Activation of methane by MH ⁺ (M = Fe, Co, and Ni): A combined mass spectrometric and DFT study

A mass spectrometric method is used to study the reaction of MH ⁺ (M = Fe, Co, and Ni) with methane to form MCH ₃ ⁺ and H ₂ over a wide temperature range from 80 to 850 K. The reaction energy barriers are measured to be 11.7, 1.9, and <0 kcal/mol for Fe, Co, and Ni, respectively. However, the exothermicities of the reactions are close for Fe, Co, and Ni: 5.4, 2.3, and 5.4 kcal/mol, respectively. Density functional theory (DFT) calculations are carried out to complement the experimental observations. The DFT calculations indicate that both the MH ⁺ reactant and the MCH ₃ ⁺ product prefer to have a 3d ^{n - 1} 4s ¹ electron configuration for their metal centers but a 3d ⁿ configuration for the metal center in its transition state, MHHCH ₃ ⁺; consequently, a crossing between high-spin (3d ^{n - 1}4s ¹) and low-spin (3d ⁿ) potential energy surfaces (PESs) takes place at both the entrance and the exit channels of the reaction. The calculated activation energies of 14.3, 4.7, and -1.7 kcal/mol are in good agreement with the experiments. The differences in the activation energies are ascribed to the differences in the energy separation between the 3d ^{n - 1}4s ¹ and the 3d ⁿ states for Fe ⁺, Co ⁺, and Ni ⁺. It costs an additional 3.1 kcal/mol for the Fe ⁺ center to alter its electron configuration from the FeH ⁺ reactant to the MHHCH ₃ ⁺ transition state; however, Co ⁺ and Ni ⁺ benefit from the change of the electron configurations by 5.7 and 13.9 kcal/mol, respectively.