Characterization of archaeological frankincense by gas chromatography-mass spectrometry

A simple gas chromatography-mass spectrometry (GC-MS) method has been developed for the characterization of frankincense in archaeological samples. After trimethylsilylation of the methanolic extract, 15 triterpenoids have been found among the chemical constituents of commercial olibanum (alpha-boswellic acid, 3-O-acetyl-alpha-boswellic acid, beta-boswellic acid, 3-O-acetyl-beta-boswellic acid, alpha-amyrin, beta-amyrin, lupeol, 3-epi-beta-amyrin, 3-epi-beta-amyrin, 3-epi-lupeol, alpha-amyrenone, beta-amyrenone, lupenone, 3alpha-hydroxy-lup-20(29)-en-24-oic acid and 3-O-acetyl-hydroxy-lup-20(29)-en-24-oic acid). These compounds have been unequivocally identified by retention time and mass spectral comparison with pure standards previously isolated, for the most part, in our laboratory. Within these triterpenes, acid ones, the corresponding O-acetates, and their products of degradation were found to be characteristic of frankincense (Boswellia resin). The presence of these unusual triterpenic compounds in an archaeological resinous sample, recovered during excavations from Dahshour site (Egypt, XIIth Dynasty), enabled us to identify unambiguously frankincense resin among several other materials. Additional chromatographic peaks of this sample were assigned to broad chemical classes using retention time and mass spectra features.     

Diverse redox-active molecules bearing O-, S-, or Se-terminated tethers for attachment to silicon in studies of molecular information storage

A molecular approach to information storage employs redox-active molecules tethered to an electroactive surface. Attachment of the molecules to electroactive surfaces requires control over the nature of the tether (linker and surface attachment group). We have synthesized a collection of redox-active molecules bearing different linkers and surface anchor groups in free or protected form (hydroxy, mercapto, S-acetylthio, and Se-acetylseleno) for attachment to surfaces such as silicon, germanium, and gold. The molecules exhibit a number of cationic oxidation states, including one (ferrocene), two [zinc(II)porphyrin], three [cobalt(II)porphyrin], or four (lanthanide triple-decker sandwich compound). Electrochemical studies of monolayers of a variety of the redox-active molecules attached to Si(100) electrodes indicate that molecules exhibit a regular mode of attachment (via a Si-X bond, X = O, S, or Se), relatively homogeneous surface organization, and robust reversible electrochemical behavior. The acetyl protecting group undergoes cleavage during the surface deposition process, enabling attachment to silicon via thio or seleno groups without handling free thiols or selenols.     

Application of heat conduction calorimetry to high explosives

This paper describes some thermal analysis experiments conducted on high explosive samples. These employ differential scanning calorimetry to monitor thermal effects at elevated temperatures (around 200 °C) and heat conduction calorimetry to record thermal effects at much lower temperatures (below 100 °C).The work shows that, due to the generally high thermal stability of many high explosive compositions, heat generation rates are very low, if detectable at all, at normal storage temperatures, even when using a very sensitive instrument. The sensitivity and reproducibility of this technique has been investigated in detail by Wilker et al. [S. Wilker, U. Ticmanis, G. Pantel, Detailed investigation of sensitivity and reproducibility of heat flow calorimetry, in: Proceedings of the 11th Symposium on Chemical Problems Connected with the Stability of Explosives, Sweden, 1998] and shown to be capable of recording heat generation rates of less than a microwatt. This allows continuous measurement of decomposition processes in nitrate ester based propellants at temperatures as low as 40 °C. However, the measurement of very low levels of heat generation is difficult, time consuming and therefore expensive. If the assumption is made that the life limiting process is invariably the slow decomposition of the energetic component, this will frequently lead to very long service lifetime predictions.A number of possible complications are identified. Firstly, due to its low detection threshold, a heat conduction calorimeter may detect other reactions which will not lead to failure, but which may still dominate the heat flow signal. Secondly, the true failure process may generate little energy and be overlooked. In view of these considerations, at present it seems unwise to rely on heat conduction microcalorimetry as the only tool for the assessment of the life of high explosive energetic systems.Based on examples of life terminating processes in high explosives during storage and use, it is clear that decomposition of the energetic material is not invariably the cause of system failure. It is also by no means the only reaction that may take place in, and be observed by, a heat conduction calorimeter.     

