“Constant-loss” relaxation response in crystals and glasses

Nowick and his associates have stated that many ionic crystals and glasses exhibit a loss per cycle which is independent of frequency over an appreciable range and have suggested that such behavior constitutes a new universality. Furthermore, much such data seem to approach an asymptotic, nearly temperature-independent ac loss at sufficiently low temperatures. In order to further evaluate these conclusions, small-signal ac relaxation data for a CaTiO₃:30% Al³⁺ ceramic material are analyzed in detail and the results compared to those published by Nowick and associates for the same material. It is found that a plausible conducting-system dispersion model based on the effective-medium approximation for hopping charges yields results globally similar to, but somewhat different in detail from, those of Nowick et al. But a response model which includes both such conducting-system response and dielectric-system dispersion well fits the data over a wide temperature range. To do so, it requires the presence of a non-zero high-frequency-limiting resistivity probably arising from localized charge motion. No constant-loss individual dispersions appear in the model, but it nevertheless yields approximately constant loss over a limited frequency range at low temperatures. It suggests that asymptotic behavior is associated with the nearly temperature-independent dielectric-dispersion contribution to the response at low temperatures, and it does not verify the Nowick conclusion that the slope of the ac conductivity approaches a constant value near 0.6 at high temperatures.     

Internal rotation and framework relaxation in ethene thiol

New and existing data on the ground and excited CS torsional states of the stable rotamers of ethene thiol have been analysed to give a potential function for internal rotation around the CS bond. The barrier between the syn and anti rotamers is found to be 800 cm^?1 and that at the planar anti conformation itself to be 12 cm^?1. the syn conformation is 50 cm^?2 more stable than the ‘quasi-planar’ anti conformation Satisfactory reproduction of the observed trends in rotational constants with torsional excitations has been achieved by introducing extensive relaxation of the CCS angle and the CS bond length during internal rotation. The CCS angle reduces by up to 5° v'hilst the CS bond length increases by up to 0.02 A during rotation from the syn to anti conformation. The use of ab initio M.O. calculations to provide an indication of the most significant aspects of structure relaxation is described.     

The preparation,properties and gas-phase molecular structure of difluoro(germylthio)phosphine

Difluoro(germylthio)phosphine, PF₂(SGeH₃), has been prepared by the reaction of S(PF₂)₂ with GeH₃Cl, and has been characterised by i.r.,Raman, n.m.r. and mass spectroscopy. Cleavage reactions with Cl₂ and HBr, donor reactions of the phosphorus atom and exchange reactions with platinum complexes have been studied. The molecular structure of PF₂(SGeH₃) in the gas phase has been determined by electron diffraction. Principal parameters (r _a) are:r(Ge–S) 225.6(4)pm;r(P–S) 211.5(8)pm;r(P–F) 159.0(9)pm; <(GeSP) 99.0(6)° <(SPF) 99.9(4)° <(FPF) 97.0(10)°. The conformation adopted is such that there are short non-bonded F...H contacts, with the PF₂ group twisted 18° from the position in which the FPF angle bisector eclipses the Ge–S bond.

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Difluor(germylthio)phosphin. Darstellung, Eigenschaften und Molekülstruktur in der Gasphase

Zusammenfassung PF₂(SGeH₃) wurde über die Reaktion von S(PF₂)₂ mit GeH₃Cl dargestellt und mittels IR,Raman, NMR and MS charakterisiert. Es wurden Spaltungs-reaktionen mit Cl₂ und HBr, Donor-Reaktionen des Phosphor und Austausch-reaktionen mit Platinkomplexen untersucht. Die Molekülstruktur von PF₂(SGeH₃) in der Gasphase wurde mittels Elektronendiffraktion bestimmt. Die Hauptparameter (r _a) sind:r(Ge–S) 225,6(4)pm;r(P–S) 211,5(8)pm;r(P–F) 159,0(9)pm; <(GeSP) 99,0(6)° <(SPF) 99,9(4)° <(FPF) 97,0(10)°.

