Studies on organotin compounds using the Del Re method : VIII. Mössbauer spectra or organotin compounds

A method for calculating the electron density at the nucleus of the tin atom in a tetravalent compound has been developed using the quantities available from the Del Re calculations. The calculated electron densities show a fair correlation with the experimental Mössbauer isomer shifts, and a value of +3.2 × 10^?4 is obtained for (ΔR/R), in excellent agreement with the value +3.30 × 10^?4 obtained from an independent method using internal electron conversion measurements. By defining an assymmetry parameter, A, which is a measure of the deviation of the calculated p-electron distribution from the hypothetical spherically-symmetric distribution, in which the p-electron would be distributed equally between the three p-orbitals, the experimental quadrupole splittings have been correlated and interpreted. Factors governing the magnitude of quadrupole splitting are also discussed.     

Studies on organotin compounds using the Del Re method : VII. Bond polarisability index and reactivity of organotin compounds

Using the Del Re method a simple reactivity index, defined as the bond polarisability index, has been derived. The observed reactivity sequence for the cleavage of the alkyl groups from the metalalkyls both in the polar and in the non-polar solvents have been interpreted, and the relative rates of these reactions correlated with this index. The relative rates of the exchange reactions between trialkylaluminium and trialkyltin hydrides have also been satisfactorily interpreted.     

A modified PCILO method

The perturbative configuration interaction using strictly localized molecular orbitals, called the modified PCILO method, for which the use of the Rayleigh-Schrödinger many-body perturbation theory with the Moller-Plesset Hamiltonian partitioning is characteristic, has been proposed in this communication. On the CNDO/2 and INDO levels of Hamiltonian approximations strictly localized molecular orbitals have been constructed by solving modified Roothaan equations. From the zero and second order energy interatomic distances and harmonic force constants for some diatomic molecules have been calculated. The linear dependence of the correlation energy on the number of valence electrons in the series of the molecules CH₄, CH₃F, CH₂F₂, CHF₃ and CF₄ is perfect.     

Production method of no-carrier-added186Re

No-Carrier-Added¹⁸⁶Re was produced using the¹⁸⁶W(p,n)¹⁸⁶Re nuclear reaction with 13.6 MeV protons on thick targets of 99.79% isotopically enriched¹⁸⁶WO₃. The theoretical excitation functions for producing¹⁸⁶Re, and possible radionuclidic impurities of¹⁸²Re,¹⁸³Re, and¹⁸⁴Re were calculated using the ALICE code. Cross-sections of the¹⁸⁶W(p,n)¹⁸⁶Re reaction were measured up to 20 MeV using the stacked target method with thin foils of natural composition tungsten metal. The experimental and theoretical excitation functions were in good agreement. Targetry used at the TIARA cyclotron, and a radiochemical separation scheme for¹⁸⁶Re are described.     

Studies on organotin compounds using the Del Re method III. Variations in tin-chlorine,tin-carbon and tin-hydrogen stretching frequencies and tin-chlorine bond distance in organotin compounds

The concept of bond order has been extended to Del Re calculations, and the bond orders of tin-chlorine and tin-carbon bonds in Me_4-nSnCl_n (n = 1 to 4) type compounds have been calculated. The tin-chlorine bond order increases progressively from 0.922 in Me₃SnCl to 0.977 in SnCl₄, and correlates satisfactorily with the experimental tin-chlorine bond distances. The tin-carbon bond order, on the other hand, remains almost constant, in agreement with the constancy of tin-carbon bond distance in the series. The average tin-chlorine, tin-carbon and tin-hydrogen stretching frequencies in similar compounds vary linearly with the calculated bond polarities indicating variation in bond polarity to be the dominating factor. The unusually low values of the tin-carbon stretching frequency for the tin-vinyl bond compared to the tin-methyl bond in Me₃ViSn and Et₂Vi₂Sn can also be explained in terms of larger polarity of the tin-vinyl bond in these compounds.     

A convenient method for synthesizing modified 4-nitrophenols

[reaction: see text] Beta-nitroenamine having a formyl group behaves as the synthetic equivalent of unstable nitromalonaldehyde upon treatment with ketones under basic conditions and leads to 2,6-disubstituted 4-nitrophenols. The present method is safer than the conventional one using sodium nitromalonaldehyde and enables the preparation of hitherto unknown nitrophenols.     

