Electrophilicity of oxalic acid monomer is enhanced in the dimer by intermolecular proton transfer.

We have analyzed the effect of excess electron attachment on the network of hydrogen bonds in the oxalic acid dimer (OA)2. The most stable anionic structures may be viewed as complexes of a neutral hydrogenated moiety HOA˙ coordinated to an anionic deprotonated moiety (OA-H)-. HOA˙ acts as a double proton donor and (OA-H)- as a double proton acceptor. Thus the excess electron attachment drives intermolecular proton transfer. We have identified several cyclic hydrogen bonded structures of (OA)2-. Their stability has been analyzed in terms of the stability of the involved conformers, the energetic penalty for deformation of these conformers to the geometry of the dimer, and the two-body interaction energy between the deformed HOA˙ and (OA-H)-. There are at least seven isomers of (OA)2- with stabilization energies in the range of 1.26-1.39 eV. These energies are dominated by attractive two-body interaction energies. The anions are vertically bound electronically by 3.0-3.4 eV and adiabatically bound by at least 1.6 eV. The computational predictions are consistent with the anion photoelectron spectrum of (OA)2-. The spectrum consists of a broad feature, with an onset of 2.5 eV and spanning to 4.3 eV. The electron vertical detachment energy (VDE) is assigned to be 3.3 eV.

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.

Intermolecular interactions between molecules in various conformational states: the dimer of oxalic acid.

We considered stability of the dimer of oxalic acid. The global minimum energy structure identified by us is stabilized by two inter- and four intramolecular hydrogen bonds, whereas the most stable structure identified in previous studies is supported by two inter- and three intramolecular hydrogen bonds. The latter structure proves to be less stable by 25 meV than the former. The global minimum stability results from a balancing act between a moderately attractive two-body interaction energy and small repulsive one-body terms. We have analyzed zero-point vibrational corrections to the stability of various conformers of oxalic acid and their dimers. We have found that minimum energy structures with the most stabilizing sets of hydrogen bonds have the largest zero-point vibrational energy, contrary to a naive anticipation based on red shifts of OH stretching modes involved in hydrogen bonds.

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.

Discovery of Most Stable Structures of Neutral and Anionic Phenylalanine through Automated Scanning of Tautomeric and Conformational Spaces.

We have developed a software tool for combinatorial generation of tautomers and conformers of small molecules. We have demonstrated it by performing a systematic search for the most stable structures of neutral and anionic phenylalanine (Phe) using electronic structure methods. For the neutral canonical tautomer we found out that the conformers with and without the intramolecular (O)H···NH2 hydrogen bond are similarly stable, within the error bars of our method. A unique IR signature of the conformer without the hydrogen bond has been identified. We also considered anions of Phe, both valence type and dipole-bound. We have found out that tautomers resulting from proton transfer from the carboxylic OH to the phenyl ring do support valence anions that are vertically strongly bound, with electron vertical detachment energies (VDE) in a range of 3.2-3.5 eV. The most stable conformer of these valence anions remains adiabatically unbound with respect to the canonical neutral by only 2.17 kcal/mol at the CCSD(T)/aug-cc-pVDZ level. On the basis of our past experience with valence anions of nucleic acid bases, we suggest that the valence anions of Phe identified in this report can be observed experimentally. The most stable conformer of canonical Phe is characterized by an adiabatic electron affinity of 53 meV (a dipole-bound state).