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 = 20,040.473 MHz, B = 6974.140 MHz, 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 = 47,380.066 MHz, B = 4738.781 MHz, and C = 4430.339 MHz; and A = 47,389.697 MHz, B = 4737.751 MHz, and C = 4429.607 MHz.