Anisotropic Hyperfine Research Articles

Ultralong spin decoherence times in graphene quantum dots with a small number of nuclear spins

We study the dynamics of an electron spin in a graphene quantum dot, which is interacting with a bath of less than ten nuclear spins via the anisotropic hyperfine interaction. Due to substantial progress in the fabrication of graphene quantum dots, the consideration of such a small number of nuclear spins is experimentally relevant. This choice allows us to use exact diagonalization to calculate the longtime average of the electron spin as well as its decoherence time. We investigate the dependence of spin observables on the initial states of nuclear spins and on the position of nuclear spins in the quantum dot. Moreover, we analyze the effects of the anisotropy of the hyperfine interaction for different orientations of the spin quantization axis with respect to the graphene plane. Interestingly, we then predict remarkable long decoherence times of more than 10ms in the limit of few nuclear spins.

Coupling of Li motion and structural distortions in olivine LiMnPO4from7Li and31P NMR

We present a detailed $^7$Li- and $^{31}$P-NMR study on single crystalline LiMnPO$_4$ in the paramagnetic and antiferromagnetic phase (AFM, $T_N \sim$ 34 K). This allows us to determine the spin directions in the field-induced spin-flop phase. In addition, the anisotropic dipolar hyperfine coupling tensor of the $^7$Li- and $^{31}$P-nuclei is also fully determined by orientation and temperature dependent NMR experiments and compared to the calculated values from crystal structure data. Deviations of the experimental values from the theoretical ones are discussed in terms of Mn disorder which is induced by Li-disorder. In fact, the disorder in the Mn-sublattice is directly revealed by our diffuse x-ray scattering data. The present results provide experimental evidence for the Li-diffusion strongly coupling to structural distortions within the MnPO$_4$ host, which is expected to significantly affect the Li-mobility as well as the performance of batteries based on this material.

Electron spin–lattice relaxation mechanisms of rapidly-tumbling nitroxide radicals

Electron spin relaxation times at 295K were measured at frequencies between 250MHz and 34GHz for perdeuterated 2,2,6,6-tetramethyl-4-piperidone-1-oxyl (PDT) in five solvents with viscosities that result in tumbling correlation times, τR, between 4 and 50ps and for three 14N/15N pairs of nitroxides in water with τR between 9 and 19ps. To test the impact of structure on relaxation three additional nitroxides with τR between 10 and 26ps were studied. In this fast tumbling regime T2-1∼T1-1 at frequencies up to about 9GHz. At 34GHz T2-1>T1-1 due to increased contributions to T2-1 from incomplete motional averaging of g-anisotropy, and T2-1-T1-1 is proportional to τR. The contribution to T1-1 from spin rotation is independent of frequency and decreases as τR increases. Spin rotation dominates T1-1 at 34GHz for all τR studied, and at all frequencies studied for τR=4ps. The contribution to T1-1 from modulation of nitrogen hyperfine anisotropy increases as frequency decreases and as τR increases; it dominates at low frequencies for τR>∼15ps. The contribution from modulation of g anisotropy is significant only at 34GHz. Inclusion of a thermally-activated process was required to account for the observation that for most of the radicals, T1-1 was smaller at 250MHz than at 1–2GHz. The significant 15N/14N isotope effect, the small H/D isotope effect, and the viscosity dependence of the magnitude of the contribution from the thermally-activated process suggest that it arises from intramolecular motions of the nitroxide ring that modulate the isotropic A values.

Characterisation of paramagnetic Mo impurities on MgO(100) single-crystalline films grown on Mo(100)

Electron paramagnetic resonance spectroscopy is used to characterise Mo(V) centres present on the surface of MgO(100) films grown on Mo(100) single-crystal surfaces at 600 K. The Mo(V) centres exhibiting a molydenyl group to reduce the charge imbalance with respect to the MgO lattice substitute Mg cations on regular terrace sites within the MgO(100) islands as deduced from angular-dependent measurements and spectral simulations. The observed anisotropic Zeeman and hyperfine interaction is comparable with Mo(V) centres supported on different powdered oxide surfaces such as alumina or silica. Analysis of the films grown at 300 K and subsequently annealed to elevated temperatures to improve the long-range order shows that the presence of the Mo sites on the surface is a thermally activated process. Comparison of the defect structures of the two preparation methods suggests that Mo plays a role in the relaxation of the lattice misfit between the Mo substrate and the MgO film.

