Theoretical Basis for Switching a Kramers Single Molecular Magnet by Circularly-Polarized Radiation.

The d-group Kramers ions, having strong zero field splitting (ZFS) with axial symmetry and a negative D value for the ZFS Hamiltonian, are widely considered as candidates for use as single molecular magnets (SMMs). An important need is the means to switch the SMM between its states in a reasonably short and predictable period of time, which is generally not available. We propose an approach, Zeeman-far infrared (ZeFIR) double resonance, in which circularly polarized alternating magnetic fields in the far infrared (FIR) range induce selective magnetic dipole transitions between different Kramers doublets of the SMM and polarized microwave (mw) pulses transfer excitation inside the upper Kramers doublet. A combination of FIR and mw pulses allows unidirectional switching between +S and -S states of the ion. The proposed approach is considered for a model quartet system with total spin S = 3/2, which seems to be the most promising object for selective resonance manipulations of its states by circularly polarized radiation.

Analytical solution of the PELDOR inverse problem using the integral Mellin transform.

We describe a new model-free approach to solve the inverse problem in pulsed double electron-electron resonance (PELDOR, also known as DEER) spectroscopy and obtain the distance distribution function between two radicals from time-domain PELDOR data. The approach is based on analytical solutions of the Fredholm integral equations of the first kind using integral Mellin transforms to provide the distance distribution function directly. The approach appears to confine the noise in the computed distance distribution to short distances and does not introduce systematic distortions. Thus, the proposed analysis method can be a useful supplement to current methods to determine complicated distance distributions.

Electron spin dynamics and spin-lattice relaxation of trityl radicals in frozen solutions.

Electron spin-lattice relaxation of two trityl radicals, d24-OX063 and Finland trityl, were studied under conditions relevant to their use in dissolution dynamic nuclear polarization (DNP). The dependence of relaxation kinetics on temperature up to 100 K and on concentration up to 60 mM was obtained at X- and W-bands (0.35 and 3.5 Tesla, respectively). The relaxation is quite similar at both bands and for both trityl radicals. At concentrations typical for DNP, relaxation is mediated by excitation transfer and spin-diffusion to fast-relaxing centers identified as triads of trityl radicals that spontaneously form in the frozen samples. These centers relax by an Orbach-Aminov mechanism and determine the relaxation, saturation and electron spin dynamics during DNP.

Bloch equations for anisotropic paramagnetic centers with spin of 1/2.

The Bloch equations are an invaluable tool in magnetic resonance for describing the dynamics of isotropic spin systems. However, when the Bloch equations are reformulated for anisotropic spin systems, much of their utility is lost because the spin evolution they describe is not physically observable. A set of Bloch-like equations are derived for these anisotropic systems in terms of the magnetic moment which is the physical property measured in magnetic resonance and other experiments. The equations describe the dynamics of the magnetic moment including relaxation and only contain parameters that are experimentally measurable.

Spin dynamics of paramagnetic centers with anisotropic g tensor and spin of 1/2.

The influence of g tensor anisotropy on spin dynamics of paramagnetic centers having real or effective spin of 1/2 is studied. The g anisotropy affects both the excitation and the detection of EPR signals, producing noticeable differences between conventional continuous-wave (cw) EPR and pulsed EPR spectra. The magnitudes and directions of the spin and magnetic moment vectors are generally not proportional to each other, but are related to each other through the g tensor. The equilibrium magnetic moment direction is generally parallel to neither the magnetic field nor the spin quantization axis due to the g anisotropy. After excitation with short microwave pulses, the spin vector precesses around its quantization axis, in a plane that is generally not perpendicular to the applied magnetic field. Paradoxically, the magnetic moment vector precesses around its equilibrium direction in a plane exactly perpendicular to the external magnetic field. In the general case, the oscillating part of the magnetic moment is elliptically polarized and the direction of precession is determined by the sign of the g tensor determinant (g tensor signature). Conventional pulsed and cw EPR spectrometers do not allow determination of the g tensor signature or the ellipticity of the magnetic moment trajectory. It is generally impossible to set a uniform spin turning angle for simple pulses in an unoriented or 'powder' sample when g tensor anisotropy is significant.

