Showing papers on "Relaxation (NMR) published in 1972"

Anomalous low-temperature thermal properties of glasses and spin glasses

Abstract: We show that a linear specific heat at low temperatures for glass follows naturally from general considerations on the glassy state. From the same considerations we obtain the experimentally observed anomalous low-temperature thermal conductivity, and we predict an ultrasonic attenuation which increases at low temperatures. Possible relationships with the linear specific heat in magnetic impurity systems are pointed out. We suggest experimental study of the relaxation of thermal and other properties.

Electron Spin Resonance: Elementary Theory and Practical Applications

Abstract: 1 Basic Principles of Electron Spin Resonance.- 1-1 Introduction.- 1-2 Energy of Magnetic Dipoles in a Magnetic Field.- 1-3 Quantization of Angular Momentum.- 1-4 Relation between Magnetic Moments and Angular Momenta.- 1-5 Interaction of Magnetic Dipoles with Electromagnetic Radiation.- 1-6 Characteristics of the g Factor.- Problems.- 2 Basic Instrumentation of Electron Spin Resonance.- 2-1 A Simple ESR Spectrometer.- 2-2 Choice of Experimental Conditions.- 2-3 Typical Spectrometer Arrangement.- 2-3a The Cavity System.- 2-3b The Source.- 2-3c The Magnet System.- 2-3d The Modulation and Detection Systems.- 2-4 Line Shapes and Intensities.- References.- Problems.- 3 Nuclear Hyperfine Interaction.- 3-1 Introduction.- 3-2 Origins of the Hyperfine Interaction.- 3-3 Energy Levels of a System with One Unpaired Electron and One Nucleus with I = 1/2.- 3-4 The Energy Levels of a System with S = 1/2 and I=1.- 3-5 Summary.- Problems.- 4 Analysis of Electron Spin Resonance Spectra of Systems in the Liquid Phase.- 4-1 Introduction.- 4-2 Energy Levels of Radicals Containing a Single Set of Equivalent Protons.- 4-3 ESR Spectra of Radicals Containing a Single Set of Equivalent Protons.- 4-4 ESR Spectra of Radicals Containing Multiple Sets of Equivalent Protons.- 4-5 Hyperfine Splittings from Other Nuclei with I = 1/2.- 4-6 Hyperfine Splittings from Nuclei with I > 1/2.- 4-7 Useful Rules for the Interpretation of Spectra.- 4-8 Other Problems Encountered in the ESR Spectra of Free Radicals.- 4-9 Second-order Splittings.- Problems.- 5 Interpretation of Hyperfine Splittings in ?-type Organic Radicals.- 5-1 Introduction.- 5-2 Molecular Orbital Energy Calculations.- 5-3 Unpaired Electron Distributions.- 5-4 The Benzene Anion and Its Derivatives.- 5-5 The Anions and Cations of the Polyacenes.- 5-6 Other Organic Radicals.- 5-7 Summary.- References-HMO Method.- Problems.- 6 Mechanism of Hyperfine Splittings in Conjugated Systems.- 6-1 Origin of Proton Hyperfine Splittings.- 6-2 Sign of the Hyperfine Splitting Constant.- 6-3 Extension of the Molecular Orbital Theory to Include Electron Correlation.- 6-4 Alkyl Radicals-A Study of Q Values.- 6-5 The Effect of Excess Charge on the Parameter Q.- 6-6 Methyl-proton Hyperfine Splittings-Hyperconjugation.- 6-7 Hyperfine Splitting by Nuclei Other than Protons.- Problems.- 7 Anisotropic Interactions in Oriented Systems with S = 1/2.- 7-1 Introduction.- 7-2 A Simple Example of Anisotropy of g.