DNP and CIDEP Study of Cross-Relaxation Processes in Short-Lived Radicals in Solution (original) (raw)
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Cross relaxation in free radicals with chemically induced electron and nuclear polarization
Molecular Physics, 1988
Electron-nuclear cross relaxation in free radicals with chemically induced nuclear polarization (CIDNP) is discussed in the fight of some unexpected chemically induced electron polarization (CIDEP) effects. Sign rules are deduced for the transfer of net CIDNP to net CIDEP and multiplet CIDNP to multiplet CIDEP. Experiments indicating that cross relaxation is not responsible for the unusual CIDEP observations are described.
Chemical Physics Letters, 1995
The kinetics of the nuclear polarization formed during the photolysis of acetone in isopropanol-d 8 were analyzed quantitatively. Model calculations show that the spin-selective recombination of radicals gives rise to the electron polarization and, with regard to the electron-nuclear cross-relaxation, are adequate to describe the formation of the net nuclear polarization of the reaction products even if the solution contains only one type of radical. For the 2-hydroxy-2-propyl radicals at a magnetic field of 7 T, fitting the theoretical results to the experimental data gives the electron relaxation time T~ = 1.0 +_ 0.2 I~s and the cross-relaxation time T x = 92 + 18 ~s. (CH3)2CO hv T , (CH3)2CO ° , T(CH3)2CO* + (OH)CH(CH3) 2 ' 2(CH3)2C(OH),
Electron Spin-Lattice Relaxation Processes of Radicals in Irradiated Crystalline Organic Compounds
The Journal of Physical Chemistry A, 2003
Electron spin-lattice relaxation times (T 1e ) for the major radicals in γ-irradiated polycrystalline samples of glycylglycine, L-alanine, 2,4,6-tri-tert-butyl-phenol, and 4-methyl-2,6-di-tert-butyl-phenol were measured as a function of temperature using pulsed EPR. CW-saturation recovery (CW-SR) were obtained at X-band (9.1 GHz) and S-band (3.0 GHz) between about 10 and 295 K. Inversion recovery, echo-detected saturation recovery (ED-SR), and pulsed electron-electron double resonance (ELDOR) curves were obtained at X-band between 77 and about 295 K. For 2,4,6-tri-tert-butyl-phenoxy radical, which has a single-line EPR spectrum, the recovery times obtained by the three methods were in good agreement and were assigned as T 1e . For the three radicals with resolved hyperfine splitting, spectral diffusion caused the recovery times observed by inversion recovery or ED-SR to be significantly shorter than T 1e obtained by CW-SR or ELDOR. Spectral diffusion processes were observed directly by pulsed ELDOR experiments, and time constants for cross relaxation and nuclear relaxation were obtained by modeling the ELDOR curves. For irradiated L-alanine and for the 4-methyl-2,6-tert-butyl-phenoxy radical at some temperatures, the effects of rapid cross relaxation on CW-SR curves could not be fully mitigated even by long saturating pulses, and T 1e could only be determined by ELDOR. For the radicals in γ-irradiated L-alanine, 2,4,6-tri-tert-butyl-phenol, and 4-methyl-2,6-di-tert-butyl-phenol, methyl group rotation makes significant contributions to T 1e at temperatures where the rate of rotation of a methyl group is comparable to the microwave frequency. Activation energies for methyl rotation were determined by modeling the temperature dependence of T 1e at X-band and S-band. In temperature ranges where methyl rotation did not dominate, T 1e was dominated by Raman, direct, or local mode processes.
