Molecular dynamics in comb-like ionene as studied by NMR

Proton and fluorine second moment and spin-lattice relaxation timesT ₁ andT _lρ have been employed to study molecular dynamics in the comb-like I-6,6-16-Me-BF₄ ionene in the temperature range from 110 up to 300 K. The existence of motions of methyl groups, trans-gauche isomerization, and/or rotation of the main- and side-chain méthylene groups, as well as isotropic reorientation of tetrafluoroborate ions were established. The observed relaxation behaviors are explained by motional models which assume Davidson-Cole asymmetrical distribution of correlation times. The best-fit motional parameters are given.     

铵根离子在水和甲醇溶液中的转动机制

Dynamics of ammonium and ammonia in solutions is closely related to the metabolism of ammoniac compounds, therefore plays an important role in various biological processes. NMR measurements indicated that the reorientation dynamics of NH₄⁺ is faster in its aqueous solution than in methanol, which deviates from the Stokes-Einstein-Debye rule since water has higher viscosity than methanol. To address this intriguing issue, we herein study the reorientation dynamics of ammonium ion in both solutions using numerical simulation and an extended cyclic Markov chain model. An evident decoupling between translation and rotation of methanol is observed in simulation, which results in the deviation of reorientation from the Stokes-Einstein-Debye rule. Slower hydrogen bond (HB) switchings of ammonium with methanol comparing to that with water, due to the steric effect of the methyl group, remarkably retards the jump rotation of ammonium. The observations herein provide useful insights into the dynamic behavior of ammonium in the heterogeneous environments including the protein surface or protein channels.     

NMR Study of the Molecular Dynamics of D-Amphetamine Sulfate Salt Powder

The temperature dependence of the proton spin–lattice relaxation times was measured for the D-amphetamine sulfate salt in the temperature range from 80 to 466 K. Two minima of T ₁ were observed and explained in terms of reorientation of the CH₃ and NH₃ groups. The T ₁ minimum attributed to the motion of methyl groups was analyzed assuming two dynamically inequivalent types of these groups in the unit cell of the studied salt. The activation parameters for the C ₃ reorientation of the methyl and ammonium groups were determined. A structural phase transition was evidenced at about 335 K in the polycrystalline sample of the D-amphetamine sulfate salt. Authors' address: Joanna Kaszyńska, Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, Poznań 60-179, Poland     

<Superscript>1</Superscript>H NMR study of molecular dynamics of acetylcholine chloride

The longitudinal relaxation timeT ₁ and the second momentM ₂ of¹H nuclear magnetic resonance line in a wide temperature range have been measured for acetylcholine chloride. Two different types of the methyl groups reorientation occurred. The first type was the hindered rotation of the methyl group denoted as C(1)H₃ about the threefold symmetry axis. The second type was the reorientation of the trimethyl group-N(CH₃)₃ around the pseudo C₃^′ axis of C(6)-N(7) bond, which accompanied the standard C₃ motion of the methyl group. The Dunn-McDowell model was applied to analyze the dynamics observed.     

Nonlinear refraction in photo-induced anisotropic states of azo-dye containing materials

The optical anisotropic states in the azo-dye Disperse Red 13 doped poly(methyl methacrylate) polymeric film are induced by polarized light and the nonlinear refractive index in these states are studied by the Z-scan technique. It is found that the n₂ of the sample can be controlled by changing both the polarization direction and the intensity of the 514 nm light. Photo-induced isomerization and reorientation of azo chromophores in polymer matrices are used to explain the results.     

<Superscript>1</Superscript>H Spin–Lattice Relaxation Study of Dynamical Inequivalence of Methyl Groups in Solid 1,2-<Emphasis Type="Italic">O</Emphasis>-(1-Ethylpropylidene)-α-<Emphasis Type="SmallCaps">d</Emphasis>-Glucofuranose

We investigated the dynamics of methyl groups in organic polycrystalline 1,2-O-(1-ethylpropylidene)-α-d-glucofuranose by the proton spin–lattice relaxation method. The temperature and nuclear magnetic resonance Larmor frequency dependence of relaxation time is presented and interpreted in terms of simple possible dynamical model for the reorientation of methyl groups: the random hopping for methyl groups, which are in a, b, and c sites in the crystal. The energy E _a of 13.5 kJ mol⁻¹ for the a-type methyl groups is typical for methyl groups in ethyl groups. In contrast, the b- and c-methyl groups characterized by the lower E _a values of 9.5 and 6.5 kJ mol⁻¹ are located in the crystal structure where the intermolecular interactions significantly influence the potential, leading to a decrease in the total energy.     