- 7-3 Systems with Orthorhombic or Lower Symmetry.- 7-4 Experimental Determination of the g Tensor in Oriented Solids.- 7-5 Anisotropy of the Hyperfine Coupling.- 7-6 Origin of the Anisotropic Hyperfine Interaction.- 7-7 Determination of the Elements of the Hyperfine Tensor.- 7-8 Corrections to Hyperfine Tensor Elements.- 7-9 Line Shapes in Nonoriented Systems.- 7-9a Line Shapes for Systems with Axial Symmetry.- 7-9b Hyperfine Line Shapes for an Isotropic g Factor, S = 1/2 and One Nucleus with I = 1/2.- Problems.- 8 Interpretation of the ESR Spectra of Systems in the Solid State.- 8-1 Generation of Free Radicals in Solids.- 8-2 ?-type Organic Radicals.- 8-2a Identification.- 8-2b Aliphatic Radicals.- 8-2c Radicals from Unsaturated Organic Compounds.- 8-3 ?-type Organic Radicals.- 8-4 Inorganic Radicals.- 8-4a Identification of Radical Species.- 8-4b Structural Information.- 8-5 Point Defects in Solids.- 8-5a Generation of Point Defects.- 8-5b Substitutional or Interstitial Impurities.- 8-5c Trapped-electron Centers.- 8-5d Trapped-hole Centers.- References.- Problems.- 9 Time-dependent Phenomena.- 9-1 Introduction.- 9-2 Spin-lattice Relaxation Time.- 9-3 Other Sources of Line Broadening.- 9-3a Inhomogeneous Broadening.- 9-3b Homogeneous Broadening.- 9-4 Mechanisms Contributing to Line Broadening.- 9-4a Electron Spin-Electron Spin Dipolar Interactions.- 9-4b Electron Spin-Nuclear Spin Interactions.- 9-5 Chemical Line-broadening Mechanisms.- 9-5a General Model.- 9-5b Electron-spin Exchange.- 9-5c Electron Transfer.- 9-5d Proton Exchange.- 9-6 Variation of Linewidths within an ESR Spectrum.- 9-6a Time-dependent Hyperfine Splitting for a Single Nucleus.- 9-6b Time-dependent Hyperfine Splittings for Systems with Several Nuclei.- 9-7 Spectral Effects of Slow Molecular Tumbling Rates.- 9-8 Spectral Effects of Rapid Molecular Tumbling Rates-Spin-rotational Interaction.- 9-9 Summary.- Problems.- 10 Energy-level Splitting in Zero Magnetic Field The Triplet State.- 10-1 Introduction.- 10-2 The Spin Hamiltonian for S = 1.- 10-3 State Energies for a System with S = 1.- 10-4 The Spin Eigenfunctions for a System with S=1.- 10-5 Electron Spin Resonance of Triplet-state Molecules.- 10-6 Line Shapes for Randomly Oriented Systems in the Triplet State.- 10-7 The "?MS = 2" Transitions.- 10-8 Triplet Ground States.- 10-9 Carbenes and Nitrenes.- 10-10 Thermally Accessible Triplet States.- 10-11 Biradicals Exchange Interaction.- 10-12 Systems with S > 1.- Problems.- 11 Transition-metal Ions. I..- 11-1 States of Gaseous Transition-metal Ions.- 11-2 Removal of Orbital Degeneracy in Crystalline Electric Fields.- 11-3 The Crystal Field Potential.- 11-4 The Crystal Field Operators.- 11-5 Crystal Field Splittings of States for P-, D- and F-state Ions.- 11-6 Spin-orbit Coupling and the Spin Hamiltonian.- 11-7 D- and F-state Ions with Orbitally Nondegenerate Ground States.- 11-7a D-state Ions 3d1(ttdl + ttgl) in 3d1(cubal + ttgl) 3d7(1s)(oct + ttgl) 3d9(oct + ttgl).- 11-7b F-state Ions 3d8(oct) 3d2(ttdl) 3d8(oct + ttgl) 3d2(ttdl + ttgl) 3d3(oct) 3d7(hs)(ttdl) 3d3(oct + ttgl).- 11-8 S-state Ions 3d5(hs)(oct) 3d5(hs)(oct + ttgl).