Quadrupole Effects with Anomalous Temperature Dependences in 127 I-Solid State NMR of (n-C4H9) 4 AsIO4 and (n-4H9) 4 PIO4
For T>310K the compounds (n-C4H9)4 AsIO4 (1) and (n-C4H9)4 PIO4 (2) reveal second order quadrupole splittings in their 127 I- wideline NMR spectra with quadrupole coupling constants e2qQ/h in the range of 2-3 MHz. By a point charge model the distortions afi from tetrahedral symmetry of the anions are calculated to be very small with Dß ≤ 0.5°. In both compounds anomalous positive temperature dependences of e2qQ/h with very high values of the temperature coefficients = +0.4•10-2 [K-1] (1) and a = +0.3•10-2 [K-1] (2) were measured. Special inter- and intramolecular mechanisms like reorientational motion, jump rotation, displacements of the electric field gradient tensor axes, breakdown of dx-px backbonding or volume of the cations are discussed to explain these effects.
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