Showing papers on "Relaxation (NMR) published in 1980"

Dipolar relaxation and nuclear Overhauser effects in nonrigid molecules: The effect of fluctuating internuclear distances

Abstract: A simple treatment of the dipolar contribution to nuclear magnetic relaxation is developed for molecules in which conformational fluctuations modulate the relevant internuclear distances. Expressions are given for AX system spectral densities using a general model in which conformational fluctuations occur as random jumps among discrete conformations, while the molecule as a whole undergoes rotational diffusion as either a spherical or a symmetric top. Approximations valid for proton spin systems are given for cases in which the jumping rates are either fast or slow compared to rotational diffusion; the results are independent of the jumping rates. Similar results are obtained for cross‐correlation spectral densities. For complex spin systems, e.g., An1Mn2Xn3..., the cross‐relaxation constants σij which couple pairs of magnetizations, depend only upon autocorrelation, spectral densities and are thus easily obtained from the AX system results. Measurement of σij by time resolved Overhauser effect experimen...

Deuteron spin alignment: A probe for studying ultraslow motions in solids and solid polymers

Abstract: Deuteron spin alignment offers a new possibility to investigate extremely slow rotational motions in solids and solid polymers. A convenient theoretical description of the creation and detection of spin alignment by application of the Jeener–Broekaert pulse sequence is given for both static and slowly time dependent quadrupole coupling, as well as for spin–lattice relaxation of spin alignment. It is shown that the NMR signal following spin alignment yields a correlation function of the time dependent quadrupole coupling. This correlation function is evaluated explicitly for a deuteron on the corner of a regular tetrahedron undergoing tetrahedral jumps. Various applications of deuteron spin alignment are demonstrated experimentally, e.g., its use to obtain undistorted deuteron line shapes, from which the orientational distribution of partially ordered solids or solid polymers may be determined. In solid polyethylene it is shown that not only the deuterons in the crystalline regions but those in the mobile ...

Time‐correlation functions for restricted rotational diffusion

Abstract: The dynamical model of rotational diffusion of rod‐shaped molecules in a conical volume is applied to the calculation of time autocorrelation functions of 1st and 2nd order spherical harmonic functions of the rod’s orientation angles. Numerical results are obtained for the various correlation functions as a function of the polar angle ϑ0 within which the molecule’s axis is confined. Applications to dielectric relaxation, dynamic light scattering, and fluorescence depolarization experiments are pointed out.

Optical dephasing and vibronic relaxation in molecular mixed crystals: A picosecond photon echo and optical study of pentacene in naphthalene and p‐terphenyl

Abstract: Picosecond photon echo and spectroscopic measurements have been used to establish that optical dephasing in the pure electronic and several vibronic transitions of pentacene in naphthalene is induced by pseudolocal phonon scattering in the ground and excited state. The frequency and low‐temperature lifetime of this local phonon are 18 cm−1 and 3.5 ps in the groundstate and 13.8 cm−1 and 11 ps in the excited state (vibron independent). The deuterium isotope effect on the local phonon frequency demonstrates its molecular character and suggests its assignment as an in‐plane librational mode. For the system pentacene in p‐terphenyl photon echo measurements indicate the existence of a librational mode of ?30 cm−1 with a ?1.5 ps lifetime. Line shift measurements on both mixed crystal systems further show that the observed temperature induced shift is due to a difference in quadratic electron–bandphonon coupling of the ground and excited state and crystal anharmonicity (thermal expansion). We further report for ...

Phase transitions in crystals of chain molecules. Relation between defect structures and molecular motion in the four modifications of n-C33H68

Abstract: n-Tritriacontane (n-C33H68) exhibits three solid–solid phase transitions before melting. Applying a variety of experimental techniques, including X-ray scattering, Raman spectroscopy, n.m.r., dielectric relaxation and quasielastic neutron scattering, it was possible to specify for all four modifications defect structures and the corresponding molecular motions.Each phase transition is accompanied by a step-like decrease in the degree of order resulting from the successive onset of rotational jumps, translational jumps in chain direction and the creation and diffusion of intrachain defects.

