Raman spectra of gases. XVIII. Internal rotational motions in ethylamine and ethylamine-d2

Abstract

The Raman spectra of gaseous CH3CH2NH2 and CH3CH2ND2 have been recorded from 100 to 600 cm−1. Both ground and excited states of the methyl and amino torsional modes for the gauche conformer have been observed. The overtone of the methyl torsion has also been observed for the gauche conformer. The methyl barriers in the gauche conformers of the CH3CH2NH2 and CH3CH2ND2 molecules were found to be 3.71±0.05 and 3.59±0.05 kcal/mole, respectively. The apparent difference in the barrier heights for these two isotopic species is attributed to a difference in the coupling between the methyl and amino torsions in these two molecules. The potential function for internal rotation around the C–N bond in CH3CH2ND2 was determined and the following potential constants found: V1=279±16 cm−1, V3=707±4 cm−1, V6=−12±3 cm−1. The energy difference between the potential energy minima of the gauche and trans conformations is 207 cm−1 (0.592 kcal/mole) with the trans being the more stable. The calculated gauche–gauche barrier is 779 cm−1 (2.23 kcal/mole) and the trans–gauche barrier is 572 cm−1 (1.64 kcal/mole). A similar potential function was determined for the CH3CH2NH2 isomer but the fit to this one-dimensional model was not as good as that obtained for the deuterium compound probably because of the greater coupling in the light molecule.