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Electron paramagnetic resonance measurements for a 10 B-containing heterocyclic phenanthrenedione radical, (C 6 F 5 ) 2 B(O 2 C 14 H 8 ), were made at X-band in 9:1 toluene:dichloromethane from 10 to 293 K and in toluene from 180 to 293 K. In well-deoxygenated 0.1 mM toluene solution at room temperature hyperfine couplings to 10 B, four pairs of protons and five pairs of fluorines contribute to a continuous wave spectrum with many resolved lines. Hyperfine couplings were adjusted to provide the best fit for spectra of the radical enriched in 10 B and the analogous radical synthesized with 10,11 B in natural abundance, resulting in small refinements of the hyperfine coupling constants previously reported for the natural abundance sample. Electron spin relaxation rates at temperatures between 15 and 293 K were similar for samples containing 10 B and natural isotope abundance. Analysis of electron spin echo envelope modulation and hyperfine correlation spectroscopy data at 80 K found A xx  = −7.5 ± 0.3, A yy  = −8.5 ± 0.3, and A zz  = −10.8 ± 0.3 MHz for 11 B, which indicates small spin density on the boron. The spin echo and hyperfine spectroscopy data for the 10 B –containing radical are consistent with the factor of 2.99 smaller hyperfine values for 10 B than for 11 B. [ABSTRACT FROM AUTHOR]

The effects of methyl rotation on electron spin–lattice relaxation times were examined by pulsed electron paramagnetic resonance for the major radicals in γ-irradiated polycrystalline α-amino isobutyric acid, dimethyl-malonic acid, and l-valine. The dominant radical is the same in irradiated dimethyl-malonic acid and α-amino isobutyric acid. Continuous wave saturation recovery was measured between 10 and 295 K at S-band and X-band. Inversion recovery, echo-detected saturation recovery, and pulsed electron–electron double resonance (ELDOR) data were obtained between 77 and 295 K. For the radicals in the three solids, recovery time constants measured by the various techniques were not the same, because spectral diffusion processes contribute differently for each measurement. Hyperfine splitting due to the protons of two methyl groups is resolved in the EPR spectra for each of the samples. Pulsed ELDOR data were obtained to characterize the spectral diffusion processes that transfer magnetization between hyperfine lines. Time constants were obtained for electron spin–lattice relaxation (T1e), nuclear spin relaxation (T1n), cross-relaxation (Tx1), and spin diffusion (Ts). Between 77 and 295 K rapid cross-relaxation (ΔMs=±1, ΔMI=∓1) was observed for each sample, which is attributed to methyl rotation at a rate that is approximately equal to the electron Larmor frequency. The large temperature range over which cross-relaxation was observed suggests that methyl groups in the radical and in the lattice, with different activation energies for rotation, contribute to the rapid cross-relaxation. Activation energies for methyl and amino group rotation between 160 and 1900 K (1.3–16 kJ/mol) were obtained by analysis of the temperature dependence of 1/T1e at S-band and X-band in the temperature intervals where the dynamic process dominates T1e. [Copyright &y& Elsevier]

Dephasing of two-pulse electron spin echoes for 0.1-0.4mM solutions of nitroxyl radicals in glassy solvents at 11K and 40K is dominated by nuclear spins in the solvent. Below about 70 K the rate of dephasing is the same for 4-oxo-2,2,6,6-tetramethylpiperin-1-yl (tempone) and for Fremy's salt (peroxylaminedisulphonate). In solvents that do not contain methyl groups the rate constant for echo dephasing increases linearly with increasing proton concentration. The shape of the echo decay curve could be simulated with a model based on dephasing due to proton flip-flops with a rate constant of 30 5HzM 1. At the same total proton concentration the electron spin echo decay rate is faster in solvents that contain methyl groups than in solvents without methyl groups. Several observations indicate that the effect of the methyl groups in the solvent on the echo dephasing is due to quantum mechanical tunnelling: (a) the echo dephasing is approximately independent of temperature between 11K and 40K, (b) the apparent spin flip-flop rate, Wm, for the methyl protons is independent of the concentration of methyl protons, (c) the estimated values of Wm for the methyl protons increase in the order hindered aliphatics < less hindered aliphatics < aromatic methyls, which is the order of decreasing barriers to classical rotation and therefore increasing tunnelling frequency, and (d) dephasing due to fluorines in CF3 groups is comparable with that for non-methyl protons. These results indicate that the nitroxyl spin echo dephasing can be used to probe the proton environment of the radical. [ABSTRACT FROM AUTHOR]

