Performance evaluation of transfer lines with general repair times and multiple failure modes

The paper proposes a decomposition method for evaluating the performance of transfer lines where machines can fail in multiple modes and can be repaired with non-exponential times. Indeed, while times to machine failure can be often modeled using exponential distributions with acceptable accuracy, times to repair are very rarely observed to be exponentially distributed in actual systems. This feature limits the applicability of existing approximate analytical methods to real production lines. In this paper, the discrete acyclic phase-type distribution is used to model the repair process, for each failure mode of the machines composing the system. The exact analysis of the two-machine system is used as a building block for the decomposition method, proposed to study multi-stage lines. Numerical results show the high accuracy of the developed method in estimating the average throughput and buffer levels.     

Tautomerism in 4-hydroxypyrimidine, S-methyl-2-thiouracil, and 2-thiouracil

The keto-enol tautomerism of 4-hydroxypyrimidine and of the related molecules S-methyl-2-thiouracil and 2-thiouracil has been investigated using synchrotron-based techniques. The populations of the constituent tautomers and thermodynamic parameters have been obtained by analysis of core-level photoemission spectra. The effect of substituents on the stability of tautomers has been revealed. Attaching additional OH (or SH) groups to the aromatic ring stabilizes the dioxo (or oxo-thione) forms. However, substitution of hydrogen in position 2 by an S-CH(3) group (that is, in going from 4-hydroxypyrimidine to S-methyl-2-thiouracil) does not significantly affect the tautomeric equilibrium.     

Molecular structure of nitrobenzene in the planar and orthogonal conformations

The molecular structure and ring distortions of nitrobenzene have been determined by gas-phase electron diffraction and ab initio molecular orbital (MO) calculations as well as from the structures of six derivatives studied by X-ray crystallography. The experimental value of the ring angle at the ipso position is = 123.4 ± 0.3° in the free molecule; this is about 1.5° less than the hitherto reported values. Regression analysis of the ring angles in the six derivatives studied by X-ray crystallography yields = 122.7(1)° for nitrobenzene in a crystalline environment. The small difference in the two values of a is interpreted as an effect of intermolecular interactions in the crystal. The value produced by the MO calculations, = 122.3° at the 6–31G^* (5D) level, is smaller than either of the experimental results. As regards the ring angles at the meta and para positions, the three techniques of structure determination consistently indicate that these are larger than 120° by a few tenths of a degree. Other important geometrical parameters from the electron diffraction study are r _g (C-C) = 1.399 ± 0.003 Å,r _g (C-N) = 1.486 ± 0.004 Å,r _g (N-O) = 1.223 ± 0.003 Å, and A sO-N-O = 125.3 ± 0.2°. X-ray diffraction experiments on 3,5-dimethyl-4-nitrobenzoic acid and 3,5-dimethylbenzoic acid and ab initio MO calculations provide solid evidence that the geometry of nitrobenzene is little affected when the nitrogroup is twisted by 90° out of the planar equilibrium conformation. This indicates that the extent of -electron transfer from the benzene ring to the nitro group is small. The barrier to rotation is estimated to be 17 ± 4 kJ mol^–1 from the electron diffraction data.     

The dimensionally reduced effective theory for quarks in high-temperature QCD

We show that QCD undergoes a partial dimensional reduction at high temperatures also in the quark sector. In the kinematic region relevant to screening physics, where the lowest Matsubara modes are close to their “mass shells”, all static Green functions involving both quarks and gluons are reproducible in the high-T limit by a renormalizable three-dimensional Lagrangian up to order ²(T) 1/1nT. This three-dimensional theory only contains explicitly the lightest bosonic and fermionic Matsubara modes, while the heavier modes correct the tree-level couplings and generate extra local vertices. We also find that the quark degrees of freedom that have been retained in the reduced theory are non-relativistic in the high-T limit. We then improve our result to order ⁴(T) through an explicit non-relativistic expansion, in the spirit of the heavy-quark effective theory. This effective theory is relevant for studying QCD screening phenomena with observables made from quarks, e.g. mesonic and baryonic currents, already at temperatures not much higher than the chiral transition temperature T_c.     

