An unusually large secondary α-deuterium kinetic isotope effect in hydride ion SN2 reactions

Theoretical calculations at the HF/6-31+G* level suggest the secondary alpha-deuterium kinetic isotope effects for hydride ion S(N)2 reactions are much larger than expected for the structure of the transition state. The secondary alpha-deuterium kinetic isotope effects for the S(N)2 reactions between sodium borohydride (hydride ion) and para-methyl- and para-chlorobenzyl chlorides are much larger than expected as the theoretical calculations suggest. It appears the secondary alpha-deuterium isotope effects are larger than expected for the structure of the S(N)2 transition state because the hydride ion is too small to affect the C-alpha-H(D) out-of-plane bending vibrations in the transition state.