2′‐Deoxy‐5‐methylisocytidine

In the title compound, 2‐amino‐1‐(2‐deoxy‐β‐d ‐erythro‐pento­furan­osyl)‐5‐methyl­pyrimidin‐4(1H)‐one, C₁₀H₁₅N₃O₄, the conformation of the N‐glycosidic bond is syn and the 2‐deoxy­ribo­furan­ose moiety adopts an unusual ^OT₁ sugar pucker. The orientation of the exocyclic C4′—C5′ bond is +sc (+gauche).     

Quantum Cosmology in Higher Derivative and Scalar-Tensor Gravity

The Bicknell theorem states that a non-linear Lagrangian can be recast in the form of a scalar-tensor theory, with a suitable potential, through a conformal transformation. In this paper, we first show that such classical equivalence remains valid at the level of the Wheeler—deWitt equation. Then, we consider a specific case, represented by a Lagrangian f(R) = R + l^–2(l²R)^4/3 whose vacuum cosmological solutions describe a non-singular Universe. The corresponding scalar-tensor theory and its cosmological solutions are written down. We find again non-singular solutions. The Wheeler—deWitt equation for this case is analyzed. The application of the Bicknell theorem leads to the interpretation of the behaviour of the scale factor in terms of the matter content, represented by the scalar field, and consequently to the energy conditions. The problem of classical and quantum regime is discussed and the classical behaviour is recovered, from the quantum solutions, near the maximum of the scale factor where the strong energy condition is satisfied.     

Phosphor‐Chalkogen‐Moleküle als Komplexliganden – Reaktionen mit NbCl5

Phosphorus Chalcogen Molecules as Complex Ligands – Reactions with NbCl₅ The reaction of P₄E₃ (E = S, Se) with NbCl₅ yields [β‐P₄S₄(NbCl₅)₂] and [P₄Se₃(NbCl₅)]. [β‐P₄S₄(NbCl₅)₂] crystallizes in the monoclinic space group P2₁/n with the lattice parameters a = 6.226(1), b = 12.971(2), c = 26.380(2) Å, β = 93.7(1) (Z = 4). In this compound a sulfur atom is introduced into the basal P₃‐ring and the resulting β‐P₄S₄ is central unit of the complex. [P₄Se₃(NbCl₅)] crystallizes in the same space group type with the lattice parameters a = 11.939(1), b = 18.603(2), c = 12.763(4) Å, β = 90.16(2)° (Z = 8). In both compounds the ligands are coordinated to basal phosphorus atoms.     

1,N6‐Etheno derivative of 7‐de­aza‐2,8‐di­aza­adenosine

In the tricyclic nucleoside 7‐(β‐d ‐ribo­furan­osyl)‐7H‐imidazo­[1,2‐c]­pyrazolo­[4,3‐e][1,2,3]­triazine, C₁₁H₁₂N₆O₄, the con­formation of the N‐gly­cosyl bond is intermediate between anti and high anti [χ = −103.5 (3)°]. The ribo­furan­ose moiety adopts a ₃T² sugar pucker (S‐type sugar) and the conformation at the exocyclic C—C bond is ap (gauche–trans). Molecules of the title compound form a three‐dimensional network via three medium–strong intermolecular hydrogen bonds (one O—H⋯N and two O—H⋯O bonds).     

Dynamics and burning limits of near-limit hot,warm, and cool diffusion flames of dimethyl ether at elevated pressures

The near-limit diffusion flame regimes and extinction limits of dimethyl ether at elevated pressures and temperatures are examined numerically in the counterflow geometry with and without radiation at different oxygen concentrations. It is found that there are three different flame regimes—hot flame, warm flame, and cool flame—which exist, respectively, at high, intermediate, and low temperatures. Furthermore, they are governed by three distinct chain-branching reaction pathways. The results demonstrate that the warm flame has a double reaction zone structure and plays a critical role in the transition between cool and hot flames. It is also shown that the cool flame can be formed in several different ways: by either radiative extinction or stretch extinction of a hot flame or by stretch extinction of a warm flame. A warm flame can also be formed by radiative extinction of a hot flame or ignition of a cool flame. A general €-shaped flammability diagram showing the burning limits of all three flame regimes at different oxygen mole fractions is obtained. The results show that thermal radiation, reactant concentration, temperature, and pressure all have significant impacts on the flammable regions of the three flame regimes. Increases in oxidizer temperature, oxygen concentration, and pressure shift the cool flame regime to higher stretch rates and cause the warm flame to have two extinction limits. At elevated temperatures, it is found that there is a direct transition between the hot flame and warm flame at low stretch rates. The results also show that, unlike the hot flame, the cool flame structure cannot be scaled by using pressure-weighted stretch rates due to the its significant reactant leakage and strong dependence of reactivity on pressure. The present results advance the understanding of near-limit flame dynamics and provide guidance for experimental observation of different flame regimes.     

DME/Oxygen wall-stabilized premixed cool flame

Wall-stabilized cool flames have been studied through numerical analysis and a series of experiments. One- and two-dimensional numerical simulations were performed to estimate the characteristics of the wall-stabilized cool flames, such as flammability and temperature/species distributions. Based on the computational results, the ignition condition of the cool flame at a fixed wall temperature has been identified with the strain rate between the van't Hoff point and the cool flame extinction point. In experiments with an impinging jet burner and a heated plate, the spontaneous ignition of the cool flame on the heated wall has been successfully established under the conditions predicted by the present numerical simulations. Spatial distributions of the HCHO concentration and flame temperature were measured through formaldehyde Planar Laser-Induced Fluorescence (HCHO-PLIF) and thermocouple measurements, respectively. It is found that the measurement data show a reasonable accordance with the simulation results with reduced low-temperature reactivity.     

SIMS Analysis of the wear of boron nitride tools for the machining of compacted graphite iron and grey cast iron

Cubic boron nitride (cBN) is a common material for tools for the machining of cast irons at high cutting speed. During the machining of compacted graphite iron (CGI) in continuous cutting the wear of the cBN tools was found to be significantly higher compared to the machining of grey cast iron. This is possibly a result of a heating of the tool surface during the cutting of CGI. One possible reason for the wear is diffusion of some elements from the cutting tool into the CGI or from the CGI into the cutting tool. SIMS measurements were carried out which prove the existence of such diffusion processes. A static model experiment has been performed by heating cBN tools to 700 °C while in contact with CGI or cast iron (CI). SIMS depth profiles of the cBN tools and of CGI/CI show that there is a diffusion of several elements in both directions (B, W and Ti from the cBN tools into the CGI or CI, Fe and Si from the CGI or CI samples into the cBN) up to a depth of 20 μm. Received: 18 March 1999 / Revised: 16 June 1999 / Accepted: 18 June 1999