Importance of Time Scale and Local Environment in Electron-Driven Proton Transfer. The Anion of Acetoacetic Acid.

Anion photoelectron spectroscopy (PES) and electron energy-loss spectroscopy (EELS) probe different regions of the anionic potential energy surface. These complementary techniques provided information about anionic states of acetoacetic acid (AA). Electronic structure calculations facilitated the identification of the most stable tautomers and conformers for both neutral and anionic AA and determined their relative stabilities and excess electron binding energies. The most stable conformers of the neutral keto and enol tautomers differ by less than 1 kcal/mol in terms of electronic energies corrected for zero-point vibrations. Thermal effects favor these conformers of the keto tautomer, which do not support an intramolecular hydrogen bond between the keto and the carboxylic groups. The valence anion displays a distinct minimum which results from proton transfer from the carboxylic to the keto group; thus, we name it an ol structure. The minimum is characterized by a short intramolecular hydrogen bond, a significant electron vertical detachment energy of 2.38 eV, but a modest adiabatic electron affinity of 0.33 eV. The valence anion was identified in the anion PES experiments, and the measured electron vertical detachment energy of 2.30 eV is in good agreement with our computational prediction. We conclude that binding an excess electron in a π* valence orbital changes the localization of a proton in the fully relaxed structure of the AA(-) anion. The results of EELS experiments do not provide evidence for an ultrarapid proton transfer in the lowest π* resonance of AA(-), which would be capable of competing with electron autodetachment. This observation is consistent with our computational results, indicating that major gas-phase conformers and tautomers of neutral AA do not support the intramolecular hydrogen bond that would facilitate ultrarapid proton transfer and formation of the ol valence anion. This is confirmed by our vibrational EELS spectrum. Anions formed by vertical electron attachment to dominant neutrals undergo electron autodetachment with or without vibrational excitations but are unable to relax to the ol structure on a time scale fast enough to compete with autodetachment.

Photoelectron spectroscopic and computational study of hydrated pyrimidine anions.

The stabilization of the pyrimidine anion by the addition of water molecules is studied experimentally using photoelectron spectroscopy of mass-selected hydrated pyrimidine clusters and computationally using quantum-mechanical electronic structure theory. Although the pyrimidine molecular anion is not observed experimentally, the addition of a single water molecule is sufficient to impart a positive electron affinity. The sequential hydration data have been used to extrapolate to -0.22 eV for the electron affinity of neutral pyrimidine, which agrees very well with previous observations. These results for pyrimidine are consistent with previous studies of the hydrated cluster anions of uridine, cytidine, thymine, adenine, uracil, and naphthalene. This commonality suggests a universal effect of sequential hydration on the electron affinity of similar molecules.

Communication: Remarkable electrophilicity of the oxalic acid monomer: an anion photoelectron spectroscopy and theoretical study.

Our experimental and computational results demonstrate an unusual electrophilicity of oxalic acid, the simplest dicarboxylic acid. The monomer is characterized by an adiabatic electron affinity and electron vertical detachment energy of 0.72 and 1.08 eV (±0.05 eV), respectively. The electrophilicity results primarily from the bonding carbon-carbon interaction in the singly occupied molecular orbital of the anion, but it is further enhanced by intramolecular hydrogen bonds. The well-resolved structure in the photoelectron spectrum is reproduced theoretically, based on Franck-Condon factors for the vibronic anion → neutral transitions.

The reaction rates of O2 with closed-shell and open-shell Al(x)⁻ and Ga(x)⁻ clusters under single-collision conditions: experimental and theoretical investigations toward a generally valid model for the hindered reactions of O2 with metal atom clusters.

In order to characterize the oxidation of metallic surfaces, the reactions of O2 with a number of Al(x)(-) and, for the first time, Ga(x)(-) clusters as molecular models have been investigated, and the results are presented here for x = 9-14. The rate coefficients were determined with FT-ICR mass spectrometry under single-collision conditions at O2 pressures of ~10(-8) mbar. In this way, the qualitatively known differences in the reactivities of the even- and odd-numbered clusters toward O2 could be quantified experimentally. To obtain information about the elementary steps, we additionally performed density functional theory calculations. The results show that for both even- and odd-numbered clusters the formation of the most stable dioxide species, [M(x)O2](-), proceeds via the less stable peroxo species, [M(x)(+)···O2(2-)](-), which contains M-O-O-M moieties. We conclude that the formation of these peroxo intermediates may be a reason for the decreased reactivity of the metal clusters toward O2. This could be one of the main reasons why O2 reactions with metal surfaces proceed more slowly than Cl2 reactions with such surfaces, even though O2 reactions with both Al metal and Al clusters are more exothermic than are reactions of Cl2 with them. Furthermore, our results indicate that the spin-forbidden reactions of (3)O2 with closed-shell clusters and the spin-allowed reactions with open-shell clusters to give singlet [M(x)(+)···O2(2-)](-) are the root cause for the observed even/odd differences in reactivity.

Combined photoelectron, collision-induced dissociation, and computational studies of parent and fragment anions of N-paranitrophenylsulfonylalanine and N-paranitrophenylalanine.

