Electron-nuclear Coupling Research Articles

Dynamic nuclear polarization in suspensions of asphaltene obtained from MC-30 liquid asphalt

Nuclear magnetic resonance and the Overhauser effect type dynamic nuclear polarization were used to study some colloidal suspensions of asphaltene obtained from MC-30 liquid asphalt in a weak magnetic field of 1.437mT. Intermolecular spin–spin interactions occur between diffusing nuclear spins in the solvent medium and the unpaired electron spin in the asphaltene micelles. The samples were prepared in ten pure and six mixed solvent media. The Electron Paramagnetic Resonance spectrum of the asphaltene was obtained and the saturation experiments were applied to the samples. The dipolar interaction is predominant for all suspensions. Solvent molecules can diffuse freely in and out of the asphaltene micelles making dipolar interaction. Some solvent molecules, in the same sample, can attach to the asphaltene particles for an extremely short time, forming complexes and making scalar interaction. The variation of the nuclear-electron coupling parameter with the low frequency relaxation component for each pure solvent medium and the solvent effects of CCl4 and CS2 which do not contain proton (1H nucleus) were investigated using mixed solvent media. The results are compared with some other studies.

Nonrelativistic dynamics of particles with characteristic spins and anomalous magnetic moments in a homogeneous magnetic field

A detailed theoretical treatment of the effect of a homogeneous external magnetic field on a general molecule is carried out in this work at the nonrelativistic level. The Hamiltonian considered here is, although nonrelativistic, explicitly spin-adapted through order v2/c2. The effects of the anomalous magnetic moment of each particle, hyperfine interactions and spin–orbit interactions have been systematically included in it. This Hamiltonian is reexpressed in a preferred set of coordinates so as to facilitate the removal of the center of mass motion by exploiting the properties of the total pseudomomentum. The reduced Hamiltonian operator at zero total pseudomomentum represents the dynamics of the electrons and the nuclei belonging to a neutral molecule in relative coordinates. The same operator can be separated as a sum of the electronic Hamiltonian, the nuclear Hamiltonian, and the electronic–nuclear coupling. The effect of a strong magnetic field on molecular vibration and rotation is then described for a general diatomic molecule. The interelectronic and the electron–nuclear kinetic coupling terms are obviously responsible for the known phenomenal increase in the bond dissociation energy at a strong magnetic field under the Born–Oppenheimer approximation. The case of nuclear spin isomerism has been discussed in the specific context of ortho and para hydrogen. This leads to a prediction of the critical field strength beyond which ortho hydrogen may be formed as the major product even at an extremely low temperature. The basic tenets of the adiabatic approximation are examined, and the influence of the electronic–nuclear kinetic coupling on the evaluation of the vibronic interaction parameters is discussed in an Appendix.

Photoinduced electron transfer and its magnetic field dependence in supramolecular complex of zinc(II) tetraphenylporphyrin–viologen chain-linked compound with 2,3,6-tri- O-methyl-β-cyclodextrin

Photoinduced electron transfer in a supramolecular complex of zinc(II) tetraphenylporphyrin–viologen covalently linked compound (ZnP–V 2+) with 2,3,6-tri- O-methyl-β-cyclodextrin in acetonitrile–water (1:1, v/v) was investigated. On increasing the magnetic fields, the lifetime of photogenerated biradical (ZnP + –V + ) increased and reached an asymptotic value under high magnetic fields (>100 mT). The effects were explained by the electron-nuclear hyperfine coupling and spin–lattice relaxation mechanisms. The observed forward electron transfer was quite slow compared with the free ZnP–V 2+, and was increased by the magnetic fields. These results were explained by the fact that the photoinduced electron transfer was an equilibrium between the 3 ZnP *– V 2+ and ZnP + –V + , and this free energy change was quite a small value (ca. −1.3 kJ/mol).

