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-state environment. Computer simulations have been performed to calculate the cage-induced changes to the gas phase potential energy surface for this reaction. The calculations are in a good agreement with the experimental results.

Stepwise conformational cooling towards a single isomeric state in the four internal rotors system 1,2-butanediol

“…9 kJ mol À1 , was found to be thermally activated in matrices between 30 and 36 K for 1,2-difluoroethane. 19 An analogous transformation between the most stable forms of the GT and GG 0 families (7 and 4) has slightly higher predicted energy barrier values: 12.1 and 13.4 kJ mol À1 at the DFT and MP2 levels, respectively. Finally, the highest predicted barriers for the rotation around the CCCC axis separate 9 (G 0 G family) from its counterparts 6 (G 0 G 0 ) and 1 (G 0 T).…”

Conformational distortion of α-phenylethyl amine in cryogenic matrices – a matrix isolation VCD study

“…39 Experiments indicate that barriers of about 2.4 kcal mol À1 can be passed at a matrix temperature of 30 K. 40,41 Based on the calculated transition states, all interconversion barriers between PEA conformers were predicted to be considerably lower than this threshold, thus at 20 K all of them should be passable. 39 Experiments indicate that barriers of about 2.4 kcal mol À1 can be passed at a matrix temperature of 30 K. 40,41 Based on the calculated transition states, all interconversion barriers between PEA conformers were predicted to be considerably lower than this threshold, thus at 20 K all of them should be passable.…”

Matrix Photochemistry of Cycloheptatriene: Site Effects

“…Matrix isolation is also amenable to molecular dynamics simulations, which help in analyzing possible site effects, as demonstrated recently. − Much insight was gained, particularly in identifying and characterizing different trapping sites and relating them to reaction feasibility and even different reaction rates. Some recent experimental studies have emphasized the importance of site effects in understanding thermal and photochemical effects on the reactivity of matrix trapped molecules. Site effects reported in this work are also qualitatively supported by MD simulations.…”