Reorientational Correlation Times Research Articles

Deuteron Magnetic Resonance Study of Order and Dynamics in Liquid Crystal Polymers

Abstract Thermotropic nematic polyesters, specifically deuterated at different positions of the polymer chain are studied by multiple pulse dynamic NMR. Analysis of the NMR experiments is achieved, employing a comprehensive model, based on the stochastic Liouville equation. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. In the anisotropic melt the correlation times for chain reorientation and trans-gauche-isomerization are in the range of 0.1–10 ns. Decreasing the temperature of the solid polymer first freezes the intermolecular motions. Thus, below the glass transition only intramolecular motions such as trans-gauche isomerization and ring flips can be detected. The chain order parameter of the nematic melt is S(ZZ) = 0.85. In addition, the chains adopt a highly extended conformation, evidenced by a trans population of n(t) = 0.8 throughout the entire spacer. This microorder is retained, when the polym...

Effect of ion-solvation on the internal dynamics of the ethanol molecule as studied using NMRB?coefficients

The modification of reorientational correlation times for the various kinds of chemically non-equivalent protons of the ethanol molecule as a result of ionsolvation has been investigated with the help of NMR B′ coefficients. For this purpose, the proton spin-lattice relaxation times of specifically deuterated ethanols, containing only one type of chemically equivalent protons were measured in neat ethanols and their lithium chloride and iodide solutions at a number of concentrations. The measurements were also extended over a wide temperature range below room temperature. Some results of various other chlorides were also included for comparison. The results are discussed in the light of pertinent theories available at present. At 250 K the rotational correlation time of the molecule in pure ethanol is 64 ps and the correlation times of the CH2 and CH3 groups are 8 and 3 ps, respectively. In the solvation sphere of Li+ the correlation times for the molecule and the respective groups are about 4, 7, and 4 times as long. However, the internal reorientation times are found, in some cases, to be virtually zero which we take as an indication that the description of the internal motion by current theories is not satisfactory.

Molecular dynamics simulation of liquid CH3F, CHF3, CH3Cl, CH3CN, CO2and CS2with new pair potentials

Results of molecular dynamics simulations are reported for CH3F, CHF3, CH3Cl, CH3CN, CO2 and CS2. The intermolecular pair potentials used in these calculations were derived with a new model which was introduced recently. The calculated data on thermodynamics, static structure, self-diffusion coefficient and reorientational correlation times are compared with experimental and other molecular dynamics results.

Extended diffusion of rigid asymmetric molecules

The extended diffusion theory of molecular motions in fluids has been applied to the case of rigid asymmetric molecules. Illustrative calculations of the second rank reorientational correlation time matrix and the correlation factors associated with NMR spin-rotation relaxation are presented as functions of the angular momentum correlation time. Analytic expressions for these correlation times and factors in the diffusion limit are also given.

A spin-lattice relaxation study of the molecular reorientation of 2-methylindole as a function of viscosity and complex formation

The viscosity (η) dependence of the 13C spin-lattice relaxation time (T1) of C-3 of 2-methylindole was investigated by changing the temperature (T) and by changing the solvent system. The observed η/T dependence of the effective reorientation correlation time (calculated from T1) is compared with those derived from theory based on hydrodynamic rotation with "stick" and with "slip" boundary conditions. This data implies that the reorientation of 2-methylindole obeys a near-slip condition in 1,2-dichloroethane. The 13C T1 of C-3 of 2-methylindole was also determined as a function of the fraction complexed with 1,3,5-trinitrobenzene (TNB) at 35 °C in 1,2-dichloroethane. The observed T1 in this case, depends on the rates of reorientation of the molecule in the free and the bound states, the rate of association of the molecules, the rate of dissociation of the complex, as well as increases in viscosity brought about by increases in the concentration of TNB. Analysis of the data, by application of the Anderson–Fryer formulation, gives a rate of reorientation in the bound state of 55 ns−1 at 35 °C and 0.69 cP as compared to a value of 150 ns−1 in the free state under the same conditions.

Quadrupolar interactions in amines as determined by 1H nuclear magnetic resonance: mono- and dimethylamines

Nitrogen-14 quadrupole interactions have been observed in the nonexchanging amino proton 1H nmr spectra of rigorously purified liquid mono- and dimethylamines over the temperature range −82 °C to +81 °C. The proton lineshapes have been analyzed to yield 14N spin-lifetimes (τ) as a function of temperature. Activation energies and correlation times for molecular reorientation in the liquids have been calculated from these data and the results compared with those from 14N nmr measurements.

