Molecular motion in solid ethyltrimethylammonium halides

Proton magnetic resonance spectra have been studied over a wide temperature range for polycrystalline ethyltrimethylammonium halides [C₂H₃N(CH₃)₃]⁺X⁻, where X⁻ = Cl⁻, Br⁻ and I⁻. For the bromide and its ethyl-deuterated analogue, the proton relaxation times T₁ were also measured vs. temperature. Analysis of the experimental data yields information on the activation parameters for molecular motion within the cation and of the cation as a whole. The potential barriers determined increase as anion size decreases.     

Molecular motion in solid polymers detected by photon correlation spectroscopy

Molecular relaxation in the 10–10⁶ Hz region in a solid amorphous polymer PMMA has been detected by photon correlation spectroscopy. Relaxation times are found to depend strongly on sample annealing and sample temperature equilibration. The main relaxation frequencies, determined around the glass-rubber transition. T_g, are in good correspondence with values obtained by other methods.     

Molecular motion in poly(L-histidine) in the solid state

Molecular motions in poly(L -histidine) (PLH) and its hydrochloride in the solid state have been studied by NMR and dielectric measurements. Four relaxation processes, β,γ,δ, and ε, are observed for PLH. The δ relaxation is assigned to rotation of an imidazole ring about the C^β? C^γ bond, since the observed activation energy of 2.7 kcal/mole agrees with the calculated energy barrier for rotation of the central imidazole ring about C^β? C^γ in an imidazole trimer model and the experimentally determined dielectric relaxation strength is consistent with the theoretically estimated value based on the two-state transition theory. The γ relaxation was attributed to the restricted rotational motion about C^α? C^β. The β relaxation is related to motion of water molecules bound by PLH. The ε relaxation is assigned to the wagging mode of imidazole groups in the defect region as observed for polymers containing pendent aromatic rings. No relaxation is observed in the hydrochloride of PLH due to the increased interaction between imidazolium cations as side groups. This is confirmed by the comparison of dipole moments of protonated and deprotonated imidazoles estimated by molecular-orbital calculations.     

Ultraviolet absorption spectra of ammonia and deuterated ammonia in solid argon

The matrix isolated spectrum of the 2100 Å system of ammonia is reexamined and extended to include the dependence of the absorption spectrum on the temperature of the matrix. Assuming that the observed spectra reflect the populations of the O⁺ and O⁻ vibrational levels in the ground electronic state, the observed temperature dependence of the spectrum of ammonia in argon are more satisfactorily explained for trapped ammonia molecules free to rotate but without nuclear spin interconversion.     

On the interaction between ammonia and carbon dioxide in solid argon and solid nitrogen

Infrared spectra of nitrogen and argon matrices containing ammonia and carbon dioxide have been recorded. A 1:1 ammonia carbon dioxide complex has been observed. The geometry of the complex has been deduced from its infrared spectrum. Carbon dioxide disturbs the rotation of ammonia in argon.     

Infrared spectroscopy of the solid phases of ammonia

Thin films of solid ammonia (NH(3) and ND(3)) have been characterized using low temperature (25-110 K) Fourier-transform infrared (FTIR) spectroscopy, and the three solid phase (amorphous, metastable, and crystalline) spectra are reported. This work has been motivated by confusion in the literature about the metastable and crystalline phases as a result of an early erroneous report by Staats and Morgan [(J. Chem. Phys. 31, 553 (1959)]. Although the crystalline phase has subsequently been reported correctly, the metastable phase has not been described in the literature in detail. The unique characteristics of the metastable phase, reported here for the first time, include multiple peaks in the nu(2) and nu(3) regions and peak intensities that are dependent on the deposition temperature. This behavior may be the result of (a) preferential molecular orientations in the solid, or (b) exciton splitting due to different crystal shapes in the solid. The amorphous and metastable phases of deuterated ammonia are also reported for the first time.     

