Ultrasonic studies of attenuation and dispersion in NH4Br near the order-disorder transition

Velocity and attenuation of sound wave propagation along the [100] direction in NH₄Br were measured near its transition temperature T_c = 234.5 K at frequencies of 30–150 MHz. The experimental results for the high-frequency attenuation and the dispersive velocity shifts are found to be in good agreement with the model of a single critical relaxation time.     

Mathematics learning support and engagement in first year engineering

This paper considers the effects of both free optional mathematics learning support and engagement on the mathematics performance in a foundation mathematics subject of a cohort of engineering students entering university with poor mathematical skills. New engineering students were directed to either a foundation or standard mathematics subject based on the results of a placement test. For students in the foundation subject, it was found that high levels of learning support were associated with greater improvement over the semester. Some form of learning support was used by 57.9% of the students, a reasonably high proportion of the cohort. Some factors for this high level of use of learning support are considered. One possible factor, the placement test, appears to have had a positive effect. Engagement in the subject activities as measured by tutorial attendance and learning management system use was found to have a positive association with final mark. Students who utilized a high level of learning support were more likely to be more engaged with the subject, making it impossible to draw conclusions about improvements being solely due to the use of learning support.     

Crystal structures of two isomeric oximes. (EE)-2-methyl-3-phenyl-3-thiocyanatopropenaldoxime and (Z)-5-methyl-6-phenyl-1,3-thiazin-2-one oxime

The crystal structure of two isomeric oximes, C₁₁N₁₀N₂OS have been studied by X-ray diffraction. The open-chain thiocyanato derivative is somewhat'less dense and crystallizes with the orthorhombic space group Pbcn;a,b,c=18.718(1), 10.601(3), 11.528(1) Å,Z=8. The heterocyclic isomer occurs as pseudo-orthorhombic (Cmma)twinned crystals of space group P2₁/n, a, b, c, =12.11(1), 7.46(1), 12.12(1) Å, 100.09(1)^o,Z=4. Twinning occurs as a result of the near-equality of the monoclinica andc cell constants.     

Halogen bonding in 1,2‐dibromo‐4,5‐dimethoxybenzene and 1,2‐diiodo‐4,5‐dimethoxybenzene

An interesting case of `halogen‐bonding‐promoted' crystal structure architecture is presented. The two title compounds, C₈H₈Br₂O₂ and C₈H₈I₂O₂, have almost indistinguishable molecular structures but very different spatial organization, and this is mainly due to differences in the halogen‐bonding interactions in which the different species present, i.e. Br and I, take part. The dibromo structure exhibits a π‐bonded columnar array involving all four independent molecules in the asymmetric unit, with intercolumnar interactions governed by C—Br...Br—C links and with no C—Br...O/N interactions present. In the diiodo structure, instead, the C—I...O synthon prevails, defining linear chains, in turn interlinked by C—I...I—C interactions.     

Theoretical and electrochemical studies on organometallic symmetrical schiff base complexes of Zn(II), Cu(II), Ni(II) and Co(II)

The electronic communication between two redox centres through a Schiff base complex has been investigated in a series of ethylenediimine-bis(1-ferrocenyl-1,3-butanedionate) complexes of Zn(II) 1, Cu(II) 2, Ni(II) 3 and Co(II) 4. Cyclic voltammetry experiments of 1 and 2 exhibit a unique two-electron reversible oxidation wave, whereas in the case of 3 and 4 two and three one-electron oxidation processes are, respectively, observed. These results suggest some electronic interaction between the iron atoms of the ferrocenyl groups. DFT calculations carried out on model complexes show that for all the studied compounds the removal of the first two electrons corresponds to the oxidation processes of the iron centres in the weakly coupled ferrocenyl termini. The electronic communication between the two iron centres increases on going from 1 to 4. Finally, a re-indexation of the bands observed in the UV-Visible spectra has been carried out using TDDFT calculations.     

Experimental dipole moments for nonisolatable acetic acid structures in a nonpolar medium. A combined spectroscopic, dielectric, and DFT study for self-association in solution

Acetic acid can exist in many possible structural forms depending on its surrounding medium. A recently developed inverse problem methodology (J. Phys. Chem. B 2007, 111, 13064-13074) was utilized in order to elucidate acetic acid structures in a dilute nonpolar medium. In this regard, simultaneous and stopped-flow measurements of the bulk solution densities, refractive indices, relative permittivities, and IR spectra of acetic acid in toluene were performed at several different concentrations in a semibatch closed-loop experimental setup at 298.15 K and 0.1013 MPa. This combined IR spectroscopic and dielectric, density, and refractive index analysis was employed in order to distinguish acetic acid structures and to further determine the dipole moments of the monomer, cyclic dimer, and "lumped-sum" open dimers. The infrared spectra were first analyzed to provide qualitative understanding as well as quantitative estimates for each acetic acid species. Subsequently, the dipole moments of these species were calculated using a direct approach which was primarily based on response surface models. The present method allows the determination of individual dipole moments not only for the monomer but also for the cyclic dimer and the open dimer. The results obtained from this study experimentally show that the cyclic dimer with centrosymmetric structure has a dipole moment approximately 0 D. The results also suggest that the linear dimers are present as mixtures of linear dimers structures. The existence of the linear dimers mixture was also indicated by the experimental infrared analysis of the OH-stretching region (particularly for measurements in n-hexane as solvent) and comparison of these spectra with DFT predictions. Finally, the present methodology which incorporates simultaneous physicochemical and spectroscopic analysis is undoubtedly useful for physicochemical characterization for other nonisolatable solute species and self-associated structures in solution.     

Ozonolysis of oleic acid adsorbed to polar and nonpolar aerosol particles

Single-particle kinetic studies of the reaction between oleic acid and O 3 have been conducted on two different types of core particles: polystyrene latex (PSL) and silica. Oleic acid was found to adsorb to both particle types in multilayer islands that resulted in an adsorbed layer of a total volume estimated to be less than one monolayer. The rate of the surface reaction between surface-adsorbed oleic acid and O 3 has been shown for the first time to be influenced by the composition of the aerosol substrate in a mixed organic/inorganic particle. A Langmuir-Hinshelwood mechanism was applied to the observed dependence of the pseudo-first-order rate constant with [O 3], and the resulting fit parameters for the ozone partition coefficient ( K O 3 ) and maximum first order rate constant ( k 1,max ) suggest that the reaction proceeded faster on the less polar PSL core at lower [O 3] due to the increased residence time of O 3 on the PSL surface, but the reaction was ultimately more efficient on the silica surface at high [O 3]. Values for the uptake coefficient, gamma oleic , for reaction of oleic acid on PSL spheres decrease from 2.5 x 10 (-5) to 1 x 10 (-5) with increasing [O 3] from 4 to 25 ppm and overlap at high [O 3] with the estimated values for gamma oleic on silica, which decrease from 1.6 x 10 (-5) to 1.3 x 10 (-5). The relationship between gamma oleic and the more common expression for gamma O 3 is discussed.