Disorder effects in Mn12–acetate at 83 K

The structure of hexadeca‐μ‐acetato‐tetra­aqua­dodeca‐μ₃‐oxo‐dodecamanganese bis(acetic acid) tetrahydrate, [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄]·2CH₃COOH·4H₂O, known as Mn₁₂–acetate, has been determined at 83 (2) K by X‐ray diffraction methods. The fourfold (S₄) molecular symmetry is disrupted by a strong hydrogen‐bonding interaction with the disordered acetic acid mol­ecule of solvation, which displaces one of the acetate ligands in the cluster. Up to six Mn₁₂ isomers are potentially present in the crystal lattice, which differ in the number and arrangement of hydrogen‐bonded acetic acid mol­ecules. These results considerably improve the structural information available on this molecular nanomagnet, which was first synthesized and characterized by Lis [Acta Cryst. (1980), B 36 , 2042–2046].     

Influence of steric symmetry and electronic dissymmetry on the enantioselectivity in palladium-catalyzed allylic substitutions

Chiral P,N-ligands with pseudo-C2 and pseudo-Cs symmetry based on chiral pyrrolidine and phospholane rings or on dinaphthatodihydroazepino and dinaphthatodihydrophosphepino moieties were prepared and assessed in the palladium-catalyzed allylic substitutions of allylic acetates. Higher selectivity was achieved with pseudo-C2-symmetric ligands based on the binaphthyl skeleton than with the analogous C2-symmetric P,P- and N,N-analogues. Pseudo-C2-type ligands had properties superior to those of pseudo-meso-type ligands when 1,3-diphenyl-2-propenyl acetate was used as a substrate, whereas the reverse situation was found for 3-cyclohexenyl acetate. Chirally flexible ligands, prepared by substitution of one of the rigid binaphthyl skeletons for a flexible biphenyl system, were found to induce chirality to the same extent as a 1:1 mixture of the rigid ligands.     

What a difference ancillary thienyl makes: unexpected additional stabilization of the diruthenium(III,II) but not the diosmium(III,II) mixed-valent state in tetrazine ligand-bridged complexes

The Ru(2)(III,II) mixed-valent state is strongly stabilized in [(bpy)(2)Ru(mu-bttz)Ru(bpy)(2)](5+) (3(5+), bttz = 3,6-bis(2-thienyl)-1,2,4,5-tetrazine, as evident from lowered oxidation potentials and isolability, a strongly increased comproportionation constant K(c) = 10(16.6), and a high-energy intervalence charge transfer band at 10100 cm(-1). Curiously, no such effects were observed for the diosmium(III,II) analogue, whereas the related systems [(bpy)(2)M(mu-bmptz)M(bpy)(2)](5+), bmptz = 3,6-bis(4-methyl-2-pyridyl)-1,2,4,5-tetrazine, exhibit conventional behavior, i.e., a slightly higher K(c) value of the Os(2)(III,II) analogue. EPR signals were observed at 4 K for 3(5+) but not for the other mixed-valent species, and high-frequency (285 GHz) EPR was employed to study the diruthenium(II) radical complexes 2(3+) and 3(3+).     

Experimental analysis of floc size distributions in a 1-L jar under different hydrodynamics and physicochemical conditions

This study focuses on the relation among hydrodynamics, physicochemical conditions, and floc size. During ortho-kinetic flocculation, the floc size is controlled by a balance between hydrodynamic stress and aggregate strength. Special attention was paid to the influence of a hydrodynamic sequencing on both the aggregate strength and the flocculation processes. Experimental research was conducted in a 1-L jar for two different pH values. The hydrodynamic sequencing was made up of successive slow and rapid mixing periods, and different slow mixing intensities were studied. First, the large floc size was shown to decrease with increasing velocity gradient (G), with an expected trend (d proportional variant epsilon(-1/4)). Then, the aggregate strength was shown to depend on two main factors: the flocculation history and the physicochemical conditions, which control the cohesion forces between primary particles. Finally, flocculation processes are discussed in terms of aggregation and breakup phenomena, with relation to local hydrodynamics and physicochemical conditions.     

