Hyper conjugation

FIGURE 4 16 Hyper conjugation in ethyl cation Ethyl cation is stabilized by delocalization of the elec trons in the C—H bonds of the methyl group into the vacant 2p orbital of the posi tively charged carbon... 

A comparison of 190 with 1,3-dioxane 177 shows that the anomeric interactions in 177 are much stronger than in 190. Moreover, the balance of the computed hyper-conjugative interactions successfully accounts for the relative C—Ha and C—Hgq... 

In both compounds there are type (I) azo functions surrounded by alkyl groups and one cyano group. Upon heating, tertiary alkyl radicals and cyano alkyl radicals are formed. These radicals are relatively stable due to hyper conjugation and, in the case of cyano substituted alkyl radicals, to resonance. Therefore, azo groups (I) have a high proneness to thermal decomposition. 

Prehminary AMI calculations carried out with the MOPAC program on 18 and related molecules suggest that there are atomic orbital contributions from the heteroatom (e.g., S in 18) to the frontier molecular orbitals. It is conceivable, therefore, that there is negative hyper conjugation involving specific orbitals of S and the P centers in 18. This electronic effect may explain the unusual stabiUty towards oxidation of 18 and other heteroatom functionaUzed primary bisphosphines as described above [51]. 

The cyclopentyl cation (39) undergoes a rapid degenerate rearrangement which can be frozen out at cryogenic temperatures as shown by solid state CPMAS 13C NMR spectra.57 MP2/6-31G(d,p) calculations show that cyclopentyl cation has a twisted conformation 4058 in which the axial hydrogens are bend toward the carbocation center. This is due to the pronounced geometrical distortion caused by the hyper-conjugative interaction of the /i-cr-C-H-bond with the formally vacant 2pz-orbital at the C+ carbon of this secondary carbocation. 

In general, the greater the resonance and hyper-conjugation the greater is the stability of carbocation. The stability also depends on the field strengths. The following examples illustrate this point. 

At a slightly deeper level, the difficulty of this approach lies in its acceptance of a transition complex in which the original classification into a and tt electrons has been broken consequently pure tt electron theory is inadequate for the prediction of energy changes, and a complete analysis must await the inclusion of the a bond modifications at the point of attack. Preliminary attempts to include such effects have invoked hyper conjugation (Muller et al., 1954 Fukui et al., 1954a) and other factors (Dewar et al., 1956), but little progress has yet been made towards a more detailed theoretical interpretation based on more complete calculations. 

The question of the effect of the —CH2-group which is present after proton addition is connected with the question as to the extent to which hyperconjugation in methyl-substituted proton addition complexes has to be taken into account. This question was treated in detail in an investigation of the proton addition complex of benzene by Muller et al. (1954). In an MO calculation, the effect of this CH2-group was treated as a hyper conjugation effect. In contrast to a simple HMO calculation without overlap, the overlap between adjacent C-atoms was taken into account. The calculations were based on the model ... 

We pointed out" that the lowest unoccupied (LU)MO of 4 must have the same cylindrical symmetry (oi in the Csv point group) at the y carbon that it has at the ipso carbon. Since methyl groups stabilize carbocations principally through hyper-conjugative donation of electrons from 7i-like combinations of C—H bonds, and since the C H orbitals of a methyl group at the y carbon with n symmetry (e in Csv) are orthogonal to the LUMO, a methyl substituent at the y carbon of 4 should provide much less stabilization than a methyl at an a carbon. 

This result is surprising, since in the electronegativity scale carbon is more electronegative than hydrogen this is a resonance effect, called hyper conjugation, which is discussed in Sec. 8-9. 

The demand for 7r-aryl delocalization of the positive charge is decreasing as the hyper-conjugative cr-stabilization of the positive charge by the /1-substituents is increasing from fi- in 390 and 392 to a /i-silyl group in 389a and the strained cyclopropane C—C bonds in 391. 

Another remarkable feature of these results (Hirota and Weissman, 1960) is that hyperfine coupling to the /3-proton in the isopropyl group was only about 2-4 G. This is very small compared with the results for /3-protons in comparable alcohol radicals (Table 2), and is thought to arise because of restricted rotation about the C—C bond (Symons, 1962). Indeed, molecular models show that only the conformation of this ketyl in which the /3-proton lies in the radical plane is not severely strained. Under these conditions hyper conjugation is at a minimum, and the splitting detected may be a measure of residual coupling in the absence of hyperconjugation (Symons, 1962). 

Allinger NL, Chen K, Katzenellenbogen JA, Wilson SR, Anstead GM. Hyper-conjugative effects on carbon-carbon bond lengths in molecular mechanics (MM4). J Comput Chem 1996 17 747-755. 

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. 

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