Activation of Methane by MH+ (M = Fe, Co, and Ni): A Combined Mass Spectrometric and DFT Study

Abstract

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 n - 1 4s 1 electron configuration for their metal centers but a 3d 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) 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 n - 1 -4s 1 and the 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.