Conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of ethyl methyl sulfide

Abstract

The far-infrared spectrum of ethyl methyl sulfide in the gas phase has been recorded at a resolution of 0.10 cm −1 in the region 370-30 cm −1. The fundamental asymmetric torsions of the gauche and trans conformations have been observed at 93.4 cm −1 and 90.8 cm −1 respectively, with the gauche form having several excited-state transitions. From these data, the asymmetric torsional potential function has been recalculated with the following coefficients: V 1=310±10, V 1 −190±7, v 3 =997±17, V 4 =101±4 and V 6 = −94±7 cm −1. From this potential function, the trans to gauche, gauche to gauche and gauche to trans barriers are determined to be 1012±17 cm −1, 1184±9 cm −1 and 881±21 cm −1 respectively, with an energy difference of 131 ± 45 cm −1, the trans conformer being the more stable form in the gas phase. The enthalpy difference has also been determined experimentally from the variable temperature studies of the Raman lines and a value of 133±34 cm −1 (380±97 cal mol −10) has been obtained for the gas with the trans form more stable, but for the liquid phase the value is too small to be measured. A complete vibrational assignment is proposed for a trans/gauche equilibrium in the gas and liquid phases from the Raman (3200-10 cm −1) and infrared (3200-35 cm −1) spectra obtained for the gas, liquid and solid phases, and in the solid only the gauche conformer remains. The structural parameters, conformational stability, barriers to internal rotation and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio calculations employing both the 3-21 G* and 6-31G* basis sets. These results are compared to those obtained for some similar molecules.