Bonding, antibonding

Starting from hybrid orbitals, it is possible to construct bond orbitals by using normalized combinations. For every couple of hybrids pointing to each other along a bond direction, a bond-antibond pair can be obtained2 ... 

The in-phase (out-of-phase) combined orbitals are stabilized (destabilized) and bonding (antibonding)... 

MC-SCF treatments written in terms of coupled Fock equations [44], The simplest examples are the two-configuration SCF theory [45] used in and pi+2 atomic mixing [46], or bonding-antibonding molecular problems [47], and more generally the Clementi-Veillard electron-pair MC-SCF theory [48],... 

The NBOs are an optimized set of localized bonding, antibonding, and lone pair orbitals. The bond orbitals typically have occupancies of 1.98e in molecules with one simple Lewis structure. In C2H6, Weinhold and coworkers36 have shown that the barrier height V3 = 1025 cm-1 can be understood in the NBO terms in the form of aCH — donor-acceptor interactions. When the corresponding off-diagonal... 

Figure 1.17 The hypothetical formation of the bonding molecular orbitals of ethane from two sp -hybridized carbon atoms and six hydrogen atoms. All of the bonds are sigma bonds. (Antibonding sigma molecular orbitals — are called a orbitals — are formed in each instance as well, but for simplicity these are not shown.)... 

The relative strengths of hyperconjugative stabilizations could be rationalized with contour plots similar to Fig. 3.49. The most important features of such plots could be predicted from the transferable forms of the NBOs (cf. Figs. 3.16 and 3.25) and expected variations with electronegativity (Sections 3.2.5 and 3.2.8). As shown in Fig. 3.49, the hyperconjugating ax and ax NBOs are typically canted away from the pi NBOs, weakening their interactions compared with ordinary (7r-7r ) con-jugative stabilizations in Table 3.19. However, the dependences on bond/antibond... 

The number of CSFs is now reduced to 24. We can constrain the wave function further by requiring that there is two electrons in each of the bonding-antibonding pairs of n orbitals ... 

The reader should now complete the bonding model for CH4 by constructing a bonding-antibonding pair for each of the other three interacting pairs of atomic orbitals. 

Figure 8. Model for the trans-polyacetylene and the system of bonding - antibonding levels associated with each double-bond.

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