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Association constants for 1,2,4,5‐tetracyanobenzene and tetracyanoethylene charge‐transfer complexes with methyl‐substituted benzenes revisited
Author(s) -
Arnold B. R.,
Euler A.,
Fields K.,
Zaini R. Y.
Publication year - 2000
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/1099-1395(200011)13:11<729::aid-poc311>3.0.co;2-l
Subject(s) - tetracyanoethylene , chemistry , hexamethylbenzene , ground state , benzene , charge (physics) , thermodynamics , computational chemistry , equilibrium constant , photochemistry , organic chemistry , atomic physics , quantum mechanics , physics
The association constants for charge‐transfer (CT) complex formation of a series of methylated benzene donors with 1,2,4,5‐tetracyanobenzene and tetracyanoethylene as acceptors were measured. In several cases the values determined previously using standard analysis techniques, such as Benesi–Hildebrand or related methods, were shown to be incorrect and a new method for determining the association constants for weak complexes is presented. A systematic error occured in the determination of these constants when standard analysis was carried out on weakly bound complexes. In general, the thermodynamic stabilities have been underestimated and the extinction coefficients for the CT absorptions overestimated. Furthermore, it was demonstrated that the ground‐state stabilization of the complexes studied here is due primarily to non‐bonded interactions and that the ion‐pair contributions are minor in the ground state. A notable exception may be the tetracyanobenzene–hexamethylbenzene complex where preliminary evidence points to a significant contribution of the ion‐pair state to the ground‐state stability. This study raises significant questions about what is currently known concerning the thermodynamics of CT complexes because much of what is believed may be based on incorrectly determined constants. Copyright © 2000 John Wiley & Sons, Ltd.