A comparison of mesogenic properties for one- and two-ring dipentyl derivatives of p-carboranes, bicyclo[2.2.2]octane, and benzene

A series of nonpolar single- and two-ring dipentyl derivatives of p-carboranes, bicyclo[2.2.2]-octane and benzene was investigated in the pure state and in binary mixtures with a nematic host. The resulting virtual nematic-isotropic transition temperatures [T_NI] for single ring compounds were compared with those for two ring compounds. All [T_NI] were compared with the molecular aspect ratios X and filling fractions F obtained from MNDO calculations. The highest effectiveness in promotion of the nematic phase was found for bicyclo[2.2.2]octane and 12-vertex p-carborane and ascribed to exceptional molecular rigidity and electronic properties, respectively. Results show that a high filling fraction F and molecular stiffness are the necessary factors for a highly stable nematic phase.     

A symplectic structure for the Einstein-Maxwell field

In the space of all solutions of the coupled Einstein-Maxwell field the symplectic form Ω is defined. The construction is based on the multiphase approach to classical field theories [13], [14]. The degeneracy distribution of Ω is given and its connection with the action of the symmetry group is established.     

Polarographic behaviour of phenothiazine

Summary Phenothiazine oxidizes at the dropping mercury electrode in waterethanolic solutions, containing 7 N sulphuric acid, giving a wave corresponding to the formation of free cation radicals.The normal potential of the system phenothiazine/radical, against the reference electrode formed from mercuric sulphate in the presence of 10 N H₂SO₄ and 30% ethanol, is –240±20 mV.In alkaline solutions of pH 12.5 phenothiazine also gives a polarographic wave. This wave corresponds to the insoluble anions of compounds formed with mercury. The conditions for the quantitative polarographic determination of phenothiazine are also given.

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Zusammenfassung Es wurde bewiesen, daß in äthanolisch-wäßrigen Lösungen, die 7 n H₂SO₄ enthalten, Phenothiazin an der Quecksilber-Tropfelektrode oxydiert wird. Als Reaktionsprodukt entsteht ein freies Kation-Radikal. Das Normalpotential des Systems Phenothiazin/Radikal gegen die gesättigte Quecksilbersulfatelektrode, die 10 n H₂SO₄ und 30% C₂H₅OH enthält, beträgt –240 ± 20 mV.In alkalischen Lösungen bei pH 12,5 konnte man eine polarographische Welle registrieren, deren Eigenschaften schwerlöslichen Anionen einer Quecksilberverbindung entsprechen.Die Bedingungen für eine quantitative polarographische Phenothiazinbestimmung werden angegeben.

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Dedicated to Prof. Dr. M. von Stackelberg on his 70th birthday.     

Inhibiting effect of beta-cyclodextrin on thymine dimerization photosensitized by para-aminobenzoic acid

Beta-cyclodextrin can act as an efficient inhibitor of the photosensitized dimerization of thymine by para-aminobenzoic acid (PABA) in aqueous solution. This can be explained by considering the formation of an inclusion complex between PABA and beta-cyclodextrin.     

1H, 13C, 15N NMR and X-Ray Diffractometry in Structural Studies of Macrocyclic Lactams Containing Pyridine Moiety

Abstract

We examined complexing sites of the Pb²⁺ complex of the macrocyclic lactam 1 using ¹⁵N NMR and other spectroscopies and we have found that the amide groups undergo conformational changes to allow the complexation process to proceed via the pyridine nitrogen atom and carbonyl and ethereal oxygen atoms. X-Ray analysis of compound 1 was carried out successfully. Space group I4₁/a, a=28.332(4)Å, b=28.332(4)Å, c=10.7379(4)Å, Z=16, V=8619.3(18)ų, D_c=1.197gcm^?3, R1=0.0479 (based on 2510 reflections I>2[sgrave](I). It shows presence of intramolecular hydrogen bonds, which are broken during the complexation. Molecules form supramolecular tetragonal assemblies in the crystal, which form channels the walls of which are 7.42 Å apart.     

Anion⋅⋅⋅Anion Attraction in Complexes of MCl3− (M=Zn,Cd, Hg) with CN−

High-level ab initio calculations show that the MCl₃⁻ anions comprising Group 2B M atoms Zn, Cd, and Hg form a stable complex with the CN⁻ anion, despite the like charge of the two ions. The complexation occurs despite a negative π-hole region above the M atom of MCl₃⁻. The dimerization distorts the planar geometry of MCl₃⁻ into a pyramidal shape which reduces the negative potential above the M atom, facilitating a close approach of the two anions, with R(M⋅⋅⋅C)∼2 Å, and an overall attractive electrostatic attraction within the dimer. In the gas phase, this dimer is less stable than the pair of separated ions by some 30 kcal/mol. However, the dissociation must surmount an energy barrier of roughly 25 kcal/mol which occurs at an intermolecular distance of 4 Å. In aqueous solution, the dimerization process is exothermic and barrier-free, with a binding energy in the 11–18 kcal/mol range.