Spin exchange : principles and applications in chemistry and biology

Abstract: 1. Introduction.- 2. Theory of Spin Exchange.- 2.1 Exchange Interaction Between Two Paramagnetic Particles.- Dependence of Interaction Energy on Spin.- Spin-Hamiltonian of Exchange Interaction.- The Nature of Exchange Interaction. The Two-Electron System.- Interaction Between Many-Electron Systems. Delocalization and Spin Polarization.- Anisotropy of Exchange Interaction.- Semi empirical Estimates of Exchange Integral.- 2.2 Qualitative Description of Spin Exchange Process.- Spin Exchange.- Spin Exchange in a Two-Electron System.- Spin Density Matrix.- Spin Motion Due to Exchange Interaction.- 2.3 Formal Kinetics of Spin Exchange in Magnetically Dilute Solutions.- Binary Collision Approximation.- Rate of Bimolecular Spin Exchange Process.- Kinetics of Spin Exchange for Particles with Spins S = 1/2.- Kinetic Equations for Free Radicals Showing Hyperfine Structure (HFS) in Their ESR Spectra.- 2.4 Spectroscopic Manifestations of Spin Exchange.- Spectral Diffusion Due to Spin Exchange.- Shape of ESR Spectra.- Slow Exchange.- Fast Exchange.- Other Methods of Measuring the Rate of Spin Exchange.- Effect of Saturation.- Electron-Electron Double Resonance (ELDOR).- Electron Spin Echo.- 2.5 Theory of Sudden Collisions.- 2.5.1 Model of Sudden Collisions.- Basic Assumptions.- Equations for the Estimation of xc.- 2.5.2 Kinetic Equations.- Steady-State Equation for the Operator of Collision Efficiency.- 2.5.3 Spin Exchange Between Radicals.- Model System.- Spin Exchange Between Radicals in the Presence of Hyperfine Interaction.- 2.5.4 Spin Exchange Between Radicals with Anisotropic Interaction.- Rotation of Partners at the Moment of Their Contact.- Orientation Relaxation of Particles in Intervals Between Reencounters.- 2.5.5 Spin Exchange of Radicals with Other Paramagnetic Particles.- Spin Exchange Between Radicals and Spins SB with Long Times of Paramagnetic Relaxation.- Shape of ESR Spectra.- The Influence of the Relaxation of Spin SB on the Rate of Exchange.- Anisotropy of Spin Exchange.- 2.5.6 Spin Exchange in Biradicals.- The Model.- Kinetic Equations.- The ESR Spectrum of Nitroxide Biradicals.- 2.6 The Model of Diffusive Passage.- The Model.- An Equation for the Operator of Collision Efficiency.- The Kinetic Equations.- The Spin Exchange Between Two Kinds of Particles with Spins SA = SB = 1/2.- Comparison of Results Obtained from Models of Sudden Collisions and Those of Diffusive Passage.- 3. Experimental Measurement of Spin Exchange Rate.- 3.1 Experimental Measurement of Spin Exchange Rate Constants from ESR Spectra.- 3.1.1 Exchange Between Identical Particles with S = 1/2.- Doublet Spectrum with a Zero Initial Width.- Complex Spectra.- Determination of Rate Constants from Exchange Broadening of Complex Spectra.- Determination of Rate Constants from Exchange Shift of the Lines of Complex Spectra.- Determination of Rate Constants from Exchange Narrowing of Complex Spectra.- Peculiarities of Exchange Narrowing in Concentrated Solutions.- Comparison of Spin Exchange Rate Constants Measured by Different Techniques.- 3.1.2 Exchange Between Different Particles.- 3.2 Other Methods of Measuring the Spin Exchange Rate Constants.- 3.2.1 Continuous Wave Saturation Method.- 3.2.2 Electron-Electron Double Resonance.- 3.2.3 Electron Spin Echo.- 3.3 Separation of Exchange and Dipole-Dipole Contributions to Concentration Broadening of ESR Lines.- Theoretical Estimation of Dipole Broadening.- Experimental Estimation of Dipole-Dipole Contribution at Low Viscosity.- Separation of the Cases of Exchange and Dipole-Dipole Broadening According to Viscosity and Temperature Dependences.- 3.4 Separation of the Cases of Weak and Strong Exchange.- 4. Spin Exchange in Chemistry and Biology.- 4.1 Study of Diffusion Collisions in Solutions.- Hydrodynamic Model of Diffusion Collisions.- Potentialities of Spin Exchange.- 4.1.1 Collisions in Single-Component Solvents.- Spin Exchange Between Radicals.- Spin Exchange Between Free Radicals and Paramagnetic Complexes.- 4.1.2 Collisions in Complex Systems.- Binary Solvents.- Liquids Near Critical Points.- Polymeric and Heterogeneous Systems.- Collisions of Macroradicals.- 4.1.3 Study of Intramolecular Collisions of Paramagnetic Fragments.- 4.2 Estimation of the Energy of Exchange Interaction During Collisions.- 4.2.1 Free Radicals.- 4.2.2 Spin Exchange with Metal Complexes.- 4.2.3 Discussion of Results.- 4.3 Collisions of Charged Particles.- 4.3.1 Experimental Results.- Spin Exchange in Solutions of Anion Radical (S03)2N02-.- Spin Exchange Involving Charged Nitroxide Radicals.- Spin Exchange Between Anion Radicals in Organic Solvents.- 4.3.2 Comparison of Experiments on Spin Exchange with the Diffusion Theory of Collisions of Charged Particles in Liquid.- 4.3.3 Study of Collisions Between Aquo Complexes of Transition Metals.- Role of Counterions in Collisions of Aquo Complexes of Vanadyl.- An Example of the Process with Weak Influence of Electrostatic Repulsion.- 4.4 Biological Applications of Spin Exchange.- Paramagnetics Used as Spin Probes.- 4.4.1 Measurement of Binary Collision Rate of Molecules in Biological Systems.- 4.4.2 Study of Location of Paramagnetic Centers in Biopolymers.- Quantitative Interpretation of the Values of Kg for Spin Exchange Processes Involving Paramagnetic Centers of Biopolymers.- 4.4.3 Development of Individual ESR Spectra of Spin Labels via Spin Exchange.- 4.4.4 Spin Exchange Titration.- Kinetics of Reactions Involving Oxygen Molecules.- Coordination of Paramagnetic Ions by Macromolecules.- 4.4.5 Other Applications.- 4.5 Conclusion.- List of Frequently Used Notations and Abbreviations.- References.

