Pulsed Electron Nuclear Double Resonance Research Articles

Time-domain chirp electron nuclear double resonance spectroscopy in one and two dimensions

It is demonstrated by theory and experiment that time-domain broadband electron nuclear double resonance (ENDOR) spectroscopy can be performed by using radio frequency chirp pulses. The chirp ENDOR experiment is shown to have a sensitivity comparable to that of established pulsed ENDOR experiments while featuring at the same time a higher resolution. Two pulsed schemes are proposed for chirp ENDOR that are related to the Davies- and Mims-ENDOR experiments. A simple two-step phase cycle can remove the contributions from unwanted coherence transfer pathways and, unlike in time-domain ENDOR with pulses of fixed radio frequency, no quadrature readout is needed. The basic chirp ENDOR experiment can easily be extended by applying concepts from electron spin echo envelope modulation and nuclear magnetic resonance spectroscopy. A novel hyperfine sublevel correlation (HYSCORE) experiment based on chirp ENDOR is proposed.

Compound ES of Cytochrome c Peroxidase Contains a Trp .pi.-Cation Radical: Characterization by Continuous Wave and Pulsed Q-Band External Nuclear Double Resonance Spectroscopy

The fully oxidized state of cytochrome c peroxidase (CcP), called ES, contains two oxidizing equivalents, one as an oxyferryl heme and the other as an organic radical on an amino acid residue. The unusual electron paramagnetic resonance spectrum of ES has been shown to be due to a weak distributed exchange coupling between the two paramagnetic redox centers. Various residues have been proposed as the radical site over the years. In this paper continuous wave and pulsed Q-band electron nuclear double resonance (ENDOR) spectroscopy confirms that the radical is located on Trp-191, as previously proposed. The paper completes the characterization of the active site of compound ES as being comprized of an oxyferryl heme coupled to the Trp-191 {pi}-cation radical by a weak spin exchange. 47 refs., 11 figs., 2 tabs.

Nature and dynamics of radicals in polyaramide as studied by pulsed electron nuclear double resonance**

Chain scission is one of the factors which leads to the failure of high‐performance materials based on main‐chain liquid crystalline polymers. Stable radical species are formed wich can be studied using the ENDOR technique, thus throwing some light on the degradation mechanism, dynamics and kinetics in a range of commercial polymers

Application of Pulsed ENDOR to the Study of Radicals in a Liquid-Crystalline Copolyester

Pulsed electron nuclear double-resonance (ENDOR) techniques and pulsed triple-resonance experiments are used to study radicals in a liquid-crystalline copolyester. In accordance with the commonly observed heterogeneity of polymers, the ENDOR spectrum is quantitatively analyzed in terms of a heterogeneous distribution of similar radicals with different hyperfine couplings. Furthermore, the nuclear longitudinal and transverse relaxation of hyperfine coupled protons has been studied by transient ENDOR and triple-resonance experiments. The nuclear longitudinal relaxation indicates that, above room temperature, thermally activated small-angular motions occur on a time scale of 100 μs. The nuclear phase-memory relaxation rate increases linearly with the hyperfine coupling. A quantitative description of a motional model is given, which explains the experimental results by small-angle fluctuations of the radicals on a microsecond time scale.

17O hyperfine and quadrupole interactions for water ligands in frozen solutions of high spin Mn2+

