2,6-Diamino-5,8-diaza-7,9-dicarba-purine

[reaction: see text] The title compound, a constitutional isomer of the natural nucleobase 2,6-diaminopurine, undergoes regioselective electrophilic substitutions at carbon C-9.     

Electronic structure of high-T c superconductors and related compounds

We study a 5-band Hubbard model for the CuO₂ planes in cuprate super-conductors using Hartree-Fock mean-field theory including spiral spin density waves. For the half-filled case we recover a ZSA-like phase diagrambut with an additional new region characterized by strong covalency effects, which we call a covalent insulator region. We also provide a nonperturbative calculation ofJ _eff, the effective in-plane antiferromagnetic interaction, as a function of parameters of the model. We suggest that the high-T _c cuprates are in or very close to the covalent insulator region and within this we show that a consistent explanation of apparently conflicting high energy spectroscopic and magnetic measurements of the high-T _c cuprates can be given.     

p-Adic arithmetic procedures for exact matrix computations

Computer procedures are described for error-free matrix computations, using thep-adic arithmetic. As an example, the exact solution of a highly ill-conditioned linear system of equations is obtained, by using the Gaussian elimination method.     

Use of tetravalent manganese in solutions of sulfuric acid as an oxidant for the direct titration of nitrite

Conditions have been worked out for the direct titration of nitrite with manganese(IV) sulfate using ferroin as indicator. The method is suitable for the estimation of 8–60 mg of sodium nitrite with a standard deviation of 0.04, in an acidity range of 0.3 to 0.6 M. The special feature of this method lies in the reagent's association with acid and its relatively higher redox potential.     

A parallel wilf algorithm for complex zeros of a polynomial

A global recursive bisection algorithm is described for computing the complex zeros of a polynomial. It has complexityO(n ³ p) wheren is the degree of the polynomial andp the bit precision requirement. Ifn processors are available, it can be realized in parallel with complexityO(n ² p); also it can be implemented using exact arithmetic. A combined Wilf-Hansen algorithm is suggested for reduction in complexity.     

White light interferometric detection of unpolarized light for complete Stokesmetric optical coherence tomography

Optical coherence tomography (OCT) relies on interference between a polarized reference and the target reflection. Thus, it has generally been impossible to detect any unpolarized part in the signal. Here, we demonstrate a scheme that overcomes this limitation. Using a combination of heterodyning and filtering, we realize a polarization-sensitive OCT system capable of measuring the full Stokes vector, including the depolarized part. Based on such a system, we perform full Stokesmetric imaging of different layers in a porcine tendon sample. The complete 4 × 4 backscattering Muellermetric images of one layer are acquired and investigated.     

Kinetics of substitution in two water soluble cobalt(III)-porphyrins

Summary Kinetics of substitution of thiocyanate and pyridine in the axial positions of tetra(p-trimethylammoniumphenyl)porphinatocobalt(III) and tetra(p-sulfonatophenyl)porphinato cobalt(III) were investigated at pH 4.00 and ionic strength 0.1 M (NaNO₃) in aqueous solution. These reactions were found to be as facile as other such reactions of cobalt(III) porphyrins even though a new synthesis was employed in the present study. The rates of these reactions are proportional to the base strength of the parent free base porphyrin. Electrostatic attraction between the ligand and the porphyrin peripheral charges were also found to enhance the rates. The cause of labilization is believed to be primarily electronic and not steric.