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Steric hindrance effects on bimolecular coupling rate constants of carbon‐centered radicals
Author(s) -
Gostowski Rudy C.,
Bailey Terry,
Bonner Sabrinia D.,
Emrich Erin E.,
Steelman Stephanie L.
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<735::aid-poc310>3.0.co;2-z
Subject(s) - chemistry , steric effects , radical , dihedral angle , reaction rate constant , photochemistry , reactivity (psychology) , aryl , van der waals force , computational chemistry , substituent , organic chemistry , molecule , kinetics , medicine , hydrogen bond , physics , alternative medicine , alkyl , pathology , quantum mechanics
Carbon‐centered radicals have historically been classified as either persistent or reactive in solution. Little attention has been given to finding a relationship between this reactivity and the steric hindrance of radicals. The bimolecular coupling rate constants of some aryl‐substituted fluorenyl and xanthenyl radicals were determined utilizing fast‐scan cyclic voltammetry and digital simulations. This rate constant was found to be attenuated from the diffusion‐limited value as the twist of the aryl substituent increased; the twist was described by an appropriate dihedral angle (Θ). However, Θ was found to be insufficient to represent the steric shielding provided by the aryl groups. Instead, thickness parameters ( Z ) obtained from MOPAC RHF‐PM3 calculations correlated well ( R 2 = 0.954) with the coupling rate constants of the radicals considered. This parameter was taken as the greatest extent of the van der Waals surface in the area of the p‐orbital holding the unpaired electron. Copyright © 2000 John Wiley & Sons, Ltd.