Complex formation of isocytosine tautomers with PdII and PtII

Isocytosine (ICH) exists in solution as two major tautomers, the keto form with N1 carrying a proton (1a) and the keto form with N3 being protonated (1b). In water, 1a and 1b exist in equilibrium with almost equal amounts of both forms present. Reactions with a series of Pd(II) and Pt(II) am(m)ine species such as (dien)Pd(II), (dien)Pt(II), and trans-(NH(3))(2)Pt(II) reveal, however, a distinct preference of these metals for the N3 site, as determined by (1)H NMR spectroscopy. Individual species have been identified by the pD dependence of the ICH resonances. pK(a) values (calculated for H(2)O) for deprotonation of the individual tautomers complexes are 6.5 and 6.4 for the N3 linkage isomers of dienPd(II) and dienPt(II), respectively, as well as 6.2 and 6.0 for the N1 linkage isomers. The dimetalated species [(dienM)(2)(IC-N1,N3)](3+) (M = Pd(II) or Pt(II)) are insensitive over a wide range of pD. The crystal structure analysis of [(dien)Pd(ICH-N3)](NO(3))(2) is reported. Ab initio calculations have been performed for tautomer compounds of composition [(NH(3))(3)Pt(ICH)](2+), cis- and trans-[(NH(3))(2)PtCl(ICH)](+), as well as trans-[(NH(3))(2)Pt(ICH)(2)](2+). Without exception, N3 linkage isomers are more stable, in agreement with experimental findings. As to the reasons for this binding preference, an NBO (natural bond orbital) analysis for [(NH(3))(3)Pt(ICH-N3)](2+)strongly suggests that intramolecular hydrogen bonding between trans-positioned NH(3) ligands and the two exocyclic groups of the ICH is of prime importance. The calculations furthermore show a marked pyramidalization of the NH(2) group of ICH in the complex once the heterocyclic ligand forms a dihedral angle <90 degrees with the Pt coordination plane.     

Carbon-13 spectra of some tetrahydropyridines. The structure of the tetrahydropyridines from 3,5-lutidine 1-oxide and mercaptans in acetic anhydride

The reaction of 3,5-lutidine 1-oxide ( 1 ) with t-butyl mereaptan in acetic anhydride, with or without triethylamine, was reinvestigated. There was obtained 2-t-butylthio-3,.5-lutidine as the major product, a small quantity of 3-(t-bulylthio)methyl-5-picoline, 1-acetyl-2,3-diacetoxy-3,5-dirnethyl-6-t-butylthio-1,2,3,6-tetrahydropyridine (which represents a structure revision) and l-acetyl-2,6-dihydroxy-3-t-butylthio-3,5-dimethyl-1,2,3,6-tetrahydropyridine. A similar reaction of 1 with 1-adamantyl mercaptan furnished 2-(l-adamantylthio)-3,5-lutidine and 1-acetyl-2,3-diacetoxy-3,5-dimethyl-6-(1-adamantylthio)-1,2,3,6-tetrahydropyridine. The structures of these new tetrahydropyridines were established primarily by carbon-13 nmr spectra.     

The absorption spectrum of water near 750 nm by CW-CRDS: contribution to the search of water dimer absorption

The absorption spectrum of natural water vapour around 750 nm has been recorded with a typical sensitivity of 3 x 10(-10) cm(-1) using a cw cavity ring down spectroscopy set up based on a Ti:sapphire laser. The 13 312.4-13 377.7 cm(-1) spectral interval was chosen as it corresponds to the region where water dimer absorption was recently measured (K. Pfeisticker et al., Science, 2003, 300, 2078-2080). The line parameters (wavenumber and intensity) of a total of 286 lines of water vapor were measured by a one by one fit of the lines to a Voigt profile. For the main water isotopologue, 276 lines were measured with line intensities as weak as 5 x 10(-29) cm molecule(-1)i.e. about 50 times smaller than the weakest H(2)16O line intensities included in the 2004 edition of the HITRAN database. On the basis of the predictions of Schwenke and Partridge, all but 16 lines could be assigned to different isotopologues of water (H(2)16O, H(2)18O, and HD16O) present in natural abundance in the sample. A total of 272 energy levels of H(2)16O were determined and rovibrationally assigned to 18 upper vibrational states. Half of them had not been reported previously. The importance of the additional absorbance resulting from the observation of many new weak lines is discussed in relation to the detection of water dimer absorption and compared to the absorbance predicted by Schwenke and Partridge. The quality of the line parameters of water monomer is shown to be of crucial importance to identify the absorbance of the water dimer in the considered region.     

Thiazolo-N-hydroxyuracils

The partial Lossen degradation of the hydroxamic acid group at C-4 or C-5 of sodium 4,5-thiazoledicarbohydroxamate (IIIb) and its 2-methyl analog (IIIe) initiated a multicoursed reaction which furnished a mixture of thiazolo[4,5-d]- and thiazolo[5,4-d]-N-hydroxyuracils. The isomer distribution was sensitive to the solvent systems in which these reactions were carried out. The structure of the isomers so obtained was established by chemical and spectral methods.     

GLOBAL ANALYSIS OF FLUORESCENCE DECAY: APPLICATIONS TO SOME UNUSUAL EXPERIMENTAL AND THEORETICAL STUDIES

The methodology for the simultaneous analysis of fluorescence decay curves is shown to create the possibility for some new types of fluorescence decay experiments. These new experiments include the use of the T-format in anisotropy experiments without the need to match the timing characteristics of photomultiplier tubes. Decay surfaces obtained at multiple timing calibrations can be analyzed simultaneously to resolve widely differing decay rates. Methodologies are described for the discrimination of associative from nonassociative behavior for multiexponential anisotropy decay. A systems theory approach to the analysis of excited state reactions is described.     

Microchip electrophoresis for chiral separations

Microchip electrophoresis (MCE) is a promising new technique for the separation of enantiomers. This recently introduced technique enables chiral separations to be performed in seconds on tiny micromachined devices. This review is intended to give a brief introduction into the principles of chiral separations with MCE with regard to methodology and instrumentation. Different approaches to realize chiral separations in microfluidic devices are described and discussed. This review gives an overview of original work done in this field with emphasis on approaches to improve detection and resolution in chiral MCE.     

Accurate calibration of TXRF using microdroplet samples

TXRF has been applied in combination with VPD to the analysis of trace impurities in the native oxide layer of Si wafer surfaces down to the range of 10(8) atoms. cm(-2). Proper quantification of VPD/TXRF data requires calibration with microdroplet standard reference wafers. The precision of calibration function has been evaluated and found to allow quantification at a high level of 3 sigma confidence with microdroplet standard reference.     

Über Derivate des 5,5-Diphenyl-glykocyamidins

Zusammenfassung Es wird über die Kondensation von Benzil mit Guanidin, p-Toluolsulfonyl-guanidin und Dicyandiamid berichtet, die zu 5,5-Diphenylglykocyamidinen führt. Der Einfluß der Alkali-Menge und der Reaktionsdauer auf die Richtung der Kondensation werden untersucht. Weiters wird eine strukturbeweisende Zweitsynthese für 5,5-Diphenyl-N_exo-p-toluolsulfonyl-glykocyamidin angegeben.

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Derivatives of 5.5-diphenyl-glycocyamidine

Guanidine, p-toluenesulfonyl-guanidine or dicyandiamide react with benzil to give 5,5-diphenyl-glycocyamidines. The influence of the alkali concentration and the reaction time on the direction of the condensation is investigated. For 5,5-diphenyl-N_exo-p-toluenesulfonyl-glycocyamidine a structure proving synthesis is given.