Mannam Krishnamurthy

Electronic structure of quinoxaline-2,3(1H,4H)dione and its prototropic species in the ground and excited singlet states

Abstract The solvent-dependent electronic absorption and fluorescence spectra indicate that quinoxaline-2,3(1H,4H)dione exists in its keto form, even in non-polar solvents, but the deprotonation reaction is observed from its enol form. The protonation reaction in comparison with those of the parent quinoxaline and 1,4-dimethylquinoxaline-2,3-dione suggests that the cation is formed by the gain of a proton on one of the carbonyl groups of the molecule.

Absorptiometric and fluorimetric study of solvent dependence and prototropism of benzimidazole homologues

Electronic absorption and fluorescence spectra of benzimidazole and six of its derivatives in different solvents and at various pH values have been studied. The dual fluorescence observed in dilute acid solutions and the similarity of the fluorescence excitation spectra to the absorption spectra indicate the existence of several excited states of benzimidazoles. The emitting transition of neutral benzimidazoles is πâ†�π*; in the case of the monocations the short-wavelength fluorescence band is from the πâ†�π* transition and the long-wavelength band arises from charge transfer. The pKa values for various prototropic reactions in aqueous solutions in the S0 and S1 states have been calculated, and the proton-induced fluorescence quenching of the monocations is discussed.

A simple spectrophotometric determination of some chlorobenzoquinones with morpholine, thiomorpholine and piperazine.

A spectrophotometric method for the determination of 2,5-dichloro-, 2,6-dichloro- and 2,3,5,6-tetrachloro-1,4-benzoquinones is based on the yellow products of their reaction with morpholine, thiomorpholine and piperazine in chloroform.

Prototropic equilibria of some Harmala alkaloids in acid solutions: proton-induced fluorescence quenching of the monocations of harmaline and harmalol

Absorption and fluorescence spectra of harman homologues in acid solutions have shown the presence of mono-, di-, and tri-cations. Fluorescence spectra for the various cations at 77 K and their quantum yields at 298 K are reported. The acidity constants indicate that these alkaloids are more basic in the excited singlet state than in the ground state. The proton-induced fluorescence quenching rate constant for the monovalent cations is observed to be of the order of 109 dm3 mol–1 s–1 for harmaline and harmalol and considerably less for harman, harmine, and harmol.

Titrimetric determination of uric acid with manganese(IV) sulfate

Abstract A simple indirect titrimetric determination of uric acid with manganese(IV) sulfate has been proposed. The method is stoichiometric and accurate and can be conveniently adapted for the assay of urinary uric acid.

Some observations concerning the direct titration of nitrite with cerium(IV)

Direct titration of nitrite with cerium (IV), with ferroin as indicator, is shown to give satisfactory results if the acidity is kept between 0.033 and 0.055M at the end-point. Loss of nitrous acid owing to volatilization and decomposition is discussed. From 10 to 60 mg of sodium nitrite can be estimated with a standard deviation of 5 mug and an average error of 0.2%.

Use of tetravalent manganese in solutions of sulfuric acid as an oxidant for the direct titration of nitrite

Abstract Conditions have been worked out for the direct titration of nitrite with manganese(IV) sulfate using ferroin as indicator. The method is suitable for the estimation of 8–60 mg of sodium nitrite with a standard deviation of 0.04, in an acidity range of 0.3 to 0.6 M. The special feature of this method lies in the reagents association with acid and its relatively higher redox potential.

Analytical uses of charge transfer complexes—Determination of piperidines

Abstract A simple spectrophotometric determination of piperidine and some methyl-substituted piperidines has been proposed. The method is based on the blue-colored charge transfer complex formation of the nitrogen bases with tetrachloro-1,2-benzoquinone in chloroform. The method is rapid and sensitive to 5.5 μg ml−1 piperidine and has the applicability for the assay of piperidine hydrochloride.

PMR Spectra of Piperidine Involved in the Formation of Charge Transfer Complexes with 1,4-Benzoquinones

Abstract The proton magnetic resonance spectra of piperidine involved in the formation of charge transfer complexes with 1,4-benzoquinones, in chloroform, have been presented and discussed.

Solvent Dependent Electronic Absorption Band Shifts of Charge-Transfer Complex Formed between Piperidine and Tetrachloro-1,4-Benzoquinone

Abstract The electronic absorption spectrum of intermoiecular charge-transfer complex formed between piperidine as electron donor and tetrachloro-l,4-benzoquinone as electron acceptor was studied in 30 different solvents. The band shifts are discussed on the basis of dielectric, refractive index and polarity functions.