Is the FTIR Free-OH Band of t-Butanol only from Free OHs? Case Studies on the Binary Systems of the Alcohol with CCl4 and CHCl3.

ATR-FTIR spectroscopy and quantum chemical calculations were performed on tert-butyl alcohol (t-BuOH) and its binary solutions with CCl4 and CHCl3. 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, CCl4 was found not to be an inert solvent and, similar to CHCl3, forms hydrogen-/halogen-bonds (H-/X-bond) with t-BuOH. It was observed that the free-OH band in t-BuOH‒CHCl3 system is larger and more red-shifted than that in t-BuOH‒CCl4 system, indicating the stronger intermolecular interactions in the former system. Further, in t-BuOH‒CHCl3 system, the H-bonds are stronger than the X-bonds, while in t-BuOH‒CCl4 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 t-BuOH monomer, 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 t-BuOH molecules. Finally, all the identified species increased simultaneously via cosolvents addition, suggestive of the destabilization of the highly associated t-BuOH clusters.