Pseudosymmetry in pyridinium ­tetrachloro(oxo)pyridineniobate(V) pyridine solvate

The title compound, (C₅H₆N)[NbCl₄O(C₅H₅N)]·C₅H₅N, crystallizes as discrete ions, with a very strong linear N—H?N hydrogen‐bonding interaction between the cation and the solvate pyridine molecule [N?N 2.755 (5) Å]. All chemical species occupy crystallographic twofold axes. The ligated and solvate pyridines form ABABAB stacks in the lattice. There is pseudosymmetry which emulates a centred unit cell in Amm2, but it is not supported by the diffraction pattern, which is consistent with the correct space group Pnc2. Three crystallographic software packages suggested space group Amm2 over Pnc2, while a fourth indicated Pnc2, a subgroup of Amm2.     

Surface hydroxylation at a Zn(0001)O surface

We report the investigation of the hydroxylation of a Zn(0001)O surface by water vapour using both valence and core level spectroscopies. Hydroxylation is induced by pre-exposure of the Zn(0001) surface to oxygen and occurs in the temperature range 80–200 K. Dehydroxylation is observed above 250K in two steps: $\text{OH}{}^{\mathrm{?}}\text{(a)→o}{}^{\mathrm{?}}\text{(a)+}\text{1}\text{2}\text{H}{}_2\text{(g)}$ followed by O^?(a) → O^2?(b).     

Uniqueness in the Cauchy problem for weakly hyperbolic operators not satisfying Levi conditions

Local uniqueness of solutions to the Cauchy problem is shown for certain classes of operators whose principal part is an even power of a strictly hyperbolic operator and whose lower order terms satisfy finite order zero conditions instead of the usual Levi conditions.     

On Ramsey numbers for linear forests

For a graph G, the Ramsey number r(G) is the smallest natural number p such that given a graph H with p points. H or H? contains a G. An (n,j)-linear forest L is the disjoint union of nontrivial paths, j of which have an odd number of points, and such that the union has n points. If L is an (n,j)-linear forest, then we show that

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Nitrogen-15 nuclear magnetic resonance spectroscopic study of compounds with nitrogen?nitrogen bonds

The ¹⁵N NMR spectra of several substances with nitrogen—nitrogen bonds have been obtained at the natural-abundance level by high-resolution NMR spectroscopy. Azo (? N?N? ) nitrogens are 300–500 ppm deshielded compared with hydrazo (? NH? NH? ) nitrogens. The sensitivity of the ¹⁵N shifts in these types of substances to substituent changes are reported, along with ¹⁵N–¹H spin–spin couplings for some hydrazo compounds. Hydrogen-bonding effects arising from solvent changes on the ¹⁵N shifts of azoxybenzene are different for its two kinds of nitrogen. The ¹⁵N NMR spectrum of N,N'-dinitrosopiperazine in dimethyl sulfoxide at room temperature is consistent with the presence of two different conformations about the N? NO bonds.