From ephemers to monomers, oligomers and polymers. New methods for the generation and transformation of silanones

New efficient routes to the generation of silanones at relatively low temperatures are based on the following reactions:

1. (1) reaction of linear and branched perhydrocarbyldisiloxanes and oligosiloxanes, and of some of their C-functional derivatives, with gallium or indium iodides or bromides;

2. (2) reaction of hydrocarbylchlorosilanes R_4−nSiCl_n (n = 2–4) and SiCl₄ with dimethylsulphoxide (with or without Mg or Zn);

3. (3) Autodecomposition of organosilicon compounds containing and groups;

4. (4) reaction of R_4−nSiCl_n with metal oxides of high redox potential (with or without the presence of CH₃CN).

Insertions of dialkylsilanones into Si---O---Si and Si---O---C linkages as well as into the Si---Cl bond have been studied.     

Laser spectroscopy in analytical chemistry:light on the next millennium

To date, lasers are widely accepted tools in analytical spectroscopy, involved in various stand-alone and hyphenated techniques. Furthermore, significant progress can be noted in this field. In this paper, first of all some laser characteristics are discussed. Subsequently, five selected topics are outlined to illustrate recent achievements and future developments:

1. Laser-induced fluorescence for detection in capillary electrophoresis, including the use of ultraviolet, continuous-wave lasers in combination with wavelength-resolved emission detection; the use of diode laser-induced fluorescence in the red region of the electromagnetic spectrum and the use of Ti:sapphire lasers for multiphoton-excited fluorescence detection.

2. Degenerate four-wave mixing for detection in liquid microseparation systems (based on the coherence of laser light).

3. Fluorescence line-narrowing spectroscopy for identification purposes, a cryogenic high-resolution molecular fluorescence technique with a high potential in environmental analysis.

4. Recent developments in Raman spectroscopy (including ultraviolet-Resonance Raman and hyphenation of liquid chromatography and Raman spectroscopy).

5. Use of lasers for sample introduction in inorganic analysis based on controlled material ablation.

Semiempirical molecular orbital theory in carcinogenesis research

It is generally believed that most chemical carcinogens exert their effects through covalent modification of the nucleic acid base sites. Significant clarification of the underlying chemistry involved in these early and critical events is available through the techniques of computational chemistry. The role of semiempirical molecular orbital theory in this area is reviewed using examples focusing on

1. (i) the conversion of precarcinogens to reactive electrophiles,

2. (ii) the regiochemistry of nucleic acid adduct formation,

3. (iii) the chemical and physical consequences of nucleic acid base modification, and

4. (iv) empirical structure activity relationships.

The symbiotic relationship that can exist between the semiempirical and ab initio molecular orbital procedures is emphasized, as is the continuing niche for the semiempirical methodology in the study of systems that are too large, or in situations that are insufficiently cost effective, for the application of ab initio methods.     

Monte Carlo simulation of catalytic CO oxidation

A simple Monte Carlo model of the CO oxidation on a single-crystal catalyst surface is presented. The simulation model considers the following elementary reaction steps:

1. (1) chemisorption of a CO molecule, its surface migration and possible desorption

2. (2) physisorption of an O₂ molecule to a precursor state and its subsequent dissociative chemisorption

3. (3) activated reaction of adsorbed O and CO (the Langmuir - Hinshelwood reaction mechanism), formation of CO₂ and its rapid desorption.

The changes in the activation energy of reaction and in the adsorption energy of CO resulting from the interactions between adsorbed species are also considered. The model makes possible to monitor temperature programmed reaction spectra or reaction spectra obtained during changes of the ratio of the partial pressures of CO and O₂. The results of simulations for a Pd(111) single-crystal plane are compared with experiment.     

A computer program for searching the best model for describing different experimental systems

An algorithm for searching the best polynomial analytical function for describing different experimental systems is presented. It is based

1. (1)on the generation of all possible analytical functions of a given order, with a given number of terms and with a given number of independent variables, and

2. (2)on the calculation of the parameters of all selected functions using the linear regression method.

To show the ability of the program two different examples are given:

1. (1) searching the best univariate polynomial model, and

2. (2) modelling of the stability of a two-component mixture as a function of three factors.

