Is the Fourier Transform Infrared Free-OH Band of -Butanol Only from Free OHs? Case Studies on the Binary Systems of the Alcohol with CCl and CHCl.

Attenuated total reflection-Fourier transform infrared spectroscopy and quantum chemical calculations were performed on -butyl alcohol (-BuOH) and its binary solutions with CCl and CHCl. The study was focused on the free-OH stretching bands. Two resolution-enhancing methods, excess spectroscopy and two-dimensional correlation spectroscopy, were employed to examine the structural heterogeneity and search for the detailed contributors to the free-OH bands. Unexpectedly, CCl was found not to be an inert solvent and, similar to CHCl, formed hydrogen/halogen bonds (H-/X-bond) with -BuOH. It was observed that the free-OH band in the -BuOH-CHCl system is larger and more red-shifted than that in the -BuOH-CCl system, indicating the stronger intermolecular interactions in the former system. Furthermore, in the -BuOH-CHCl system, the H-bonds are stronger than the X-bonds, while in the -BuOH-CCl system, both interactions are similar in strength. To assign the free-OH bands, it was found that they are not only from the free OH of the -BuOH monomer, but they are also contributed by the quasi-free OH with the oxygen bonded to H or Cl and even the weakly H-bonded OH of -BuOH molecules. Finally, all the identified species increased simultaneously via cosolvent addition, suggestive of the destabilization of the highly associated -BuOH clusters.