A low-temperature electron diffraction study of structural disorder and its relationship to the Kondo effect in ThAsSe

A low-temperature electron diffraction study has been carried out on ThAsSe to search for evidence of structural disorder associated with the low-temperature non-magnetic Kondo effect. A highly structured and extremely complex characteristic diffuse intensity distribution has been observed at low temperature and interpreted in terms of a gradual charge density wave type phase transition upon lowering of temperature involving disordered As-As dimerization within (001) planes. Plausible models of the proposed As-As dimerization have been obtained using a group theoretical approach.     

A 13C NMR investigation of restricted rotation and dimerization in p-substituted nitrosobenzenes

The barrier to internal rotation in a series of p-substituted nitrosobenzenes has been determined by means of variable temperature ¹³C NMR. Substituent (X) effects on the barrier are similar to those observed for acetophenones and benzaldehydes. In addition, dimerization of the nitrosobenzenes was observed at low temperature for X = CH₃, CI and H. The barrier height is discussed in terms of contributions from resonance effects.     

The measurement and analysis of normal incidence solar UVB radiation and its application to the photoclimatherapy protocol for psoriasis at the Dead Sea, Israel

The broad-band normal incidence UVB beam radiation has been measured at Neve Zohar, Dead Sea basin, using a prototype tracking instrument composed of a Model 501A UV-Biometer mounted on an Eppley Solar Tracker Model St-1. The diffuse and beam fraction of the solar global UVB radiation have been determined using the concurrently measured solar global UVB radiation. The diffuse fraction was observed to exceed 80% throughout the year. The application of the results of these measurements to the possible revision of the photoclimatherapy protocol for psoriasis patients at the Dead Sea medical spas is now under investigation. The suggested revision would enable the sun-exposure treatment protocol to take advantage of the very high diffuse fraction by allowing the patient to receive the daily dose of UVB radiation without direct exposure to the sun, viz. receive the diffuse UVB radiation under a sunshade. This would require an increase in sun-exposure time intervals, as the UVB radiation intensity beneath a sunshade is less than that on an exposed surface.     

ULTRAVIOLET PHOTOCHEMISTRY OF THYMINE IN AQUEOUS SOLUTION

Abstract— We have investigated the ultraviolet photochemistry of thymine in aqueous solution. Four isomeric dimers are produced, and the yield of each has been measured as a function of thymine concentration, oxygen concentration, and temperature. At low thymine concentration, dimerization proceeds via the triplet state, while at high concentration it arises mainly from aggregates, probably from a singlet precursor. We have determined the ratios of rate constants for the triplet state mechanism and the quantum yield for dimerization from aggregates. The quantum yield for dimerization from the triplet state in thymine is smaller by a factor of about 10 than that in uracil, which in turn is smaller by another factor of 10 than that in orotic acid. It increases with the energy of the exciting radiation in a manner similar to the behaviour of uracil and orotic acid. On the other hand, dimer formation from aggregates is independent of photon energy. Dimerization from aggregates decreases with increasing temperature, while the total production of dimers from the triplet state is independent of temperature.     

Metallic behavior and periodical valence ordering in a MMX chain compound, Pt(2)(EtCS(2))(4)I.

