Scott H. Courtney

Photoisomerization of stilbene in low viscosity solvents: Comparison of isolated and solvated molecules

The photoisomerization of stilbene has been studied in low viscosity liquid alkanes and in the gas phase. The barrier crossing process shows no evidence of a low‐friction or energy controlled region even at the lowest liquid viscosities studied. We present evidence that the barrier crossing may be in the energy controlled region in the thermal vapor. We discuss the influence of intramolecular vibrational energy transfer on the observed dynamics and note that entropy effects should be considered in comparing experimental data with theoretical models.

Activated barrier crossing: Comparison of experiment and theory

Photochemical isomerization in stilbene and diphenyl butadiene has been studied as a model for activated barrier crossing. Experiments have been carried out from isolated molecule conditions up to 3000 atm pressure in solution-phase samples. The qualitative features predicted by Kramers theory are observed. The system undergoes a transition from energy-controlled to diffusion-controlled behavior in the high-pressure gas phase. The influences of multidimensionality, intramolecular vibrational relaxation, and frequency dependent friction are discussed.

Unimolecular reactions in isolated and collisional systems: Deuterium isotope effect in the photoisomerization of stilbene

The isomerization of t‐stilbene (stilbene h12 ) and three deuterated derivatives has been studied in a supersonic expansion, the thermal gas phase, and solution. In the jet we find that almost all effect of full deuteration (stilbene d12 ) is produced by deuteration of the two ethylinic hydrogens only (stilbene d2 ). Complete deuteration of the phenyl rings (stilbene d10 ) has rather little influence on the decay of the jet‐cooled molecule. Nonexponential decays are found at intermediate excess energies in the jet‐cooled system, with the degree of nonexponentiality decreasing with increasing excess energy. The ordering of the decay rates observed in the jet is not consistent with previous RRKM calculations of the isomerization rates of stilbene h12 and d2. Using similar parameters the calculations consistently place the stilbene d2 and stilbene d10 curves in the wrong order. Our results suggest extensive but not complete vibrational relaxation in the isolated molecule. Vibrational redistribution rapidly b...

Time-Correlated Single Photon Counting With Microchannel Plate Detectors

ABSTRACT We describe the experimental techniques and methods of analysis used in our laboratory in the study of a wide range of chemical and biological systems. In addition to standard single exponential decay time measurements we discuss the resolution of multiexponential decays and the analysis of fluorescence anisotropy experiments.

Search for the kramers theory turnover: photochemical isomerization at very low viscosities

Abstract The rate of photochemical isomerization of diphenyl butadiene has been studied as a function of viscosity and temperature in liquid ethane and propane. The lowest viscosity studied is about one order of magnitude lower than our previous study. No evidence for a turnover or decrease in rate with decreasing viscosity was found.

Isomerization of t-stilbene in alcohols

Abstract The isomerization of optically excited t-stilbene in n-alcohol solution is discussed. The data are consistent with incomplete solvation in the initial state for alcohols larger than ethanol and with stabilization of the polar transition state by rapid components of the solvent dielectric response in all alcohols. The combination of these two effects leads to a progressive decrease in apparent barrier height with increasing size of alcohol.

Unimolecular reaction rates in solution and in the isolated molecule: Comparison of diphenyl butadiene nonradiative decay in solutions and supersonic jets

The recent study of diphenyl butadiene (DPB) in supersonic jets and in solution by Shepanski et al.(1) and by Courtney and Felming(2), respectively, provides an opportunity to compare the isomerization rates measured in the isolated molecule (jet) with those measured at very low viscosity in solution. These comparisons should shed light on the vibrational energy flows between “optical” and “reactive” modes in the isolated molecule and on the connection between activated, friction dependent, models of barrier crossing in solution,(3-5) and statistical RRK (or RRKM) theories of gas phase unimolecular reactions(6).

Rotational diffusion of stilbene in alkane and alcohol solutions

An amplified colliding pulse modelocked laser system is described which is capable of subpicosecond resolution flash-photolysis studies. As an application of this system we used the anisotropic absorption technique (polarization spectroscopy) to measure the rotational reorientation of trans-stilbene. These measurements give a more accurate estimate of the microscopic friction involved in the isomerization dynamics of this molecule. When the friction is assumed proportional to the reorientation times, the isomerization rates in alkane solvents are adequately described by activated barrier-crossing theory.

Rotational reorientation and isomerization of trans‐stilbene

We have tested the validity of using solvent shear viscosity as a measure of friction involved in the dynamics of t‐stilbene. The rotational reorientation and the isomerization in n‐alkanes are considered.

Unimolecular reactions of isolated, collisional gas phase and solvated molecules: Connections between stilbene isomerization rates under supersonic jet and thermal gas phase conditions

Photochemical isomerization provides a practical testing ground for activated barrier crossing theories. Stilbene is a prototype, because (1) reaction may be initiated by a short optical pulse and the decay of the excited trans isomer followed optically, and (2) the isomerization rate is slow compared to vibrational or solvent relaxation processes in solution. The goal is to understand the effects of the medium upon the reaction kinetics proceeding from the isolated molecule to the low and the high pressure gas, and liquid solution. Towards this end time correlated single photon counting experiments of stilbene isomerization are discussed from isolated molecule conditions up to 90 atm of methane gas. In statistical theories of barrier crossing the transition state rate at temperature T, kTST(T), represents an upper limit for the reaction rate. However, contrary to expectation, for pressures greater than about 2 atm the isomerization rate was found to exceed kTST(T) by over a factor of 2 at 5 atm, and by a...