R.A.J. Driessen

Structure of CnCn+2Cn-type (n = even) β′-triacylglycerols

The crystal structures of the β′ phase of CLC (1,3-didecanoyl-2-dodecanoylglycerol) and MPM (1,3-ditetradecanoyl-2-hexadecanoylglycerol) have been determined from single-crystal X-ray diffraction and high-resolution X-ray powder diffraction data, respectively. Both these crystals are orthorhombic with space group Iba2 and Z = 8. The unit-cell parameters of β′-CLC are a = 57.368u2005(6), b = 22.783u2005(2) and c = 5.6945u2005(6)u2005A and the final R value is 0.175. The unit-cell parameters of β′-xadMPM are a = 76.21u2005(4), b = 22.63u2005(1) and c = 5.673u2005(2)u2005A and the final Rp value is 0.057. Both the β′-CLC and β′-MPM molecules are crystallized in a chair conformation, having a bend at the glycerol moiety. The zigzag planes of the acyl chains are orthogonally packed, as is typical for a β′ phase. Furthermore, unit-cell parameters of some other members of the CnCn+2Cn-type triacylglycerol series have been refined on their high-resolution X-ray powder diffraction pattern. Finally, the crystal structures are compared with the currently known structures and models of triacylglycerols.

Molecular simulations of montmorillonite intercalated with aluminum complex cations. Part I: Intercalation with [Al13O4(OH)(24+x)(H2O)(12−x)]((7−x)+)

AbstractThe structure of montmorillonite intercalated with [Al13O4(OH)24+x(H2O)12−x](7−x)+ cations (Al13(7−n x)+ for short), where x = 0,2 an 4, has been studied using the Cerius2 modeling environment. The Crystal Packer module used in the present study takes into account only the nonbonded interactions between the silicate layer and the Keggin cations. Minimization of the total sublimation energy led to the following conclusions: the structure of the interlayer (that is, the orientation of Keggin cations and the basal spacing) depends on the charge of cations (that is, on the degree of hydrolysis, x). The values of basal spacings in the range 19.38–20.27 Å have been obtained, depending on the charge and arrangement of cations in the interlayer. The dominating contribution to the total sublimation energy comes from the electrostatic interactions. Translations of Al13(7−x)+ cations along the 2:1 layers give only small fluctuations of the total sublimation energy and basal spacings. No preference for the position of Al13(7−n x)+ cations in the interlayer of montmorillonite was found during translation along the 2:1 layers. This result confirmed the inhomogeneous distribution of cations in the interlayer and turbostratic stacking of layers.

Molecular simulations of montmorillonite intercalated with aluminum complex cations; Part II, Intercalation with Al(OH) 3 -fragment polymers

The Crystal Packer module in the Cerius2 modeling environment has been used to study the structure of montmorillonite intercalated with Al(OH)3-fragment (gibbsite-like) polymers. Basal spacings in gibbsite-like polymers arranged in 2 layers in the interlayer of montmorillonite varied in the range 19.54–20.13 Å, depending on the type and arrangement of Al(OH)3 fragments. The inhomogeneous distribution of intercalating species in the interlayer and, consequently, the turbostratic stacking of layers has been found for gibbsite-like polymers as well as in the case of Keggin cations (Čapková et al. 1998). The dominating contribution to the total sublimation energy comes from electrostatic interactions for both intercalating species, gibbsite-like polymers and Keggin cations.

9α-Fluoro-11β,17α,21-trihydroxy-1,4-pregnadiene-3,20-dione-21-acetate (9-fluoroprednisolone-21-acetate)

The synthetic steroid 9α-fluoro-11β,17α,21-trihydroxy-1,4-pregnadiene-3,20-dione-21-acetate (9-fluoroprednisolone-21-acetate), formula C23H29O6F, is related to substances which are potent inducers of hepatic microsomal enzymes and anti-inflammatory agents. The structure was determined at 294 K from counter-collected data by direct methods. Crystals are tetragonal, space groupP41212,a, b=9.214(2),c=49.452(39) Å,V=4198(4), Å,Dx=1.33μg/m3 forZ=8;R (onF) 0.063 for 915 independent intensities (I>2σ1). The structure shows H-bonding and packing of the mean molecular plane approximately perpendicular to the crystallographic 4-fold axis.

Molecular simulations of trioctahedral smectites intercalated with a aluminium complex cation

Structures of hectorites and saponites intercalated with a Keggin like cation [Al13O4(OH)24(H2O)12]7+ have been studied using molecular simulations in a Cerius2 modelling environment. Present study is focused to the effect of octahedral and tetrahedral substitutions in the silicate layers on the crystal packing and interlayer porosity of intercalated trioctahedral smectites. Translation of the Keggin cation along the silicate layers results in only small fluctuations of basal spacings and energy values. No correlations were found between the interlayer position of the shifting Keggin ion and the crystal energy values for the hectorite structure because of its even charge distribution. However the interlayer translation of the Keggin ion shows more distinct, preferential positions with lower energy values for the saponite structure due to its tetrahedral substitution. Consequently a more homogeneous distribution of the Keggin ion positions is more probable for pillared saponite. This more homogeneous distribution of the pillaring cation leads to a better porosity control for pillared saponites compared to pillared hectorites.

