Spectrophotometric determination of ascorbic acid in pharmaceutical preparations and fruit juices

Conditions were established for the determination of ascorbic acid using phsophovanadotungstic acid as reagent. The method was applied to the determination of ascorbic acid in pure form, pharmaceutical preparations and fruit juices. The method is sensitive (2-24 micrograms ml-1 of ascorbic acid) and rapid and tolerates the presence of common ingredients usually found in fruit juices. The results obtained with the proposed method showed good agreement with those given by the standard method.     

Synthesis of 6-vinyl and 5-vinylproline analogues of ascomycin

6-Vinyl (12) and (5R)- and (5S)-vinylproline (18, 19) analogues of ascomycin are synthesised starting from the known suitably protected (6S)-methoxy-9-hydroxy derivative (4) of ascomycin. The strategy involves hydrolytic cleavage of the C_ε-N bond of the pipecolic acid moiety, extension of the amino acid side chain by two or one carbon units, functional group manipulations, Pd-catalysed reinstallation of the C_ε-N or C_δ-N bonds, followed by deprotection and oxidation.     

Extraction of thorium(IV) as perchlorate and chloroacetate complexes with 1-phenyl-2,3-dimethyl-5-pyrazolone (antipyrine)

Summary Thorium was extracted quantitatively with the neutral ligand 1-phenyl-2,3-dimethyl-5-pyrazolone (Apy) in the presence of perchlorate (ClO₄^-) and di/trichloroacetates (DCA/TCA) at pH 2.5 into an organic solvent. Optimization of experimental parameters like pH, equilibration time, solvents, reactant concentrations, best suited conditions for back extraction, interference due to the presence of different anions and cations and effect of metal ion concentration were studied. The composition of the complexes has been established using log-log plots and are represented as [Th(Apy)₂(ClO₄)₄], [Th(Apy)₂(TCA)₄], [Th(Apy)(H₂O)(DCA)₄]. The respective conditional stability constants of the effectively extracted complexes have also been calculated as 9.4±0.03 and 10.4±0.04, respectively . The method has been extended for the separation of thorium from a number of cations in binary and multicomponent mixtures. The method is simple, rapid, selective and has a good reproducibility (±0.5%).     

Field and Frequency Effects in a Nematic Mixture of Negative and Positive Dielectric Anisotropy

The molecular alignment in the nematic liquid crystal mixture of p-ethoxy benzylidene p′-n-butyl aniline (EBBA) and p-ethoxy benzylidene p′-cyano aniline (EBCA) has been investigated by measuring the dielectric constant at 1 MHz. Detailed investigation has been carried out for a composition of 2.74%, of EBCA in EBBA for different electric and magnetic field strengths and at different frequencies of the applied electric field. Both parallel and perpendicular electric and magnetic field configurations have been employed. Temperature and frequency dependence of the dielectric anisotropy have been studied.

Two interesting features have been observed. I) In the crossed field configuration complete molecular alignment could be observed by suitably changing the electric field strength. The range of electric field strength required to change the molecular ordering, is frequency dependent. This range shows a minimum in the neighbourhood of 2 kHz. 2) It has also been observed that in the parallel field configuration minimum dielectric constant change is observed apparently independent of frequency and electric field strength. Detailed results of the field and frequency effects and of optical studies in this mixture are reported.     

Alignment studies on p-ethoxy phenyl azophenyl butyrate in the presence of electric and magnetic fields

Dielectric permittivity studies on the nematic liquid crystal p-(ethoxy phenyl azophenyl) butyrate were carried out at 1 MHz. The effect of parallel and perpendicular magnetic fields on the alignment of the molecules was studied. The threshold voltage for domain formation in the presence of a magnetic field is estimated.     

Review of electronic transport models for thermoelectric materials

Thermoelectric devices have gained importance in recent years as viable solutions for applications such as spot cooling of electronic components, remote power generation in space stations and satellites etc. These solid-state devices have long been known for their reliability rather than their efficiency; they contain no moving parts, and their performance relies primarily on material selection, which has not generated many excellent candidates. Research in recent years has been focused on developing both thermoelectric structures and materials that have high efficiency. In general, thermoelectric research is two-pronged with (1) experiments focused on finding new materials and structures with enhanced thermoelectric performance and (2) analytical models that predict thermoelectric behavior to enable better design and optimization of materials and structures. While numerous reviews have discussed the importance of and dependence on materials for thermoelectric performance, an overview of how to predict the performance of various materials and structures based on fundamental quantities is lacking. In this paper we present a review of the theoretical models that were developed since thermoelectricity was first observed in 1821 by Seebeck and how these models have guided experimental material search for improved thermoelectric devices. A new quantum model is also presented, which provides opportunities for the optimization of nanoscale materials to enhance thermoelectric performance.     

Effective equidistribution of eigenvalues of Hecke operators

In 1997, Serre proved an equidistribution theorem for eigenvalues of Hecke operators on the space S(N,k) of cusp forms of weight k and level N. In this paper, we derive an effective version of Serre's theorem. As a consequence, we estimate, for a given d and prime p coprime to N, the number of eigenvalues of the pth Hecke operator Tp acting on S(N,k) of degree less than or equal to d. This allows us to determine an effectively computable constant Bd such that if J₀(N) is isogenous to a product of Q-simple abelian varieties of dimensions less than or equal to d, then N?Bd. We also study the effective equidistribution of eigenvalues of Frobenius acting on a family of curves over a fixed finite field as well as the eigenvalue distribution of adjacency matrices of families of regular graphs. These results are derived from a general “all-purpose” equidistribution theorem.     

Linear independence of digamma function and a variant of a conjecture of Rohrlich

Let ψ(x) denote the digamma function. We study the linear independence of ψ(x) at rational arguments over algebraic number fields. We also formulate a variant of a conjecture of Rohrlich concerning linear independence of the log gamma function at rational arguments and report on some progress. We relate these conjectures to non-vanishing of certain L-series.     

In situ nucleophilic substitution reaction of N,N-dialkylaminoethyl-2-chlorides monitored by gas chromatography/mass spectrometry

The detection and identification of degradation products of scheduled chemicals, which are characteristic markers of Chemical Warfare agents (CWAs), plays a key role in verification analysis. Identification of such non-scheduled but specific markers of CWAs helps in deciphering the kind of agent that was present in the sample submitted for off-site analysis. This paper describes the stability of N,N-dialkylaminoethyl-2-chlorides, which are precursors for highly toxic chemicals like VX, in different solvents. These compounds are stable in chloroform, acetonitrile, hexane and dichloromethane but tend to undergo in situ nucleophilic substitution reaction in the presence of alcohols giving the corresponding alkyl ether. The study shows that N,N-dialkylaminoethyl alkyl ethers can be used as markers of N,N-dialkylaminoethyl-2-chlorides. A detailed degradation study of these compounds in the presence of alcohols was carried out and it was found that the reaction follows pseudo-first order kinetics. Electron ionization mass spectral data for the methyl ethers of all the compounds are briefly discussed.