A new one-dimensional (1-D) halogen-bridged mixed-valence diplatinum(II,III) compound, Pt(2)(EtCS(2))(4)I (3), has been successfully synthesized from [Pt(2)(EtCS(2))(4)] (1) and [Pt(2)(EtCS(2))(4)I(2)] (2). These three compounds have been examined using UV-visible-near-IR, IR, polarized Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray crystal structure analyses (except for 1). Compound 3 was further characterized through electrical transport measurements, determination of the temperature dependence of lattice parameters, X-ray diffuse scattering, and SQUID magnetometry. 3 crystallizes in the monoclinic space group C2/c and exhibits a crystal structure consisting of neutral 1-D chains with a repeating -Pt-Pt-I- unit lying on the crystallographic 2-fold axis parallel to the b axis. The Pt-Pt distance at 293 K is 2.684 (1) A in the dinuclear unit, while the Pt-I distances are essentially equal (2.982 (1) and 2.978 (1) A). 3 shows relatively high electrical conductivity (5-30 S cm(-1)) at room temperature and undergoes a metal-semiconductor transition at T(M-S) = 205 K. The XPS spectrum in the metallic state reveals a Pt(2+) and Pt(3+) mixed-valence state on the time scale of XPS spectroscopy ( approximately 10(-17) s). In accordance with the metal-semiconductor transition, anomalies are observed in the temperature dependence of the crystal structure, lattice parameters, X-ray diffuse scattering, and polarized Raman spectra near T(M-S). In variable-temperature crystal structure analyses, a sudden and drastic increase in the Pt-I distance near the transition temperature is observed. Furthermore, a steep increase in U(22) of iodine atoms in the 1-D chain direction has been observed. The lattice parameters exhibit significant temperature dependence with drastic change in slope at about 205-240 K. This was especially evident in the unit cell parameter b (1-D chain direction) as it was found to lengthen rapidly with increasing temperature. X-ray diffraction photographs taken utilizing the fixed-film and fixed-crystal method for the metallic state revealed the presence of diffuse scattering with line shapes parallel to the a* axis indexed as (-, n + 0.5, l) (n; integer). Diffuse scattering with k = n + 0.5 is considered to originate from the 2-fold periodical ordering corresponding to -Pt(2+)-Pt(2+)-I-Pt(3+)-Pt(3+)-I- or -Pt(2+)-Pt(3+)-I-Pt(3+)-Pt(2+)-I- in an extremely short time scale. Diffuse lines corresponding to 2-D ordering progressively decrease in intensity below 252 K and are converted to the diffuse planes corresponding to 1-D ordering near T(M-S). Furthermore, diffuse planes condensed into superlattice reflections below T(M-S). Polarized Raman spectra show temperature dependence through a drastic low-energy shift of the Pt-I stretching mode and also through broadening of bands above T(M-S).     

The reversible formation of a single-bonded (C(60)(-))(2) dimer in ionic charge transfer complex: Cp*(2)Cr.C(60)(C(6)H(4)Cl(2))(2). The molecular structure of (C(60)(-))(2)

A new ionic complex of C60 with decamethylchromocene, Cp*2Cr.C60(C6H4Cl2)2 (1), has been obtained. The fullerides are monomeric in 1 at room temperature, whereas they form a single-bonded (C60-)2 dimer at low temperatures, the structure of which has been studied by the X-ray diffraction on a single crystal at 100 K. The length of the intercage C-C bond is 1.597(7) A and the interfullerene distance is equal to 9.28 A. A phase transition attributed to the reversible C60*- dimerization is observed in the 220-200 K range. The transition is accompanied by changes in the unit cell parameters, the decrease of the magnetic moment from 4.20 muB (S = 3/2, 1/2) to 3.88 muB (S = 3/2) and the appearance of EPR signal from Cp*2Cr+, simultaneously.     

Photochromic behaviour of ozonated and photoirradiated cellulose studied by fluorescence spectroscopy

Samples of ozonated pure cotton cellulose have been subjected to three extended periods of irradiation with monochromatic light at 350 nm, with intervening dark periods. The changes during the treatments were monitored using fluorescence spectroscopy. Photochromic behaviour, comprising a fairly rapid emission intensity decrease during irradiation and a slower recovery of the emission intensity in the dark at ambient temperature, was observed. Starting from the completion of the first irradiation/dark treatment the intervening dark reaction almost completely restored the situation prevailing before irradiation. The photochromism observed for a reference sample of cellulose not pretreated with ozone and for microcrystalline cellulose was similar but differed both in amplitude and in fluorescence recovery during the dark periods. The ozonated sample was also irradiated with the entire spectrum of a medium pressure Hg lamp. This treatment caused a strong increase in the emission intensity and a red-shift of the emission maximum. The changes caused by ozonation and irradiation were also studied by diffuse reflectance FT-IR and UV-visible reflectance spectroscopy.     

