Hyperconjugation cross

Figure 8.14 Cross-conjugated systems connected with a n-system and cross-hyperconjugated systems connected with a pseudo-7t system.

Emanuelsson, R., Wallner, A., Ng, E. A. M., Smith, J. R., Nauroozi, D., Ott, S., Ottosson, H. (2013). Cross-Hyperconjugation An Unexplored Orbital Interaction between n-Conjugated and Saturated Molecular Segments. 

Fig. 3. Explanation of the level crossing in Fig. 2 a. a increases from left to right. The A level is stabilized by the decrease in through-space interaction through-bond coupling (hyperconjugation) with the a and a levels of the interposed CH2 destabilizes the S level.

Figure 3.52 Leading hyperconjugative stabilizations in CFH2CH = CH2, showing the torsional dependence of n-o (solid lines) and a-n interactions (dotted lines) for the C—F (crosses) and two C—FI bonds (triangles, circles) of the—CFF12 group. The sum of all six interactions is shown as the heavy solid line (squares), which may be compared with the total barrier potential in Fig. 3.51.

The presence of these higher cross-terms tend to improve the ability of the force field to predict the properties of unusual systems (such as those which are highly strained) and also to enhance its ability to reproduce vibrational spectra. It must be noticed, however, that any of the cross terms listed above have been proven to be truly of the form in which they are written. No attempts have been reported to derive that or any other form of the coupling between different geometry distortions and to estimate the corresponding constants from some independent point of view. The class III force field will also take into account further features such as electronegativity and hyperconjugation. We shall turn to these problems later. 

The stereoselectivity of cyclopropane ester formation could also be effected by using reagents supported on linear or cross-linked polymers. The most important effects were noted with chloromethylated polymers cross-linked with divinylbenzene. The role of hyperconjugation in determining the stereochemistry of nucleophilic cyclopropanation of electrophilic alkenes has been studied and predicted In terms of equation 57 the... 

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. 

We have suggested " that a Class 3 force field be defined as one which contains chemical effects, in addition to the physical effects so far discussed. Chemical effects would include such things as hyperconjugation, the electronegativity effect, the anomeric and Bohlmann effects, and so on. These effects depend upon exactly which atom occupies a position, that is, they depend not only on ordinary mechanical quantities, but also on specific properties of oxygen, for example, compared with nitrogen, or with carbon. These effects can, of course, be properly represented by suitable cross terms in the force constant matrix. The origins of these terms, however, have a definite chemical basis. 

Atoms in Molecules Electron Transfer Calculations Electronic Wavefunctions Analysis Hyperconjugation Intermolecular Interactions by Perturbation Theory Localized MO SCF Methods Natural Orbitals NMR Chemical Shift Computation Ab Initio Rotational Barriers Barrier Origins Valence Bond Curve Crossing Models. 

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