ChemInform Abstract: Determining the Orientation and Localization of Membrane‐Bound Peptides

AbstractReview: 121 refs.

An ENDOR and DFT analysis of hindered methyl group rotations in frozen solutions of bis(acetylacetonato)-copper(ii)

ENDOR spectroscopy and DFT calculations have been used to thoroughly investigate the ligand hyperfine couplings for the bis(acetylacetonato)-copper(ii) complex [Cu(acac)2] in frozen solution. Solutions of [Cu(acac)2] were prepared under anhydrous conditions, and EPR revealed that the g and (Cu)A values were affected by traces of water present in the solvent. The ligand (H)Ai hyperfine couplings were subsequently investigated by CW and pulsed ENDOR spectroscopy. Anisotropic hyperfine couplings to the methine protons ((H)Ai = 1.35, -1.62, -2.12 MHz; a(iso) = -0.80 MHz) and smaller couplings to the fully averaged methyl group protons ((H)Ai = -0.65, 1.658, -0.9 MHz; a(iso) = 0.036 MHz) were identified by simulation of the angular selective ENDOR spectra and confirmed by DFT. Since the barrier to methyl group rotation was estimated to be ca. 5 kJ mol(-1) by DFT, rapid rotation of these -CH3 groups, even at 10 K, leads to an averaged value of (H)Ai. However, variable temperature X-band Mims ENDOR revealed an additional set of hyperfine couplings which showed a pronounced temperature dependency. Using CW Q-band ENDOR, these additional couplings were characterised by the hyperfine parameters (H)Ai = 3.45, 2.9, 2.62 MHz, a(iso) = 2.99 MHz and assigned to a hindered methyl group rotation. This hindered rotation of a sub-set of methyl groups occurs in 120° jumps, such that a large A(dip) and a(iso) component is always observed. Whilst the majority of the methyl groups undergo free rotation, a sub-set of methyl groups experience hindered rotation in frozen solution, through proton tunnelling. This hindered rotation appears to be caused by weak outer-sphere solvent interactions with the complex.

Electron spin resonance investigation of H2+, HD+, and D2+ isolated in neon matrices at 2 K

Various isotopologues of nature's simplest molecule, namely H(2)(+), HD(+), and D(2)(+), have been isolated in neon matrices at 2 K for the first time and studied by electron spin resonance (ESR). Over many years, hundreds of matrix isolation experiments employing a variety of deposition conditions and ion generation methods have been tried to trap the H(2)(+) cation radical in our laboratory. The molecule has been well characterized in the gas phase and by theoretical methods. The observed magnetic parameters for H(2)(+) in neon at 2 K are: g(∥) ≈ g(⊥) = 2.0022(1); A(iso)(H) = 881(7) MHz; and A(dip)(H) = 33(3) MHz. Reasonable agreement with gas phase values of the isotropic hyperfine interaction (A(iso)) is observed; however, the neon matrix dipolar hyperfine interaction (A(dip)) is noticeably below the gas phase value. The smaller matrix value of A(dip) is attributable to motional averaging of the H(2)(+) radical in the neon matrix trapping site--an occurrence that would prevent the full extent of the hyperfine anisotropy from being measured for a powder pattern type ESR sample.

Rabi oscillation and electron-spin-echo envelope modulation of the photoexcited triplet spin system in silicon

We report on a pulsed electron paramagnetic resonance (EPR) study of the photoexcited triplet state ($S=1$) of oxygen-vacancy centers in silicon. Rabi oscillations between the triplet sublevels are observed using coherent manipulation with a resonant microwave pulse. The Hahn echo and stimulated echo decay profiles are superimposed with strong modulations known as electron-spin-echo envelope modulation (ESEEM). The ESEEM spectra reveal a weak but anisotropic hyperfine coupling between the triplet electron spin and a ${}^{29}$Si nuclear spin ($I=1/2$) residing at a nearby lattice site, that cannot be resolved in conventional field-swept EPR spectra.

Theoretical Study of Hyperfine Interactions in Small Arsenic-Containing Radicals

Various density functional theory (DFT) and ab initio MP2 and CCSD methods were employed in calculations of arsenic isotropic and anisotropic hyperfine parameters in three small radicals, AsH(2), AsO(2), and H(2)AsO. Convergent basis sets for these calculations were specially derived starting from Dunning's correlation-consistent sets. DFT methods proved to be particularly appropriate choice, because the results obtained with the suitable functionals are in accordance with the available experimental values. Additionally, mechanisms of hyperfine couplings were investigated by examining individual orbital contributions to spin density. The spin polarization mechanism in AsH(2) was studied by evaluating one- and two-electron integrals in spin-restricted and unrestricted case. Apart from nonrelativistic, scalar-relativistic Douglas-Kroll-Hess DFT calculations were performed to examine relativistic impacts on spin density distribution.

