Dipolar Relaxation Time Research Articles

Intramolecular character of the intrapair dipolar order relaxation in the methyl deuterated nematic para-azoxyanisole

The proton intra-pair dipolar order relaxation time (T1D) was measured for nematic methyl-deuterated para-azoxyanisole (PAAd6) as a function of both the temperature and concentration in perdeuterated PAA (PAAd14), at 27 MHz. Since the results coincide for all measured concentrations in all the nematic temperature range, we conclude that the intermolecular contribution to the relaxation of the dipolar energy in this compound is negligible, at the studied frequency. The observed temperature dependence of T1D is typical of the order fluctuations of the director (ODF), which clearly indicates that the ODF is the relevant mechanism producing dipolar order relaxation, in accordance with previous field cycling experiments.

On the applicability of the Redfield high-temperature theory for proton dipolar order relaxation in liquid crystals

Nuclear magnetic resonance experiments on the Larmor frequency dependence of the intrapair dipolar order relaxation time T1D showed the existence of discrepancies between the usual two-spin theory and the experimental data. Field-cycling experiments of T1D in methyl deuterated para azoxyanisole (PAA-d6) in the nematic phase [J. Chem. Phys. 110, 8155 (1999)] showed that the difference cannot be assigned to the common assumption of isolated spin pairs or to the neglecting of the alkyl chain protons in the theoretical interpretation. Though the applicability of the spin temperature and short correlation time assumptions have not been justified for nematic liquid crystals, they have been used in all the available theoretical approaches for dipolar order relaxation. In this work, we calculate T1D of PAA-d6 within the Redfield high-temperature theory of spin–lattice relaxation but avoid the usual assumptions of short correlation times and random phase. First, we find that this fact does not alter the expression for T1D in the simple case of an ensemble of isolated spin pairs. Then we take the four benzene protons as the spin system and find that this refinement gives no relevant contribution able to explain the discrepancies between theory and experiments. Also, we find that the random phase assumption becomes redundant in the calculation of T1D, whenever the relevant spin Hamiltonian is nondegenerate. Thus, in these cases the spin temperature hypothesis is justified in liquid crystals, where the spin system contains few spins. We conclude that the Redfield high-temperature relaxation theory fails to describe T1D in thermotropic liquid crystals, since it underestimates the prominent role of the slow cooperative motions.

Symmetry breaking in the relaxed S(1) excited state of bianthryl derivatives in weakly polar solvents.

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to investigate the nature of the relaxed S(1) state of 9,9'-bianthryl (AA), 10-cyano-9,9'-bianthryl (CAA), and 10,10'-dicyano-9,9'-bianthryl (CAAC). Changes in both the real, Deltaepsilon' (dielectric constant), and imaginary, Deltaepsilon' ' (dielectric loss), components of the complex permittivity have been measured. The dielectric loss transients conclusively demonstrate the dipolar nature of S(1) for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and even in the nonpolar solvents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations in the solvent environment. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The time scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolecular, flip-flop dipole reversal via a neutral excitonic state. The dipole moment of the transient dipolar state is estimated to be ca. 8 D, that is much lower than the value of ca. 20 D determined from the solvatochromic shifts in the fluorescence in intermediate to highly polar solvents which corresponds to close to complete charge separation. For the asymmetric compound, CAA, a dipole moment close to 20 D is found in all solvents, including n-hexane. Dipole relaxation in this case occurs on a time scale of several hundred picoseconds and is controlled mainly by diffusional rotation of the molecules. The mechanism and kinetics of formation of the dipolar excited states are discussed in the light of these results.

Conformational dynamics in polyethylene under isochoric conditions: A molecular dynamics simulation study

Conformational dynamics in polyethylene (PE) have been studied via molecular dynamics simulations under isochoric conditions at four specific volumes (1.26, 1.20, 1.15 and 1.10 cm3 g−1) as a function of temperature (300–500 K). The autocorrelation function (ACF) decay for virtual perpendicular dipolar relaxation and rates of conformational transitions were monitored. The isochoric Kohlrausch–Williams–Watts (KWW) relaxation times determined from the ACFs show Vogel–Fulcher (V-F) behavior with temperature. From the pressures determined at each T, V point, constant pressure relaxation times vs temperature could also be constructed along several isobars by interpolation. The resulting temperature dependent activation energies at constant volume and at constant pressure are compared. At temperatures where the isochoric and isobaric P, V values coincide, i.e., where the ischoric and isobaric V-F curves intersect, the ratios of the constant volume activation energies to those at constant pressure are found to be ∼0.8. This high ratio can be considered as an indicator of activated barrier crossing dynamics dominating in comparison to the role of free volume. The dipolar ACF relaxation times diverge from the conformational transition rates as the temperature is lowered (under isochoric conditions) and analogously with increasing pressure (under isothermal conditions). The spatial heterogeneity of the conformational transitions increases with decreasing temperature and an increase in self-correlation of the conformational transitions ensues. These signatures of approaching vitrification are consistent with those previously found for PE under isobaric conditions. Qualitatively, decreasing the temperature at constant volume, increasing the pressure at constant temperature, and decreasing the temperature at constant pressure result in very similar behavior in the conformational dynamics.

