Trans /gauche isomerization rate

The rate of trans-gauche isomerization is evaluated using the following state function of the dihedral angles [49] ... 

Table 20.2 Equilibrium constants N autho/N tran,s for the trans/gauche isomerism of 1,2-dibromoethane at 25 °C. Source JE Leffler and E Crunwald, Rates and Equilibria of Organic Reactions, Dover Publications, New York, 1989.

Chachaty and co-workers [8.20, 8.37, 8.38] were first to describe correlated internal motions in alkyl chains of surfactant molecules that form lyotropic liquid crystals. The last section described an extension of the master equation method of Wittebort and Szabo [8.4] to treat spin relaxation of deuterons on a chain undergoing trans-gauche jump rotations in liquid crystals. This method was also followed by Chachaty et al. to deal with spin relaxation of nuclei in surfactants. However, they assumed that the conformational changes occur by trans-gauche isomerization about one bond at a time. In their spectral density calculations (see Section 8.3.1), they used a transition rate matrix that was constructed from the jump rate Wi, W2, and Ws about each bond. Since W3 is much smaller than Wi and W2, the time scale of internal motions was practically governed by Wi and W2 of each C-C bond. Since... 

Isomerizations are very simple model reactions that are among the most common chemical processes. Solvent elfects on these reactions have been studied extensively in the bulk phase. The study of solvent effects near the interface is especially interesting, since the gauche-trans equilibrium can involve two species with significantly different dipole moments consequently, a substantial interfacial effect can be expected. The isomerization reaction of 1,2-dichloroethane near the water/vacuum interface [249], and of 1,2-dichloroethane and of dialanine near the water/hexane interface [249, 250] has been studied. An increase of the gauche —> trans isomerization rate is observed near the interface (for a review see Ref. 251). 

Figure 3.1-10 illustrates how step-scan FTIR can be used to monitor more complicated photochemical reactions in SCFs. Visible photolysis of trans-[CpMo(CO)3l2 in scCOa generates [CpMo(CO)3] radicals which recombine at a diffusion controlled rate to form the stable trans and unstable gauche forms of [CpMo(CO)3]2. Gauche-[CvMo(CO)-i 2 slowly isomerizes to trans-[CpMo(CO)3l2 (Scheme 3.1-2). 

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