Hyperconjugation spectroscopic effects

We now discuss systematic hyperconjugative effects on orbital composition and stabilization, torsion barriers, and spectroscopic properties, for the CH2=CHX species summarized in Table 3.22. 

The small stabilization is attributable to hyperconjugation with the o- and o -orbitals formed between the phosphorus and its substituent, and is therefore of hyper-conjugative nature. This was recognized from early photoelectron spectroscopic investigations [167], but since the extent of the effect is small, the literature mainly contains cautious statements about its nature. The interaction between the n-system, the sPH-orbitals, and the phosphorus lone pair can clearly be seen in Fig. 3. 

Stereoelectronic effects can have a profound effect on the ground-state structure of molecules, and can often help to explain counter-intuitive conformational preferences or spectroscopic features. Their effect on the energy of transition states is, however, less straightforward to predict. As stated by the Curtin-Hammett principle [75] (Section 1.4), reactions will proceed via energetically unfavorable conformers if these are more reactive (as is often the case) than better stabilized conformers. In such instances ground-state stabilization of certain conformers or the weakening of bonds by hyperconjugation will not necessarily be predictive for the outcome of a reaction. 

The effect of hyperconjugation on spectroscopic parameters associated with the IT and n states of 1-6 is Illustrated by the data in Table II. In particular, the attention of the reader is directed t02the relative values of the Coulomb Integral (J) which is equal to <7r (1) e /r. Tr (2)>, the mutual repulsion of two electrons (1 and... 

The spectroscopic manifestation of symmetry effects in cyclic hyperconjugation was reported in electron spin resonance (ESR) spectra. " Symmetric LUMOs show cooperative effects to hyperconjugative interactions, while interactions with antisymmetric LUMOs are cancelled by symmetry (Figure 8.28). Such effects can be extended to spin-paired molecules, Wheland intermediates of electrophilic aromatic substitution, metal cation/arene complexes, and n -cyclopentadieneylmetal compounds. ... 

All three previously discussed types of negative and neutral hyperconjugation (n interactions) have been implicated in spectroscopic consequences of the Perlin effect. 

The NMR spectroscopic data of the -sUyl-substituted a-mesitylvinyl cations corroborate the hyperconjugative charge-delocalizing ability of -silyl groups. Comparison with -alkyl- and -H-substituted a-mesitylvinyl cations gives a measure of the magnitude of the -silyl effect in these type of carbocations. The NMR spectroscopic data of the a-mesitylvinyl cations 372-375 and 378, 383-385 with various yS-substituents together with data for the sp -hybridized mesityl carbocations 386 and 387 are summarized in Table 15. 

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