Interpretation of short-circuit discharge current in film capacitors based on an equivalent circuit

Misfire phenomena in a discharge circuit composed of a capacitor and thyristor were investigated for operating frequencies to 198 Hz. Misfire was found to occur in polyethylene terephthalate (PET) capacitor circuits at low temperatures, whereas polypropylene (PP) capacitor circuits did not misfire under any conditions. Transient currents in the range 1 mA to 40 A were measured using a digital oscilloscope. A PET capacitor was shown to discharge initially with a given time constant, but subsequently discharge at a lower rate, extending the discharge period. It is this delayed gradual decay that causes misfire, and it is associated with residual charges trapped in the bulk of the PET film. The correlation between the transient discharge current and dielectric dispersion in PET and PP capacitors was investigated over a wide frequency range with the aid of the Debye model for dipolar polarization. It was found that the time dependence of the discharge current is related closely to the broad distribution of dipole relaxation time, with a discrepancy between the discharge current and the dielectric response attributed to charge trapping.

Decay of Dipolar Order in Diamagnetic and Paramagnetic Proteins and Protein Gels

Magnetic relaxation in solids may be complicated by the creation and loss of dipolar order at finite rates. In tissues the molecular and spin dynamics may be significantly different because of the relatively high concentration of water. We have applied a modified Jeneer–Broekaert pulse sequence to measure dipolar relaxation rates in both dry and hydrated protein systems that may serve as magnetic models for tissue. In lyophilized and dry serum albumin, the dipolar relaxation time, T1D is on the order of 1 ms and is consistent with earlier reports. When hydrated by deuterium oxide, the dipolar relaxation times measured were on the order of tens of microseconds. When paramagnetic centers are included in the protein, the Jeneer–Broekaert echo decay times became the order of the decay time for transverse magnetization, i.e., the order of 10 μs or less. In the hydrated or paramagnetic systems, the dipolar relaxation times are too short to require inclusion in the quantitative analysis of magnetization transfer experiments.

Transient NQR signals in the effective field of multipulse trains

This paper is a theoretical analysis of time-dependent nutation and echo signals in the effective field of multipulse trains in NQR. The results of experimental investigations of the same aspects were reported in earlier papers. The various features of dipole-dipole interactions in three-level quadrupole spin systems are discussed. It is shown that, in contrast to NMR, the dipole Hamiltonian in the interaction representation determined by the quadrupole Hamiltonian contains only three diagonal components of the dipole-dipole interaction tensor. On the other hand, the strong inhomogeneous broadening characteristic of NQR hinders exact measurement of these components by ordinary methods. The theoretical analysis suggests that the decay of the echo-signal envelope in the effective field of multipulse trains is determined solely by the dipole relaxation time, which serves as justification of a new experimental method used in measuring the characteristics of the dipole-dipole interaction tensor in spin systems with an inhomogeneously broadened spectrum.

13C NMR relaxation time measurements of macrocyclic ether complexes. The association of benzo[18]crown-6 with NaClO4·H2O in acetonitrile

Abstract Association constants for the complex Na+/benzo[18]crown-6 in acetonitrile solution were determined from 13C-NMR relaxation time measurements at different temperatures. The dipolar contributions to relaxation were separated by nuclear Overhauser enhancement measurements. The contributions are split into parts originating from uncomplexed and from complexed macrocycles assuming the 1:1 complex formation between Na+ and crown ether and by measuring solutions of different concentrations. The measured data were analysed to yield the respective relaxation times and the association constants as a function of the temperature. The results depend on the carbon site in the macrocycle thus showing preferences for binding of Na+. The dipolar relaxation times of the complex were explained on the basis of pseudorotational motion of the macrocycle which was proved with the MM+ geometry optimisation.

