VICINAL CARBON-PROTON COUPLING-CONSTANTS - ANGULAR-DEPENDENCE AND FLUORINE SUBSTITUENT EFFECTS

Data sets of vicinal carbon-proton coupling constants (3)J(CH) have be en calculated for propane, 1-fluoropropane and 2-fluoropropane by mean s of SCF ab initio methods using various standard Gaussian-type basis sets and a double zeta basis set [4s2pld/2slp] with additional tight s functions. The major contribution to (3)J(CH) arises from the Fermi c ontact term. The remaining contributions, spin dipolar, orbital parama gnetic and orbital diamagnetic, are very small. A satisfactory agreeme nt with experimental data is obtained after multiplying the best calcu lated values by a factor close to 0.8. The dependence of the calculate d (3)J(CH) couplings upon the torsion angles phi(C-13(alpha)-C-beta-C- gamma-H), between the coupled nuclei, and phi(alpha)(X-C-13(alpha) -C- beta-C-gamma), between a substituent X at C-alpha and the C-gamma carb on, as well as the effect of a fluorine substituent bonded to the carb ons C-alpha, C-beta, or C-gamma, have been analysed using equations fo rmulated from a substituent effect model. The two-dimensional angular dependence on phi and phi(alpha), for the (3)J(CH) coupling of propane is described by an equation including 11 terms with a r.m.s. deviatio n a of 0.13 Hz. Likewise, the effect upon (3)J(CH) Of a fluorine attac hed to C-alpha depends on both angles phi and phi(alpha),. This effect is described by an equation of five terms with a sigma of 0.31 Hz. On the other hand, the effects upon (3)J(CH) Of a fluorine bonded to C-b eta or C-gamma depend mainly on the angle phi and are described by equ ations including, respectively, four and seven terms. The correspondin g sigma deviations for these equations are 0.13 and 0.28 Hz. The exper imental effects upon (3)J(CH) of a substituent OH are satisfactorily r eproduced by these equations when the different electronegativity of o xygen and fluorine is taken into account. (C) 1998 Elsevier Science B. V.