- Problems.- 12 Transition-metal Ions. II. Electron Resonance in the Gas Phase.- 12-1 Ions in Orbitally Degenerate Ground States.- 12-1a D-state Ions 3d1(oct) 3d1(oct + ttgl), ? > > ? > > ? 3d1(oct + ttgl), ? > > ? ? ? 3d1(oct + trgl) 3d5(1s)(oct + ttgl) 3d9(ttdl + ttgl) 3d6(hs)(oct).- 12-1b F-state Ions 3d2(oct) 3d2(oct + trgl) 3d7(hs)(oct).- 12-1c Jahn-Teller Splitting 3d9(oct) 3d7(1s)(oct).- 12-2 Elements of the 4d and 5d Groups (Palladium and Platinum Groups).- 12-3 The Rare-earth Ions.- 12-4 The Actinide Ions.- 12-5 Deficiencies of the Point-charge Crystal Field Model Ligand-Field Theory.- 12-6 Electron Resonance of Gaseous Free Radicals.- 12-7 The Practical Interpretation of ESR Spectra of Ions in the Solid State.- Problems.- 13. Double-resonance Techniques.- 13-1 An ENDOR Experiment.- 13-2 Energy Levels and ENDOR Transitions.- 13-3 Relaxation Processes in Steady-state ENDOR.- 13-4 An ENDOR Example: The F Center in the Alkali Halides.- 13-5 ENDOR in Liquid Solutions.- 13-6 ENDOR in Powders and Nonoriented Solids.- 13-7 Electron-electron Double Resonance.- Problems.- 14. Biological Applications of Electron Spin Resonance.- 14-1 Introduction.- 14-2 Substrate Free Radicals.- 14-3 Flavins and Metal-free Flavoproteins.- 14-4 Photosynthesis.- 14-5 Heme Proteins.- 14-6 Iron-sulfur Proteins.- 14-7 Spin Labels.- Appendix A. Mathematical Operations.- A-1 Complex Numbers.- A-2 Operator Algebra.- A-2a Properties of Operators.- A-2b Eigenvalues and Eigenfunctions.- A-3 Determinants.- A-4 Vectors: Scalar, Vector, and Outer Products.- A-5 Matrices.- A-5a Addition and Subtraction of Matrices.- A-5b Multiplication of Matrices.- A-5c Special Matrices and Matrix Properties.- A-5d Dirac Notation for Wave Functions and Matrix Elements.- A-5e Diagonalization of Matrices.- A-6 Tensors.- A-7 Perturbation Theory.- A-8 Euler Angles.- Problems.- Appendix B. Quantum Mechanics of Angular Momentum.- B-1 Introduction.- B-2 Angular-momentum Operators.- B-3 The Commutation Relations for the Angular-momentum Operators.- B-6 Angular-momentum Matrices.- B-7 Addition of Angular Momenta.- B-8 Summary.- Problems.- C-1 The Hamiltonian for the Hydrogen Atom.- C-2 The Spin Eigenfunctions and the Energy Matrix for the Hydrogen Atom.- C-3 Exact Solution of the Determinant of the Energy Matrix (Secular Determinant).- C-4 Selection Rules for High-field Magnetic-dipole Transitions in the Hydrogen Atom.- C-5 The Transition Frequencies in Constant Magnetic Field with a Varying Microwave Frequency.- C-6 The Resonant Magnetic Fields at Constant Microwave Frequency.- C-7 Calculation of the Energy Levels of the Hydrogen Atom by Perturbation Theory.- C-8 Wave Functions and Allowed Transitions for the Hydrogen Atom at Low Magnetic Fields.- Problems.- Appendix D. Experimental Methods Spectrometer Performance.- D-1 Sensitivity.- D-2 Factors Affecting Sensitivity and Resolution.- D-2a Modulation Amplitude.- D-2b Modulation Frequency.- D-2c Microwave Power Level.- D-2d The Concentration of Paramagnetic Centers.- D-2e Temperature.- D-2g Microwave Frequency.- D-2h Signal Averaging.- D-3 Absolute Intensity Measurements.- Problems.- Table of Symbols.- Name Index.

