Hyperconjugation defined

It is important to point out once again that explanations (rationalizations) of isotope effects which employ arguments invoking hyperconjugation and/or steric effects are completely equivalent to the standard interpretation of KIE s in terms of isotope independent force constant differences, reactant to transition state. In turn, these force constant differences describe isotope dependent vibrational frequencies and frequency differences which are not the same in reactant and transition states. The vibrational frequencies determine the partition functions and partition function ratios in the two states and thus define KIE. The entire process occurs on an isotope independent potential energy surface. This is not to claim that the... 

Mulliken s model is entirely compatible with the descriptions given previously in this Section, since charge shifts that result from polarization are already taken into account in forming the cr-complex by subsequent changes in hybridization. Fukui s model, upon which the definition of the superdelocalizability is based, resembles Mulliken s only in the use of a pseudo-TT orbital , and the formulation of the hyperconjugation problem is quite different, since )3 is taken by Fukui to be small, so that 8f can be defined by perturbation formulae. In particular, the bonding of the pseudo-TT orbital in Fukui s model primarily involves the least bound, or frontier orbitals, whereas in Mulliken s model the most bound MO is involved. 

Since medium effects alter intra-molecular hyperconjugative interactions,147,393,394 which, in turn affect many spin-spin couplings, it can be expected that couplings that are defined by hyperconjugative interactions will... 

Since the Es value is determined by the relative activation free energy from the unsaturated initial state to the saturated tetrahedral intermediate state of the ester hydrolysis, Hancock and his coworkers considered that a hyperconjugation effect of a-hydrogen may contribute to the estimate of Es values 19). To separate the hyperconjugation effect from the true steric effect , they defined the parameter E° (corrected steric) as Eq. 20, assuming that the hyperconjugation effect is proportional to the number of a-hydrogen atoms, nH. By definition, E (Me) = 0. 

Talvik and Palm made a similar analysis, taking into account hyperconjugation of a-CH and a-CC bonds (710R445), and defined s° constants ... 

To express the collective solvent reaction coordinate as in equation (6), it is necessary to define the specific diabatic potential surface for the reactant and product state. This, however, is not a simple task, and there is no unique way of defining such diabatic states. What is needed is a method that allows the preservation of the formal charges of the fragments of reactant and product resonance states. At the same time, solvent effects can be incorporated into electronic structure calculations in molecular dynamics and Monte Carlo simulations. Recently, we developed a block-localized wave function (BLW) method for studying resonance stabilization, hyperconjugation effects, and interaction energy decomposition of organic molecules.20-23 The BLW method can be formulated to specify the effective VB states.14... 

Fig. 11 Hyperconjugative stabilization of the oxocarbenium ion of 2-deoxy sugars. Using the empirically derived angular dependence of hyperconjugation, where hypercomugation oc exp(cos 0), and 6 is the dihedral angle defined by p-orbital-Cl -C2 -H2 S, the total amount of hyperconjugative stabilization is plotted as a function of the angle 6.

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