     

Synthesis of 1,2,4-triazol-5-ylidenes and their interaction with acetonitrile and chalcogens

Two new, more convenient methods for the synthesis of 1,2,4-triazol-5-ylidenes are described. Four new 1,2,4-triazol-5-ylidenes have been prepared using these methods: 1-(1-adamantyl)-3,4-diphenyl-1,2,4-triazol-5-ylidene (2a), 1-(1-adamantyl)-3-phenyl-4-(p-bromophenyl)-1,2,4-triazol-5-ylidene (2b), 1-(1-adamantyl)-3-phenyl-4-(alpha-naphthyl)-1,2,4-triazol-5-ylidene (2c), and 1-(1-adamantyl)-3,4-di(p-bromophenyl)-1,2,4-triazol-5-ylidene (2d). The X-ray crystal structures of 2d and the precursor salt 1-(1-adamantyl)-3,4-di(p-bromophenyl)-1,2,4-triazolium bromide (1e) are described. Compound 2a reacts with CH(3)CN via C-H insertion to form 1-(1-adamantyl)-3,4-diphenyl-5-cyanomethyl-5H-1,2,4-triazoline (3), and 2a and 2d react with elemental sulfur and elemental selenium, respectively, to form the corresponding thione (4) and selenone (5).     

The elimination of metal(I) hydride in the mass spectra of some metal(II) compounds

Metal(I) hydrides are eliminated as neutral species in the electron impact ionization mass spectra of copper(II) and palladium(II) complexes of ethylene-N,N′-3-benzoylprop-2-en-2-amine. Deuterium labelling shows that the hydrogen atom of the metal(I) hydride is derived predominantly from the ethylene bridge both for ion source reactions and for metastable ion transitions. Evidence supporting the proposed rationalization for elimination of metal(I) hydride is provided by the observation of an analogous reaction in the mass spectrum of (ethylene-N,N′-salicylaldiminato)copper(II). The mass spectrum of ethylene-d₄-N,N′-3-benzoylprop-2-en-2-amine shows an unusual rearrangement to give [C₇H₅D₂]⁺ ions involving a formal phenyl-to-methylene transfer.     

Ab initio calculation of the potential functions for internal rotation around the CS bonds in simple ethene thiols

Ab initio calculations with complete geometry optimisation have been used to study internal rotation in compounds of the type XCH = CHSH, X = CN, H, CH₃ and F, with X located trans to the sulphur atom. Potential functions for the CS torsion have been obtained for each case and it has been established that the dominant framework changes accompanying internal rotation in these molecules involve the CCS angle and CS bond length. Furthermore, it has been shown that the nature of the substituent X significantly affects the molecular conformation of the SH group. The observed trends are discussed in terms of a simple model.     

The microscopic reaction dynamics and branching ratio in the F + HCOOH and F + H2CO reactions

Infrared chemiluminescence under conditions of arrested relaxation has been applied to the study of the hydrogen and deuterium abstraction reactions of HCOOH, DCOOH and H₂CO with F atoms. Two distinctly different modes of product excitation are observed, depending upon whether the reaction proceeds via the formyl or carboxyl hydrogen. Reaction at the formyl hydrogen (or deuterium) causes substantial inversion in the diatomic product internal energy distributions. The F + H₂CO and F + DCOOH reactions respectively channel 56% and 54% of the available energy into vibration in the product diatomic when they occur at the formyl site. In both cases the product energy distributions are qualitatively similar to those observed in direct reactions of triatomic systems on repulsive energy surfaces. In contrast to these, reaction at the carboxyl hydrogen of DCOOH gives an HF2 product vibrational distribution having a Boltzmann equilibrium shape at a temperature of 4300 K. The ratio of HF to DF product from the F + DCOOH study shows that reaction occurs at the carboxyl hydrogen approximately twice as often as at the formyl site. Comparison with triatomic reactions involving the same mass-combinations implies that abstraction of the formyl hydrogen occurs via single-collision, direct encounters, whereas reaction at the carboxyl site involves a long-lived complex in which extensive randomisation of the reaction exoergicity among all the product vibrational modes can occur.     

Characterization by three-dimensional representation of sequential transitions of metastable ions in a double focusing mass spectrometer

The ion currents due to products of selected ion reactions of toluene and 3-chlorophenol, occurring in the source or flight tube of a double focusing mass spectrometer for which the electric sector precedes the magnetic sector, have been represented by plotting them against two variables that reflect independently the effects of the sectors. The daughter ion produced from the molecular ion of 3-chlorophenol that decays by sequential losses of CO in the field free region preceding the electric sector and Cl˙ in the field free region preceding the magnetic sector gives rise to a peak with a distinctive shape on the resultant three-dimensional surface. A peak due to the daughter ion arising from the corresponding sequential losses of H˙ and C₂H₂ from the molecular ion of toluene could not be detected. A portion of the ion kinetic energy spectrum which would be recorded by a detector at the energy resolving slit is derived from the surface for 3-chlorophenol and is in satisfactory agreement with a published spectrum.