After synthesizing the compounds N-paranitrophenylsulfonylalanine (NPNPSA) and N-paranitrophenylalanine (NPNPA), the photoelectron spectrum of the valence anion of N-paranitrophenylsulfonylalanine (NPNPSA)(-), was measured and the collision-induced dissociation (CID) pathways of deprotonated N-paranitrophenylsulfonylalanine (NPNPSA-H)(-) and deprotonated N-paranitrophenylalanine (NPNPA-H)(-) were determined. Pertinent calculations were conducted to analyze both sets of experimental data. From the valence anion photoelectron spectrum of (NPNPSA)(-), the adiabatic electron affinity (AEA) of NPNPSA was determined to be 1.7 ± 0.1 eV, while the vertical detachment energy (VDE) of (NPNPSA)(-) was found to be 2.3 ± 0.1 eV. Calculations for four low lying conformers of (NPNPSA)(-) gave AEA values in the range of 1.6-2.1 eV and VDE values in the range of 2.0-2.4 eV. These calculations are in very good agreement with the experimental values. While the NPNPA anion (NPNPSA)(-) was not observed experimentally it was studied computationally. The six low lying (NPNPSA)(-) conformers were identified and calculated to have AEA values in the range of 0.7-1.2 eV and VDE values in the range of 0.9-1.6 eV. CID was used to study the fragmentation patterns of deprotonated NPNPA and deprotonated NPNPSA. Based on the CID data and calculations, the excess charge was located on the delocalized π-orbitals of the nitrobenzene moiety. This is made evident by the fact that the dominant fragments all contained the nitrobenzene moiety even though the parent anions used for the CID study were formed via deprotonation of the carboxylic acid. The dipole-bound anions of both molecules are studied theoretically using the results of previous studies on nitrobenzene as a reference.

Negative ions of p-nitroaniline: photodetachment, collisions, and ab initio calculations.

The structures of parent anion, M(-), and deprotonated molecule, [M-H](-), anions of the highly polar p-nitroaniline (pNA) molecule are studied experimentally and theoretically. Photoelectron spectroscopy (PES) of the parent anion is employed to estimate the adiabatic electron affinity (EAa = 0.75 ± 0.1 eV) and vertical detachment energy (VDE = 1.1 eV). These measured energies are in good agreement with computed values of 0.73 eV for the EAa and the range of 0.85 to 1.0 eV for the VDE at the EOM-CCSD∕Aug-cc-pVTZ level. Collision induced dissociation (CID) of deprotonated pNA, [pNA - H](-), with argon yielded [pNA - H - NO](-) (i.e., rearrangement to give loss of NO) with a threshold energy of 2.36 eV. Calculations of the energy difference between [pNA - H](-) and [pNA - H - NO](-) give 1.64 eV, allowing an estimate of a 0.72 eV activation barrier for the rearrangement reaction. Direct dissociation of [pNA - H](-) yielding NO2(-) occurs at a threshold energy of 3.80 eV, in good agreement with theory (between 3.39 eV and 4.30 eV). As a result of the exceedingly large dipole moment for pNA (6.2 Debye measured in acetone), we predict two dipole-bound states, one at ~110 meV and an excited state at 2 meV. No dipole-bound states are observed in the photodetachment experiments due the pronounced mixing between states with dipole-bound and valence character similar to what has been observed in other nitro systems. For the same reason, dipole-bound states are expected to provide highly efficient "doorway states" for the formation of the pNA(-) valence anion, and these states should be observable as resonances in the reverse process, that is, in the photodetachment spectrum of pNA(-) near the photodetachment threshold.

Photoelectron spectroscopy of the 6-azauracil anion.

We report the photoelectron spectrum of the 6-azauracil anion. The spectrum is dominated by a broad band exhibiting a maximum at an electron binding energy (EBE) of 1.2 eV. This spectral pattern is indicative of a valence anion. Our calculations were carried out using ab initio electron propagator and other many-body methods. Comparison of the anion and corresponding neutral of 6-azauracil with those of uracil shows that substituting a nitrogen atom for C-H at the C6 position of uracil gives rise to significant changes in the electronic structure of 6-azauracil versus that of uracil. The adiabatic electron affinity (AEA) of the canonical 6-azauracil tautomer is substantially larger than that of canonical uracil. Among the five tautomeric, 6-azauracil anions studied computationally, the canonical structure was found to be the most stable. The vertical detachment energies (VDE) of the canonical, valence-bound anion of 6-azauracil and its closest "very-rare" tautomer have been calculated. Electron propagator calculations on the canonical anion yield a VDE value that is in close agreement with the experimentally determined VDE value of 1.2 eV. The AEA value of 6-azauracil, assessed at the CCSD(T) level of theory to be 0.5 eV, corresponds with the EBE value of the onset of the experimental spectrum.

Photoelectron spectroscopy of the molecular anions, ZrO-, HfO-, HfHO-, and HfO2H-.

Negative ion photoelectron spectra of ZrO(-), HfO(-), HfHO(-), and HfO(2)H(-) are reported. Even though zirconium- and hafnium-containing molecules typically exhibit similar chemistries, the negative ion photoelectron spectral profiles of ZrO(-) and HfO(-) are dramatically different from one another. By comparing these data with relevant theoretical and experimental studies, as well as by using insights drawn from atomic spectra, spin-orbit interactions, and relativistic effects, the photodetachment transitions in the spectra of ZrO(-) and HfO(-) were assigned. As a result, the electron affinities of ZrO and HfO were determined to be 1.26 ± 0.05 eV and 0.60 ± 0.05 eV, respectively. The anion photoelectron spectra of HfHO(-) and HfO(2)H(-) are similar to one another and their structural connectivities are likely to be H-Hf-O(-) and O-Hf-OH(-), respectively. The electron affinities of HfHO and HfO(2)H are 1.70 ± 0.05 eV and 1.73 ± 0.05 eV, respectively.