Ultrafast dynamics of barrier crossing: Step-wise solvation effect on isomerization of trans-stilbene in alkane clusters.

摘要

The ultrafast spectroscopic study of molecular clusters in supersonic beams can provide valuable information on the structure, energetics and dynamics of molecular aggregates in gas phase and in solution. This information will shed light on important issues such as how molecules interact, how energy flows in solvated systems, and how chemical reactions progress.; Although microscopic friction and solvation in barrier crossing reactions is of fundamental importance in molecular dynamics, their roles are not well understood. This is mainly due to the fact that few comprehensive investigations of this subject have been performed. In this thesis, the detail studies of a prototypical barrier crossing reaction—the photoisomerization of jet-cooled trans-stilbene—in size-selected n-alkane clusters, using the picosecond pump-probe ionization TOF mass spectrometry and transient technique, are reported. The microcanonical nonradiative decay rate constants at the S₁ manifold for trans-stilbene-hexanen and trans-stilbene-octane _n (n = 1,2) complexes, including certain deuterated variants, were measured as a function of excitation energy, with the energy range defined by tuning the pump wavelength from the 0-0 transition of trans-stilbene to ∼3200 cm−1 higher in energy. The experimental results were modeled with standard RRKM theory, nonadiabatic RRKM theory and Kramers-type theory for microcanonical systems. It was found that the excess energy dependence results could be accounted for very well by the nonadiabatic RRKM theory, from which analysis the barriers to isomerization for all of the trans-stilbene n-alkane clusters were found to be lower than that of the parent molecule by ∼50%. The analysis revealed that not only can the differences in the rates among the four trans-stilbene-hexane1 isotopic species studied (combinations of trans-stilbene- h₁₂ and -d₁₂ with n-hexane-h₁₄ and -d ₁₄) be attributed to energy friction, a term describing how energy is “drained away” from the reaction coordinate as a result of the change in the vibrational density of states, but the reduction in the nonradiative rates of the 1:2 complexes, relative to that of the 1:1 complexes, can also be attributed to the same energy friction.; Finally, in the same studies, the cluster binding energies of trans-stilbene-hexane_n and trans -stilbene-octane_n were also determined.