Synthesis of enantiomerically pure β-azidoselenides starting from natural terpenes

Several unsaturated natural terpenes have been easily converted, in good yields, into the corresponding enantiomerically pure β-azidoselenides by addition of the electrophilic selenium reagent PhSeOTf in the presence of sodium azide. These reactions are stereospecific anti additions, which occur with a Markovnikov orientation. Examples of the synthetic importance of these β-azidoselenides are also reported.     

Symmetry and Phase-Selected NMR Spectra of Liquid Crystalline Samples

It is demonstrated that the NMR spectra of liquid crystalline samples can be simplified by using multiple quantum filtering. In a system of N spin- nuclei, the N or (N-1)-multiple quantum filtered spectra (NQF or (N-1)QF) contain lines which originate only from transitions among the eigenstates belonging to the highest symmetry class of the spin permutation group. In addition the NQF spectra are divided further into two sets of lines which differ in phase by 180°. A method for simulating and analysing multiple quantum filtered spectra is described, with examples from molecules with up to eight interacting spins.     

Phytochemical study of Cistus libanotis L.

n continuation of our ongoing study on Mediterranean Flora, we focused the attention on Cistus genus. These plants possess interesting secondary metabolites and are used in many fields, principally in perfumery and more recently as raw material for food supplements (botanicals). n this article, we report the phytochemical analysis of Cistus libanotis L. from Tunisia. Among the diterpenes, labdane compounds resulted absent, in favour of two clerodanes, one of that never reported in Cistus sp. The main representative compounds were found to be several flavonoids with various grades of O-methylation. Other interesting components were two cinnamic esters of borneol, reported here for the first time in Cistus. The identified compounds confirm in part the reported biological properties and add chemotaxonomic data to this complicated genus.     

An Experimental and Theoretical Investigation on Pentacoordinated Cobalt(III) Complexes with an Intermediate S=1 Spin State: How Halide Ligands Affect their Magnetic Anisotropy

Understanding the factors that control the magnitude and symmetry of magnetic anisotropy should facilitate the rational design of mononuclear metal complexes in the quest for single‐molecule magnets (SMMs), based on a single metal ion, with high blocking temperatures and large energy barriers. The best strategy is to define magnetostructural correlations through the investigation of a series of metal complexes. It has been demonstrated that the main contribution to the magnetic anisotropy arises from the spin‐orbit coupling (SOC) effect in metal‐ion‐based systems, so current studies focus particularly on the use of both ligands and metal ions possessing a large SOC. In this context, we report a unique series of halide Co^III complexes, [CoL(X)], with X=Cl, Br, I ( CoX ) and L=2,2′‐(2,2′‐bipyridine‐6,6′‐diyl)bis(1,1‐diphenylethanethiolate), which possess a rare intermediate S=1 spin ground state. The S=1 Co^III complexes are attractive species because they possess a remarkably large axial zero‐field splitting (defined by D from the following Hamiltonian: H =DS_z²), as well as the halide ligands inducing large SOC constants. The single‐crystal X‐ray structures reveal that the CoBr and CoI complexes are isostructural with the previously described CoCl complex. Their coordination sphere displays a distorted pentacoordinated square pyramidal geometry, with the halide located in the Co^III axial position. Large positive D values of 35, 26, and 18 cm^?1 are found for CoCl , CoBr , and CoI , respectively, through analysis of the magnetic susceptibility data as a function of temperature. To rationalize this trend, theoretical calculations based on both density functional theory (DFT) and complete active space self‐consistent field (CASSCF) methods are performed successfully. Both the sign and magnitude of D are predicted remarkably well by these theoretical approaches. The DFT calculations also show that the resulting D values originate from a balance of several contributions, and that many factors, including differences in their structural properties and in the contribution of the halide, should be taken into account to explain the trend of D in this series of complexes.