Geometries of type Cn and F4 with flag-transitive automorphism groups

Let be a finite thick geometry of type C_n (n 4) or F₄. We prove that is a building iff Aut() is flag-transitive.     

Crystal and molecular structure of nitrosyl(1,10-phenanthroline)-bis(triphenylphosphine)iridium(I) Dihexafluorophosphate, [Ir(NO)(phen)(PPh3)2][PF6]2

Summary The structure of [Ir(NO)(phen)(PPh₃)₂][PF₆]₂ has been determined from x-ray diffractometer data. The compound crystallizes in space groupPnam with four molecules in a unit cell witha = 19.924(12),b = 14.793(9) andc = 16.348(9) A. Full-matrix least-squares refinement has led to a final R value of 0.061 for the 4796 observed reflections. The structure consists of well-separated ions, and the geometry around the metal is trigonal bipyramidal with nitrosyl and bidentate 1,10-phenanthroline (in spite of the very narrow bite angle of 75.8°) ligands occupying the equatorial positions and the triphenylphosphine ligands the axial positions. The cation has an imposed crystallographicm symmetry. Important bond lengths are as follows: Ir-P, 2.391(3): Ir-N (nitrosyl) 1.700(12): Ir-N (1,10-phenanthroline) 2.103(12) and 2.142(11): N-O, 1.201(18)A. The nitrosyl ligand is linear [Ir-N-O = 179.9(9)°] so that this complex can be formulated as an NO⁺ complex of iridium(I).     

Hemithio-analogues of meisenheimer complexes and sulphide group activation of aromatic substitution by alkoxides

EtO^? in DMSO adds to ethylthiopicrate at C-3 to generate an ephemeral

-adduct which ends into 1-ethylthio-4-ethoxy-2,5-dinitrobenzene, whilst ethyl picrate gives two adducts of attack of EtS^? at C-3 or C-1.     

On the 2 and the 3-connectedness of ample divisors on a surface

Let X be a nonsingular complex projective surface and let D be an ample divisor on X such that the associated invertible sheaf is spanned by its global sections. We prove that D is 2-connected apart from a few cases we explicitly describe. We also provide a corresponding result for the 3-connectedness when D²10 and for the 4-connectedness when D²17 and D is very ample.     

Sugli insiemi di permutazioni strettamente k-transitivi (k ≥ 3)

Summary Let G be a sharply 3-transitive permutation set on a finite set E of even cardinality and let 1 be in G. The following theorems are proved. G is one of the known examples if and only if there exists a non-identity normal subgroup N of G and an element of E such that NG G.G is a group if and only if G for every G and for every G and for every G .By using the classification of finite single groups a result concerning sharply k-transitive permutation sets k>3 is also proved.

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Dedicato a Guido Zappa in occasione del suo 70° compleanno

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Lavoro eseguito nell'ambito dei progetti finanziati dal Ministero della Pubblica Istruzione.     

The crystal structure of tris(acetonitrile)nitrosylbis(triphenylphosphine)iridium(III) dihexafluorophosphate. An unusual iridium nitrosyl hexacoordinate complex

Summary The structure of [Ir(NCMe)₃(NO)(PPh₃)₂][PF₆]₂ has been ] determined by x-ray methods. Crystals are orthorhombic, space groupPca 2₁ , witha = 21.753(14),b = 11.678(10),c = 18.474(12) Å and Z = 4. The structure has been solved from diffractometer data and refined by full-matrix leastsquares to R = 0.076 for 2776 observed reflections. The cation is a hexacoordinate and not a pentacoordinate species as expected. The extra acetonitrile molecule,trans to the nitrosyl ligand, is much more weakly bound to the metal atom [Ir-N 2.360(26) against 1.965(20) and 1.912(14) Å for the other two acetonitriles]. The nitrosyl is bent [Ir-N-O 111(1)° Å] and acts as the formally one-electron donor NO^–.     

Energetics of the Preyssler anion's molecular orbitals: quantifying the effect of the encapsulated-cation's charge

The ground state electronic properties of metal-exchanged Preyssler heteropolyoxoanions [M(n+)P(5)W(30)O(110)](n-15), in which the encapsulated M(n+) ions are the spherical, diamagnetic ions Na(+), Ca(2+), Sr(2+), Y(3+), La(3+) and Th(4+), are studied using a combination of electrochemical, optical, and NMR experiments. We have designed experiments that focus on the influence of the charge (n) of the encapsulated cations, which themselves have no redox response, and its effect upon the W-O framework MOs. As n increases, the cluster anions accept electrons into their LUMOs with increasing ease, and their lowest-energy LMCT bands reveal a corresponding blue shift, which is indicative of an increase of the LUMO-HOMO energy splitting with increasing n. (183)W NMR spectra are used to identify the atomic origin of the LUMO states, which are shown to be composed primarily of orbitals from the ring of 5 W atoms near M(n+). The cation charge correlates directly and linearly with the half-wave potentials of the first redox couples, the LMCT band energies, and the W chemical shifts. We have combined this suite of experimental results to construct an energy level diagram of the frontier MOs for the Preyssler cluster anions. In so doing, we provide a fundamental perspective that is not otherwise available on the cation's role with specific regard to the electronic behavior of the W-O orbitals. These results are expected to provide benchmarking information as theorists begin to study these large POM systems.     

Mass spectrometry of heterocyclic compounds—III. Electron-impact-induced fragmentation of 1,2-benzisothiazole and some 3-substituted derivatives

The mass spectra of fifteen 1,2-benzisothiazoles are reported; their fragmentation patterns have been investigated by labelling experiments, high resolution mass measurements and defocused metastable ion detection. The parent compound of the series, 1,2-benzisothiazole, eliminates HCN after partial inter-ring hydrogen scrambling. The extent of scrambling has been determined for normal daughter ions produced at different electron beam energies and also for metastable daughter ions, and is compared with the data reported for benzothiazole. Some 3-substituted derivatives show unusual fragmentation patterns. The mechanisms of these processes are under further investigation.     

Amide-water interactions in cosolvent systems. I. Solubilities and apparent molar volumes of methyl paraben

Solubility and apparent molar volume data are used to demonstrate effects of amide alkylation on amide-water interactions at 25° C. Precise measurements were made of the apparent molar volumes of the amides in binary amide-water mixtures using a dilatometric technique. The results show that the apparent molar volumes of alkyl-substituted amides in water pass through a minimum at an amide concentration which varies inversely with the degree of alkylation. Further studies showed that the solubilities of methyl paraben (methyl-p-hydroxybenzoate) and naphthalene in various amide-water solvent systems increased in characteristic fashion with amide alkylation.     

Convenient synthesis of nucleoside and isonucleoside analogues

[reaction: see text]. A very simple methodology to stereoselectively achieve tricyclic isonucleosides (nucleobase = thymine, uracil, and 5-fluoruracil) and 3'-C-branched nucleosides (nucleobase = theophylline) was performed by means of a DBU-mediated addition process using a readily available 2-bromo sugar. The mechanism for these transformations implies the loss of both substituents at C-2 and C-3 on the sugar moiety, and although it seems that DBU is probably involved, its involvement has not yet been ascertained. Cytosine did not react under these conditions.