Proton Spin Relaxation of a Liquid Crystal with a Glassy Cholesteric State

1990 ◽  
Vol 45 (9-10) ◽  
pp. 1077-1084 ◽  
Author(s):  
D. Pusiol ◽  
F. Noack ◽  
C. Aguilera
Keyword(s):  
Spin Relaxation ◽  
Rotational Diffusion ◽  
Glassy State ◽  
Larmor Frequency ◽  
Proton Spin ◽  
Relaxation Dispersion ◽  
Proton Relaxation ◽  
Relaxation Processes ◽  
Molecular Reorientation ◽  
Cholesteric Phase

Abstract Field-cycling and standard pulsed NMR techniques have been used to study the frequency dependence of the longitudinal proton spin relaxation time T x in the crystalline estradiol compound (+)3,1,7-ß-bis-(4n-butoxybenzoyloxy)-estra-1,3,5-(10)-trien or BET, which is a mesogenic material with a chiral molecular structure. From the measured Larmor frequency and temperature depen-dences we conclude that, at low NMR frequencies in the cholesteric phase, T1 reflects in addition to the relaxation process familiar from nematic liquid crystals (director fluctuation modes) another slow mechanism theoretically predicted for cholesteric systems, namely diffusion induced rotational molecular reorientation. These relaxation processes are not or much less effective in the crystalline and glassy state, where they are frozen. Also the high NMR frequency relaxation dispersion strongly differs between the cholesteric mesophase and the not liquid crystalline samples. This is interpreted by a change from essentially translational self-diffusion to rotational diffusion controlled proton relaxation.

scholarly journals Nitrogen Nuclear Quadrupole Resonance Dips in the Proton Spin Relaxation Dispersion of Nematic and Smectic Thermotropic Liquid Crystals

1992 ◽  
Vol 47 (11) ◽  
pp. 1105-1114 ◽  
Author(s):  
D. J. Pusiol ◽  
R. Humpfer ◽  
F. Noack
Keyword(s):  
Spin Relaxation ◽  
Liquid Crystalline ◽  
Larmor Frequency ◽  
Quadrupole Coupling Constant ◽  
Proton Spin ◽  
Relaxation Dispersion ◽  
Coupling Constants ◽  
Smectic Phases ◽  
Nematic State ◽  
Quadrupole Coupling

Abstract The Larmor frequency dependence of the proton spin relaxation time, obtained by means of the fast field-cycling NMR technique, has been used to study the 14N quadrupole coupling constant K and its asymmetry parameter η in the nematic and smectic phases of some liquid crystalline azoxybenzenes (PAA, BAB, HAB, HpAB), cyanobiphenyls (8CB, 9CB, 11CB) and oxycyanobiphenyls (9 OCB). Due to fast molecular reorientations, the effective quadrupole coupling constants are relatively small, whereas surprisingly the asymmetry parameters are rather large. The temperature dependence of both K and η within the mesophases, as well as their discontinuities at the different mesophase transitions, can be interpreted by the anisotropy of molecular rotations. It is found that temperature effects are significantly more pronounced for the (biaxial) smectic-C phase of the heptyloxyazoxybenzene (HpAB) than for the (uniaxial) smectic-A phase of the various investigated cyano- and oxycyanobiphenyls. As a rule, η turned out smaller in the smectic than in the nematic state, whereas K has similar values in both phases

Protonenspinrelaxation und Wasserbeweglichkeit in Muskelgewebe / Proton Spin Relaxation and Mobility of Water in Muscle Tissue

1973 ◽  
Vol 28 (1-2) ◽  
pp. 59-62 ◽  
Author(s):  
G. Held ◽  
F. Noack ◽  
V. Pollak ◽  
B. Melton
Keyword(s):  
Spin Relaxation ◽  
Muscle Tissue ◽  
Continuous Distribution ◽  
Larmor Frequency ◽  
Close Relation ◽  
Rana Esculenta ◽  
Proton Spin ◽  
Protein Solutions ◽  
Two Phase ◽  
Muscle Water

