Sybolt Harkema

Syn-1,2-dialkylated calix[4]arenes: general intermediates in the NaH/DMF tetraalkylation of calix[4]arenes

In DMF or acetonitrile with NaH as a base at room temperature the tetraalkylation of calix[4]arenes 1a and 1b proceeds via the syn-1,2-disubstituted products to the tetraalkylated calix[4]arenes in the cone conformation. With KH as a base the tetraalkylated calix[4]arenes are predominantly formed in the partial cone conformation, and the reaction proceeds via both the syn-1,3-di and the syn-1,2-disubstituted products. Also the solvent influences the pathway via which tetraalkylation takes place. The syn-1,2-disubstituted calix[4]arenes 4-6 can be isolated in 15-55 % from the NaH/DMF or MeCN reactions when only 2.2 equiv of the electrophile are used.

Molecular-dynamics simulation of crystalline 18-crown-6: thermal shortening of covalent bonds

Molecular-dynamics simulations of crystalline 18-crown-6 have been performed in a study of the apparent thermal shortening of covalent bonds observed in crystal structures. At 100 K, a shortening of 0.006 _+ 0.001 A for C----C and C----O bonds was obtained. This result was found to be independent of details of the force field and the simulation. There was agreement between computational and experimental values for the thermal parameters, as well as for the molecular geometry (bond and dihedral angles) of 18-crown-6. Some differences are attributed to the inability of the force field to reproduce hydrogen-bonding geometries. Simulation at 295 K resulted in an estimated shortening of 0.019_+ 0.005 A. Thus at room temperature for C--C bonds (apparent) thermal shortening and (real) chemical shortening, resulting from the electronegative oxygen substituents, are of the same order of magnitude. In the simulation at 295 K occasional dihedral transitions were observed, which may reflect the proximity of the melting point (312 K).

Selective functionalization of calix[4]arenes at the upper rim

Methods are described for the selective diametrical functionalization of calix[4]arenes at the upper rim by transfer of functionality and selective substitutions at the para positions of the phenol rings. The crystal structure of 26,28-dimethoxy-11,23-dinitrocalix[4]arene is described.

Superconducting thin films of MgB2 on Si by pulsed laser deposition

Superconducting thin MgB2 films have been prepared using pulsed-laser deposition. We have studied the influences of deposition conditions such as pressure and temperature, the substrate-material, and annealing-procedures. Various approaches have been pursued to obtain the right Mg content in the film during ablation and annealing. Special care has been taken to avoid oxidation of Mg in the laser plasma and deposited film, by optimizing the background pressure of Ar gas in the deposition chamber. The annealing procedure was found to be the most critical to obtain superconducting films.

Structural changes of segmented copolyetheresteramides with uniform aramid units induced by melting and deformation

The structural changes induced by melting and deformation of segmented copolymers with uniform bisesteraramid units and poly(tetramethyleneoxide) (PTMO) based soft segments are investigated. With DSC, WAXS and IR it was shown that the uniform bisesteraramid units self assemble in the melt through the formation of hydrogen bonds. This leads to fast and nearly complete crystallization into lamellae with a thickness of only one bisesteraramid repeat unit (1.8 nm). IR-dichroism measurements showed that during the first 300% strain the bisesteraramid units preferentially orient transverse to the polymer chain direction, after which they become oriented in the polymer chain direction. This is attributed to the high aspect ratio of the thin aramid crystalline lamellae. The following model for the structural changes during deformation was proposed: first the bisesteraramid crystalline network is disrupted and the lamellae are, due to their high aspect ratio, preferentially oriented perpendicular to the polymer chain direction. At the same time, the bisesteraramid lamellae are broken up in their lateral dimension into eventually ‘square’ crystallites. These small crystallites are oriented parallel to the polymer chain direction. The orientation of the polymers was also studied with synchrotron WAXS measurements. After approximately 300% strain, the polyether phase strain crystallizes. The bisesteraramid orientation is irreversible while the PTMO orientation disappears upon strain release. PTMO1000 based soft segments and PTMOm soft segments strain crystallize reversibly and PTMO segments longer than 1000 g/mol strain crystallize irreversibly.