Molecular motions in thermotropic liquid crystals studied by NMR spin-lattice relaxation

Nuclear magnetic resonance relaxation experiments with field cycling techniques proved to be a valuable tool for studying molecular motions in liquid crystals, allowing a very broad Larmor frequency variation, sufficient to separate the cooperative motions from the liquidlike molecular diffusion. In new experiments combining NMR field cycling with the Jeener Broekaert order-transfer pulse sequence, it is possible to measure the dipolar order relaxation time (T1D), in addition to the conventional Zeeman relaxation time (T1Z) in a frequency range of several decades. When applying this technique to nematic thermotropic liquid crystals, T1D showed to depend almost exclusively on the order fluctuation of the director mechanism in the whole frequency range. This unique characteristic of T1D makes dipolar order relaxation experiments specially useful for studying the frequency and temperature dependence of the spectral properties of the collective motions.

Femtosecond time resolved impulsive stimulated Raman scattering in KDP-type Ferroelectrics

Femtosecond time resolved impulsive stimulated Raman scattering has been applied to the low lying B2 mode of KH2PO4 and KD2PO4. The time dependent signals of both crystals show an exponential decay, meaning that both modes are relaxational in nature. The crossing angle of the exciting beams was varied to study the wavenumber dependence of the excited mode. The relaxation time of the individual dipoles of the relaxational mode was calculated to be τ = 0·131 ps for KH2PO4 and 1·06 ps for KD2PO4. Here τ0 is the relaxation time of individual dipoles which is given in the equation of the relaxation time τ = τ0 T 0/(T–T 0).

Order fluctuations of the director in nematic thermotropic liquid crystals studied by nuclear magnetic resonance dipolar relaxation

Larmor frequency dependent NMR studies of dipolar order relaxation in liquid crystals have seldom been tried in the past. Using conventional static magnetic field techniques, the experiment cannot be extended to the low Larmor frequency (νL) regime due to limitations in the signal-to-noise ratio of the dipolar echo. In this work, we present an experimental study of the dipolar relaxation time in the frequency range 103–7×107 Hz in nematic thermotropic liquid crystals. To extend the study to such low frequencies, we used the Jeener–Broekaert pulse sequence combined with fast field-cycling NMR technique. For frequencies higher than 105 Hz, the dipolar relaxation time T1D(νL) follows the νL1/2-law that is characteristic of order fluctuations of the director (OFD) in nematics. In contrast, the Zeeman relaxation is driven by faster and less correlated motions, specially in the MHz frequency range. The relaxation of dipolar energy was measured to be remarkably faster than the one predicted by the usual semiclassical model of isolated spin pairs. Conceivably, the failure of the usual two-spin model should be sought in the absence of multispin interactions and multispin correlations. We propose that the OFD are the dominant relaxation mechanism for the dipolar order, even in the MHz regime. This result turns T1D(νL) experiment in a useful NMR technique for the study of slow molecular dynamics in mesophases.

Two-time clock NMR observation of the ferroelectric transition in the organic compound cyclohexane-1,1′-diacetic acid

The ferroelectric transition in the molecular organic ferroelectric material cyclohexane-${1,1}^{\ensuremath{'}}$-diacetic acid has been studied with a ${}^{1}H$ NMR two-time clock experiment. On the spin-lattice relaxation time scale ${(10}^{8}\mathrm{Hz})$ the molecular motions are observed as slow in both the paraelectric and the ferroelectric phase so that the instantaneous value of the crystal symmetry is observed in both phases and no symmetry breaking effectively takes place at ${T}_{c}.$ On the kHz time scale of the NMR line shape and the dipolar relaxation time experiments, the phase transition is observed and confirms the model of a dynamically disordered structure in the paraphase.

Translational diffusion in phospholipid bilayer membranes

The translational diffusion in artificial phospholipid bilayers has attracted much interest in the past. This process is widely accepted to be of importance for the diffusion of biological membrane components. The lipid bilayer is known as the permeability barrier in biological membranes. So far the diffusion of the membrane components has been determined indirectly by sophisticated methods, e.g., by the help of fluorescent probes. We present here experimental data on the orientational relaxation of the lipid headgroup in alternating electric fields. The dipole relaxation time measured by high-frequency spectroscopy is shown to be related to the self-diffusion coefficient. We have determined the electric dipole relaxation of two phospholipids and of their mixture. The dielectric relaxation time and thus the diffusion coefficient was measured as a function of temperature in both the liquid and the gel crystalline phase. The mixture even allows one to study the relaxation and the diffusion behavior in a state where both gel and liquid phases coexist. The experimental study has revealed the potential of high-frequency dielectric spectroscopy to determine the diffusion coefficient and its thermal activation energy in phospholipid membranes where the dipolar relaxation of the headgroup is associated with a site change.

