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.