Studies of molecular motions and interactions in acetonitrile

Microwave and far-infrared absorption coefficients are reported for acetonitrile over the whole concentration range in carbon tetrachloride solution. The data have been used to compute dipole reorientational correlation functions and relaxation times as a function of concentration. These show that τ_1R ^T increases non-linearly as the acetonitrile concentration increases (in contrast to the values of τ_2R ^T obtained from light scattering measurements) and as the viscosity decreases. This behaviour is examined in terms of intermolecular correlations between rotating dipoles. Intensity data are used to provide a check on the importance of induced dipole absorption in the far-infrared spectra and to calculate ‘static’ correlation factors g ⁽¹⁾. Both g ⁽¹⁾ and the dynamic correlation factor f ⁽¹⁾ reflect the effects of strong molecular interactions in these solutions.     

Nuclear relaxation,molecular motions and interactions

The ¹⁴N quadrupolar relaxation time for pyrrole as the pure liquid and in solution in 1,4-dioxan, pyridine, 3,5-lutidine and 2-fluoropyridine has been determined from the line-shape analysis of the imido proton magnetic resonance signal. The dependence of the line shape on solute concentration and temperature was studied and the activation parameters for molecular reorientation determined.     

NMR relaxation study of molecular motions in liquid chlorobenzotrifluorides

The temperature dependences of the ¹H and ¹⁹F nuclear spin-lattice relaxation times T₁ in liquid o-, m-, and p-chlorobenzotrifluorides have been measured. The analysis of the temperature dependences of the ¹H spin-lattice relaxation times leads to the conclusion that the overall molecular reorientational motion in o-, m-, and p-benzotrifluorides is nearly the same. Data for ¹H and ¹⁹F spin-lattice relaxation times of o-chlorobenzotrifluoride jointly lead to the determination of the individual contributions to relaxation rate in the entire temperature range studied. A knowledge of these contributions for o-chlorobenzotrifluoride, together with the assumption of equal correlation times for overall molecular reorientation in o- and p-chlorobenzotrifluorides, leads to the determination of the spin-internal-rotation interaction contribution to relaxation for p-chlorobenzotrifluoride in the same range of temperature.     

Influence of molecular motions on multipulse NMR spectra

Spin-lattice relaxation in multipulse experiments is considered. Experimental values of T₁₁ (WHH-4) as a function of temperature are presented.     

NMR investigations of molecular motions inp-n-hexyloxybenzylidene-p′-n-propylaniline

Proton spin-lattice relaxation times,T ₁, have been measured in the smectic phases, S _G ² , S _G ¹ and S_A, and in the nematic phase of HxBPA, in the temperature range, 220K<T<360 K. In the S _G ¹ and S _G ² phases,T ₁ has been measured at 15 and 40 MHz. The (S _G ¹ →S _G ² ) and (S _G ² →S _G ¹ ) transitions are clearly seen as discontinuities inT ₁. The former transition is seen to be due to possible freezing or change of hydrocarbon chain motions of the molecule. Our data in the S _G ¹ phase have been fitted to a model in which anisotropic rotational diffusion of the end hydrocarbon chains as also that of the rigid part of the molecule are considered. In the nematic phase, at 351 K, the observed behaviour ofT ₁, measured in the frequency range, 5 to 40 MHz, agrees well with the theoretical predictions of Uklejaet al and Freed, who take into account long range collective order fluctuations and local reorientations. Using these theories, the correlation time for short range reorientations has been calculated from our results to be 4.3 × 10⁻¹⁰ and 1.1 × 10⁻⁹ s respectively.     

Effective interactions in molecular dynamics simulations of lysozyme solutions

On the basis of ab-initio calculations, we predict the effect of conformation and molecule-electrode distance on transport properties of asymmetric molecular junctions for different electrode materials M (M = Au, Ag, Cu, and Pt). The asymmetry in these junctions is created by connecting one end of the biphenyl molecule to conjugated double thiol (model A) and single thiol (model B) groups, while the other end to Cu atom. A variety of phenomena viz. rectification, negative differential resistance (NDR), switching has been observed that can be controlled by tailoring the interface state properties through molecular conformation and molecule-electrode distance for various M. These properties are further analyzed by calculating transmission spectra, molecular orbitals, and orbital energy. It is found that Cu electrode shows significantly enhanced rectifying performance with change in torsion angles, as well as with increase in molecule-electrode distances than Au and Ag electrodes. Moreover, Pt electrode manifests distinctive multifunctional behavior combining switch, diode, and NDR. Thus, the Pt electrode is suggested to be a good potential candidate for a novel multifunctional electronic device. Our findings are compared with available experimental and theoretical results.     

1H NMR studies on molecular motions of 4-aminopyridinium and pyrrolidinium cations in new ferroics

The aim of the study was to check the effect of the cation on the molecular dynamics of the anion, which is not directly observed, in different phases of the following compounds: (C4H8NH2)SbCl6(C4H8NH2)Cl, (C4H8NH2)SbCl6 and (4-apyH)ClO4, (4-apyH)SbCl4.     