Dynamic phase shifts in nanoscale distance measurements by double electron electron resonance (DEER).

The off-resonant pump pulse used in double electron electron resonance (DEER) measurements produces dynamic phase shifts that are explained here by simple analytic and vector descriptions of the full range of signal behaviors observed during DEER measurements, including: large phase shifts in the signal; changes in the position and shape of the detected echo; and changes in the signal intensity. The dynamic phase shifts depend on the width, amplitude and offset frequency of the pump pulse. Isolated radicals as well as pairs or clusters of dipolar-coupled radicals have the same dynamic phase shift that is independent of pump pulse delay in a typical measurement. A method of calibrating both the pump pulse offset frequency and the pump pulse field strength is outlined. A vector model is presented that explains the dynamic phase shifts in terms of precessing magnetization that is either spin locked or precessing about the effective pump field during the pump pulse. Implications of the dynamic phase shifts are discussed as they relate to setting up, calibrating and interpreting the results of DEER measurements.

Two-dimensional hyperfine sublevel correlation spectroscopy: powder features for S=1/2, I=1.

The lineshapes of two-dimensional magnetic resonance spectra of disordered or partially ordered solids are dominated by ridges of singularities in the frequency plane. The positions of these ridges are described by a branch of mathematics known as catastrophe theory concerning the mapping of one 2D surface onto another. We systematically consider the characteristics of HYSCORE spectra for paramagnetic centers having electron spin S=1/2 and nuclear spin I=1 in terms of singularities using an exact solution of the nuclear spin Hamiltonian. The lineshape characteristics are considered for several general cases: zero nuclear quadrupole coupling; isotropic hyperfine but arbitrary nuclear quadrupole couplings; coincident principal axes for the nuclear hyperfine and quadrupole tensors; and the general case of arbitrary nuclear quadrupole and hyperfine tensors. The patterns of singularities in the HYSCORE spectra are described for each case.

Water concentration profiles in membranes measured by ESEEM of spin-labeled lipids.

Electron spin-echo envelope modulation (ESEEM) spectroscopy of phospholipids spin-labeled systematically down the sn-2 chain was used to detect the penetration of water (D2O) into bilayer membranes of dipalmitoyl phosphatidylcholine with and without 50 mol % cholesterol. Three-pulse stimulated echoes allow the resolution of two superimposed 2H-ESEEM spectral components of different widths, for spin labels located in the upper part of the lipid chains. Quantum chemical calculations (DFT) and ESEEM simulations assign the broad spectral component to one or two D2O molecules that are directly hydrogen bonded to the N-O group of the spin label. Classical ESEEM simulations establish that the narrow spectral component arises from nonbonded water (D2O) molecules that are free in the hydrocarbon chain region of the bilayer membrane. The amplitudes of the broad 2H-ESEEM spectral component correlate directly with those of the narrow component for spin labels at different positions down the lipid chain, reflecting the local H-bonding equilibria. The D2O-ESEEM amplitudes decrease with position down the chain toward the bilayer center, displaying a sigmoidal dependence on position that is characteristic of transmembrane polarity profiles established by other less direct spin-labeling methods. The midpoint of the sigmoidal profile is shifted toward the membrane center for membranes without cholesterol, relative to those with cholesterol, and the D2O-ESEEM amplitude in the outer regions of the chain is greater in the presence of cholesterol than in its absence. For both membrane types, the D2O amplitude is almost vanishingly small at the bilayer center. The water-penetration profiles reverse correlate with the lipid-chain packing density, as reflected by 1H-ESEEM intensities from protons of the membrane matrix. An analysis of the H-bonding equilibria provides essential information on the binding of water molecules to H-bond acceptors within the hydrophobic interior of membranes. For membranes containing cholesterol, approximately 40% of the nitroxides in the region adjacent to the lipid headgroups are H bonded to water, of which ca. 15% are doubly H bonded. Corresponding H-bonded populations in membranes without cholesterol are ca. 20%, of which ca. 6% are doubly bonded.