Molecular rheology of concentrated polymer systems. I

Abstract: The primitive chain model of Doi and Edwards is generalized to include the short-time relaxation process. Stress relaxation after a sudden imposition of strain is studied in detail. It is shown that in the linear region (small strain) stress relaxation occurs in two steps, the relaxation of chain segments between the fixed entanglement points, and the relaxation of the entanglement points, in accordance with the conventional picture, whereas in the nonlinear region (large strain) there appears a new relaxation process between the above two. The characteristic time of this process is the Rouse relaxation time which the entire chain would have if there were no entanglements, and increases with the square of the molecular weight. This result is consistent with experimental observations.

Self-diffusion of n-paraffins in NaX zeolite

Abstract: Self-diffusion coefficients for intracrystalline diffusion of n-CnH2n+2 molecules (n= 1–18) adsorbed in NaX zeolite have been measured by means of the n.m.r. pulsed field gradient technique as a function of temperature (–150 to +220°C) and of sorbate concentration. While the self-diffusion coefficients decrease over two orders of magnitude, the activation energy remains constant with increasing pore-filling factor. Only for methane and ethane are significant changes in the activation energies observed. For the light paraffins (n⩽ 7) the pre-exponential factors of Henry constants and self-diffusion coefficients can be estimated from the activation energy of self-diffusion. For n 7 the pre-exponential factors of the self-diffusion coefficients of the sorbate (as well as of the liquid) obey the relation D(n)≈D(n0) exp[–0.3(n—n0)].The concentration dependence of the self-diffusion coefficients can be described quantitatively using a modification of the free volume theory. In contrast to its original application to pure liquids where the mean jump lengths are assumed to be constant, the peculiarity of zeolitic diffusion is taken into account by equating the jump lengths with the cube root of the molecular free volume.Additional information about the microdynamics of diffusion is obtained by including the results of n.m.r. relaxation analysis. On the basis of these investigations, values for mean molecular jump lengths can be determined. They are in satisfactory agreement with the values estimated on the basis of the free volume theory.