The frequency dependence of the proton spin relaxation in muscle tissue shows that the mobility of the muscle water must be described by a continuous distribution of jumping times instead of the usually assumed two-phase model. Measurements on frog muscles (rana esculenta and rana pipiens, gastrocnemius) in the Larmor frequency range 3 kHz to 75 MHz can be understood quantitatively by a log-gaussian distribution, which supports a close relation to protein solutions.

scholarly journals Effects of Hydrogen Bonding on the Rotational Dynamics of Water-Like Molecules in Liquids: Insights from Molecular Dynamics Simulations

2020 ◽  
Vol 73 (8) ◽  
pp. 734
Author(s):  
W. A. Monika Madhavi ◽  
Samantha Weerasinghe ◽  
Konstantin I. Momot
Keyword(s):  
Molecular Dynamics ◽  
Hydrogen Bonding ◽  
Hydrogen Sulfide ◽  
Spin Relaxation ◽  
Rotational Diffusion ◽  
Molecular Geometry ◽  
Md Simulations ◽  
Water Molecules ◽  
Molecular Reorientation ◽  
Nmr Spin Relaxation

Rotational motion of molecules plays an important role in determining NMR spin relaxation properties of liquids. The textbook theory of NMR spin relaxation predominantly uses the assumption that the reorientational dynamics of molecules is described by a continuous time rotational diffusion random walk with a single rotational diffusion coefficient. Previously we and others have shown that reorientation of water molecules on the timescales of picoseconds is not consistent with the Debye rotational-diffusion model. In particular, multiple timescales of molecular reorientation were observed in liquid water. This was attributed to the hydrogen bonding network in water and the consequent presence of collective rearrangements of the molecular network. In order to better understand the origins of the complex reorientational behaviour of water molecules, we carried out molecular dynamics (MD) simulations of a liquid that has a similar molecular geometry to water but does not form hydrogen bonds: hydrogen sulfide. These simulations were carried out at T=208K and p=1 atm (~5K below the boiling point). Ensemble-averaged Legendre polynomial functions of hydrogen sulfide exhibited a Gaussian decay on the sub-picosecond timescale but, unlike water, did not exhibit oscillatory behaviour. We attribute these differences to hydrogen sulfide’s absence of hydrogen bonding.

NUCLEAR SPIN RELAXATION IN LIQUID AND SOLID METHANE: ISOTOPE EFFECTS

1965 ◽  
Vol 43 (6) ◽  
pp. 986-1000 ◽  
Author(s):  
Gerald A. De Wit ◽  
Myer Bloom
Keyword(s):  
Relaxation Time ◽  
Melting Point ◽  
Spin Relaxation ◽  
Isotope Effects ◽  
Rotational Diffusion ◽  
Lattice Relaxation ◽  
Spin Lattice Relaxation ◽  
Molecular Reorientation ◽  
Temperature Range ◽  
Deuteron Spin

The deuteron spin–lattice relaxation time T1 and spin–spin relaxation time T2 have been studied in CD4 and CD3H between 55 °K and 110 °K. T1 was found to increase very slowly with temperature over the entire temperature range for CD4 with no measurable change being observable at the melting point. Since the deuteron spin relaxation is produced by intramolecular quadrupolar interactions, these results are in strong disagreement with the Debye rotational diffusion model often used to describe molecular reorientation. These results have been used to reanalyze the proton T1 data for CH4−nDn previously given by Bloom and Sandhu. The contributions to T1 from intermolecular dipolar interactions were found to be in close agreement with theory. Contributions from the spin–rotation interaction were found to be extremely small or zero in this temperature range. The effects of translational diffusion on the proton and deuteron T1 and T2 just below the melting point are also discussed.

scholarly journals Proton spin relaxation in dilute methane gas: A symmetrized theory and its experimental verification

