Lalji Dixit

Quantitative Analysis by Derivative Electronic Spectroscopy

Abstract Absorption and emission spectroscopy in the ultraviolet and visible regions, apart from being the earliest physical techniques of analysis, have great utility in solving a variety of structural and analytical problems [l-41. However, in many cases the quantitative determination of individual components in a mixture by UV-VIS spectroscopy becomes very difficult owing to the spectral similarities and overlapping of weak absorption bands of one component by strong absorption bands of other components. It was recently recognized that derivative spectrophotometry, introduced about three decades ago [5-81 and originally proposed by Rutherford [9], can be a very useful analytical tool for characterizing an analyte band that is overlapped by other absorption bands with different halfwidths. On differentiation, the intensity of the absorption band with a small halfwidth increases more than that of a band with a large halfwidth [10]. The central idea behind the development of the science of derivatized spe...

Vibrational Band Intensities of Hydrocarbons

Abstract The experimental observables of a vibrational spectrum are depicted either in the form of their positions, i.e., frequency relating the energy required in a given quantum transition, or as the intensities of absorption and scattering related to their transition probabilities. Expressed in terms of molecular parameters, the frequencies depend on the geometry, atomic masses, and intramolecular forces [11 while the band intensities [2] reflect changes in dipole moment (infrared) or polarizabilities (Raman) which are caused during a vibrational displacement and are related to movement of electronic charges within the individual bonds. The mathematical basis for determining vibrational frequencies was well established as early as 1940 by Wilson [3] and others [4, 51. Applying the interpretation of vibrational spectra has become routine in the multitudinous disciplines of chemical literature [6–26]. Accounts of infrared and Raman spectra [27–29], collection of literature [30], and reasonable sets of in...

Infrared intensities and molecular structures of inorganic and coordination compounds

Abstract The observed infrared absorption band of a vibrating molecule may be characterized by several parameters, including the position, i.e., frequency, the intensity of the absorption areas, and the shapes o r band half-width. Whereas the frequency of a band depends upon the geometrical distribution of masses undergoing oscillation and the forces holding the nuclei together, the intensity is governed mainly by the charge redistribution within the bond under specific vibrational transition. During recent years the intensities of infrared bands have attracted considerable interest in the field of vibrational spectroscopy of polyatomic molecules. In earlier articles [1,2] we discussed the vibrational band intensities pertaining to some typical classes of hydrocarbons, and the present article is devoted to the infrared intensities. of inorganic and coordination compounds. A careful literature survey reveals that progress in measuring the absolute infrared intensities of inorganic and coordination compound...

On the molecular polarizabiltties and intermolecular dispersion energies of deuterated hydrocarbons and related compounds

An effort has been made to discuss the effect of replacing hydrogen by deuterium on electronic polarizabilities and intermolecular dispersion energies of sixteen organic compounds. Standard one-dimensional delta-potential function model of chemical binding has been used. It has been found that the model is capable enough to account for major electronic changes of (C−H) bond region in the form of probability amplitudes of electrons so as to explain H/D isotope effect on the polarizabilities (αM) and intermolecular dispersion energies (Ed).

Determination of alkylnaphthalenes in petroleum fractions by second-derivative ultraviolet spectrophotometry

The determination of naphthalene and its alkyl derivatives in aviation turbine fuels (ATF) and other petroleum products is important not only with respect to their performance but also for establishing heart cuts for producing intermediate chemicals for the manufacture of dyes, pesticides and agrochemicals. Normal and second-derivative UV spectra of naphthalenes, biphenyls and other interfering aromatics have been studied with a view to developing a method for determining naphthalene and α- and β-methyl-, dimethyl- and trimethylnaphthalenes. The characteristic signals due to the substituted naphthalenes were identified in terms of second-derivative signals in a number of straight-run and cracked products of varying nature. Working relationships were derived for each of the naphthalenes in terms of their signal amplitudes. The method was tested on a number of straight-run and cracked petroleum products and it was found that the results calculated by the proposed method are in close agreement with those given by the zero-order UV spectrophotometric method.

