The role of electron exchange in cidep

An alternative vector model of chemically induced electron polarization is introduced and a scheme for exhibiting the role of exchange is described.     

Self-diffusion and electron spin exchange of hydrophobic probes in dilute solution

Electron spin exchange rate constants have been measured by ESR spectroscopy for a nitroxide spin probe in a number of solvents, including water. The apparent collision rate constants (k _c ^′ ) calculated from the spin exchange rate constants showed marked deviations from the Smoluchowsky equation (k _c ^′ η=const), which were greatest in solvents of lowest viscosity. These effects are attributed to inefficiency of the spin exchange process. Self-diffusion coefficients (D) were measured for diamagnetic analogs of the nitroxide spin probe in similar solvent systems by pulsed field gradient NMR spectroscopy. TheD values gave reasonable agreement when corrected for viscosity (Dη=const). Collision rate constants calculated fromD were in good agreement with those measured by ESR in solvents of high viscosity. Thek _c ^′ value for the spin probe in water was significantly lower than that in isoviscous organic solvents. This effect is discussed in terms of a hydrophobic hydration shell for the spin probe which acts as an additional barrier to collision.     

Pressure effects on the electron spin relaxation of several radicals in solution

The pressure effect on the decay rate of chemically induced dynamic electron spin polarization (CIDEP) was investigated on several free-radical intermediates in photolysis, and the spin-lattice relaxation times for these radicals were estimated from the decay rates of CIDEP signals at various pressures. The spin-lattice relaxation rates were retarded by increasing external pressure. From the pressure dependence of the spin-lattice relaxation rates the activation volume was estimated. The activation volumes of these radicals divide into two groups; ≈30 cm³ mol⁻¹ for negative ions and ≈10 cm³ mol⁻¹ for neutral radicals.     

Nuclear spin relaxation study of aqueous raffinose solution in the presence of a gadolinium contrast agent

Paramagnetic enhancement of nuclear spin-lattice relaxation rates (PREs) was measured in aqueous solution of the trisaccharide raffinose in the presence of a gadolinium(III) complex, GdDTPA-BMA, used as a magnetic resonance imaging contrast agent. The relaxation enhancement of aqueous protons was measured over a broad range of magnetic fields, using field-cycling apparatus in addition to conventional spectrometers. The nuclear magnetic relaxation dispersion profile thus obtained was interpreted with a recently developed model, allowing for both inner- and outer-sphere relaxation. The relaxation enhancement for the carbon-13 nuclei in raffinose was studied under high-resolution conditions at three magnetic fields, whereas the sugar proton PRE was measured at two fields. The PRE of the sugar nuclei could be interpreted in a consistent way, assuming that it was caused by the outer-sphere mechanism. The electron spin relaxation was found to be a less important source of modulation of the electron-nuclear dipole-dipole interaction than the mutual translational diffusion.     

Nuclear spin relaxation in benzyl fluoride : v- 19f intra and intermolecular relaxation of benzyl fluoride in solution

¹⁹F relaxation times of benzyl fluoride in acetone-d₆ and in methanol-d₄ were measured and extrapolated at infinitely dilute solution. The fluorine relaxes through intramolecular dipole-dipole (DDa) and spin-rotation (SR) mechanisms in acetone-d₆ ,and through DDa,SR and inter-molecular dipole-dipole mechanisms in methanol-d₄. The DDa contribution can be recalculated from the overall and internal reorientational motions through ²D measurements on the same solutions.The separation of the different contributions are consistent with those made on the proton relaxation times and correlates more closely with poor solvation of benzylfluoride in acetone-d₆ and with greater solvation in methanol-d₄.     

Photochemical and cidep studies of the reactions of naphthalene and 2,6-Di-tert-butyl-p-cresol in solution

The photoreaction of the solution system of naphthalene and 2,6-di-tert-butyl-p-cresol was investigated by a combination of optical spectroscopy, ESR and time-resolved CIDEP techniques. ESR signal of 2,6-di-tert-butyl-4-methylphenoxy radical was detected during 308 nm irradiation. Three different types of polarization mechanisms contribute to the CIDEP of the phenoxy radicals. In the initial stage RPM with singlet precursor predominates, whereas RPM with F-pairtnplet precursor persists at later stage. The contribution from TM is superimposed during all the time regions but to a small extent. Both the CIDEP intensity at the initial stage as well as the depletion rate of naphthalene during photolysis were found to be proportional to the square of the light intensity, which indicates that most phenoxy radicals were produced via a biphotonic process.     

