Biomedical subjects

Krzysztof Ejsmont

Publications and source records attributed to Krzysztof Ejsmont.

5 recordsLinked to original sources

PubMed2004

Relation between the substituent effect and aromaticity.

Molecular geometries of benzene and its 18 monosubstituted derivatives were optimized at B3LYP/6-311+G** level of theory. The changes of pi-electron delocalization of the benzene fragment were estimated by use of aromatic stabilization energies (ASE) based on different homodesmotic reaction schemes, geometry-based HOMA model, magnetism-based NICS, NICS(1), NICS(1)zz, and an electronic delocalization index, PDI, derived from the AIM theory. Apart from aromatic stabilization energies the other descriptors of aromaticity vary to a very small extent, indicating high resistance of the pi-electron structure to the substituent effect. This is somewhat analogous to a tendency of benzene systems to retain their initial pi-electron structure during the reaction course that leads to aromatic substitution.

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PubMed2004

trans-4-Bromo-ONN-azoxybenzene at 100 K.

The crystal structure of the alpha isomer of trans-4-bromoazoxybenzene [systematic name: trans-1-(bromophenyl)-2-phenyldiazene 2-oxide], C(12)H(9)BrN(2)O, has been determined by X-ray diffraction. The geometries of the two molecules in the asymmetric unit are slightly different and are within approximately 0.02 A for bond lengths, approximately 2 degrees for angles and approximately 3 degrees for torsion angles. The azoxy bridges in both molecules have the typical geometry observed for trans-azoxybenzenes. The crystal network contains two types of planar molecules arranged in columns. The torsion angles along the Ar-N bonds are only 7 (2) degrees, on either side of the azoxy group.

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PubMed2003

Bis(glycylglycinium) oxalate at 100 K.

The structure of the title compound, 2C(4)H(9)N(2)O(3)(+).C(2)O(4)(2-), which has been determined by X-ray diffraction, contains discrete glycylglycine (HGly-Gly)(+) cations in general positions and oxalate anions which lie across centres of inversion. Although the geometry of the (HGly-Gly)(+) cation is not significantly different compared with other structures containing this residue, a few changes in conformation are observed which indicate the presence of molecular interactions. The molecular network in the crystal consists of one nearly linear O-H...O, five N-H...O and two weak C-H.O hydrogen bonds.

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PubMed2003

Synthesis, structures, and conformational characteristics of calixarene monoanions and dianions.

The synthesis, complete characterization, and solid state structural and solution conformation determination of calix[n]arenes (n = 4, 6, 8) is reported. A complete series of X-ray structures of the alkali metal salts of calix[4]arene (HC4) illustrate the great influence of the alkali metal ion on the solid state structure of calixanions (e.g., the Li salt of monoanionic HC4 is a monomer; the Na salt of monoanionic HC4 forms a dimer; and the K, Rb, and Cs salts exist in polymeric forms). Solution NMR spectra of alkali metal salts of monoanionic calix[4]arenes indicate that they have the cone conformation in solution. Variable-temperature NMR spectra of salts HC4.M (M = Li, Na, K, Rb, Cs) show that they possess similar coalescence temperatures, all higher than that of HC4. Due to steric hindrance from tert-butyl groups in the para position of p-tert-butylcalix[4]arene (Bu(t)C4), the alkali metal salts of monoanionic Bu(t)C4 exist in monomeric or dimeric form in the solid state. Calix[6]arene (HC6) and p-tert-butylcalix[6]arene (Bu(t)C6) were treated with a 2:1 molar ratio of M(2)CO(3) (M = K, Rb, Cs) or a 1:1 molar ratio of MOC(CH(3))(3) (M = Li, Na) to give calix[6]arene monoanions, but calix[6]arenes react in a 1:1 molar ratio with M(2)CO(3) (M = K, Rb, Cs) to afford calix[6]arene dianions. Calix[8]arene (HC8) and p-tert-butylcalix[8]arene (Bu(t)()C8) have similar reactivity. The alkali metal salts of monoanionic calix[6]arenes are more conformationally flexible than the alkali metal salts of dianionic calix[6]arenes, which has been shown by their solution NMR spectra. X-ray crystal structures of HC6.Li and HC6.Cs indicate that the size of the alkali metal has some influence on the conformation of calixanions; for example, HC6.Li has a cone-like conformation, and HC6.Cs has a 1,2,3-alternate conformation. The calix[6]arene dianions show roughly the same structural architecture, and the salts tend to form polymeric chains. For most calixarene salts cation-pi arene interactions were observed.

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PubMed2002

Orthorhombic polymorphs of two trans-4-aminoazoxybenzenes.

The two isomeric compounds 4-amino-ONN-azoxybenzene [or 1-(4-aminophenyl)-2-phenyldiazene 2-oxide], i.e. the alpha isomer, and 4-amino-NNO-azoxybenzene [or 2-(4-aminophenyl)-1-phenyldiazene 2-oxide], i.e. the beta isomer, both C(12)H(11)N(3)O, crystallized from a polar solvent in orthorhombic space groups, and their crystal and molecular structures have been determined using X-ray diffraction. There are no significant differences in the bond lengths and valence angles in the two isomers, in comparison with their monoclinic polymorphs. However, the conformations of the molecules are different due to rotation along the Ar-N bonds. In the alpha isomer, the benzene rings are twisted by 31.5 (2) and 14.4 (2) degrees towards the plane of the azoxy group; the torsion angles along the Ar-N bond in the beta isomer are 24.3 (3) and 23.5 (3) degrees. Quantum-mechanical calculations indicate that planar conformations are energetically favourable for both isomers. The N-H.O hydrogen bonds observed in both networks may be responsible for the deformation of these flexible molecules.

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