Fourier transform infrared spectroscopy and the analytical study of works of art for purposes of diagnosis and conservation

This paper is a study of the analytical capacity of Fourier transform infrared spectroscopy (FT-IR) for any type of samples from works of art. The analytical information obtained with this technique on organic and inorganic compounds is extremely useful in the preliminary studies necessary for diagnosis and to decide on the conservation process. This paper reports the analysis and study with FT-IR on samples extracted from the microlayers in several works of art from different periods (16th to 18th century) comprising wall paintings and canvas. The most outstanding of these being the frescoes by A. Palomino from two ceilings in the Santos Juanes church. The analytical procedures for the different components of the works studied such as

1. (a) varnish applied to wood panels and canvas paintings,

2. (b) binding media and pigments used by the artists,

3. (c) inerts, mortars, stuccos and grounds have been optimized. The FT-IR technique offers a quick analysis of microsamples (less than 0.5 mg) and is able to characterise the different molecular groups which provide information on the nature of the different materials of organic and inorganic origin used by the artist and thus permits the diagnosis of pathologies requiring conservation treatment.

XIIth international meeting on radiation processing Avignon 25–30 March 2001 (Polymer irradiation: past–present and future)

Radiations are used efficiently and economically for the production of new or modified polymers. The following processes are considered:

1. Radiation curing;

2. Radiation cross-linking;

3. Radiation grafting.

These processes are commonly used today in industry and provide a broad range of new potential applications in various fields. The history of their development is briefly reported. The chemical reactions underlying these processes are described.

(1) Radiation curing is used commercially on a large scale for the production of improved coatings, lacquers and inks. The process can be conducted at very high speeds. Curing of magnetic formulations leads to particularly stable products, which compete favourably with more conventional materials.

(2) Radiation cross-linking is an established technology in the wire and cable industry. It emparts to the modified insulators improved resistance to solvents, to ageing and to elevated temperatures. The resulting cross-linked network also reduces the migration of fillers and thereby stabilizes in time any message imprinted with magnetic or colored pigments dispersed in a polymer.

(3) Radiation grafting is a powerful method for modifying more profoundly the properties of a polymer and for creating numerous, entirely new materials. The chemical modification can be applied at will into the bulk of the material or limited to a surface zone of any desired depth. This method can be used for instance, for introducing polar groups in the bulk or on the surface of non-polar polymers, for increasing or reducing the wettability of a polymer, for imparting a better compatibility of a polymer to a specific coating and the like.

The irradiation of water-soluble polymers in aqueous solutions, with or without the addition of another monomer gives rise to a variety of cross-linked gels which find useful applications in the biomedical field. Other promising applications will be considered.     

Time-resolved spectroscopy of long-path fluorescence and scattering for measuring total absorption of fluids

Optical methods are appropriate tools to detect organic micro-pollutants in fluids. A new technique is introduced which uses the decay of interaction processes like fluorescence and elastically scattered radiation by a fluid. Principally two different parameters are determined:

1. (i) the decay-time of the conventional interaction τ_C, which occurs at relatively short path-lengths of the incidence beam in the fluid, and

2. (ii) the decay-time τ_MP of the multi-path-saturation interaction originating at long path-lengths, e.g. in multi-path-reflection cuvettes, where the incidence beam is fully absorbed by the fluid.

A relation between the decay-time and the absorption coefficient of a fluid is theoretically derived. A simple preliminary experiment is performed considering distilled water polluted with non-fluorescent azobenzene and fluorescent quinine-sulphate. A nitrogen laser has been used to generate the fluorescence and scattering signals. The reciprocal value of the difference between the decay-time of the multi-path and conventional signals, 1/(τ_MP − τ_C), yields the total absorption coefficient directly. In comparison to the conventional absorption technique the decay-time method is characterized by a higher sensitivity.     

