Curvature tunnelling corrections

A study of the reaction between Me-OBr and Cl has been carried out at the QClSD(T)/6-311-l-l-G(3df,2p)//MP2/6-311-l-G(d,p) level of theory using canonical variational TST with a small curvature tunneling correction. The calculations predict an anti-E2 elimination accompanied by a small amount of a syn-El elimination reaction that increases in importance as the temperamre is increased and an even smaller amount of S 2 0 reaction giving CH3-OCI. The energy barriers, transition structures, rate constants, and temperature dependences of the rate constants between 200 and 3000 K are given for the three reactions. 

Attempts to calculate theoretical values for the isotope effects and their temperature dependence were made using a linear activated complex model and a Sato potential energy surface. Various tunneling corrections were applied but only the Bell model ° predicts the curvature observed in log (fcio/ ii) versus l/T. Similar theoretical isotope effect predictions were found using a non-linear transition state model. 

Melissas and Truhlar (1993a) studied the kinetic isotope effects (CD4/CH4) of Equation (71) using TST, CVT, and interpolated VTST (IVTST), which uses the small curvature tunneling (SCT) correction (Melissas and Truhlar 1993b). Their calculations show that accuracy of the KIE prediction increased dramatically from TST to IVTST. 

Alternative corrections are Eckart tunneling, multidimensional zero-curvature tunneling, and multidimensional small-curvature tunneling (Truhlar et al. 1982), in increasing order of accuracy. The last two involve points on the PES other than the first order saddle point and have more sophisticated calculations. 

If accurate tunneling corrections are required, the situation rapidly becomes more complicated. Issues related to separability of motion along the reaction coordinate, curvature of the reaction coordinate, and multidimensional tunneling arise and must be dealt with. Marcus and Coltrin [51] found that reaction path curvature forces the reaction to cut... 

The results discussed in the previous section reveal that the reaction rate corresponding to the formation of major by-products of the oxidation reaction is important to determine the lifetime of dimethylphenol in the atmosphere. The rate constants are calculated using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) corrections over the temperature range of 278-350 K. As described in Figure 19.2, the formation of product channels consists of four reaction channels. The rate constants for the formation of alkyl radical (11), peroxy radical (12), m-cresol and the product channels are designated as k, ki2, and kp, respectively, and are summarized in Tables 19.2 and 19.3. The reaction path properties and rate constant obtained for the most favorable product channels, P5 and P6, are discussed in detail. 

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