Qualitative Consideration of the HOMO—LUMO Interaction

The HO—LU interaction came early to the notice of theoreticians. Hiickel 74) pointed out the role of LU in the alkaline reduction of naphthalene and anthracene. Moffitt S) characterized the formation of SO3, SO2CI2, etc. by the reactions of SO2 as an electron donor with the S-atom-localiz-ing character of HO MO. Walsh considered that the empirical result of producing nitro compounds in the reaction of the nitrite anion with the carbonium ion should be attributed to the HO of the NO2 anion which is localized at the nitrogen atom. 

Quite independently, of these fragmentary remarks, a distinctive role of HO (and later LU and SO, too) in unsaturated molecules was pointed out 43) in a general form and with substantiality (cf. Chap. 2). With respect to the molecular complex formation, the theory of charge-transfer force was proposed 47). A cluetograsp the importance of HO—LU interaction was thus brought to light simultaneously both from the side of ionic reaction and from the side of molecular complex formation. 

The MuUiken theory of overlap and orientation principle (cf. Chap. 2) predicts that stabilization in the molecular complex formation should essentially be determined by the overlap of the donor HO and the acceptor LU. The iodine complex of trimethylamine will take the form 

The shape of the complex of benzene and silver cation is also explicable in a similar manner. The HO MO s of benzene are degenerate (eij) and have the symmetry as follows  

A more complicated example has been discussed by Tsubomura . The stability of the quinkyirone-type complex is ascribed to the symmetry 

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As in benzene, the substituents in ortho and para will be better conjugated than those in meta. These qualitative aspects of regioselectivity have been studied quantitatively by quantum mechanical studies. Further the molecular orbital considerations have shown that the favoured transition state will be that in which there is strongest interaction between the HOMO of the diene and LUMO of the dienophile. In the most common cases the dienophile bears an electron withdrawing substituent and the diene an electron releasing one. Here the interation is between the LUMO of the dienophile and the HOMO of diene. 

This preference can be rationalized by qualitative molecular orbital considerations that assume that the favored transition state will be that in which there is the strongest interaction between the HOMO of the diene and the LUMO of the... 

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