The magnetic couplings of 17O in H217O coordinated to high spin Mn2+ in a frozen aqueous solution were determined using the complementary magnetic resonance techniques of pulsed and continuous wave (cw) ENDOR (electron nuclear double resonance), ESEEM (electron spin echo envelope modulation), and PFSEPR [pulse field sweep electron paramagnetic resonance (EPR)]. Several complications arise from the high electron spin multiplicity of the d5, Mn2+ ion and the high nuclear spin multiplicity, I=5/2, of the 17O nucleus. At the applied magnetic field strengths in 9 GHz EPR studies, the zero-field splitting of the S=5/2 Mn2+ ion in aqueous frozen solution is small relative to the electron spin Zeeman interaction so that the MS=±1/2,±3/2,±5/2 electron spin states all contribute to the ENDOR spectrum. This results in a complex spectrum in which the 17O ENDOR powder pattern arising from the MS=±1/2 manifolds are separately resolved but the powder patterns from the MS=±3/2,±5/2 manifolds overlap the multiple 1H ENDOR lines arising from all six MS manifolds [X. Tan, M. Bernardo, H. Thomann, and C. P. Scholes, J. Chem. Phys. 98, 5147 (1993)]. Given this complexity, a combination of complementary spectroscopic techniques and numerical simulations are used to deconvolute the overlapping spectra and to assign the spectral lines. The ENDOR spectra provided an experimental description of H217O hyperfine couplings to high spin Mn2+ in a frozen solution. The ESEEM results are consistent with the first-order assignments of the ENDOR lines and demonstrate the feasibility of ESEEM measurements of 17O ligand hyperfine couplings to Mn2+. Simulations of the 17O ENDOR hyperfine patterns of aqueous frozen solutions of Mn2+, especially those near 20 MHz, indicated an A-tensor anisotropy of A⊥=−6.5±0.5 MHz and A∥=−9.5±0.5 MHz, consistent with couplings observed by single crystal ENDOR of H217O ligated to Mn2+ doped in [LaMg(NO2)12⋅24(H2O)]. More detailed simulations of the ENDOR pattern below 10 MHz indicated the need for quadrupole couplings consistent with those measured by single crystal ENDOR and with those determined by gas phase measurements on H17OD. Simulations of the ENDOR spectra recorded by the cw and pulsed techniques have delineated important features of the techniques which must be taken into account for a quantitative analysis of the ENDOR amplitudes. It is expected that the general ENDOR conditions employed and the theory developed will be useful in frozen solution studies of 17O involved as a ligand to Mn2+ in enzymes.

Structure of the intrinsic shallow electron center in AgCl studied by pulsed electron nuclear double resonance spectroscopy at 95 GHz.

In this Letter we report the first direct determination of the wave function of the intrinsic shallow electron center in silver chloride. A model is suggested in which an electron is shallowly trapped by two adjacent silver ions on a single cationic site. The information has been obtained by pulsed electron nuclear double resonance at 95 GHz and illustrates the potential of high-frequency EPR techniques for the study of defects in semiconductors.

Pulsed 95 GHz high-field EPR heterodyne spectrometer with high spectral and time resolution

Various time resolved EPR methods are applied to different test samples to demonstrate the abilities of pulsed high-field EPR spectroscopy. Two-pulse-echo field swept EPR spectroscopy on a nitroxide radical shows the increased spectral resolution by separating different spin systems by their relaxation properties. Additionally N14 electron-spin-echo-envelope-modulation (ESEEM) is observed for these systems at fields as high as 3.5 T. Thus, the N14 hyperfine interaction couplings can be probed by ESEEM and pulsed ENDOR (electron-nuclear-double-resonance) experiments. The sensitivity of pulsed ENDOR experiments is compared with cw-ENDOR. The different linewidths and amplitudes of the two methods are discussed. Transient nutation experiments on light induced triplet states demonstrate the high sensitivity and time resolution of high-field pulsed EPR. The sensitivity and time resolution of our 95 GHz spectrometer are determined and compared with pulsed X-band EPR spectrometer performances.

Pulsed electron nuclear double resonance study of molecular motions of radicals in a high-strength liquid-crystalline polymer

Radicals in a liquid-crystalline copolyester are studied by pulsed electron paramagnetic resonance (EPR) and pulsed electron nuclear double resonance (ENDOR). For the first time transient ENDOR techniques are used to investigate molecular motion of radicals in a disordered solid. The nuclear phase memory relaxation of hyperfine coupled protons is measured in a temperature region between 250 and 350 K in the solid state. Remarkably, the relaxation rate increases linearly rather than quadratically with the strength of the hyperfine coupling | A zz |. This results from a thermally activated process, characterized by small-angle fluctuations of radicals with an amplitude of the order of 1°. The corresponding activation energy of 24 kJ/mol indicates that the radicals are chain ends in the polymer.

