Natural resonance theory: I. General formalism

✍ Scribed by Glendening, E. D.; Weinhold, F.

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 341 KB
• Volume: 19
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/(sici)1096-987x(19980430)19:6<593::aid-jcc3>3.0.co;2-m

✦ Synopsis

The proposed ''natural resonance theory'' does not meet the requirements of a theoretically correct approach. As Ž . we are dealing with pure quantum mechanical states described by a well-defined wave function, the use of incoherent superposition 3.1 of localized density matrices to characterize the given system is a mistake from the conceptual point of view and cannot be justified Ž even by appealing to ''chemically reasonable'' numerical results. But it is also questionable whether the results are really Ž . ''chemically reasonable''; e.g., the expansion 3.1 cannot account for the significant negative value of the 1᎐4 -density matrix element . of the butadiene molecule, to which one can assign an important chemical significance.

In quantum mechanics it is of meaning to consider expansions of the wave functions, but it is not allowed to simulate the first-order density matrix as a superposition of independent first-order density matrices of selected idealized structures. This gives a fundamental difference between the old resonance theory and the present formalism. In my opinion, the MO language is inadequate to discuss the concept of ''resonance''; it has the concept of ''delocalization'' instead.

Ž . A brief response to the referee's points: We concur and have attempted to emphasize that there is ''a fundamental difference between the old resonance theory and the present formalism.'' What is ''chemically reasonable'' will ultimately be judged by comparisons with experiment, not with supposed Huckel-type analogs. We do not concur that there is a fundamental distinction Ž . between ''resonance'' and ''delocalization'' insofar as both terms express departures from a single idealized Lewis structure , nor that MO theory is somehow more adequate to discuss the latter than the former.