Ab Initio Study of the Rotational-torsional Spectrum of Methyl Formate

The molecular structure of methyl formate is determined from ab initio calculations. The molecule presents two conformers (cis and trans) with a 5.3 kcal mol 1 difference in energy. In the most stable cis conformer, the carbonyl group eclipses the methyl group. The internal rotation barriers are V3(cis) 1⁄4 368 cm 1 and V3(trans) 1⁄4 26 cm 1 for the methyl group and V CO 1⁄4 4826 cm 1 for the CO group. The dependence of the spectroscopic parameters on the torsional motions is detailed. The rotational-torsional energy levels have been calculated variationally up to J 1⁄4 6 using a flexible model depending on the two torsional modes. Far-infrared frequencies and intensities are determined at room temperature. The rotational parameters have been computed to beA 1⁄4 20;040:473MHz,B 1⁄4 6974:140MHz, C 1⁄4 5350:705 MHz, DJ 1⁄4 0:510 kHz, DJK 1⁄4 1:566 kHz, and DK 1⁄4 0:619 kHz; and A 1⁄4 20;040:492 MHz, B 1⁄4 6974:399 MHz, C 1⁄4 5350:851 MHz, DJ 1⁄4 2:070 kHz, DJK 1⁄4 14:712 kHz, and DK 1⁄4 5:898 kHz at the symmetric and E components of the cis ground state, respectively. The corresponding values for trans–methyl formate are A 1⁄4 47;380:066MHz,B 1⁄4 4738:781MHz, andC 1⁄4 4430:339MHz; and A 1⁄4 47;389:697MHz,B 1⁄4 4737:751MHz, and C 1⁄4 4429:607 MHz. Subject headinggs: astrochemistry — molecular data