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 s hielding of F-19 in xenon and krypton difluoride in solution were stud ied by F-19 NMR spectroscopy. For both molecules, linear relationships were observed between the one-bond secondary isotope shift of the F-1 9 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 an d the quadratic mean bond displacement, (Delta r)(2). On the basis o f 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 Ang strom(-2), respectively, were determined. Further, the temperature dep endence of the Xe-129 and F-19 chemical shifts and the Xe-129,F-19 ind irect 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 indepe ndent of temperature over the range 243-303 K, the mean value being 56 44.2 +/- 0.6 Hz.