Temperature-Dependent Studies of Condensed Phase Reaction Dynamics.

Abstract

Transient absorption spectroscopy was used to perform temperature-dependent studies of the reaction dynamics of cis-1,3,5-hexatriene (Z-HT), 7-dehydrocholesterol (DHC) and iso-bromoiodomethane (iso-CH2BrI) in various solvents. The activation barrier for the ground-state isomerization of cZt-1,3,5-hexatriene to tZt-1,3,5-hexatriene was measured in methanol/propanol (17.4 ± 2.4 kJ/mol), and in cyclohexane/hexadecane (23.5 ± 2.5 kJ/mol). It was found to be independent of which alcohol or alkane was used but dependent on solvent type. Calculations to replicate experimental results were performed in Gaussian 03. In methanol the calculations accurately predicted the barrier height within the error bars of the measurement, while in cyclohexane the calculated results were lower than experimental results. The discrepancy suggested that a more complex calculation was needed in order to explain the experimental results. The decay of the excited-state absorption (ESA) of DHC was studied in methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, n-heptane and n-hexadecane. The decay of the ESA is biexponential, with a fast component of ca. 0.4 – 0.65 ps and a slow component 1.0 - 1.8 ps depending on the solvent. The relative amplitudes are also influenced by the solvent. Temperature dependent results suggested an intrinsic intramolecular barrier to ring-opening and a solvent dependent barrier arising from friction of the environment on the reaction coordinate. The low viscosity solvents set an upper limit for the intrinsic barrier of ca. 4 kJ/mol. A more complete analysis suggests that the intrinsic barrier is ca. 2 kJ/mol The decay of iso-CH2Br—I was studied in 1-butanol, 2-butanol, methanol and acetonitrile. The results are interpreted using a model consisting of a bimolecular reaction between iso-CH2Br-I and CH2BrI and a solvent-assisted unimolecular decay. Rate constants are calculated for the diffusion-controlled bimolecular reaction using three approximations for the diffusion coefficient of the solute particles. Comparison of calculated and measured values suggested that at ≤135 mM, iso-CH2Br--I decays primarily through a solvent-assisted isomerization in 2-butanol and ≥135 mM the decay is dominated by a diffusion-limited bimolecular collision with another species in 1-and 2-butanol. In methanol and acetonitrile, the decay is presumed to be a bimolecular reaction between contact-pairs and is essentially temperature independent.