Infrared spectroscopy of acetone-methanol liquid mixtures: Hydrogen bond network

Abstract

Acetone and methanol mixtures covering the whole solubility range are studied by Fourier transform infrared attenuated total reflectance spectroscopy. The strong bathochromic shifts observed on methanol OH and acetone CO stretch IR bands are related to hydrogen bonds between these groups. Factor analysis separates the spectra into four acetone and four methanol principal factors. A random molecular model developed for the acetone-water system [Max and Chapados, J. Chem. Phys. 119, 5632 (2003); 120, 6625 (2004)] was modified for the acetone-methanol system. This model, which takes into account H bonds accepted by methanol and acetone, is made up of 12 methanol and 11 acetone species. The 23 species abundances are regrouped according to evolving patterns or spectral similarities to compare them to the eight experimental factors. Methanol acetone mixtures are almost but not exactly random: the methanol oxygen atoms have stronger capacities than acetone to accept H bonds from methanol in the proportion 1.5 to 1. Since oxygen atoms are in excess, all labile hydrogen atoms will form H bonds. As acetone is added to methanol, its OH stretch band blueshifts as the number of accepted H bonds decreases. When methanol gives one H bond and accepts one, an H-bonding network is formed that was coined "chained organization." However, the acetone molecules do not sequester any methanol molecules by breaking or increasing the H-bond methanol network. Similarly, the methanol molecules do not sequester any acetone molecules. Consequently no acetone-methanol complex is formed in the mixtures. Gaussian simulation of the four principal factors in the methanol OH stretch region gave three distinct absorption regimes consisting of the OH stretch bands and their satellites that are identified as MeOH(1), MeOH(2), and MeOH(3) (subscript indicates the number of H, covalent and H bond, which surround the oxygen). These regimes are related to those identified in the water-acetone system as OH(2), OH(3), and OH(4).