Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy

Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide (HCN), carbon monoxide (CO), formaldehyde (H2CO), and water (H2O) have been observed from 500 to 2400 GHz. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of HCN and CO. The quantification of the measurement was validated by the means of a calibration gas containing CO. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of HCN equal to 9 parts in 10(6).     

A kinetic model for Brownian motion

Summary The Fokker-Planck equation for the distribution function of a Brownian sphere is derived from the exact hierarchy of kinetic equations for a massive sphere in a bath of smaller spheres, using a multiple-time-scale analysis. Our earlier derivation is specialized to the limiting cases where the bath is either an ideal or Boltzmann gas. The resulting simplifications allow more physical insight, and lead to explicit expressions for the friction coefficient. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Crystallographic and magnetic order analysis of a banded limonite

Mössbauer and XRD studies of the light and dark brown parts of a banded limonite showed the presence of kaolinite in the light brown regions. Relaxation behaviour was very well described by a recently proposed model based on magnetic ordering of clusters. The difference in relaxation behaviour of the samples was attributed to different concentrations of non-magnetic impurities.     

Far infrared generation by CO2 lasers frequencies subtraction in a ZnGeP2 crystal

a pulsed radiation power of the order of 1W in the submillimeter range (102–110 µm) was reached for the first time, with the aid of a ZnGeP₂ crystal, at the difference frequency of CO₂ laser radiations.     

On the Brownian motion of a massive sphere suspended in a hard-sphere fluid. II. Molecular dynamics estimates of the friction coefficient

The friction coefficient exerted by a hard-sphere fluid on an infinitely massive Brownian sphere is calculated for several size ratios , where and are the diameters of the Brownian and fluid spheres, respectively. The exact microscopic expression derived in part I of this work from kinetic theory is transformed and shown to be proportional to the time integral of the autocorrelation function of the momentum transferred from the fluid to the Brownian sphere during instantaneous collisions. Three different methods are described to extract the friction coefficient from molecular dynamics simulations carried out onfinite systems. The three independent methods lead to estimates of which agree within statisticalerrors (typically 5%). The results are compared to the predictions of Enskog theory and of the hydrodynamic Stokes law. The former breaks down as the size ratio and/or the packing fraction of the fluid increase. Somewhat surprisingly, Stokes' law is found to hold withstick boundary conditions, in the range 1/4.5 explored in the present simulations, with a hydrodynamic diameterd=. The analysis of the moleuclar dynamics data on the basis of Stokes' law withslip boundary conditions is less conclusive, although the right trend is found as / increases.     

Ab initio static and molecular dynamics study of 4-styrylpyridine.

We report an in‐depth theoretical study of 4‐styrylpyridine in its singlet S₀ ground state. The geometries and the relative stabilities of the trans and cis isomers were investigated within density functional theory (DFT) as well as within Hartree–Fock (HF), second‐order Møller–Plesset (MP2), and coupled cluster (CC) theories. The DFT calculations were performed using the B3LYP and PBE functionals, with basis sets of different qualities, and gave results that are very consistent with each other. The molecular structure is thus predicted to be planar at the energy minimum, which is associated with the trans conformation, and to become markedly twisted at the minimum of higher energy, which is associated with the cis conformation. The results of the calculations performed with the post‐HF methods approach those obtained with the DFT methods, provided that the level of treatment of the electronic correlation is high enough and that sufficiently flexible basis sets are used. Calculations carried out within DFT also allowed the determination of the geometry and the energy of the molecule at the biradicaloid transition state associated with the thermal cis?trans isomerization and at the transition states associated with the enantiomerization of the cis isomer and with the rotations of the pyridinyl and phenyl groups in the trans and cis isomers. Car–Parrinello molecular dynamics simulations were also performed at 50, 150, and 300 K using the PBE functional. The studies allowed us to evidence the highly flexible nature of the molecule in both conformations. In particular, the trans isomer was found to exist mainly in a nonplanar form at finite temperatures, while the rotation of the pyridinyl ring in the cis isomer was incidentally observed to take place within ≈1 ps during the simulation carried out at 150 K on this isomer.     

A novel extended covalent tripod for assembling nine-coordinate lanthanide(III) podates: a delicate balance between flexibility and rigidity

The introduction of long semirigid spacers between the capping carbon atom of the tripod and the unsymmetrical tridentate binding units provides the novel, extended covalent podand tris-[2-[2-(6-diethylcarbamoylpyridin-2-yl)-1-ethyl-1H-benzoimidazol-5-yl-methoxy]ethyl]methane (L(15)). Reaction of L(15) with lanthanide(III) in acetonitrile produces stable podates [Ln(L(15))](3+) (Ln=La-Lu) in which three tridentate binding units are facially organized. These wrap around the nine-coordinate pseudo-tricapped trigonal-prismatic metal ions. The crystal structure of [La(L(15))](ClO(4))(3) (18, LaC(67)H(82)N(12)O(18)Cl(3), trigonal, R3c, Z=6) reveals the formation of a C(3)-symmetrical triple-helical podate. Two slightly different arrangements of the flexible ethylenoxy parts of the spacer are observed in the solid state in agreement with the formation of two conformational isomers (M:m) in a 4:1 ratio. A qualitative analysis of the aromatic diamagnetic anisotropies affecting the NMR signals of [Ln(L(15))](3+) (Ln=La, Y, Lu) in solution, combined with the quantitative determination of electron-induced relaxation in the paramagnetic complex [Nd(L(15))](3+), demonstrate that the solid state structure is maintained in solution. This leads to a mixture of two triple-helical conformers of similar stabilities and that do not interconvert on the NMR timescale between 243 and 343 K. Particular attention has been given to the structural programming of extended covalent tripods for facially organizing unsymmetrical tridentate binding units around Ln(III). Photophysical measurements show that L(15) efficiently protects the metallic coordination spheres and sensitizes Eu(III) and Tb(III) upon UV irradiation.     

Ethene epoxidation selectivity inhibited by twisted oxametallacycle: a DFT study on Ag surface-oxide

Competitive ethene oxidation pathways are presented for a p(4 x 4) surface-oxide phase on Ag(111) obtained from density functional theory (DFT) calculations. Both parallel routes are found to proceed from a common oxametallacycle intermediate (OMME) in agreement with previous mechanistic studies on low coverage O adatom phase, although acetaldehyde (AcH) is favored by almost 2 kcal/mol. An even more striking difference with pure metal surface appears with the oxide regeneration pathways, which are found non-rate controlling. Furthermore, a kinetic model is developed on the basis of these DFT calculations and yields 96% selectivity in favor of AcH for a simulation in realistic catalytic conditions (600 K and respective partial pressures of 1 atm for ethene and oxygen reactants). As a key finding, this low ethene epoxide selectivity is proposed to be directly linked to the conformational barrier of the pivotal intermediate. In fact, the elasticity of the ultrathin oxide adlayer enables a twisted OMME structure as a true minimum, which agrees well with orbital prerequisite of the concerted H migration toward AcH. On the contrary, the desired selective ring closure forming ethene epoxide (EO) requires conformational inversion although the eclipsed form lies 2 kcal/mol above.