Vicinal carbon-proton coupling constants. Angular dependence and fluorine substituent effects

Abstract

Data sets of vicinal carbon-proton coupling constants 3 J CH have been calculated for propane, 1-fluoropropane and 2-fluoropropane by means of SCF ab initio methods using various standard Gaussian-type basis sets and a double zeta basis set [ 4s2p1d 2s1p ] with additional tight s functions. The major contribution to 3 J CH arises from the Fermi contact term. The remaining contributions, spin dipolar, orbital paramagnetic and orbital diamagnetic, are very small. A satisfactory agreement with experimental data is obtained after multiplying the best calculated values by a factor close to 0.8. The dependence of the calculated 3 J CH couplings upon the torsion angles φ( 13C αC βC γH) , between the coupled nuclei, and φ α ( X 13 C α  C β  C γ ), between a substituent X at C α and the C γ carbon, as well as the effect of a fluorine substituent bonded to the carbons C α , C β , or C γ , have been analysed using equations formulated from a substituent effect model. The two-dimensional angular dependence on φ and φ α for the 3 J CH coupling of propane is described by an equation including 11 terms with a r.m.s. deviation σ of 0.13 Hz. Likewise, the effect upon 3 J CH of a fluorine attached to C α depends on both angles φ and φ α . This effect is described by an equation of five terms with a σ of 0.31 Hz. On the other hand, the effects upon 3 J CH of a fluorine bonded to C β or C γ depend mainly on the angle φ and are described by equations including, respectively, four and seven terms. The corresponding σ deviations for these equations are 0.13 and 0.28 Hz. The experimental effects upon 3 J CH of a substituent OH are satisfactorily reproduced by these equations when the different electronegativity of oxygen and fluorine is taken into account.