Near-Thermal Reactions of Au+(1S,3D) and AuX+ with CH3X (X = Br, I): A Combined Experimental and Computational Analysis

Abstract

Reactions of Au⁺(¹S,³D) and AuX⁺ with CH₃X (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⁺(¹S) exhibits, among others, halogen abstraction, HX elimination, and association. By comparison, Au⁺(³D) 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 CH₃Br resulted in association exclusively, whereas the AuI⁺/CH₃I and AuBr⁺/CH₃Br systems exhibited halogen loss as the dominant process. By contrast, all possible bimolecular pathways occurred in the reaction of AuBr⁺ with CH₃I. 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⁺/CH₃Y 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.