Motional Correlation Times Research Articles

Structural Relaxation Time of a Polymer Glass during Deformation.

In order to determine the structural relaxation time of a polymer glass during deformation, a strain rate switching experiment is performed in the steady-state plastic flow regime. A lightly cross-linked poly(methylmethacrylate) glass was utilized and, simultaneously, the segmental motion in the glass was quantified using an optical probe reorientation method. After the strain rate switch, a nonmonotonic stress response is observed, consistent with previous work. The correlation time for segmental motion, in contrast, monotonically evolves toward a new steady state, providing an unambiguous measurement of the structural relaxation time during deformation, which is found to be approximately equal to the segmental correlation time. The Chen-Schweizer model qualitatively predicts the changes in the segmental correlation time and the observed nonmonotonic stress response. In addition, our experiments are reasonably consistent with the material time assumption used in polymer deformation modeling; in this approach, the response of a polymer glass to a large deformation is described by combining a linear-response model with a time-dependent segmental correlation time.

Water Dynamics in Dextran-Based Hydrogel Micro/Nanoparticles Studied by NMR Diffusometry and Relaxometry.

Water dynamics in mesoporous dextran hydrogel micro/nanoparticles was investigated by means of nuclear magnetic resonance (NMR) techniques. High-resolution 1H NMR spectra and pulsed field gradient (PFG) NMR diffusometry measurements obtained on swollen state dextran micro/nanogel revealed the existence of different fractions of water molecules based on their interaction with the gel matrix. In addition to the translational diffusion of bulk water, two more diffusion processes characterized with self-diffusion coefficients 1 and 2 orders of magnitude smaller than that of bulk water were identified. 1H spin-lattice relaxation dispersion profiles obtained for a broad range of Larmor frequencies using fast field cycling (FFC) and conventional NMR relaxometry techniques allowed us to further clarify the mechanisms of molecular motion. According to the water proton pool fractions and associated self-diffusion coefficients, it is shown that the relaxation contribution associated with reorientation-mediated translational motions (RMTDs) dominates the relaxation dispersion observed at intermediate frequencies. At very low frequencies, the spin-lattice relaxation rate is dominated by the slow solid-gel dynamics probed by the water molecules interacting with the pores' surface hydroxyl groups due to the rapid chemical exchange between surface hydroxyl groups and free water. The correlation time for the thumbling-like motion of the dextran gel was found to be in the submillisecond range. The values of the self-diffusion and coherence lengths associated with motion of water molecules interacting with the solid-gel particles are consistent with the particle size and pore size distributions obtained for the studied dextran gels.

Molecular simulations and NMR reveal how lipid fluctuations affect membrane mechanics

Lipid bilayers form the main matrix of functional cell membranes, and their dynamics underlie a host of physical and biological processes. Here we show that elastic membrane properties and collective molecular dynamics (MD) are related by the mean-square amplitudes (order parameters) and relaxation rates (correlation times) of lipid acyl chain motions. We performed all-atom MD simulations of liquid-crystalline bilayers that allow direct comparison with carbon-hydrogen (CH) bond relaxations measured with NMR spectroscopy. Previous computational and theoretical approaches have assumed isotropic relaxation, which yields inaccurate description of lipid chain dynamics and incorrect data interpretation. Instead, the new framework includes a fixed bilayer normal (director axis) and restricted anisotropic motion of the CH bonds in accord with their segmental order parameters, enabling robust validation of lipid force fields. Simulated spectral densities of thermally excited CH bond fluctuations exhibited well-defined spin-lattice (Zeeman) relaxations analogous to those in NMR measurements. Their frequency signature could be fit to a simple power-law function, indicative of nematic-like collective dynamics. Moreover, calculated relaxation rates scaled as the squared order parameters yielding an apparent κC modulus for bilayer bending. Our results show a strong correlation with κC values obtained from solid-state NMR studies of bilayers without and with cholesterol as validated by neutron spin-echo measurements of membrane elasticity. The simulations uncover a critical role of interleaflet coupling in membrane mechanics and thus provide important insights into molecular sites of emerging elastic properties within lipid bilayers.

