Mulliken hyperconjugation

Mulliken s model of the transition state (Pickett et al., 1953 Muller et al., 1954) is related indirectly to the structure of u-complexes. It is based on the idea of hyperconjugation effects stabilizing the transition state, and, although not directly invoking reactivity indices, must be mentioned as perhaps the most satisfactory description given so far. 

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. 

Measurements by electron impact methods have led to experimental stabilization energies of a number of alkylcarbonium ions. Muller and Mulliken (1958) compared these with calculated values obtained by a procedure based on the LCAO-MO approximation, the large stabilization energies found for carbonium ions being attributed to the combined effects of hyperconjugation and charge redistribution (Table 1). 

Scheme 2.113 Synthesis and structure of TAS+ (tris(di-methylamino)sulfonium) per-fluoro-1,3-cyclobutanide top). Calculated Mulliken partial charges q e) for a related. P model anion middle) suggest V negative hyperconjugation bottom) [37].

In describing the complex intermediate, hyperconjugation can be introduced by the method of Muller, Pickett, and Mulliken.4 In this procedure the first atom of the reagent R and the hydrogen atom of the carbon where reaction takes place form a "pseudo-atom. This complex can be represented in benzene as ... 

It is with no disrespect to the late Professor Coulson s very good friend, Professor R. S. Mulliken, that we quote Coulson s affectionate remark in the lecture This used to be known as the Baker-Nathan effect but later, thanks to Mulliken s ability to invert names, as hyperconjugation". 

Negative hyperconjugation, 274, 275 Neighboring group effect, 82 Net Charge SHMO, 91-92 Mulliken, 236 Nickel tetracarbonyl, 252 Nitrene (FIN) electronic states, 116-117 S—T gap, 116... 

Mulliken R. S. (1939). Intensities of electronic transitions in molecular spectra. IV. Cyclic dienes and hyperconjugation. Journal of Chemical Physics, 7,339—352. 

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