Structural and dynamical aspects of the phase transition in the new thiourea thiazolium bromide inclusion compound.

A new thiourea thiazolium bromide inclusion compound is presented here. Detailed investigations of its phase transition were performed by differential scanning calorimetry, x-ray diffraction, and dielectric and nuclear magnetic resonance spectroscopy methods, completed by calculation of the steric hindrances for molecular reorientations and simulations of the second moment of the nuclear magnetic resonance line by the Monte Carlo method. A second order ferrielectric structural phase transition has been detected at 190.5 K as thiazolium cations collectively reorient inside channels. The dynamics is discussed in terms of inequivalent energy barriers associated with cation rotation as the symmetry breaking occurs. Oscillations of thiourea molecules and NH(2) groups have been also observed.

Redetermination of the structure and dielectric properties of bis(thiourea) pyridinium iodide--a new ferroelectric inclusion compound.

The crystal structure of bis(thiourea) pyridinium iodide (T(2)PyI) was previously determined at 295 and 110 K [Prout, Heyes, Dobson, McDaid, Maris, Mueller & Seaman (2000). Chem. Mater. 12, 3561-3569] and the two phases were described in the space groups Cmcm and P2(1)cn, respectively. Because differential scanning calorimetry revealed two phase transitions, at 161 and 141 K, a redetermination of the structure of T(2)pyI at 295, 155 and 110 K has been undertaken, and the following sequence of space groups obtained: Cmcm (I) --> C2cm (II) --> P2(1)cn (III). The high- (I) and low-temperature (III) phases confirmed the results reported in the previous study. In the new intermediate phase II, the mirror plane perpendicular to the x axis vanishes and the crystal structure loses the centre of symmetry. In phases I and II the pyridinium cations are strongly dynamically disordered, while in the low-temperature phase III the cations are well ordered. In all three phases the thiourea-iodine hydrogen-bonded sublattice is very well ordered. Dielectric measurements show that the intermediate and low-temperature phases are ferroelectric and that 161 K is the Curie point of a new ferroelectric crystal.

Ferroelectric ordering in imidazolium perchlorate.

Imidazolium perchlorate has been synthesized and studied over a wide range of temperatures by differential scanning calorimetry, x-ray diffraction, proton magnetic resonance, optical observation, and dielectric spectroscopy. Polymorphic solid-solid phase transitions have been disclosed at 487, 373, and 247 K. The crystal structure at 298 K has been determined as trigonal, space group R3m, Z=1 with a=5.484(1) A and alpha=95.18(2) degrees. The imidazolium cations are strongly disordered, while the perchlorate ions are well ordered. At 385 K the crystal structure remains trigonal, space group R3m, a=5.554(1) A and alpha=95.30(2) degrees . Both ionic sublattices are orientationally disordered. Temperature evolution of the molecular dynamics of the imidazolium cation has been characterized. In spite of a high cationic disorder, dielectric measurements have revealed the polar properties of the crystal. It appears to be a new ferroelectric compound with the Curie point at 373 K. The spontaneous polarization originates predominantly from the behavior of slightly distorted perchlorate anion.

Asymmetric transformation of N-nitrosamines by inclusion crystallization with optically active hosts.

Several N-nitrosopiperidines with chirality solely due to a hindered rotation about the N-N bond were resolved to enantiomers by inclusion crystallization with optically active diols (TADDOLs). The absolute configuration of the guest nitrosamines was deduced from the X-ray crystal structures of the inclusion complexes. The enclathrated nitrosamines were liberated by a competitive complexation of the host diols with piperazine. The optical activity of the resolved nitrosamines is manifested by their CD spectra. A simple chirality rule was proposed for a rationalization of the observed Cotton effect sign corresponding to the n-pi* electronic transition. The optically active nitrosamines are configurationally labile compounds and gradually racemize in solution but they are indefinitely stable in the solid state. The first-order kinetics of the racemization in solution allowed us to assign the N-N rotation barriers by simple polarimetric measurements.

The crystal structure and thermotropic liquid-crystal properties of N-n-undecyl-D-gluconamide.

N-n-Undecyl-D-gluconamide, C17H35O6, crystallizes in space group P1, with one molecule in a unit cell a = 5.2267(6), b = 19.628(9), c = 4.7810(4) A, alpha = 93.23(2), beta = 95.60(1), gamma = 89.58(2) degrees, V = 487.35 A3, Dx = 1.19 g.cm-3. The crystal lattice is isostructural with N-n-heptyl-D-gluconamide having monolayer head-to-tail molecular packing. The molecules have a V-shaped conformation. The hydrogen bonding of the gluconamide moieties includes a four-link homodromic cycle. The transition to a smectic A liquid-crystal phase at 156.7 degrees is preceded by two crystal-to-crystal phase transitions at 77.2 degrees and 99.4 degrees. The long d-spacing of the intermediate crystal phase of 39 A, and the d-spacing of the liquid-crystal phase of 32 A, are consistent with a transition to a bilayer head-to-head molecular packing.

