Temperature Dependence of the 127I Quadrupole Coupling in Periodates with Alkyl- or Aryl-Substituted Cations of Main Group V

Abstract

127I solid state NMR studies of metaperiodates with alkyl- or aryl-substituted cations of main group V revealed second order quadrupole effects with quadrupole coupling constants in the range of 2 - 7 MHz. In (C2H5)4PIO4 and (C2H5)4AsIO4 the 127I quadrupole coupling constants show normal temperature coefficients ᾱ in the order of -10-4 K-1 for 186 K < T < 300 K. For the metaperiodates with aryl-substituted cations (C6H5)4P+, (C6H5)4As+ and (C6H5)4Sb+ complicated temperature dependences of the 127I quadrupole interaction with phase transitions and anomalous positive temperature coefficients +0.2 · 10-2K-1 ≦ ᾱ ≦ +0.8 · 10-2K-1 were observed. The relevance of possible mechanisms for the anomalous temperature coefficients, like breakdown of dη - pη backbonding, negative pressure coefficients, charge transfer, orientational change of the EFG-tensor axes system and influence of reorientational motions is discussed for these compounds, including results from an X-ray investigation of (C6H5)4AsIO4 and 1H-solid state NMR measurements.