An Isolated Complex of Ethyne and Gold Iodide Characterized by Broadband Rotational Spectroscopy and Ab initio Calculations

A molecular complex of C₂H₂ and AuI has been generated and isolated in the gas phase through laser ablation of a gold surface in the presence of an expanding sample containing small percentages of C₂H₂ and CF₃I in a buffer gas of argon. Rotational, B₀, centrifugal distortion, Δ_J and Δ_JK, and nuclear quadrupole coupling constants, Ξ_aa(Au), Ξ_bb(Au) - Ξ_cc(Au), Ξ_aa(I), and Ξ_bb(I) - Ξ_cc(I), are measured for three isotopologues of C₂H₂···AuI through broadband rotational spectroscopy. The complex is C_2v and T-shaped with C₂H₂ coordinating to the gold atom via donation of electrons from the π-orbitals of ethyne. On formation of the complex, the C≡C bond of ethyne extends by 0.032(4) Å relative to r(C≡C) in isolated ethyne when the respective r₀ geometries are compared. The geometry of ethyne distorts such that (∗-C-H) (where ∗ indicates the midpoint of the C≡C bond) is 194.7(12)° in the r₀ geometry of C₂H₂···AuI. Ab initio calculations at the CCSD(T)(F12∗)/AVTZ level are consistent with the experimentally determined geometry and further allow calculation of the dissociation energy (D_e) as 136 kJ mol^-1. The Ξ_aa(Au) and Ξ_aa(I) nuclear quadrupole coupling constants of AuI and also the Au-I bond length change significantly on formation of the complex consistent with the strong interaction calculated to occur between C₂H₂ and AuI.