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 V-3( cis) = 368 cm(-1) and V-3( trans) = 26 cm(-1) for the methyl group and V-CO = 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 = 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 be A = 20, 040. 473MHz, B = 6974.140MHz, C = 5350.705 MHz, D-J = -0.510 kHz, D-JK = 1.566 kHz, and D-K = -0: 619 kHz; and A = 20, 040. 492 MHz, B = 6974: 399 MHz, C = 5350: 851 MHz, D-J = 2: 070 kHz, D-JK = 14: 712 kHz, and D-K = 5: 898 kHz at the symmetric and E components of the cis ground state, respectively. The corresponding values for trans - methyl formate are A = 47; 380: 066MHz, B = 4738: 781MHz, and C = 4430: 339 MHz; and A = 47; 389: 697 MHz, B = 4737: 751 MHz, and C = 4429: 607 MHz.