Orientation et dynamique moléculaire de sondes paramagnétiques en phase lamellaire dibutylephosphate de sodium/eau

Aqueous solutions of sodium dibutylphosphate (NaDBP) give rise to lamellar liquid crystalline phase between 280 and 340 K for NaDBP concentrations ranging from 3 to 4 mol L−1. The orientation and dynamic behaviour of several paramagnetic probes such as the vanadyl ion, tempoamine, tempone and tempobutanamide dissolved in this phase have been investigated by esr.The vanadyl ion binds to the phosphate group, forming a complex, the z magnetic axis of which is on the average perpendicular to the lamellar plane. The molecular mobility at the H2O/DBP− interface is given by the reorientation correlation time of the z axis, which is derived from the total width of the spectrum, using the empirical relation [Formula: see text]. The parameters a = 2.95 × 10−11 s and b = −1.42 are obtained from the viscosity dependence of the esr line widths and of the anisotropic hyperfine couplings.The tempone and tempoamine are localized in the polar domains of the mesophase, reorienting rapidly about the N—O direction which is, on the average, parallel to the director of the lamellar plane. The anisotropic motion of these radicals is assigned to hydrogen bonding which occurs, according to the cases, with water molecules or phosphate groups. Some correlations are established between the esr of spin probes and 2H and 31P nmr experiments on substrate molecules.

Raman spectroscopic studies on the molecular reorientation and vibrational relaxation in liquid ethylene

We report detailed reorientation (ø2R ) and vibration correlation functions derived from the Raman v 2(Ag ) and v 3(Ag ) bands of liquid ethylene between the freezing (92 K) and boiling (169 K) points. Rotation is essentially ‘free’ for ∼0·2 ps (even though the band profiles are essentially lorentzian) and the ø2R functions are in poor agreement with those predicted by the J-diffusion model since τ J values fitted to the long time part of the correlation functions are much lower than 0·2 ps. The reorientation correlation times follow the liquid viscosity with a κ factor of 0·17—reflecting the relative weakness of the intermolecular torques in this liquid. The vibrational part of v 3 shows the effects of ‘motional narrowing’ observed when phase relaxation occurs via relatively long ranged potentials. The Kubo environmental relaxation times τe are least an order of magnitude longer than that expected if phase relaxation occurred via collisional interaction. Their temperature and viscosity dependence are also consistent with the Kubo model of dephasing by homogeneous broadening due to long ranged potentials but not with the modulation of such potentials by reorientational motions.

Molecular reorientation and vibrational relaxation in dibromomethane

AbstractThe polarized and depolarized Raman spectra of the ν2 (CH2 bend) and ν3 (CBr stretch) vibrations in dibromomethane have been studied as a function of temperature throughout the liquid range. Reorientational correlation times derived from the two modes were simllar in both magnitude and dependence on temperature. This result, together with other structural data, suggests that the rotational dynamics of this molecule in the liquid phase are quasi symmetric‐top in character. Vibrational dephasing times calculated from the two lineshapes were also of the same magnitude. However, modulation times determined via the Kubo lineshpae formalism varied markedly between the vibrations. This provides evidence that vibrational dephasing of the bending and stretching modes are influenced by different intermolecular forces in the liquid.

Solution dynamics of the osmium cluster [Os3(µ-H)2(CO)10] by13C and17O nuclear magnetic resonance. Estimation of the13C chemical shift anisotropy and17O quadrupole coupling constant

Application of the saturation-transfer technique to 13C n.m.r. spectra of 13C-enriched [Os3(µ-H)2(CO)10] shows that only localised 13C exchange occurs, involving two of the four types of carbonyl ligand. Equations are derived to describe the recovery of the exchanging AX2 spin system towards equilibrium, after an initial perturbation. When the initial perturbation is a selective inversion of A or X2, values for the 13C relaxation parameters and the A X2 exchange rate constant can be determined. The rate constant for this localised process is 0.36 s–1 at 300 K in CDCl3 solution. An estimate of the quadrupole coupling constant for 17O in the complex (1.59 MHz), together with 17O spin-lattice relaxation time measurements, yielded a value for the correlation time for molecular reorientation of 22 ± 4 ps. The 13C spin-lattice relaxation rates are strongly field dependent and values for the 13C shielding anisotropy in the region of 383 ± 50 p. p. m. were obtained.

Reorientational motion, collision-induced scattering, and vibrational relaxation in liquid carbonyl sulfide

The rotational and vibrational behavior of neat liquid carbonyl sulfide under its own vapor pressure is investigated through Raman line shape studies of the symmetric ν1 C=S stretching mode over the temperature range T=183 to 303 K. The reorientational motion is found to be near the diffusional limit. Various methods of determining the reorientational correlation time τθ are compared and their validity discussed. The contribution of collision-induced scattering to the ν1 mode rotational line shape is examined semiquantitatively. It is found that the relative contribution of collision-induced scattering to the overall rotational line shape increases with increasing temperature and decreasing density, whereas the second moment of the collision-induced line shape decreases. Assuming the separability of the collision-induced scattering contribution and the reorientational contribution to the observed line shape, the experimental data are discussed qualitatively in terms of the dipole–induced–dipole (DID) theory of Balucani and Vallauri. The vibrational relaxation mechanism for the ν1 mode is attributed to dephasing and interpreted by Kubo line shape theory. The dephasing is found to be in the ‘‘motionally narrowed’’ regime and discussed qualitatively in terms of attractive dipolar forces.