Investigations of ultrafast nuclear response induced by resonant and nonresonant laser pulses

We analyze the nonstationary vibrational states prepared by ultrashort laser pulses interacting with a two electronic level molecular system. Fully quantum mechanical expressions are derived for all the moments of the coordinate and momentum operators for the vibrational density matrices associated with the ground and excited electronic states. The analysis presented here provides key information concerning the temperature and carrier frequency dependence of the moments, and relates the moments to equilibrium absorption and dispersion line shapes in a manner analogous to the “transform methods” previously used to describe resonance Raman scattering. Particular attention is focused on the first two moments, for which simple analytical expressions are obtained that are computationally easy to implement. The behavior of the first two moments with respect to various parameters such as the pulse carrier (center) frequency, pulse width, mode frequency, electron-nuclear coupling strength, and temperature is investigated in detail. Using rigorous analytical formulas, we also discuss the laser pulse induced squeezing of the nuclear distributions as well as the pulse induced vibrational heating/cooling in the ground and excited states. The moment analysis of the pump induced state presented here offers a convenient starting point for the analysis of signals measured in pump–probe spectroscopy. The moment analysis can also be used, in general, to better understand the material response following ultrashort laser pulse excitation.

Molecular oxygen spin-lattice relaxation in solutions measured by proton magnetic relaxation dispersion.

Proton spin-lattice relaxation rate constants have been measured as a function of magnetic field strength for water, water-glycerol solution, cyclohexane, methanol, benzene, acetone, acetonitrile, and dimethyl sulfoxide. The magnetic relaxation dispersion is well approximated by a Lorentzian shape. The origin of the relaxation dispersion is identified with the paramagnetic contribution from molecular oxygen. In the small molecule cases studied here, the effective correlation time for the electron-nuclear coupling may include contributions from both translational diffusion and the electron T(1). The electron T(1) for molecular oxygen dissolved in several solvents was found to be approximately 7.5 ps and nearly independent of solvent or viscosity.

Solid effect in the electron spin dressed state: A new approach for dynamic nuclear polarization

We describe a new type of solid effect for dynamic nuclear polarization (DNP) that is based on simultaneous, near resonant microwave (mw) and radio frequency (rf) irradiation of a coupled electron nuclear spin system. The interaction of the electron spin with the mw field is treated as an electron spin dressed state. In contrast to the customary laboratory frame solid effect, it is possible to obtain nuclear polarization with the dressed state solid effect (DSSE) even in the absence of nonsecular hyperfine coupling. Efficient, selective excitation of dressed state transitions generates nuclear polarization in the nuclear laboratory frame on a time scale of tens of μs, depending on the strength of the electron–nuclear coupling, the mw and rf offset and field strength. The experiment employs both pulsed mw and rf irradiation at a repetition rate comparable to T1e−1, where T1e is the electronic spin lattice relaxation time. The DSSE is demonstrated on a perdeuterated BDPA radical in a protonated matrix of polystyrene.

Isotope effect of hydrogen and lithium hydride molecules. Application of the dynamic extended molecular orbital method and energy component analysis

In order to explore the isotope effect including the nuclear–electronic coupling and nuclear quantum effects under the one-particle approximation, we apply the dynamic extended molecular orbital (DEMO) method and energy component analysis to the hydrogen and lithium hydride isotope molecules. Since the DEMO method determines both electronic and nuclear wave functions simultaneously by variationally optimizing all parameters embedded in the basis sets, the virial theorem is completely satisfied and guarantees the relation of the kinetic and potential energies. We confirm the isotope effect on internuclear distances, nuclear and electronic wave functions, dipole moment, the polarizability, and each energy component. In the case of isotopic species of the hydrogen molecule, the total energy decreases from the H2 to the T2 molecule due to the stabilization of the nuclear–electronic potential component, as well as the nuclear kinetic one. In the case of the lithium hydride molecule, the energy lowering by replacing 6Li with 7Li is calculated to be greater than that by replacing H with D. This is mainly caused by the small destabilization of electron–electron and nuclear–nuclear repulsion in 7LiH compared to 6LiH, while the change in the repulsive components from 6LiH to 6LiD increases.