An NMR relaxation study of magnesium borohydride in solid

AbstractThe spin–lattice relaxation phenomenon is a good tool for obtaining the thermodynamical parameters of solid metal borohydrides. In the process of deriving the motional parameters from the NMR experimental data, it is very important to adequately interpret the spin–lattice relaxation data. By more precisely considering the quadrupolar interaction of 11B and intermolecular 1H‐1H dipole–dipole interactions, the activation energies and correlation times of the rotational motions of BH4 group in solid magnesium borohydride could be more accurately determined, and the spin–lattice relaxation phenomenon of the quadrupolar nucleus could be more appropriately understood.

A method to construct the dynamic landscape of a bio-membrane with experiment and simulation

Biomolecular function is based on a complex hierarchy of molecular motions. While biophysical methods can reveal details of specific motions, a concept for the comprehensive description of molecular dynamics over a wide range of correlation times has been unattainable. Here, we report an approach to construct the dynamic landscape of biomolecules, which describes the aggregate influence of multiple motions acting on various timescales and on multiple positions in the molecule. To this end, we use 13C NMR relaxation and molecular dynamics simulation data for the characterization of fully hydrated palmitoyl-oleoyl-phosphatidylcholine bilayers. We combine dynamics detector methodology with a new frame analysis of motion that yields site-specific amplitudes of motion, separated both by type and timescale of motion. In this study, we show that this separation allows the detailed description of the dynamic landscape, which yields vast differences in motional amplitudes and correlation times depending on molecular position.

Model-Free or Not?

Relaxation in nuclear magnetic resonance is a powerful method for obtaining spatially resolved, timescale-specific dynamics information about molecular systems. However, dynamics in biomolecular systems are generally too complex to be fully characterized based on NMR data alone. This is a familiar problem, addressed by the Lipari-Szabo model-free analysis, a method that captures the full information content of NMR relaxation data in case all internal motion of a molecule in solution is sufficiently fast. We investigate model-free analysis, as well as several other approaches, and find that model-free, spectral density mapping, LeMaster’s approach, and our detector analysis form a class of analysis methods, for which behavior of the fitted parameters has a well-defined relationship to the distribution of correlation times of motion, independent of the specific form of that distribution. In a sense, they are all “model-free.” Of these methods, only detectors are generally applicable to solid-state NMR relaxation data. We further discuss how detectors may be used for comparison of experimental data to data extracted from molecular dynamics simulation, and how simulation may be used to extract details of the dynamics that are not accessible via NMR, where detector analysis can be used to connect those details to experiments. We expect that combined methodology can eventually provide enough insight into complex dynamics to provide highly accurate models of motion, thus lending deeper insight into the nature of biomolecular dynamics.

Nuclear magnetic relaxation studies of BH4 in metal Borohydrides

AbstractA different approach of nuclear magnetic resonance (NMR) relaxation of the rotational motion of BH4 tetrahedron in light metal borohydrides was introduced. Previous NMR relaxation studies were mainly performed using an approximation which is based on a single exponential transition approach for the spin–lattice relaxation of the spin under quadrupolar interaction. In this new approach, a relaxation rate equation was derived by strictly taking into account both electric quadrupolar and magnetic dipolar contributions to the transitions between Zeeman energy states of quadrupolar 11B nucleus that was coupled to many proton spins. Two exponential relaxation times derived from our theoretical treatment were applied to the analysis of 11B spin–lattice relaxation of LiBH4 and Ca(BH4)2. The activation energies and the correlation times of the reorientational motions of BH4 were evaluated from the two experimental spin–lattice relaxation times. The results were compared to previous activation energies and correlation times.

Molecular Dynamics in Ionic Liquid/Radical Systems.

Molecular dynamics of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Emim-Tf2N) with either of the four organic stable radicals, TEMPO, 4-benzoyloxy-TEMPO, BDPA, and DPPH, is studied by using Nuclear Magnetic Resonance (NMR) and Dynamic Nuclear Polarization (DNP). In complex fluids at ambient temperature, NMR signal enhancement by DNP is frequently obtained by a combination of several mechanisms, where the Overhauser effect and solid effect are the most common. Understanding the interactions of free radicals with ionic liquid molecules is of particular significance due to their complex dynamics in these systems, influencing the properties of the ion-radical interaction. A combined analysis of EPR, DNP, and NMR relaxation dispersion is carried out for cations and anions containing, respectively, the NMR active nuclei 1H or 19F. Depending on the size and the chemical properties of the radical, different interaction processes are distinguished, namely, the Overhauser effect and solid effect, driven by dominating dipolar or scalar interactions. The resulting NMR relaxation dispersion is decomposed into rotational and translational contributions, allowing the identification of the corresponding correlation times of motion and interactions. The influence of electron relaxation time and electron-nuclear spin hyperfine coupling is discussed.

