C-H···Au^- Interaction: To Be or Not to Be a Genuine Hydrogen Bond?

The ability of a gold ion to act as a proton acceptor in C-H hydrogen bonding remains an open question. Although increasing evidence has pointed to the existence of C-H···Au hydrogen bonding, benchmark measurements of its bond strength are scarce. Here, we present a gas-phase photoelectron velocity-map imaging spectroscopy study of the archetypical M^-···CH₃CN (M = Cu, Au) hydrogen bonded complexes. In combination with density functional theory (DFT) calculations, the interaction strengths are determined to be 0.43 eV (9.9 kcal mol^-1) for Cu^-···CH₃CN and 0.51 eV (11.8 kcal mol^-1) for Au^-···CH₃CN. Energy-decomposition analysis indicates that electrostatic interactions provide the dominant contribution, accounting for approximately 60% of the total interaction energy in both complexes. These results establish a benchmark for C-H···metal hydrogen bonding and clarify the physical nature of these interactions.