A THEORETICAL SEMIEMPIRICAL (AM1) STUDY OF REDUCTIVE ELECTRON-TRANSFER - RADICAL-ANION STABILITY, BOND-DISSOCIATION ENERGIES AND PROPERTIESOF THE RADICALS FORMED

Authors
Citation
F. Taddei, A THEORETICAL SEMIEMPIRICAL (AM1) STUDY OF REDUCTIVE ELECTRON-TRANSFER - RADICAL-ANION STABILITY, BOND-DISSOCIATION ENERGIES AND PROPERTIESOF THE RADICALS FORMED, Gazzetta chimica italiana, 126(8), 1996, pp. 529-537
Citations number
46
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry
Journal title
ISSN journal
0016-5603
Volume
126
Issue
8
Year of publication
1996
Pages
529 - 537
Database
ISI
SICI code
0016-5603(1996)126:8<529:ATS(SO>2.0.ZU;2-E
Abstract
Molecular structure, intrinsic stability and dissociation energies of the C-Cl bond in the radical anions of benzyl chloride, 2a, benzal chl oride, 3a, and benzotrichloride, 4a, have been studied theoretically w ith the semi-empirical AM1 method. While the energy profile of the met hyl chloride radical anion la as a function of the C-Cl coordinate tur ns out to be purely dissociative, in agreement with results of more so phisticated theoretical approaches, for compounds 2a-4a the energy pro files display flat minima at a bond distance slightly higher than that of the corresponding neutral molecules. Bond dissociation energies (D H') derived from heats of formation calculated with the AM1 method are positive and lower than those of the corresponding neutral molecules (DH). These DH' values nevertheless seem rather unreliable, and this i s probably due to the fact that the total molecular energies of neutra l molecules, radicals and radical anions, calculated at this level of theory, are poorly correlated. When empirical corrections are introduc ed, smaller DH' values are obtained: for 2a the DH' values are distinc tly negative; for 3a and 4a they range between slightly negative and s lightly positive. Although the absolute DH' values can hardly be thaug ht reliable, their trend becomes more reliable as the number of haloge n atoms in the reaction centre increases, and also as a result of ring substitution. The effect of ring substituents on the calculated bond dissociation energies of several molecules ArZ-Y, where the Z-Y are th e C-H, O-H, S-H, C-Cl, C-Br bonds, has been evaluated and found to rep roduce experimental trends, where comparison is feasible. The effect o f substituents on the radical anions is markedly higher than in the ne utral molecules and in radicals, and shows that electron-withdrawing s ubstituents enhance their stability, in agreement with a number of exp erimental observations, and raise the energy necessary to break the C- Cl bond.