Secondary isotope effect on the 19 F shielding in xenon and krypton difluorides in solution

The effects of the various xenon and krypton isotopes on the nuclear shielding of 19 F in xenon and krypton difluoride in solution were studied by 19 F NMR spectroscopy. For both molecules, linear relationships were observed between the one‐bond secondary isotope shift of the 19 F nucleus, 1 Δ; 19 F( m ′/128 Xe) and 1 Δ 19 F( m ′/78 Kr), and the relative mass factor, ( m ′ − m )/ m ′, and also between the shifts and the quadratic mean bond displacement, 〈(Δ r ) 2 〉. On the basis of the latter correlation and the linear relationship between 〈Δ r 〉 and 〈(Δ r ) 2 〉 for the linear combination of the first and second derivatives of the 19 F shielding, the values −1742 and −5804 ppm Å −2 , respectively, were determined. Further, the temperature dependence of the 129 Xe and 19 F chemical shifts and the 129 Xe, 19 F indirect spin‐spin coupling of XeF 2 were determined. The 129 Xe chemical shift is extremely sensitive to temperature with Δδ/Δ T = −0.4718 ppm K −1 , whereas for 19 F Δδ/Δ T = +0.0042 ppm K −1 . In contrast, the 129 Xe, 19 F indirect spin–spin coupling is independent of temperature over the range 243–303 K, the mean value being 5644.2 ± 0.6 Hz.