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\u2018Whether a metal can truly act as a genuine proton acceptor for a C\u2013H donor has long remained an open and controversial question in chemistry,\u2019 says Jun Chen<\/a> at the Fujian Institute of Research on the Structure of Matter in China. Looking to answer this question Chen and colleagues conducted spectroscopic analysis of gold<\/a> complexes and discovered that these species feature hydrogen bonds as strong as those formed by O\u2013H and N\u2013H groups. Understanding these interactions could allow chemists to design better catalysts or host\u2013guest systems that rely on molecular recognition.<\/p>\n Gold is an ideal hydrogen bond acceptor, owing to relativistic effects contracting the 6s orbital, which localises electron density and makes it more directional for a better interaction with a hydrogen bond donor. In fact, gold atoms routinely form hydrogen bonds with conventional donors, such as O\u2013H, N\u2013H and F\u2013H. Yet capturing a true C\u2013H\u00b7\u00b7\u00b7Au bond is challenging, as the interaction is weaker and, in many cases, these atoms are simply close to each other in space rather than directly forming a bond.<\/p>\n Spectroscopic analysis of gas phase gold anions bound to acetonitrile molecules has now allowed Chen and his team to better probe these interactions. \u2018[The cyanide group in acetonitrile] enhances the C\u2013H acidity without reacting with the metal, allowing us to successfully isolate and characterise the complex,\u2019 explains Chen.<\/p>\n Photoelectron spectroscopy and computational analysis revealed that the bond strength of the C\u2013H\u00b7\u00b7\u00b7Au bond was around 0.50eV, with Chen explaining that this is \u2018perfectly comparable to many conventional O\u2013H or N\u2013H anion hydrogen bonds\u2019. \u2018This directly challenges the common assumption that only strongly polarised, conventional X\u2013H groups can act as effective hydrogen bond donors towards metal anions,\u2019 he says.<\/p>\n Further analysis showed that electrostatics accounted for around 60% of the interaction, while dispersion and induction effects had small contributions (26% and 16%, respectively).<\/p>\n