A Theoretical Investigation of a-Carbon Kinetic Isotope Effects and Their Relationship to the Transition-State Structure of SN2 Reactions

摘要

The transition structures and α-carbon ~(12)C/~(13)C kinetic isotope effects for 22 SN2 reactions between methyl chloride and a wide variety of nucleophiles have been calculated using the BlLYP/aug-cc-pVDZ level of theory. Anionic, neutral, and radical anion nucleophiles were used to give a wide range of SN2 transition states so the relationship between the magnitude of the α-carbon kinetic isotope effect and transition-state structure could be determined. The results suggest that the α-carbon ~(12)C/~(13)C kinetic isotope effects for SN2 reactions will be large (near the experimental maximum) and that the curve relating the magnitude of the KIE to the percent transfer of the α-carbon from the nucleophile to the leaving group in the transition state has a broad maximum. This means very similar KIEs will be found for early, symmetric, and late transition states and that one cannot use the magnitude of these KIEs to estimate transition-state structure.