The ESR spectrum and structure of the dimer radical cation of dimethyl selenide (Me2Se-SeMe+2) in a γ-irradiated single crystal

The dimer radical cation of dimethyl selenide has been identified in a γ-irradiated single crystal of the parent compound, its main (^evenSe) ESR spectrum showing hyperfine coupling to twelve equivalent hydrogens. Also, the relative intensity of the naturally abundant⁷⁷Se satellite spectrum is in agreement with a hyperfine interaction involving two anisotropically equivalent selenium atoms. The principal values and direction cosines of the⁷⁷Se hyperfine andg tensors have been determined and are shown to be satisfactorily interpreted in terms of a centrosymmetric structure, the unpaired electron occupying an antibonding (σ^*) orbital built almost entirely from the p_σ orbitals of the two selenium atoms. Analysis of theg tensor components indicates that the CSeC planes are perpendicular to the direction of the Se-Se bond. It is also shown that the isotropic¹H splitting andg factor obtained previously from solution ESR studies of the dimethyl sulphide dimer cation are consistent with the structure advanced here for the selenium dimer. The results seem to rule out the possibility of any significant d orbital participation in the bonding.     

Anomalous density effect near the interface between two media

A theoretical investigation is presented for the local energy loss of a relativistic charged particle and theK-shell excitation cross section σ _K when the particle crosses a sharp interface between two different media. The logarithmic increase of the local energy loss andσ _K with the particle energy is shown to differ essentially from the case of a homogeneous medium. The calculations ofσ _K for aluminum and copper foils are in good agreement with the experimental data.     

Hyperfine structure of N2 (B 3Π g andA 3Σ u + ) from LIF measurements on a beam of metastable N2 molecules

Crossing an intense beam of nitrogen molecules in the metastable N₂(A) state with the beam from a CW dye laser, laser-induced fluorescence was observed in the first positive system of N₂,B ³Π _g ?A ³Σ _u ⁺ . About 300 lines of the (10, 6) band were studied at sub-Doppler resolution (15 MHz FWHM). From the well-resolved hyperfine structure of the lines, the hyperfine splittings of both the upper and the lower state were derived for a range of rotational quantum numbers up toJ=12. Using multiple independent determinations of each splitting via lines belonging to different branches, the hfs could be measured with an accuracy of about 2 MHz. Fitting known theoretical expressions for the hyperfine energies to the data, the following nuclear coupling constants were obtained (in MHz): For theA state,v=6: α=12.86, β=?11.40,e ² q ₀ Q=?2.5. For theB state,v=10:K ₁₁=86.46,D ₁₁=12.67,D _1?1=?44.64,G ₁₁=69.18,Q ₁₁=0.64,Q _1?1=1.38. The hfs is mostly due to nuclear magnetic dipole interactions. For theA state the results are essentially in agreement with hfs constants derived from RF resonance experiments, but are superior as regards the data fit over the entireJ range covered. For theB state, the results are new and are interpreted in terms of a simple LCAO model. The Fermi contact coupling constant is in good agreement with unpublished SCF results by V. Staemmler. The striking dependences of the hfs splitting on the fine structure levels, Λ sublevels and onJ are explained both quantitatively and in terms of vector models.     

Core-valence correlation potentials based on density functional theory. Applications to valence-electron-only calculations on Na and K diatomics

The core-valence correlation potential has been derived for Na and K employing atomic calculations which make use of the density functional formula worked out by Lee, Yang and Parr based on Colle-Salvetti approach. The numerical potential is fitted with a small number of Gaussians leading to a very simple expression for an one-electron corevalence correlation operator? _cv . The core-valence correlation corrections can be computed by applying? _cv on a quite general class of wavefunctions. Applications of the? _cv operator within the framework of valence-electron-only calculations using effective Hamiltonians are presented for Na and K atoms, for Na₂, K₂, NaK and their cations. Almost all the corrections calculated for the physical properties due to the core-valence correlation lead to results which are in good agreement with those obtained from much more sophisticated treatments and experimental data.     

Distribution ofK α L n satellite yields for mediumZ elements bombarded with16O ions

K _α X-ray spectra were measured with a Du Mond type curves crystal spectrometer for₄₂Mo and₄₆Pd targets bombarded with oxygen ions at 5.5 MeV/u energy (i.e. at velocity closely matching those of the targetL-shell electrons). The distribution ofL-shell vacancies produced in collisions with simultaneousK-shell ionization is deduced from the measured yield distribution of theK _α X-ray satellites. The distribution shows deviations from the binomial ones. The deviations can be accounted for by assuming that theL-shell vacancy production is due to two uncorrelated processes: the direct ionization by impact and the electron capture from theL-shell of the target atom into theK-shell of the projectile, both occurring at nearly central collisions. The corresponding probability values are deduced using simple statistical considerations.     

