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 reactio ns between different nucleophiles and methyl and ethyl chloride and fl uoride have been calculated at the HF/6-13+G level. The secondary alp ha-deuterium kinetic isotope effects for these reactions were calculat ed with Sim's BEBOVIB-IV program. The results demonstrate that the mag nitude 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 isot ope effect is essentially constant for each substrate while the bendin g vibration contribution varies with the nucleophile and the looseness of the S(N)2 transition state. Thus, the out-of-plane bending vibrati on model for relating the magnitude of secondary alpha-deuterium kinet ic isotope effects to transition-state structure is correct. The bendi ng vibration contribution to the isotope effect is greater in the ethy l substrate reactions than in the methyl substrate reactions. As a res ult, larger isotope effects and looser transition states are found for the S(N)2 reactions of larger substrates. Looser transition states an d larger isotope effects are also observed for the S(N)2 reactions wit h softer nucleophiles.