Crystal structure and charge distribution of pyrazine: effects of extinction, thermal diffuse scattering and series termination

The crystal structure and electronic charge distribution of pyrazine (1,4-diazabenzene) has been determined at 184 K by X-ray methods. The structural results of Wheatley [Acta Cryst. (1957), 10, 182-187] have been confirmed. A clear indication of bonding effects is obtained. Neither positional and thermal parameters nor difference-Fourier maps are affected by extinction. The effect of thermal diffuse scattering (TDS) on positional parameters is also negligible. However, after correction for TDS, thermal parameters increase significantly. The difference-Fourier map is influenced by TDS as well as the inclusion of high-order Fourier terms.

A novel approach to inherently chiral calix[4]arenes by direct introduction of a substituent at the meta position

A novel method for the preparation of inherently chiral calix[4]arenes is described by direct introduction of a substituent in para-acetamido substituted calix[4]arenes. Bromination and nitration of mono(acetamido)calix[4]arenes 5, 6 afforded calix[4]arenes 7?10, in which the substituent was selectively introduced adjacent to the acetamido moiety, in 58?98% yield. Bromination of bis(acetamido)calix[4]arene 13 gave a mixture from which dibromo- (14) and tribromocalix[4]arene 15 were isolated in 10% and 22% yield, respectively. The structure of 14 was confirmed by single-crystal X-ray structure determination. Nitration of 13 gave 4,16-dinitro- (16) and 4,18-dinitrocalix[4]arene 17 in 53% and 18% yield, respectively.

Potential of mean force by thermodynamic integration: molecular-dynamics simulation of decomplexation

“Umbrella sampling” has been incorporated in the thermodynamic integration method to obtain a potential of mean force by slow growth molecular-dynamics simulations. The method was tested for liquid argon, for which good agreement was obtained with a standard potential of mean force, as derived from the radial pair-correlation function. For a sodium chloride ion-pair in aqueous solution the calculations showed resonable agreement with a literature result. The method was also applied to the decomplexation of 18-crown-6 and a potassium cation in aqueous solution.

The crystal and molecular structure of ammonium titanyl oxalate

Ammonium titanyl oxalate monohydrate, (NH4)2 TiO(C2O4)2·H2O, is monoclinic with cell parameters A = 13.473(2), B = 11.329(1), C = 17.646(2) A, β = 126.66(1)°. The space group is P21/c with Z = 8, dc = 1.808 g cm−3 and dm = 1.80 g cm−3. The crystal structure was determined from single-crystal diffractometer data and refined by least-squares methods using isotropic thermal parameters. The conventional R factor was 7.0% for 2466 used reflections. The anions consist of cyclic tetranuclear complexes [TiO O (C2O4)2]48− with symmetry. The titanium atoms are six-coordinated with two bridging oxygen atoms cis to one another and four oxygen atoms of bidentate oxalate groups, together forming a distorted octahedron. The titanium atoms of the tetramer are linked through oxygen atoms at two neighbouring apices of each octahedron. The actual name of the compound should therefore be: ammonium cyclotetra-di-μ-oxo-cis-dioxalatotitanate(IV)tetrahydrate. The di-μ-oxo-tetratitanium unit is an eight-membered ---O---Ti---O--- ring with Ti-O distances of successively 1.840(7), 1.785(7) and 1.855(6), 1.788(8) A, repeated by the centre of symmetry, and the orientation of the atoms suggests the presence of dπ---pπ 3-center 2-electron bonds in each Ti---O---Ti set. The Ti---O distances trans to the bridging oxygen atoms are much longer: 2.060(7), 2.101(7), 2.081(7) and 2.116(7) A respectively, as is to be expected from the charge displacement due to the dπ---pπ bonding. Most O---Ti---O angles in the distorted octahedra differ considerably from 90°. As was found by difference Fourier synthesis and thermal analysis, half of the crystal water is held very loosely compared to the other half.

Project on comparison of structural parameters and electron density maps of oxalic acid dihydrate

Results obtained from four X-ray and five neutron data sets collected under a project sponsored by the Commission on Charge, Spin and Momentum Densities are analyzed by comparison of thermal parameters, positional parameters and X - N electron density maps. Three sets of theoretical calculations are also included in the comparison. Though several chemically significant features are reproduced in all the experimental density maps, differences in detail occur which caution against overinterpretation of the maps. Large differences between vibrational tensor elements Uij are observed which can often not be corrected by the scaling of all temperature parameters in a set. Positional parameters are reproducible to precisions of 0.001 A or better. The biggest discrepancies between theoretical and experimental deformation density maps occurs in the lone-pair regions where peaks are higher in the theoretical maps. However, this comparison may be affected by inadequacies in the thermal-motion formalism which must be invoked before experimental and theoretical maps can be compared in a quantitative way.