Deuterium NMR in the solid-state and in solution of the molecular motion of the bases in poly(I) and poly(I)·poly(C)

To provide information regarding the confirmational flexibility of nucleic acids, in particular the rate and amplitude of base motions, we have observed the deuterium NMR from single-stranded and double-stranded polynucleotides. Poly(I) was deuterated at the 8-position of the base, and the deuterium NMR was examined in solution (at 23.0 and 55.4 MHz) and for hydrated and dry fibers (at 23.0 MHz). In the solid state, the deuterium signal of dry poly(I) exhibits a powder pattern with the maximal expected quadrupolar splitting, while the relatively short spin-lattice relaxation time indicates the prescence of a rapid internal reorientation of the C-D bond with an amplitude of that motion of at least ±2.4 degrees. Hydrating the poly(I) fibers to the extent of eight molecules of water per nucleotide results in the disappearance of the deuterium signal, apparently due to a decreased spin-spin relaxation time shorter than the instrumental dead-time (even using the quadrupolar echo technique); this could occur if conformational fluctuations are occuring at a rate comparable to the deuterium quadrupole interaction strength, i.e., 175 kHz. In solution, a theoretical fit to the measured Lorentzian linewidths and spin-lattice relaxation times necessitates the inclusion of at least two motional correlation times, with a subnanosecond internal motion. Double-stranded poly(I)·poly(C) yielded a solid state spectrum similar to poly(I), albeit with a longer T 1, which reduced the lower limit for the amplitude of an internal motion to ±1.9 degrees. The 2H signal from the poly(I)·poly(C), hydrated to a degree of approx. eight molecules of water per base pair, retained its solid-state lineshape (with a reduced T 1 value, indicating increased internal mobility of the bases with a lower limit on amplitude of ±4.7°). In solution, however, the 2H-NMR signal from poly(I)·poly(C) became virtually undetectable, even in solid-echo experiments, when the echo was observed after 52 μs. This indicates that the spin-spin relaxation time of the deuterium nucleus must be close to its theoretical minimum of about 9 μs, and the correlation time for an isotropic reorientation of the C-D vector can be estimated to be between 0.2 and 200μs.

An effective pair potential for liquid acetonitrile

A six site potential model has been developed for liquid acetonitrile, based on atomic interaction parameters taken from studies of other liquids and partial charges derived by ab initio calculations. When used in molecular dynamics simulations, the model yields results in good agreement with experimental data on the thermodynamics, structure, self diffusion coefficients and reorientational correlation times of the liquid.

Information theory applied to time correlation functions in fluids

Information theory is used to predict the reorientational time correlation functions for a linear molecule in a fluid given only the moment of inertia and the mean squared torque. Comparison with ‘exact’ computer simulation results for a variety of fluids show that the method is remarkably successful—even when the torque is small. It is just as successful, for instance, as a much more elaborate memory-function perturbation theory which uses the same information. The method is quite general and can be usefully applied to any time correlation function.

L'enrobage moléculaire: complexation du cation sodium par le dibenzo-24-couronne-8

Structure and microdynamics in solution of the dibenzo-24-crown-8 – sodium perchlorate complex have been studied by Na-23 and C-13 nmr. In all solvents (acetone, acetonitrile, pyridine, nitromethane, propylene carbonate, and chloroform-d) and in the binary mixtures acetone/pyridine, acetonitrile/nitromethane, propylene carbonate/methanol, the crown wraps around the sodium cation and expels the conjugate anion and the solvent molecules from the cation coordination sphere. Measurements of Na-23 quadrupolar and C-13 dipole–dipole relaxation times give the effective correlation time of the complex in CDCl3 solution (65 ± 11 ps) and the Na-23 quadrupolar coupling constant in all those solvents and binary mixtures (0.8–1.2 MHz). The sodium coordination shell is completely frozen during a time superior to the reorientational correlation time.