1976 ◽  
Vol 54 (16) ◽  
pp. 1712-1727 ◽  
Author(s):  
P. A. Beckmann ◽  
M. Bloom ◽  
I. Ozier
Keyword(s):  
Relaxation Rate ◽  
Time Evolution ◽  
Spin Relaxation ◽  
Collision Cross Section ◽  
Larmor Frequency ◽  
Proton Spin ◽  
Centrifugal Distortion ◽  
Methane Gas ◽  
Exponential Relaxation ◽  
Nuclear Larmor Frequency

Nuclear spin relaxation in low density methane gas is investigated theoretically and experimentally. A theory is developed in which full account is taken of the tetrahedral symmetry of the molecule. For a nuclear Larmor frequency of 30 MHz, the time evolution of the nonequilibrium magnetization is measured as a function of density between approximately 0.005 and 17 amagats at temperatures of 110, 150, and 295 K. In all cases, exponential relaxation is observed. By using the theory in conjunction with the known spin rotation constants and rotational energy levels of CH4, the measured values of the relaxation rate R1 have been fit very well at each temperature, both for the maximum value of R1 which contains no adjustable parameters and for the density dependence of R1 which contains a single parameter taken to be the collision cross section for molecular reorientation. The centrifugal distortion splittings of the rotational levels are shown to have an important influence on the observed values of R1 at 30 MHz and. more generally on the dependence of the time evolution of the nonequilibrium magnetization on density and frequency. On the basis of the theory, a new type of 'relaxation rate spectroscopy' is proposed. Non-exponential relaxation is predicted to occur at low densities when the nuclear Larmor frequency is tuned to a centrifugal distortion splitting.

NMR Investigation of Order Fluctuations, Self-Diffusion and Rotational Motions in the Nematic Liquid Crystal MBBA by T1-Relaxation Spectroscopy

1977 ◽  
Vol 32 (1) ◽  
pp. 61-72 ◽  
Author(s):  
V. Graf ◽  
F. Noack ◽  
M. Stohrer
Keyword(s):  
Liquid Crystal ◽  
Optimization Procedure ◽  
Larmor Frequency ◽  
Proton Spin ◽  
Relaxation Dispersion ◽  
Self Diffusion ◽  
T1 Relaxation ◽  
Computer Optimization ◽  
Rotational Motions ◽  
Experimental Findings

Abstract We report on measurements of the proton spin T1 relaxation dispersion at various temperatures in the nematic phase of the liquid crystal MBBA in the Larmor frequency range from 2 kHz to 270 MHz, which exceeds previous studies by more than 3 orders of magnitude. The new results cannot be interpreted in terms of either order fluctuations or self-diffusion as recently proposed by Doane et al.1 and Blinc et al.2, respectively. Instead, the dispersion and its temperature dependence indicate the significance of at least three relaxation mechanisms, namely order fluctuations (OF), self-diffusion (SD) and rotation of the molecular ellipsoids about the short axis (R). The combined OF-SD-R model presented in this work allows a quantitative analysis of the experimental findings! The correlation times and activation energies of the three molecular reorientations evaluated from the T1 dispersion by means of a computer optimization procedure are in essential agreement with data provided by other spectroscopic methods [light scattering, tracer technique, dielectric relaxation], but differ from former NMR conclusions.

A Site-Specific Study of the Magnetic Field-Dependent Proton Spin Relaxation of an Iridium N-Heterocyclic Carbene Complex

2017 ◽  
Vol 231 (4) ◽  
Author(s):  
Andrey N. Pravdivtsev ◽  
Alexandra V. Yurkovskaya ◽  
Pavel A. Petrov ◽  
Konstantin L. Ivanov
Keyword(s):  
Spin Relaxation ◽  
Proton Spin ◽  
Relaxation Dispersion ◽  
Wide Field ◽  
Field Range ◽  
Carbene Complex ◽  
Relaxation Measurements ◽  
A Site ◽  
Nuclear Magnetic