Uncertainty estimation in Co gamma-ray dosimetry at JAERI involving a two-way dose intercomparison study with NPL in the dose range 1–50 kGy

The uncertainties in ⁶⁰Co gamma-ray dosimetry at JAERI have been estimated by reviewing irradiation and calibration procedures. The uncertainty in dose delivered by the JAERI calibration system has been estimated to be 2.2% (2σ) and the uncertainty in dose measurement using JAERI alanine/ESR transfer dosimeters has been estimated to be 3.4% (2σ). A two-way dose intercomparison study between NPL and JAERI employing alanine-PS, alanine-paraffin, and dichromate dosimeters as transfer dosimeters was performed to confirm the estimated uncertainty over the dose range 1–50 kGy. Dose intercomparison results using both alanine and dichromate dosimeters show agreement within about 2%, which gives confidence in the estimated uncertainty in ⁶⁰Co gamma-ray dosimetry at JAERI.     

Solar UVB Dosimetry by Amplification of Short and Long Segments in Phage LD DNA

Abstract— DNA UVB dosimeters, consisting of minidots of dried bacteriophage DNA placed on a UV-transparent polymer film, were analyzed by polymerase chain reactions (PCR). Ultraviolet-B dosimetry obtained with amplification of phage LD DNA of segments of 1.08 kilobase pairs (kbp) and 2.24 kbp was compared with that obtained with amplification of a 0.5 kbp segment (H. Yoshida and J. D. Regan, 66, 82-88, Photochem. Photobiol 1997). The number of lesions in each segment induced by UV radiation is proportional to the size of the amplified segments; thus, the average lesion frequency per unit dose was greatest in the 2.24 kbp and least in the 0.5 kbp segment. The average lesion frequency per unit dose was 3.5 times 10_-2 and 11.9 times 10_-2 m₂ kJ_-1 for 1.08 kbp and 2.24 kpb, respectively, compared to that for 0.5 kbp of 1.7 times 10_-2 m₂ kJ_-1 at 50 ng DNA. Dependability of DNA dosimeters, containing 50 ng and 100 ng, was tested by placing the DNA dosimeters for a time period of either 1 or 2 days outdoors on 8-12 January at Melbourne, FL. The daily dose was obtained directly with amplification of the 2.24 kbp segment and the 2 day continuous dose was obtained with amplification of the 1.08 kbp segment. Although the average lesion frequencies were different, both 50 ng and 100 ng DNA dosimeters provided about the same UVB dose, equivalent to the dose applied with a solar UVB simulator. The total UVB dose for 4 days obtained by amplification of the 1.08 and 2.24 kbp segments was 19.4-20.8 kJ m_-2, which is within experimental error with the 4 day continuous dose obtained with 0.5 kbp segments. The average daily dose obtained by 0.5 kbp and 1.08 kbp agreed with the average daily dose directly obtained with 2.24 kbp.     

An improved method to predict second cross virial coefficients

A previous method of the author to calculate the second cross virial coefficients is re-considered to improve the reliability of the predictions and to reduce the number of empirical rules. The method is based on the reduced second cross coefficient at the normal boiling temperature, , whose value is always assumed equal to unity. This value is then extrapolated to the experimental temperatures using only two empirical constants: K₁, a corrective multiplying factor of and K, in an exponential term as a multiplying factor of temperature. To improve the reliability of the method, literature experimental data are grouped in three binary classes:

- non-polar or slightly polar fluids;

- at least one strongly polar fluid;

- strong interactions of the acid–base type.

Only the critical constants and the normal boiling temperature are required as input parameters.

Deviations of calculated results from experimental one are in the range 25–40 cm³ mol⁻¹ for the first and the second class and below 300 cm³ mol⁻¹ for the third class.     

Three-dimensional polymer gel dosimetry: basic physical properties of the dosimeter

This study concentrated on assessment of the basic physical properties of a polymer gel dosimeter evaluated by NMR. For this, BANG-2 type polymer gel was prepared. The dosimeters were irradiated by ⁶⁰Co gamma photons and by 4, 6 and 18 MV X-ray photons for doses in the range 0–50 Gy. The multi-echo CPMG sequence was used for the evaluation of T₂-relaxation times in the irradiated gel dosimeters. Dependence of 1/T₂ in terms of the following factors was studied: absorbed dose, energy of applied radiation, temperature during NMR evaluation, time between irradiation and NMR evaluation and strength of the magnetic field. An exponential dependence of the 1/T₂ response on absorbed dose in the range 0–50 Gy was observed, while in the range 0–10 Gy the data could be fitted by a linear function. This paper also describes the dependence of 1/T₂ response on: radiation energy, strength of magnetic field and time from irradiation of the dosimeters to NMR evaluation. Increase of gel dosimeter 1/T₂ response with the decrease of the temperature during NMR evaluation has been qantitatively described. The polymer gel dosimetry system used in this study proved that it is a reliable system for three-dimensional dose distribution measurement.     