Advanced electron paramagnetic resonance techniques for the study of coal structure

A combination of advanced transient electron paramagnetic resonance (e.p.r.) techniques have been applied to the study of the chemistry of coals of widely varying rank. These pulsed e.p.r. experiments allow detailed studies of the relaxation behaviour of the carbon radicals in coals, including the phase memory time ( T m ) and the electron spin lattice relaxation time ( T 1 e ) which significantly augment the information obtained by conventional constant wave (c.w.) e.p.r. methods. Pulsed electron-nuclear double resonance allows quantitative determination of the various hyperfine interactions which the radicals have with protons on the same and adjacent molecules. Another particularly striking extension of previous e.p.r. techniques is to use the dynamics of the electron spins to track the relaxation properties of protons in their vicinity — the first instance of probing these nuclei which are inaccessible by n.m.r. methods.

Fourier-transform hyperfine spectroscopy

A new pulsed electron nuclear double resonance technique for the measurement of hyperfine interactions is proposed. The experiment is based on burning transient holes into an inhomogeneously broadened EPR line by microwave and radio frequency pulses and detection of the hole pattern via an FID. In the spectrum obtained, each group of equivalent nuclei is represented by one peak at the hyperfine frequency, independent of the nuclear spin quantum number.

Multiple Quantum Nuclear Coherences in Pulsed ENDOR Spectroscopy

AbstractExperimental and theoretical aspects of multiple quantum (MQ) NMR coherence phenomena for paramagnetically coupled nuclei are presented. The spin interactions responsible for MQ NMR coherences observed in pulsed EPR and pulsed ENDOR (Electron Nuclear Double Resonance) are contrasted with those responsible for MQ coherences in pulsed NMR spectroscopy. MQ NMR coherences in pulsed ENDOR spectroscopy are demonstrated experimentally and described using an operator formalism approach. Detailed discussions of the phase incrementation technique for detecting MQ NMR coherences in ENDOR spectroscopy, different microwave excitation schemes for the preparation of nuclear spin order, off‐resonance excitation, and the evolution of MQ coherences under free precession are presented. Experimental results on a single crystal of irradiated malonic acid and on the conjugated polyene polymer poly acetylene are presented.

Pulsed electron-nuclear double resonance methodology

Pulsed electron-nuclear double resonance methodology

Stimulated echo time domain electron nuclear double resonance

The observation of nuclear spin coherences in a pulsed electron nuclear double resonance (ENDOR) experiment is reported. The coherences are detected indirectly as time-dependent modulations of the electron spin stimulated echo envelope. A theoretical description of these double resonance coherence experiments for spin systems coupled by nonsecular interactions is developed, and is employed to calculate the expected time domain signals assuming indirect detection of the nuclear coherences. The resolution and sensitivity of this technique is compared with other, related ENDOR techniques, and the prospects for more sophisticated time domain experiments are briefly assessed.

A pulsed ENDOR probehead with the bridged loop-gap resonator: Construction and performance

A probehead for pulsed electron-nuclear double resonance (ENDOR) experiments with radio-frequency fields up to 20 mT is introduced. The probehead consists of a bridged loop-gap resonator for X-band microwave frequencies surrounded by a solenoidal rf coil that generates the radio-frequency field, and serves as microwave radiation shield. A broad band and a tuned circuit for pulsed ENDOR experiments between 1–200 and 1.5–26 MHz, respectively, are described. Details of the probehead design and characteristics of the rf circuits are given. The performance of the probehead is demonstrated by pulsed ENDOR spectra in solids and liquids between 8 and 150 MHz and by an ENDOR experiment that requires a stepping of the static magnetic field strength.

Pulsed electron nuclear double and triple resonance schemes

AbstractThe following pulsed electron nuclear double (ENDOR) and triple resonance schemes are discussed: Population transfer ENDOR, TRIPLE resonance, coherence transfer ENDOR, spin echo double resonance (SEDOR) and stimulated echo double resonance.

Bloch-Siegert shift, Rabi oscillation, and spinor behavior in pulsed electron-nuclear double-resonance experiments.

It is demonstrated that three different coherence phenomena, namely Rabi oscillations, the Bloch-Siegert shift, and spinor behavior, appear simultaneously in pulsed electron-nuclear double-resonance (ENDOR) experiments. New pulse sequences for the separation of these different effects are proposed.