Gain without inversion for gamma radiation

Gain without inversion for a level-mixing scheme is studied for gamma-optics. In this scheme nuclear level mixing is created by misalignment of a dc magnetic field with respect to the c-axis of a noncubic crystal. Axially symmetric electron nuclear coupling and nuclear quadrupole interaction with an electric field gradient produce two electro-nuclear levels that are equally mixed and split, with the energy gap dependent on the tilting angle of the magnetic field. By laser excitation of an electron transition, nuclear spin coherence can be created between these two levels. A condition is found for the predominant population of the dark state by spontaneous emission. This state is a particular superposition of the mixed states, such that, by selection rules, the transition from it cannot be excited by γ-radiation. If all absorbing nuclei are in the dark state, resonant γ-absorption is suppressed in the sample. At the same condition, γ-emission of the excited nuclei is allowed because the corresponding transition terminates in another component of the mixed states superposition. The constraints on the tilting angle and reciprocal gap between two mixed electro-nuclear levels are found, setting the limits to these values beyond which the gain without inversion becomes impossible.

Demonstration of a conserved histidine and two water ligands at the Mn2+ site in Diocleinae lectins by pulsed EPR spectroscopy.

Lectins from the Diocleinae subtribe, including Canavalia brasiliensis, Canavalia bonariensis, Canavalia grandiflora, Cratylia floribunda, Dioclea grandiflora, Dioclea guianensis, Dioclea rostrata, Dioclea violacea, and Dioclea virgata, have been recently isolated and characterized in terms of their carbohydrate binding specificities. Although all of the lectins are Man/Glc specific, they possess different biological activities. In the present study, electron paramagnetic resonance (EPR) spectroscopy demonstrates that all nine Diocleinae lectins contain Mn2+. The spectra of C. floribunda and D. rostrata suggest Mn2+ site symmetry different from that of the other seven lectins. However, electron spin-echo envelope modulation (ESEEM) spectroscopy indicates that all nine lectins are coordinated to a histidyl imidazole, with similar electron-nuclear coupling to the Mn2+-bound imidazole nitrogen. ESEEM also demonstrates ligation of two water molecules to Mn2+ in all nine Diocleinae lectins. Thus, the EPR and ESEEM data indicate the presence of a Mn2+ binding site in the above Diocleinae lectins with a conserved histidine residue and two water ligands.

Enhancement of the skyrmionic excitations due to the suppression of Zeeman energy by optical orientation of nuclear spins

An effective experimental method for compensation of Zeeman energy of two-dimensional (2D) electrons based on optical pumping of nuclear spins is proposed and used to enhance skyrmionic excitations under conditions of integer and fractional quantum Hall effect. We demonstrate that photoexcitation by circularly polarized light can result in a strong spin orientation of nuclei along (or against) the direction of the external magnetic field and that the energy of the hyperfine contact interaction between 2D electrons and optically oriented nuclear spins can be comparable to the electronic Zeeman energy. It is shown that a compensation of the Zeeman energy by the hyperfine interaction at fixed magnetic field results in an enhancement of skyrmionic excitations and the dependence of the skyrmionic radius as a function of the ratio between Zeeman and Coulomb energies is measured. A strong increase of the skyrmion radius is observed for ${E}_{Z}{/E}_{C}<0.005,$ and the quenching of the skyrmion at ${E}_{Z}{/E}_{C}=0.011\ifmmode\pm\else\textpm\fi{}0.001$ is established. Strong electron-nuclear coupling was used for optical detection of the nuclear magnetic resonance from the variation of the polarized luminescence intensity.

