The interaction of an H(2)O molecule with cluster models of fractured silica surfaces was studied by means of quantum mechanical calculations. Two clusters representing homolytic cleavage (triple bond Si(*) and triple bond SiO(*)) and two representing heterolytic cleavage (triple bond Si(+) and triple bond Si-O(-)) of silica surfaces were modeled. Vibrational frequencies of the reactants and products of these silica surfaces reacting with H(2)O have been calculated and compare favorably with experiment. Comparisons of the Gibbs free and potential energies for the model ionic and radical states were made, and the radical pair of sites was predicted to be more stable by approximately -70 to -85 kJ/mol, depending on the computational methodology. These calculations suggest that when silica is fractured in a vacuum homolytic cleavage is favored. Reaction pathways were investigated for these four model surface sites interacting with H(2)O. The reaction of H(2)O with triple bond SiO(*) was predicted to generate OH(*). Rate constants for these reactions were also calculated and predict a rapid equibrium for the reaction triple bond SiO(*) + H(2)O --> triple bond SiOH + OH(*). Stability of a finite number of triple bond SiO(*) sites at equilibrium in the above reaction with H(2)O was also predicted, which implies a long-term ability of silica surfaces to produce OH(*) radicals if the sites of the broken bonds do not repolymerize to form siloxane groups.

The low coverage reversible potential for underpotential deposition of hydrogen on platinum in acid electrolyte is calculated quantum mechanically with two approaches, (i) one using the calculated reaction energy for the overall reaction and an added constant and (ii) the other based on a model of the electrochemical interface. The former yields 0.40 V and the latter 0.48 V, both close to the observed value of ∼0.40 V. Electrode-potential-dependent activation energies are calculated using the interface model for the reduction of the hydronium ion to form the Pt−H bond and for the reverse oxidation reaction over the −0.3 to 3.0 V range. To make this possible, the model explicitly excludes the hydrogen evolution reaction at the cathodic end and water oxidation at the anodic end of the potential range. Cathodic and anodic symmetry factors are determined from the slopes of the activation energy curves, which are used as models for the corresponding activation free energy curves. They are extended into the Marcus-inverted regions for oxidation and reduction. The symmetry factors add to 1.0 in the normal region and in the inverted regions and are not linear functions of the electrode potential.

The 1:1 adduct of arginine with 2,5-dihydroxybenzoic acid (DHB) has been studied in the gas phase and in the solid state. Experimentally, the ionization energy (IE) of the 1:1 cluster was determined by wavelength-dependent laser ionization of clusters formed by seeding DHB and arginine into a supersonic jet expansion. Ionization laser power studies performed at several discrete wavelengths established the upper and lower limits for the 1:1 cluster IE and dissociation energy. Subsequent one-color scanned-wavelength laser ionization studies allowed an experimental establishment of the 1:1 cluster IE of 7.193 eV. A combination of molecular dynamics/simulated annealing calculations on the 1:1 cluster followed by density functional theory geometry optimizations using reasonably large basis sets yielded 15 distinct minima on the potential energy surface, all within 5.2 kcal/mol in energy at the B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G** level. The Boltzmann-averaged IE at the same level is 7.11−7.14 eV, in excellent agreement with experiment. Cocrystals of arginine and DHB have been grown, and the crystal structure has been solved. The dominant intermolecular interaction in the cocrystal is a double hydrogen bond (salt bridge) between the guanidinium group of arginine and the (deprotonated) carboxylate group of DHB. This is exactly the same interaction that is found in the lowest-energy structure of the gas-phase 1:1 adduct. The electronic structure of the solid-state cocrystal has been modeled using a cluster approach.

Reversible potentials, U°, have been calculated based on reaction energies, E r, for the intermediate steps in the outer-sphere oxygen reduction reaction to water. The working formula is U° = (−E r eV-1 + c) V, where U° is the reversible potential and the reference energy of the electron is −4.6 eV on the physical (vacuum) or 0 V on the hydrogen scale. Results using a 6-31G** basis in the B3LYP hybrid density functional theory are shown to be comparable to ab initio MP2 results obtained previously for acid solution with c = 0.49 for the former and 0.50 for the latter. Both methods are shown to work for basic solution but, unlike in acid, the c values are different, 0.58 for B3LYP and 0.76 for MP2, reflecting differences in the two computational methods in treating anions.

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