A new route to 2-styrylbenzimidazoles

An attempt was made to prepare 2-benzylquinoxalin-3-one by hydrolyzing the azlactone, 2-phenyl-4-benzylidene-5-oxazolone to β-phenylpyruvic acid and then treating this in situ with o-phenylenediamine (OPDA). The initial hydrolysis apparantly proceeded only as far as opening the azlactone ring forming 2-benzamidocinnamic acid which condensed with OPDA to form a substituted styrylbenzimidazole.     

Electron–Electron interaction energy in homonuclear diatomic molecules

The derivative of the electron–electron potential energy U_ee with respect to internuclear separation R is studied for light homonuclear diatomic molecules at equilibrium. It is readily related to nuclear–nuclear potential energy U_nn, the force constant K, and the electron–nuclear potential energy U_en. An approximate expression, based on the simplest form of density functional theory, is then used to eliminate dU_en/dR|_Re. The result thus obtained for dU_ee/dR|_Re transcends an earlier proposal of Kryachko by including a term 2/3R_eK, with K the force constant. Numerical tests at SCF–RHF level are presented for nine homonuclear diatomic molecules.     

Rearrangements of 3-heterobicyclo[3.2.0]hept-6-enes

Studies directed at a synthesis of dihydrothiepin 1b have resulted in the elucidation of several factors which effect cyclobutene ring opening in the 3-heterobicyclo[3.2.0]hept-6-ene ring system. We report the unexpected rearrangement of 4a, 4b, 13b and 13c to the synthetically useful a-vinyl-2,5-dihydrothiophenes 7a, 7b, 15a and 15b, respectively. Conversion of 4a to 6 is suggested to occur by a 1,3-rearrangement of 4a to isomeric 3-thiabicyclo[3.2.0]hept-6-ene 19 followed by cyclobutene ring opening in 19.     

Analysis of betamethasone, dexamethasone and related compounds by liquid chromatography/electrospray mass spectrometry

A reversed-phase high-performance liquid chromatography/electrospray ionisation mass spectrometry (HPLC/ESI-MS) method has been developed to conclusively differentiate the epimers betamethasone and dexamethasone and various esterification products (betamethasone and dexamethasone 21-acetate, betamethasone and dexamethasone 21-phosphate, betamethasone 17-valerate, betamethasone 21-valerate and betamethasone 17,21-dipropionate) in counterfeit drugs. Good separation with baseline resolution of all epimers or isomers was obtained on a Zorbax Eclipse XDB or Luna C8 column, using a step gradient with mobile phases of 0.05 M ammonium acetate and acetonitrile. Betamethasones can also be distinguished by the relative abundance of their m/z 279 ion in the positive electrospray tandem mass spectra. The LC/MS or LC/MS/MS method developed was successfully applied to the analysis of drug product samples, i.e. creams and tablets.     

Efficient flow injection and sequential injection methods for spectrophotometric determination of oxybenzone in sunscreens based on reaction with Ni(II)

Spectrophotometric determination of a widely used UV-filter, such as oxybenzone, is proposed. The method is based on the complexation reaction between oxybenzone and Ni(II) in ammoniacal medium. The stoichiometry of the reaction, established by the Job method, was 1:1. Reaction conditions were studied and the experimental parameters were optimized, for both flow injection (FI) and sequential injection (SI) determinations, with comparative purposes. Sunscreen formulations containing oxybenzone were analyzed by the proposed methods and results compared with those obtained by HPLC. Data show that both FI and SI procedures provide accurate and precise results. The ruggedness, sensitivity and LOD are adequate to the analysis requirements. The sample frequency obtained by FI is three-fold higher than that of SI analysis. SI is less reagent-consuming than FI.     