Polarizability model of acidity of zeolites

A new model has been developed for the estimation of structural acidity parameters and the acid amount of zeolites using valence electron density, atomic polarizability, and chemical composition. Based on this model, the computed zeolite acidity has been compared with the data obtainable from Sandersons electronegativity model and also with available experimental data. The general agreement between our calculated values and those reported in the literature has been found to be very good in the case of mordenites. It has been established that electronegativity equalization (EEq) in zeolites could be equally explained via the electronic polarization process governed by the proposed equation: EEq = 1.67 [(NP/αP)NQ/αQ)NR/αR)]1/(p+q+r), where N is the valence electron density and a the atomic polarizabilities of the constituting atoms expressed in the unit of volume, and p, q, r are the number of atoms representing a molecule of general formula PinpQqRr The relative polarizing power of structures has also been reported as a function of Al content. This means that increased bond polarization will result in enhanced acidic behavior of the protons upon interaction with basic substrate molecule. This is why relative polarizing powers, α′p′, are directly related to the Al content, the acidity [H+], and shrinkage effect of bond lengths (Dt−0).

Characterization and compositional study of lighter fractions from coal-derived liquids

Abstract Synfuel fractions boiling in the range IBP-150 °C and 150–250 °C have been characterized. Individual component and hydrocarbon type distributions have been carried out quantitatively employing high resolution capillary gas chromatography and mass spectrometric techniques independently. More than 180 compounds including 60 olefins have been identified and quantified in the IBP-150 °C fraction only. A comparative study of hydrocarbon structures present in synfuel and crude oil fractions has been made and revealed that the cyclic character of coal-derived oils is due to predominance of hydroaromatics and cycloolefins rather than naphthenes alone. Moreover, for most of the homologous series of cyclic and aromatic structures, parent compounds are relatively more abundant in coal-derived oil than in natural crude petroleum and cracked petroleum fractions. The fractions have been evaluated for their suitability as gasoline/kerosene blending components in view of their estimated octane number and smoke point respectively.

New approach to acid catalysis and hydrocarbon - zeolite interactions

Abstract Adsorption of hydrocarbons on zeolites has been the subject of serious concern owing to its important catalytic consequences in petroleum refining, cracking, fuel synthesis and processing. Besides thermodynamic links between zeolites and hydrocarbons during adsorption, diffusion, and cracking, the inclusion of framework and surface parameters, e.g., genus of periodic minimal surface and atomic connectivity, have been postulated in establishing nonspecific type interactions. Keeping this in view, the earlier proposed model that establishes the concept of atomic polarizabilities in zeolite science and acid catalysis (Dixit and Rao, Zeolites, 16, 4, 287–293, 1996) has been extended to examine heats of adsorption of paraffins, naphthenes, olefins and aromatics on the zeolite surfaces. The formalism thus developed has been used to evaluate heats of adsorption and activation energies of hydrocarbon species on the surfaces of faujasites, ZSM-5, mordenite and silicalite by proposing a new relation that reads: Δh=αM β.γ+Θf where αM is the average polarizability, β and Θf are the constants, and y the average curvature of the zeolite surface. The agreement between theoretical and experimental values of heats of adsorptions of C1– C20 paraffins have been found to be very good in the case of ZSM-5 and relates well with their activation energies for cracking.

Spectroscopy in the Measurements of Acido - Basic Properties of Solids

INTRODUCTION In this article. applications of spectroscopic techniques shall be discussed in relation to the measurements of acido-basic properties of solid materials Solids, such as alumina [1], magnesia [2], silica alumina [3],aluminosilicates [4], and zeolitic materials with T-atoms as Ga, Ge, Si, Al, P [5], mixed oxides [6], clays and pillared clays [7], hetero-polyatomic systems having Kaggine structures [8], crystals of spinel type [9], and other super acid forming systems [10], exhibit either acidic or basic (or both) features dependent upon their constitution and nature. These solids, used as chemical accelerants (catalysts), are characterized by several of their intrinsic properties, viz., acidity, basicity, electrostatic vacancies, and surface genus [11-15]. Such properties form a very important theme in acid-base catalysis and play a very crucial role in deciding the catalytic nature of solids [16-20]. Hence, the diagnostics of such hidden domains of solids has been a subject of serious concern...

Development of a derivative u.v. spectroscopic method for the analysis of principal aromatic components in fuels

Abstract Normal and second-derivative ultraviolet absorption spectra of several narrow cuts prepared from a kerosene feedstock boiling 135–240°C were studied. The nature of the absorption profiles of normal and second-derivative spectra is explained in terms of occurrence and existence of principal aromatic components, namely alkylbenzenes, indans, tetralins and alkylnaphthalenes. The derivative signals were further utilized for the quantitative estimation of these constituents. The results are compared with the data obtained by conventional u.v. spectroscopic measurements and possible new relations in terms of second-derivative signal amplitudes are proposed. The role of derivative spectroscopy in the analysis of aromatics present in straight-run kerosenes/ATF or reformed kerosenes is assessed.