Studies on the structures and interactions of glutathione in aqueous solution by molecular dynamics simulations and NMR spectroscopy

All-atom molecular simulations and temperature-dependent NMR have been used to investigate the conformations and hydrogen bonds of glutathione (GSH) in aqueous solution. The simulations start from three different initial conformations. The properties are characterized by intramolecular distances, radius of gyration, root-mean-square deviation, and solvent-accessible surface. GSH is highly flexible in aqueous solutions in the simulations. Moreover, conformations can covert between “extended” and “folded” states. Interestingly, the two different hydrogen atoms in cysteine (H_N2) and glycin (H_N3) show different capabilities in forming NH?OW hydrogen bonds. The temperature-dependent NMR results of the different amide hydrogen atoms also show agreements with the MD simulations. Competing formation of GSH hydrogen-bonding interactions in aqueous solutions leads to hydrogen-bonding networks and the distribution of conformations. These changes will affect the activity of GSH under physiological conditions.     

Pulsed ESR and molecular motions

The possibilities of applying three different pulsed ESR techniques have been considered: 1. Phase relaxation measurements by electron spin echo (ESE) affords the estimation of the correlation time of the motion in the region up to 10^?5 s. The results of theoretical and experimental analysis of the effect of methyl group rotation in nitroxide radicals have been proposed. 2. The method of pulsed saturation involving detection of ESE signal allows the range of the measured times to be extended up to the values of about 10^?2 s. The rotation of CH₂ group in the CH₂CO₂ ^? radical and that of the CH₃ group in the CH₃CHCO₂ ^? radical have been investigated. 3. The method of pulsed saturation combined with pulsed scanning of H₀ allows the analysis of the rotationally induced redistribution of the pulsed saturation throughout the ESR spectra of the radicals. This version of pulsed ESR has been used to study the mobility of nitroxide spin labels.     

Homothetic motions and vacuum Robinson-Trautman solutions

The approach to symmetries given by Kerr and Debney (1970) is applied to the class of vacuum Robinson-Trautman solutions. The coordinate freedom is extended and used to define the pull-backs of local diffeomorphisms which are then shown to be homothetic motions. The resulting homothetic Killing's equations are solved to give a form of all homothety groups for this class.     

Raman and Rayleigh spectroscopy and molecular motions

Raman and Rayleigh scattering experiments on HCl, DCl, HBr and DBr molecules (pure liquid or isotopically diluted) have been carried out at room temperature. Results are discussed in the light of the existing theories on molecular motions in the liquid phase. The comparison of Rayleigh and Raman scattering results shows that reorientational processes are not sufficient to explain completely the profile of anisotropic Raman scattering. According to a recent theory due to Bratos, the rotational correlation function can be obtained only after elimination of the vibrational correlation function determined from the isotropic Raman scattering. In this last case broadening may arise from three causes: translational motion, resonance broadening and vibration-rotation coupling. A comparative study of the band profile of anisotropic Rayleigh and Raman scattering shows that the wings in both cases come from essentially the same origin while a band moment analysis is consistent with Gordon's theory according to which they are of reorientational origin.     

Numerical solutions for single passage NMR on nuclei experiencing small electric quadrupole interactions

The adiabatic rapid-passage NMR equations are numerically solved for a small additional quadrupole interaction. The full range of the ratio of rf interaction strengths to quadrupole interactions strengths is considered for any electric field gradient orientation. The results forI=1 are particularly easy to describe and are used to predict the single-passage results for different orientations of the electric field gradient with respect to the static magnetic field.     

Slow molecular motions and solid echo in hydrate crystals

The temperature dependences of the solid-echo amplitude have been measured in stilbite single crystal at temperatures of slow diffusion motions of water molecules. The obtained experimental data agree well with the derived theoretical expressions describing the two-pulse response of the isolated two-spin system performing jumps between two equilibrium positions.     

Nuclear spin relaxation and molecular motions in sebacate polyesters

In this study, pulsed NMR techniques were used to probe the molecular motions occurring in poly(hexamethylenesebacate) (HMS), and its amorphous isomer poly(2-methyl-2-ethyl-1,3-propylenesebacate (MEPS), and in particular, to examine the perturbations in the molecular motions of HMS and MEPS that arise as a result of block copolymerization. The results show a low-temperature β-relaxation due to local motion of the methylenes of the chain backbone, an α-relaxation associated with the glass transition, and an α_c-relaxation due to the melting of the crystalline phase. Both α-and β-relaxations in the block copolymer are only slightly perturbed, and this suggests that the blocks are incompatible. The β-relaxation of HMS is confined to the amorphous regions of the polymer. Although sample preparation had little effect on the degree of crystallinity, large differences in the relaxation behavior were observed when the sample preparation was varied.