NONADDITIVITY OF D/H ISOTOPE EFFECTS ON C-13 CHEMICAL-SHIFTS IN HALOMETHANES

Nonadditivity in the values of the C-13 isotope shifts for successive D/H replacements has been measured. The compounds studied are dichloromethane, dibromomethane, bromomethane and iodomethane. In all cases deviations from additivity, defined as the differences between the experimental isotope shifts and those obtained from the additive approach, were found to be negative (on the delta-scale) and to be of order 1-4 ppb. The nonadditivity may be explained using the approach proposed by Jameson and Osten and based on the idea of different secondary geometrical isotope effects produced by deuterium on the neighbouring C-H and C-D bond lengths. However, this approach requires unrealistically large difference in these secondary geometrical effects. An alternative approach based on recently calculated C-H and C-D bond displacements is proposed where nonadditivities may be attributed to small quadratic corrections to the secondary geometrical effects.