Bond-bending isomerism of AuI: competition between covalent bonding and aurophilicity.

We report a joint photoelectron spectroscopy and theoretical investigation of the gaseous AuI cluster, which is found to exhibit two types of isomers due to competition between Au-I covalent bonding and Au-Au aurophilic interactions. The covalent bonding favors a bent IAuIAuI structure with an obtuse Au-I-Au angle (100.7°), while aurophilic interactions pull the two Au atoms much closer, leading to an acutely bent structure (72.0°) with an Au-Au distance of 3.08 Å. The two isomers are separated by a small barrier and are nearly degenerate with the obtuse isomer being slightly more stable. At low temperature, only the obtuse isomer is observed; distinct experimental evidence is observed for the co-existence of a combination of isomers with both acute and obtuse bending angles at room temperature. The two bond-bending isomers of AuI reveal a unique example of one molecule being able to oscillate between different structures as a result of two competing chemical forces.