Direct Dynamics Simulation of Dissociation of the [CH3‑‑I--OH]− Ion–Molecule Complex

Direct dynamics simulations were used to study dissociation of the [CH3--I--OH]− complex ion, which was observed in a previous study of the OH– + CH3I gas phase reaction ( J. Phys. Chem. A 2013, 117, 7162 ). Restricted B97-1 simulations were performed to study dissociation at 65, 75, and 100 kcal/mol and the [CH3--I--OH]− ion dissociated exponentially, in accord with RRKM theory. For these energies the major dissociation products are CH3I + OH–, CH2I– + H2O, and CH3OH + I–. Unrestricted B97-1 and restricted and unrestricted CAM-B3LYP simulations were also performed at 100 kcal/mol to compare with the restricted B97-1 results. The {CH3I + OH–}:{CH2I– + H2O}:{CH3OH + I–} product ratio is 0.72:0.15:0.13, 0.81:0.05:0.14, 0.71:0.19:0.10, and 0.83:0.13:0.04 for the restricted B97-1, unrestricted B97-1, restricted CAM-B3LYP, and unrestricted CAM-B3LYP simulations, respectively. Other product channels found are CH2 + I– + H2O, CH2 + I–(H2O), CH4 + IO–, CH3 – + IOH, and CH3 + IOH–. The CH3 – + IOH singlet products are only given by the restricted B97-1 simulation and the lower energy CH3 + IOH– doublet products are only formed by the unrestricted B97-1 simulation. Also studied were the direct and indirect atomic-level mechanisms for forming CH3I + OH–, CH2I– + H2O, and CH3OH + I–. The majority of CH3I + OH– were formed through a direct mechanism. For both CH2I– + H2O and CH3OH + I–, the direct mechanism is overall more important than the indirect mechanisms, with the roundabout like mechanism the most important indirect mechanism at high excitation energies. Mechanism comparisons between the B97-1 and CAM-B3LYP simulations showed that formation of the CH3OH---I– complex is favored for the B97-1 simulations, whereas formation of the HO–---HCH2I complex is favored for the CAM-B3LYP simulations. The unrestricted simulations give a higher percentage of indirect mechanisms than the restricted simulations. The possible role of the self-interaction error in the simulations is also discussed. The work presented here gives a detailed picture of the [CH3--I--OH]− dissociation dynamics and is very important for unraveling the role of [CH3--I--OH]− in the dynamics of the OH–(H2O) n=1,2 + CH3I reactions.