AbstractWe report a study of proton spin relaxation of an Iridium N-heterocyclic carbene complex [Ir(COD)(IMes)Cl] complex (where COD=1,5-cyclooctadiene, Imes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). This compound is a pre-catalyst of the most efficient complex allowing the signal amplification by reversible exchange (SABRE) effect, relevant for enhancing weak signals in nuclear magnetic resonance (NMR). An important feature of the study is a combination of relaxation measurements over a wide field range with high-resolution NMR detection. As a result, we are able to measure nuclear magnetic relaxation dispersion (NMRD) curves in the field range 0.1 mT–16.4 T (corresponding to the frequency range 4 kHz–700 MHz) for individual protons in the complex under study. This attractive possibility enables determination of the motional correlation times,

Proton NMR Relaxation Study of Molecular Motions in a Liquid Crystal with a Strong Polar Terminal Group

1993 ◽  
Vol 48 (8-9) ◽  
pp. 851-860 ◽  
Author(s):  
P. J. Sebastião ◽  
A. C. Ribeiro ◽  
H. T. Nguyen ◽  
F. Noack
Keyword(s):  
Theoretical Calculations ◽  
Proton Nmr ◽  
Nmr Relaxation ◽  
Larmor Frequency ◽  
Terminal Group ◽  
Relaxation Dispersion ◽  
Proton Relaxation ◽  
Self Diffusion ◽  
Smectic A ◽  
Field Cycling

Abstract Liquid crystalline compounds containing a cyano terminal group often exhibit peculiar molecular organizations of their mesophases. In this work we present proton NMR relaxation studies, performed by means of standard NMR and fast field-cycling NMR techniques, in the nematic (N) and bilayered smectic-A phase (SA2) of 4-pentyl-phenyl 4'-cyanobenzoyloxy-benzoate. The field-cycling measurements were used to clarify the relaxation behaviour in the low Larmor frequency range, where conventional techniques are not applicable. Self-diffusion and rotational reorientations are found to be the essential relaxation mechanisms at MHz frequencies in the smectic mesophase, while the contribution of collective modes appears only at lower frequencies in the kHz range. In the nematic mesophase the order director fluctuations mechanism dominates the relaxation dispersion up to 10 MHz, where the rotational reorientations become important, with minor corrections from the self-diffusion process. The agreement between the experimental findings and model fits could be improved by an additional relaxation mechanism in the kHz regime, ascribed to the interaction between protons and fast relaxing quadrupolar nitrogen 14N nuclei. Though all four processes are present in the nematic and smectic-A2 phases, the overall T1 frequency dependence is quite different in the two cases. This behaviour is discussed in terms of available theoretical calculations of the proton relaxation dispersion in liquid crystals, and it is also compared with data known from other cyano compounds.

Investigation of Molecular Motion in Smectic Phases of the Liquid Crystal TBBA by NMR Relaxation Dispersion

1980 ◽  
Vol 35 (9) ◽  
pp. 924-929 ◽  
Author(s):  
Th. Mugele ◽  
V. Graf ◽  
W. Wülfel
Keyword(s):  
Nmr Relaxation ◽  
Larmor Frequency ◽  
Proton Spin ◽  
Relaxation Dispersion ◽  
Self Diffusion ◽  
Smectic Phases ◽  
Field Cycling ◽  
The Difference ◽  
Nmr Methods ◽  
Fast Field

Abstract The proton spin T1 relaxation dispersion in the smectic A and C phase of TBBA, and for comparison also in the nematic phase, have been studied using time dependent fast field-cycling techniques in the Larmor frequency range from νp = 100 Hz to 44 MHz. Our measurements considerably extend recent ones by Blinc et al., performed with other NMR methods for frequencies ≧ 140 kHz. The new experimental data are consistent with the reported ones for Sm C but not for Sm A, the difference being that the essential T1 dispersion observed with our technique occurs at much lower frequencies, namely below about 100 kHz. As a consequence, the relaxation dispersion for both smectic phases looks very similar. It can be described quantitatively in terms of relaxation by "nematic-like" order fluctuations, self-diffusion, and by a third molecular mechanism with (for simplicity) Debve-like power spectrum, which is possibly a second type of order fluctuation or a molecular rotation about the short axis. The analysis reveals surprisingly far going parallels between the spin relaxation of simple smectics and that of high-temperature nematics like PAA.

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