Infrared and Raman spectra, conformational stability, vibrational assignment, ab initio calculations and r0 structural parameters for N-methylpropargyl amine

Abstract

The infrared spectra (3200 to 50 cm⁻¹) of gaseous and solid and Raman spectra (3200 to 10 cm⁻¹) of the liquid and solid N-methylpropargyl amine (4-aza-1-pentyne), HCCCH₂N(CH₃)H, have been recorded. All three possible conformers, i.e., methyl group (Me-trans) hydrogen atom (H-trans), and lone pair (LP-trans) trans to the ethynyl group, have been identified in the fluid phase with only the conformer with the LP-trans remaining in the solid. Variable temperature (−105 to −150 °C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data the enthalpy difference has been determined to be 360 ± 72 cm⁻¹ (4.31 ± 0.86 kJ mol⁻¹), between the most stable LP-trans conformer and the second most stable Me-trans rotamer. An enthalpy difference of 100 ± 20 cm⁻¹ is estimated between the Me-trans form and the least stable H-trans conformer. These results are in reasonable agreement with the ab initio predictions at all levels of calculations. It is estimated that there is only 8% of the H-trans form and 14% of the Me-trans conformer present at ambient temperature. A complete vibration assignment is proposed for the LP-trans conformer which is based on infrared band contours, infrared intensities, Raman activities, and group frequencies, which is supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) and DFT(B3LYP) calculations. Proposed assignments are also made for several of the fundamentals of the other two conformers. The conformational stabilities, barriers to internal rotation, 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 LP-trans rotamer along with ab initio predicted structural values, r₀ parameters have been obtained for this conformer. The results are discussed and compare to the corresponding properties of some similar molecules.