Showing papers by "Jack H. Freed published in 1993"

Full determination of the rotational diffusion tensor by electron paramagnetic resonance at 250 GHz

Abstract: High-frequency (250 GHz) electron paramagnetic resonance (EPR) spectra in the limit of motional narrowing have been studied for a nitroxide spin probe diffusing in a low-viscosity isotropic solvent. The enhanced sensitivity of the 250-GHz spectrum to rotational modulation of the g tensor reveals new details of the microscopic motion of the spin probe. Specifically, it is shown how independent linear constraints imposed by line-width measurements at 250 GHz and a lower frequency may be used to fully determine the diffusion tensor R in the general case R x ¬=;R y ¬=;R z

250-GHz EPR of nitroxides in the slow-motional regime : models of rotational diffusion

Abstract: A 250-GHz electron paramagnetic resonance (EPR) study of the slow rotational diffusion of two spin probes in toluene, viz., perdeuterated 2,2,6,6-tetramethyl-4-piperidone (PDT) and 3-doxylcholestane (CSL) is presented. EPR spectra were obtained in the slow-motional and near-rigid limit regions, which corresponds to rotational correlation times 10 -10 >τ R >10 -6 s. These two probes differ significantly in size and shape, permitting a detailed exploration of the sensitivity of 250-GHz EPR to different aspects of the molecular dynamics such as rotational anisotropy and non-Brownian diffusion. Nonlinear least-squares fitting based on full stochastic Liouville calculations provides a sensitive means for discriminating amongst motional models

A 250-GHz ESR Study of Highly Distorted Manganese Complexes+

Abstract: ESR spectra at 250 GHz and magnetic fields ranging from 45 to 95 kG have been measured for the compounds Mn(y-picoline)4X2 and Mn(o-phenanthroline)zXz, where X = C1, Br, and I. These spectra are in the high-field limit and well resolved; hence, they are inherently simple to analyze. We find these spectra to be very sensitive to the precise values of the zero-field splitting (zfs) parameters. Thus D and p (E/D) can be estimated directly from the spectrum, with very accurate values obtained using third-order perturbation theory for the electron spin Hamiltonian of high-spin Mn2+. The contributions of all five allowed electron spin transitions are observed and successfully simulated. The y-picoline complexes have axial symmetry and D increases from 0.186 to 0.999 cml with the size of the halogen. The o-phenanthroline complexes show a wide range of rhombic distortion and E increases linearly with the magnitude of D. The chloride compound is nearly axial (D = 0.124 cml, p = 0.04), while the iodide compound is highly distorted (D = 0.590 cm-l, p = 0.246). To demonstrate the applicability of the high-frequency ESR technique to biological samples, we have also measured the high-field spectrum of Mn(I1) protoporphyrin IX and determined its zfs parameters to be D = 0.775 cml and 7 = 0.048.

A two‐dimensional Fourier transform electron‐spin resonance (ESR) study of nuclear modulation and spin relaxation in irradiated malonic acid

Abstract: Nuclear modulation in electron‐spin‐echo spectroscopy is conventionally studied by one‐dimensional electron‐spin‐echo envelope modulation (1D‐ESEEM). Two‐dimensional Fourier transform electron‐spin resonance (2D‐FTESR) studies of nuclear modulation have the promise of enhancing the spectral resolution and clarifying the key details of the relaxation processes. We present a 2D‐FTESR study on single proton nuclear modulation from γ‐irradiated malonic acid single crystals to test the validity of the Gamliel–Freed theory and to assess the value of the new methods. The two pulse spin‐echo correlation spectroscopy (SECSY) spectra as a function of orientation of the single crystal show very good agreement with the Gamliel–Freed theory extended to the general case of nonaxially symmetric hyperfine interaction. It is very simply affected by spin relaxation, such that relative intensities are essentially unaffected. Thus SECSY‐ESR can most reliably be utilized for studying nuclear modulation. Stimulated SECSY provi...

Translational diffusion in a smectic‐A phase by electron spin resonance imaging: The free‐volume model

Abstract: The method of dynamic imaging of diffusion (DID)‐ESR (electron spin resonance) has been utilized to study the anisotropy of translational diffusion of spin probes in the smectic A phase of a eutectic liquid crystal, S2. In particular, the nearly spherical perdeuterated‐TEMPONE (PDT) and the rigid and elongated cholestane spin label (CSL) molecules were studied. Whereas D⊥ (the coefficient of diffusion perpendicular to the nematic director) showed simple Arrhenius dependence for both probes, diffusion parallel to the director displayed two different temperature regimes with a changeover of D∥ at t*≊26–27 °C. The regime above (below) t* is characterized by weak (strong) translational ordering. For CSL the ratio D⊥/D∥<1 above t* which indicates nematiclike behavior, but below t* the behavior is more smecticlike, i.e., D⊥/D∥≳1; for PDT D⊥/D∥≳1 over the whole temperature range. A free volume model is developed to interpret the activation energies associated with D⊥ and D∥ (i.e., E⊥ and E∥) in terms of the orie...

250- and 9.5-GHz EPR studies of an electride and two alkalides

Abstract: The EPR spectra of polycrystalline samples of Cs+( 18-crown-6)2X-, in which X- = e-, Na-, or Cs-, were studied at both X-band (9 GHz) and at 250 GHz. The high frequency affords much better g-factor resolution and reveals at least two types of asymmetric electron sites in each of these compounds. The defect electrons detected in the alkalides Cs+( 18C6)2Na- and Cs+( 18C6)2Cs-appear to be located at isolated centers in the crystal lattice with the strongest signal originating from electrons trapped at anion vacancies. These species exhibit broadening due to unresolved superhyperfine interactions at the periphery of the trapping site. In contrast, the X-band spectra of the two-electron sites in the electride appear to be exchange-narrowed, indicating that the electron spins have some degree of mobility within the crystal lattice. Comparison of the 9- and 250-GHz spectra of the electride places an upper bound of about 1 X lo7 s-l on the rate of spin exchange. The temperature and saturation behavior of the electride at X-band are consistent with the YF-centern model, in which electrons serve to balance the charge of the complexed cations, with the center of charge of each electron at an anionic site. The strongest signal in Cs+( 18C6)ze- has nearly the sameg-anisotropy as one of the signals from the isostructural sodide, Cs+( 18C6)2Na-, indicating that at least one of the electron-trapping sites detected in the two crystal lattices is the same. The temperature dependence of the two EPR signals from Cs+(18C6)2Cs- at X-band suggests an activated process that populates two types of unpaired electron sites from a common spin-paired precursor.