1,3-Alternate calix[4]arene with para-phenylene spacers connecting nitroxide monoradicals and high-spin (S= 1) diradicals provides tetraradical and octaradical scaffolds that possess conformations with slow electron spin relaxation rates (1/T1). Such scaffolds may facilitate tuning of relaxation rates that are more favorable for MRI or DNP applications. [ABSTRACT FROM AUTHOR]

Between 65 and 175 K, nitroxyl radicals with spirocyclohexyl groups at the 2- and 6-positions of the piperidine ring exhibit spin echo dephasing rates that are slower than for nitroxyl radicals with 2,5-gem-dimethyl or 2,6-gem-dimethyl substituents that are currently used as spin labels, and are slow enough to permit DEER measurements at temperatures up to about 125 K for spin labels with analogous ring structures. [ABSTRACT FROM AUTHOR]

Multiharmonic electron paramagnetic resonance spectroscopy was demonstrated for two samples with both narrow and broad lines: (i) α,γ-Bisdiphenylene-β-phenylallyl (BDPA) with Δ B pp of 0.85 G plus ultramarine blue with Δ B pp of 17 G, and (ii) a nitroxide radical immobilized in sucrose octaacetate. Modulation amplitudes up to 17 G at 41 kHz were generated with a rapid scan coil driver and Litz wire coils that provide uniform magnetic field sweeps over samples with heights of 5 mm. Data were acquired with a 2-D experiment in the Xepr software through the transient signal path of a Bruker E500T and digitized in quadrature with a Bruker SpecJet II. Signals at the modulation frequency and its harmonics were calculated by digital phase-sensitive detection. The number of harmonics with signal intensity greater than noise increases as the ratio of the modulation amplitude to the narrowest peak increases. Spectra reconstructed by the multiharmonic method from data obtained with modulation amplitudes up to five times the peak-to-peak linewidths of the narrowest features have linewidths that are broadened by up to only about 10% relative to linewidths in spectra obtained at low modulation amplitudes. The signal-to-noise improves with increasing modulation amplitude up to the point where the modulation amplitude is slightly larger than the linewidth of the narrowest features. If this high a modulation amplitude had been used in conventional methodology the linewidth of the narrowest features would have been severely broadened. The multiharmonic reconstruction methodology means that the selection of the modulation amplitude that can be used without spectral distortion is no longer tightly tied to the linewidth of the narrowest line. [ABSTRACT FROM AUTHOR]

Abstract: The Frank polyphase sequence has been applied to pulsed EPR of triarylmethyl radicals at 256MHz (9.1mT magnetic field), using 256 phase pulses. In EPR, as in NMR, use of a Frank sequence of phase steps permits pulsed FID signal acquisition with very low power microwave/RF pulses (ca. 1.5mW in the application reported here) relative to standard pulsed EPR. A 0.2mM aqueous solution of a triarylmethyl radical was studied using a 16mm diameter cross-loop resonator to isolate the EPR signal detection system from the incident pulses. [Copyright &y& Elsevier]

The radicals in six 60 Co γ-irradiated solids: malonic acid, glycylglycine, 2,6 di-t-butyl 4-methyl phenol, l -alanine, dimethyl malonic acid, and 2-amino isobutyric acid, were studied by rapid scan electron paramagnetic resonance at L-band (1.04 GHz) using a customized Bruker Elexsys spectrometer and a locally-designed dielectric resonator. Sinusoidal scans with widths up to 18.2 mT were generated with the recently described coil driver and Litz wire coils. Power saturation curves showed that the rapid scan signals saturated at higher powers than did conventional continuous wave signals. The rapid scan data were deconvolved and background subtracted to obtain absorption spectra. For the same data acquisition time the signal-to-noise for the absorption spectra obtained in rapid scans were 23–37 times higher than for first-derivative spectra obtained by conventional continuous wave electron paramagnetic resonance. [ABSTRACT FROM AUTHOR]

An algorithm is derived and demonstrated that reconstructs an EPR spectral--spatial image from projections with arbitrarily selected gradients. This approach permits imaging wide spectra without the use of the very large sweep widths and gradients that would be required for spectral--spatial imaging with filtered back projection reconstruction. Each projection is defined as the sum of contributions at the set of locations in the object. At each location gradients shift the spectra in the magnetic field domain, which is equivalent to a phase change in the Fourier-conjugate frequency domain. This permits solution of the problem in the frequency domain. The method was demonstrated for 2D images of phantoms consisting of (i) two tubes containing 14N and 15N nitroxide and (ii) two tubes containing a pH sensitive trityl radical at pH 7.0 and 7.2. In each case spectral slices through the image agree well with the full spectra obtained in the absence of gradient. [ABSTRACT FROM AUTHOR]