Thermodynamic study on molecular association of some organic dyestuffs in aqueous solution

The temperature effects on the absorption spectra of acridine orange and methylene blue in aqueous solution at various concentrations have been observed, and it was found that the increase in temperature has an effect on the spectra similar to that of the dilution. The heats of dimerization of the dyestuffs in aqueous solution have been measured directly by the calorimetric titration method, and ΔH_d for acridine orange and methylene blue were found to be ?49.6 and ?83.3 kJ mol^?1, respectively. The entropy of dimerization values were also calculated to be ΔS_d = ?54.9 and ?211.6 J K^?1 mol^?1 for acridine orange and methylene blue, respectively. It has been concluded that the stacking interactions are primarily responsible for the self-association of the dye molecules.     

On the synthesis and characterization of ZnO/MgO nanocomposite by thermal evaporation technique

ZnO/MgO nanocomposites have been synthesized by an easy and cost effective thermal evaporation technique. Various growth temperatures ranging from 800 to 900 °C were tried. It is observed that the process temperature plays a key role in the formation of ZnO/MgO nanocomposite and the proper formation of ZnO/MgO nanocomposite occurs at 875 °C temperature as confirmed by X-ray diffraction studies. Scanning electron microscopic images indicate that the ZnO/MgO nanocomposite is formed as agglomerated nanoparticles distributed over a large area. Energy dispersive X-ray analyses also reveal that the Mg composition in the synthesized nanocomposite strongly depends on the process temperature. Photoluminescence (PL) spectrum exhibits a blue shift for the ZnO/MgO nanocomposite synthesized at 875 °C indicating the incorporation of Mg into the ZnO crystal lattice. A higher PL intensity ratio of band-edge to deep band emission has been observed for this sample indicating the presence of low crystalline defects.     

Fourier transform infrared spectroscopy and theoretical study of dimethylamine dimer in the gas phase

Dimethylamine (DMA) has been studied by gas-phase Fourier transform infrared (FTIR) spectroscopy. We have identified a spectral transition that is assigned to the DMA dimer. The IR spectra of the dimer in the gas phase are obtained by spectral subtraction of spectra recorded at different pressures. The enthalpy of hydrogen bond formation was obtained for the DMA dimer by temperature-dependence measurements. We complement the experimental results with ab initio and anharmonic local mode model calculations of monomer and dimer. Compared to the monomer, our calculations show that in the dimer the N-H bond is elongated, and the NH-stretching fundamental shifts to a lower wavenumber. More importantly, the weak NH-stretching fundamental transition has a pronounced intensity increase upon complexation. However, the first NH-stretching overtone transition is not favored by the same intensity enhancement, and we do not observe the first NH-stretching overtone of the dimer. On the basis of the measured and calculated intensity of the NH-stretching transition of the dimer, the equilibrium constant for dimerization at room temperature was determined.     

Cyclic voltammetry of solid diphenylamine crystals immobilized on an electrode surface and in the presence of an aqueous solution

Diphenylamine solid crystals have been mechanically attached to platinum or gold electrodes by four different methods and investigated by cyclic voltammetry in the presence of aqueous acidic media. It is demonstrated that the oxidative dimerization and polymerization (observed in non-aqueous solutions or mixed solvent systems) can also be accomplished by using diphenylamine microcrystals in contact with the electrode metal in the presence of aqueous solutions. Formation of a thin polymer layer has been observed even in the areas of the electrode that were not in direct contact with the diphenylamine microparticles. This finding can be explained by surface diffusion of the organic molecules or by oversaturation of the solution close to the diphenylamine microcrystals. A gradual growth of the voltammetric peaks characteristic of the polymer has been observed, while the irreversible wave due to the formation of cation radicals and their dimerization has shown a stationary behaviour. The redox transformation of the oligomeric compound formed at the surface is accompanied by a clear colour change: at pH 0 the colourless reduced form is converted to the bright-blue (violet) oxidized form. Electronic Publication     

