Bonding-antibonding interactions

FIGURE 1.1.3 Illustration of the bonding-antibonding interactions between the HOMO/ LUMO levels of two ethylene molecules in a cofacial configuration we also illustrate the formation of the valence and conduction bands when a large number of stacked molecules interact. 

The TT interactions reinforce the metal-carbon bond (bonding interactions M—C) but weaken the CO bond (antibonding interactions C—O) (3-17b). This electronic reorganization can be represented by tbe Lewis structures shown in 3-18. 

Earlier attempts by molecular orbital (MO) methods, including db initio MO calculation, failed to reproduce the observed facts as reviewed by Abe and Mark in their paper. Recently, however, Brunck and Weinhold gave a qualitative explanation for the gauche preference in terms of the vicinal bond-antibond interaction by using INDO MO s expressed as a linear combination of bond orbitals, and Ohsaku and Imamura correctly predicted the preferred conformation of poly(oxyethylene) by the CNDO/2 MO method under the tight-binding approximation. The possibility... 

FIGURE 1.28. (a) Representation of ethane in terms of hybrid AOs (b) unperturbed MOs for a paraffin (c) effect of first-order perturbations (d) effect of bonding CC-bonding CH interactions (e) effect of bonding-antibonding interactions. 

In Section 7.4 we considered the molecules formed by second-row elements with H. In eaeh ease there was only one heavy atom situated at the junction of all the symmetry elements, i.e. at the point of the point group. This allows the symmetry labels for the p-orbitals of this atom to be taken direetly from the right-hand column of the standard eharaeter table in Appendix 12. To form the MO diagram, we then considered the SALCs for the H(ls) orbitals and matched symmetry labels to identify the MOs that will give bonding-antibonding interactions. 

As noted above (equations 9 and 10), each pair of valence NHOs /ia, /ib leads to a complementary pair of valence bond (/)ab) and antibond ( ab) orbitals. Although the latter orbitals play no role in the elementary Lewis picture, their importance was emphasized by Lennard-Jones and Mulliken in the treatment of homonuclear diatomic molecules. Since valence antibonds represent the residual atomic valence-shell capacity that is not saturated by covalent bond formation, they are generally found to play the leading role in noncovalent interactions and delocalization effects beyond the Lewis structure picture. Indeed, it may be said that the NBO treatment of bond-antibond interactions constitutes its most unique and characteristic contribution toward extending the Lewis structure concepts of valence theory. Although the NBO hybrids and polarization coefficients are chosen to minimize the role of antibonds, the final non-zero weighting of non-Lewis orbitals reflects their essential contribution to wavefunction delocalization. 

It is important to emphasize that the bond-antibond interactions depicted in equation (17) or equation (21) are the only physically significant delocalizations in Hartree-Fock theory. The remaining symmetry adaptation that converts LMOs into MOs (i.e., that converts equation 20b into equation 20a) is only decorative, with no physical effect on the HF wave-function or other measurable property. 

Table 1 Bond-Antibond Interaction Contributions to the Internal Rotation Barrier of Ethane (kcal mor ), Level HF/6-31 G(d)...

The bond-antibond interactions, following Weinhold, have been estimated by second order perturbation theory, i.e., — i). where is the Fock matrix element between bond i and antibond j. The major barrier forming interactions are given in Table 11 (Table 1 for ethane). 

Table 12 Effect of Bond-Antibond Interaction Deletion on the Acetaldehyde Cme-Caid Bond Length (A) ...

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