title: A re-investigation of (4)JFF and (5)JFF nuclear spin-spin couplings in substituted benzenes, a novel conformational tool.
abstract: A theoretical analysis of the (4)JFF and (5)JFF couplings in fluorobenzenes separates the σ and π components of the substituent coefficients. The π bond mechanism is dominant but the σ bond mechanism must be included to give accurate values of the couplings. For monosubstituted difluorobenzenes the (4)JFF and (5)JFF couplings can be predicted from the calculated π densities by linear equations. The use of additive substituent effects allows the prediction of the meta(4)JFF couplings for multisubstituted compounds. The π dependence of the (4)JFF coupling in 2,6-difluorobenzenes provides a novel and simple method of determining the torsional angle of the C1 substituent and the benzene ring for non-symmetrical functional groups (acetyl, carboxymethyl, dimethylamino, amide, nitro etc.). This could be used to determine the geometries of such molecules in biological systems. The π dependence of the (4)JFF coupling is also of importance in the charged species of 2,6-difluoroanilinium ((4)JFF 2.1 Hz) and 2,6-difluoro-N,N,N-trimethylanilinium ((4)JFF 0.0 Hz) due to the very different π electron densities.
authors.affiliation: Department of Chemistry, The University of Liverpool, Crown St., P. O. Box 147, Liverpool L69 7ZD, UK. rabraham@liv.ac.uk macooper1607@gmail.com.
authors.forename: Raymond J
authors.initials: RJ
authors.lastname: Abraham
authors.forename: M Ashley
authors.initials: MA
authors.lastname: Cooper