Investigating the role of halogen-bonded complexes in microsolvated Y^-(H₂O)_n + CH₃I S_N2 reactions

The dynamics of bimolecular nucleophilic substitution (S_N2) reactions in the gas phase are of great interest and several new mechanisms have been observed recently by theoretical studies. Here we investigate a recent-discovered S_N2 reaction mechanism, called front-side complex (FSC) or halogen-bonded complex (XC) mechanism that couples the traditional front-side attack (FSA) and back-side attack (BSA) Walden-inversion mechanism. This XC-pathway begins with a front-side attack on the leaving group, then goes through a bending transition state (XTS) that is followed by Walden-inversion, and results in a configuration inverted product. We characterized the potential energy surface of the microsolvated Y^-(H₂O)_n=0,1,2 + CH₃I S_N2 reaction using the B97-1/ECP/d method, where Y = HO, F, Cl, Br, and I, and n is the number of water molecules. It is found that the XCs have a deeper well depth than the back-side attack (BSA) pre-reaction complexes for HO^-/F^- nucleophiles, indicating that the system can easily become trapped in the halogen-bonded complex well. The barriers of both FSA- and BSA-paths increase with incremental solvation, whereas the change of XTS depends on the type of nucleophile. When Y = HO/F, the order of the barriers is BSA < XC < FSA for n = 0-2, and the order inverts to XC < BSA < FSA for n = 1 (Y = Br/I) and n = 2 (Y = Cl/Br/I), where the latter suggests an increasing participation of the halogen-bonded complex in the S_N2 reactions. Comprehensive analyses on the structure, charge distribution, and energetics of XC and XTS are provided. This work indicates that the halogen-bonded complex mechanism may be common for alkyl iodides and the information on the potential energy surface is useful in understanding the dynamics behavior of the title and analogous reactions.