Condensed heterocycles containing the pyrimidine nucleus

Continuing earlier studies designed to obtain derivatives of 1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one and of the isomeric 7-one of pharmacological interest, some novel compounds 2 and derivatives of 6,7,8,9-tetrahydro-5H-1,3,4-thiadiazolo[2,3-b]quinazolin-5-one ( 3 ) were prepared. Derivatives of pyrimido[2,1-b]benzothiazol-2-one ( 6 ) and of the isomeric 4-one derivatives 8 were also synthesized. Structural identification was obtained by ¹H-nmr, ir and mass spectra.     

Laser Ablation Molecular Isotopic Spectrometry: Strontium and its isotopes

The experimental details are reported of Laser Ablation Molecular Isotopic Spectrometry (LAMIS) and its application for performing optical isotopic analysis of solid strontium-containing samples in ambient atmospheric air at normal pressure. The LAMIS detection method is described for strontium isotopes from samples of various chemical and isotopic compositions. The results demonstrate spectrally resolved measurements of the three individual ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr isotopes that are quantified using multivariate calibration of spectra. The observed isotopic shifts are consistent with those calculated theoretically. The measured spectra of diatomic oxide and halides of strontium generated in laser ablation plasmas demonstrate the isotopic resolution and capability of LAMIS. In particular, emission spectra of SrO and SrF molecular radicals provided clean and well resolved spectral signatures for the naturally occurring strontium isotopes. A possibility is discussed of using LAMIS of strontium isotopes for radiogenic age determination.     

Laser Ablation Molecular Isotopic Spectrometry: Parameter influence on boron isotope measurements

Laser Ablation Molecular Isotopic Spectrometry (LAMIS) was recently reported for optical isotopic analysis of condensed samples in ambient air and at ambient pressure. LAMIS utilizes molecular emissions which exhibit larger isotopic spectral shits than in atomic transitions. For boron monoxide ¹⁰BO and ¹¹BO, the isotopic shifts extend from 114 cm⁻¹ (0.74 nm) to 145–238 cm⁻¹ (5–8 nm) at the B²Σ⁺ (v = 0) → X²Σ⁺ (v = 2) and A²Π_i (v = 0) → X²Σ⁺ (v = 3) transitions, respectively. These molecular isotopic shifts are over two orders of magnitude larger than the maximum isotopic shift of approximately 0.6 cm⁻¹ in atomic boron. This paper describes how boron isotope abundance can be quantitatively determined using LAMIS and how atomic, ionic, and molecular optical emission develops in a plasma emanating from laser ablation of solid samples with various boron isotopic composition. We demonstrate that requirements for spectral resolution of the measurement system can be significantly relaxed when the isotopic abundance ratio is determined using chemometric analysis of spectra. Sensitivity can be improved by using a second slightly delayed laser pulse arriving into an expanding plume created by the first ablation pulse.     

Energetic and topological analysis of the reaction of Mo and Mo2 with NH3, C2H2, and C2H4 molecules

The Density functional theory has been applied to characterize the structural features of Mo(1,2)-NH(3),-C(2)H(4), and -C(2)H(2) compounds. Coordination modes, geometrical structures, and binding energies have been calculated for several spin multiplets. It has been shown that in contrast to the conserved spin cases (Mo(1,2)-NH(3)), the interaction between Mo (or Mo(2)) and C(2)H(4) (or C(2)H(2)) are the low-spin (Mo-C(2)H(4) and -C(2)H(2)) and high-spin (Mo(2)-C(2)H(4) and -C(2)H(2)) complexes. In the ground state of Mo(1,2)-C(2)H(4) and -C(2)H(2), the metal-center always reacts with the C-C center. The spontaneous formation of the global minima is found to be possible due to the crossing between the potential energy surfaces (ground and excited states with respect to the metallic center). The bonding characterization has been performed using the topological analysis of the Electron Localization Function. It has been shown that the most stable electronic structure for a pi-acceptor ligand correlates with a maximum charge transfer from the metal center to the C-C bond of the unsaturated hydrocarbons, resulting in the formation of two new basins located on the carbon atoms (away from hydrogen atoms) and the reduction of the number of attractors of the C-C basin. The interaction between Mo(1,2) and C(2)H(4) (or C(2)H(2)) should be considered as a chemical reaction, which causes the multiplicity change. Contrarily, there is no charge transfer between Mo(1,2) and NH(3), and the partners are bound by an electrostatic interaction.     

