy conformation for H2P2O72- with two intramolecular H-bonds and C2 symmetry and three minimum energy structures for H3P3O102-. The lowest energy structure of H3P3O102- has three intramolecula...

We report a photodetachment photoelectron spectroscopy study of a doubly charged anion, ZrF62-, in the gas phase at two photon energies: 193 and 157 nm. The adiabatic detachment energy, i.e., the second electron affinity of ZrF6, was measured to be extremely high, 2.9 (0.2) eV. The vertical detachment energy (VDE) was measured to be 3.4 eV. Two excited states were also observed for the ZrF6- singly charged anion with VDE's at 4.0 and 4.6 eV, respectively. The repulsive Coulomb barrier, and hence the intramolecular Coulomb repulsion in ZrF62-, was estimated from the photon-energy-dependent spectra to be {approx}2.4 eV. The ZrF62- dianion was observed to be metastable with a half-life of {approx}4 seconds under our experimental conditions, confirming theoretical predictions that ZrF62- is thermodynamically unstable against loss of an F-, despite of its extremely high electronic stability. We also performed experiment for the singly charged ZrF5-, but could not observe photodetachment even at 157 n m, the highest photon energy available to us. Thus, we estimated that the electron affinity of ZrF5 is larger than 7.8 eV, the highest electron affinity confirmed experimentally.

Multiply charged anions are common in the condensed phase, and their existence has been generally taken for granted. But until very recently, these species have only rarely been observed in the gas phase. A new experimental technique, using photodetachment photoelectron spectroscopy, electrospray, and ion-trap mass spectrometry, has been developed in the author's laboratory to probe multiply charged anions in the gas phase. In this article, the principles of this technique and some of the initial results will be presented and discussed. The difference between photodetachment of singly and multiply charged anions is emphasized. Photodetachment photoelectron spectroscopy is ideal to probe the chemical and physical properties of free multiply charged anions. Among the initial findings, the repulsive Coulomb barriers existing in multiply charged anions have been directly observed. The relationship between the intramolecular electron−electron repulsion and the potential barrier is elucidated, culminating in th...

We present a combined photoelectron spectroscopic and theoretical study of SO4- and HSO4- and their corresponding neutral species in the gas phase. We measured very high electron affinities for SO4 and HSO4, which are 5.10(0.10) and 4.75(0.10) eV, respectively. Theoretical calculations of the neutral and anionic species using both density functional and ab initio molecular orbital theory confirm the high electron affinities of the two species. The theoretically predicted vertical and adiabatic detachment energies are in good agreement with the experimental measurements. We find that both SO4- and SO4 have C2v symmetry, whereas both HSO4- and HSO4 have Cs symmetry. We predict significant geometry changes between the equilibrium geometries of the anion species and the neutral species for both systems that are consistent with the broad nature of the measured photoelectron spectra.

We report photodetachment photoelectron spectroscopy (PES) of gaseous copper phthalocyanine (CuPc) tetrasulfonate quadruply charged anions, [CuPc(SO3)4]4-, and its monoprotonated and -sodiumated triply charged anions, [CuPc(SO3)4H]3- and [CuPc(SO3)4Na]3-. The [CuPc(SO3)4]4- tetraanion was found to possess a negative electron binding energy of −0.9 eV, whereas the trianions have binding energies of 1.0 and 1.2 eV for the sodiumated and protonated species, respectively. The PES spectral features of the three multiply charged anions were observed to be similar to that of the parent CuPc neutral molecule, except that the anions have lower binding energies due to the presence of the negative charges (−SO3-). The data thus suggested a stepwise tuning of the molecular electronic energy levels of the CuPc molecule through charging, wherein the molecular orbital energies of the parent molecule were systematically pushed up by the negative charges. We further carried out semiempirical calculations, which provided i...

The authors present a combined experimental and theoretical investigation of alkali metal sulfate ion pairs and their doubly charged dimers in the gas phase: Na{sup +}SO{sub 4}{sup 2{minus}}, K{sup +}SO{sub 4}{sup 2{minus}}, (NaSO{sub 4}){sub 2}{sup 2{minus}}, (KSO{sub 4}){sub 2}{sup 2{minus}}, and NaK(SO{sub 4}){sub 2}{sup 2{minus}}. They produced these anions using an electrospray technique and measured their photoelectron spectra at three photon energies, 355, 266, and 193 nm. The photoelectron spectra of each anion exhibit two detachment features, which approximately correspond to detachment from the HOMO and HOMO-1 of the SO{sub 4}{sup 2{minus}} group perturbed by the cations. The electron binding energies of the dimer dianions are lower than those of the monomers because of the strong electron-electron repulsion in the dianions. The authors also observed the repulsive Coulomb barriers in the dianions and their effects on the photodetachment spectra. They used density functional theory and ab initio molecular orbital theory to obtain the geometric and electronic structure of the ion pairs. The calculated electron binding energies and orbital energy separations are in reasonable agreement with the experimental values. The authors found that the MSO{sub 4}{sup {minus}} ion pairs have C{sub 3v} symmetry with a C{sub 2v} structure slightly higher inmore » energy. In the dimer dianions the two alkali metal cations bridge the two SO{sub 4}{sup 2{minus}} groups.« less