Hyperconjugation torsional 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. 

Figure 3.51 also contains a dissection of the total energy ( totai) into Lewis (ii(L)) and non-Lewis (ElSL>) components. The localized Lewis component E" corresponds to more than 99.3% of the full electron density, and so incorporates steric and classical electrostatic effects in nearly exact fashion. Yet, as shown in Fig. 3.51, this component predicts local minima (at 70° and 180°) and maxima (at = 0° and 130 ) that are opposite to those of the full potential. In contrast, the non-Lewis component E (NL) exhibits a stronger torsional dependence that is able to cancel out the unphysical behavior predicted by (L), leading to minima correctly located near 0° and 120°. Thus, the hyperconjugative interactions incorporated in E(SL> clearly provide the surprising stabilization of 0° and 120° conformers that counter the expected steric and electrostatic effects contained in ElL>. 

A vibrational degree of freedom may be replaced by internal rotation (torsion) around a a bond. In this case the microwave spectrum of the molecule is modified by torsion-rotation interaction. By studying this effect on the rotational spectrum, the internal rotation potential barrier can be determined. The hindering potential of CH3N3 was found to be V3 = 695 20 cal/mole (the subscript 3 stands for the 3-fold axis of the hindering potential). The potential is rather small but is not smaller than the value expected from a hyperconjugation effect . 

The better known hyperconjugative interaction that relates to all C-0 torsions, including C-2-0-2 is the Bohlmann effect first identified from characteristic con-formationally dependent shifts in the IR spectra of amines.168 169 This effect is a result of electron donation from lone pairs on N or O to the a orbital of adjacent C-H bonds. This effect is maximal for an anti relationship between the relevant lone pair and the C-H bond. One such interaction is always present in gauche H-C-O-C conformations and is one of the factors that stabilize such conformations. 

A physical interpretation has been ascribed to each of the three terms in the MM2 torsional expansion from an analysis of ab initio calculations on simple fluorinated hydrocarbons. The first, onefold term corresponds to interactions between bond dipoles, which are due to differences in electronegativity between bonded atoms. The twofold term is due to the effects of hyperconjugation (in alkanes) and conjugation effects (in alkenes), which provide double bond character to the bond. The threefold term corresponds to steric interactions between the 1,4 atoms. It was found that the additional terms in the torsional potential were especially important for systems containing heteroatoms, such as the halogenated hydrocarbons and molecules containing CCOC and CCNC fragments. 

This is a kind of torsion-stretch cross term but different from the one where the central bond changes with torsion angle. There has been some considerable debate about the existence and origin of the hyperconjugative effects, but low-temperature X-ray crystallographic experiments on appropriate compounds together with ab initio calculations certainly reveal a detectable effect. 

Evidently, the C-H bond should hyperconjugate more strongly in acetaldehyde than it does in propene, and the C-H bond length difference as we go from 0° to 90° in torsion angle should increase more. And that s what we find. The calculations show that in this case the C-H bond length increase in acetaldehyde is 0.0047 A, vs. 0.0030 A in propene. This particular type of bond stretching has also been referred to in the literature as the carbonyl effect ... 

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