Transition states and calculations

The analytical potential surface Eg can also be used for examination of dynamical effects. This can be done in the same way as outlined in Chapter 3, propagating trajectories downhill from the transition state and calculating t of eq. (3.31). A typical downhill trajectory for our reaction is shown in Fig. 5.8. 

FIGURE 68. Free-radical substitution reactions of hydridotellurides HTeEH3 (E = Si, Ge, Sn) and HTeSi(SiH3)3 with alkyl groups R. The optimized transition states and calculated activation energies are given in Figure 69... 

Poor agreement is observed between the experimental and theoretical values [calculated by a combined method, that is, quantum-chemical calculation of the activation energy (Table 37.7 and Table 37.8) and the frequencies of the vibrations of the bonds in the prereaction complex and in the transition state and calculation of the rate constants on the basis of RRKM theory] for certain processes where the effects of electron correlation and the contributions of the excited electronic configurations are not predominant [68-73]. 

Equations to calculate AGf from the force constants of bonds and AG from the static and tfte optical dielectric constants of the so vent and the size of the reactants are given in the literature (1,2). These parameters are, however, not well known in the transition state, and calculations of rate constants, using them, are therefore not very satisfactory. 

The a(fwd) is 19 kJ, and the for the reaction as written is 392 kJ. Draw a reaction energy diagram, predict a structure for the transition state, and calculate... 

Detailed analyses of the above experiments suggest that the apparent steps in k E) may not arise from quantized transition state energy levels [110.111]. Transition state models used to interpret the ketene and acetaldehyde dissociation experiments are not consistent with the results of high-level ab initio calculations [110.111]. The steps observed for NO2 dissociation may originate from the opening of electronically excited dissociation chaimels [107.108]. It is also of interest that RRKM-like steps in k E) are not found from detailed quantum dynamical calculations of unimolecular dissociation [91.101.102.112]. More studies are needed of unimolecular reactions near tln-eshold to detennine whether tiiere are actual quantized transition states and steps in k E) and, if not, what is the origin of the apparent steps in the above measurements of k E). 

There are excellent references for transition state search calculations and in ethods ... 

Dimethylborane+propene C2 and 2-propyldimethyl borane depict the regioisomeric transition state and addition product. Calculate the energies of these species relative to those of the alternative transition state and product. Given these energy differences, and the experimental observation that this addition is almost completely selective for the anti-Markovnikov product, does it appear that this reaction is under kinetic or thermodynamic control Explain. 

Calculate activation energies for the preferred addition mode of each reagent. (Data for borane, 9-BBN and cis-4-methylpent-2-ene are available.) Which reaction will be faster Is the faster reaction more or less regioselective than the slower reaction Compare the structures of the two transition states and identify specific interactions that can account for differences in regioselectivity and reactivity. Use space-filling models. 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

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