Conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of methyl propargyl ether

Abstract

The Raman spectra (3400 to 10 cm−1) of gaseous, liquid and solid and the infrared spectra (3500 to 35 cm−1) of gaseous and solid methyl propargyl ether, HCCCH2OCH3, have been recorded. Furthermore, the far-infrared spectrum of the gas has been recorded in the region of 370 to 30 cm−1 at a resolution of 0.10 cm−1. From the vibrational spectrum two conformers have been identified in the fluid phases at ambient temperature with the gauche rotamer the more stable conformer and the only one present in the annealed solid. The fundamental asymmetric torsions of the gauche and trans conformations have been observed at 113.2 and 102.0 cm−1, respectively, with the gauche form having three excited state transitions falling to lower frequency. From these data, the asymmetric torsional potential function has been obtained with the following coefficients: V1 = 325 ± 24, V2 = −662 ± 23, V3 = 1149 ± 14, V4 = −102 ± 6 and V6 = −46 ± 6 cm−1. From this potential function, the trans to gauche, gauche to gauche and gauche to trans barriers are determined to be 664, 1831 and 1021 cm−1, respectively, with an enthalpy difference between the conformers of 357 ± 61 cm−1 (1021 ± 174 cal mol−1 and the gauche conformer the more stable form in the gas. The enthalpy difference has been determined experimentally for the liquid from the variable-temperature studies of the Raman spectrum and a value of 153 ± 10 cm−1 (437 ± 29 cal mol−1) has been obtained with the gauche form again the more stable rotamer. The methyl torsional transitions of the gauche and trans rotamers have been observed at 166.0 and 214.2 cm−1, respectively, and on the basis of the one-dimensional model the barrier to internal rotation of the methyl group is determined to be 707 ± 45 cm−1 (2.02 kcal mol−1) and 991 ± 67 cm−1 (2.83 kcal mol−1) for the gauche and trans conformers, respectively. A complete vibrational assignment is proposed for a gauche/trans equilibrium in the gas and liquid phases. 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 the RHF/4–31G*, RHF/6–31G*, MP2/6–31G* and/or MP2/6–31++G** basis sets. These results are compared to those obtained for some similar molecules.