Structure analysis of intercalated smectites using molecular simulations

Abstract Structures of di- and trioctahedral smectites intercalated with a Keggin like cation [Al 13 O 4 (OH) 24 (H 2 O) 12 ] 7+ have been studied using molecular mechanics simulations in the Cerius 2 modelling environment. The present study is focused on the effect of substitutions and distortions in the silicate layers on the crystal energy in these two types of intercalated layer structures. Detailed analysis of charge distribution has been carried out to explain the differences in behaviour of these two intercalated smectites. Tetrahedral substitution in the smectite layer reduces locally the negative charge of the smectite outside oxygen layer. This charge reduction favours the creation of preferential interlayer positions of the Keggin ion. The consequence of these preferential interlayer positions is a more homogeneous distribution of the intercalating ion.

Structure determination of two metal-organic complexes from high-resolution synchrotron powder diffraction data.

The crystal structures of [1,2-bis(2,6-diisopropylphenylimino)acenaphthene-N,N]carbonylchlororhodium(I) (1) and [N,N-ethylene-bis(3-methylsalicylideneiminato)-O,N,N,O](tetrahydrofurfuryl)-cobalt(II) (2) have been determined from high-resolution synchrotron X-ray powder diffraction data. Compound 1 is the first neutral Rh complex, in contrast with findings in the literature, containing a bidentate nitrogen ligand, and compound 2 is the first three-dimensional structure of a (five-coordinated) tetrahydrofurfurylcobalt(III) complex. Grid-search and Rietveld refinement have been used to determine and refine the structures, respectively. Crystals of 1 are orthorhombic, space group Pbca, Z = 8, with cell parameters a = 21.729 (2), b = 27.376 (3), c = 11.580 (1) A. Crystals of 2 are monoclinic, space group P2(1)/n, Z = 4, a = 16.6701 (6), b = 9.4170 (4), c = 13.7088 (7) A and beta = 96.520 (3) degrees. Chemical diagrams for the two compounds are given. Soft restraints were applied during Rietveld refinement; for 1 converging to R(p) = 8.4%, R(w) = 11.0%, GoF = 2.3, and for 2 converging to R(p) = 8.5%, R(w) = 11.4%, GoF = 7.6.

Molecular simulations of smectites intercalated with a zirconium complex ion.

Molecular mechanics simulations using the Cerius2 modeling environment have been applied to study the structures of dioctahedral smectites intercalated with the zirconium cation [Zr4(OH)12(H2O)12]4+. The substitutions in the silicate layers influence the sublimation energy in these types of intercalated layer structures. Charge distribution in the smectite layer was analyzed in relation to the interlayer structure. Tetrahedral substitutions in the smectite layer create preferential interlayer positions of the [Zr4(OH)12(H2O)12]4+ ion. A regular distribution of the tetrahedral substitutions in the smectite layer results in a better intercalant ordering in the interlayer space. Such a more homogeneous intercalant distribution results in a better interlayer pore size control.

Progress in structure determination of larger organic and organo-metallic compounds from powder diffraction data

The method of the joint probability distribution function has been used to derive accurate phase estimates for proteins via: a) the method of the isomorphous derivatives (SIR, MIR); b) the anomalous dispersion techniques (SAD, MAD); c) the method of the isomorphous derivatives when anomalous dispersion effects are present (SIRAS, MIRAS). For SIR, MIR, SIRAS and MIRAS cases the program automatically provides phase values (and a structural model) starting from the diffraction data. In the SAD and MAD cases the program first locates the anomalous scatterers, automatically assigned structure factor phases and refines them, and lastly provides a structural model. Some experimental results will be described.

Structure Analysis of Intercalated Clays Using Combination of Molecular Simulations, Powder Diffraction and IR Spectroscopy

Combination of molecular simulations with x-ray powder diffraction and TR spectroscopy has been used to study the structure of montmorillonites, intercalated with aluminium complex cations. Two different intercalating species have been investigated: (1) Keggin cation - ideal and hydrolysed and (2) gibbsite-like polymers, arranged in two layers in the interlayer of montmorillonites. The results of molecular simulations showed, that for Keggin cations, the crystal packing depends on the degree of hydrolysis, exhibiting the basal spacings within the range 19.38 - 20.27 (10(-10)m). I, case of gibbite-like polymers, arranged in two layers in the interlayer, basal spacings within the range 19.58 - 20.06 (10(-10)m) have been found, in dependence on the mutual position of Al-OH polymers. Results of molecular simulations showed, that no two-dimensional ordering of complex cations and no reggular stacking of layers can occure in the interlayer of montmorillonites. All the conclusions of modelling were in agreement with the results of XRD analysis.