Crystal structure and thermal expansion of N,N,N′,N′-tetrakis-[(1H,2,4-triazol-1-yl)methyl]-ethane-1,2-diamine

Structural Chemistry - The crystal structure of N,N,N′,N′-tetrakis-[(1H,2,4-triazol-1-yl)methyl]-ethane-1,2-diamine has been fully determined at six different temperatures by X-ray...     

Deregulation and welfare in airline markets: An analysis of frequency equilibria

This paper models airline competition as a two-stage game in frequency and prices, allowing for asymmetric frequency equilibria. The approach follows the spatial multiproduct oligopoly literature. The dynamic structure gives airlines an incentive to choose asymmetric frequency equilibria such that price competition is reduced. This feature is most pronounced in the case of inelastic demand, for which a maximum differentiation result is derived. We apply the model in a simulation study of airline deregulation of the Amsterdam—Maastricht market in The Netherlands, calculating welfare effects for various types of post-deregulation entry.     

Tyrosine alkyl esters as prodrug: the structure and intermolecular interactions of <Emphasis Type="SmallCaps">l</Emphasis>-tyrosine methyl ester compared to <Emphasis Type="SmallCaps">l</Emphasis>-tyrosine and its ethyl and <Emphasis Type="Italic">n</Emphasis>-butyl esters

l-Tyrosine alkyl esters are used as prodrugs for l-tyrosine. Although prodrugs are often designed for their behavior in solution, understanding their solid-state properties is the first step in mastering drug delivery. The crystal structure of l-tyrosine methyl ester has been determined and compared to published structures of l-tyrosine and its ethyl and n-butyl esters. It is almost isostructural with the other esters: it crystallizes in the orthorhombic chiral space group P2₁2₁2₁, a = 5.7634(15) Å, b = 12.111(2) Å, c = 14.3713(19) Å, V = 1003.1(4) Å³ with Z′ = 1. Their main packing motif is a C(9) infinite hydrogen-bond chain, but the conformation of l-tyrosine methyl ester is different from the other two: eclipsed versus U-shaped, respectively. The published structure of the ethyl ester, which was incomplete, has been confirmed by X-ray powder diffraction data. Because l-tyrosine methyl ester is very stable (28 years stored at room temperature), and its hydrolysis rate is relatively low, it should be one of the better prodrugs among the alkyl esters of tyrosine.     

Polyethylene fiber formation in tubular flow. II. Discussion of precursor formation and nucleation time analysis

This is a continuation of the preceeding paper, Part I, and presents a discussion of the nature of the precursor structure formation process observed in the flow-induced crystallization experiments described in I. A discussion of stress-induced crystallization theory as applied to these experiments is also given and a first-order analysis of crystal nucleation rates is presented. Conclusions regarding the nature of flow-induced crystallization and our current ability to quantitatively model the overall process are also presented.     

Synthesis of methyltrioxorhenium(VII) arylamine complexes and mono- and bis(ortho)-chelated arylaminorhenium(VII) trioxides

From the reaction of MeReO₃ with the neutral arylamine C₆H₅CH₂NMe₂ and the aryldiamine C₆H₄(CH₂NMe₂)₂−1,3, have been isolated in good yields the 1/1 adduct complex [MeReO₃ · C₆H₅CH₂NMe₂], 1, and the 2/1 adduct complex [(MeReO₃)₂ · C₆H₄(CH₂NMe₂)₂− 1,3], 2, respectively. The X-ray molecular structure of 2 shows that both rhenium centres have a trigonal bipyramidal geometry and in the axial positions of each rhenium centre are one of the NMe₂ units of the aryldiamine ligand and a methyl group. The mono(ortho)-chelated arylaminorhenium trioxide complex [ReO₃(C₆H₄CH₂NMe₂−2], 3, can be synthesized by a transmetallation reaction of ClReO₃ with [ZnC₆H₄CH₂NMe₂−2₂] in a 2:1 molar ratio. In a similar way the bis(ortho)-chelated arylaminorhenium trioxide complex [ReO₃C₆H₃(CH₂NMe₂)₂−2,6], 4, can be synthesized by addition of a mixture of [Li₂C₆H₃(CH₂NMe₂)₂−2,6₂] and ZnCl₂ to ClReO₃. Complexes 3 and 4 have been isolated as white solids in 66% and 81% yields respectively. The rhenium centre in complex 4 has a bicapped tetrahedral geometry in which the monoanionic C₆H₃(CH₂NMe₂)₂−2,6⁻ ligand is pseudo-facially bonded with a characteristic N1-Re-N2 angle of 107.7(3)°, a Re-C_ipso bond length of 2.112(11) Å and Re-N1 and Re-N2 bond lengths of 2.518(9) Å and 2.480(8) Å respectively.     

Uniaxial negative thermal expansion in crystals of tienoxolol

The thermal expansion of tienoxolol has been investigated by X-ray powder diffraction up to its melting temperature. The data indicate that the expansion is anisotropic and even negative in one direction of the unit cell. The supramolecular structure formed by hydrogen-bonds reflects that of a trellis, which explains the observed behavior of tienoxolol crystals.     

Phenomenology of polymorphism and topological pressure–temperature diagrams

Although polymorphism of drug molecules is often studied with extensive and excellent experimental data, pressure appears to be a forgotten variable. In this article, an analysis is provided of the phase behavior of Atovaquone using available literature data. A pressure–temperature diagram is constructed topologically by way of the Clapeyron equation. The method leads to the conclusion that Atovaquone phase I and III behave enantiotropically, like α- and β-sulfur do in their paradigmatic P–T diagram, and that phase I is stable at room temperature and under “ordinary” pressure.     

Polymorphism and solvation of indomethacin

Indomethacin crystallizes from solutions in tetrahydrofuran as a solvate exhibiting the mole ratio 1 indomethacin:2 tetrahydrofuran. Upon heating, desolvation into indomethacin phase I occurs through partial amorphization and transitory formation of a phase, which is different from the crystallographically known polymorphs. The X-ray powder diffraction pattern of the solvate was tentatively indexed on a triclinic lattice (a = 31.454(5) Å, b = 17.883(3) Å, c = 10.551(2) Å, α = 70.55(2)°, β = 105.31(2)°, γ = 136.70(1)°). Assuming Z = 6 (1 indomethacin + 2 tetrahydrofuran) formula units per unit cell, the solvate’s specific volume is similar to the value calculated using additivity.     

Morphology control of poly(vinylidene fluoride) thin film made with electrospray

Thin polymer films of poly(vinylidene fluoride) were prepared with electrospray. Effects of solvent, initial spray concentration, temperature, solution conductivity, and polymer size on the film morphology were studied with AFM. The two main factors controlling polymer film morphology are the droplet size of the spray and the viscosity of the solution at deposition. These factors determine the flow of the polymer-solvent mixture over the substrate, the density of the film, and its smoothness. The solvent is a key parameter of the entire process. It affects spray stability, polymer solubility, droplet size of the spray, and viscosity of the solution at deposition. Solvents with a low vapor pressure provide a wider window for optimization of other parameters and are therefore preferred over solvents with high vapor pressure. The viscosity at deposition is mainly controlled with the initial spray concentration, polymer size, temperature, and droplet size. The droplet size is best controlled by the conductivity of the solution and the flow rate of the spray.