A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes,

Abstract

A time-resolved Fourier transform electron spin resonance (TR FT ESR) method for determining absolute rate constants of reactive, free radical reactions is described. The phase memory times TM of a family of reacting radicals were measured precisely by electron spin−echo (ESE) detection. In all cases the ESE signal decays show a simple pseudo-first-order behavior. Rate constants of the reactions were extracted from the linear relationship between the reciprocal of the TM values and the alkene concentrations. The validity of the method is demonstrated for the measurement of the rate constant for addition of different phosphinoyl and 2-hydroxy-2-propyl radicals to several acrylates, methyl methacrylate, and vinyl pivalate in organic solvents at room temperature. The results are in excellent agreement with those obtained by other kinetic investigations employing time-resolved steady-state electron spin resonance (TR CW ESR) and optical absorption spectroscopy. The technique is shown to be applicable to systems with rate constants between 104 and 108 M-1 s-1, a range covered by the alkenes examined.