Viscosity isomerization dynamics

XVm. Isomerization Dynamics in Viscous Liquids A Study of the Viscosity Effects via MCT... 

From stochastic molecnlar dynamics calcnlations on the same system, in the viscosity regime covered by the experiment, it appears that intra- and intennolecnlar energy flow occur on comparable time scales, which leads to the conclnsion that cyclohexane isomerization in liquid CS2 is an activated process [99]. Classical molecnlar dynamics calcnlations [104] also reprodnce the observed non-monotonic viscosity dependence of ic. Furthennore, they also yield a solvent contribntion to the free energy of activation for tlie isomerization reaction which in liquid CS, increases by abont 0.4 kJ moC when the solvent density is increased from 1.3 to 1.5 g cm T Tims the molecnlar dynamics calcnlations support the conclnsion that the high-pressure limit of this unimolecular reaction is not attained in liquid solntion at ambient pressure. It has to be remembered, though, that the analysis of the measnred isomerization rates depends critically on the estimated valne of... 

The dynamic RIS model developed for investigating local chain dynamics is further improved and applied to POE. A set of eigenvalues characterizes the dynamic behaviour of a given segment of N motional bonds, with v isomeric states available to each bond. The rates of transitions between isomeric states are assumed to be inversely proportional to solvent viscosity. Predictions are in satisfactory agreement with the isotropic correlation times and spin-lattice relaxation times from 13C and 1H NMR experiments for POE. 

The prospensity for the formation of bends in the flexible spacer in polyesters containing naphthyl units is examined by the study of the fluorescence of a series of diesters. The dependence of the degree of intramolecular excimer formation on the length of the aliphatic spacer, under circumstances where the dynamics of rotational isomerism in the flexible spacer is suppressed, is evaluated by extrapolation of the measurements to infinite viscosity n. The extrapolated results exhibit an odd-even effect, with the more intense excimer emission being observed when the number of methylene groups is odd. The odd-even effect is rationalized by an RIS analysis of the diesters. 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

Attempts have been made to identify primitive motions from measurements of mechanical and dielectric relaxation (89) and to model the short time end of the relaxation spectrum (90). Methods have been developed recently for calculating the complete dynamical behavior of chains with idealized local structure (91,92). An apparent internal chain viscosity has been observed at high frequencies in dilute polymer solutions which is proportional to solvent viscosity (93) and which presumably appears when the external driving frequency is comparable to the frequency of the primitive rotations (94,95). The beginnings of an analysis of dynamics in the rotational isomeric model have been made (96). However, no general solution applicable for all frequency ranges has been found for chains with realistic local structure. 

In the case of electron transfer reactions, besides data on the dynamic Stokes shift and ultrafast laser spectroscopy, data on the dielectric dispersion (w) of the solvent can provide invaluable supplementary information. In the case of other reactions, such as isomerizations, it appears that the analogous data, for example, on a solvent viscosity frequency dependence 17 ( ), or on a dynamic Stokes fluorescence shift may presently be absent. Its absence probably provides one main source of the differences in opinion [5, 40-43] on solvent dynamics treatments of isomerization. 

In the limit of very large viscosity, such as the one observed near the glass transition temperature, it is expected that rate of isomerization will ultimately go to zero. It is shown here that in this limit the barrier crossing dynamics itself becomes irrelevant and the Grote-Hynes theory continues to give a rate close to the transition theory result. However, there is no paradox or difficulty here. The existing theories already predict an interpolation scheme that can explain the crossover to inverse viscosity dependence of the rate... 

The rate constant for the isomerization of /ra/i.v-stilbene in the S, state is also affected by solvent viscosity and has served as a favorite prototype for the investigation of solvent dynamics in fast monomolecular kinetic processes (Saltiel and Sun, 1990). 

Interest is shown still in the isomerization of stilbene and its derivatives. A study of the pathways for the cis -trans isomerization of 4-nitro, 4,4-dinitro, and 4-nitro-4 -methoxy stilbenes has been reported. The dynamics of the photoisomerization of stilbenes in hydrocarbon solution has been studied and a comparison of the trans -cis isomerization of stilbene in low viscosity liquid alkanes and in the gas phase has been carried out. The photoisomerization of stilbene in straight chain alcohols has provided evidence for the existence of rotational relaxation in the excited and the ground states. An analysis of the isomerization rates of photoexcited stilbene has been made and the photobehaviour of poly deuteriated stilbene has been studied. ... 

Chandler and co-workers successfully used stochastic dynamics in their studies of -alkanes and cyclohexane isomerization in solution. The method used is based on the BGK theory. The assumption is that the primary form of interaction between the solvent molecules and the isomerizing system is in the form of hard collisions. These collisions, when they occur, randomize the velocity of one of the isomerizing molecule s atoms. The computational implementation of this is quite simple at random times, based on the collision frequency (which is taken to be proportional to the solvent viscosity), instantaneously change one random atom s velocity to one selected from the Boltzmann distribution at the temperature of interest. Then continue running dynamics until the next collision occurs, at which time another... 

The applications of this stochastic picture to the trans-gauche isomerization of -alkanes and the boat-chair isomerization of cyclohexane have demonstrated the usefulness of this approach. In both cases, the transmission coefficients calculated from stochastic dynamics agreed quite well with those from the (later) molecular dynamics calculations, given that there can be an uncertainty in the correct value of the collision frequency to use in comparing with the full molecular dynamics in solution. Stochastic dynamics therefore can allow the rapid calculation of reaction dynamics over a wide range of solvent densities and/or viscosities. 

One of the features of stilbene photochemistry is its essentially strong dependence on medium polarity and temperature the competition between fluorescence and trans-cis isomerization has been shown to be extremely sensitive to medium viscosity. Solvent polarity can affect both the dynamics and the pathway of the reaction. The dipolar character of asymmetrically substituted stilbenes and polarizability of the traws-stilbene transition state can explain the sensitivity of the photoisomerization rate to medium polarity [5, 6, 12, 31, 66-69]. 

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