The permanent electric dipole moment of gold chloride, AuCl

The [19.20]0(+)-X-1 sigma(+) (0,0) band system of gold chloride, AuCl, has been studied using optical Stark spectroscopy. The [19.20]0(+) state is analysed as a (3)pi electronic state, and the observed Stark shifts analysed to determine ground and excited electronic state permanent electric dipole moments, mu(el). A considerably smaller mu(el) of 0.32 +/- 0.17 D for the [19.20]0(+) (v = 0) state, in comparison to that of 3.69 +/- 0.02D for the X-1 sigma(+) (v = 0) state is observed. The experimental assignment of the [19.20]0(+) state to a component of the (3)pi state has been corroborated by high-level quantum-chemical calculations using exact two-component theory for treating relativistic effects and the equation-of-motion coupled-cluster approach for describing the electronic excited state. A close inspection of the electronic wave functions for the (3)pi states of gold monohalides reveals significant participation of excitations from the halogen valence p orbitals to the anti-bonding molecular orbitals mainly localised on the gold atom. This leads to a charge transfer from halogen to gold and is responsible for the dramatic reduction of dipole moment in the (3)pi states in comparison to the ground states as observed in the Stark-shift analysis. It has been further demonstrated that this ligand to metal charge transfer increases along the F to I series and leads to predicted dipole moments in the (3)pi states of AuBr and AuI that point towards the gold atoms, qualitatively different than might be anticipated.