IR studies of van der Waals interactions on perfluoro-t-butyl alcohol OH: Part I. Environment effects on monomeric OH and self-associates

Abstract

The frequency shift Δν of the perfluoro-t-butyl alcohol (PFTB) OH stretching in low temperature rare gas matrices has been studied. Δν depends on the square root of the critical temperature of the rare gases ( T C2) 1 2 . Other matrix results for CH 3OH, HCl and HF can be plotted on the same diagram taking into account the square root of the pair potential of the different oscillators ( T C1) 1 2 . The common straight line Δν∼ ( T C1· T C2) 1 2 starts at 130 K. The cause of this effect is discussed. This relation establishes the validity of the Badger—Bauer rule, the proportionality between Δν and the van der Waals interaction energy, which is already known for H bonds. The free open end OH of PFTB self associates shows a 72% larger matrix sensitivity. This seems to indicate higher van der Waals interactions by OH groups which are polarized by H bonds. Recent experimental demonstrations of H bonds seem to be able to extend the cooperativity between H bonds and van der Waals interactions. The preference of PFTB dimers for the linear conformation is demonstrated. The effects of PFTB H bonds in self associates on Δν due to the different van der Waals effects of rare gas matrices are smaller than of free OH. The first effect may be more like a solvent effect due to different inner pressures of the matrices, and the second effect a direct consequence of the inter- molecular potentials.