Conformational Isomerization of 1,2-Difluoroethane in Solid Argon: Cage-Modified Reaction Barriers

Abstract

Thermally induced rotational isomerization of trans-1,2-difluoroethane into the more stable gauche conformer in Ar matrices was studied over the temperature range 30−36 K by infrared spectroscopy. Photochemical generation of 1,2-difluoroethane in the matrix by two different mechanisms forms the product in singly and doubly substitutional argon lattice sites. Only the more stable doubly substitutional site is populated by the ordinary vapor deposition technique. At 33 K, the first-order rate constant for isomerization in the single site is 1.0 × 10-5 s-1 whereas the corresponding value for the double site is 5 × 10-3 s-1. Measurements of the temperature-dependent isomerization rates show that the activation energies for molecules in single and double sites are 15.0 ± 1.5 and 9.7 ± 0.8 kJ/mol, respectively. The latter value is close to a recent estimate of the gas phase reaction barrier (8.86 kJ/mol). This study provides an important example of how a reaction barrier is significantly modified by a solid-sta...