Neutron scattering studies of the solid tetramethyl compounds of silicon,germanium and tin

A study is presented of both quasi-elastic and inelastic neutron scattering on Si(CH₃)₄, Ge(CH₃)₄ and Sn(CH₃)₄. Within the applied resolution no quasi-elastic scattering was detected for the silicon compound and only a small broadening of the elastic line for the germanium compound at 149 and 178 K was determined, in agreement with NMR data. From the quasi-elastic scattering between 81 and 211 K of Sn(CH₃)₄ an activation energy for 3-fold jump reorientation of the methyl group, E_a = (3.4 ± 0.2) kJ/mol, was deduced.For all three compounds a very similar lattice vibration spectrum was found. The inelastic neutron spectrum between 10 and 24 meV of Ge(CH₃)₄ and Sn(CH₃)₄ and Sn(CH₃)₄, measured with a very good resolution, revealed some new peaks and made a more accurate determination of the torsiònal transitions of the methyl groups possible. It is shown that the barrier heights for methyl reorientation, determined from these transitions correspond very well with activation energies found in NMR experiments and from the quasi-elastic scattering in this investigation.     

Overall and internal motions of substituted benzenes studied by means of selective proton magnetic longitudinal relaxation time measurements

Selective T ₁ measurements at different temperatures on seven substituted benzenes in CDCl₃ solution show that for these molecules the rotational diffusion model applies. Anisotropic reorientation is important if the molecules contain two polar substituents para to each other. Anomalies in the temperature dependence of the T ₁ values in a 2-methyl substituted phenol are explained by intermolecular hydrogen bonds, which strongly influence the molecular motion. In a 2,6-dimethyl substituted phenol this effect is absent. The correlation times and the energy barriers for methyl rotation of methyl and methoxy groups are determined. The reorientation of methoxy groups around the aryl oxygen bond is slower than the molecular motion. The T ₁ values of ring protons and substituents can in some cases be used for spectral assignment. Expressions are given for the T ₁ value of a ring proton relaxed by an ortho methyl or methoxy group and for the T ₁ value of a reorienting methoxy group in the case of dipolar relaxation and axially symmetric behaviour of the molecule.     

Nonexponential nuclear spin-lattice relaxation in the isotropic phase of thermotropic liquid crystals

In the isotropic phase of nematic and smectic liquid crystals T₁ of CH3 and chain protons is larger than that of ring and ring-neighboured protons being caused by fast CH₃ reorientation and internal motions in chains, respectively.     

Unusual effects in variable temperature powder NMR spectra of the methyl group protons in 9,10-dimethyltriptycene-d12

Variable temperature ¹H wide line NMR spectra of polycrystalline 9,10-dimethyltriptycene-d₁₂ deuterated in the aromatic positions were studied. The spectra show different patterns in an unrepeatable dependence on the way of preparation of the powdered samples. Simultaneously, no anomalies were seen in the MAS and CPMAS proton-decoupled room-temperature ¹³C spectra as well as in powder X-ray diffraction patterns. The effects observed in the ¹H spectra are tentatively explained in terms of a phenomenological model. For one of the examined samples it afforded a consistent interpretation of the entire series of temperature dependent spectra in terms of structural non uniformity of the solid material studied. Quantum character of the stochastic dynamics of the methyl groups in the investigated compound was confirmed, although these dynamics are close to the classical limit where the familiar random jump model applies.     

