Conformational Cooling in Dimethyl Sulfite – Influence of the Matrix Gas and Matrix Site Effect

BORBA, GOMEZ ZAVAGLIA, ANDREA AND R. FAUSTO

Coimbra, Portugal

Congreso; 8th Photochemistry Meeting of the Portuguese Chemical Society. Coimbra; 2005

Sociedad Portuguesa de Química

Organic sulfites have demonstrated practical applications as insecticides and plant growth regulators, among others uses.1 Dimethyl sulfite (DMSO3) is the simplest member of the family and has been used as insect repellent and also experimentally identified as a mutagenic agent. It is a biologically meaningful odorant and is present, for instance, in male preputial glands. DMSO3 has also been described as attractive for conspecifics in rats.2 In this work, conformational cooling of dimethyl sulfite has been studied. For this aim, the compound was isolated in argon, krypton and xenon matrices at different temperatures of the cold window. According to calculations at the DFT/B3LYP/aug-cc-pVQPZ level, the three most stable conformers are GG, GT and GG’, with relative energies of 0, 0.83 and 1.18 kJ mol-1. The experimentally relevant barriers of interconversion, calculated at the MP2/6-31++G(d,p) level are: 1.90 kJ mol-1 for GG’→GT and 9.64 kJ mol-1 for GT→GG. In both argon and krypton matrices, the three conformers could be trapped, while in xenon matrices, the highest energy conformer, GG’, could not be trapped, due to conformational cooling effects (conversion to GT conformer due to annealing during deposition of the matrix). Deposition of DMSO3 in matrices of the three gases at different temperatures of the cold window was done and the kinetics of conformational interconversion investigated. Very interestingly, a two-step mechanism for conversion of GG’ into the most stable form, GG, passing through the GT conformer in the first step, was observed in argon and krypton matrices. The observed conformational isomerization processes start to occur at different temperatures, which can be correlated with the relative values of the energy barriers for the two processes, GG’→GT and GT→GG. In addition, the effects of different matrix inclusion sites on the isomerization processes were also studied. This constitutes one of the first direct experimental observations of successive conformational isomerization taking place into cryogenic noble-gas matrices made hitherto. References 1. H. Woerden. Chem. Rev. 1963, 63, 557. 2. A. Gawienowski and M. Stacewicz-Sapuntzakis. Behav. Biol. 1978,  23, 267.