Computational investigations of the gas phase reactions between hydrogen chloride and protonated alkyl chlorides

Ab initio calculations have been carried out for the gas phase identity nucleophilic substitution reactions between HCl and the protonated alkyl chlorides, MeClH+, EtClH+, (PrClH+)-Cl-i, and (BuClH+)-Cl-t. Geometries of stationary points along the reaction coordinate have been optimized at the B3LYP/6-31G(d), MP2/6-31G(d) and MP4/6-31G(d) level of theory. Additional single point energy calculations based on the MP2 optimized geometries are carried out at the full MP4 (SDTQ) level and improved energies have been calculated with the MP2/6-31++G(d,p). The potential energy profiles for both nucleophilic substitution (S(N)2) and elimination (E2) pathways have been investigated. The C-Cl bond for the tertiary butyl system is significantly longer than the other alkyl groups. Calculated potential energy barrier of the backside (S(N)2(B)) substitution reactions for HCl/RClH+ system decreases in the order: Me > Et > Pr-i > Bu-t. The frontside substitution (S(N)2(F)) reaction barrier is higher than the backside substitution (S(N)2(B)) reaction barriers, but the difference between the S(N)2(F) and S(N)2(B) becomes smaller when the number of methyl substituents increases. Elimination reactions (E2) are thermodynamically unfavorable. (c) 2007 Elsevier B.V. All rights reserved.