Concerning the interconnections of some parameters of self-organizing oxyhydrate gels and their experimental determination

The appearance of a pulsating current in the elongated cell filled with the zirconium oxyhydrate gel under short-circuited electrodes and constant mixing was shown. The pulsating character of the current is expressed in the form of current splashes. The reason for the appearance of current splashes is the self-organization of the gel with time; to be more exact, the reason is the constant conformational reorganization of the oxyhydrate matrix. When magnetic field acts on the zirconium oxyhydrate gel, increases in both the current values and the current splashes are observed. The increase of the gel particle sizes in a magnetic field is detected. Thus, it is of interest to consider the connection between the period of appearance of current splashes, splashes amplitudes and the sizes of gel aggregate regions.

In this regard, it is important to find out the connection between the parameters of the self-organizing gel cell. We suppose that:

1. In the process of self-organizing, the concentration of monomers forming the matrix in a gel area may change with time.

2. Solution of the task is independent of time. Generally speaking, it is possible to find a time-dependant solution of the task, but it is hardly worth doing.

3–45 MeV/u ion beam dosimetry using thin film dosimeters

Four kinds of film dosimeters well-characterized for low linear energy transfer (LET) radiations were applied to 3–45 MeV/u ions. The dose–responses relative to those for low LET radiations are almost one at a stopping power of about 10 MeV/(mg/cm²) and gradually become smaller with an increase in the stopping power. The overall uncertainty in ion beam dosimetry using these characterized dosimeters is better than ±5% (1σ) including uncertainty in fluence measurement (±2%). Lateral- and depth-dose profile measurements were made using one of the dosimeters (Gafchromic) with a spatial resolution of better than 1 and 10 μm, respectively.     

MOS ionizing radiation dosimeters: from low to high dose measurement

Metal-oxide-silicon dosimeters with a very thick gate oxide in a stack-connected configuration are studied with respect to their capability to measure a low dose. It is shown that if the temperature is controlled to ±5°C during reading a dose as low as 10⁻⁴ Gy can be measured with 10% accuracy.     

Fluorine-19 NMR spectroscopy of polyhalonaphthalenes. Part II polychloropolyfluoronaphthalenes via nucleophilic fluoride dechlorination

Six new polychloropolyfluoronaphthalenes have been identified by F-19 NMR whose empirical formulae are C₁₀ Cl_8−n F_n where n is 5 and 6 two trichloropentafluoronaphthalenes and four dichlorohexafluoronaphthalenes:-

• 1,3,8-trichloro-2,4,5,6,7-pentafluoronaphthalene,

• 1,3,6-trichloro-2,4,5,7,8-pentafluoronaphthalene,

• 1,3-dichloro-2,4,5,6,7,8-hexafluoronaphthalene,

• 1,8-dichloro-2,3,4,5,6,7-hexafluoronaphthalene,

• 1,6-dichloro-2,3,4,5,7,8-hexafluoronaphthalene and

• 2,7-dichloro-1,3,4,5,6,8-hexafluoronaphthalene.

In the partly fluorinated compounds such as 1,3,6,8-tetrachloro- 2,4,5,7-tetrafluoronaphthalene, nucleophilic fluoride dechlorination occurs at both and β positions. Initial substitution in octachloronaphthalene is more selective and occurs predominantly at the -position; both subsequent exchanges occur at sites two carbons away from the initial site. The results support the idea that the transition state is stabilised by chlorine at the sites of electron density, ortho and para to the site of nucleophilic attack.     