On the applicability of the HSAB principle through the use of improved computational schemes for chemical hardness evaluation

Finite difference schemes, named Compact Finite Difference Schemes with Spectral-like Resolution, have been used for a less crude approximation of the analytical hardness definition as the second-order derivative of the energy with respect to the electron number. The improved computational schemes, at different levels of theory, have been used to calculate global hardness values of some probe bases, traditionally classified as hard and soft on the basis of their chemical behavior, and to investigate the quantitative applicability of the HSAB principle. Exchange acid-base reactions have been used to test the HSAB principle assuming the reaction energies as a measure of the stabilization of product adducts.     

Synthesis of periphery-functionalized dendritic molecules using polylithiated dendrimers as starting material

A general method for the functionalization of Si-Cl terminated carbosilane dendritic molecules via organolithium or organomagnesium reagents is described. Quantitative exchange of the bromine atoms of 4-bromophenyl-functionalized dendrimers affords polylithiated species that are valuable starting materials for further functionalization, e.g., into pyridyl alcohols. The latter were successfully applied as catalyst precursors in a ruthenium-mediated ring-closure metathesis reaction.     

Laser ablation in analytical chemistry-a review

Laser ablation is becoming a dominant technology for direct solid sampling in analytical chemistry. Laser ablation refers to the process in which an intense burst of energy delivered by a short laser pulse is used to sample (remove a portion of) a material. The advantages of laser ablation chemical analysis include direct characterization of solids, no chemical procedures for dissolution, reduced risk of contamination or sample loss, analysis of very small samples not separable for solution analysis, and determination of spatial distributions of elemental composition. This review describes recent research to understand and utilize laser ablation for direct solid sampling, with emphasis on sample introduction to an inductively coupled plasma (ICP). Current research related to contemporary experimental systems, calibration and optimization, and fractionation is discussed, with a summary of applications in several areas.     

Are metal alkoxides linear owing to electrostatic repulsion?

Density functional theory calculations on [LnCp2APh] (Ln = La, Lu; Cp = eta 5-C5H5; A = O, S; Ph = C6H5) suggest that the linearity of the Ln-O-C vectors arises largely as a result of electrostatic repulsion between the alpha carbon and the trivalent metal centre.     

Interaction of Cu(+) and Cu(2+) ions with alpha-alanine. A density functional study

The Cu(+) and Cu(2+) preferred binding sites on alpha-alanine and their affinity values for this amino acid were determined at the density functional level using three different hybrid exchange correlation potentials and the 6-311++G** basis set. The results demonstrated that the two ions both give stable complexes with alpha-alanine but the stability order of the metalated species and the coordination sites are different depending on the nature of the cation. In particular, the Cu(+)-alpha-alanine ground-state structure is characterized by an eta(2)-N,O coordination with the nitrogen and oxygen atoms belonging to the amino and carbonyl groups, respectively. In contrast, the most stable complex of the Cu(2+)-alpha-alanine system has an eta(2)-O,O coordination with the cation bonded to the -CO(2) (-) moiety of the zwitterionic form of the amino acid. Comparison with the Cu(+) and Cu(2+) affinity values for glycine, computed at the same levels of theory, demonstrated that the relative values do not change significantly as different hybrid functionals are used, although the absolute affinities are strongly influenced by the choice of the hybrid potential.