X-ray spectra and electronic structure of solid ammonia

Intermolecular interactions in solid ammonia are investigated in a combined X-ray spectral and quantum chemical study. Theoretical NK_α spectra are constructed on the basis of MNDO calculations of the ammonia molecule and (NH₃)₇ and (NH₃)₁₃ clusters modeling solid ammonia; the spectra are in satisfactory agreement with the experimental X-ray spectra. Fragment analysis of the clusters with respect to the central ammonia molecule is carried out. It is shown that intermolecular electronic interactions in solid ammonia are most effective in MOs of e symmetry (σ-binding of nitrogen and hydrogen atoms). The fragment 2a₁ orbital contributes to the MO structure of the clusters to the least extent. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 721–726, July–August, 1996. Translated by L. Smolina     

On the OH torsional motion in solid pentachlorophenol

After theoretical and experimental studies, an assignment is made at 409 cm^?1 for the OH torsional vibration in solid pentachlorophenol. An optimization method is applied to calculate the V₂ torsional barrier, which provides larger values than those obtained from the conventional methods used by other authors.     

Molecular motion in zinc hydrazone grid complexes

Four new grid forming hydrazone ligands substituted with methoxy and dimethylamino groups were synthesised. Combination of these ligands with zinc triflate in acetonitrile resulted in self-assembly to form grids as indicated by ¹H NMR and ES-MS. ¹H NMR also showed thermally induced rotation of the intercalated phenyl ring in both the new grids, and in three previously reported grid compounds. Of these seven, five were amenable to study by variable temperature ¹H NMR. Though the observed rates varied by more than an order of magnitude depending upon ligand structure and level of deprotonation, activation energies were similar (～60 kJ/mol) for all complexes studied. This suggests a model in which dissociation of the central pyrimidine ligand precedes phenyl group rotation with an enthalpy of dissociation near zero. The rate of rotation of the phenyl ring increases with an introduction of electron-donating substituents on the phenyl ring, possibly due to an increased repulsion between π systems.     

Molecular orientational motion in discotic liquid crystals

Abstract

The spectral densities of motion for the aromatic and chain deuterons of the discotic mesogen hexahexyoxytriphenylene (THE6) have been reported in the literature for a frequency of 46 MHz. Most spectral densities J_p (pω₀, 90°) have been obtained from samples consisting of a planar distribution of domains in which the directors were perpendicular to the magnetic field Limited data J_p (pω₀) have also been available from single-domain samples with the director aligned parallel to the magnetic field. We have applied the small-step rotational diffusion model of Nordio et al. to the data from the aromatic deuterons of THE6-ard in its uniaxial columnar D_ho phase, to describe the spinning (D _∥, rotational diffusion constant about the planar normal to the disc) and the tumbling (D?, rotational diffusion constant of the planar normal) motions of the molecular core. Although this model has been successfully used for rod-like nematic liquid crystals, its use has not been attempted for discotic liquid crystals. The model seems to indicate that molecular reorientation has slowed down in the D_ho phase, giving frequency dependence to the spectral densities. This can be explained by the high orientational order of the molecules. We are able to account for the four spectral densities J ₁ (ω₀), J ₁ (ω₀, 90°), J ₂ (2ω₀) and J ₂(2ω₀, 90°) with a calculated ratio D∥/D? of about 1. This is quite different from that of rodlike liquid crystals.     

Molecular motion and spin relaxation in polydiethylsiloxane

Quantitative comparison of previously published NMR spin-relaxation data for polydiethylsiloxane with theoretical predictions for a variety of motional processes allowed both the nature and time scale of molecular motions to be identified. At the lowest temperatures, methyl reorientation produced a T₁ minimum and was found to proceed with an activation energy of 2.4 kcal/mole in both amorphous and crystalline phases. Reorientation of the ethyl groups in the amorphous phase was observed at a higher temperature with an activation energy of 9.3 kcal/mole. Relaxation in the melting region was influenced by flexing and stretching of the helical polymer chain. The maximum angular displacement of the chain was estimated to be 24°, with an activation energy for this process of 2.6 kcal/mole.     

Molecular motion in the supercooled liquid state: Small molecules in slow motion

It is shown that the motion of small molecules in the supercooled liquid state may occur at reorientation rates 10⁸ to 10¹¹ sec slower than in normal low viscosity solvents. The detailed dielectric behaviour of anthrone and nitrobenzene in supercooled ortho-terphenyl is described, and interpreted as a cooperative process. The important implications of this work in relation to the behaviour of solid polymers is briefly discussed.     

