Measuring Electron Delocalization in Aromatic Molecules

Patrick Bultinck 1Robert Ponec 2Ramon Carbó-Dorca 1,3

1. Ghent University (UGENT), Krijgslaan 281, Gent 9000, Belgium
2. Czech Academy of Sciences, Institute of Chemical Process Fundamentals (ICPF), Rozvojová 135, Prague 16502, Czech Republic
3. Universitat de Girona (UDG), Campus de Montilivi, Girona 17071, Spain

Abstract

Aromaticity, although introduced long time ago, remains a challenging chemical concept. The basic reason is that there is no clear definition based on quantum chemical description, yet it persists as a very powerful idea, allowing the interpretation of many chemical phenomena. As a consequence there exist many indices aimed at quantifying molecular aromaticity. An especially intriguing finding is that some of these measures are mutually orthogonal or divergent, an unacceptable situation in front of the necessary aromaticity index uniqueness. Moreover, there exist many ambiguities about how to exactly compute many of these measures.

The present paper introduces two novel approaches, solving many of the problems related to other ways to assess aromaticity. The first method is based on powers of the density matrix and proper projection operators, allowing to quantify aromaticity as a 6-center chemical interaction (SCI) [1-3]. The second method is based on Molecular Quantum Similarity [4] measures, which permit to compare quantitatively the first order density matrices of molecular fragments to a reference system, thereby obtaining a new aromaticity index.

Both new indices are found to correlate very well. Lack of correlation with some other indices is discussed and shown to be due to shortcomings of or debatable arbitrary choices in many other techniques. The SCI model is critically examined for clamped benzenes and distorted benzenes. The grounds for the lack of correlation with NICS aromaticity indices are examined and the effects on the multidimensional character of aromaticity reported.

[1] P. Bultinck, R. Ponec, S. Van Damme, J. Phys. Org. Chem. 2005, 18, 706.
[2] R. Ponec, P. Bultinck, A. Gallegos-Saliner, J. Phys. Chem. A. 2005, 109, 6606.
[3] P. Bultinck, R. Ponec, R. Carbó-Dorca, J. Comput. Chem. 2006, in press.
[4] P. Bultinck, X. Girones, R. Carbó-Dorca, Rev. in Comput. Chem. (Eds: K.B. Lipkowitz, R. Larter and T. Cundari), John Wiley, Hoboken (USA), 21, 127, 2005.

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Presentation: oral at 18th Conference on Physical Organic Chemistry, Symposium 1, by Patrick Bultinck
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-05-10 09:03
Revised:   2009-06-07 00:44
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