Radical recombination following the photodissociation of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone in isotropic solution was monitored using time-resolved infrared spectroscopy. The rate of radical recombination was determined to decrease in the presence of a magnetic field of greater than 5 mT and to increase in the presence of a magnetic field smaller than 5 mT.
The recombination reactions of free radicals formed from the photolysis of a series of polymerisation photoinitiators were studied using time-resolved infrared spectroscopy. All molecules showed Zeeman magnetic field effects (MFEs) in the field range 0-37 mT and those molecules that produced radical pairs with average hyperfine couplings greater than 5 mT showed substantial inverted field effects at fields of less than 10 mT (so-called low field effects, LFEs). Monte Carlo simulations with full treatment of all the isotropic hyperfine couplings in the spin Hamiltonian reproduced well the observed field effects. The use of the usual analysis based on the calculated B1/2 value for the radical pair was found to be inappropriate in systems with substantial LFEs, but simple correlations between this B1/2 value and the observed field features were established.
A rapidly switched (<10 ns) magnetic field was employed to directly observe magnetic fields from f-pair reactions of radical pairs in homogeneous solution. Geminate radical pairs from the photoabstraction reaction of benzophenone from cyclohexanol were observed directly using a pump-probe pulsed magnetic field method to determine their existence time. No magnetic field effects from geminate pairs were observed at times greater than 100 ns after initial photoexcitation. By measuring magnetic field effects for fields applied continuously only after this initial geminate period, f-pair effects could be directly observed. Measurement of the time-dependence of the field effect for the photolysis of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone in cyclohexanol using time-resolved infrared spectroscopy revealed not only the presence of f-pair magnetic field effects but also the ability of the time dependence of the MARY spectra to observe the changing composition of the randomly encountering pairs throughout the second order reaction period.
A robust macrocyclic tetraamide complex of Th(IV) binds phosphate diesters and promotes cleavage of RNA and phosphate diesters at 37 °C in the pH range 5.00-7.90.
Magnetic field effects on the photolysis of homogeneous solutions containing (2,4,6-trimethylbenzoyl)diphenylphoshine oxide, MAPO, and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, BAPO, were studied using time-resolved infrared spectroscopy. The two molecules display distinctly different field dependences in conflict with established photochemistry. Time-resolved EPR was employed to examine the photochemistry in detail, resulting in the detection of previously unobserved radical species when BAPO was photoexcited in alcoholic solvents. Plausible reaction mechanisms were used to suggest candidate species that may be responsible for the new EPR signals. DFT calculations were then used to evaluate the likelihood of formation of these species and to estimate their hyperfine coupling constants for comparison with the recorded spectral data. The most likely identities of the new species are a two-coordinate phosphorus radical anion for the species with an observed hyperfine coupling of 2.9 mT and a four coordinate phosphorus centred radical for the species with the large 49.8 mT coupling.
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