Gold cluster bends between two isomers

Collaborating research groups in the US and China have observed a rare type of isomerism in a gas phase Au2I3 cluster. The cluster exhibits two stable structures with the same electronic configuration and potential energy surface that vary in only their Au–I–Au bond angles, a phenomenon that the researchers have dubbed ‘bond-bending isomerism’.1

bond bending isomers

Attraction between the gold atoms partially compensates for the energetically unfavourable Au–I–Au bond angle in the acute isomer

Lai-Sheng Wang’s group at Brown University first saw evidence for the existence of two isomers in the photoelectron spectra of the compound. Temperature-dependent studies showed peaks that were absent at low temperature and grew in intensity when the cluster was warmed, behaviour characteristic of a higher-energy isomer. Jun Li’s group at Tsinghua University then used ab initio computational methods to determine which conformations of Au2I3would give rise to the experimental spectra. The results identified a ground state with an obtuse Au–I–Au bond angle of 100.7° and a low-lying acute isomer with an angle of 72.0°.

‘In chemistry, most molecules only have one structure,’ explains Li. Observing bond-bending isomerism is like ‘finding that one person has two bodies. It is really strange.’ An analogous phenomenon involving structures with different bond lengths, bond-stretch isomerism, was first proposed in 1972,2 but many early reports of experimental observations of bond-stretch isomerism were later disproven, and the researchers describe their results as the first experimental proof that this kind of structural isomerism can occur.

This surprising dual orientation is due to aurophilic interactions in the cluster. In the acute isomer, attraction between the two gold atoms partially compensates for the energetically unfavourable contraction of the covalent Au–I–Au bond. ‘It is against all common intuition that heating this anionic species then does not lead to a widening, but to a narrowing of the Au–I–Au angle to generate an isomer with a significant aurophilic interaction,’ comments Hubert Schmidbaur, emeritus professor of inorganic and analytical chemistry at the Technical University of Munich in Germany. Wang and Li’s groups now hope to find new gas-phase bond-bending isomers to extend their study.


1 L Wang et al, Chem. Sci., 2015, DOI: 10.1039/c5sc03568f This article is open access

2 W D Stohrer and R J Hoffmann, J. Am. Chem. Soc., 1972, 94, 779 (DOI: 10.1021/ja00758a017)


Related Content

What is a bond?

30 January 2014 Premium contentFeature

news image

There’s more to bonding than covalent, ionic and the lines we draw between atoms on paper. Philip Ball takes on the expandi...

First experimental evidence of a boron fullerene

14 July 2014 Research

news image

Distorted 40-boron atom fullerene detected mixed with quasiplanar isomer

Most Commented

Ethanol to butanol conversion shows sustainable potential

13 January 2016 Research

news image

Borrowed hydrogen chemistry drives reaction to obtain useful fuel from biomass

Israeli chemists urge government to ban chemical weapons

21 July 2016 News and Analysis

news image

Open letter presses prime minister to ‘remove the curse of chemical weapons from the face of the Earth’