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.