Sequential activation of methane by Ir^+: An IRMPD and theoretical investigation

The sequential activation of up to 4 CH_4 molecules by Ir^+ is investigated through a gas-phase infrared multiple photon dissociation (IRMPD) experiment and theoretical calculations. A molecular beam apparatus was used to generate Ir^+ by laser ablation and expose it to controlled amounts of CH_4. Product ions were irradiated with IR light from a free electron laser over the 500–1800 cm^(−1) spectral range and photodissociation was monitored using a time-of-flight mass spectrometer. Experimental spectra were obtained for five distinct species: [Ir,3C,8 H]^+, [Ir,3C,10 H]^+, [Ir,4C,10 H]^+, [Ir,4C,12 H]^+, and [Ir,O,3C,12 H]^+. To identify these species, B3LYP/def2-TZVPPD geometry optimizations were performed on a variety of possible structures, with computed IR spectra compared to the experimental IRMPD spectra. This has led to the following assignments: [Ir,3C,8 H]^+ = IrCH_2(CH_3)_2^+, [Ir,3C,10 H]^+= HIr(CH_3)_3^+, [Ir,4C,10 H]^+ = Ir(CH_3)_2(C_2H_4)^+, [Ir,4C,12 H]^+ = a mixture of HIr(CH_3)(C_2H_4)^+(CH_4), HIr(CH_3)_2(C_2H_5)^+, Ir(CH_3)_4^+, and (H_2)Ir(CH_3)_2(C_2H_4)^+, and [Ir,O,3C,12 H]^+ = (H_2O)HIr(CH_3)_3^+. Notably, evidence for C-C coupling is observed upon reaction with a fourth methane. Mechanisms for the formation of the observed products were also explored computationally by examining the reaction coordinate pathways for the reactions of methane with HIrCH^+, IrCH_2^+, Ir(CH_3)_2^+, HIrCH_2(CH_3)^+, HIr(CH_3)_3^+, and IrCH_2(CH_3)_2^+.