Some Investigations of Intramolecular Screening Effects on 1H and 19F N.M.R. Chemical Shifts

Abstract

The investigations reported herein concern the evaluation of a and 195 chemical shifts by semi-empirical methods. Cyclic molecules have been primarily used in these investigations. The spectral parameters required have, for some of the compounds used, been reported previously and those for the other compounds, a series of novel highly fluorinated bicyclo(2,2,1)heptanes and heptenes, are reported herein, It has generally been assumed that 1H shifts in saturated hydrocarbons depend on secondary fields produced at a resonant nucleus by magnetically anisotropic neighbouring bonds. It has been shown that a quantity, conveniently called the 'apparent' anisotropy in bond magnetic susceptibility, can be derived from n.mr. measurements, and better used to interpret the 1H shifts in a range of saturated compounds. The magnitudes of the 'apparent' anisotropies and origins of the equivalent dipoles for the C-C, C-H, C-F and C-C1 bonds have been determined. The magnitudes are found to be +4.3 x 10-6 and between O and -1.2 x 10-6 and -7.8 x 10-6 cm3.mole-1 respectively and the equivalent dipole in the last three cases shown to act at the carbon atom of the bond. 19F shifts in saturated compounds have been attributed to intramolecular electric fields produced by neighbouring bonds at the resonant nucleus and evaluated using an equation of the form: Δσ=-AΔEz-B(ΔE2 +Δ<E2 ) Significantly better correlations with experimental results are (ii) demonstrated if the equation used is of the form: Δσ=-χΔEz - YΔE2-ZΔ<E2> Values representing X, Y and Z and the location of the point dipole associated with the C-F bonds, have been deduced. It is found that the latter originates at c.a. 0.629A along the bond from the carbon atom and X = -90 x 10-12, Y = -1270 x 10-18 ; Z = +146.5 x 10-18 e.s.u. Better agreement with experimental results is observed when possible contributions to <E2> from C-C bonds are neglected.