Probing Side-Chain Dynamics in Proteins by NMR Relaxation of Isolated 13C Magnetization Modes in 13CH3 Methyl Groups.

The dynamics of methyl-bearing side chains in proteins were probed by 13C relaxation measurements of a number of 13C magnetization modes in selectively 13CH3-labeled methyl groups of proteins. We first show how 13C magnetization modes in a 13CH3 spin-system can be isolated using acute-angle 1H radio-frequency pulses. The parameters of methyl-axis dynamics, a measure of methyl-axis ordering (Saxis2) and the correlation time of fast local methyl-axis motions (τf), derived from 13C relaxation in 13CH3 groups are compared with their counterparts obtained from 13C relaxation in 13CHD2 methyl isotopomers. We show that in high-molecular-weight proteins, excellent correlations are obtained between the [13CHD2]-derived Saxis2 values and those extracted from relaxation of the 13C magnetization of the I = 1/2 manifold in 13CH3 methyls. In smaller proteins, a certain degree of anticorrelation is observed between the Saxis2 and τf values obtained from 13C relaxation of the I = 1/2 manifold magnetization in 13CH3 methyls. These parameters can be partially decorrelated by inclusion in the analysis of relaxation data of the I = 3/2 manifold 13C magnetization.

Solid-State NMR Spectroscopy Study of Molecular Dynamics of Pyridine 1-Oxide Encapsulated in p-tert-Butylcalix[4]arene.

The mixture of guests 25% pyridine 1-oxide (PNO) and 75% nitrobenzene (NB) encapsulated in a host cavity of p-tert-butylcalix[4]arene (p-tBC) is a suitable system for the study of intermolecular weak interactions, and the preferred orientation of the guest molecules within the host was derived. Variable temperature deuterium nuclear magnetic resonance line shape and spin-lattice relaxation studies were performed on three samples of 25% PNO as a guest (selectively and entirely deuterated) with 75% NB encapsulated in p-tBC as a host system. It is found that the PNO molecular motion supports the two-site jump model and is highly mobile throughout the temperature range from -130 to 20 °C. PNO reorients about its C2 molecular symmetry axis followed by reorientation around the compound's C4 axis of symmetry of a host. Calculation showed that besides precise molecular jumping, the guest also experiences the vibrational motion with a variation angle dispute about 15°. The correlation times for the molecular guest motion and two-dimensional exchange spectroscopy confirmed a fast exchange limit inside the cavity. These molecular dynamics follow an Arrhenius behavior motion from which the small activation energy is delivered that strongly suggests a relatively weak intermolecular interaction between the host and guest species. PNO has definite dipole orientational motion in the cavity where its aromatic covalent bond structure minimizes contact with the host p-tert-butyl groups. Experimentally found, PNO-d5 deuterons in the single crystal collapse at 55 and 125° at ambient temperature. The first splitting of 124 kHz belongs to D3(D5), the second splitting of 100 kHz belongs to D2(D6), and the last splitting of 52 kHz belongs to D4 of the ring of PNO inside the p-tBC.

1-Amino 2-Naphthol Modified Solvent Filled Carbon Nanotubes for Enhanced Electrochemical Sensing of Bioanalytes