Synthesis and characterisation of [(C5Me4R)2NbS2]2M complexes (M = Fe, Co; R = Me, Et) : organometallic tetrathiometalates with niobocene ligands

Irradiation of Cp₂^* Nb(η²---S₂)H (Cp^* = C₅Me₅) 1a in the presence of Fe(CO)₅ gives the CO-free complex [Cp₂^*NbS₂]₂Fe 2a. The core of 2a contains an FeS₄ tetrahedron which is ligated by two niobocene ligands as shown by X-ray diffraction analysis. In the reaction of 1a or Cp₂^xNb(η²---S₂)H (CP^x = C₅Me₄Et) 1b with Co₂(CO)₈, compounds 3a and 3b of the same type are formed. Electrochemical studies of 2a and 3a,b show that they undergo three reversible 1e⁻ steps. The oxidation of 3b exerts a considerable influence on its absorption spectrum. A qualitative EHMO analysis is in agreement with a strong delocalisation of electron density over the whole NbS₂MS₂Nb system.     

Dμ—A new concept in industrial low-energy electron dosimetry

Irradiation with low-energy electrons (100–300 keV) results in dose gradients across the thickness of the dosimeters that are typically used for dose measurement at these energies. This leads to different doses being measured with different thickness dosimeters irradiated at the same electron beam, resulting in difficulties in providing traceable dose measurements using reference dosimeters. In order to overcome these problems a new concept is introduced of correcting all measured doses to the average dose in the first micrometer—D_μ. We have applied this concept to dose measurements with dosimeters of different thickness at two electron accelerators operating over a range of energies. The uncertainties of the dose measurements were evaluated, and it was shown that the dose in terms of D_μ was the same at each energy for all dosimeters within the measurement uncertainty. Using the concept of D_μ it is therefore possible to calibrate and measure doses from low-energy electron irradiations with measurement traceability to national standards.     

Absorbed dose distributions in a tissue-equivalent absorber for bremsstrahlung produced at the beamlines of the European Synchrotron Radiation Facility

The absorbed-dose distributions for bremsstrahlung, incident on a tissue-equivalent phantom, were measured with LiF : Mg,Ti thermoluminescent dosimeters at two insertion device beamlines of the European Synchrotron Radiation Facility (ESRF). The measurements were carried out for two different electron beam energies of 4 and 6 GeV. The corresponding bremsstrahlung spectra and power were measured using a high-resolution lead glass total absorption calorimeter. The results are compared with similar measurements carried out at other facilities. The normalized bremsstrahlung absorbed dose in a cross-sectional area of 100 mm², at a depth of 150 mm of the phantom, was measured as 6.1 and 3.6 kGy h⁻¹ W⁻¹ for the corresponding bremsstrahlung spectra of 4  and 6 GeV.     

The effect of dose on light-sensitivity of radicals in alanine EPR dosimeters

In this work we present a study of light-induced effects on free radicals and their transformations in gamma-irradiated pure L-alanine and in commercially available alanine detectors: rods, pellets and films. Samples irradiated to doses from 2 Gy to 4000 kGy were exposed to light from a fluorescent lamp and to ordinary daylight. The observed changes in EPR spectra of the samples were analyzed with regard to their intensity and shape. The shape analysis was based on numerical decomposition of the measured spectra into model spectra reflecting contributions of R1, R2 and R3 radical populations in the samples. The illumination of alanine dosimeters resulted in significant decrease of the central EPR line and was accompanied by distinct variations in the shape of EPR spectra. The rate of light-induced decay in spectra amplitude was found to be dependent on dose of ionizing radiation--the sensitivity to light was decreasing with increase in dose in all detectors in the 2-5x10(5) Gy dose range. The exposure of gamma-irradiated (to 300 Gy) alanine to normal, diffused daylight resulted in decay of the signal amplitude at rate about 0.5% per week. It was shown that decay in the R1 component was responsible for the observed reduction of the spectra amplitude. The observed increase in R2 contributions in samples exposed to light confirmed a hypothesis of R-->R2 radical transformations promoted by visible light. The reported effects indicate a necessity of protection of irradiated dosimeters from their prolonged exposure to light.