Nonlinear spin dynamics in ferromagnets with electron-nuclear coupling

Nonlinear spin motion in ferromagnets is considered with nonlinearity due to three factors: (i) the sample is prepared in a strongly nonequilibrium state, so that evolution equations cannot be linearized as would be admissible for spin motion not too far from equilibrium, (ii) the system considered consists of interacting electron and nuclear spins coupled with each other via hyperfine forces, and (iii) the sample is inserted into a coil of a resonant electric circuit producing a resonator feedback field. Due to these nonlinearities, coherent motion of spins can develop, resulting in their ultrafast relaxation. A complete analysis of mechanisms triggering such a coherent motion is presented. This type of ultrafast coherent relaxation can be used for studying intrinsic properties of magnetic materials.

Deuteron spin-lattice relaxation in theMn12O12-acetate magnetic cluster

The dynamics of the fully deuterated polycrystalline molecular cluster nanomagnet ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}{[(\mathrm{C}\mathrm{D}}_{3}{\mathrm{C}\mathrm{O}\mathrm{O})}_{16}{(\mathrm{D}}_{2}{\mathrm{O})}_{4}{]\mathrm{\ensuremath{\cdot}}2\mathrm{C}\mathrm{D}}_{3}{\mathrm{COOD}\mathrm{\ensuremath{\cdot}}4\mathrm{D}}_{2}\mathrm{O}$ has been studied in a field of 6.3 T via deuteron spin-lattice relaxation time ${(T}_{1})$ measurements between room temperature and 4 K. The relaxation is due to the fluctuations in the hyperfine electron-nuclear coupling. The deuteron magnetization recovery was found to be of the stretched exponential type and the temperature dependence of the spin-lattice relaxation time parameter ${T}_{1}$ showed two pronounced minima, one around 70 K and another around 10 K. The two modes detected in this study are intermediate between the slow magnetization reorientation mode detected by susceptibility measurements and the fast spin modes detected by inelastic neutron scattering. Whereas the slow correlation time ${\ensuremath{\tau}}_{1}$ is of the order of days in the vicinity of the superparamagnetic blocking temperature 5 K, the fast correlation time ${\ensuremath{\tau}}_{2}$ is here still in the ${10}^{\ensuremath{-}6} \mathrm{s}$ range.

Magnetic Relaxation Dispersion of7Li: II. Complex Formation with Nitroxides in the Aqueous Phase

Measurements of7Li nuclear spin–lattice relaxation times are made at applied magnetic field strengths from 0.25 mT to 7.05 T, in order to determine directly the form of the frequency-dependent spectral densities that modulate relaxation. This magnetic resonance dispersion (MRD) technique provides detailed information regarding molecular dynamics down to the picosecond time scale.7Li MRD measurements made on aqueous lithium ion in the presence of small concentrations of nitroxide free radicals give direct evidence supporting the formation of a coordination complex. The dipole–dipole electron-nuclear coupling is modulated by both translational and rotational diffusive motions, and both of these contributions are resolved. However, scalar coupling arising from the presence of paramagnetic electron spin density at the nucleus dominates the nuclear relaxation. Changes in the pH and free radical moiety are compared with dynamical variables, geometric constraints, and formation constants obtained from a model of nuclear relaxation. The calculated bimolecular formation constants are on the order of 2 × 10−3M−1, and the relative accuracy of this parameter is tested.

Electron Spin Echo Envelope Modulation and Extended X-ray Absorption Fine Structure Studies of Active Site Models of Oxygenated Cobalt-Substituted Hemoproteins: Correlating Electron-Nuclear Couplings and Metal−Ligand Bond Lengths

