Studies of solvent effects on reaction dynamics using ultrafast transient absorption spectroscopy.

Abstract

Ultrafast transient absorption spectroscopy was used to investigate the solvent dependent reaction dynamics of two prototypical chemical systems: (1) The ring-opening reaction of 1,3-cyclohexadiene, the isolated chromophore in Provitamin D, and (2) The photolysis of various Vitamin B₁₂ cofactors. We investigated the influence of solvent polarity on the ground state conformational relaxation of 1,3,5-<italic>cis</italic> hexatriene subsequent to the ring opening of 1,3-cyclohexadiene in methanol and 1-propanol solvents. Comparisons to the conformational relaxation in alkane solvents studied earlier demonstrated a surprising influence of solvent polarity on single bond isomerization. Temperature dependent transient absorption measurements were performed on 1,3,5-<italic>cis</italic> hexatriene in cyclohexane and 1-propanol to determine the effect of solvent polarity on the activation energy barrier for ground state single bond isomerization. These measurements conclude that the polar solvent lowers the energy barrier for single bond isomerization allowing conformational relaxation to proceed faster in alcohol solvents compared to alkane solvents. With no perceived polar transition state for single bond isomerization, this result disagrees with the conventional view of solvation and differentiates the single bond isomerization dynamics of polyenes from alkanes. Transient absorption spectroscopy was also utilized to study the solvent effects in the photolysis of various B₁₂ cofactors in different environments. We investigated the solvent dependent photolysis of adenosylcobalamin, methylcobalamin, and cyanocobalamin in water and ethylene glycol as a function of solvent temperature. In comparing the radical cage escape of adenosylcobalamin and cyanocobalamin, we determined a larger than expected hydrodynamic radii for the diffusing radicals in water compared to ethylene glycol, thus making necessary a revised perspective of solvent interaction with the diffusing radical. In addition, we investigated the role of the solvent environment in electronic internal conversion from the lowest electronic excited state to the ground state in cyanocobalamin and methylcobalamin. This study reveals the unique electronic nature of alkylcobalamins, which undergo photolysis, in comparison to non-alkylcobalamins that do not undergo photolysis. Finally, we examined the role of solvent environment on adenosylcoblamin bound to glutamate mutase, making comparisons to solvent effects on free adenosylcobalamin.