Separation of isotropic and anisotropic hyperfine constants in disordered systems by analysis of electron paramagnetic resonance lines at two different microwave frequencies: Application to the molecular structure around excess electrons in γ-irradiated 10M sodium hydroxide alkaline ice glass

Hyperfine interactions of narrow-line trityl radical with solvent molecules

The electron nuclear dipolar interactions responsible for some dynamic nuclear polarization (DNP) mechanisms also are responsible for the presence formally in CW EPR spectra of forbidden satellite lines in which both the electron spin and a nuclear spin flip. Such lines arising from (1)H nuclei are easily resolved in CW EPR measurements of trityl radicals, a popular family of DNP reagents. The satellite lines overlap some of the hyperfine features from (13)C in natural abundance in the trityl radical, but their intensity can be easily determined by simple simulations of the EPR spectra using the hyperfine parameters of the trityl radical. Isotopic substitution of (2)H for (1)H among the hydrogens of the trityl radical and/or the solvent allows the dipolar interactions from the (1)H on the trityl radical and from the solvent to be determined. The intensity of the dipolar interactions, integrated over all the (1)H in the system, is characterized by the traditional parameter called reff. For the so-called Finland trityl in methanol, the reff values indicate that collectively the (1)H in the unlabeled solvent have a stronger integrated dipolar interaction with the unpaired electron spin of the Finland trityl than do the (1)H in the radical and consequently will be a more important DNP route. Although reff has the dimensions of distance, it does not correspond to any simple physical dimension in the trityl radical because the details of the unpaired electron spin distribution and the hydrogen distribution are important in the case of trityls.

Measuring Ti(III)-carotenoid radical interspin distances in TiMCM-41 by pulsed EPR relaxation enhancement method

Interspin distances between the Ti(3+) ions and the carotenoid radicals produced inside TiMCM-41 pores by photoinduced electron transfer from 7'-apo-7'-(4-carboxyphenyl)-beta-carotene (coordinated to Ti(3+)), canthaxanthin (formed as a random distribution of isomers), and beta-ionone (model for a short-chain polyene) to Ti(3+) framework sites were determined using the pulsed EPR relaxation enhancement method. To estimate the electron transfer distances, the temperature dependence of relaxation rates was analyzed in both siliceous and metal-substituted siliceous materials. The phase memory times, T(M), of the carotenoid radicals were determined from the best fits of two-pulse ESEEM curves. The spin-lattice relaxation times, T(1), of the Ti(3+) ion were obtained from the inversion recovery experiment with echo detection on a logarithmic time scale in the temperature range of 10-150 K. The relaxation enhancement for the carotenoid radicals in TiMCM-41 as compared to that in MCM-41 is consistent with an interaction between the radical and the fast relaxing Ti(3+) ion. For canthaxanthin and beta-ionone, a dramatic effect on the carotenoid relaxation rate, 1/T(M), occurs at 125 and 40 K, respectively, whereas for carboxy-beta-carotene 1/T(M) increases monotonically with increasing temperature. The interspin distances for canthaxanthin and beta-ionone were estimated from the 1/T(M) - 1/T(M0) difference, which corresponds to the Ti(3+) contribution at the temperature where the maximum enhancement in the relaxation rate occurs. Determination of the interspin distances is based on calculations of the dipolar interaction, taking into consideration the unpaired spin density distribution along the 20-carbon polyene chain, which makes it possible to obtain a fit over a wider temperature interval. A distribution of the interspin distances between the carotenoid radical and the Ti(3+) ion was obtained with the best fit at approximately 10 A for canthaxanthin and beta-ionone and approximately 9 A for 7'-apo-7'-(4-carboxyphenyl)-beta-carotene with an estimated error of +/-3 A. The interspin distances do not depend on 1/T(M) - 1/T(M0) for carboxy-beta-carotene which shows no prominent peak in the relaxation rate over the temperature range measured.

The Structure of the Hydrated Electron. Part 1. Magnetic Resonance of Internally Trapping Water Anions: A Density Functional Theory Study

Density functional theory is used to rationalize magnetic parameters of hydrated electron trapped in alkaline glasses as observed using electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) spectroscopies. To this end, model water cluster anions (n=4-8 and n=20, 24) that localize the electron internally are examined. It is shown that hyperfine coupling tensors of H/D nuclei in the water molecules are defined mainly by the cavity size and the coordination number of the electron; the water molecules in the second solvation shell play a relatively minor role. An idealized model of the hydrated electron (that is usually attributed to L. Kevan) in which six hydroxyl groups arranged in an octahedral pattern point toward the common center is shown to provide the closest match to the experimental parameters, such as isotropic and anisotropic hyperfine coupling constants for the protons (estimated from ESEEM), the second moment of the EPR spectra, and the radius of gyration. The salient feature is the significant transfer (10-20%) of spin density into the frontal O 2p orbitals of water molecules. Spin bond polarization involving these oxygen orbitals accounts for small, negative hyperfine coupling constants for protons in hydroxyl groups that form the electron-trapping cavity. In Part 2, these results are generalized for more realistic geometries of core anions obtained using a dynamic one-electron mixed quantum/classical molecular dynamics model.