Solid-State ¹²⁹Xe and ¹³¹Xe NMR Study of the Perxenate Anion XeO₆^4-

Abstract

Results of the first solid-state 131Xe NMR study of xenon-containing compounds are presented. The two NMR-active isotopes of xenon, 129Xe (I=1/2) and 131Xe (I=3/2), are exploited to characterize the xenon magnetic shielding and quadrupolar interactions for two sodium perxenate salts, Na4XeO6.xH2O (x=0, 2), at an applied magnetic field strength of 11.75 T. Solid-state 129/131Xe NMR line shapes indicate that the local xenon environment in anhydrous Na4XeO6 adopts octahedral symmetry, but upon hydration, the XeO6(4-) anion becomes noticeably distorted from octahedral symmetry. For stationary, anhydrous samples of Na4XeO6, the heteronuclear 129/131Xe-23Na dipolar interaction is the principal contributor to the breadth of the 129/131Xe NMR lines. For stationary and slow magic-angle-spinning samples of Na4XeO(6).2H2O, the anisotropic xenon shielding interaction dominates the 129Xe NMR line shape, whereas the 131Xe NMR line shape is completely dominated by the nuclear quadrupolar interaction. The xenon shielding tensor is approximately axially symmetric, with a skew of -0.7+/-0.3, an isotropic xenon chemical shift of -725.6+/-1.0 ppm, and a span of 95+/-5 ppm. The 131Xe quadrupolar coupling constant, 10.8+/-0.5 MHz, is large for a nucleus at a site of approximate Oh symmetry, and the quadrupolar asymmetry parameter indicates a lack of axial symmetry. This study demonstrates the extreme sensitivity of the 131Xe nuclear quadrupolar interaction to changes in the local xenon environment.

Perovskites with the Framework‐Forming Xenon

“…[7] In that case,h owever, the line broadening would positively correlate with the XeÀNa distances in the corresponding crystal structures,w hich is not supported by the experimental data;t he NMR spectrum of crystal hydrate K 2 Na 2 (XeO 6 )·8 H 2 Ot aken for reference purposes exhibits very sharp and narrow 129 Xe resonances,i ns pite of the substantially smaller XeÀNa distance of 3.31 relative to the large d(XeÀNa) in the perovskites (Table 1). [7] In that case,h owever, the line broadening would positively correlate with the XeÀNa distances in the corresponding crystal structures,w hich is not supported by the experimental data;t he NMR spectrum of crystal hydrate K 2 Na 2 (XeO 6 )·8 H 2 Ot aken for reference purposes exhibits very sharp and narrow 129 Xe resonances,i ns pite of the substantially smaller XeÀNa distance of 3.31 relative to the large d(XeÀNa) in the perovskites (Table 1).…”

“…Further examination of the 129 Xe NMR spectra reveals one more structure‐related feature of KCa(XeNaO 6 ): the apparent splitting of the 129 Xe resonance in its spectrum as compared with the isotropic shifts of KSr(XeNaO 6 ) and KBa(XeNaO 6 ) (Figure 3). The main resonances between −774 and −770 ppm in all three perovskites can be unambiguously assigned to the symmetric octahedral (XeO 6 ) environment;7 at the same time, the origin of the high‐frequency shoulder at −743 ppm in the spectrum of KCa(XeNaO 6 ) appears enigmatic. The large difference (31 ppm) between the shifts of the main resonance and the shoulder cannot be assigned to the distortions in the (XeO 6 ) octahedra: the latter would fall well beyond the structurally determined distortion values (Table 1).…”

“…[7] In that case,h owever, the line broadening would positively correlate with the XeÀNa distances in the corresponding crystal structures,w hich is not supported by the experimental data;t he NMR spectrum of crystal hydrate K 2 Na 2 (XeO 6 )·8 H 2 Ot aken for reference purposes exhibits very sharp and narrow 129 Xe resonances,i ns pite of the substantially smaller XeÀNa distance of 3.31 relative to the large d(XeÀNa) in the perovskites (Table 1). Them ain resonances between À774 and À770 ppm in all three perovskites can be unambiguously assigned to the symmetric octahedral (XeO 6 )environ- Angewandte Chemie ment; [7] at the same time,t he origin of the high-frequency shoulder at À743 ppm in the spectrum of KCa(XeNaO 6 ) appears enigmatic.T he large difference (31 ppm) between the shifts of the main resonance and the shoulder cannot be assigned to the distortions in the (XeO 6 )octahedra:the latter would fall well beyond the structurally determined distortion values (Table 1). That size-dependent broadening of resonances is ak nown phenomenon in the solid-state NMR of nanoparticles and is related to structural defects and lattice strains occurring at the particle surface.…”

“…Thec loser inspection of 129 Xe MAS NMR spectra ( Figure 3) reveals further structural features of the studied perovskites.A tf irst, the 129 Xe resonances corresponding to the first coordination shell of Xe (xenon-oxygen bonds in the XeO 6 octahedra) are significantly broadened for all the three compounds.T he comparable degree of broadening has been formerly reported in the 129 Xe NMR spectra of sodium perxenates;t he latter effect has been ascribed to the heteronuclear dipolar coupling between 129 Xe and 23 Na nuclei. [7] In that case,h owever, the line broadening would positively correlate with the XeÀNa distances in the corresponding crystal structures,w hich is not supported by the experimental data;t he NMR spectrum of crystal hydrate K 2 Na 2 (XeO 6 )·8 H 2 Ot aken for reference purposes exhibits very sharp and narrow 129 Xe resonances,i ns pite of the substantially smaller XeÀNa distance of 3.31 relative to the large d(XeÀNa) in the perovskites (Table 1). Them ost plausible explanation for the observed NMR signal broadening is the nanoscale size (50-200 nm) of xenon perovskite crystallites (Supporting Information).…”

Gas-Phase Ion Chemistry of the Noble Gases: Recent Advances and Future Perspectives

“…This species is, however, definitely less stable and structurally less compact than the xenon covalent anions observed in the condensed phase. 14,[205][206][207][208][209] The pentacoordinated XeF 5 -, in particular, is a covalent species of D 5h symmetry, with short Xe-F distances of 2.012 Å. 205 Consistently, the Faffinity of XeF 4 is theoretically estimated to be as large as 247.3 kJ mol -1 .…”