Raman and infrared spectra, conformational stability and Ab initio calculations for n-propylamine

The Raman (3500–10 cm−1) and infrared (3500–50 cm−1) spectra of gaseous and solid n‐propylamine, CH3CH2CH2NH2, and its deuterated analog, CH3CH2CH2ND2, were recorded. Additionally, the Raman spectra of the liquids were measured and qualitative depolarization values were obtained. From the fact that several distinct Raman lines disappear on goning from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of several conformers, with the trans–trans conformer being the most stable form in the solid. The first term indicates the position of the methyl group relative to the CN bond, whereas the second term indicates the position of the lone electron pair on the nitrogen relative to the central CC bond. Relying mainly on group frequencies, depolarization data and relative intensities of Raman and infrared bands, a complete vibrational assignment is proposed for the trans–trans conformer. Ab initio calculations were carried out on these molecules, employing both 3–21G and 6–31G* basis sets. The results obtained using these basis sets are compared, and used in normal coordinate calculations. The results of the ab initio calculations support the vibrational assignment, and also indicate that it is the trans–trans and trans–gauche conformers which are the more stable forms of n–propylamine and have nearly equal energies.