Computational insights into CH3MX (M = Cu, Ag and Au; X = H, F, Cl, Br and I)

Abstract

The C–X bond activation products, CH₃MX (M = Cu, Ag and Au; X = H, F, Cl, Br and I) formed by the insertion of coinage metal atoms into C–X bonds of methane and halomethanes, were investigated by density functional theory (DFT). Equilibrium geometries, harmonic vibrational frequencies, and energies were calculated. Bader's atoms-in-molecule (AIM), natural population charge (NPA) and fuzzy bond orders (FBO) calculations were performed to investigate the bonding interactions in CH₃MX. As X varies from F to I, the thermodynamic stability of CH₃MX with respect to CH₃X + M increases, and the order of the thermodynamic stability for different coinage metals is CH₃CuX > CH₃AuX > CH₃AgX. Although the CH₃MX (M = Cu, Ag and Au; X = Cl, Br and I) were predicted to be more stable thermodynamically than the others (e.g. CH₃MH and CH₃MF) observed in matrix isolation experiments, they have not been identified experimentally yet, and one of the probably key reasons is that their vibrational fingerprints (ν_C–M and ν_M–X) are so low that they are beyond the detection limit of an infrared spectrometer. AIM analyses show that both C–M and M–X bonds in CH₃MX exhibit mainly closed-shell interaction character, and partial covalent character contributes to them. The BCP of M–H bond just locates at the boundary between the charge concentration region and the charge depletion region, which lead to the covalent character of M–H bond being overestimated by the AIM topological parameters.