Stefan Stoyanov

Analysis of the Overlapping Bands in UV-Vis Absorption Spectroscopy

The analysis of the overlapped bands in UV-Vis absorption spectroscopy provides valuable information about the structure of the molecule and its environment. The resolution of individual bands in the UV-Vis region is a complicated task since the absorption bands are generally strongly overlapped, they have different half-band widths, and their number is difficult to estimate. In the present study the possibilities of using Levenbergs method for analysis of overlapping bands in UV-Vis spectroscopy are discussed, and a logical algorithm for their resolution is developed. This approach is applied to the analysis of the vibrational structured long-wavelength bands of trans-stilbene and its derivatives.

Tautomeric equilibrium in 1-phenylazo-2-naphthol—A quantitative study

Abstract A new approach for quantitative analysis of the tautomeric azoquinonehydrazone equilibria was applied to 1-phenylazo-2-naphthols. The spectral characteristics of both tautomeric forms were calculated from the absorption spectra, taking into account the solvent polarity. Using the PPP-MO method, the calculated electronic transitions for both tautomeric structures were compared with the corresponding experimental values.

Azo-quinonehydrazone tautomerism in 2-phenylazo-1-naphthol

Abstract The tautomeric equilibrium of 2-phenylazo-1-naphthol in 13 different solvents is analysed using a previously developed quantitative approach. The spectral characteristics of both tautomeric forms and the tautomeric constants KT are determined and compared with those of the isomers 1-phenylazo-4-naphthol and 1-phenylazo-2-naphthol. The long wavelength band at about 500 nm, determining the colour, corresponds to the hydrazone form and consists of three vibrational sub-bands. The low sensitivity towards changes in solvent polarity and the highest content of hydrazone form within the three isomeric phenylazonaphthols explains the wide use of dyes containing a 2-phenylazo-1-naphthol moiety. PPP-MO calculations were in good agreement with the experimental spectral data for both tautomeric forms. The redistribution of the π-electronic density during the S0 → S1 transition is calculated for both tautomeric forms, and its influence on substituent effects, dye-solvent interaction and colour strength is discussed.

Resolution of overlapping UV-visible absorption bands : quantitative analysis of tautomeric equilibria

Abstract A quantitative approach for the analysis of two-component mixtures with unknown spectra of the pure components, based on resolution of the UV-visible absorption spectra into individual bands is proposed and its reliability is confirmed. The proposed method is applied for the analysis of the tautomeric equilibrium of 1-phenylazo-4-naphthol. The molar parts and individual spectra of both tautomeric forms and tautomeric constant K t are determined. The proposed general approach could be applied for quantitative analysis of any other two-component equilibria including aggregation, acid-base processes, complex formation, where the absorption spectra of the individual forms cannot be obtained experimentally.

Quantitative analysis of tautomeric equilibrium in 1-phenylazo-4-naphthols—a new approach

Abstract A new quantitative approach, based on the analysis of overlapping bands, in determining the tautomeric equilibrium constant and the individual spectral curves of the pure tautomeric forms of 1-phenylazo-4-naphthol and its derivatives is applied. The influence of some important factors affecting the tautomeric equilibrium, such as solvent polarity, substituents in the phenyl ring, etc., is evaluated.

Approach for Increased Information from the Second-Derivative Spectra in UV-Vis Absorption Spectroscopy

The resolution of overlapping bands in the UV-Vis absorption spectra leading to determination of their basic parameters () provides important information about the energies and probabilities of the electronic transitions. In the analysis of UV-Vis absorption spectra recorded linearly in wavelength by means of modern spectrophotometers with holographic gratings, the analytical shape describing individual bands is asymmetric. This factor leads to certain limitations in determining their number with the use of the second-derivative spectra and decreased effectiveness of the computing procedure. It was found that the loss of information in d2A/dλ2 is due to its long-wavelength attenuation in comparison with d2A/dν˜2. An analytical equation connecting d2A/dλ2 with d2A/dν˜2 is proposed, which restores the information from the second derivative in the long-wavelength region.

