Richard D Bates

Polarization of solvent nuclei by nitroxide spin labels at low magnetic fields

Abstract The application of nitroxide spin labels in low-field dynamic nuclear polarization studies of solvent-solute interactions is examined. Data obtained by saturating the nitroxide EPR line at a field-frequency combination determined after employing the Breit-Rabi formalism are most satisfactory provided low radical concentrations are used. Ratio method determinations of ultimate enhancements are appropriate in most cases, although application of an exchange-based model by using data on the broadening of spectra as the spin-label concentration is increased may provide a means of determining ultimate enhancements when the ratio method is not applicable. Studies with several nitroxide radicals show that similar species give polarization results that do not change when substituents far removed from the localized unpaired electron are changed. Perfluoropyridine is shown to exhibit increased scalar-dominated polarization with more aromatic nitroxides as has been observed for hexafluorobenzene; the dipolar-dominated enhancements for both monofluorobenzene and 2-fluoropyridine are also similar.

Medium effects on ESR spectra in studies of hydrogen-bonded transient solvent-solute complexes

The effect of solvent polarity on the observed hyperfine coupling constants of nitroxide radicals has been investigated both in the presence of and in the absence of hydrogen donors. With inert solvents, the observed hyperfine coupling constant increases linearly with (ϵ - 1)/(ϵ + 1). With hydrogen donors, the observed hyperfine coupling constants are much greater, in part because of the dielectric constant of the medium and in part because of the equilibrium between free and complexed radicals. Correcting for the effect of the dielectric constant of the medium on the observed hyperfine coupling constants in the determination of association constants of transient complexes is illustrated for the donors C3H7OH, C4H9OH, CF3CH2OH, C6H5OH, (CH3)3COH, and CHCl3. Association constants for these donors with TMPO at room temperature in benzene are found to be in the range from 0.68 to 2.04 M−1.

Characterization of silica surfaces by free radical spin labels

Abstract Silanol groups on the surface of various silicon oxide materials have been labeled for esr study by bonding or adsorption of free radicals. The number of bonded radicals was correlated with the relative concentrations of vicinal and isolated silanols. When the vicinal silanols have been removed by treatment above 600°C, the labeling was found to be quantitative and complete. The improved label attachment in the latter case indicates that steric crowding is not a consideration, but that the resistance of vicinal silanols to labeling reagents is responsible for limited labeling at low temperatures. The ratio of trichlorosilane di-labels to dichlorosilane mono-labels was observed to approach the theoretical limiting value 2.0 above 600°C, and the variation of this ratio may be a basis for quantitative determinations under conditions of incomplete labeling. In contrast to covalently bonded labels, adsorbed labels were found to attach to both vicinal and isolated silanols, and observed silanol concentrations are closely comparable to the bonded label values above 600°C. Labeling susceptibility of the silica surface was found to be drastically modified by pretreatment with inorganic and organometallic reagents. The potential usefulness of spin-labeling in electronics was illustrated by labeling of silanols in quartz surfaces (piezoelectric resonators), on oxidized silicon (MOS integrated circuits) and in evaluation of various surface passivation techniques.

Effects of partially fluorinated hydrogen donors on ESR spectra in studies of hydrogen-bonded transient solvent-solute complexes

Abstract The effects of five protonated and eight partially fluorinated hydrogen donors on observed hyperfine coupling constants of TMPO have been determined. Correction of the observed splitting for changes in the polarity of the medium leads to values of the hyperfine coupling constants for the donor:radical complex and of the association constants for complex formation that follow the trend in acidities of the donors. The behavior of partially fluorinated donors, which give more pronounced relaxation effects in NMR studies and more varied polarization effects in dynamic nuclear polarization studies, is consistent with the trend in acidities of the donors. Association constants range from 0.68 ± 0.04 M −1 for n -C 4 H 9 0H with TMPO to 6.20 ± 0.01 M −1 for C 3 F 7 COOH with TMPO at room temperature.

Dynamic polarization of 7Li nuclei in solutions containing radical ions

Dynamic nuclear polarization parameters, obtained at 75 G, are reported for 7Li ions in collision with several radical anions and with one radical cation. All systems show large negative 7Li N.M.R. enhancements indicative of weak scalar relaxation. However, radical induced relaxation rates derived from 7Li T 1 measurements suggest stronger complexing of lithium ions with radical anions than with the radical cation as would be expected from simple coulombic considerations. Translational modulation of the dipolar interaction best accounts for proton and radical cation dipolar relaxation rates while rotational modulation best accounts for the corresponding radical anion rates; this supports the interpretation above. A model for the lithium radical collision is proposed which implies that, for radical anions, scalar coupling is only apparently weak and that the low 7Li scalar relaxation rates observed result from scalar correlation times (τc = 10-8-10-9 s) longer than any yet observed by this technique. The m...

Effects of nitroxide free radicals on proton donor chemical shifts

Abstract Chemical-shift changes in samples containing the proton donors CF 3 CH 2 OH and C 6 F 5 OH and a nitroxide free radical have been measured. The values of the association constants for the donor:radical complexes determined from them are smaller than those measured by observing the changes in hyperfine coupling constants in the EPR spectrum of the same radical:donor systems because the chemical-shift measurements are sensitive to all interactions between the radical and donor solvent molecule nuclei, not just those arising from the formation of the transient hydrogen-bonded complex.

Effects of inert cosolvents on hydrogen bonding between nitroxide radicals and proton donors

Abstract The effects of inert (non-hydrogen donor) cosolvents on transient complex formation between a nitroxide radical and two hydrogen donors are examined by EPR methods. For both C 6 H 5 OH and CF 3 CH 2 OH the complex association constants are lower and the radical-donor hyperfine coupling constants are higher in more polar cosolvents. The results are interpreted by using a model based on donor acidity and the role of the cosolvent in solvating each of the species in solution.

Spin density delocalization from a stable free-radical ligand onto diamagnetic metal ions

The novel air-stable, free-radical di-imine ligand 4,4,5,5-tetramethyl-2-(2-pyridyl)-Δ2–imidazoline-1-oxyl forms stable 1 : 1 complexes with bivalent, Zn, Cd, and Hg; upon complexation, e.s.r. ligand hyperfine couplings increase from 4·25 G to 5·2 G for the co-ordinating imine nitrogen, decrease from 9·25 G to 7·6 G for the nitroxide nitrogen, and metal hyperfine splitting of 9·6 G and 46 G is observed due to I=½ isotopes of Cd and Hg, respectively.