Chain dynamics of bilayer n-decylammonium chloride studied by deuteron NMR spectroscopy

N-decylammonium chloride is a polymorphic solid which exhibits a variety of layered phases. The molecular dynamics and phase transitions in these phases have been studied using²H NMR of selectively deuterated n-decylammonium in the chain positions 1 and 6 and in the ammonium group. Measurements were performed over the temperature range of the interdigitated phase I, the non-interdigitated monoclinic phase , the tetragonal phase , and the monotropic phase , obtained upon cooling the sample from the -phase. In phase I, below 321 K, the hydrocarbon chain is rigid except for ND₃ rotation about its own symmetry axis. In phase , between 321 K and 325 K, a fast motion of the hydrocarbon chains is assigned to an exchange between two pairwise nonequivalent positions. The chains are slightly tilted in this phase. In phase above 325 K the chains rotate freely around their long axes experiencing additional transgauche conformational dynamics. In the non-interdigitated phase the spectra can be interpreted by assuming a reorientation between two variously occupied potential wells about the axis determined by the C-N bond, which is believed to be parallel to the normal bilayer axis.     

Methyl rotation in acetamide: the transition from quantum mechanical tunneling to classical reorientation studied by inelastic neutron scattering

Quasielastic and inelastic incoherent neutron scattering has been used to study in detail the transition from quantum mechanical tunneling motion to classical reorientation of the methyl groups in rhombohedral acetamide CH₃CONH₂. The temperature dependence of the low temperature quasielastic and inelastic scattering due toE ^a E ^b and AE transitions of the tunneling methyl groups has been investigated with eV resolution and — together with the higher temperature quasielastic scattering-compared with theoretical predictions. Microscopic theories are capable to describe most of the experimental observations at low temperatures. A heuristic theoretical approach accounts well for the high temperature results.     

Raman study of NH4NO3 and ND4NO3—250–420K

Raman spectra of NH₄NO₃, and ND₄NO₃, were studied from 250 to 420K. The results show that there are four phases separated by first order transitions. No evidence of the previously reported phase II' was observed.The present results combined with the results of other experiments present the following picture of the state of order of the molecules.In phase I, the highest temperature phase, the NH₄⁺ groups are in a free rotation and the nitrate groups are likely in random reorientation among 12-equivalent positions. In phase II, the NH₄⁺ groups are likely in rapid random reorientation under the local force field of S₄ symmetry. The nitrate groups are in hindered rotation but are disordered with one of the O-N bonds directed in one sense or the other along the c-axis. In phase III, the absence of the librational mode indicates that the NH₄⁺ groups are in nearly free rotation but the rotational motion is restricted by the local force field of C₃ symmetry. The nitrate groups are probably ordered as suggested by the well polarized character of the modes associated with the nitrate groups. In phase IV, the nitrate groups are ordered with their molecular planes perpendicular to the b-axis. The NH₄⁺ groups are in orientational disorder but may undergo bindered rotations. An optical mode was observed to couple to an anomalous mode which is believed to be a zone edge acoustical mode.     

Raman study of (NH4)2CuCl4·2H2O—I: Dynamics and structure in low temperature phase

We report the results of a Raman scattering study of (NH₄)₂CuCl₄2H₂O at 300, 205 and 100°K, in order to elucidate the dynamics and phase transition in this double salt. A group theoretical calculation of the symmetry vectors, in the high temperature phase (D_4h¹⁴), is made and the various modes are identified. The deuterated compound (ND₄)₂CuCl₄·2D₂O has also been investigated to help in identifying the modes involving motion of the ammonium ions and water molecules. Through a careful analysis of the spectra at 100°K, the space group in the low temperature phase has been established as D_2d³. The important consequence of this result is that this leads to parallel spatial ordering of ammonium tetrahedra in this compound in the low temperature phase.     

Proton magnetic relaxation and molecular motion in polycrystalline amino acids

A proton magnetic relaxation study has been carried out on a further seven polycrystalline amino acids from 130 to 500 K at 60·2 MHz, supplemented by measurements of the spectrum down to 4 K on five of them. These amino acids all have one or two methyl groups in their side chain, and exhibit two relaxation minima. Clear evidence is given that the relaxation minima at lower temperatures are attributable to reorientation of the methyl groups, while those at higher temperatures are attributable to reorientation of the -NH₃ groups in the zwitterion configuration of the molecules. Values of the relaxation constants, activation energies and time factors which best characterize the kinetics of both motions are tabulated. Effects of methyl group tunnelling are found for methionine and valine.     