Carbon-13 nuclear magnetic resonance spectroscopy

Abstract: Partial table of contents: Introduction, Theory, and Methods. 13C NMR Spectral Characteristics. Aliphatic Compounds. Aromatics. Organic Functional Groups. Ions, Radicals, and Complexes. Polymers. Relaxation Studies. Biomolecules. Special Techniques and Applications. Appendix. Index.

Electrical Relaxation in Na2O·3SiO2 Glass

Abstract: The electrical relaxation associated with alkali diffusion in Na2O·3SiO2 glass was studied from 0.2 Hz to 700 kHz at –1° to 163°C. A formalism for analysis of electrical relaxation in conducting dielectrics which associates the nonexponential decay of the electric field to zero and the dispersions in the dielectric constant and the conductivity with a distribution of relaxation times for the electric field was developed and is shown to be in qualitative accord with current molecular theories of electrical relaxation in alkali silicate glasses. A relation between the dc conductivity, the limiting high-frequency dielectric constant, and the average electric field or conductivity relaxation time was derived and is verified experimentally for the Na2O·3SiO2 glass. The distribution of electric-field relaxation times for the glass is broad, asymmetric on a logarithmic scale, and weighted in favor of the shorter relaxation times; the distribution narrows with increasing temperature. A reduced electrical relaxation curve which can be used to compare electrical and mechanical relaxations in Na2O·3SiO2 glass was generated.

Hydration of monosaccharides: A study by dielectric and nuclear magnetic relaxation

Abstract: NMR studies indicate that the relaxation rate of17O-enriched water is enhanced in monosaccharide solutions, and it is greater in hexose solutions than with a pentose, ribose. Three dielectric relaxations have been isolated and assigned to bulk water

Nitrogen NMR Spectroscopy

Abstract: Publisher Summary Nitrogen nuclear magnetic resonance (NMR) is finding new applications. This chapter presents some of the applications of the nitrogen NMR. The element nitrogen in its natural occurrence has two isotopes; 14N and 15N. The nuclei of both possess magnetic moments; consequently, both give nuclear magnetic resonance (NMR) spectra and additionally may affect the spectra of other nuclei. The NMR spectra of nitrogen nuclei may provide an important contribution to the development of the general theory of chemical shifts, by allowing a rigorous check of current theories. This arises from the variety of nitrogen valence states occurring in the molecules, including more or less non-bonding electron pairs, many types of bonds of varying polarity and hydrogen bonding, as well as the common occurrence of nitrogen atoms in both neutral and ionic species. 14N nucleus has an electric quadrupole moment and this is responsible for producing broad-line NMR spectra. Broad lines often occur both in the spectrum of the 14N nucleus and in the spectrum of any nucleus spin coupled to the nitrogen nucleus that may preclude the detection of any spin-spin coupling between these nuclei. The electric quadrupole relaxation process of the 14N nucleus may influence the NMR spectra of nuclei that are spin-coupled to it. The most frequently encountered example is the N-H coupling. The chapter presents some examples of nitrogen chemical shifts in the organic and inorganic molecules. However, spin-spin coupling, involving nitrogen nuclei, is readily observed for the 15N isotope than for 14N. As 15N nuclei possess a spin of 1/2 and no quadrupole moment, these give NMR spectral patterns similar to those arising from lH and 19F interactions.

Optical Free Induction Decay

Abstract: A simple application of the Stark-pulse technique, developed by Brewer and Shoemaker, demonstrates optical free induction decay-the optical analog of free induction decay in NMR. A molecular sample which is coherently prepared by a cw laser beam exhibits such a decay when it is suddenly switched out of resonance by a Stark field. Observations are presented for a nondegenerate Doppler-broadened infrared transition of N${\mathrm{H}}_{2}$D, where the sample is optically thin, and the decay behavior can be compared quantitatively with a solution of the Bloch equations. When the molecular sample is prepared under steady-state conditions, the solutions are analytic; for pulse excitation, a numerical solution is required. The treatment invokes a hard-collision relaxation model. Such characteristics as the abrupt termination of the decay and the related edge echo, which result from Doppler dephasing, can be examined for Stark pulses of finite extent.