My research interest is mainly on developing cost-effective electrochemical sensors for bioanalytes sensing. For example, bioanlytes such as ascorbic acid, dopamine, and uric acid present together in human body poses challenge for selective sensing of one over other. As these molecules play a vital role in controlling the body’s metabolism, it is paramount to develop a sensor tool with high sensitivity towards these bioanlytes for diagnosing diseases, including AIDS, Parkinsonism, etc. Development of fast and reliable methods for sensing these biomolecules is of considerable importance in offering rapid diagnosis.1–3 In the present experimental work, it is shown for the first time that liquid-filled multi-walled carbon nanotubes (MWCNT) exhibit remarkably enhanced characteristics for sensor applications, the detection threshold being lowered by as much as an order of magnitude, while detection sensitivity is doubled for a number of molecules including ascorbic acid, dopamine, and uric acid. 1H NMR relaxometry at a moderately low field, i.e., 14.6 MHz (0.34 T), confirms that the improved sensor characteristics are correlated with the filling of MWCNTs with solvents such as acetonitrile, dimethyl sulfoxide, etc. The molecular motions are slowed down when solvent molecules fill MWCNTs, thus increasing motional correlation times, and hence, longitudinal relaxation rate increases under extreme narrowing conditions. The filling of MWCNTs appears to relate to solvent-CNT interactions that are in line with the principle of hard and soft acids and bases, and also correlates with solvent dipole moments.4 We have further enhanced the detection limit and selectivity towards ascorbic acid by modifying the MWCNTs surface with amino naphthol. References K. Iriyama, M. Yoshiura, T. Iwamoto, and Y. Ozaki, Anal. Biochem., 141, 238–243 (1984).L. Jothi, S. Neogi, S. kumar Jaganathan, and G. Nageswaran, Biosens. Bioelectron., 105, 236–242 (2018).M. Jahan, Z. Liu, and K. P. Loh, Adv. Funct. Mater., 23, 5363–5372 (2013).T. Gurusamy, A. Banerjee, N. Chandrakumar, and K. Ramanujam, J. Electrochem. Soc., 166, B1186–B1195 (2019).

Features of the Structural Organization and Sorption Properties of Cellulose

An improved model of the layered structure of cellulose microfibrils is proposed, taking into account the presence of slit-like pores between its structural elements. The scheme of the formation of donoracceptor intramolecular, intermolecular and interlayer hydrogen bonds of one glucopyranose unit in the cellulose crystallite is presented. The mechanism of specific adsorption interactions of water molecules in a monolayer with active centers located on the hydrophilic surfaces of elementary fibrils is described. The energy of dipole-dipole interactions was calculated depending on the distance between the active center and the adsorbate water molecule. The thermodynamic parameters characterizing the state of the adsorbate in the adsorption layers are determined by the proton magnetic relaxation method and sorption measurements. The possibility of determining the net heat of adsorption in a bilayer taking into account the Arrhenius nature of the correlation times of the thermal molecular motions of the adsorbate was established. An increase in the entropy of adsorbed water during the adsorption process was revealed. It was found that during adsorption, a part of the inner regions of crystallites transitions to their surface with the participation of cellulose hydroxyl groups, and during desorption, the reverse process is observed.

1-Amino 2-Naphthol Modified Solvent Filled Carbon Nanotubes for Enhanced Electrochemical Sensing of Bioanalytes

Our research interest is mainly on developing cost-effective electrochemical sensors for bioanalytes sensing. For example, bioanlytes such as ascorbic acid, dopamine, and uric acid present together in human body poses challenge for selective sensing of one over other. As these molecules play a vital role in controlling the body’s metabolism, it is paramount to develop a sensor tool with high sensitivity towards these bioanlytes for diagnosing diseases, including AIDS, Parkinsonism, etc. Development of fast and reliable methods for sensing these biomolecules is of considerable importance in offering rapid diagnosis.1–3 In the present experimental work, it is shown for the first time that liquid-filled multi-walled carbon nanotubes (MWCNTs) exhibit remarkably enhanced characteristics for sensor applications, the detection threshold being lowered by as much as an order of magnitude, while detection sensitivity is doubled for a number of molecules including ascorbic acid, dopamine, and uric acid. 1H NMR relaxometry at a moderately low field, i.e., 14.6 MHz (0.34 T), confirms that the improved sensor characteristics are correlated with the filling of MWCNTs with solvents such as acetonitrile, dimethyl sulfoxide, etc. The molecular motions are slowed down when solvent molecules fill MWCNTs, thus increasing motional correlation times, and hence, longitudinal relaxation rate increases under extreme narrowing conditions. The filling of MWCNTs appears to relate to solvent-CNT interactions that are in line with the principle of hard and soft acids and bases, and also correlates with solvent dipole moments.4 We have further enhanced the detection limit and selectivity towards ascorbic acid by modifying the MWCNTs surface with amino naphthol.

Unusual Overhauser Dynamic Nuclear Polarization Behavior of Fluorinated Alcohols at Room Temperature.

