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 T M 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 T M 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 10 4 and 10 8 M -1 s -1 , a range covered by the alkenes examined.

Addition Rate Constants of Phosphorus- and Carbon-Centered Radicals to Double Bond of Monomers as Studied by a Pulsed Electron Paramagnetic Resonance Method.

“…The procedures employed were based on the method reported by Weber and Turro. 34 The phase memory time, T M *, of a target radical was measured by the EPR apparatus under pulsed-microwave operation as follows. Electron spin echo (ESE) of the target radical was generated by a two-pulse sequence initiated at t ns after the laser flash (laser flash − t − π/2 − τ − π − τ − echo).…”

“…10,31−33 Moreover, Weber and Turro reported that a pulsed EPR technique enables us to determine rate constants of addition reactions between initiator radicals and monomers. 34 In their procedures, the rate constants were determined by analyzing the linear relationship between monomer concentration and phase memory time of the reacting radical, the latter of which can be measured with high precision by the electron spin−echo detection method. 34 In the present study, the mechanisms for addition reactions of initiator radicals to various monomers were studied by the pulsed EPR spectroscopic method along with quantum chemical calculations for enthalpy and polar effects.…”

“…34 In their procedures, the rate constants were determined by analyzing the linear relationship between monomer concentration and phase memory time of the reacting radical, the latter of which can be measured with high precision by the electron spin−echo detection method. 34 In the present study, the mechanisms for addition reactions of initiator radicals to various monomers were studied by the pulsed EPR spectroscopic method along with quantum chemical calculations for enthalpy and polar effects. Four different parent molecules were chosen as sources of initiator radicals for addition reaction: three of them produce carboncentered radicals, and the other one gives a phosphoruscentered radical.…”

Rate constant measurements for initial addition reactions of radicals at the propagation step of photo-polymerization as studied by pulsed EPR spectroscopy

“…Photo‐polymerization is described by the following schemes, R · + Mon R‐(Mon) 1 · (the first step) R‐(Mon) 1 · + Mon R‐(Mon) 2 · (the second step) R‐(Mon) n − 1 · + Mon R‐(Mon) n · (chain growth) and the early stage focused here is the first and the second steps. The rate constant k 1 for initiator radical R · at the first step were determined by using various spectroscopic methods such as photo‐acoustic spectroscopy, EPR, and transient absorption method . However, as for the time‐resolved spectroscopic study on the rate constant k 2 for the second step, there is no literature to the best of our knowledge.…”

“…The T M * can be measured with high precision through the two‐pulse electron spin echo (ESE) technique and therefore, k value can be determined from monomer concentration dependence of T M * −1 . This method has been applied to determine rate constants for addition reactions between initiator radicals and some monomers …”