The theory of magnetic resonance

Abstract: Mathematical and Quantum-Mechanical Background General Two-Spin (1/2,1/2) Systems NMR Two-Spin (1/2,1/2) Systems ESR Two-Spin (1/2, 1/2) Systems Anisotropic Hamiltonians Multispin Systems High-Spin Systems Mossbauer Resonance Atomic Spectra and Crystal Field Theory Line Shapes Double Resonance Electron Nuclear Double Resonance Electron-Electron Double Resonance Dynamic Polarization Nuclear-Nuclear Double Resonance Acoustic, Muon and Optical Magnetic Resonance Spin Labels Fourier Transform Nuclear Magnetic Resonance Index.

Redistribution of resonance radiation. I - The effect of collisions.

Abstract: The techniques of modern line-broadening theory are used to investigate the scattering of polarized radiation in the rest frame of an atom undergoing collisions. The formulation explicitly includes both elastic and inelastic (quenching) collisions. When the lower state has zero width, a form for the redistribution function similar to that of Zanstra is obtained, but with the redistribution in the neighborhood of the resonance line being caused solely by elastic collisions. In the limit of no collisions, but with both levels of finite lifetime, the result of Weisskopf and Woolley is obtained. The effect of level-degeneracy is also explicitly included; in this case the results are a function of the polarization of the light and the different relaxation rates for the multipolar components of the atomic states.

Critical-Point Anomalies in the Electron-Paramagnetic-Resonance Linewidth and in the Zero-Field Relaxation Time of Antiferromagnets

Abstract: A theory is presented for the anomalies in the electron-paramagnetic-resonance (EPR) linewidth and zero-field relaxation time of antiferromagnets. The analysis applies to the paramagnetic state immediately above the N\'eel point in systems where the dominant spin-spin interaction is the isotropic exchange coupling. It is assumed that the dipolar coupling is the principal source of anisotropy. The EPR linewidth and the relaxation rates for fluctuations in the total magnetization are separated into critical and noncritical parts; the latter are approximated by their values in the high-temperature limit. The anomalous increases in the linewidths and the relaxation rates are shown to arise from processes in which a fluctuation in the total magnetization decays into two fluctuations of the staggered magnetization via the dipolar coupling. The predictions of the theory are compared with linewidth measurements in RbMn${\mathrm{F}}_{3}$, Mn${\mathrm{F}}_{2}$, MnO, and MnS.

Dielectric Properties of Some Nematic Liquid Crystals with Strong Positive Dielectric Anisotropy

Abstract: Measurements of the temperature dependence of the dielectric constants e‖, e⊥, and eis of some nematic liquid crystals with strong positive dielectric anisotropy (e‖ ≃ 3 e ⊥) are reported. The low frequency dispersion regions of the dielectric constants parallel to the unique axes were measured to be in the range from 10 kHz to 10 MHz. A correlation between the lengths of similar molecules and their relaxation frequency in the nematic state was found. The height of the potential barrier which hinders the reversal of the rodlike molecules in the nematic order was determined by measuring the temperature dependence of the low frequency relaxation and comparison with the temperature dependence of viscosity.

Rotational Diffusion of Symmetric Top Molecules in Liquids

Abstract: The extended M‐ and J‐diffusion models previously proposed to describe the reorientation of linear and spherical molecules are used to treat liquids composed of symmetric top molecules. The reorientational correlation functions and their Fourier transforms are determined and shown to reduce, in the J‐diffusion model in the limit of very short angular momentum correlation times τJ, to the results obtained by solution of the anisotropic diffusion equation. The applicability of the diffusion equation results, outside the domain of validity of the diffusion equation, are examined in detail. Magnetic spin relaxation due to motional modulation of spin‐rotational interactions is also considered within the framework of the extended diffusion models, and the inertial motion of the molecules and the precessional motion of the angular momentum are shown to influence the contributions due to the modulation of the anisotropic part of the spin‐rotational interactions.