Magnetic after-effects in Fe3O4and vacancy-doped magnetite

Abstract: The magnetic relaxation spectrum of stoichiometric and vacancy-doped magnetite has been investigated in the temperature range between 4·2 and 130 K. The observed relaxation processes are classified as short- and long-range rearrangements of Fe2+ and Fe3+ ionic states within domain walls. It is proposed that below 35 K a coherent tunnelling process governs the mobility of electrons, but that above 50 K is replaced by a thermally activated hopping process leading to the Verwey transition at 123 K.

Zero-field spin relaxation of. mu. /sup +/ as a probe of the spin dynamics of AuFe and CuMn spin-glasses

Abstract: The zero-field spin relaxation of positive muon is measured in spin-glasses AuFe and CuMn. A stochastic theory of muon spin relaxation in a random dilute spin system is formulated to deduce the correlation time of Fe (or Mn) moments. The observed correlation time increases rapidly from 10/sup -10/ sec (atTapprox.1.2T/sub g/) to 10/sup -5/ (at Tapprox.0.5T/sub g/), showing a sharp slowing down of spin fluctuation around T/sub g/.

Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. I. CHCl3, CHBr3, CH2Cl2, CH2Br2

Abstract: Visible absorption spectra (14 700–19 100 cm−1) measured with the thermal lensing spectrometer and near‐infrared absorption (5 600–14 000 cm−1) measured with a conventional spectrophotometer are reported for CHCl3, CHBr3, CH2Cl2, and CH2Br2. Relatively strong absorption peaks are identified as overtones of the C–H stretching vibrations in these compounds. The overtone spectra are analyzed in terms of the local mode (LM) model, which treats the molecule as a set of loosely coupled anharmonic oscillators localized on individual C–H bonds. Relatively less intense peaks are observed and are assigned as combinations of a local mode C–H vibration and some lower frequency (normal) mode of the molecule. Fermi resonance is seen to occur in the spectra of CHCl3 and CHBr3, resulting in anomalously high intensities for the combination bands involved. Significant anharmonic local–normal coupling constants are seen for these same molecules. These coupling constants appear to indicate the presence of strong physical coupling of the two motions involved. The importance of these coupled motions in the intramolecular relaxation of vibrational energy is suggested.

Multiple transitions in isotactic polypropylene around and above the glass transition

Abstract: Gas chromatography, thermal expansion, dynamic mechanical relaxation and thermally stimulated current measurements were used to study multiple transitions in isotactic polypropylene. The crystalline phase relaxation Tαc was decomposed into two components, Tαc1 and Tαc1, in order of decreasing temperature. The activation energy of the Tαc mode was found to be lower than that of the relaxation observed around the glass transition temperature. Thermally stimulated current (TSC) measurements revealed the existence of an apparent double liquid-liquid transition at 39°C and 68 °C in undrawn polypropylene and at 28°C and 55°C in drawn polypropylene. These TSC peaks correspond to a relaxation time following a Vogel equation whose critical temperatures let us predict an apparent double glass transition in polypropylene.