The structure of fluorinated alcohols is a matter of considerable interest in view of wide-ranging biomolecular applications. The microheterogeneity of fluorinated alcohols in the liquid state, in particular, has been a matter of debate and discussion in recent years using experimental and theoretical methods, including neutron or X-ray diffraction, as well as density functional theory (DFT) and molecular dynamics (MD) simulations. Here, we show that 1H and 19F Overhauser dynamic nuclear polarization (ODNP) buildup curves in solution state at room temperature show unusual behavior that could offer a novel approach to investigate the structural heterogeneity and dynamics of such homogeneous liquids with improved sensitivity. A detailed analysis of multiexponential ODNP buildup curves as a function of microwave irradiation time is shown to evidence microheterogeneity in such systems. Experimental ODNP buildup rates are interpreted using simple motional models that yield the motional correlation times of the relevant species in solution. It may be emphasized that this information is not available from standard approaches of high-resolution nuclear magnetic resonance (NMR) spectroscopy. While the present study focuses on fluorinated alcohols, it is to be anticipated that this approach would be valuable in the study of molecular assemblies in the solution state, including peptides, surfactant systems, etc.

Spin locking in liquid entrapped in nanocavities: Application to study connective tissues.

Study of the spin-lattice relaxation in the spin-locking state offers important information about atomic and molecular motions, which cannot be obtained by spin lattice relaxation in strong external magnetic fields. The application of this technique for the investigation of the spin-lattice relaxation in biological samples with fibril structures reveals an anisotropy effect for the relaxation time under spin locking, T1ρ. To explain the anisotropy of the spin-lattice relaxation under spin-locking in connective tissue a model which represents a tissue by a set of nanocavities containing water is used. The developed model allows us to estimate the correlation time for water molecular motion in articular cartilage, τc=30μs and the averaged nanocavity volume, V≃5400nm3. Based on the developed model which represents a connective tissue by a set of nanocavities containing water, a good agreement with the experimental data from an articular cartilage and a tendon was demonstrated. The fitting parameters were obtained for each layer in each region of the articular cartilage. These parameters vary with the known anatomic microstructures of the tissue. Through Gaussian distributions to nanocavity directions, we have calculated the anisotropy of the relaxation time under spin locking T1ρ for a human Achilles tendon specimen and an articular cartilage. The value of the fitting parameters obtained at matching of calculation to experimental results can be used in future investigations for characterizing the fine fibril structure of biological samples.

Anisotropy of the electron spin–lattice relaxation. PO32− radical in glycinium phosphite gly·H3PO3 crystal

Electron spin–lattice relaxation has been measured for PO32− radical in glycinium phosphite gly·H3PO3 crystal and its deuterated analogue in temperature range 40–300 K. Angular dependence of the relaxation rate was measured in three crystal planes at room temperature. The Debye cut-off temperature has been calculated as ΘD = 97 K, which indicates that the temperature dependence of 1/T1 is governed by radical local vibrations localized in optical phonon modes range. Theories of possible 1/T1 angular dependence are reviewed. The anisotropy of spin–lattice relaxation rate in our crystal is assumed to be a result of local magnetic field fluctuations due to electron–proton dipolar coupling. Theoretical evaluation of 1/T1 were performed for coupling with protons at distances up to 0.46 nm. Not perfect agreement was found with motion correlation time τc = 2·10−9 s.

The structural and dynamical role of water in natural organic matter: A 2H NMR and XRD study