Reactivity differences of indomethacin solid forms with ammonia gas

The present study deals with the acid-base reaction of three solid-state forms of the nonsteroidal antiinflammatory drug indomethacin with ammonia gas. X-ray powder diffraction, optical microscopy, gravimetry, and spectroscopic methods were employed to establish the extent of the reaction as well as the lattice changes of the crystal forms. The glassy amorphous form readily reacts with ammonia gas to yield a corresponding amorphous ammonium salt. In addition, the metastable crystal form of indomethacin (the alpha-form) also reacts with ammonia gas, but produces the corresponding microcrystalline ammonium salt. This reaction is anisotropic and propagates along the a-axis of the crystals. The stable crystal form (the gamma-form), however, is inert to ammonia gas. Amorphous indomethacin can react with ammonia gas because it has more molecular mobility and free volume. The reactivity differences between the alpha- and gamma-forms are dictated by the arrangement of the molecules within the respective crystal lattices. The recently determined crystal structure of the metastable alpha-form of indomethacin (monoclinic P2(1) with Z = 6, V = 2501.8 A(3), D(c) = 1.42 g.cm(-3)) has three molecules of indomethacin in the asymmetric unit. Two molecules form a mutually hydrogen-bonded carboxylic acid dimer, while the carboxylic acid of the third molecule is hydrogen bonded to one of the amide carbonyls of the dimer. The carboxylic acid groups of the alpha-form are exposed on the [100] faces and are accessible to attack by ammonia gas. After one layer of molecules reacts, the reactive groups in the subsequent layer are accessible to the ammonia gas. This process proceeds along the a-axis until the ammonia gas has penetrated the entire crystal. In contrast to the alpha-form, the gamma-form has a centrosymmetric crystal structure in which the hydrogen-bonded carboxylic acid dimers are not accessible to ammonia gas because they are caged inside a hydrophobic shield comprising the remainder of the indomethacin molecule. In view of the significantly lower density of the stable gamma-form as compared to the metastable alpha-form (1.37 and 1.42 g cm(-3), respectively), it became apparent that the reactivity of the crystal forms depends exclusively on the molecular arrangement and not on the packing density of the indomethacin crystals.     

Nuclear magnetic relaxation and molecular motion in solid lysozyme

Nuclear magnetic relaxation data for both proton and carbon-13 nuclei in solid lysozyme are analysed together to obtain information on local internal motions in protein. For this analysis the “model-free” approach is used. Three types of internal motion appear to determine the observed nuclear relaxation in protein. They may be attributed to local rotations of methyl groups around symmetry axes, the motion of main and side chain atoms like in rigid lattice, and large-amplitude motions of side groups (mainly, methylene groups). Conclusions on hydrated water influence on local dynamics of protein are made.     

Rotational motion in LiBH4/LiI solid solutions

We investigated the localized rotational diffusion of the (BH(4))(-) anions in LiBH(4)/LiI solid solutions by means of quasielastic and inelastic neutron scattering. The (BH(4))(-) motions are thermally activated and characterized by activation energies in the order of 40 meV. Typical dwell times between jumps are in the picosecond range at temperatures of about 200 K. The motion is dominated by 90° reorientations around the 4-fold symmetry axis of the tetrahedraly shaped (BH(4))(-) ions. As compared to the pure system, the presence of iodide markedly reduces activation energies and increases the rotational frequencies by more than a factor of 100. The addition of iodide lowers the transition temperature, stabilizing the disordered high temperature phase well below room temperature.     

An infrared spectroscopy study of the phase transition in solid ammonia

The infrared spectra of solid ammonia at different phases and the existence of a metastable phase have been in controversy for the last fifty years. In order to address this problem, we studied the infrared spectra of solid ammonia in an ultrahigh vacuum chamber at distinct temperatures. Having prepared amorphous ammonia at 10 K, we observed a transition from the amorphous phase to the cubic crystalline phase at 57 K; successive re-cooling from 85 K back to 10 K confirms the presence of crystalline ammonia. No metastable phase has been detected.     

An FTIR study of the rotation of ammonia in solid nitrogen

The infrared spectra of NH₃, NH₂D, NHD₂ and ND₃ in solid nitrogen have been studied with 0.05 cm^?1 resolution. All isotopic species of ammonia rotate around their c axis. The matrix actively participates in this motion. The resulting potential barrier is six-fold, showing that the trapping site has high symmetry. The ammonia inversion, which is strongly perturbed by the nitrogen matrix, is completely inhibited by large concentrations of polar molecules.