The stereochemistry of the water molecules in the hydrates of small biological molecules.

An examination of the stereochemistry of the water molecules in the hydrates of amino acids and peptides, carbohydrates, purines and pyrimidines, and nucleosides and nucleotides, reveals a variety of hydrogen-bonded configurations within a radius of 3.0 A from the water oxygen atom. Water molecules which accept one hydrogen bond are more common than those that accept two, by a factor of 1.4. There are nine examples where the water is not a hydrogen-bond acceptor, but only one where it does not donate two hydrogen bonds. Of the 621 OWH...A bonds examined, 15% were three centered and 2% were four centered or three-center bifurcated. The amino-acid and peptide hydrates displayed the greatest variety with 15 different hydrogen-bond configurations. The coordination of the donor and acceptor atoms within 3.0 A of the water oxygen atom ranged from two to seven.

The structures of 1-deoxy-(N-methyloctanamido)-D-glucitol (MEGA-8) and 1-deoxy-(N-methylundecanamido)-D-glucitol (MEGA-11).

1-Deoxy-(N-methyloctanamido)-D-glucitol, C15H31NO6 (MEGA-8), crystallizes in space group P2(1)2(1)2(1), Mr = 321.4, a = 4.865 (1), b = 9.186 (3), c = 39.097 (9) A, V = 1747.24 A3, Z = 4, Dx = 1.22 Mg m-3, lambda (Cu K alpha) = 1.5418 A, mu = 0.78 mm-1, F(000) = 704, R = 0.035 for 1318 reflections. 1-Deoxy-(N-methylundecanamido)-D-glucitol, C18H37-NO6 (MEGA-11), crystallizes in space group P1, Mr = 363.5, a = 4.950 (1), b = 5.6027 (8), c = 19.162 (4) A, alpha = 83.19 (2), beta = 89.76 (2), gamma = 76.28 (2) degrees, V = 512.64 A3, Z = 1, Dx = 1.18 Mg m-3, lambda (Mo K alpha) = 0.7107 A, mu = 0.093 mm-1, F(000) = 200, R = 0.061 for 1898 reflections. The glucitol C-atom-chain conformation is different in the two structures. In MEGA-8 it is fully extended, whereas in MEGA-11 it is bent. The alkyl C-atom chains are fully extended in both structures. The molecular packing is different. In MEGA-8 it is head-to-head bilayer with intercalating alkyl chains, whereas in MEGA-11 it is monolayer head-to-tail with non-intercalating alkyl chains. The hydrogen bonding in MEGA-8 is a finite chain; in MEGA-11 it includes a homodromic four-bond cycle.

A model for the hydrogen-bond-length probability distributions in the crystal structures of small-molecule components of the nucleic acids.

The probability distributions of the N-H...O = C and O-H...O = C hydrogen-bond lengths observed in the crystal structures of the purines, pyrimidines, nucleosides and nucleotides have been fitted to a one-dimensional hydrogen-bond potential-energy function. In order to obtain a quantitative correspondence between the experimental and theoretical distributions, it is necessary to include with the usual hydrogen-bond-type potential-energy function, an effective crystal-packing force and two thermodynamical parameters of the crystal lattice, the Debye temperature and the Gruneisen constant.

The crystal structure of the C-nucleoside, 1,3-dimethyl-8-beta-D-ribofuranosylxanthine monohydrate.

1,3-Dimethyl-8-beta-D-ribofuranosylxanthine monohydrate, C12H16N4O6.H2O, is monoclinic, P21, with two molecules in a unit cell having a = 8.186(1), b = 19.222(3), c = 4.7655(8) A, beta = 103.79(1) degrees, V = 728.2 A, and Dx = 1.506 g.cm-3. The structure was solved by the direct methods, using MoK alpha radiation and refined with anisotropic temperature-factors. The final agreement factors were R = 0.061, Rw = 0.042 for 2254 structure amplitudes having [Fo[ greater than 2 sigma (Fo). The ribose ring has the C-2'-exo-C-3'-endo (2T3) conformation with pseudorotation parameters P = -3.2, tau m = 39.9 degrees, and a gauche-gauche conformation around the ribose C-4'-C-5' bond. The beta-link conformation is anti, with O-4'-C-1'-C-8-N-9 = -162.6 degrees. The pyrimidine and imidazole rings are planar, making an interplanar angle of 1.0(1) degree. The molecular conformation is stabilized by an unsymmetrical, three-center hydrogen bond from N-7-H to O-5'-H as the major component and to O-4'-H as the minor component. The hydrogen bonding includes a cooperative cycle involving the HN-C-C=O moiety, two hydroxyl groups, and a water molecule.