Kenichi Fukui Nobel Lecture
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- Chemistry 1981
16 Chemistry 1981 the nitrogen lone-pair so as not to disturb the π conjugation will evidently be more advantageous than the addition to higher occupied π orbitals which may intercept the π conjugation. Thus, the reason why proton dare not add to the positions of large amplitude of π HOMO in this case will easily be understood. It is not completely satisfactory to dispose of a disagreement between the HOMO-LUMO argument and the experimental fact formally as an exception to the theory. A so-called exception does possess its own reason. To investigate what the reason is will possibly yield a novel finding. The HOMO-LUMO interaction argument was recently pointed out 44 to be
in an auxiliary sense useful for the interpretation of the sign of a reaction constant and the scale of a substituent constant in the Hammett rule 45 which
has made an immeasurable contribution to the study of the substituent effect in chemical reactivity. In the cyclic addition, like Diels-Alder reactions and 1,3- dipolar additions, the relative easiness of occurrence of reactions, various subsidiary effects, and interesting phenomena like regioselection and periselec- tion were interpreted with considerable success simply by the knowledge of the height of the energy level of HOMO and LUMO, the mode of their extension, their nodal structure, etc. 46 - I defined these in a mass: the "orbital pattern". Other topics that have been discussed in terms of HOMO-LUMO interac- tions are thermal formation of excited states, 47 singlet-triplet selectivity, 48 the
chemical property of biradicals and excited molecules, 49 the interaction of the central atom and ligands in transition metal complexes, 50
the interaction of three or more orbitals 51 and so forth. Inagaki et al. included in the theory the polarization effect in HOMO and LUMO due to the mixing in of other orbitals and gave an elucidation for a number of organic chemical problems which were not always easy to explain. The unique stereoselection in the transannular cross-bond formation, the lone-pair effect, the d orbital effect, and the orbital polarization effect due to substituents were the cases. 52 As was partly discussed above, the method of orbital interaction was applied not only to the ground electronic state but to the excited states, giving an explanation of the path of even complicated photochemical isomerizations. 13 21
In a majority of ca ses the HOMO and the LUMO of the ground-state molecule were also found to be the essential orbitals. Even the ground-state reaction of a strong electron acceptor (or donor) causes a mixing in of an ionized electron configuration or an excited electron configuration in another molecule. In consequence, a partial HOMO-HOMO or LUMO-LUMO inter- action, which would be trivial if there were no influence of the acceptor (or donor), becomes important in stabilizing the interacting system.*’ The problems so far discussed have been limited to chemical reactions. However, the HOMO-LUMO interaction must come into relation also with other chemical phenomena in almost the same mechanism - with the exception of one different point that they usually do not bring about so remarkable a change in the nuclear configuration as in the case of chemical reactions. N OW
city”- one of the simplest, but the hardest-to-interpret problems. There seem to be few problems so annoying to theoreticians as the explanation of this K. Fukui 17 chemically classical concept. I greatly appreciate the contribution of Dewar’s theory 53, 54
based on a quantitative energy values argument. Here, however, I want to give a quality comment through a totally different way of consider- ation. It is easily ascertained 55 in Fig. 4 that in benzene, naphthalene, phenan- threne, etc., any virtual division of the molecule into two always produces the parts in which their HOMO and LUMO overlap in-phase at the two junctions. Fig. 4. The HOMO-LUMO phase relationship in virtual division of aromatic hydrocarbons. (SOMO: a singly occupied MO of a radical) 18 Chemistry 1981 But these circumstances are not seen in anthracene which is usually looked upon as one of the typical representatives of aromatic compounds. Hosoya 55 pointed out from the comparison with phenanthrene indicated in Fig. 4, that the ring growth of type (II) was less stable than that of (I), It is well known that anthracene occasionally exhibits a reactivity of olefin-like additions. In view of so-called Hückel’s (4n+2)-rule mentioned above, an anthracene molecule has 14π electrons and fulfils the stability condition for “aromaticity.” Actually, if one considers a molecule of anthracene with the two inside bonds deleted,
it is really seen that the HOMO and the LUMO of the two parts overlap in an in-phase manner at both of the junctions: In this way, it is understood that the two bonds which were deleted above exerted a certain unfavourable influence for aromaticity. Such an influence bears a close resemblance to that of impurity scattering in the wave of a free electron moving in a metal crystal. This discussion seems to be a digression but, as a matter of fact, it relates to the essential question as to how an electron in a molecule can delocalize. As will be mentioned later, Anderson 56 solved the question how an electron in a Yüklə 271,11 Kb. Dostları ilə paylaş:
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