Apparatus and method for detecting cancer in tissue

Abstract: An apparatus and method in which a tissue sample is positioned in a nuclear induction apparatus whereby selected nuclei are energized from their equilibrium states to higher energy states through nuclear magnetic resonance. By measuring the spin-lattice relaxation time and the spin-spin relaxation time as the energized nuclei return to their equilibrium states, and then comparing these relaxation times with their respective values for known normal and malignant tissue, an indication of the presence and degree of malignancy of cancerous tissue can be obtained.

Spin Relaxation in Nematic Solvents

Abstract: The electron spin relaxation processes for oblate and prolate symmetric top paramagnetic probes dissolved in liquid crystals are studied by solving the diffusion equation for molecules subjected to an orienting potential, expressed as a series expansion of Legendre polynomials. The line shape variations upon ordering have been calculated for vanadyl complexes and nitroxide radicals, and the effect of the contributions of quadratic and quartic terms of the potential energy function, and of the diffusion tensor anisotropy have been examined.

On a T1 and T1ρ Study of Molecular Motion and Phase Transitions in the Tetramethylammonium Halides

Abstract: The temperature dependences of methyl group and (CH3)4N+ ion reorientations in the tetramethylammonium salts [(CH3)4N+X− where X is Cl, Br, I] were studied by means of the proton spin‐lattice relaxation times in the static and rotating frame (T1 and T1ρ). The T1 values calculated for the motions are in good agreement with the double minima observed for each compound. Activation energies and frequency factors were obtained for both motions from the temperature dependence of T1 in the three salts. Rotating frame spin‐lattice relaxation times were also measured over temperatures from 200 to 380°K for (CH3)4NCl, giving results in agreement with those for T1. A discontinuity in the T1 of (CH3)4NCl was found near 418°K. This led us to make a preliminary DTA investigation with results indicating that a first‐order phase transition may occur near 434°K and another definitely at 528 ± 1°K. The appearance of the phase above 434°K is accompanied by a modulation on the free induction envelope which we ascribe to piez...

Intermediate Level Structure in Highly Excited Electronic States of Large Molecules

Abstract: In this paper we consider the radiative decay of excited electronic states of a large molecule which correspond to the dense intermediate level spacing situation, encountered for the second excited singlet state of some aromatic hydrocarbons, which is separated from the first excited singlet state by a small (3000 to 4000 cm$^{-1}$) electronic energy gap. Intramolecular interstate coupling and the interaction with the radiation field were handled by a self-consistent extension of the Wigner-Weisskopf approximation. We were able to derive a general expression for the time and energy resolved decay spectrum of a highly excited state of a large molecule, which yields information concerning the decay pattern, for the fluorescence lifetime(s) and for the corresponding quantum yields, in different spectral regions. Weak and strong coupling limits can be distinguished and defined in terms of the magnitude of the non-adiabatic interstate coupling terms relative to the spacing and the width of the zero states. In the intermediate level spacing in a large molecule, the radiative decay of the excited state should be considered in terms of the decay of a finite set of coupled levels and intramolecular relaxation between these states does not occur in the 'isolated' molecule. Resonance fluorescence is amenable to experimental observation from the second excited singlet state of a large molecule which corresponds to this intermediate case. The weak and strong coupling situations can be experimentally distinguished on the basis of energy resolved lifetime measurements. General criteria have been provided for the observation of quantum interference effects in the radiative decay for different coupling schemes. The present theoretical results provide a proper interpretation of some recent experimental data concerning the radiative decay of the second excited state of some aromatic molecules (3,4-benzpyrene and naphthalene).

Nuclear Magnetic Resonance Measurements at High Pressure

Abstract: Instrumentation for the measurement of spin lattice relaxation times in liquids at pressures up to 5 kilobar and temperatures from −50 to 350°C is described. The experimental setup allows the use of the Fourier transform technique to obtain high resolution spectra and/or relaxation of chemically shifted nuclei under these extreme conditions.