Compositionally modulated Fermi surfaces, structured diffuse scattering and ternary derivatives of 1T-TaS2

Ternary derivatives of 1T-TaS₂ have been synthesized and the variation in the highly structured diffuse intensity distributions characteristic of such materials carefully monitored to investigate the effect that such substitution has upon the band structures and Fermi surfaces (FSs) of the materials. Removal of d electrons via the replacement of Ta ions with lower valent transition metal ions leads to a systematic increase in the radii of the characteristic structured diffuse intensity distribution. Extended Hückel tight binding calculations of the FSs of the doped samples are carried out and used to predict possible nesting wave-vectors. The results are in reasonably good agreement with the radii of the experimentally observed diffuse intensity distributions.     

Spectrophotometric and thermodynamic study on the dimerization equilibrium of ionic dyes in water by chemometrics method

The monomer-dimer equilibrium and thermodynamic of several ionic dyes (Neutral Red, Nile Blue A, Safranine T and Thionine) has been investigated by means of spectrophotometric and chemometrics methods. The dimerization constants of these ionic dyes have been determined by studying the dependence of their absorption spectra on the temperature in the range 20-75 degrees C at concentrations of Neutral Red (1.73 x 10(-5) M), Nile Blue A (3.94 x 10(-5) M), Safranine (6.59 x 10(-5) M) and Thionine (6.60 x 10(-5) M). The monomer-dimer equilibrium of these dyes has been determined by chemometrics refinement of the absorption spectra obtained by thermometric titrations performed. The processing of the data carried out for quantitative analysis of undefined mixtures, based on simultaneous resolution of the overlapping bands in the whole set of absorption spectra. The enthalpy and entropy of the dimerization reactions were determined from the dependence of the equilibrium constants to the temperature (van't Hoff equation).     

Competitive isomerization and dimerization in co-crystals of 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol and sorbic acid: a new look at stereochemical requirements for [2+2] dimerization

Competitive [2+2] photodimerization and E-->Z isomerization reactions occur in a co-crystal of 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol upon irradiation with 325 nm light. At 90 K both reactions are observed, whereas at 280 K the dimerization reaction is very fast and inhibits isomerization as the nature of the chromophore is affected by the reaction. The temperature dependence of the stereospecificity of the dimerization reaction is related to the large sliding motion required to bring the reacting molecules into juxtaposition. The progress of the reactions has been monitored by photocrystallographic methods.     

The kinetics of photosensitized thymine dimerization

Since thymine dimerization is the main photochemical lesion occuring in uv irradiated DNA, an understanding of the mechanism of dimerization is biologically significant. Both photosensitized and direct dimerization are important in DNA, but because photosensitized thymine dimerization has been less thoroughly investigated, this has been the major topic of study in this laboratory. By comparing experimental results with those obtained by computer simulation, attempts have been made to deduce mechanisms for photosensitization by acetone, acetophenone, and benzophenone. Photolysis of photosensitized solutions was performed using a xenon lamp and quantitative detection of dimer was achieved using h.p.l.c. techniques. A program designed to solve differential rate equations was used for the computer simulation of reaction mechanisms. Based on the results obtained it has been confirmed that acetone photosensitization over the entire range of thymine concentration considered (10^?4 mol dm^?3 to 10^?2 mol dm^?3) proceeds via diffusion controlled triplet transfer from the photosensitizer to thymine, followed by bimolecular collision of ground and excited state thymine monomers. For acetophenone and benzophenone photosensitization this method applied at low thymine concentrations, but at higher concentrations (<10^?3 mol dm^?3) predicted far lower yields than those observed experimentally. The effect of thymine base stacking was then considered, but it was found that this did not significantly increase the dimer yield. A mechanism involving association of thymine and photosensitizer molecules was therefore proposed. This mechanism was found to give reasonably good agreement between experimental and computed data. On the basis of present data the authors regard this as the most likely mechanism for thymine dimerization and work is in progress to confirm this proposal.     

