Conformational analysis, barriers to internal rotation and inversion, vibrational assignment, ab initio calculations, and r0 structural parameters of ethylmethylamine

Abstract

Variable temperature (−110 to −150 °C) studies of the infrared spectra between 4000 and 50 cm −1 of ethylmethylamine, CH 3CH 2N(H)CH 3, dissolved in liquid krypton have been recorded. Additionally, the infrared spectrum (4000–50 cm −1) of the solid has been recorded. All three possible conformers, i.e. N-methyl group (Me- trans, or T), hydrogen atom (H- trans, or G′), and lone pair (LP- trans, or G) trans to the methyl group of the ethyl moiety, have been identified with only the Me- trans conformer remaining in the solid. From the variable temperature studies the enthalpy difference has been determined to be 335±35 cm −1 (4.01±0.42 kJ/mol) between the most stable Me- trans conformer and the least stable LP- trans rotamer. An enthalpy difference of 177±27 cm −1 (2.12±0.32 kJ/mol) is estimated between the H- trans form and the most stable Me- trans conformer. These results are somewhat lower but still in reasonable agreement with the ab initio predictions at all levels of calculations. It is estimated that there is only 12±2% of the LP- trans form and 26±3% of the H- trans conformer present at ambient temperature. A complete vibration assignment is proposed for the Me- trans conformer which is supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations. The conformational stabilities, barriers to internal rotation and inversion, and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio calculations. Utilizing previously reported microwave rotational constants for two isotopomers of the Me- trans rotamer along with ab initio predicted structural values, r 0 adjusted parameters have been obtained for this conformer. The results are discussed and compare to the corresponding properties of some similar molecules.