Effect of the frequency of intramolecular motions on the NMR relaxation in liquid state temperature regime

Equations for the spectral densities of complex motion of a spin pair undergoing internal motion and isotropic/anisotropic overall rotation have been considered. The fluctuations of the interproton distances, caused by internal motion, have been taken into account in the theoretical equations. A method allowing a distinction between the isotropic and the anisotropic overall rotation of molecules has been proposed. The effect of the activation parameters of internal motions (known from the solid state study) on the measured T ₁ relaxation of the ¹³C and ¹H–¹H cross-relaxation rates has been analysed for methyl-β-D-galactopyranoside in DMSO-d6 solution. The conformational trans-gauche jumps of the methylene group are not fast enough to affect the T ₁ value of carbon C6 in the liquid state temperatures regime. Only the methyl group rotation is a very fast internal motion. This motion influences the carbon C7 relaxation and methyl protons–anomeric proton cross-relaxation. The values of interatomic distances between anomeric H(C1) and H(C5) as well as the three methyl protons H(C7) have been calculated from the cross-relaxation rates. The distance H(C1)–H(C7) fluctuates due to the rotation of methyl group. The application of the ‘model-free approach’ to study molecular dynamics in solutions is discussed.     

Isomers of C46 fullerene with carbyne chains

The dynamics of the processes of isomerization and decomposition of C₄₆ fullerene has been studied by computer simulation in the real-time regime. At an intermediate stage of evolution of the cluster (before it loses the spheroidal shape), isomers in which groups of adjacent pentagons and hexagons of C-C bonds are connected to each other with two-, four-, and six-atomic carbyne chains have been revealed. An analysis of the potential energy hypersurface has demonstrated that the isomers are quite stable and some of them can be observed experimentally.     

Proton magnetic relaxation studies of molecular diffusion in the liquid crystal 40.5

Nuclear magnetic relaxation spectroscopy is used to study the molecular dynamics in a liquid crystal butoxy benzylidene pentylaniline (40.5) in the frequency range 4 to 30 MHz and the results are compared with two other members of the same homologous series (viz 40.8 and 40.6). Spin lattice relaxation time studies indicate that molecular self diffusion (SD) and reorientation processes (R) dominate the relaxation process and their relative contributions are quantified. This contrasts with the case where order director fluctuations (ODF) effectively mediate relaxation process and all the three processes are found to be important in 40.6 in a similar frequency range.T _1D in 40.5 in the nematic phase shows temperature dependence indicating that ODF that is present at low frequencies might be diffusion assisted. These relaxation data are analysed in theS _B phase of this compound also to obtain contributions to the relaxation process. These results are also analysed to obtain different parameters associated with the above dynamical processes.     

Proton magnetic relaxation and molecular motion in polycrystalline amino acids

A proton magnetic resonance and relaxation study has been carried out on seven polycrystalline amino acids from 130 to 500 K at 60·2 MHz. These amino acids contain no methyl groups and all exhibit a single relaxation minimum. The measurements are well accounted for by the Kubo-Tomita theory of relaxation assuming a single correlation time which follows a simple activation law. In all cases the source of relaxation is provided by reorientation of the -NH₃ groups in the zwitterion form of the molecules; other protons are relaxed by spin-exchange with those of the -NH₃ group. The measured relaxation constants are consistent with inter-proton distances in the -NH₃ groups recently measured by neutron diffraction.     

Magnetic and Mössbauer studies of the DyFe11Mo compound

A detailed magnetic and Mössbauer study focusing on intrinsic magnetic and anisotropy properties of the DyFe₁₁Mo compound is reported. The compound shows a spin reorientation phase transition at T_sr=220 K. Anomalies in physical properties such as saturation magnetization, AC-susceptibility and hyperfine field at T_sr were identified, analysed and are discussed in terms of the individual site anisotropy model.