Chlorine-35 quadrupole resonance in sodium tetrachloroaurate(III) dihydrate at high pressure

Abstract

The 35Cl quadrupole resonance in Na[AuCl4]·2H2O and its deuteriated derivative has been studied as a function of both temperature and pressure. Of the four crystallographically non-equivalent chlorine atoms, two show unique temperature and pressure coefficients; of these, one appears to be weakly hydrogen bonded to the water molecules, and the other is very probably the trans chlorine, the electronic effect being transmitted by a trans influence. The appreciable deuteriation shifts are temperature dependent and are attributed both to hydrogen-bond expansion and the effects of torsional oscillations of the water molecules. The high-pressure behaviour of the 35Cl frequencies is explained in terms of a polarization of the non-bonding 3pπ electrons; under pressure, the local symmetry and electronic environment of three of the Cl atoms in the [AuCl4]– ion cause the non-bonding electron clouds, principally 3px perpendicular to the [AuCl4]– plane, to contract, which increases their quadrupole resonance frequencies, that of the fourth (the chlorine trans to the hydrogen-bonded atom) moving in the opposite direction, corresponding to an expansion of its 3px orbital. The relationship of these conclusions to the electronic mechanism of the trans influence is briefly discussed.