Isometries of JB-algebras

A bijective linear mapping between two JB-algebrasA andB is an isometry if and only if it commutes with the Jordan triple products ofA andB. Other algebraic characterizations of isometries between JB-algebras are given. Derivations on a JB-algebraA are those bounded linear operators onA with zero numerical range. For JB-algebras of selfadjoint operators we have: IfH andK are left Hilbert spaces of dimension ≥3 over the same fieldF (the real, complex, or quaternion numbers), then every surjective real linear isometryf fromS(H) ontoS(K) is of the formf(x)=UoxoU ⁻¹ forx inS(H), whereτ is a real-linear automorphism ofF andU is a real linear isometry fromH ontoK withU(λh)=τ(λ)U(h) for λ inF andh inH. Supported by Acción Integrada Hispano-Alemana HA 94 066 B     

Trace enrichment and measurement of platinum by flow injection inductively coupled plasma mass spectrometry

A flow injection system incorporating an alumina microcolumn has been coupled to inductively coupled plasma mass spectrometry (ICP-MS) for on-line preconcentration and determination of platinum (IV) in natural waters. Depending on the nature of the sample, a nominal preconcentration factor of up to 600 can be achieved by eluting with 50l of 2 mol/l NH₄OH. The limit of detection after a 5 min preconcentration time was 4 ngl^-1, with a relative standard deviation of 4% (100 ngl^-1 working solution). The proposed method was assessed for the determination of platinum (IV) in natural waters, motor car exhaust and some common analytical reagents.     

Evaluation of biological sample mineralisation methods for the determination of fluorine by graphite furnace molecular absorption spectrometry

Various mineralisation methods were evaluated as means of treating different liquid and solid biological samples for the determination of fluorine by the formation of aluminium monofluoride in an electrothermal graphite furnace and molecular absorption spectrometry (AIF-MAS). Simple sample dilution and the use of 0.01 M Al3+ + 0.01 M Sr2+ solution as a matrix modifier are sufficient to determine the fluorine content in most liquid samples, although some require the addition of 0.3 M ammonium nitrate to the matrix modifier solution in order to diminish background absorbance. In solid samples, treatment methods routinely used with fluoride ion-selective electrodes such as microdiffusion, furnace ashing - microdiffusion and oxygen flask combustion, were tested for compatibility with AIF-MAS. The results were compared with those obtained with a fluoride ion-selective electrode. The proposed mineralisation methods were checked for applicability to different plants, foodstuffs and other biological materials. Some of the methods gave an over-all precision of better than 10%, which is often acceptable, and all methods gave recoveries above 80%. Differences between labile + ionic fluoride and total fluorine can be established by sample treatment.     

Reactions of 2-amino-2-thiazolines with isocyanates and isothiocyanates. Chemical and computational studies on the regioselectivity, adduct rearrangement, and mechanistic pathways

2-Amino-2-thiazoline derivatives bearing alkyl or aryl substituents at exocyclic nitrogen have been condensed with different isocyanates and isothiocyanates. The addition occurs at ring endocyclic nitrogen in a regiospecific manner to afford kinetic and enthalpy-favored adducts. The unequivocal assignment of these structures has been confirmed by X-ray diffraction analyses of several compounds. The endo adducts do not rearrange on heating with the sole exception of adducts in which the exocyclic nitrogen remains unsubstituted. Trapping experiments in the presence of other isocyanates or isothiocyanates produce the formation of new endo adducts by acyl exchange in the reaction mixture. Semiempirical PM3 calculations full corroborate the higher stability of endo or exo adducts depending on the substitution pattern. The formation of adducts is compatible with a stepwise reaction mechanism, for which semiempirical transition structures could be located in the potential energy surface, and the global energetics of the process have been determined. The formation of the endo adducts proceeds with a smaller activation barrier.     

1,3-dipolar cycloadditions of aromatic azoxy compounds to strained cyclo-alkenes

The 1,2,3-oxadiazolidines resulting from the addition of 4,4′-dicyano-azoxybenzene to trans-cyclooctene or cis, trans-cycloocta-1,5-diene are not stable, bu suffer 1,3-dipolar cycloreversion to give an azomethine imine; this intermediate is either captured by a second molecule of the strained cycloalkene to give 1:2 adducts in high yields or it tautomerizes to an enehydrazine. 4,4′-Dinitroazoxybenzene and benzo[c]cinnoline N-oxide react analogously.     

N-confused calix[4]pyrroles

This is a first review devoted to N-confused calix[4]pyrroles (NCCPs). NCCPs are a relatively recent arrival to the family of the pyrrole-based anion binding macrocycles, being for the first time identified in 1999. Yet, in a relatively short time these calix[4]pyrrole (CP) isomers attracted attention of the community of research groups interested in anion binding and sensing. This is because they are relatively easy to synthesize, but mainly because they posses anion-binding properties that are different from that of regular calix[4]pyrroles. The difference in anion-binding properties stems from a different binding mode between the NCCP and anion. While the regular CPs adopt in the complex an ideal cone-like conformation where all four pyrroles-NHs engage in hydrogen bonding to the anion, the inverted pyrroles do not allow forming the cone. NCCPs bind anions via a confused cone (CC), by three NH hydrogen bonds with an anion and a CH–anion contact. This different binding mode results also in different anion-binding affinity and selectivity compared to regular CPs. Also, the inverted pyrroles offer a unique possibility for selective chemical modification of the receptor. The corresponding colorimetric sensors were tested for anion binding applications. The results of colorimetric assays for anions are presented and potential applications discussed.