Theoretical Characterization of Dimethyl Carbonate at Low Temperatures

Highly correlated ab initio methods (CCSD(T) and RCCSD(T)-F12) are employed for the spectroscopic characterization of the gas phase of dimethyl carbonate (DMC) at low temperatures. DMC, a relevant molecule for atmospheric and astrochemical studies, shows only two conformers, cis-cis and tran-cis, respectively, of C-2v and C-s symmetries. cis-cis-DMC represents the most stable form. Using RCCSD(T)-F12 theory, the two sets of equilibrium rotational constants have been computed to be A(e) = 10493.15 MHz, B-e = 2399.22 MHz, and C-e = 2001.78 MHz (cis-cis) and to be A(e) = 6585.16 MHz, B-e = 3009.04 MHz, and C-e = 2120.86 MHz (trans-cis). Centrifugal distortions constants and anharmonic frequencies for all of the vibrational modes are provided. Fermi displacements are predicted. The minimum energy pathway for the cis-cis -> trans-cis interconversion process is restricted by a barrier of similar to 3500 cm(-1). DMC displays internal rotation of two methyl groups. If the nonrigidity is considered, the molecule can be classified in the G(36) (cis-cis) and the G(18) (trans-cis) symmetry groups. For cis-cis-DMC, both internal tops are equivalent, and the torsional motions are restricted by V-3 potential energy barriers of 384.7 cm(-1). trans-cis-DMC shows two different V-3 barriers of 631.53 and 382.6 cm(-1). The far-infrared spectra linked to the torsional motion of both conformers are analyzed independently using a variational procedure and a two-dimensional flexible model. In cis-cis-DMC, the ground vibrational state splits into nine components: one nondegenerate, 0.000 cm(-1) (A(1)), four quadruply degenerate, 0.012 cm(-1) (G), and four doubly degenerate 0.024 cm(-1) (E-1 and E-3). The methyl torsional fundamentals are obtained to lie at 140.274 cm(-1) (v(15)) and 132.564 cm(-1) (v(30)).