Nuclear spin relaxation in free radicals as revealed in a stimulated electron spin echo experiment
Applied Magnetic Resonance, 2005
Electron spin echo envelope modulation (ESEEM) in a three-pulse stimulated echo experiment, when the time interval between the first and second pulses r is varied, is att¡ to a spontaneous change of the electron spin Larmor frequency in the time interval T between the second and third pulses, due to the longitudinal relaxation of nearby nuclei. It is observed for nitroxide radicals in glassy matrices in the temperature range of 130-240 K. Nuclear relaxation is assumed to arise from fluctuation of the proton hyperfine interaction, due to fast rotation of the methyl groups. This contribution to ESEEM and the conventional one that is induced by the simultaneous excitation of allowed and forbidden electron spin transitions were found to be multiplicative. As the latter does not depend on the time T, both contributions can be easily separated. The rate of nuclear spin re-[axation was determined, and corre[ation time of methyl group rotation was estimated by Redfield theory of spin relaxation. Arrhenius parameters of this motion were estimated on the basis of these data and those at 77 and 90 K, where the previously developed approach was used (
The Journal of Physical Chemistry, 1993
Time-resolved electron spin resonance (TR ESR) has been used to investigate the chemically induced dynamic electron polarization (CIDEP) generated by the interaction of stable free radicals with the triplet states of benzophenone, benzil, and 2-acetylnaphthalene. The stable radicals were mono-, di-, tri-, and tetranitroxyl free radicals possessing the 2,2,6,6-tetramethylpiperidineN-oxyl moiety. All of the stable radical systems investigated were found to be emissively polarized by interaction with the triplet states, and the phase of polarization was independent of the sign of zero-field splitting (D) of the interacting triplet molecule. Possibleand likely mechanisms of polarization transfer (creation) resulting from the interaction of photoexcited triplet molecules with nitroxyls in the strong electron exchange are discussed. The emissive CIDEP of nitroxyls observed in the interactions with triplet benzil, which has D > 0, provides strong support for the operation of the radical-triplet pair mechanism. Within the time scale of T R ESR experiments (-lO-'-lO" s) no significant variation in the shape of the CIDEP spectra of the nitroxyls was observed, either in viscous media or in micelles. It is concluded that intramolecular spin exchange (or conformational change) of polynitroxyls occurs much faster than the time resolution of the experiment,
Chemical Physics Letters, 1986
Two different mechanisms of creation of nuclear polarisation in radical reaction products are studied by means of the h&frequency field influence on intermediate radical pairs (stimulated nuclear polarisation, SNP) and on intermediate short-lived radicals (dynamic nuclear polarisation, DNP). Criteria are formulated for distinguishing the contributions of DNP and SNP effects, and experimental demonstration of this possibility is presented for photoinduced reactions of quinones.
Electron-nuclear spin transitions in free phosphonyl radicals detected via CIDNP
Applied Magnetic Resonance, 2004
Electron-nuclear spin transitions in short-lived phosphonyl radicals have been investigated experimentally by nuclear magnetic resonance detection of nuclear polarization in diamagnetic reaction products in low magnetic fields (15-80 mT) for 3~p-centered radicals formed in laser photolysis of (2,4,6-t¡ oxide and 2,4,£ acid dimethyl ester. A theoretical model on the basis of the numerical sotution of the kinetic equation for the density matrix of a radical with one nonzero hyperfine coupling constant has been employed to study the main peculiarities of this effect and to account for the data quantitatively.
Angewandte Chemie International Edition, 2010
Dynamic nuclear polarization (DNP) is attracting considerable attention as a method to increase NMR sensitivity. [1] Although the basic theory has been known for a long time, [2] the field is rapidly evolving with the introduction of new technologies as well as new radicals that enable new applications. The radical used as the polarizing agent determines the polarization-transfer mechanism. Supramolecular interactions take place between the radical, the glassing solvent, and the molecule undergoing polarization. However, understanding of the first events that occur in DNP experiments has been limited by the lack of structural diversity in the radicals used so far. Herein we introduce polychlorotriphenylmethyl (PTM) radicals as a new class of DNP polarizing agents. The presence of chlorine nuclei enables a different polarization mechanism to take place from that observed for other commonly used trityl radicals, such as OX63 (Scheme 1).
Chemical Physics, 1991
LF (low-field) SNP spectra are reported from the carbonyl carbon m dibenzyl ketone photolyzed m an aqueous micellar solution with sodmm dodecyl sulfate. Spectra obtained usmg resonant radtofrequencies of 0.33 and 1.53 GHz are affected by adtabahc transitions within the region of ST_ level-crossing of the radical pair and by fltp-flop transtttons in the corresponding weak magnetic fields. Numerical simulation indicates that all the observable features of the spectra are accounted for by a combmation of these transittons and spin relaxation induced by the electron spm exchange interaction. Unexpectedly, no additional broadening caused by the electron dipole-dipole or other anisotroptc interacttons is evident in them.
Chemical Physics Letters, 1998
The low magnetic field electron-nuclear resonance transitions, detected via changes in the nuclear polarization of radical reaction products, have been investigated. Due to the mixing of the wavefunctions in low magnetic fields such transitions are allowed as electron resonance transitions but at the same time lead to significant changes of nuclear polarization. The photolysis of 2,4,6-trimethylbenzoylphosphonic acid dimethyl ester has been used as a model reaction. It proceeds via the Ž. intermediate dimethoxyphosphonyl radical which has a large hyperfine constant 69.6-70 mT. It has been shown that this effect can be applied for the investigation of low field electron and nuclear polarization. q 1998 Elsevier Science B.V.