Ammonium-azonium tautomerism in some N,N-dialkylaminoazo dyes. Part 1: General considerations

Abstract The UV-Vis properties in neutral and acid media of some N,N-dialkylaminoazobenzenes containing electron donor substituents in the unsubstituted benzene ring are discussed. A redistribution of the individual vibronic band intensities within the long wavelength absorption π-π ∗ band due to specific solvation of the molecules in neutral media is observed and the corresponding vibrational sub-bands are assigned. In acid media, the increased positive halochromism is explained within the charge transfer (CT) models of the corresponding chromogenic systems. The tautomeric equilibrium constants, K t and pKa values of the ammonium and azonium tautomeric forms are evaluated.

Thione-Disulfide Interchange of some Heterocyclic Tautomeric Thiones and their Symmetrical Disulfides

SummaryThe UV/Vis spectroscopic properties of some symmetrical disulfides derived from potentially tautomeric thiones are investigated. Reversed thione-disulfide transformation is observed, and the influence of several factors including the nature of solvent, concentration, and UV irradiation, is studied. Possible implication of the tautomeric thiol form and the importance of this thione-disulfide redox system in biological aspects is suggested. A general scheme including monomer-dimer equilibrium, thione-thiol tautomeric equilibrium, and reversible thiol-disulfide redox behaviour is proposed in order to explain the factors affecting the overall thione-disulfide transformation.ZusammenfassungDie UV/Vis-spektroskopischen Eigenschaften einiger symmetrischer Disulfide, die als Derivate von potentiell tautomeren Thionen auftreten, wurden untersucht. Es wurde eine reversible Thion-Disulfid-Transformation beobachtet. Der Einfluß einiger Faktoren wie Natur des Lösungsmittels, Konzentration und UV-Bestrahlung wurden erforscht. Die Bedeutung des untersuchten Redoxsystems in biologischer Hinsicht wird diskutiert. Um die Auswirkung der einzelnen Faktoren auf die Umsetzung zu erklären, wird ein allgemeines Schema vorgeschlagen (Monomer-Dimer-Gleichgewicht, Thion-Thiol-Gleichgewicht, reversibles Thiol-Disulfid-Redoxsystem).

Step by step filter — an approach for noise reduction in the derivative UV-visible spectra

Abstract Methods for obtaining the more informative second derivative d 2 A(λ) dv 2 from absorption spectra linear in λ are discussed. It is found that the noise level in the derivatives obtained is high, especially in the long-wavelength region. The efforts to reduce the noise level by use of classical smoothing procedures lead to irregular deformation of the derivative curves, which is stronger in the short-wavelength and weaker in the long-wavelength region. An alternative method for obtaining the more informative second derivative d 2 A(λ) dv 2 , as well as higher derivatives, called “general step by step” filter (GSBSF), is proposed. Its use considerably decreases the noise level in the derivative spectra without loss of spectral information in the short-wavelength region. This method could be used in UV-visible-near-infrared (UV-Vis-NIR) absorption spectroscopy, circular dichroism spectroscopy, UV-Vis densitometry, etc.

Ammonium-azonium tautomerism in some N,N-dialkylaminoazodyes—II. Compounds containing more than two protonation sites

Abstract The UV-vis properties in neutral and acidic media of some N , N -dialkylaminoazo dyes containing strong electron donor substituents or an extended conjugated system, leading to more than two protonation sites, are investigated in order to clarify the influence of these structural changes on the position of the ammonium-azonium tautomeric equilibrium. The observed absorption maxima associated with the colour changes of neutral and protonated molecules are assigned within the charge-transfer (CT) model. The tautomeric equilibrium constants K T and p K a values of the ammonium and azonium tautomeric forms of N , N -dialkylaminoazobenzenes containing strong electron donor substituents are evaluated.