Abstract: X-band rapid-scan EPR spectra were obtained for dilute aqueous solutions of nitroxyl radicals 15N-mHCTPO (4-hydro-3-carbamoyl-2,2,5,5-tetra-perdeuteromethyl-pyrrolin-1-15N-oxyl-d12) and 15N-PDT (4-oxo-2,2,6,6-tetra-perdeuteromethyl-piperidinyl-15N-oxyl-d16). Simulations of spectra for 15N-mHCTPO and 15N-PDT agreed well with the experimental spectra. As the scan rate is increased in the rapid scan regime, the region in which signal amplitude increases linearly with B 1 extends to higher power and the maximum signal amplitude increases. In the rapid scan regime, the signal-to-noise for rapid-scan spectra was about a factor of 2 higher than for unbroadened CW EPR, even when the rapid scan spectra were obtained in a mode that had only 4% duty cycle for data acquisition. Further improvement in signal-to-noise per unit time is expected for higher duty cycles. Rapid scan spectra have higher bandwidth than CW spectra and therefore require higher detection bandwidths at faster scan rates. However, when the scan rate is increased by increasing the scan frequency, the increase in noise from the detection bandwidth is compensated by the decrease in noise due to increased number of averages per unit time. Because of the higher signal bandwidth, lower resonator Q is needed for rapid scan than for CW, so the rapid scan method is advantageous for lossy samples that inherently lower resonator Q. [Copyright &y& Elsevier]

Abstract: In rapid scan EPR the magnetic field is scanned through the signal in a time that is short relative to electron spin relaxation times. Previously it was shown that the slow-scan lineshape could be recovered from triangular rapid scans by Fourier deconvolution. In this paper a general Fourier deconvolution method is described and demonstrated to recover the slow-scan lineshape from sinusoidal rapid scans. Since an analytical expression for the Fourier transform of the driving function for a sinusoidal scan was not readily apparent, a numerical method was developed to do the deconvolution. The slow scan EPR lineshapes recovered from rapid triangular and sinusoidal scans are in excellent agreement for lithium phthalocyanine, a trityl radical, and the nitroxyl radical, tempone. The availability of a method to deconvolute sinusoidal rapid scans makes it possible to scan faster than is feasible for triangular scans because of hardware limitations on triangular scans. [ABSTRACT FROM AUTHOR]

The validity of the popular point-dipole approximation for interpretation of the zero-field splitting (ZFS) parameter (0-value) in EPA spectroscopy is studied. This approximation is of central importance for the determination of distances by analysis of EPR data. In this work, a detailed experimental (EPR spectroscopy and X-ray crystallography) and theoretical study for a model system (2,2',5,5'-tetra(tert-butyl)- 4,4'-bis(ethoxy-carbonyl)-3,3'-bipyrrolyl-1,1'-dioxyl) was performed to understand the scope and limitations of the point-dipole model in EPA spectroscopy. For this diradical, the radical-radical distance derived with the point-dipole approximation deviates significantly (by ∼40%) from the results derived from the X-ray analysis. Explicit quantum chemical calculation of the D-value on the basis of B3LYP density functional calculations leads to excellent quantitative agreement with the measured D-value. The quantitative accuracy of the employed methodology was confirmed for two additional systems that have previously been experimentally characterized. We therefore analyzed the contributions to the D-value of the target system in detail. This analysis leads to insight into the reasons for the failure of the point-dipole approximation. The analysis was then extended to an in silico study of five classes of model systems. Linkers of varying length and bond saturation were introduced between the radical-carrying groups. This allows for the analysis of the distance dependence of the D-parameter as well as the through-bond and through-space spin-spin interaction. From these results we established the limitations of the point-dipole approximation. The results of this analysis demonstrate that even very modest amounts of spin delocalization can cause significant deviations from pure point-dipole behavior and consequently cause the EPA derived distances to deviate from the N-O midpoint distance by up to several angstroms. If unsaturated linkers are used, the distance dependence of D does not follow the inverse cubic behavior predicted by the point-dipole model. However, for commonly used nonaromatic nitroxide rings connected by a saturated linker, the point-dipole approximation works well. Among the various point-dipole variants tested in this work for delocalized spins, the most successful one is based on distributed point-dipoles with spin populations derived from quantum chemical calculations. The distance dependence of the isotropic Heisenberg exchange parameter J has also been studied theoretically. The decay was found to be monoexponential with a decay constant of ∼ 1 Å-1. Thus at linker lengths between 6-8 carbon atoms between a nitroxide radical pair, a switch from the strong to the weak exchange limit is predicted. [ABSTRACT FROM AUTHOR]