EPR Spectra of a Mono- and a Hetero-Diradical in Nematic and Isotropic Phases

A hybrid hydrazine–nitroxide monoradical and the corresponding heterohydrazyl– nitroxide diradical were used as probes in 4-cyano-4′-pentylbiphenyl liquid crystal. EPR data (hyperfine constants, rotational time, and anisotropy parameter values) are reported in the range of 25°C–55°C. It was shown by EPR spectrometry that the nitroxide moiety gives an anisotropic triplet line, while the hydrazyl moiety cannot be detected under these conditions.

Determining the orientation and localization of membrane-bound peptides.

Many naturally occurring bioactive peptides bind to biological membranes. Studying and elucidating the mode of interaction is often an essential step to understand their molecular and biological functions. To obtain the complete orientation and immersion depth of such compounds in the membrane or a membrane-mimetic system, a number of methods are available, which are separated in this review into four main classes: solution NMR, solid-state NMR, EPR and other methods. Solution NMR methods include the Nuclear Overhauser Effect (NOE) between peptide and membrane signals, residual dipolar couplings and the use of paramagnetic probes, either within the membrane-mimetic or in the solvent. The vast array of solid state NMR methods to study membrane-bound peptide orientation and localization includes the anisotropic chemical shift, PISA wheels, dipolar waves, the GALA, MAOS and REDOR methods and again the use of paramagnetic additives on relaxation rates. Paramagnetic additives, with their effect on spectral linewidths, have also been used in EPR spectroscopy. Additionally, the orientation of a peptide within a membrane can be obtained by the anisotropic hyperfine tensor of a rigidly attached nitroxide label. Besides these magnetic resonance techniques a series of other methods to probe the orientation of peptides in membranes has been developed, consisting of fluorescence-, infrared- and oriented circular dichroism spectroscopy, colorimetry, interface-sensitive X-ray and neutron scattering and Quartz crystal microbalance.

Determination of the Proton Environment of High Stability Menasemiquinone Intermediate in Escherichia coli Nitrate Reductase A by Pulsed EPR

Escherichia coli nitrate reductase A (NarGHI) is a membrane-bound enzyme that couples quinol oxidation at a periplasmically oriented Q-site (Q(D)) to proton release into the periplasm during anaerobic respiration. To elucidate the molecular mechanism underlying such a coupling, endogenous menasemiquinone-8 intermediates stabilized at the Q(D) site (MSQ(D)) of NarGHI have been studied by high-resolution pulsed EPR methods in combination with (1)H2O/2H2O exchange experiments. One of the two non-exchangeable proton hyperfine couplings resolved in hyperfine sublevel correlation (HYSCORE) spectra of the radical displays characteristics typical from quinone methyl protons. However, its unusually small isotropic value reflects a singularly low spin density on the quinone carbon α carrying the methyl group, which is ascribed to a strong asymmetry of the MSQ(D) binding mode and consistent with single-sided hydrogen bonding to the quinone oxygen O1. Furthermore, a single exchangeable proton hyperfine coupling is resolved, both by comparing the HYSCORE spectra of the radical in 1H2O and 2H2O samples and by selective detection of the exchanged deuterons using Q-band 2H Mims electron nuclear double resonance (ENDOR) spectroscopy. Spectral analysis reveals its peculiar characteristics, i.e. a large anisotropic hyperfine coupling together with an almost zero isotropic contribution. It is assigned to a proton involved in a short ∼1.6 Å in-plane hydrogen bond between the quinone O1 oxygen and the Nδ of the His-66 residue, an axial ligand of the distal heme b(D). Structural and mechanistic implications of these results for the electron-coupled proton translocation mechanism at the Q(D) site are discussed, in light of the unusually high thermodynamic stability of MSQ(D).

Persistent optical nuclear spin narrowing in a singly charged InAs quantum dot

We review the investigation of the hole-assisted dynamical nuclear spin polarization mechanism in a singly charged InAs quantum dot. Using coherent dark state spectroscopy, we measure the locking of the Overhauser field to a value determined only by the laser frequencies. Importantly, we review data that the locking effect can suppress nuclear spin fluctuations. We determine the onset time of the nuclear spin narrowing effect and its persistence absent laser interactions by directly measuring the enhancement of the electron spin coherence. This nuclear field locking effect can be explained in terms of an anisotropic hyperfine coupling between the hole spin and the nuclear spins.

