Absence of first order electronic transitions in liquid metals

High pressure shock wave data on a wide variety of metals indicates that electronic transitions are continuously distributed in the liquid phase and accompanied by melting maxima. A qualitative explanation for this behavior is suggested.     

Thermal conductivity of undoped trans-polyacetylene films

The low temperature variation of the thermal conductivity of transpolyacetylene is reported. The thermal conductivity, which though it is comparable to that of a semi-crystalline polymer at 100 K, varies linearly with the temperature below this temperature. The temperature variation of the phonon mean free path is found to be similar to that of an oriented polyethylene.     

The investigation of coating materials for the detection of nitrobenzene with coated quartz piezoelectric crystals

Powdered activated charcoal, quadrol tetrabase and polyethylene glycol (PEG)-400 and -750 are used as coatings on 15-MHz quartz piezoelectric crystals for the detection of nitrobenzene in air. The response to nitrobenzene over the range 2–10 ppm (2–10 × 10^?6 mol mol^?1)_was almost linear for all the coatings except charcoal which exhibited marked non-linearity, as well as greater sensitivity than the other coatings. The charcoal response was linear over the range 0.7–7.6 ppm nitrobenzene. For the other coatings, sensitivity increased in the order tetrabase < quadrol < PEG-400 < PEG-750.     

The accurate determination of total sulphur and disulphide and polysulphide compounds in petroleum products by metal reduction and flame emission spectrometry

Conditions for the determination of total sulphur and disulphide/polysulphide in petroleum products are described. The sulphur is reduced by heating with sodium or Devarda's alloy under reflux with subsequent liberation of H₂S and measurement of the chemiluminescent S₂ emission intensity in a hydrogen—argon diffusion flame. The precision and accuracy are good. Applications to light distillates and waxy residues are discussed.     

Structure-based prediction of the conductance properties of ion channels

The HOLE procedure allows the prediction of the absolute conductance of an ion channel model from its structure. The original prediction method uses an empirically corrected Ohmic method. It is most successful, with predictions being reliable to within a factor of two. A new modification of the procedure is presented in which the self-diffusion coefficients of water molecules from molecular dynamics simulation are used to replace the empirical correction factor. A "prediction" of the conductance for the porin OmpF by the new method is made and shown to be very close to the experimental value. HOLE also allows the prediction of the effect that the addition of non-electrolyte polymers will have on channel conductance. The method has great potential to yield structural information from data provided by single channel recordings but needs further validation by making measurements on channels of known structure. Preliminary results are given of single channel records establishing the effects of non-electrolytes on the conductance of gramicidin D channels. As an example of the potential uses of the procedure application is made to examine the oligomerization of alpha-toxin (alpha-hemolysin) channels. A model for the alpha-toxin hexamer, based on the crystal structure for the heptamer, is generated using molecular mechanics methods. The compatibility of the structures with single channel conductance data is assessed using HOLE.     

Structural relaxation in dense hard-sphere fluids

The long-time decay of the shear-stress autocorrelation function is shown to be quantitatively related to the decay of correlations between the orientation of bonds connecting colliding pairs of particles. Within computational uncertainties, we find that orientational correlations in high-density fluids decay as a stretched exponential in time, with an exponent that is independent of density. However, at low densities the decay is exponential. In two-dimensional systems the decay is exponential, even at high density.     

Investigations into the use of a reverse scan in a quadrupole ion trap mass spectrometer

The forward scan (i.e. an increasing RF voltage ramp for the mass-selective instability scan) is commonly used as an analytical scan for ion detection with quadrupole ion trap instruments. A number of phenomena have been observed while using this scan technique. These include space charge effects resulting in the delayed ejection of ions from the ion trap, and the fragmentation of fragile ions producing very broad peaks. Here the use of a reverse scan (i.e. a decreasing RF voltage ramp) is examined to determine the effect of the above phenomena in this acquisition method. With regard to space charge effects, the apparent reduction of the carbon isotope spacing below one Thomson (for singly charged ions) that is observed with the forward scan is now replaced by an apparent increase in this spacing. The reverse scan, which optimizes at lower axial modulation ejection voltages than the forward scan, allows for the intact ejection of fragile ions under its typical operating conditions whereas the forward scan results in fragmentation. Reducing the axial modulation voltage for the ejection of ions in the forward scan results in less dissociation of the fragile ions during ion ejection, but with the observation of ghost peaks due to incomplete ejection of all of the ions at the resonance ejection condition. While performing the reverse scan experiment, the formation of product ions from dissociation of the MH(+) ion has also been observed.     

The azulene-to-naphthalene rearrangement revisited: a DFT study of intramolecular and radical-promoted mechanisms

Intramolecular and radical-promoted mechanisms for the rearrangement of azulene to naphthalene are assessed with the aid of density functional calculations. All intramolecular mechanisms have very high activation energies (>/=350 kJ mol(-1) from azulene) and so can only be competitive at temperatures above 1000 degrees C. Two radical-promoted mechanisms, the methylene walk and spiran pathways, dominate the reaction below this temperature. The activation energy for an orbital symmetry-allowed mechanism via a bicyclobutane intermediate is 382 kJ mol(-1). The norcaradiene-vinylidene mechanism that has been proposed in order to explain the formation of small amounts of 1-phenyl-1-buten-3-ynes from flash thermolysis of azulene has an activation energy of 360 kJ mol(-1); subtle features of the B3LYP/6-31G(d) energy surface for this mechanism are discussed. All intermediates and transition states on the spiran and methylene walk radical-promoted pathways have been located at the B3LYP/6-31G(d) level. Interconversion of all n-H-azulyl radicals via hydrogen shifts was also examined, and hydrogen shifts around the five-membered ring are competitive with the mechanisms leading to rearrangement to naphthalene, but those around the seven-membered ring are not. Conversion of a tricyclic radical to the 9-H-naphthyl radical is the rate-limiting transition state on the spiran pathway, and lies 164.0 kJ mol(-1) above that of the 1-H-azulyl radical. The transition state for the degenerate hydrogen shift between the 9-H-azulyl and 10-H-azulyl radicals is 7.4 kJ mol(-1) lower. Partial equilibration of the intermediates in the spiran pathway via this shift may therefore occur, and this can account for the surprising formation of 1-methylnaphthalene from 2-methylazulene. The rate-limiting transition state for the methylene walk pathway involves the concerted transfer of a methylene group from one ring to the other and lies 182.3 kJ mol(-1) above that of the 1-H-azulyl radical. It is shown that rearrangement via a combination of 31% methylene walk and 69% spiran pathways can account semiquantitatively for all the products from 1-(13)C-azulene, 9-(13)C-azulene, and 4,7-(13)C(2)-azulene, in addition to accounting for the products from methylazulenes, and the formation of naphthalene-d(0) and -d(2) from azulene-4-d. It is also pointed out that a small extension to the spiran pathway could provide an alternative explanation for the formation of 1-phenyl-1-buten-3-ynes.