Theoretical Studies of Competitive Gas-Phase SN2 and E2 Reactions of NCCH2CH2Cl with OH- and SH-

Abstract

The gas-phase reactions of OH- and SH- with NCCH2CH2Cl have been studied with ab initio methods using the 6-31+G* basis sets. The electron correlation effect was accounted for at the MP2 level of theory. Both the SN2 and E2(anti) reactions are exothermic and have negative activation barriers when Cl- is the leaving group. Overall, E2 is preferred to SN2, and E2(anti) is favored over E2(gauche) for both OH- and SH-, indicating the preference of an antiperiplanar transition state in the gas phase. Since SH- is a weaker gas-phase nucleophile than OH-, the transition state occurs later on the reaction coordinate. The results of this work are consistent with those of gas-phase experimental studies which indicated that first-row nucleophiles undergo both SN2 and E2 reactions, whereas second-row nucleophiles have lower abilities to induce elimination reactions. Substitution of a CN group for a β-hydrogen of CH3CH2Cl to NCCH2CH2Cl is found to enhance both the SN2 and E2 reactivities and makes the β-H-elimination become more E1cb-like. Incorporation of the electron correlation effect at higher levels, MP3, MP4D, MP4DQ, and MP4SDQ, gives essentially the same order of reactivities as found at the MP2 level, except the preference of SN2 over E2(anti) with SH- at the higher level of theory.