High resolution 1H solid state NMR studies of polyethyleneterephthalate

Abstract: Molecular motions and spatial properties of the solid polymer polyethyleneterephthalate have been investigated using high resolution 1H solid state NMR techniques. The longitudinal spin relaxation time T1ρ of protons (1H) in the rotating frame was measured for a spin locking field ranging from 5 to 20 G. The decay of the 1H magnetization indicated the existence of two distinct T1ρ’s and their field dependence shows that they are associated with two mobile phases of the polymer. The 1H magnetization also relaxes under the dipolar narrowed Carr–Purcell (DNCP) multipulse sequence with two dintinct T1y relaxation times. The ratios T1y’s and T1ρ’s deviate significantly from the expected theoretical values. The combined experiment with magic angle spinning and the DNCP sequence followed by homonuclear dipolar decoupling reveals the individual T1y relaxation of the resolved methylene and aromatic protons. These two species of protons were found to relax with the same T1y’s, thus implying that spin diffusion must have taken place under the homonuclear dipolar decoupling multipulse. The qualitative description of spin diffusion under homonuclear decoupling is given. The combined experiment with spin locking and the DNCP sequence yields the correspondence between the two T1ρ’s and the two T1y’s. The long T1ρ corresponds to the short T1y whereas the short T1ρ corresponds to the long T1y. Communication between the two spatial phases via spin diffusion was also observed in this experiment by monitoring the recovery of the 1H magnitization associated with the short T1ρ after it has been eliminated during the spin locking. The total 1H magnetization is allowed to equilibrate in the laboratory frame for a variable time much shorter than T1 after the spin locking field has been turned off. The spatial relationship between the two phases is discussed.

Conformational mobility of deoxyribonucleic acid, transfer ribonucleic acid, and poly(adenylic acid) as monitored by carbon-13 nuclear magnetic resonance relaxation.

Abstract: The molecular motion of DNA, the native form of tRNA, and partially denatured poly(A) has been investigated by carbon-13 nuclear magnetic resonance (13C NMR). The nuclear Overhauser effect of the RNA samples was measured at 25.1 and 50.3 MHZ, and the spin-lattice relaxation time of all the samples was measured at 50.3 MHZ. The NMR data indicate that the local motion of the ribose carbons is much less restricted than that of the bases for DNA and tRNA. The local motion correlation times of the ribose carbons are in the range of 1-7 ns for the samples investigated. The local motion correlation times of the different nucleic acids are quite similar with the exception that the 2' carbon of DNA and poly(A) is apparently less restricted than that for tRNA. The local motion correlation times of the ribose carbons, except perhaps the 2', do not appear to be strongly coupled to the conformation of the polynucleotide. The 13C NMR results can be combined with those of other investigations to obtain a consistent picture of the internal and overall motions of polynucleotides which have a backbone that is much more flexible than that of the bases.

Photoelectron energy distribution following UV laser ionization of gas phase benzene

Abstract: Photoelectrons from KrF (249 nm) and ArF (193 nm) laser induced ionization of gas phase benzene have been observed. The ionization is shown to result from absorption of two photons only. The intensity dependence of the ArF results indicates that collisionless intramolecular relaxation following absorption of the first photon is detectable with this technique.(AIP)

Hydrogen diffusion in β-LaNi5 hydride

Abstract: The coefficient of self-diffusion D of hydrogen in LaNi5H65 was measured in the temperature range 331 K ⩽ T ⩽ 375 K using a modified pulsed field gradient nuclear magnetic resonance (NMR) technique The NMR relaxation times T1 and T1ρ were also measured in the temperature range 77 K ⩽ T ⩽ 375 K D was found to follow the Arrhenius relation D = (014 cm 2 s −1 ) exp [{−40 kJ ( g atom H ) −1 RT} ] The activation energy EA of 40 kJ (g atom H)−1 determined from direct D measurements agrees well with EA determined from the slope of the short correlation time side of the T1ρ minimum The observed T1 (60 MHz) and T1ρ (45 kHz) minima also scale according to an activation energy of 40 kJ (g atom H)−1 The long correlation time sides of both T1ρ and T1, however, indicate an activation energy for hydrogen motion of approximately 20 kJ (g atom H)−1 T1 was also found to contain contributions from the conduction electron relaxation mechanism (T1ET = 283 s K) The results of direct D measurements and of relaxation time measurements are shown to be consistent and are crudely interpreted in terms of a diffusion process involving more than one activation energy The effects of LaNi5 surface segregation (to La2O3 and nickel) on the NMR relaxation time measurements are also discussed