Electron spin echo envelope modulation (ESEEM) and extended X-ray absorption fine structure (EXAFS) spectroscopic studies of oxygenated cobalt (oxyCo) [tetraphenylporphyrin(TPP)][1-methylimidazole (1-MeIm)], an active site model of oxyCo-substituted globins (functional and EPR-active [S = 1/2] analogues of oxygen carrying hemoproteins), are carried out in order to examine the correlation of oxygen affinity with electron-nuclear coupling parameters and metal−ligand bond lengths. ESEEM demonstrates that the magnitude of the electron-nuclear hyperfine and nuclear quadrupole couplings to the directly-coordinated 14N of 1-MeIm decrease (Aiso, from 3.54 to 3.04 MHz; e2qQ, from 2.39 to 2.08 MHz) as the solvent compositon is varied from 0 to 50% (v/v) dichloromethane in toluene. For oxyCo[(o-R)1TPP][1-MeIm] (where R = −H, −NHCOC(CH3)3, −NHCOCH3, or −NHCONHC6H5, an ortho substitutent on one of the four meso phenyls of TPP), couplings to the axial nitrogen decrease (Aiso, from 3.54 to 3.07 MHz; e2qQ, from 2.39 to 2...

Temperature dependence of solid-liquid scalar interactions in aqueous char suspensions by nonstationary DNP at low magnetic field

The temperature dependence of DNP enhancements in aqueous suspensions of hardwood chars by nonstationary DNP at low magnetic field has been investigated. With increasing temperature, the sign of the water proton DNP enhancement changed from negative to positive, indicating a change in the relative contributions from exchange (scalar) and dipole-dipole nuclear-electron coupling. A cross-over temperature was found at which the contributions of the dipole-dipole and exchange interactions canceled each other and the DNP effect disappeared. A model of molecular migration at the solid-liquid interface is constructed to account for these observations. Correlation of experimental and theoretical results has been achieved through consideration of the proton-electron spin dipole-dipole interaction and chemical exchange of solvent molecules. The increase with temperature in the contribution of the contact interaction to proton relaxation is a result mainly of a decrease in the lifetime τ h of water in the short-lived hydrated complex at the hardwood char interface. The enthalpy of activation E h = 13.5 kcal/mol for surface exchange diffusion as well as the lifetime of water molecules on the surface τ h(300K) = 1.9 × 10 −4s and hyperfine scalar constant a = 5.3 × 10 −3MHz are estimated. The results are characteristic of weak short-lived chemisorption bonds at the solid-liquid interface.

A multidimensional Landau-Zener description of chemical reaction dynamics and vibrational coherence

We discuss the observation of coherent nuclear oscillations in chemical and biological systems subsequent to excitation with an ultrashort laser pulse. The experimentally observed coherent signals arising from rapid, photon initiated, chemical reactions are considered separately from vibrational coherences directly excited by the (electric) field interventions of the light pulse. In order to treat the chemical reaction, a modified Landau-Zener theory is developed that is quasi-classical along the traditional reaction coordinate but fully quantum mechanical in all other degrees of freedom. In this work, we explore explicitly the effects of electronic state coupling, electron-nuclear coupling, reaction velocity, vibrational damping and temperature on the product electronic state population development and on the coherent oscillations of the nuclear coordinates. Recent experimental studies of the ligand photolysis reaction in heme proteins are presented as examples.

Electronic and Nuclear Dynamics of the Accessory Bacteriochlorophylls in Bacterial Photosynthetic Reaction Centers from Resonance Raman Intensities

Resonance Raman spectra and absolute Raman scattering intensities of the Franck-Condon coupled vibrational modes of the accessory bacteriochlorophylls (B) in the photosynthetic reaction center from Rb. sphaeroides have been obtained with excitation in their 800 nm Qy absorption band. Although the relative Raman intensities are unchanged when the temperature is reduced from 278 to 95 K, the absolute Raman scattering intensity is found to increase by a factor of4 at 95 K. A self-consistent multimode vibronic model for the excitedstate properties of B has been developed that provides a unique fit to the absorption band and the Raman cross sections at both temperatures using a total S (linear electron-nuclear coupling factor) of 0.32 and an effective electronic dephasing time of54 fs at 278 K and210 fs at 95 K. The Raman scattering cross sections are consistent with an electronic structure model where the two B chromophores absorb and scatter independently. The Raman cross sections scale as 1/T 2 in the 95-298 K range, suggesting that the pure dephasing time, if modeled as a Gaussian (slow-modulation limit), decreases linearly with temperature. At room temperature, the electronic dephasing time is controlled by bath-induced dephasing processes rather than the energy transfer time from the excited state of B. The electronic and nuclear relaxation parameters derived here provide a more quantitative picture of the time scale and structural relaxation of the chromophores involved in energy transfer in reaction centers.

