Internal rotation in ethylamine: A treatment as a two-top problem

Abstract

The infrared absorption spectra of eight isotopic ethylamine molecules, i.e., CH3CH2NH2, CD3CH2NH2, CH3CD2NH2, CD3CD2NH2, CH3CH2ND2, CD3CJ2ND2, CH3CD2ND2, and CD3CD2ND2, have been examined in the vapor phase in the 300–100 cm−1 region. Several Q-branch peaks were observed for each isotopic species and assigned to the torsional oscillations of the methyl and amino groups of the trans and gauche isomers. The energy levels were calculated on the basis of a coupled two-top system. The analysis yielded a probable potential function for the internal rotation about the C–N bond of the form V (α) = (316.5/2)(1−cosα)−(11.3/2)(1−cos2α)+(713.7/2)(1−cos3α) −(25.0/2)(1−cos4α)+(25.0/2)(1−cos5α)−(3.7/2)(1−cos6α). The difference between the potential energy minima of the trans and gauche conformations is about 230 cm−1, the trans being the more stable form. It has been concluded that the axis of internal rotation of the amino group does not coincide with the N–C bond but is along a line about 4.5° from the N–C bond and in the plane bisecting the NHH triangle.

Conformational and structural studies of n‐propylamine from temperature dependent Raman and far infrared spectra of xenon solutions and ab initio calculations

“…This value was in agreement with earlier determined values of 107 ± 70 cm –1 (1.28 ± 0.84 kJ/mol) from electron diffraction study, 110 ± 50 cm –1 (1.32 ± 0.60 kJ/mol) from a microwave investigation, and 100 ± 10 cm –1 (1.20 ± 0.12 kJ/mol) from an infrared study of ethylamine in an argon matrix . However, several of the earlier studies reported the gauche conformer as the more stable form, and it is rather interesting that second‐order Møller–Plesset (MP2, full)/6‐311+G(2d,2p) calculations predict the gauche conformer to be the more stable form by 66 cm –1 (0.79 kJ/mol). However, without the diffuse functions the trans form is predicted to be the more stable conformer by 32 cm –1 (0.38 kJ/mol).…”

Molecular mechanics (MM4) study of amines

“…However, the same group shortly after that concluded 51 that the trans conformation was in fact more stable by 0.66 kcal/mol. This molecule, for all of its apparent simplicity, is complicated by the fact that the rotational barrier is rather low, as is the inversion barrier for the amino group, and those two motions are coupled.…”

“…Durig and Li 30b reached substantially the same conclusion independently from a study of the infrared spectrum (trans stable by 0.59 kcal/mol). Additionally, these authors concluded that the rotational barrier of the methyl group for the gauche form is 3.71 6 0.5 kcal/mol, while Tsuboi et al 51 report that the trans methyl barrier has a height of 3.74 kcal/mol, and the gauche is 3.56 kcal/mol. Durig also reports the gauche barrier for the N,N-dideutero compound has a height of 3.59 kcal/mol, and in agreement with that obtained by microwave studies, 3.57 kcal/mol (the microwave studies are cited as unpublished).…”

Conformational and structural studies of 2,2-difluoroethylamine from temperature dependent infrared spectra of xenon solution and ab initio calculations

“…This value was in agreement with earlier determined values of 107 ± 70 cm À1 (1.28 ± 0.84 kJ/mol) from electron diffraction study [2], 110 ± 50 cm À1 (1.32 ± 0.60 kJ/mol) from a microwave investigation [3], and 100 ± 10 cm À1 (1.20 ± 0.12 kJ/mol) from an infrared study of ethylamine in an argon matrix [4]. However, several of the earlier studies [5][6][7][8] reported the gauche conformer as the more stable form, and it is rather interesting that MP2(full)/6-311 + G(2d,2p) calculations predict [1] the gauche conformer to be the more stable form by 66 cm À1 (0.79 kJ/mol). However, without the diffuse functions the trans form is predicted to be the more stable conformer by 32 cm À1 (0.38 kJ/mol).…”