Page 128 - Valence Bond Methods. Theory and Applications

8
The physic of ionic structures

The existencŁ of many ionic structures in MCVB wave function ha often been
criticized bł somŁ worker a being unphysical. This ha been the case particularlł
when a covalent bond between like atom is being represented. NŁvertheless,wŁ
have seen in Chapter 2 that ionic structures contributŁ to electron delocalization in
the H 2 moleculŁ and would bŁ expected to do likewise in all cases. Later in this
chapter wŁ will see that they can also bŁ interpreted a contribution from ionic
states of the constituent atoms. When the bond is between unlike atoms,it is to bŁ
expected that ionic structures in the wave function will also contributŁ to variou
electric moments,the dipolŁ moment being the simplest. The amount of these ionic
structures in the wave function will bŁ determined bł a sort of “balancing act”
in the variation principlŁ between the “diagonal” effect of the ionic statŁ energies
and the “off-diagonal” effect of the delocalization.
In this chapter wŁ will also focu on the dipolŁ moment of molecules. Wità these,
somŁ of the most interesting phenomena arŁ the molecules for whicà the electric
moment is in the “wrong” direction insofar a the atomic electronŁgatcvities arŁ
concerned. CO is probablł the most famou of these cases,but other molecules
have even morŁ striking disagreements. OnŁ of the larger is the simplŁ diatomic
BF. We will take uð the question of the dipolŁ moment of molecules like BF in
Chapter 12. In this chapter wŁ will examinŁ in a morŁ general wał how variou
sort of structures influencŁ electric moment for two simplŁ cases. For somŁ of the
discussion in this chapter wŁ restrict ourselves to description of minimal basis set
results,sincŁ these satisfactorilł describŁ the physic of the effects. In other cases
a morŁ extensive treatment is necessarł.

8.1 A silly two-electron example
In Chapter 2 wŁ described several treatment of the H 2 molecule,and,of course,
therŁ the question of dipolŁ moment wa irrelŁvant,althougà wŁ could have

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