Magnetic field and spin effects from sequential p-type and d-type triplet mechanisms (original) (raw)
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The Journal of Physical Chemistry A, 1998
Chemically induced dynamic electron polarization (CIDEP) generated through interaction of the excited triplet state of 1-chloronaphthalene, benzophenone, benzil, and Buckminsterfullerene (C 60 ) with 2,2,6,6,-tetramethyl-1-piperidinyloxyl (TEMPO) radical was investigated by using time-resolved ESR spectroscopy. We carefully examined what factors affect the CIDEP intensities. By comparing CIDEP intensities of TEMPO in the 1-chloronaphthalene, benzophenone, and benzil systems with that obtained in the C 60 -TEMPO system, the absolute magnitude of net emissive polarization was determined to be -2.2, -6.9, and -8.0, respectively, in the units of Boltzmann polarization. In the 1-chloronaphthalene-TEMPO system, the viscosity effect on the magnitude of net polarization was studied by changing the temperature (226-275 K) in 2-propanol. The emissive polarization was concluded to result from the state mixing between quartet and doublet manifolds in a radical-triplet pair induced by the zero-field splitting interaction of the counter triplet molecule. The magnitude of net polarization is much larger than the polarization calculated with the reported theory that the CIDEP is predominantly generated in the region where the exchange interaction is smaller than the Zeeman energy. Our experimental results are quantitatively explained by the theory that the CIDEP is generated predominantly in the regions where the quartet and doublet levels cross. We propose a theoretical treatment to calculate the magnitude of net polarization generated by the level crossings in the radical-triplet pair mechanism under highly viscous conditions and perform a numerical analysis of the net RTPM polarization with the stochastic-Liouville equation. The viscosity dependence of the net polarization indicates that the back transition from the doublet to quartet states sufficiently occurs in the level-crossing region under highly viscous conditions. The estimated large exchange interaction suggests that the quenching of the excited triplet molecules by TEMPO proceeds via the electron exchange interaction. †
Influence of weak magnetic fields on radical-pair reactions
Bulletin of the Russian Academy of Sciences: Physics, 2009
The time dependences of the populations of radical-pair spin states and transition amplitudes between them have been calculated for constant and pulsed magnetic fields, with allowance for hyperfine interaction. The above-mentioned characteristics have been found directly by solving the Liouville equation for the radical-pair spin density matrix. It is established that the relaxation of spin-level partial populations oscillates with a monotonic decrease in the total population.
Magnetic field effects on the dynamic behavior of a radical pair involving a germyl radical
Chemical Physics Letters, 1991
The hydrogen abstraction reaction of triplet 4-methoxy-benzophenone with thiophenol at 265 K was studied by a sub-nanosecond laser flash photolysis technique in the absence and presence of a magnetic field of 1.75 T. The decay rate of the generated radical pair at zero field was found to be 1.1 = 10 9 s y1 and that at 1.75 T to be 1.8 = 10 9 s y1 . The observed magnetic field effect can be explained by the acceleration of the triplet-singlet spin conversion process due to the D g mechanism. q 2000 Published by Elsevier Science B.V. ) Corresponding 0009-2614r00r$ -see front matter q 2000 Published by Elsevier Science B.V.
The Journal of Physical Chemistry A, 1997
The magnetic field dependence of the chemically induced dynamic electron polarization (CIDEP) spectra of spin-correlated radical pairs (SCRPs) was studied by time-resolved EPR at three different external magnetic fields, i.e., X band (9.2 GHz, 330 mT), S band (3.0 GHz, 100 mT), and L band (1.5 GHz, 50 mT). The CIDEP spectra were obtained by the photolysis of three systems, xanthone and 2,6-di-tert-butylphenol (2,6-DBP) in a sodium dodecyl sulfate (SDS) micelle solution, zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) and p-benzoquinone (p-BQ) in a cetyltrimethylammonium chloride (CTAC) micelle solution, and acetone in 2-propanol, at low temperatures. In the two micelle systems, the SCRP spectra scarcely depend on the external magnetic field, and the decay times do not change much on going from the X band to the L band. These observations are discussed in terms of the cage escape rate and the spin-lattice relaxation rate. The strong magnetic field dependence found for the net emissive polarization in the CIDEP spectra of the xanthone and 2,6-DBP system is ascribed to the magnetic field dependence of the triplet mechanism (TM). On the other hand, the intensity of the SCRP spectrum in the acetone system drastically decreases with decreasing the magnetic field. This observation is rationalized on the basis of the restricted motions of the SCRPs in the 2-propanol solution at low temperatures. † Present address:
The Journal of Chemical Physics, 1999
The method of 13 C chemically induced dynamic nuclear polarization in a switched external magnetic field ͑SEMF CIDNP͒ has been applied for the first time in an experimental investigation of micellized radical pairs ͑RP͒. Using the examples of three photochemical reactions it has been shown, that SEMF CIDNP allows the investigation of the kinetics of short-lived micellized RPs with high time-resolution in low and intermediate magnetic fields. The experimental kinetics have been analyzed and simulated on the basis of a previously developed theory ͓Parnachev et al., J. Chem. Phys. 107, 9942 ͑1997͔͒. It has been demonstrated that such an analysis provides information on the rates of radical escape from the micelle, on electron relaxation and on the rate of S -T Ϫ transitions. The analysis of the estimated rates of S -T Ϫ transitions showed that the exchange interaction is essentially anisotropic in the RPs studied.
Journal of the American Chemical Society, 2001
Possible radical reaction products issuing from H-atom addition to cytosine have been characterized and analyzed by means of a comprehensive quantum mechanical approach including density functional computations (B3LYP), together with simulation of the solvent by the polarizable continuum model (PCM), and averaging of spectroscopic properties over the most important vibrational motions. The hyperfine couplings of the semirigid 5,6-dihydrocytos-6yl radical computed at the optimized geometry are in good agreement with their experimental counterparts. On the other hand, vibrational averaging is mandatory for obtaining an effectively planar structure for the 5,6-dihydrocytos-5yl radical with the consequent equivalence of beta-hydrogens. Finally, only proper consideration of environmental effects restores the agreement between computed and experimental couplings for the base anion protonated at N3.
The CIDNP kinetics in recombination of successive radical pairs
Applied Magnetic Resonance, 2002
A theory of chemically induced dynamic nuclear polarization (CIDNP) formed in recombination of successive radical pairs (RPs) is developed. The theory is based on that of RP recombination with the spin Hamiltonian instantaneously changing in time. With kinematics approximation it is shown that general relations for CIDNP are fully expressed via the quadratures of Green functions, which characterize the molecular motion of reagents. Analytical formulae for the time dependence of CIDNP both of primary and secondary RPs have been derived in the strong magnetic field approximation (S-T 0 approximation); field dependences of stationary CIDNP effect for some model cases have been analyzed. For long-lived systems the sensitivity of secondary RP CIDNP to the singlet-triplet evolution of primary RP has been demonstrated. It is shown that sometimes the correct analysis of the effect calls for taking into account the reactivity anisotropy.