A Markov model for relaxation and exchange in NMR spectroscopy

A two-state Markov noise process for lattice fluctuations and chemical exchange dynamics is used to derive a stochastic Liouville equation describing the evolution of the spin-density operator in nuclear magnetic resonance spectroscopy. Relaxation through lattice fluctuations and chemical exchange processes is incorporated into the theory at the same fundamental level, and the results are valid for all time scales provided that lattice fluctuations are much faster than chemical exchange kinetics. Time-scale separation emerges as an essential feature from the lowest-order perturbation expansion of the average resolvent in the Laplace domain.     

A model for thermal spin relaxation in rotating ferromagnetic clusters

A model for thermal spin relaxation in isolated ferromagnetic clusters is proposed and investigated theoretically by means of nonequilibrium thermodynamics. It is shown that thermal agitation mediates relaxation of the spin towards the total angular momentum vector of the cluster so that the clusters are magnetically polarized in the direction of their rotational axis. A relaxation mechanism via thermal moment of inertia fluctuations is proposed. The results are discussed for a Fe₁₂₅-cluster.     

Fast photodriven electron spin coherence transfer: a quantum gate based on a spin exchange j-jump

Photoexcitation of the electron donor (D) within a linear, covalent donor-acceptor-acceptor molecule (D-A(1)-A(2)) in which A(1) = A(2) results in sub-nanosecond formation of a spin-coherent singlet radical ion pair state, (1)(D(+?)-A(1)(-?)-A(2)), for which the spin-spin exchange interaction is large: 2J = 79 ± 1 mT. Subsequent laser excitation of A(1)(-?) during the lifetime of (1)(D(+?)-A(1)(-?)-A(2)) rapidly produces (1)(D(+?)-A(1)-A(2)(-?)), which abruptly decreases 2J 3600-fold. Subsequent coherent spin evolution mixes (1)(D(+?)-A(1)-A(2)(-?)) with (3)(D(+?)-A(1)-A(2)(-?)), resulting in mixed states which display transient spin-polarized EPR transitions characteristic of a spin-correlated radical ion pair. These photodriven J-jump experiments show that it is possible to use fast laser pulses to transfer electron spin coherence between organic radical ion pairs and observe the results using an essentially background-free time-resolved EPR experiment.     

Coupled motional model for spin relaxation in 5CB liquid crystal

Using the master equation method we examine the effects of correlated internal rotations in an alkyl chain on its deuterium spectral densities of motion. The overall reorientation of molecules is coupled to the internal motions in their alkyl end chains by the use of conformation-dependent diffusion axes. The pentyl chain that is attached to a cylindrical biphenyl core in 4-n-pentyl-4′-cyanobiphenyl is studied and the results are compared with the previously published decoupled diamond-lattice model.     

Proton spin relaxation in a smectic liquid crystal

A proton spin relaxation study in the liquid crystal ethyl-[(methoxybenzylidene)-amino] cinnamate is presented. A “phase change” is observed at ≈ 103°C within the smectic A phase. Some liquid-like mobility exists below this temperature.     

Electron spin resonance studies of spin-labelled polymers—I segmental relaxation of polystyrene in solution

From an analysis of the widths of the hyperfine lines in the ESR spectrum of a nitroxidelabelled polystyrene in toluene solution, the correlation times for rotational diffusion at three temperatures have been measured. The values agree well with published data from NMR studies of polystyrene in solution. The value of the activation energy for the relaxation process, 4·3 kcal. mole⁻¹, is close to published values for dielectric relaxation of para-substituted polystyrenes. These comparisons provide strong evidence for associating the ESR correlation times with some form of segmental motion of the polymer backbone.     

A study of the proton exchange in tissue water by spin relaxation in the rotating frame

The exchange rate of the water protons in mouse tissues was determined for the first time. The process was studied by proton spin relaxation in the rotating frame. The exchange correlation time was found to be ≈5 × 10^?6 sec. The actual residence time of a water proton on one water molecule in tissue was estimated to be ≈ 10^?6 sec. It is shown that because of the fast exchange the estimate (≈10%) of the amount of ordered water is too low.     

Molecular motion in some radiolabelled dipeptides: a muon spin relaxation study

Activation parameters were determined for the dynamics of radicals formed by muonium addition to glycylglycine (GlyGly; H₃N⁺CH₂CONHCH₂CO₂^?) and the doubly protected alanylalanine derivative [Boc‐AlaAla‐Bz; Bu^tOCONHCH(Me)CONHCH(Me)CO—O—CH₂Ph]. GlyGly forms an adduct by muonium addition to the amide carbonyl group which isomerizes by flipping the muon between opposite sides of the molecule, requiring an activation energy of 20.4 kJ mol^?1. In Boc‐AlaAla‐Bz, muonium addition to the benzene ring of the benzyl (—CH₂Ph) group occurs, exhibiting an activation energy of 9.4 kJ mol^?1, believed to be from torsion about the C—Ph bond. Copyright © 2002 John Wiley & Sons, Ltd.