Highly stereoselective C-allylation of an enantiopure alpha-sulfinyl thioacetamide

The alkylation of the lithium enolate of enantiopure alpha-cyclohexylsulfinyl thioacetamide 1 with allyl bromides 5 possessing an electron-withdrawing group at the vinylic position does not occur at the sulfur center - as expected in the sulfur series - but at the carbon center through conjugate addition followed by bromide elimination. The modest to excellent 1,2-asymmetric induction achieved by the alkylsulfinyl group (dr up to 100:0) is explained by an electronic model.     

Two new terpyridine dimanganese complexes: a manganese(III,III) complex with a single unsupported oxo bridge and a manganese(III,IV) complex with a dioxo bridge. Synthesis, structure, and redox properties

Two new terpyridine dimanganese oxo complexes [Mn(2)(III,IV)(mu-O)(2)(terpy)(2)(CF(3)CO(2))(2)](+) (3) and [Mn(2)(III,III)(mu-O)(terpy)(2)(CF(3)CO(2))(4)] (4) (terpy = 2,2':6,2' '-terpyridine) have been synthesized and their X-ray structures determined. In contrast to the corresponding mixed-valent aqua complex [Mn(2)(III,IV)(mu-O)(2)(terpy)(2)(H(2)O)(2)](3+) (1), the two Mn atoms in 3 are not crystallographically equivalent. The neutral binuclear monooxo manganese(III,III) complex 4 exhibits two crystallographic forms having cis and trans configurations. In the cis complex, the two CF(3)CO(2)(-) ligands on each manganese adopt a cis geometry to each other; one CF(3)CO(2)(-) is trans to the oxygen of the oxo bridge while the second is cis. In the trans complex, the two coordinated CF(3)CO(2)(-) have a trans geometry to each other and are cis to the oxo bridge. The electrochemical behavior of 3 in organic medium (CH(3)CN) shows that this complex could be oxidized into its corresponding stable manganese(IV,IV) species while its reduced form manganese(III,III) is very unstable and leads by a disproportionation process to Mn(II) and Mn(IV) complexes. Complex 4 is only stable in the solid state, and it disproportionates spontaneously in CH(3)CN solution into the mixed-valent complex 3 and the mononuclear complex [Mn(II)(terpy)(2)](2+) (2), thereby preventing the observation of its electrochemical behavior.     

A high-frequency and high-field EPR study of new azide and fluoride mononuclear Mn(III) complexes

The isolation, structural characterization and electronic properties of three new six-coordinated Mn(III) complexes, [Mn(bpea)(F)(3)] (1), [Mn(bpea)(N(3))(3)] (2), and [Mn(terpy)(F)(3)] (3) are reported (bpea = N,N-bis(2-pyridylmethyl)-ethylamine; terpy = 2,2':6',2' '-terpyridine). As for [Mn(terpy)(N(3))(3)] (4) (previously described by Limburg J.; Vrettos J. S.; Crabtree R. H.; Brudvig G. W.; de Paula J. C.; Hassan A.; Barra A-L.; Duboc-Toia C.; Collomb M-N. Inorg. Chem. 2001, 40, 1698), all these complexes exhibit a Jahn-Teller distortion of the octahedron characteristic of high-spin Mn(III) (S = 2). The analysis of the crystallographic data shows an elongation along the tetragonal axis of the octahedron for complexes 1 and 3, while complex 2 presents an unexpected compression. The electronic properties were investigated using a high-field and high-frequency EPR study performed between 5 and 15 K (190-575 GHz). The spin Hamiltonian parameters determined in solid state are in agreement with the geometry of the complexes observed in the crystal structures. A negative D value found for 1 and 3 is related to the elongated tetragonal distortion, whereas the positive D value determined for 2 is in accordance with a compressed octahedron. The high E/D values, in the range of 0.103 to 0.230 for all complexes, are correlated with the highly distorted geometry present around the Mn(III) ion. HF-EPR experiments were also performed on complex 1 in solution and show that the D value is the only spin Hamiltonian parameter which is slightly modified compared to the solid state (D = -3.67 cm(-1) in solid state; D = -3.95 cm(-1) in solution).