Temperature Drift of Conformational Equilibria of Butyl Alcohols Studied by Near-Infrared Spectroscopy and Fully Anharmonic DFT

Conformational isomerism of aliphatic alcohols with respect to the internal rotation of C-O(H) group and its impact on near-infrared (NIR) spectra has been known in the literature. However, no attempt has ever been made to investigate systematically whether and how the conformational flexibility of the aliphatic chain determines the observed NIR data of aliphatic alcohols. In the present study NIR spectra of four kinds of butyl alcohols, 1-butanol, 2-butanol, isobutanol, and tert-butyl alcohol, were investigated in diluted (0.1 M) CCl4 solutions. The experimental NIR spectra of butyl alcohols were accurately reproduced and explained in a fully anharmonic DFT study by means of generalized second-order vibrational perturbation theory (GVPT2). Entire conformational populations were taken into account in each case. On the basis of the theoretical study, influences of conformational flexibility with respect to internal rotations not only about the C-O bond, but also about the C-C bonds have been well evidenced in the experimental spectra. The conformational isomerism affects significantly the shape of NIR spectra. The temperature-dependent NIR spectra of butyl alcohols show changes in the band shape and a blue-shift of the overtone band due to the stretching mode of free OH group, and its intensity decreases with increasing temperature. These effects can be closely monitored by two-dimensional correlation spectroscopy (2D-COS). In this work, the experimental 2D-COS patterns have been successfully reproduced, based on DFT calculated NIR spectra of conformational isomers of the studied molecules and their Boltzmann coefficients over the corresponding temperature range. Thus, the experimentally observed effects are fully reflected in the DFT study, which leads to the conclusion that the main factor in the temperature-dependent spectral changes of 2v(OH) band of aliphatic alcohols in the diluted phase, where no self-association occurs, is played by the changes in the relative population of their conformational isomers.