Near-Thermal Reactions of Au(S,D) and AuX with CHX (X = Br, I): A Combined Experimental and Computational Analysis.

Reactions of Au(S,D) and AuX with CHX (X = I and Br) were performed in the gas phase by utilizing a selected-ion drift cell reactor. These experiments were done at room temperature as well as reduced temperature (∼200 K) at a total pressure of 3.5 Torr in helium. Rate coefficients, product sequencing, and branching fractions were obtained for all reactions to evaluate reaction efficiencies and higher-order processes. Reactions of both Au states proceed with moderate efficiencies as compared to the average dipole orientation model with these neutral substrates. Results from this work revealed that, dependent on the reacting partner, Au(S) exhibits, among others, halogen abstraction, HX elimination, and association. By comparison, Au(D) participates primarily in charge transfer and halogen abstraction. Dependent on the halogen ligand, AuX ions induce several processes, including association, charge transfer, halogen loss, and halogen substitution. AuI reacting with CHBr resulted in association exclusively, whereas the AuI/CHI and AuBr/CHBr systems exhibited halogen loss as the dominant process. By contrast, all possible bimolecular pathways occurred in the reaction of AuBr with CHI. Observed products indicate that displacement of bromine by iodine on gold is favored in ionic products, consistent with the thermochemical preference for formation of the Au-I bond. All AuX reactions proceed at maximum efficiency. Potential energy surfaces calculated at the B3LYP/def2-TZVPP level of theory for the AuX reactions are in good agreement with the available thermochemistry for these species and with previously calculated structures and energetics. Experimental and computational results are consistent with a mechanism for the AuX/CHY systems where bimolecular products occur either via direct loss of the halogen originally on Au or via a common intermediate resulting from methyl migration in which the Au center is three-coordinate.