Valence bonds activation barrier

The hydrogen bond formation decreases the frequency of the O—H bond valence vibration (see Section 4.2.3). Two configurations of tertiary hydroperoxides are known E- and Z-configurations. The activation barrier for transition from Z- to /i-configuration is found to be equal to 195 kJ mol 1 (quantum-chemical calculation [64]). 

Thus, the radius of the atom carrying the free valence has a substantial influence on the activation barrier to the addition reaction the greater the radius of this atom, the higher the activation energy. Apparently, this effect is due to the repulsion in the transition state, which is due to the interaction between the electron shells of the attacked double bond and the atom that attacks this bond. 

Table 5. Post-HF activation barriers for the insertion reaction of ethene into the Zr-CH3 bond of the HjSifCpEZrCH species. All the reported insertion barriers were obtained through single point calculations on the MP2 geometries of Tables 3 and 4 (corresponding to run 3 in this Table). In the valence calculations the Is orbitals on the C atoms, the orbitals up to 2p on the Si atom and up to the 3d on the Zr atom where not included in the active orbitals space. In the full MP2 calculations all occupied orbitals were correlated.

Molecular dynamics free-energy perturbation simulations utilizing the empirical valence bond model have been used to study the catalytic action of -cyclodextrin in ester hydrolysis. Reaction routes for nucleophilic attack on m-f-butylphenyl acetate (225) by the secondary alkoxide ions 0(2) and 0(3) of cyclodextrin giving the R and S stereoisomers of ester tetrahedral intermediate were examined. Only the reaction path leading to the S isomer at 0(2) shows an activation barrier that is lower (by about 3kcal mol ) than the barrier for the corresponding reference reaction in water. The calculated rate acceleration was in excellent agreement with experimental data. ... 

The predicted course of reaction between a heteronuclear pair of atoms is shown in Figure 7.2. Promotion is once more modeled with isotropic compression of both types of atom. The more electropositive atom (at the lower quantum potential) reaches its valence state first and valence density starts to migrate from the parent core and transfers to an atom of the second kind, still below its valence state. The partially charged atom is more readily compressible to its promotion state, as shown by the dotted line. When this modified atom of the second kind reaches its valence state two-way delocalization occurs and an electron-pair bond is established as before. It is notable how the effective activation barrier is lowered with respect to both homonuclear (2Vq)i barriers to reaction. The effective reaction profile is the sum of the two promotion curves of atoms 1 and 2, with charge transfer. 

The trends in the activation barriers were explained440,442 using the valence bond configuration mixing model443 444, as shown schematically in Figure 32a. According to this... 

The existence of an activation barrier is determined by restructuring of the valence bonds in a chemical transformation, that is, the value of E is related to the electron-vibrational structure of the molecules and the... 

As stated above, reliable studies of enzyme catalysis require accurate results for the difference between the activation barriers in enzyme and in solution. The early realization of this point led to a search for a method that could be calibrated using experimental and theoretical information of reactions in solution. It also becomes apparent that in studies of chemical reactions, it is more physical to calibrate surfaces that reflect bond properties (i.e., valence bond-based (VB-based) surfaces) than to calibrate surfaces that reflect atomic properties (e.g., molecular orbital-based surfaces). Furthermore, it appears to be very advantageous to force the potential surfaces to reproduce the experimental results of the broken fragments at infinite separation in solution. This can be easily accomplished with the VB picture. The resulting EVB method has been discussed extensively elsewhere,21 22 but its main features will be outlined below, because it provides the most direct microscopic connection to concepts of physical organic chemistry. 

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