LIF measurement of the vibrational dependence of the N2(B 3Π g ) hyperfine splitting parameters

From very high resolution (8 MHz FWHM) LIF measurements, the hyperfine coupling constants of N₂(A ³Σ _u ⁺ ) and N₂(B ³Π _g ) have been obtained for three pairs of vibrational quantum numbersv′,v″. TheA-state constants are in very good agreement with accurate literature data. The vibrational dependence of some of theB-state constants is found to be much more pronounced. This is qualitatively explained in terms of the electronic structure of the two states.     

K X-ray satellites of uranium bombarded with 500 MeV neon

K X-ray satellite spectra produced during the bombardment of uranium with 500 MeV neon, have been investigated with a high resolution crystal spectrometer. The experimental results are compared with theoretical predictions for vacancy production computed in the semiclassical separate atom perturbation theory and corrected forL subshell coupling and electron transfer from targetL shell into projectileK shell. The agreement between theory and experiment is rather good. It turns out, that the reduction ofL vacancy production due toL subshell coupling is essentially compensated by theL electron transfer from target to projectile. The present work is an extension of a previous study of theK α₁ satellite spectra of tantalum bombarded with 403 MeV nitrogen.     

On transport properties of hot liquid and supercritical water and their relationship to the hydrogen bonding

The dynamic viscosity η of water at temperatures along the saturation line is fitted with an expression taking into account the relative void volume, the enthalpy of vaporization, and the hydrogen bonding, the latter through the Kirkwood dipole orientation correlation parameter, g_K. A similar expression is given for the fitting of viscosity data of supercritical water, except that the explicit temperature dependence of ln η on 1/RT is now negative. The self-diffusion coefficients D of water along the saturation line up to the critical point are shown to depend on the fractions of non- and singly hydrogen-bonded water molecules. The high pressure (110 MPa) values of D of supercritical water are a smooth extension of the values for lower temperature water at the same pressure.     

The pressure dependence of the dielectric constant and density of acetonitrile at three temperatures

The static dielectric constant of liquid acetonitrile is reported at 10, 25, and 40°C and at pressures up to 3 kbar. Densities of pure acetonitrile were measured where needed in order to analyze the dielectric measurements using the Kirkwood-Fröhlich equation. The Kirkwood correlation factorg _K was found to be less than unity with a positive temperature but negative pressure coefficient, suggesting an increasing anticorrelation of dipoles with decreased temperature and increased pressure. Theg _K factors are analyzed by a dipole pair-bonding model as outlined by Dannhauser and Flueckinger in which the lowg _K values are attributed to the presence of dimers consisting of completely anticorrelated dipoles. The implication of this model on other measurements, especially MNR relaxation studies, is examined.     

Single crystal Raman study of potassium oxalate monohydrate,K2C2O4. H2O,and potassium oxalate monodeuterate,K2C2O4. D2O

Raman spectra of poly crystalline and single crystal K₂C₂O₄. H₂O and K₂C₂O₄. D₂O have been recorded at room temperature. From an earlier neutron diffraction study it is known that the space group is C⁶₂h. The water molecule occupies a C₂ site and the oxalate ion a C₁ site. The assigned water vibrations show small factor group splitting between g modes (Raman active) and u modes (IR active). The internal oxalate vibrations are found to have wavenumbers in good agreement with those reported from Raman studies of other oxalates.     

Atomistic simulation of the glass transition of di-substituted polysilanes

Molecular dynamics has been used to determine the glass transition temperature of the amorphous phase of five di-substituted polysilanes from plots of specific volume versus temperature. In each case, good agreement was obtained between the simulation values and the reported DSC results. The effect of amorphous cell dimensions and equilibration time on T_g has been investigated. The use of larger cells provides better agreement with experimental T_g and probably more accurate densities as suggested by earlier studies. The effect of pressure on the T_g of two different polysilanes was also investigated. Although experimental data for comparison is unavailable, values obtained for dT_g/dp are consistent with those reported for other polymers. Vectorial autocorrelation analysis was used to explore the mobility of the polysilane main chains and side groups relative to polyalkanes, polyphosphazenes, and polysiloxanes.     

The effect of water on the physicochemical and mechanical properties of gelatin

Strips of gelatin have been prepared by extrusion at different water contents varying from 20 to 50% H₂O (dry weight basis, d.w.b.). The processes of subsequent hydration or dehydration of these strips were followed by dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction and NMR relaxation measurements. A comparison of the calculated dependence of theT _g of gelatin (T _g anhydrous, 200?C) on water content (using the Ten Brinke and Karasz equation) with experimental results derived from DMTA showed that freshly extruded material followed the theoretical plot below 25% H₂O (d.w.b.), but at higher water contents, the7 _g deviated positively, probably due in part to the effect of delayed re-equilibration of water content after thawing of separated ice crystals. The experimental results determined after storage for one week fell on a different line, with aT _g of 145?C for anhydrous gelatin Possibly, theT _g is elevated by crystallization — a view supported by the WAXS spectra. The NMR relaxation results also showed a profound mobilization of the gelatin protons at water contents greater than 25% d.w.b.     

