The temperature dependence of the Raman spectrum and gauche interactions of tri-N-butylamine: a conformational study

Abstract

Raman spectra of tri-n-butylamine show pairs of bands whose temperature-dependent intensities clearly suggest their assignment to different conformers in simultaneous equilibria. These spectroscopic data are interpreted and correlated with structural information obtained from statistical analysis of gauche skeletal arrangements in tri-n-butylamine. The average numbers of gauche interactions in various tri-n-alkylamines are used to evaluate partial molal volumes which show excellent agreement with experimental data, thus imparting statistical meaning and usefulness to the concept of "average conformation".As the temperature decreases, the band at 904 cm−1 in pure liquid tri-n-butylamine increases in intensity at the expense of the band at 880 cm−1. Both of these bands are ascribed to CH2 and CH3 rocking vibrations. In solution, the conformer represented by the band at 904 cm−1 is favoured by polar solvents, whereas the conformer represented by the 860 cm−1 band is favoured by non-polar solvents.The relative intensity changes with increasing temperature of the bands at 1456 and 1440 cm−1 is explained by an increase in the number of CH2 groups adjoining two gauche bonds and a decrease in the number of CH2 groups adjoining a trans and gauche bond.Temperature dependence of the relative intensities of bands in the CH stretching region suggest, by comparison with mono- and di-n-butylamine, that the increase in the number of alkyl chains around the nitrogen atom makes molecular packing less restrictive and unique, giving rise to mesophases, as the overall shapes of the molecules become more spherical.