Conformational studies of n-propylamine by combined ab initio MO calculations and Raman spectroscopy

Abstract

The results of ab initio SCF-MO calculations performed with a 3-21G(N*) basis set, for fully optimized geometries of five conformations of n-propylamine, are presented. The calculated relative order of total energies for these conformers is TT≈GG′>TG>GT>GG. At 300 K, the Boltzmann distribution of populations is 18, 37, 20, 19 and 7%, respectively. Raman spectra of n-propylamine and n-propylamine-N- d 2 in the liquid phase exhibit a number of bands whose temperature-dependent intensities clearly suggest the occurrence of different conformers in simultaneous equilibria. Deuteration of the amine group originates pairs of Raman bands at 428 and 440 cm −1 and at 863 and 885 cm −1. The bands at 428 and 885 cm −1 are favoured by reduction of temperature. Normal coordinate calculations permit the assignment of the Raman and i.r. spectra in good agreement with experimental evidence. Among the five possible conformers of n-propylamine, it is possible to detect the presence of at least three conformations in the liquid phase, corresponding to the skeletal trans (TT and GT) and at least one of the skeletal gauche (TG, GG or GG′) forms. In the solid phase, only the bands ascribed to the TT form were observed. The ab initio results for the isolated molecule show that the all- trans conformation, TT, and the conformation GG′ have the smallest energies. On the other hand, the vibrational results for the liquid and solid phases indicate that the all- trans conformation, TT, is the more populated form. In addition, this conformer presents the highest calculated dipole moment, in good agreement with the liquid phase Raman spectroscopic results which point out that this conformation is favoured by polar solvents. Intermolecular interactions operating in the liquid n-propylamine, possibly of the hydrogen bonding type, are responsible for altering the relative order of conformational stability as predicted by the ab initio SCF-MO results for the isolated molecule.