Relative energies of substituted benzene valence isomers

Ab initio calculations employing the STO-3G basis set are used to obtain the relative energies of the benzene valence isomers and some selected monosubstituted derivatives. We find that 3,3'-bicyclopropenyl, the least stable of the five (CH)₆ examined, is slightly more stable in the anti conformation than the gauche (Φ = 45°) conformation in agreement with experiment. Substituents are calculated to produce significant changes in the relative energies of the benzene valence isomers. The ground-state isomerization of 1-Dewar benzeneearbinyl cation to benzyl cation is more exothermic than the aromatization of Dewar benzene, but is, in contrast to the latter, symmetry-allowed.     

Coupling of a Rydberg electron capture ion source with a quadrupole mass filter

The coupling of a Rydberg electron capture ion source with a Nermag R10-10H quadrupole mass filter is described. Details are given of the addition to this instrument of a creation cell for atoms excited in Rydberg states. Within the Nermag ion source, such atoms allow attachment of electrons of well-defined thermal energy. SF(6) was used for optimization of the main experimental parameters (gas pressures and voltages applied to the electrodes). The procedure by which Rydberg electron attachment was confirmed is described. A polychlorobiphenyl compound was used to illustrate the performance of this ionization technique. Ion formation was observed in the absence of fragmentation.     

Microdetermination of phosphate with EDTA

An indirect microdetermination of phosphate via EDTA titration is described, which can be applied to minerals, soils, fertilizers, biological samples, drugs and organo-phosphorus compounds. The method is based on the precipitation of phosphate as the very insoluble silver orthophosphate (K_sp = 1.3 × 10^?20), dissolution of this precipitate in a solution of potassium cyanonickelate and titration of the nickel displaced by silver. The phosphate content is obtained indirectly by multiplying the number of ml of the titrant by a factor. The method takes about an hour after the phosphate is brought into solution. The accuracy is about 1% for samples containing 5–50 mg PO₄^3? and about 3–5% for samples with 100 to 5000 μg PO₄^3?.     

Estimating entropies from molecular dynamics simulations

While the determination of free-energy differences by MD simulation has become a standard procedure for which many techniques have been developed, total entropies and entropy differences are still hardly ever computed. An overview of techniques to determine entropy differences is given, and the accuracy and convergence behavior of five methods based on thermodynamic integration and perturbation techniques was evaluated using liquid water as a test system. Reasonably accurate entropy differences are obtained through thermodynamic integration in which many copies of a solute are desolvated. When only one solute molecule is involved, only two methods seem to yield useful results, the calculation of solute-solvent entropy through thermodynamic integration, and the calculation of solvation entropy through the temperature derivative of the corresponding free-energy difference. One-step perturbation methods seem unsuitable to obtain entropy estimates.     

A New Family of Nonstoichiometric Layered Rare-Earth Tin Antimonides, RESn(x)()Sb(2) (RE = La, Ce, Pr, Nd, Sm): Crystal Structure of LaSn(0.75)Sb(2)

A new class of nonstoichiometric layered ternary rare-earth tin antimonides, RESn(x)()Sb(2) (RE = La, Ce, Pr, Nd, Sm), has been synthesized through reaction of the elements at 950 degrees C. In the lanthanum series LaSn(x)()Sb(2), tin can be incorporated from a maximum content of x approximately 0.7 or 0.8 to as low as x approximately 0.10. The structure of lanthanum tin diantimonide with the maximum tin content, LaSn(0.75)Sb(2), has been determined by single-crystal X-ray diffraction methods. It crystallizes in the orthorhombic space group -Cmcm with a = 4.2425(5) ?, b = 23.121(2) ?, c = 4.5053(6) ?, and Z = 4. The isostructural rare-earth analogues were characterized by powder X-ray diffraction. The structure of LaSn(0.75)Sb(2) comprises layers of composition "LaSb(2)" in which La atoms are coordinated by Sb atoms in a square-antiprismatic geometry. Between these layers reside chains of Sn atoms distributed over three crystallographically independent sites, each partially occupied at about 20%. The structure of LaSn(0.75)Sb(2) can be regarded as resulting from the excision of RE-Sb and Sb-Sb bonds in the related structures of binary rare-earth diantimonides, RESb(2), and then intercalation of Sn atoms between layers.