Variable temperatures 1H NMR technique on the kinetics and thermodynamic parameters of equilibrium between the Z and E isomers in a stable phosphorus ylide involving an imidazole

Abstract

Analysis of ¹H NMR spectra of a special ylide involving an imidazole at variable temperatures has been employed to study the kinetics of the interchangeable process in the equilibrium between the two Z- and E-isomers of a stable phosphorus ylide. Activation data (k₋₁, k₁, k_total, E_a1, E_a−1, E_atotal, ΔH^‡, ΔG^‡, ΔS^‡) and thermodynamic parameters (K_e, ΔH°, ΔG° and ΔS°) for the forward and reverse steps of the opposing reaction were obtained by means of the variable temperature ¹H NMR technique and T-jump method. The ¹H NMR results indicate that the interchangeable process of rotational isomers follows first-order kinetics with respect to both the forward and reverse reactions. The process was also entropy controlled.