Molecular dynamics in solid l-adrenaline by proton NMR

Proton NMR measurements of the spectrum, second moment, spin-lattice relaxation time T 1 and dipolar relaxation time T 1D were carried out on polycrystalline l-adrenaline at 14 and 25 MHz between 55 and 400 K. Between 70 K and 250 K relaxation is dominated by C 3 reorientation of the single methyl group in each molecule, characterized by an activation energy 8.3±0.3 kJ/mole. Below 70 K tunnelling assisted relaxation is significant, characterized by an excitation energy of 1.9±0.2 kJ/mole. Above 250 K an additional molecular motion becomes significant, with activation energy above 28 kJ/mole, attributed to conformational motion of the methylene group in the ethylamine side chain.

Microwave X-band permittivity measurements on 12-tungstophosphoric acid hexahydrate

Complex permittivity measurements of 12-tungstophosphoric acid hexahydrate (WPA·6H 2O) have been performed in the X-band region at room temperature. Permittivity data were obtained by measuring reflection ( S 11) and transmission ( S 21) parameters with a network analyzer and also by using a short-circuited-waveguide. Results on powder samples are compared with the results on compressed samples using the model of mixtures. From Cole–Cole plots, a dipole relaxation time τ=1.9×10 −11 s was determined and attributed to polyatomic ion reorientation.

Extended diffusion theory of reorientation of symmetric top molecules with internal rotation

The extended diffusion (ED) theory for the reorientational dynamics of a symmetric top molecule with internal rotation is reinvestigated. Based on a unified ED picture, we express, by using the convolution theorem of the Laplace transform, the reorientational correlation time with overall and internal free rotor correlation functions. In this work, Bull’s “equal angular momentum correlation time assumption” for both overall and internal rotations is removed. Limiting expressions, for diffusion limit of the overall rotation, are derived and shown to reduce to the results of previous works. As an application, our numerical calculations are compared with experiments on 13C nuclear-magnetic-resonance dipolar relaxation times of toluene. Angular momentum correlation times, thus obtained from the comparison, show that our calculations agree with the previous work of Chung et al.[Chem. Phys. Lett. 93, 499 (1982)].

Evaluating Polarization in Dielectrics with Continuously Distributed Dipole Relaxation Time by Discharge Current Transient Spectroscopy

The application of discharge current transient spectroscopy (DCTS) to the evaluation of the steady-state polarization P s(τ) in dielectrics with continuously distributed dipole relaxation time (τ) is discussed. The method proposed in Jpn. J. Appl. Phys. 35 (1996) 2216 can be used to evaluate P s(τ) only in dielectrics with discrete τ. Using the method discussed here, P s(τ) corresponding to the transient discharge current, which obeys a power law with respect to discharge time, can easily be evaluated.

Hydrogen motion in hydrogenated amorphous silicon (a-Si:H)

Dipolar spin lattice relaxation time, T 1D, measurements of 1H in a-Si:H have been used to probe the local motion of hydrogen in a-Si:H. Variations of T 1D with doping level reveal a trend similar to the changes in hydrogen diffusion constants measured in similar samples using secondary ion mass spectroscopy (SIMS). The temperature variations of T 1D indicate that, if the local motion is thermally activated, the activation energies are at least an order of magnitude smaller than those measured using SIMS. The ‘diffusion constants’ for local hydrogen motion inferred from T 1D measurements are many orders of magnitude faster than the diffusion constants obtained from SIMS. The T 1D measurements are made on the timescale of milliseconds while the SIMS measurements are performed on a scale of several hours to days. A phenomenological model provides a plausible connection between these two measurements, which are made on very different timescales.

A Simple Graphical Method for Evaluating Dipole Relaxation Time in Dielectric

A simple graphical method for evaluating the dipole relaxation times (τ) in a dielectric is proposed. From isothermal measurements of the transient discharge current due to dipole relaxation, τ can be obtained graphically by means of discharge current transient spectroscopy (DCTS). In other words, the time at which the DCTS signal is a maximum corresponds to τ. Since τ and the steady-state polarization (P s) in dielectrics at electric field (F p) and temperature (T) can be obtained using DCTS, the dependencies of τ and P s on F p or T can be investigated in detail.

1H NMR dipolar relaxation times and the derivation of internuclear distance

The theory of nuclear spin relaxation via the dipole-dipole coupling mechanism is discussed. Various methods of measuring relaxation rates are described and shown to provide a means for measuring interhydrogen distances in molecules. The methods include the combined use of selective and biselective pulses to invert single resonances together with nonselective pulses. The theory is given with applications from the literature. A modification of the methods—in which one resonance is left noninverted—is described, and some applications of this more efficient approach are shown. The distances obtained from such inversion-recovery relaxation rate measurements are compared with values measured from nuclear Overhauser effect build-up rates. © 1996 John Wiley & Sons, Inc.