Natural organic matter (NOM) is an important component in many near-surface geochemical environments, and its properties are greatly affected by the incorporation of water. Because of its importance, the macroscopic behavior and effects of water in NOM and soil organic matter (SOM) have been extensively studied using a wide range of experimental and computational methods. The molecular scale structural and dynamical behavior of water in these materials, however, is less well understood. This paper presents a variable temperature 2H NMR and XRD study of water in Suwannee River NOM and its fulvic acid (FA) and humic acid (HA) fractions that provides new insight into the dynamical behavior of structurally different types of water and exchangeable hydrogen environments in NOM. The results provide a basis for future studies of more complex natural organic materials and the interaction of organic materials with mineral surfaces. Room temperature 2H NMR spectra of samples hydrated in 2H2O and then dehydrated, distinguish 2H2O molecules that are in rapid reorientational motion (correlation times, νc, > 105 Hz), 2H exchanged onto carboxylic sites of the NOM that do not undergo rapid reorientation at frequencies >∼103 Hz, and 2H exchanged onto phenolic and possibly other alcohol sites of the NOM that undergoes rapid, but anisotropic, dynamical reorientation. For samples exposed to water and not dried, the XRD results collected at temperatures from 173 to 298 K show the formation of ice-1h in samples exposed to 100% relative humidity (R.H.) but not in samples exposed to 43% R.H. 2H NMR of those samples collected at temperatures from 313 K to 173 K show the presence of multiple sites. Near room temperature, the spectra contain a narrow resonance for mobile water undergoing rapid isotropic motion, and a broader symmetrical resonance probably due to a combination of more dynamically restricted water molecules and 2H exchanged onto phenolic and alcohol functional groups undergoing rapid anisotropic motion. The 43% R.H. samples also yield a broader, quadrupole-dominated, resonance for 2H exchanged onto functional groups of the NOM. With decreasing temperature the resonances for dynamically restricted water molecules and 2H exchanged onto phenolic and alcohol functional groups become broader, reflecting a decreasing rate of exchange between the water molecules and functional groups and a decreasing rate of reorientation of the 2H2O molecules. The formation of ice-1h is directly reflected in the 2H spectra of the 100% R.H. samples as a resonance with a quadrupole coupling constant (QCC) of ∼180 kHz. For the 43% R.H. samples, there is also a broad, poorly resolved resonance with typical QCCs of ∼180 kHz for which the relative signal intensity increases with decreasing temperature. This signal represents 2H2O molecules that are not crystallized in ice-1h but have greatly reduced reorientation frequencies at low temperature and a hydrogen bonding network with hydrogen bond strengths similar to, but somewhat weaker than, ice-1h. Such molecules are also likely to be present in the 100% R.H. samples. At both R.H.s, some of the 2H2O molecules do not freeze and retain their isotropic motion down to 173 K, the lowest temperature investigated.

Reorientation of DABCO (1,4-diazabicyclo[2.2.2]octane) in halogen-bonded molecular complex DABCO-2(C6F5I)

Temperature dependence of 1H NMR spin-lattice relaxation time is investigated of a 1:2 molecular complex of DABCO (1,4-diazabicyclo[2.2.2]octane) and iodopentafluorobenzene formed by the halogen-bonding between nitrogen and iodine atoms. Besides the known phase transition at 230 K from polar (space group Pc) to nonpolar (space group P21/c) phases, another first-order phase transition was suggested for the first time at 129 K. The correlation time of reorientational motion of DABCO molecule is determined as a function of temperature. The activation energies Ea of the motion were estimated to be 20 kJ mol−1 and 11 kJ mol−1 for the crystallographically nonequivalent DABCO molecules in the intermediate-temperature phase.

Correction to: Characterization of fibril dynamics on three timescales by solid-state NMR.

In our recent publication (Smith et al., J Biomol NMR 65:171-191, 2016) on the dynamics of HET-s(218-289), we reported on page 176, that calculation of solid-state NMR R1ρ rate constants using analytical equations based on Redfield theory (Kurbanov et al., J Chem Phys 135:184104:184101-184109, 2011) failed when the correlation time of motion becomes too long.

Reoriention of diprotonated DABCO (1,4-Diazabicyclo[2.2.2]octane) cation and proton transfer in organic ferroelectric adduct DABCO-2(2-Chlorobenzoic acid)

Temperature dependences of 1H NMR as well as 35Cl NQR spin-lattice relaxation times T1 were investigated of a ferroelectric molecular adduct with Tc = 323 K, in which 1,4-diazabicyclo[2.2.2]octane (DABCO) is sandwiched between two 2-chlorobenzoic acid (2-ClBA). The NQR frequencies clearly show that proton transfer from 2-ClBA to DABCO is occurred and the molecular adduct consists of diprotonated DABCO cation and two 2-chlorobenzoate anions. The correlation time of reorientational motion of the diprotonated DABCO molecule was determined as a function of temperature. The activation energy Ea of the motion was estimated as 22 kJ mol−1 below Tc. The steep decrease of the NQR T1 with Ea = 50 kJ mol−1, observed above ca. 280 K in the ferroelectric phase, suggests a slow fluctuation of electric field gradient at chlorine nucleus.