Electron nuclear dynamics of H++H2 collisions at Elab=30 eV

Proton collisions with hydrogen molecules at 30 eV in the laboratory frame is a simple ion-molecule system exhibiting a number of distinct processes such as inelastic scattering, charge transfer, rearrangement, and dissociation. The electron nuclear dynamics (END) theory which allows full electron nuclear coupling and which does not restrict the system from reaching any of the possible product channels, is applied to this sytem to produce transition probabilities, differential, and integral (vibrationally resolved) cross sections. Comparisons with experiment demonstrate that END, even in its simplest implementation, with a single determinantal state for the electrons and with classical nuclei, yields results that are competitive with other theoretical approaches.

Temperature, Orientation, and Solvent Dependence of Electron Spin-Lattice Relaxation Rates for Nitroxyl Radicals in Glassy Solvents and Doped Solids

T 1 was measured by saturation recovery for nitroxyl radicals as a function of temperature, solvent, and position in the CW spectrum. Long saturating pulses were used such that contributions to the saturation recovery curve were dominated by T 1 rather than by the tumbling correlation time or the nitroxyl nitrogen nuclear spin relaxation time. Data between about 20 and 150 K were fitted to log (1/ T 1) = (2.34 ± 0.15) × log ( T) + (7.0 ± 0.3). This temperature dependence is consistent with a two-phonon Raman process, although the relevant vibrations are likely to be intramolecular rather than lattice modes. At higher temperatures and in solvents with weak solute-solvent and or solvent/solvent interactions, there is increasing mobility of the nitroxyl, which causes T 1 to be more strongly temperature dependent. T 1 values are orientation dependent: T 1 is longer when the external magnetic field is along the molecular z axis and shorter when the magnetic field is in the perpendicular plane. This orientation dependence is consistent with the orientation dependence of spin-orbit coupling and with symmetry arguments concerning the orientation dependence of vibrations. Deuteration of the nitroxyl, deuteration of solvent, or replacement of natural-abundance nitrogen by nitrogen-15 in the nitroxyl caused less than 10% change in T 1, which indicates that inter- or intramolecular electron-nuclear couplings are not the dominant factors in nitroxyl spin-lattice relaxation in the temperature regimes examined.

Exchange of chloride by bromide in the manganese photosystem-II complex studied by cw- and pulsed-EPR

Abstract Cl − is an essential cofactor in Photosystem-II (PS-II) oxygen evolution. By using electron spin echo envelope spectroscopy it is shown that, in some circumstances, magnetic couplings between the Mn cluster, in the S 2 -state, and those of the observed. In order to extract only the electron nuclear couplings between the Mn cluster, in the S 2 -state, and those of the halide, a comparison was done between chloride- and bromide-reconstituted samples. In salt-washed, CaCl 2 - or CaBr 2 -reconstituted Photosystem-II membranes, frequencies similar to the Larmor frequency of bromide and chloride were detected. When Cl − and Br − reconstituted PS-II membranes are compared to membranes that have also been Ca 2+ -depleted, and EGTA-treated, these features in the frequency spectra are absent. After chloride depletion at high pH in the presence of sulfate, no difference could be observed in the frequency spectra of bromide- and chloride-reconstituted samples. These results are discussed in view of possible different Cl − binding sites in PS-II.