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 3p_π 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 3p_x 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 3p_x orbital. The relationship of these conclusions to the electronic mechanism of the trans influence is briefly discussed.