Molecular reorientation with internal extended rotational diffusion in liquids

An extended diffusion model is applied to describe the internal rotation about the figure axis of a symmetric top molecule whose overall reorientation is undergoing anisotropic rotational diffusion. An explicit analytical expression for the internal correlation time τ(k)int as a function of the angular momentum correlation time τj is obtained. The overall reorientational correlation time τc in the presence of the internal extended diffusional rotation is also obtained analytically as a function of τ(j). This expression is free from any restriction on the range of validity for τj.(AIP)

Determination of Pitch in a Cholesteric DSCG-Water Lyomesophase by NMR Techniques

Abstract Nuclear Magnetic Resonance (NMR) techniques are used to determine the pitch in a cholesteric lyomesophase prepared by adding of l-alanine to a solution of disodiumcromoglycate (DSCG) in water. The method, which is particularly suitable for the study of bulk samples, consists of a combination of two independent types of measurements: (i) The effective reorientational correlation time of the director, due to molecular translational diffusion, is determined by a lineshape analysis of the deuterium resonances of the solvent D2O and/or deuterated l-alanine. (ii) The translational self-diffusion coefficients of both these constitutents is determined by a pulsed field-gradient spin-echo experiment. From the results of these measurements, the pitch and its dependence on the chiral dopant concentration is determined.

Molecular mobilities and the lowered osmolality of the chromaffin granule aqueous phase

Carbon-13 spin-lattice relaxation times, T 1, have been measured in whole adrenal medullary tissue slices, in suspensions of isolated chromaffin granules, in the reconcentrated chromaffin granule lysate, and in various model solutions containing catecholamines, ATP, chromogranins and Ca 2+. Reorientational correlation times have been calculated at 10°C using T 1 data and nuclear Overhauser enhancemments for protonated carbons on both catecholamines and nucleotides. Correlation times in all media are relatively short and characteristic of highly fluid aqueous phases. Adrenalin and ATP exhibit substantial differences in correlation times in all media, however, the ratio γ R(ATP):γ R(catecholamine) ranging from 2.4 in simple 3:1 adrenalin-ATP solutions to 4 in intact chromaffin granules. This difference, as well as the relatively high absolute reorientational mobilities of both components, confirms the importance of labile ionic interactions between ATP and catecholamines, but rules out the presence of high concentrations of base-stacked structures. Participation of the chromogranins in ternary complexes with catecholamines and ATP appears to be of minor importance. Ionic interactions to the protein are not reflected in either 13C T 1 values or chemical shifts of arginine or glutamate sidechain resonances, or in the 13C chemical shifts of ATP or catecholamines. Very labile protein-ATP binding appears to be reflected in the correlation time measurements, however, which show selective immobilization of ATP relative to catecholamine in the presence of soluble protein. Osmotic measurements indicate that solutions containing adrenaline, ATP and Ca 2+ are highly nonideal, but probably not sufficiently so to account fully for the osmotic stabilization of the chromaffin granule aqueous phase. Even in the absence of specific intermolecular complexation, the chromogranins, through their polyelectrolyte properties, exert a significant influence on the intragranular osmolality. The osmotic lowering due to polyion-counterion interactions has been estimated semiquantitatively using a theory developed by Oosawa.

Raman spectra of halogen‐substituted methanes in the liquid state. 2—Band widths of the totally symmetric vibration bands

AbstractThe isotropic and anisotropic Raman band widths of the totally symmetric vibrations of halogen‐substituted methanes have been measured in the liquid state at room temperature. The isotropic CH and CD stretching vibration bands broaden with increasing molecular weight and this is qualitatively explained by the Fischer‐Laubereau dephasing model. The isotropic band widths of the deformation vibrations are correlated with the reorientational correlation times and are attributed to broadening due to interactions dependent on the orientation, such as the dipole–dipole interaction.

Dynamic structure of bovine adrenal cortex microsomal membranes studied by time-resolved fluorescence anisotropy of all-trans-1,6-diphenyl-1,3,5-hexatriene.

The motional properties of all-trans-1,6-diphenyl-1,3,5-hexatriene embedded in the microsomal membranes and in extracted lipid liposomes from bovine adrenal cortex are studied by fluorescence anisotropy decay measurements. The results demonstrate the existence of a nonvanishing value of the anisotropy (r infinity) in both systems. The r infinity values are significantly higher in the total membranes than in the protein-depleted liposomes, following the trend of the steady state anisotropy values previously reported (Gallay, J., Vincent, M., de Paillerets, C., Rogard, M., and Alfsen, A. (1981) J. Biol. Chem. 256, 1235-1241). This indicates that the depolarizing rotations of the probe are more restricted in the presence of proteins. No detectable effect of proteins on the reorientational correlation time can be observed. The temperature dependence of r infinity is not similar in both systems. In the complete membranes, a sharp decrease of r infinity value takes place between 27 and 32 degrees C, corresponding to 1/3 of the total curve of r infinity versus temperature is translated to lower temperatures. Thermal dependence of the reorientational correlation time is essentially the same in lipids and in complete membranes with an activation energy of 5 kcal mol-1. The results suggest that proteins play an essential role in organizing the deepest lipid region in the microsomal membranes from bovine adrenal cortex.