Methyl internal rotation in the α‐, β‐ and γ‐picolines‐CHD2 in the gas phase

Abstract

AbstractGas‐phase infrared and Raman spectra of the three isomeric selectively deuteratedγ‐, β‐ and α‐picolines (C5ND4CHD2) were recorded in the CH stretching region. They are each characterized by a strong band and a weaker band at a lower frequency, the intensity of which increases progressively from the Para (γ‐) to the ortho (α‐) isomer. These bands exhibit simple Raman profiles but their infrared contours are of A and C type, respectively. A quantum treatment of these spectra, assuming an anharmonic coupling of the ν(CH) mode with the internal rotation of the CHD2 group, is developed in the adiabatic approximation. This model leads to a good fit of the experimental Raman band shapes and a good picture of the observed infrared spectra. The frequency of the intense band is assigned to the frequency of the ν(CH) mode during the almost free rotation of the CHD2 group; the weaker band corresponds to the frequency of the CH bond in the plane perpendicular to the pyridine ring (γ‐picoline) or in a plane tilted by about ± 120° from the pyridine ring (β‐ and α‐picoline). The frequency difference between ν∥(CH in the molecular plane) and ν⟂ (CH in the plane perpendicular to the molecular plane) is found to increase from γ‐ to α‐picoline, as does the internal rotation barrier height; the most stable conformations of the methyl group with respect to the pyidine ring are thus determined. These results provide new evidence for the importance of hyperconjugation effects in such methylated aromatic molecules.