NMR relaxation processes of 31P in macromolecules

Abstract: 31P-Nmr relaxation parameters (spin-lattice relaxation time, linewidth, and nuclear Overhauser effect) were obtained at three different frequencies for poly(U) and a well-defined (145 ± 3 base-pair) fragment of DNA in solution. Data sets for the two samples were analyzed by theories which included relaxation by the mechanisms of 31P chemical shift anisotropy as well as by 1H-31P dipole–dipole interaction. Neither data set could be satisfactorily described by a single correlation time. A model of a rigid rotor most nearly fits the data for the DNA molecule. Parameters obtained from the least-square fit indicate (1) that the DNA undergoes anisotropic reorientation with a correlation time τ0 = 6.5 × 10−7 sec for the end-to-end motion, (2) the ratio of diffusion constants D∥/D⊥ is 91, and (3) that the linewidth is due to chemical shift dispersion to the extent of 0.5 ppm. Some deviations of the calculated from the observed values suggested that significant torsional and bending motions may also take place for this DNA. Another model which contains isotropic motion but with a broad distribution of correlation times was required to fit the data for poly(U). A log − χ2 distribution function of correlation times [Scheafer, J. (1973) Macromolecules6, 881–888] described well the motion of poly(U) with the average correlation time τ = 3.3 × 10−9 sec and a distribution parameter p = 14.

ac conductivity of tungsten phosphate glasses

Abstract: The electrical conductivity in tungsten phosphate glasses of three different compositions (67 mole% of W${\mathrm{O}}_{3}$-33 mole% ${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$, 70 mole% M${\mathrm{O}}_{3}$-30 mole% ${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$, and 81 mole% W${\mathrm{O}}_{3}$-19 mole% ${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$) has been measured in the frequency range 100 Hz to 3.60 GHz and in the temperature range 77-500\ifmmode^\circ\else\textdegree\fi{}K. The measured ac conductivity at low temperature is almost independent of temperature but shows strong dependence on frequency according to the relation $\ensuremath{\sigma}(\ensuremath{\omega})=A{\ensuremath{\omega}}^{s}$ where the exponent $s$ has been observed to be less than unity. At higher temperatures the frequency dependence becomes weak at low frequencies and remains strong at higher frequencies. The weak frequency dependence is due to the contribution of dc conductivity to the measured ac conductivity. A clear Debye-type dielectric relaxation loss peak is observed by taking out the contribution of dc conductivity, but the frequency dependence of conductivity remains less than quadratic at low frequencies indicating some distribution of relaxation times. This behavior is confirmed by the variation of dielectric constant with frequency and temperature. The temperature and frequency dependence of ac conductivity can be adequately explained in W${\mathrm{O}}_{3}$-${\mathrm{P}}_{2}$${\mathrm{O}}_{5}$ systems by considering the contributions from two mechanisms, one giving an almost linear dependence of conductivity on frequency, and the other having a narrower distribution of relaxation times giving rise to clear but broad dielectric loss peaks.

Estimation of the Intra-Band Relaxation Time in Undoped AlGaAs Injection Laser

Abstract: The intra-band relaxation time of the carriers was estimated from the spontaneously emitted profile for AlGaAs injection laser which has an undoped active region. The relaxation time is estimated in the range of τin=(0.7~1.0)×10-13 s at 298 K by comparison between theoretical analysis and experimental measurement. The relaxation time increases slightly at low temperatures and is estimated as τin=(0.7~1.5)×10-13 s at 100 K, but is almost constant with variation of the injection level. The band-tail effect is explained by the intra- band relaxation of electrons.

Second-order quadrupolar and low-dimensionality effects upon NMR resonance spectra

Abstract: This paper deals with some formal problems which have been encountered in the study of ion dynamics in solid electrolytes with NMR. A time-dependent theory for second-order quadrupolar effects is presented which demonstrates that second-order contributions to the first moment of a line can give the same dynamical information on atomic motion as the spin-lattice relaxation rates. The effect of reduced dimensionality upon motional correlation functions is discussed. The results for two-dimensional and three-dimensional correlation functions, as calculated in the continuum diffusion limit, show that a simple relationship exists between the temperature and frequency dependence of the NMR parameters. Phenomenological rules are proposed to extract from the data an activation energy for the motion described by the correlation functions. A companion paper applies the theory to the $^{23}\mathrm{Na}$ and $^{27}\mathrm{Al}$ resonance in Na $\ensuremath{\beta}$-alumina, for which Na ion transport occurs in a plane.