SECONDARY ISOTOPE EFFECT ON THE F-19 SHIELDING IN XENON AND KRYPTON DIFLUORIDES IN SOLUTION

The effects of the various xenon and krypton isotopes on the nuclear shielding of F-19 in xenon and krypton difluoride in solution were studied by F-19 NMR spectroscopy. For both molecules, linear relationships were observed between the one-bond secondary isotope shift of the F-19 nucleus, (1) Delta F-19(Xe-m'/128) and (1) Delta F-19(Kr-m'/78), and the relative mass factor, (m' - m)/m', and also between the shifts and the quadratic mean bond displacement, [(Delta r)(2)]. On the basis of the latter correlation and the linear relationship between [Delta r] and [(Delta r)(2)] for the linear combination of the first and second derivatives of the F-19 shielding, the values -1742 and -5804 ppm Angstrom(-2), respectively, were determined. Further, the temperature dependence of the Xe-129 and F-19 chemical shifts and the Xe-129,F-19 indirect spin-spin coupling of XeF2 were determined. The Xe-129 chemical shift is extremely sensitive to temperature with Delta delta/Delta T = -0.4718 ppm K-1, whereas for F-19 Delta delta/Delta T = +0.0042 ppm K-1. In contrast, the Xe-129,F-19 indirect spin-spin coupling is independent of temperature over the range 243-303 K, the mean value being 5644.2 +/- 0.6 Hz.