MD study of SN1 reactivity of 2-chloro-2-methylpropane in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate.

The S(N)1 ionization reaction RX --> R(+) + X(-) for 2-chloro-2-methylpropane in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate ([emim](+)P(-)(6)) is studied via molecular dynamics computer simulations. By employing a two-state valence-bond description for electronic structure variations of the reaction complex, the free energy curve relevant to its SN1 ionization in [emim](+)P(-)(6)) is computed via the thermodynamic integration method and compared with those in water and in acetonitrile. It is found that the detailed reaction mechanism differs among the three solvents. To be specific, the dissociation of 2-chloro-2-methylpropane in [emim](+)P(-)(6)) is a stepwise process consisting of the formation of a solvent-separated ion pair and ensuing dissociation, while that in acetonitrile appears to proceed without any stable reaction intermediates. The S(N)1 pathway in water on the other hand is characterized by the formation of a contact ion pair, followed by dissociation to free ions. The activation free energy in water is much lower than those in [emim](+)P(-)(6)) and acetonitrile. Between the two latter solvents, the barrier height is lower in [emim](+)P(-)(6)) than in acetonitrile, indicating that the S(N)1 reactivity of 2-chloro-2-methylpropane would be higher in [emin](+)P(-)(6)) than in acetonitrile. Its implication for solvolysis in these solvents is briefly discussed.