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

Abstract

Abstract. In va¡ studies of the spin dynamics in radical pairs, benzoyl-type radicals have been one of the two paramagnetic pair species. Their electron spin relaxation has been assumed to be slow enough to be neglected in the data analysis. This assumption is checked by measuring the electron spin relaxation in a sequence of three acyl radicals (benzoyl, 2,4,6-trimethylbenzoyl and hexahydrobenzoyl) by time:resolved electron paramagnetic resonance spectroscopy. In contrast to the assumed slow relaxation, rather short spin-lattice relaxation times (100--400 ns) ate found for benzoyl and 2,4,6-trimethylbenzoyl radicals from the decay of the integral initial electron polarization to thermal equilibrium at different temperatures and viscosities. The relaxation is induced by a spin-rotation coupling arising from two different types of radical movements: overall rotation of the whole radical and hindered internal rotation of the CO group. The predominant second contribution depends on the barrier of the internal rotation. The obtained results are well explained in the frame of Bull's theory when using a modified rotational corretation time r~ The size of the spin-rotation coupting due to the internal CO group rotation in benzoyl radicals is estimated to be I C~l = 1510 MHz. I n t r o d u c t i o nThe electron spin relaxation rate is one of the major parameters which determine mechanism and size of magnetic spin effects in photochemically initiated radical reactions, especially in viscous and micelle solutions [1,2]. The theoretical analysis of magnetic, isotope as well as CIDNP magnetic field dependences requires knowledge of the mechanism of electron spin relaxation for the radicals under study. The solvent viscosity and temperature dependences of the various possible relaxation mechanisms (due to modulation of hyperfine interaction [hfi] and g-tensor anisotropy, spin-rotation interaction, dipole-dipole interaction, etc.) are substantially different. 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

Flexible Biradicals in Liquid and Supercritical Carbon Dioxide: The Exchange Interaction, the Chain Dynamics, and a Comparison with Conventional Solvents

“…This suggests that the rate of appearance of the bis(alkyl) spectrum (by decarbonylation) or the kinetics of disappearance of the acyl−alkyl spectrum (chemical reaction or spin relaxation 23 ) are different in the two solvents. A solvent-dependent decarbonylation rate has been observed before in conventional solvents for radicals of similar structure, as have different spin relaxation rates in acyl radical centers in different solvents . When the temperature is raised to the supercritical region (>31 °C), spectra due to biradical 1b become observable in both solvents at 0.8 μs, as shown in Figure D.…”

Strongly Entangled Triplet Acyl–Alkyl Radical Pairs in Crystals of Photostable Diphenylmethyl Adamantyl Ketones

“…An alternative mechanism based on coupled electron–nuclear spin flips determined by hyperfine coupling (hfc) interactions are known to be effective when the S-T gap is relatively small, at relatively long inter-radical distances. Spin-flipping mechanisms caused by the dynamic modulation of magnetic fields through spin–lattice relaxation requires fast molecular dynamics, such as spin-rotation interactions and solvent motion, , which are ineffective under the rigidity of the crystal lattice. Finally, while it may be expected that the most efficient ISC mechanism for a triplet radical pair in a crystalline solid will be based on spin–orbit coupling (SOC), it is anticipated that the collinear arrangement of the two singly occupied orbitals will be unfavorable, as there is a limited change in angular momentum mediated by electron exchange and/or mixing of zwitterionic states. − While there is much work to be done to ascertain the initial polarization, spin sublevel equilibration, and relative contributions of the different ISC mechanisms, it is clear that the restrictions imposed by the crystal lattice are able to maintain these spin systems for much longer times in comparison to any other medium.…”

Addition of Benzoyl Radicals to Butyl Acrylate: Absolute Rate Constants by Time-Resolved EPR

“…Moreover, changes in the spin-spin relaxation times can be neglected within this viscosity range. 25,29 The relative macroscopic kinematic viscosity (determined by capillary viscosimetry) drastically increases at butyl acrylate concentrations > 1.25 M (Figure 5). In parallel, the linear relationship between the chemical lifetime of the benzoyl radical and acrylate concentration is altered: The reactivity of the benzoyl radical toward butyl acrylate is reduced by ∼1 order of magnitude at high viscosity.…”

“…Addition of TEMPO (a stable nitroxide radical which can be utilized for the measurement of microscopic mobilities) to the reaction solution did not lead to any anisotropic features of its three-line EPR spectrum, revealing that the local mobility of the radicals is still unaffected (TEMPO is of compatible size as the photoinitiator radicals). Moreover, changes in the spin−spin relaxation times can be neglected within this viscosity range. , The relative macroscopic kinematic viscosity (determined by capillary viscosimetry) drastically increases at butyl acrylate concentrations > 1.25 M (Figure ). In parallel, the linear relationship between the chemical lifetime of the benzoyl radical and acrylate concentration is altered: The reactivity of the benzoyl radical toward butyl acrylate is reduced by ∼ 1 order of magnitude at high viscosity.…”