Theoretical and experimental studies of CIDNP kinetics in recombination of radical pairs by the method of switched external magnetic field. III. Free radicals in homogeneous solution

Abstract

The method of chemically induced dynamic nuclear polarization in a switched external magnetic field (SEMF CIDNP) is applied here for the first time to an experimental study of short-lived neutral radicals in homogeneous solutions. With three photochemical reactions it is exemplified, that SEMF CIDNP allows investigations of the kinetics of the transient species with high time-resolution as well as a determination of their spin relaxation times in low magnetic fields. A theoretical approach is developed, which permits simulation and analysis of the experimental data. In weak magnetic fields (0.5–2.0 mT) the effective spin-lattice relaxation times for the decay of the chemically induced spin polarizations in benzyl, tert-butyl, and 2-hydroxy-2-propyl radicals are found to be T1=(3.8±0.5) μs, T1=(7.8±0.5) μs, and T1=(2.5±0.5) μs, respectively, in benzene solution at room temperature. They are in fair agreement with relaxation times determined by time-resolved X-band electron paramagnetic resonance spectroscopy at strong magnetic fields (≈350 mT).

Electron spin relaxation by spin-rotation interaction in benzoyl and other acyl type radicals

“…In particular, for several methyl-containing compounds with hindered rotation of the CH 3 group Ellis et al [37] found a nearly linear relationship between TI,SR and the potential barrier V 0 of the rotation from 13C relaxation measurements at fixed temperature (38~ T1,sR = 25.61 (1 + 0.382 V0), (5) where T1.sR is given in seconds and V 0 is the barrier height in kilocalories per mole, determined by microwave spectroscopic methods. The relation is remarkable in view of the variety of compounds involved, with reported barriers as high as 5.4 kcal/mol.…”

“…For example, the dependence of the electron spin relaxation rate due to the modulation of the hfi anisotropy is proportional to the solvent viscosity, whereas it is inversely proportional to the viscosity in the case of relaxation due to spin-rotation interaction [3]. Recently, it has been shown that in going to very low magnetic fields the electron spin relaxation due to the modulation of the hfl anisotropy should go to zero [4], whereas electron spin relaxation due to spinrotation interaction should increase twice [5,6]. Thus, knowledge of the dominating electron spin relaxation mechanism is of great importante in the data analysis.…”

Switched external magnetic field CIDNP studies of coupling reaction of carbon-centered radicals with TEMPO

“…13,16,18,19 However, the time resolution of traditional CIDNP is limited by the duration of rf-pulse in NMR and as a rule cannot exceed 1 ms. A modified version of TR CIDNP has been developed recently. [20][21][22][23] The technique is called CIDNP in a switched external magnetic field (SEMF CIDNP) and it allows the measurements of rate constants of radical reaction with a much higher time resolution while keeping the information about reaction products. In this method, 20 after a reaction has been initiated with a laser pulse, CIDNP is formed during a variable time delay t in a first (low) magnetic field B 1 of a separate magnet, and then, after switching, CIDNP formation continues in a second (low) field B 2 for a fixed time t 0 .…”

“…The method has been successfully applied to study recombination of radical pairs in micellar solutions, 21 and to investigate short-lived neutral radicals and ion-radical pairs in homogeneous solutions. 22 This paper deals with SEMF CIDNP measurements of the rate constants of the coupling of several alkyl radicals (Scheme 1) with the persistent nitroxyl radical TEMPO (2,2,6,6-tetramethylpiperidinyl-N-oxyl). Chemical behaviour, reaction rate constants and ESR parameters of radicals shown in Scheme 1 are known for the most part.…”

Chemically Induced Dynamic Nuclear Polarization during the Thermolysis of Alkoxyamines: A New Approach to Detect the Occurrence of H-Transfer Reactions

“…[11][12][13][14]. Previously, Chemically Induced Dynamic Electron Polarization (CIDEP) [15,16] and CIDNP and its modification (CIDNP with fast switching of magnetic field) have been used in NMP research to measure the rate constants of coupling between acrylate-type radicals and the persistent nitroxide [17,18]. In those experiments, the carbon centered radicals under investigation were generated by the photolysis of ketone precursors in the presence of persistent nitroxides.…”