Hyperconjugative charge transfer

The ambiguity about which non-orthogonal subunits receive credit for unaccounted density in the overlap region is the source of the many reported differences between alternative computational dissections. The associated overlap density can be assigned to the filled orbital (and counted towards steric effects) or to the unfilled orbital (and counted towards hyperconjugative charge-transfer). All methods that harbor such overlap ambiguities are expected to differ sharply from NBO-based assessments of intramolecular or inter-molecular interactions. 

Brown (1959) has presented a charge transfer model of the transition state for electrophilic reactions which differs appreciably from that proposed by Fukui and his collaborators and leads to the definition of a new reactivity index termed the Z value . The model is based on a more conventional formulation of the charge transfer mechanism, which avoids the complete transfer of electrons associated with v = 0,1,2 in Fukui s model. There is no dependence on the formation of a pseudo tt orbital in the transition state, nor is hyperconjugation invoked. A wave function for a charge transfer complex is written as a linear combination of a wave function < o describing the unperturbed ground state of the molecule under attack, and a function which differs from (Pq in the replacement... 

Because the precise energies of charge-transfer interactions are sensitive to small structural modifications, purely intuitive predictions often turn out to be wrong. In tetrafluorohydrazine, for instance, hyperconjugation of the type wN — should... 

In order to explain the isomerization of neopentane to isopentane on platinum films, Anderson and Avery [34) proposed a mechanism involving, as precursor, an a,a,y-triadsorbed species, and, in the transition state, a 71 complex of the Dewar type, attached to the surface by two carbon-metal bonds. By simplified Hiickel molecular orbital (MO) calculations, they showed that hyperconjugative effect and partial charge transfer to the metal could account for the relative isomerization rates of the various molecules studied (neopentane > isobutane > n-butane) (Scheme 19). 

All explanations of the anomeric effect based on negative hyperconjugation involve charge transfer. This should result in a difference in electron density... 

Charge transfers that may accompany the anomeric effect, though unmeasurable by the X-ray diffraction method, may result in changes in spectral properties of molecules. It is probably impossible to relate a given spectral property directly to the anomeric effect in its phenomenological sense. An alternative approach should then be applied, namely to relate the spectral observation to negative hyperconjugation (as the most probable cause of the anomeric effect). 

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