Infrared Spectroscopy of Au(Acetylene) n + Complexes in the Gas Phase

Abstract

Au(C₂H₂)_n ⁺ (n = 1–6) ion–molecule complexes are produced in the gas phase via pulsed laser vaporization in a supersonic expansion of acetylene and argon. Cations are size selected and studied with infrared photodissociation spectroscopy in the C–H stretching region (3000–3500 cm⁻¹). Insight into the structure and bonding of these species is obtained from the number of infrared active bands, their relative intensities and their frequency positions. Density functional theory calculations provide structures for these complexes and predicted spectra are compared to the experiment. The combined data indicate that gold cation has a primary coordination number of two with respect to acetylene binding, and a secondary coordination sphere that is completed with a third ligand. Larger complexes (n = 4–6) are formed by solvation of the Au(C₂H₂)₃ ⁺ core ion with acetylene, in a pattern like that seen previously for Cu(C₂H₂)_n ⁺ complexes. Small differences in the spectra between corresponding copper and gold cation complexes are explained by theory, but only when relativistic corrections are included for the gold complexes.