Absorption and Emission Line-Shape Functions for Driven Atoms

Abstract: The effect of a driving or pump field on the emission and absorption line-shape functions for an atom is discussed. The pump field is assumed to oscillate at a frequency near the resonance frequency for transitions between a single pair of atomic states, and transitions between one of these states and any other state of the atom are analyzed. The pump field is treated classically, and atomic relaxation is treated in general terms. The emission and absorption line-shape function (the latter is defined as the rate of absorption of energy from a weak signal field, applied in addition to the pump field, as a function of the signal-field frequency) are found by evaluating the relevant two-time atomic correlation functions in the Markoff approximation. In the limit of high pump-field intensity, both the absorption and the emission spectra are doubly peaked at frequencies which differ from the usual resonance frequency by $\ifmmode\pm\else\textpm\fi{}\frac{1}{2} \ensuremath{\Omega}$, where $\ensuremath{\Omega}$ is the frequency of the pump-field-induced oscillations in the populations of the two strongly coupled states. The absorption and the emission spectra are represented by essentially the same function in the limit of high pump-field intensity, as they are also in the limit of vanishing pump-field intensity. For intermediate pump-field intensities, however, the two functions are quite different in form, and no simple proportionality exists between them.

[29] Nuclear relaxation measurements of water protons and other ligands

Abstract: Publisher Summary This chapter discusses the way nuclear relaxation rates are measured and only those aspects of the theory that elucidate the methods and their applications. The measurement of the relaxation rates of magnetic nuclei is a specialized branch of nuclear magnetic resonance spectroscopy, which, especially when carried out with paramagnetic probes, can provide thermodynamic, structural, and kinetic information on enzyme complexes. Specifically, the stoichiometry and dissociation constants of binary and ternary complexes of enzymes with paramagnetic ions, paramagnetic substrate analogs, and diamagnetic substrates may be measured. Coordination schemes and interatomic distances between enzyme-bound paramagnetic metal ions or substrate analogs and the substrate molecules have been determined. The exchange rates of substrates into paramagnetic and diamagnetic environments on enzymes have been estimated. By analogy, when a population of magnetic nuclei is placed in a magnetic field, the magnetic vectors experience a torque and precess about the direction of the field. Energy may be applied to this system to align the magnetic vectors of the nuclei to precess in phase with each other.

Evidence for Rb‐Rare‐Gas Molecules from the Relaxation of Polarized Rb Atoms in a Rare Gas. Experimental Results

Abstract: Relaxation experiments have been performed between 0–200 G on optically polarized Rb atoms in argon, krypton or xenon, at pressures ranging from 0.1 to 20 torr. They clearly show the existence of Rb–Ar, Rb–Kr and Rb–Xe molecules of low binding energy (smaller than kT). Molecular parameters such as lifetime, spin‐orbit coupling constant, formation rate, constant of mass action, etc., are measured, or evaluated when they cannot be directly measured. Experimental values of the diffusion coefficient of Rb in the gas (except xenon) and of the disorientation cross sections by usual binary collisions are also obtained.

ESR and NMR in Aqueous and Methanol Solutions of Copper(II) Solvates. Temperature and Magnetic Field Dependence of Electron and Nuclear Spin Relaxation

Abstract: ESR line‐shape measurements of Cu(ClO4)2 solutions in methanol and water at X‐ and Q‐band frequencies are reported. In both frequencies, above room temperature, the over‐all linewidth increases with increasing temperature, while at low temperatures the slopes level off and (at Q band) even change signs. The results are quantitatively interpreted in terms of two main electronic relaxation mechanisms, viz., modulation of the spin rotation and anisotropic g‐tensor interactions. From the magnetic parameters of the complexes determined in frozen solution and the field dependence of the linewidth, the correlation times are determined. It is found that in both solvents the anisotropic g tensor is modulated by the fast hopping of the complex distortion axis rather than by the molecular tumbling of the complex. Assuming a random jump of the distortion axis between the three octahedral principal directions, the kinetic parameters of this hopping process are calculated to be τi(25°C)=1.6× 10−11 sec−1, ΔEi=1.2 kcal/m...