Infrared Spectroscopy of Gold Carbene Cation (AuCH2 +): Covalent or Dative Bonding?

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 123; no. 41; pp. 8932 - 8941
• Main Authors: Armentrout, P. B, Stevenson, Brandon C, Yang, Fan, Wensink, Frank J, Lushchikova, Olga V, Bakker, Joost M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 17-10-2019
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.9b08049

The present work explores the structure of the gold carbene cation, AuCH2 +, using infrared multiple photon dissociation action spectroscopy and density functional theory (DFT). Unlike several other 5d transition-metal cations (M+ = Ta+, W+, Os+, Ir+, and Pt+) that react with methane by dehydrogenation to form MCH2 + species, gold cations are unreactive with methane at thermal energies. Instead, the metal carbene is formed by reacting atomic gold cations formed in a laser ablation source with ethylene oxide (cC2H4O) pulsed into a reaction channel downstream. The resulting [Au,C,2H]+ product photofragmented by loss of H2 as induced by radiation provided by the free-electron laser for intracavity experiments in the 300–1800 cm–1 range. Comparison of the experimental spectrum, obtained by monitoring the appearance of AuC+, and DFT calculated spectra leads to the identification of the ground-state carbene, AuCH2 + (1A1), as the species formed, as previously postulated theoretically. Unlike the covalent double bonds formed by the lighter, open-shell 5d transition metals, the closed-shell Au+ (1S, 5d10) atom binds to methylene by donation of a pair of electrons from CH2(1A1) into the empty 6s orbital of gold coupled with π back-bonding, i.e., dative bonding, as explored computationally. Contributions to the AuC+ appearance spectrum from larger complexes are also considered, and H2CAu+(c-C2H4O) seems likely to contribute one band observed.