A new method for the preparation of modified oligonucleotides

[reaction-see text] N-Nitrothymidine can be transformed into a phosphoramidite building block suitable for oligonucleotide synthesis using the standard phosphite triester solid-phase approach. The N-nitrothymidine residues remain stable during the elongation cycles and react smoothly with primary amines, furnishing oligonucleotides containing N3-modified thymidines. A number of N3-substituted oligonucleotides have been prepared using this methodology, some of them incorporating aminoalkyl or hydroxyalkyl groups.     

A modified micro-dumas method for rapid determination of nitrogen

Summary The Pregl-Dumas method for the estimation of nitrogen has been modified to make it suitable for the rapid performance of a number of analyses in series. The usual combustion tube for carbon and hydrogen determination is used, and the tube filling and procedure are modified.

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Zusammenfassung Die Stickstoffbestimmung nach Pregl-Dumas wurde abgeändert, um die schnelle Durchführung von Serienanalysen zu ermöglichen. Das für Kohlenstoffbestimmungen übliche Verbrennungsrohr wird benutzt. Die Substanz wird in einem Schiffchen eingeführt, auf das eine Rolle aus oxydiertem Kupferdrahtnetz folgt.

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Résumé Modification de la méthode dePregl-Dumas pour permettre le dosage rapide de l'azote en série. Emploi du tube habituellement utilisé pour le microdosage du carbone et de l'hydrogène avec modification du remplissage et de la technique.

     

A modified standard addition method in X-ray fluorescence spectrometry

A modified standard addition method for single element determination by X-ray fluorescence spectrometry has been studied. The attenuation properties of the standard added samples are kept constant by adding decreasing amounts of an attenuation modifier along with increasing amounts of a standard. In this way the standard addition curve will be a straight line in cases where the ordinary standard addition curve is non-linear, and linear regression can be used to evaluate the concentration of the analyte. Standard additions of oxides of a number of elements, with and without modifier, have been made to cellulose powder or a mixture of aluminium oxide and polyethylene as matrices in order to test the method. The method has been applied to the determination of zinc in fly-ash from a steel work and of iron in cement. The fly-ash contained about 5% of zinc and the cement samples between 2 and 5% of Fe(2)O(3). The results were compared with those obtained by ICP-AES after decomposition of samples in lithium tetraborate or lithium metaborate and dissolution of the melt in 10%(v/v) nitric acid. The results agreed within 2%, relative, for fly-ash and within 3-6%, relative, for cement samples.     

A modified spectrophotometric method for determining sulfate with 2-aminoperimidine

A simplified procedure is described in which the 2-perimidinylammonium sulfate is collected on a teflon filter and then dissolved with concentrated nitric acid. The yellow solution obtained is made alkaline and the absorbance of the resulting violet solution is measured at 550 nm. The calibration graph is linear over the range 10–50 μg of sulfate for 5-ml samples (r.s.d. ? 2.9%). Interference levels from phosphate and carbonate are reported and the method is applied to well water and fresh snow.     

A modified Ehrenfest method that achieves Boltzmann quantum state populations

A modification of the hybrid quantum/classical mean-field or Ehrenfest method is proposed which allows us to reproduce the Boltzmann quantum state populations when simulating the vibrational energy transfer between quantum systems and classical thermal baths. The modified Ehrenfest results are shown to be able to simulate the energy fluxes between the solute vibration and the classical solvent in the proper directions, depending on their relative temperatures.     

A modified method for calculating quantum yields of photochemical reactions

The previously proposed approach to calculation of quantum yields of photochemical reactions has been improved. The new version uses a different procedure of accounting for the asymmetry (anharmonicity) of potential wells of the molecules involved in the photochemical conversion. The form of a doubleminimum potential energy surface has been considered in a more correct manner. It has been shown that the number of model parameters is reduced by two thirds compared with the model used previously, adequately reflecting the characteristics of structural transformations of polyatomic molecules in which a relatively small part of the total number of coordinates are transformed. The quantum yields of photochemical transformations of six dienes into their cyclic isomers have been calculated. Quantitative agreement of the calculated values with the experimental data has been achieved for all the reactions. It has been shown that the model parameters have good transferability in a series of related molecules, thereby rendering the simulation of photochemical processes by the method in question predictive.     

A modified radiometric method for plutonium determination in irradiated fuel

A modified radiometric method for the determination of plutonium in irradiated nuclear fuel is described. The analysis consists of total plutonium alpha-activity, activity ratio²³⁸Pu:(²³⁹Pu+²⁴⁰Pu) and burnup determination. The error of plutonium determination is practically identical with that of a classical radiometric method. The method suggested is suitable for large sample series of the same type of irradiated nuclear fuel.     

