MO, Antibonding

Fig. 3.14 Generalized molecular orbital diagram possible electronic transitions in organic molecules, associated absorption regions and examples of common chromophores (modified according to Bottcher, 1991) n, non-bonding MO antibonding MOs are indicated by. ...

Fig. 1.20 The overlap of two 2 > atomic orbitals for which the atomic nuclei are defined to lie on the z axis (a) direct overlap along the z axis gives a Ggilp ) MO (bonding) (b) the formation of the Uu lpz) MO (antibonding) (c) sideways overlap of two Ipx atomic orbitals gives a Ti ilp ) MO (bonding) (d) the formation of Kg 2py.) MO (antibonding). Atomic nuclei are marked in black and nodal planes in grey.

Figure 7-18. Schematic representation of the LCAO scheme in a, T-only calculation for ethylene, The AOs

I he results of their calculations were summarised in two rules. The first rule states that at least one isomer C with a properly closed p shell (i.e. bonding HOMO, antibonding I. U.MO) exists for all n = 60 - - 6k (k = 0,2,3,..., but not 1). Thus Qg, C72, Cyg, etc., are in lhi-< group. The second rule is for carbon cylinders and states that a closed-shell structure is lound for n = 2p(7 - - 3fc) (for all k). C70 is the parent of this family. The calculations Were extended to cover different types of structure and fullerenes doped with metals. 

When aos are eombined to form mos, eore, bonding, nonbonding, antibonding, and Rydberg moleeular orbitals ean result. The mos (j) are usually expressed in terms of the eonstituent atomie orbitals Xa iii the linear-eombination-of-atomie-orbital-moleeular-orbital (LCAO-MO) manner ... 

For example, the three NH bonding and three NH antibonding orbitals in NH3, when symmetry adapted within the C3V point group, cluster into ai and e mos as shown in the Figure below. The N-atom localized non-bonding lone pair orbital and the N-atom Is core orbital also belong to ai symmetry. 

To eonsider why the two-orbital two-eleetron single bond formation ease ean be more eomplex than often thought, let us eonsider the H2 system in more detail. In the moleeular orbital deseription of H2, both bonding <5g and antibonding mos appear. 

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