Linear free energy relationship and kinetic isotope effects as measures for the transition-state variation - A case of the neophyl system

Ab initio calculations at the MP2/6-3]G* level and density functional theory (B3LYP/6-31 I +G**) calculations have been performed on acid-catalyzed ionizations of substituted neophyl alcohols to investigate whether a variation of the transition-state (TS) structure is reflected in the kinetic isotope effects (KIE) and linear free energy relationship. The effect of substituents on KIEs, TS structures, and activation and reaction energies was calculated. This Study revealed that a Curved Br phi nsted-type Plot Could arise for a single-step process from the variation of TS structure with the substituent, whereas the Hammett plots with a dual-parameter treatment can not detect Such TS variation. The variation of KIEs at various positions of neophyl alcohol reflects the variation of TS structures in a manner consistent with the More O'Ferrall - Jencks type reaction diagram analyses.