Natural and artificial ageing of an alkyd based wood finish

Wood protection in exterior use is generally achieved with a stain. This protective product is often obtained from an alkyd resin. A natural and artificial agening have been studied by measuring the glass transition temperature (T _g) of the finish variations in terms of time. In both ageings, theT _g variations are the same; there is an increase inT _g during the first steps of ageing and then a stabilization. A behaviour equation is proposed and is perfectly suitable to both ageings. Time constants are calculated. This equation shows that the artificial ageing machine chosen is a good means of artificial ageing. It allows the reproduction and the acceleration by 10 times of phenomena which are observed during the natural ageing.     

The relation between relaxed enthalpy and volume during physical aging of amorphous polymers and selenium

This paper reports physical aging results for PMMA, PMMA/PEO blends, PS, PC, PVC and PET (semicrystalline). Also included in this study is amorphous selenium. Temperature down-jumps from equilibrium above T_g to a temperature below T_g were carried out. Relaxed enthalpy, Δh and volume contraction, Δv, were measured. From the aging records, the constant ratio Δh/Δv = K_a was evaluated. For the polymeric samples K_a values of about 2 GPa were observed, thus similar to the inverse value of the isothermal compressibility close to T_g. Similarly for Se the K_a value obtained from Δh and Δv was in fair agreement with its isothermal compressibility.     

Ab initio calculations of electron-spin magnetic moments for Li,Be and B hydrides inX 2∑+ states

Electron-spin magnetic moments, in the form ofg-shifts, have been computed at the ROHF level for theX ²∑⁺ states of LiH⁺, BeH²⁺, LiH^?, BeH and BH⁺. A perturbative approach, complete to second-order in appropriate Breit-Pauli operators, has been used. Retention of two-centre integrals has proven vital. First-order terms are important, especially in describing the negativeg _∥ shifts observed experimentally in²∑⁺ molecules. The relativistic mass correction dominates in first-order, except for LiH^? where the two-electron spin-Zeeman gauge correction supersedes. Second-order terms contribute negatively, and only to the Δg _⊥ component. Along the isoelectronic series LiH^? → BeH → BH⁺, the magnitude of Δg _⊥ increases due to the dependence of spin-orbit coupling on nuclear charge. The relation ofg-shifts to electronic structure and bonding is explored.     

The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsg _xx andg _xxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of theh _xx andh _xxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects. Preliminary data were presented at the International Conferences on Chemical Thermodynamics at Baden (1973) and Montpellier (1975). The experimental part was carried out at the Istituto Chimico of the University of Trieste. To whom correspondence should be addressed.     

Thermal stability and lifetime estimates of a high temperature epoxy by Tg reduction

Thermal degradation of a high temperature epoxy network is studied in terms glass transition temperature (T_g) reduction over a temperature window encompassing the T_g of the network. The T_g is shown to decrease as the network is thermally aged at elevated temperatures in air and in argon. The duration of the aging experiments is extended to long time such that the absolute T_g reduction approaches a long time reduction plateau. Degradation is dominated by non-oxidative pyrolysis with a small contribution from diffusion limited thermal oxidative degradation at the surface. A time–temperature superposition is constructed from the extent of T_g reduction of samples aged in air and the thermal shift factors are shown to have Arrhenius scaling behavior. An activation energy is extracted that agrees with previous activation energy measurements derived from other property measurements of the same network aged under similar conditions. The agreement of the activation energy with past results shows that T_g reduction is controlled by the same degradation mechanism and may be used as an observable for lifetime estimates when thermal degradation is pyrolytic in nature. The extent of T_g reduction is modeled with an autocatalytic rate expression and compared to previous property measurements to show the difference in sensitivity of observable material properties on degradation.     

On the vibrational dependence of electron impact ionization of diatomic molecules

The dependence of the Na₂ electron impact ionization rate is measured as a function of vibrational excitation in a crossed molecule-electron beamm arrangement at collision energiesE _coll ≤ 3 eV above the ionization threshold. Specific vibrational distributions in theX ¹∑ _g ⁺ state with average vibrational energies of 0.17 eV, 0.276 eV, and 0.349 eV, are prepared via Franck-Condon pumping using a narrow-band cw laser. Enhancement of the ionization rate is observed only at impact energies near the ionization threshold where the ionization rate increases linearly as a function of vibrational excitation. Analysis of the experimental data is based on three model calculations. The first of these calculations equates vibrational energy with kinetic energy and agrees well with the experimental data. A second, more refined model allows for differences in state-to-state ionization rates and uses Franck-Condon factors to estimate transition probabilities, but leads to a less favorable agreement. The third one employs a semi-classical formulation of the Franck-Condon principle. It provides the best agreement with the experimental data. In contrast with an earlier study of electron impact ionization of diatomic molecules [20], we find no evidence of dynamical modification of the ionization rate, due to vibrational motion of the nuclei, at the present level of accuracy of our data and analysis.