Conformations of dimethyl carbonate and its complexes with water: A matrix isolation infrared and ab initio study

Conformations of dimethyl carbonate (DMC) were studied using matrix isolation infrared spectroscopy. Infrared spectra of DMC trapped in inert gas matrixes, using an effusive source at 298 and 423 K, showed evidence of both the ground state (cis–cis), and higher energy (cis–trans) conformers. Experiments were also performed using a supersonic jet source to deposit the matrix, to look for conformational cooling in the expansion process. The structures and vibrational frequencies of these conformers were computed at the B3LYP/6-31++G** level of theory. Natural bond orbital analyses were performed to understand the role of the delocalization interactions in conformational preferences. Complexes of DMC with H2O were also studied. A 1:1 DMC–H2O complex was identified in the matrix isolation experiments, where the carbonyl oxygen of DMC served as the proton acceptor for the hydrogen bonded complex. This observation was corroborated by computations performed on the complex at the B3LYP/6-31++G** level. Our computations also indicated another minimum, corresponding to an alkoxy bonded DMC–H2O complex, which was less exothermic; however, this complex was not identified in our experiments. Atoms-in-molecules theory was also performed to understand the nature of the intermolecular interaction in the DMC–H2O complex.