Theoretical study of the photochemical fragmentation of methanethiol

Recent experimental/theoretical studies have demonstrated efficient photodissociation of CH3SH to CH3S+H and to CH3+SH. Avoided crossings have been implicated in the photodissociation process. This work demonstrates the existence of a surface of conical intersections of two states of the same symmetry relevant to the photodissociation offering new insights into the mechanism of the decomposition process. Using wave functions comprised of greater than 1.3 million configuration state functions convincing evidence is provided that the surface of conical intersections is a property of the system rather the particular expansion employed in this work. The conical intersections are determined directly, that is without prior determination of the potential energy surfaces in question.

On the role of conical intersections of two potential energy surfaces of the same symmetry in photodissociation. I. CH3SH→CH3S+H and CH3+SH

Molecular structure and global description of the potential energy surface

The gradient pathways of the reactions of nucleophilic addition of H2O and HF molecules to formaldehyde in the gas phase and in the XH…H2CO…HC(O)OH complex (X=OH, F) were calculated by theab initio RHF/6-31G**, MP2(fc)/6-31G**, and MP2(full)/6-311++G** methods. Both reactions proceed concertedly. The formation of H-bonded bimolecular pre-reaction complexes is the initial stage of the gas-phase reactions; at the same time, no indications of the formation of stable π-complexes were found on the potential energy surfaces of systems under study. The calculated energy barriers to the gasphase reactions exceed 40 kcal mol−1, while those to reactions in the complex XH…H2CO…HC(O)OH (X=OH, F) become more than halved.

Pathways of the reactions of nucleophilic addition of H2O and HF molecules to formaldehyde in the gas phase and in the complex with formic acid:ab initio calculations

A theoretical study of a model of N-tert-butyl-α-phenylnitrone (PBN) as active oxygen species scavenger

The dehydrogenation reaction paths of methanethiol CH3SH were analyzed by abinitio MO calculations. The geometries and energies of the reactants, transition states, and products have been determined on the singlet potential energy surface of the ground state. We also analyze the similar dehydrogenation reactions of methanol (CH3OH) and ethanol (CH3CH2OH) for the purpose of comparison. The activation energy and the heat of reaction calculated by incorporating the effect of electronic correlation shows that the product H2C=S is chemically more unstable than the product H2C=O. "Reaction ergodography" along the intrinsic reaction coordinate (IRC) for these dehydrogenation paths clarifies the distinct differences in atomic movement. The path of methanethiol has two characteristic steps before the transition state, while neither paths of the two carbinols have such multisteps. The electronic intramolecular interaction between subsystems of the reactant molecule is investigated in terms of Interaction Hybrid Mol...

Cheng-Da Zhao

Papers

Reaction ergodography for dehydrogenation reaction of methanethiol