Valence bond theory distortion

Why then bother about much more expensive QM-based models One reason is that MM may only lead to accurate results for molecules of the same type used for the optimization and validation of the force field, i.e. extrapolation is seen to be dangerous if not impossible [9], This also extends to transition states and shortlived, unstable intermediates and therefore to chemical reactivity. Since electrons are not considered explicitly in MM, electronic effects related to structural distortions, specific stabilities and spectroscopy cannot be modeled by MM. However, in all other areas, there is no good reason for not using a well-optimized and validated MM model. Also, there are MM-based approaches to deal with most of the deficiencies listed above [9,20-28]. In the last decade, there have been a number of approaches, which have, based on simple rules [29], valence bond theory [30-33] and ligand-field theory [20-23], allowed the simplification of the force-field optimization and validation procedures and/or inclusion of electronic effects in MM models. 

A recent summary of the history and dynamics of the theoretical models of benzene39 cites a view that even though the current molecular orbital (MO) view of benzene seems complete and ultimate while the valence bond (VB) view seems obsolete, the recent findings about zr-distortivity in benzene indicate that the benzene story is likely to take additional twists and turns that will revive the VB viewpoint (see footnote 96 in ref 39). What the present review will show is that the notion of delocalized zr-systems in Scheme 1 is an outcome of both VB and MO theories, and the chemical manifestations are reproduced at all levels. The use of VB theory leads, however, to a more natural appreciation of the zr-distortivity, while the manifestations of this ground state s zr-distortivity in the excited state of delocalized species provides for the first time a physical probe of a Kekule structure .3... 

The bond charge parameter formulation developed by Van Straten and Smit [92] describes infi-ared intensities in terms of static bond charges and bond charge fluxes induced by vibrational distortions. In the mathematical procedure many common features with the )paratus of the valence-optical theory are present. 

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