Highly correlated ab initio study of the far infrared spectra of methyl acetate.

Highly correlated ab initio calculations (CCSD(T)) are used to compute gas phase spectroscopic parameters of three isotopologues of the methyl acetate (CH(3)COOCH(3), CD(3)COOCH(3), and CH(3)COOCD(3)), searching to help experimental assignments and astrophysical detections. The molecule shows two conformers cis and trans separated by a barrier of 4457 cm(-1). The potential energy surface presents 18 minima that intertransform through three internal rotation motions. To analyze the far infrared spectrum at low temperatures, a three-dimensional Hamiltonian is solved variationally. The two methyl torsion barriers are calculated to be 99.2 cm(-1) (C-CH(3)) and 413.1 cm(-1) (O-CH(3)), for the cis-conformer. The three fundamental torsional band centers of CH(3)COOCH(3) are predicted to lie at 63.7 cm(-1) (C-CH(3)), 136.1 cm(-1) (O-CH(3)), and 175.8 cm(-1) (C-O torsion) providing torsional state separations. For the 27 vibrational modes, anharmonic fundamentals and rovibrational parameters are provided. Computed parameters are compared with those fitted using experimental data.