A modified sequential extraction method for arsenic fractionation in sediments

A modified sequential extraction method was developed to characterize arsenic (As) associated with different solid constituents in surficial deposits (sediments), which are unconsolidated glacial deposits overlying bedrock. Current sequential extraction methods produce a significant amount of unresolved As in the residual fraction, but our proposed scheme can fractionate >90% of the As present in sediments. Sediment samples containing different As concentrations (3–35 μg g⁻¹) were used to assess the developed method. The pooled amount of As recovered from all the fractions using the developed method was similar (83–122%) to the total As extracted by acid digestion. The concentrations of As in different fractions using the developed scheme were comparable (89–106%) to the As fractions obtained by other existing methods. The developed method was also evaluated for the sequential extraction of other metals such as copper (Cu), cobalt (Co), chromium (Cr) and strontium (Sr) in the sediment samples. The pooled concentrations of these four individual metals from all the fractions were similar (96–104%) to their total concentrations extracted by acid digestion. During method development, we used extractants that did not contain chloride to eliminate formation of polyatomic ions of argon chloride (⁴⁰Ar³⁵Cl) that interfered with ⁷⁵As when analyzed using inductively coupled plasma mass spectrometer (ICP-MS). The results suggest that the developed method can reliably be employed for complete As and other metals’ fractionation in sediments using ICP-MS.     

A modified HPLC method for the determination of ochratoxin A by fluorescence detection

A high-performance liquid chromatographic method (HPLC) with fluorescent detector is described for the determination of ochratoxin A (OTA). A mobile phase consisting of acetonitrile:water:acetic acid (99:99:2, v/v/v) was used for the resolution of the compound on a C(18) Hypersil column. The retention time for OTA and diflunisal which was used as an internal standard (IS) were 11.7 and 12.8 min, respectively. The method is selective, reliable, reproducable with relative standard deviation (RSD) of 1.70 and linear in the range of 2.5 x 10(-9)-1.5 x 10(-8) M OTA. The limit of detection (LOD) and limit of quantification (LOQ) were 2.5 x 10(-10) M corresponding to 0.1 ng mL(-1) and 8.2 x 10(-10) corresponding to 3.3 ng mL(-1), respectively. Recovery studies were 81.2 +/- 1.9 (SD). The method was applied for analysis of OTA in wheat, corn, red pepper, cheese and wine. The proposed method can be used for the routine analysis of OTA in food and animal feed.     

A modified method for the determination of methylmercury by gas chromatography

A simple modification of the West?? extraction procedure for methylmercury and its determination by gas chromatography (GC) is presented. The cysteine clean-up step has been modified, with use of cysteine-impregnated paper instead of cysteine solution. Methylmercury bromide is extracted from the sample into toluene and is selectively adsorbed on the cysteine paper. Interfering compounds are washed from the paper with toluene. The isolated methylmercury is set free with sulphuric acid containing bromide, extracted into benzene and determined by GC. The modification of the extraction procedure results in good recovery and reproducibility for various biological and environmental samples, good sensitivity with a detection limit of 0.1 ng/g, avoidance of difficulties arising from emulsion formation, cleaner chromatograms, and faster analysis. It is particularly suitable for determination of low levels of MeHg.     

A simple method for controlled immobilization of proteins on modified SAMs

This paper describes a method for modifying self-assembled monolayers (SAMs) with the nitrilotriacetic acid (NTA) group for subsequent immobilization of hexahistidine tagged proteins. The method has two important improvements over previous ones; firstly it avoids the need to carry out a complex synthesis of the chelator alkanethiols prior to deposition because the reactions are performed in situ on a preassembled SAM. This in situ approach also avoids phase segregation of alkanethiols with different functional groups, especially bulky ones such as NTA and tri(ethylene glycol), since a simple SAM is employed as the starting material. The approach reported here uses mercaptohexadecanoic acid to form a well-ordered homogeneous carboxyl-terminated SAM on a gold surface. The carboxyl group was then condensed with an NTA derivative containing an amino group to form a peptide bond. The product is a surface that, after chelating Ni(2+) ions, binds histidine tagged proteins. The loading of NTA groups can be controlled by choice of reaction conditions thereby removing the need for a second alkanethiol to dilute the surface density of chelator groups and prevent molecular crowding. Both factors allow rapid attainment of optimal protein loading. Fluorescence imaging demonstrated that (His)(6) enhanced green fluorescent protein was reversibly immobilized and importantly, was functional on the surface. Furthermore, data from surface plasmon resonance, cyclic voltammetry and fluorescence spectrometry provided additional information on the specific and reversible immobilization of (His)(6) proteins on the NTA-modified SAM surface.