Stabilizing orbital interaction

We shall first consider how nonbonded interactions influence bond angles in molecules. Our approach will be illustrated by reference to the model systems difluoro-methane and 1,1-difluoroethylene. In these problems, we shall consider not only stabilizing orbital interactions but also overlap repulsion in order to demonstrate some interesting trends which obtain in these angle problems. 

The ethane molecule can be constructed by union of two pyramidal methyl radical fragments. The interaction diagram is shown in Fig. 16 and the key stabilizing orbital interactions are depicted below. 

Fig. 17. Sigma type stabilizing orbital interactions in syn and anti 1,2-difluoroethane...

Fig. 18. Pi type stabilizing orbital interactions in syn and anti 1,2-difluoroethane. The symmetry labels are assigned with respect to a mirror plane (syn conformer) or a rotational axis (anti con-former)...

Fig. 21. Stabilizing pi orbital interactions in propene The key stabilizing orbital interactions are sketched below...

Fig. 24. Stabilizing orbital interactions in cis and trans 1,2-difluoro-l,2-dihydroxy ethylene. Symmetry labels are with respect to a rotational axis (trans isomer) and mirror plane (cis isomer)...

Fig. 35. Dominant stabilizing orbital interactions in the boat and chair conformers of...

Fig. 39. Stabilizing orbital interactions involved in the syn and anti transition states of the Sn2 reaction...

The very marked anomeric effect shown by 2-alkoxytetrahydropyran-3-ones (199) has been interpreted in terms of stabilizing orbital interactions (77NJC79). The influence of the heteroatom and the carbonyl group on the stabilization of the conformer in which the alkoxy group is axial is apparent. 

The interaction energy is further split up into three physically meaningful components as discussed earlier (1) the classical electrostatic interaction A Vc stat between the unperturbed charge distributions of the prepared fragments which is usually attractive, (2) the Pauli repulsive orbital interactions AEpauii, and (3) the stabilizing orbital interaction AEoi (Eq. [38]) ... 

The foregoing statements concerning stabilizing orbital interactions can be extended to transition states. For them, they can be formulated more concisely and more generally using the term frontier orbitals, as introduced in the discussion of Figure 15.3 ... 

The second term in Eq. (2), AEp ui , refers to the repulsive interactions between the fragments, which are caused by the fact that two electrons with the same spin cannot occupy the same region in space. AEp ui is calculated by enforcing the Kohn-Sham determinant on the superimposed fragments to obey the Pauli principle by antisymme-trization and renormalization. The stabilizing orbital interaction term, which is... 

Figure 9. Stabilizing orbital interactions in equatorial 2-ammoniotetrahydropyrans 76e (according to ref. 8).

Bond length contraction in Y—C—X systems has been observed as a rule when Y and X are electronegative substituents. A good example is provided by polyfluoromethanes (cf. Section III.A.3). Brockway (194) observed that C—F bonds are shortened by about 0.06A on going from CHjF to CF4. Though stabilizing orbital interactions (6,70), or MOVB-based explana-... 

Table 2.4 Number and strength (given in parentheses in kcal mol ) of the stabilizing orbital interactions in I... 

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