Valence bond charge transfer

There have been some attempts to compute nonlinear optical properties in solution. These studies have shown that very small variations in the solvent cavity can give very large deviations in the computed hyperpolarizability. The valence bond charge transfer (VB-CT) method created by Goddard and coworkers has had some success in reproducing solvent effect trends and polymer results (the VB-CT-S and VB-CTE forms, respectively). 

Apart from purely electronic effects, an asymmetric nuclear relaxation in the electric field can also contribute to the first hyperpolarizability in processes that are partly induced by a static field, such as the Pockels effect [55, 56], and much attention is currently devoted to the study of the vibrational hyperpolarizability, can be deduced from experimental data in two different ways [57, 58], and a review of the theoretical calculations of p, is given in Refs. [59] and [60]. The numerical value of the static P is often similar to that of static electronic hyperpolarizabilities, and this was rationalized with a two-state valence-bond charge transfer model. Recent ab-initio computational tests have shown, however, that this model is not always adequate and that a direct correlation between static electronic and vibrational hyperpolarizabilities does not exist [61]. 

Suitieri97 has also evaluated the anharmonic contributions to the nuclear relaxation y for some push-pull polyenes using analytical methods in a valence bond charge transfer model. Saal and Ouamerali98 have investigated the vibrational ft of N-fluorophemyl-2,5-dimethypyrrole in the double harmonic... 

The two-form model has its roots in the valence-bond charge-transfer (VB-CT) model derived by Mulliken [84] and used with minor modifications by Warshel et al. for studying reactions in solutions [114]. Goddard et al. applied this VB-CT model to study the nonlinear optical properties of tire charge-transfer systems. [27, 59]. The analysis of the relationship between electronic and vibrational components of the hyperpolarizabilities within the two-state valence-bond approach was presented by Bishop et al. [17]. Despite the limitations of the VB-CT model, it is very simple and gives some insight into mutual relationships between nonlinear optical responses through the various orders. 

Bishop, D.M., Champagne, B., Kirtman, B. Relationship between static vibrational and electronic hyperpolarizabilities of rr-conjugated push-pull molecules within the two-state valence-bond charge-transfer model. J. Chem. Phys. 109, 9987-9994 (1998)... 

Chen, G., Lu. I).. Goddard, III. W.A. Valence-bond charge-transfer solvation model for nonlinear optical properties of organic molecules in polar solvents. J. Chem. Phys. 101, 5860-5864 (1994)... 

Hydrogen bonding, charge transfer and valency changes... 

The value vr is regarded as a measure of the extent to which the electron in the rth AO takes part in the bond formation with other atoms. In contrast with this, pT is the part of population in the rth AO which is living there and responsible for the interaction with outside. Hence, in view of the role of the frontier orbital in the charge-transfer interaction, it is reasonable to take, as the frontier density, the valence-inactive part 7S>. Namely,... 

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