EPR study of a gamma-irradiated (2-hydroxyethyl)triphenylphosphonium chloride single crystal

In this study, gamma-irradiated single crystals of (2-hydroxyethyl)triphenylphosphonium chloride [CH2CH2OH P(C6H5)3Cl] were investigated with electron paramagnetic resonance (EPR) spectroscopy at room temperature for different orientations in the magnetic field. The single crystals were irradiated with a 60Co-γ-ray source at 0.818 kGy/h for about 36 h. Taking the chemical structure and the experimental spectra of the irradiated single crystal of the title compound into consideration, a paramagnetic species was produced with the unpaired electron delocalized around 31P and several 1H nuclei. The anisotropic hyperfine values due to the 31P nucleus, slightly anisotropic hyperfine values due to the 1H nuclei and the g-tensor of the radical were measured from the spectra. Depending on the molecular structure and measured parameters, three possible radicals were modeled using the B3LYP/6-31+G(d) level of density-functional theory, and EPR parameters were calculated for modeled radicals using the B3LYP/TZVP method/basis set combination. The calculated hyperfine coupling constants were found to be in good agreement with the observed EPR parameters. The experimental and theoretically simulated spectra for each of the three crystallographic axes were well matched with one of the modeled radicals (discussed in the text). We thus identified the radical C˙H2CH2 P(C 6H5)3 Cl as a paramagnetic species produced in a single crystal of the title compound in two magnetically distinct sites. The experimental g-factor and hyperfine coupling constants of the radical were found to be anisotropic, with the isotropic values g iso = 2.0032, G, G, G and G for site 1 and g iso=2.0031, G, G G and G for site 2.

Computation of Hyperfine Tensors for Dinuclear MnIIIMnIV Complexes by Broken‐Symmetry Approaches: Anisotropy Transfer Induced by Local Zero‐Field Splitting

Based on broken-symmetry density functional calculations, the (55)Mn hyperfine tensors of a series of exchange-coupled, mixed-valence, dinuclear Mn(III) Mn(IV) complexes have been computed. We go beyond previous quantum chemical work by fully including the effects of local zero-field splitting (ZFS) interactions in the spin projection, following the first-order perturbation formalism of Sage et al. [J. Am. Chem. Soc. 1989, 111, 7239]. This allows the ZFS-induced transfer of hyperfine anisotropy from the Mn(III) site to the Mn(IV) site to be described with full consideration of the orientations of local hyperfine and ZFS tensors. After scaling to correct for systematic deficiencies in the quantum chemically computed local ZFS tensors, good agreement with experimental (55)Mn anisotropies at the Mn(IV) site is obtained. The hyperfine coupling anisotropies on the Mn(III) site depend sensitively on structural distortions for a d(4) ion. The latter are neither fully reproduced by using a DFT-optimized coordination environment nor by using experimental structures. For very small exchange-coupling constants, the perturbation treatment breaks down and a dramatic sensitivity to the scaling of the local ZFS tensors is observed. These results are discussed with respect to ongoing work to elucidate the structure of the oxygen-evolving complex of photosystem II by analysis of the EPR spectra.

Two-Dimensional 1H HYSCORE Spectroscopy of Dimanganese Di-μ-oxo Mimics of the Oxygen-Evolving Complex of Photosystem II

The solar water-splitting protein complex, photosystem II, catalyzes one of the most energetically demanding reactions in nature by using light energy to drive water oxidation. The four-electron water oxidation reaction occurs at the tetranuclear manganese-calcium-oxo cluster that is present in the oxygen-evolving complex of photosystem II. The tetranuclear manganese-calcium-oxo cluster is comprised of mixed-valence Mn(III) and Mn(IV) ions in the ground state. The oxo-manganese dimer, [H(2)O(terpy)Mn(III)(μ-O)(2)Mn(IV)(terpy)OH(2)](NO(3))(3) (terpy = 2,2':6',2″-terpyridine) (1), is an excellent biomimetic model that has been extensively used to gain insight on the molecular structure and mechanism of water oxidation in photosystem II. In this work, weak magnetic interactions between the protons of the two terminal water ligands and the paramagnetic dimanganese "di-μ-oxo" core of 1 are quantitatively characterized using two-dimensional hyperfine sublevel correlation (HYSCORE) spectroscopy. For the water molecule that is directly coordinated at the Mn(III) ion, the two protons are found to be magnetically equivalent and exhibit near axial hyperfine anisotropy. In contrast, for the first time, we demonstrate that the two protons of the water molecule that is directly coordinated at the Mn(IV) ion are inequivalent. We obtain the isotropic and anisotropic components of the hyperfine interaction for each proton. A comparison of the HYSCORE spectra measured in the presence and absence of acetate ions provides unambiguous evidence that only one molecule of acetate binds to 1 by replacing a terminal water molecule that is coordinated at the Mn(III) ion.

