A THEORETICAL-STUDY OF THE RELATIONSHIP BETWEEN SECONDARY ALPHA-DEUTERIUM KINETIC ISOTOPE EFFECTS AND THE STRUCTURE OF S(N)2 TRANSITION-STATES

The reactant and transition-state structures for several S(N)2 reactions between different nucleophiles and methyl and ethyl chloride and fluoride have been calculated at the HF/6-13+G* level. The secondary alpha-deuterium kinetic isotope effects for these reactions were calculated with Sim's BEBOVIB-IV program. The results demonstrate that the magnitude of these isotope effects is determined by an inverse stretching vibration contribution and a normal bending vibration contribution to the isotope effect. The stretching vibration contribution to the isotope effect is essentially constant for each substrate while the bending vibration contribution varies with the nucleophile and the looseness of the S(N)2 transition state. Thus, the out-of-plane bending vibration model for relating the magnitude of secondary alpha-deuterium kinetic isotope effects to transition-state structure is correct. The bending vibration contribution to the isotope effect is greater in the ethyl substrate reactions than in the methyl substrate reactions. As a result, larger isotope effects and looser transition states are found for the S(N)2 reactions of larger substrates. Looser transition states and larger isotope effects are also observed for the S(N)2 reactions with softer nucleophiles.