Quantum Energy Flow andtrans-Stilbene Photoisomerization:  an Example of a Non-RRKM Reaction

Abstract

We apply multireference ab initio quantum chemistry and microcanonical transition state (RRKM) theory with quantum energy flow corrections from local random matrix theory (LRMT) to determine the kinetics of trans-stilbene photoisomerization. With a single ab initio potential energy surface and no adjustable parameters, simultaneous agreement with experiment of the microcanonical isomerization rates for the d 0 , d 2 , d 10 , and d 12 isotopomers is obtained. We are also able to reproduce the pressure dependence of the thermal rate. Laser cooling effects on the isomerization rate are calculated and found to be quite small. The S 1 /S 2 energy gap at the transition state is found to be quite large (0.86 eV), suggesting that nonadiabatic effects are negligible. Using the ab initio results in a simple RRKM theory without corrections for finite quantum energy flow does not lead to agreement with experiment. We conclude that non-RRKM effects are essential to understand photoisomerization of trans-stilbene and that these can be predicted using LRMT.

On the excitation wavelength dependence and Arrhenius behavior of stilbene isomerization rates in solution

“…The focus of this work is on trans-stilbene in solution, especially on the excitation wavelength (k exc ) dependence [1,2] of isomerization kinetics, and on how the kinetics can be modeled with theoretical Arrhenius rates [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] k ¼ A expðÀE b =k B TÞ ð 1Þ…”

“…Thus at 300 K, the gas-phase rate is about 1 ns À1 whereas in hexane it is more than 10 times faster [7][8][9][10][11][12][13][14]. Various explanations of this observation have been proposed [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], the restricted IVR mechanism [11][12][13][14][25][26][27] being now widely accepted. It suggests that stilbene torsion about the C@C bond is weakly coupled to other vibrational modes, resulting for isolated molecules in a slow energy transfer, and accordingly slow isomerization rates.…”

“…At equilibrium T solv = T int the uncertainty is hidden, but generally T solv -T int and the problem becomes apparent. Below we explore the both possibilities whereas the current view is that A int , T int enter the Arrhenius equation [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].…”

Communication: Optical cooling of trans-stilbene

“…[25][26][27][28] (iv) Intramolecular vibrational redistribution (IVR) may be slow (restricted) in isolated stilbene, but becomes faster (unrestricted) in solution due to solute-solvent interactions. 15,[29][30][31][32] (v) Solute-solvent collisions may directly increase the isomerization rate; and (vi) dynamic polarization in the excited state. 33,34 In this Communication, we show that optical cooling (case (iii) above) plays a significant role in the "stilbene enigma."…”

“…Theoretical computation of the extent of cooling is quite difficult. Based on an ab initio force field and a harmonic model, Leitner et al 31 claimed that cooling was negligible, at most of the order of a few cm depending on the pulse duration of the exciting laser. As noted though in Ref.…”