Electronic Structure of a Weakly Antiferromagnetically Coupled MnIIMnIIIModel Relevant to Manganese Proteins: A Combined EPR,55Mn-ENDOR, and DFT Study

An analysis of the electronic structure of the [Mn(II)Mn(III)(μ-OH)-(μ-piv)(2)(Me(3)tacn)(2)](ClO(4))(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-μ-hydroxo bridge, bis-μ-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn(III). All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the (55)Mn-ENDOR constrains the (55)Mn hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn(II) and Mn(III) ions. The large effective hyperfine tensor anisotropy of the Mn(II), a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn(III) ion strongly perturbs the zero-field energy levels of the Mn(II)Mn(III) complex. An estimate of the fine structure parameter (d) for the Mn(III) of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn(II) ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn(III) is consistent with the known coordination environment of the Mn(III) ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T 1∼ T 2 in the range of 0.50–1.1 μs at ambient temperature. Both spin–lattice relaxation T 1 and spin–spin relaxation T 2 are longer for 15N- than for 14N-nitroxides. The dominant contributions to T 1 are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T 1 on nitrogen nuclear spin state m I was observed for both 14N and 15N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.

Possible Mass Enhancement by Multipole Fluctuations Excited via the Singlet–Triplet Crystal Electric Field States in PrOs4Sb12: Sb-NMR Studies Using a Single Crystal

121,123 Sb nuclear magnetic resonance (NMR) measurements of a high-quality single crystalline PrOs 4 Sb 12 have been carried out in magnetic fields up to 98 kOe and in the temperature range of 0.4–30 K. From the analysis of the 121 Sb Knight shift ( K ) and magnetic susceptibility at H ≈90 kOe, the transferred hyperfine coupling constants ( A THF ) are obtained. The Sb isotropic hyperfine coupling constant and anisotropic hyperfine coupling tensor are determined as ≈27.6 Oe/µ B and ≈(149.9,42.3,-192.2) Oe/µ B , respectively. The anisotropic part originates from the spin–dipolar field due to the local spin density at Sb 5 p orbital transferred through the hybridization with 4 f electrons. The results are consistent with the Sb 12 5 p molecular orbital with a u symmetry predicted from the band structure calculation. A THF for H ∥[001] is dependent on the magnetic field, suggesting the coupling of the Sb nuclear dipole moment with not only dipole moments but also octupole moments of the Pr 4 f electrons with the same time reversal. The NMR relaxation rate depends both on magnetic field and temperature, and the temperature dependence of 1/ T 1 is explained by the phenomenological expression 1/( T 1 T A THF 2 )∝(1/ T ) exp [-Δ NMR ( H )/ T ] with excitation gap Δ NMR ( H ). Here, we found that Δ NMR ( H ) is proportional to the largest excitation gap between Γ 1 and Γ t - of the triplet Γ 4 (2) state under the magnetic field. These results indicate that the crystal electric field (CEF) excitation and the degeneracy of the triplet state are important for understanding Sb-NMR results. It is strongly indicated that the mass enhancement in PrOs 4 Sb 12 is caused by the scattering conduction electrons with multipole fluctuations via the singlet–triplet CEF excitation.

Anisotropic Hyperfine Interactions Limit the Efficiency of Spin-Exchange Optical Pumping ofHe3Nuclei

We use accurate ab initio and quantum scattering calculations to demonstrate that the maximum ³He spin polarization that can be achieved in spin-exchange collisions with potassium (³⁹K) and silver (¹⁰⁷Ag) atoms is limited by the anisotropic hyperfine interaction. We find that spin exchange in Ag-He collisions occurs much faster than in K-He collisions over a wide range of temperatures (10-600 K). Our analysis indicates that measurements of trap loss rates of ²S atoms in the presence of cold ³He gas may be used to probe anisotropic spin-dependent interactions in atom-He collisions.