A Simple and Convenient Method for the Synthesis of 3,5‐disubstituted 1,2,4‐thiadiazoles via Oxidative Dimerization of Primary Thioamides

A simple and efficient protocol has been developed for the preparation of 3,5‐diaryl‐1,2,4‐thiadiazoles in high yields through the oxidative dimerization of primary thioamides in aqueous medium at room temperature.     

Photodimerization of Coumarin Functionalized Poly(alkyl Acrylate) and Poly(alkyl Methacrylate) Random Copolymers: Influence of Copolymer Composition on Photocrosslinking

Coumarin‐containing poly(alkyl (meth)acrylates) were prepared via 70–80% esterification of hydroxy‐containing acrylic copolymers, then solution cast into thin films and photocrosslinked via the dimerization of coumarin derivatives with UVA light. The coumarin‐modified polymers crosslinked upon exposure and exhibited gel fractions between 74 and 99%. Coumarin dimerization efficiency increased with higher polymer mobility at the irradiation temperature. The effects of light intensity and irradiation time in photo‐dimerized systems followed the Bunsen‐Roscoe reciprocity law indicating that coumarin photodimerization depended only on dose. Thus, low intensities are overcome with longer times. This is an important advantage over photoinitiated free radical crosslinking which depends on irradiation intensity to the ½ power.     

固体氩中一氧化氮的低温氧化

本文研究了在低温固体Ar中NO和O~2的反应。采用Ar作基质, 将NO和O~2分层沉积或混合沉积在低温基板上, 通过逐渐升温来控制扩散速率, 在温度10K-35K范围内, 从样品付里叶变换红外光谱的变化, 首次观察了NO和O~2反应中间体的生成和转化, 由此给出了反应机理, 即NO在Ar基质中首先发生扩散和聚合, 生成cis-(NO)~2, 随后与氧反应生成asym-N~2O~3和iso-N~2O~4, 而asym-N~2O~3进一步氧化生成N~2O~4(D~2~h)。     

Four-center type photopolymerization in the solid state. II. Polymerization mechanism of 2,5-distyrylpyrazine

The polymerization mechanism of trans,trans-2,5-distyrylpyrazine (DSP) has been investigated and some crystal changes along with the polymerization process have been observed through polarizing microscope and x-ray diffraction pattern. Information has been obtained on the active species, polymerization reaction type, and other factors such as light intensity, reaction temperature, or crystalline state. The polymerization of DSP occurs only in the solid state by photoirradiation. Reduced viscosity increases gradually with the increase of conversion and increases sharply above 80% conversion. Polymerization rate increases with the increase of light intensity and temperature. On the other hand, reduced viscosity decreases with the increase of temperature but does not depend on light intensity within the range investigated. The polymer obtained at low conversion as well as at high conversion has high crystallinity, and the direction of polymer axes is simply related to that of monomer crystal. It was concluded that the four-center type polymerization of DSP proceeds topochemically by a photochemically induced stepwise mechanism.     

Dimerization and polymerization of isoprene at high pressures

The high-pressure reactivity of isoprene has been studied at room temperature up to 2.6 GPa by using the diamond anvil cell technique in combination with Fourier transform infrared spectroscopy. Both dimerization and polymerization reactions take place above 1.1 GPa. At this pressure, the two processes are well separated in time, the dimerization being the only one occurring in the first 150 h. Both processes simultaneously occur as the pressure increases. The reaction product is composed of a volatile fraction, identified as sylvestrene, and a transparent rubberlike solid formed by cis-1,4- and 3,4-polyisoprene. The activation volume of the dimerization reaction has been obtained from the kinetic data. The photoinduced reaction, studied at room temperature for two different pressures, takes place through a two-photon absorption process, and the threshold pressure is lowered to 0.5 GPa. At this pressure, both the dimerization and polymerization processes occur, but the dimerization is not as selective as in the purely pressure-induced reaction. 4-Ethenyl-2,4-dimethylcyclohexene is obtained in addition to sylvestrene. By increasing the pressure, the photoinduced reaction becomes more selective, and the monomer is quantitatively transformed into the same polymer obtained in the purely pressure-induced reaction.