Density functional energy decomposition into one- and two-atom contributions

The present work provides a generalization of Mayer's energy decomposition for the density-functional theory (DFT) case. It is shown that one- and two-atom Hartree-Fock energy components in Mayer's approach can be represented as an action of a one-atom potential V(A) on a one-atom density rho(A) or rho(B). To treat the exchange-correlation term in the DFT energy expression in a similar way, the exchange-correlation energy density per electron is expanded into a linear combination of basis functions. Calculations carried out for a number of density functionals demonstrate that the DFT and Hartree-Fock two-atom energies agree to a reasonable extent with each other. The two-atom energies for strong covalent bonds are within the range of typical bond dissociation energies and are therefore a convenient computational tool for assessment of individual bond strength in polyatomic molecules. For nonspecific nonbonding interactions, the two-atom energies are low. They can be either repulsive or slightly attractive, but the DFT results more frequently yield small attractive values compared to the Hartree-Fock case. The hydrogen bond in the water dimer is calculated to be between the strong covalent and nonbonding interactions on the energy scale.     

Stereoselective synthesis of 4 alpha-hydroxy-8,12-guaianolides from santonin

Hydroxyester 2, easily obtained from santonin (1), has been transformed into 10 alpha-hydroxyguai-3-en-8,12-olide 6, a good intermediate for the synthesis of natural 8,12-guaianolides. Compound 6 was obtained from 2 by photochemical rearrangement of its acetyl derivative 7, stereoselective hydrogenation on Pd/C, reduction, regioselective elimination, hydrolysis, and lactonization. The synthesis of the natural guaianolides 3-5 was carried out in two sequences in which the regioselective elimination of a hydroxyl group at C10 with triflic anhydride or SOCl2 to afford, respectively, the endo or exo double bond on C10 and the regioselective opening of the C3-C4 alpha-epoxide were the key steps.     

Constant neutral loss scanning for the characterization of glycerol phosphatidylcholine phospholipids

The product-ion spectra of the sodiated molecules of glycerol phosphatidylcholine phospholipids (GPC) were obtained, using fast-atom bombardment (FAB) as the ionization method, in a tandem mass spectrometer. The product-ion spectra of these sodiated molecules of GPCs were found to differ significantly from those of the protonated GPC molecules. This difference is due to the absence of the ion of m/z 184 (protonated-phosphocholine moiety) and to the presence of an ion resulting from the loss of trimethylamine (m=59 Da) from the polar head group, which is the dominant fragmentation. This characteristic neutral loss provides a means of identification of this class of phospholipids and of differentiation from other phospholipid classes in complex mixtures by performing a constant-neutral-loss scan of 59.     

Viscoelastic and thermal characterization of crosslinked PVC

Soft PVC is employed for the manufacturing of a wide range of products with different properties and a relatively low cost. The utilization of soft PVC is restricted by the poor thermal, chemical and mechanical resistance properties. Also, plasticizer migration can modify the properties or can make useless the materials for some applications because of toxicity or a general loss of properties. PVC crosslinking is the most effective way to improve mechanical and transport properties of rigid or flexible PVC at high temperatures, but at the same time the thermal stability of PVC may be significantly reduced. In this work, the crosslinking reaction of plasticized poly(vinyl chloride) (PVC) through difunctional amines was studied. The mechanisms involved in the crosslinking reaction were explained by Fourier transform infrared (FTIR) analysis. The thermal activated crosslinking reaction was studied by cone and plate rheometry, analyzing the evolution of viscoelastic properties of the suspension as a function of time and temperature. The effect of the addition of crosslinking agents on the thermal stability of the polymer was studied by thermogravimetric analysis (TGA), which revealed that crosslinking reactions promote thermal degradation phenomena in the polymer matrix. This is attributed to the formation of HCl and other species promoting polymer degradation during crosslinking, thus leading to higher weight loss during thermal treatment with respect to unmodified PVC plastisols. This was also confirmed by an evident yellowing after crosslinking, especially at higher temperatures.     

Effect of drying temperature and blanching on the degradation of chlorophyll a and b in mint (Mentha spicata Huds.) and basil (Ocimum basilicum): Analysis by high performance liquid chromatography with photodiode array detection

Summary The effect of drying conditions on the preservation of chlorophyll pigments in mint and basil have been investigated in order to determine the effects of drying temperature and whether or not there was a prior blanching.Pigments extracted from fresh and dried samples were analyzed by reversed phase high performance liquid chromatography coupled to a photodiode array detector; isocratic separation was performed on a Zorbax ODS C18 column.The purity of the chromatographic peaks of chlorophylls and breakdown products was investigated. The visible spectra of standard samples of chlorophylls and pheophytins were compared, using least squares normalization with those of peaks from the extracts of fresh and dried mint and basil. The study has shown that chlorophylls were better preserved when drying was preceded by a short blanching; if samples were not blanched before drying, the degradation of chlorophylls a and b was best prevented by drying at low temperatures.     

DFT and NMR parameterized conformation of valeranone

A Monte Carlo random search using molecular mechanics, followed by geometry optimization of each minimum energy structure employing density functional theory (DFT) calculations at the B3LYP/6-31G* level and a Boltzmann analysis of the total energies, generated accurate molecular models which describe the conformational behavior of the antispasmodic bicyclic sesquiterpene valeranone (1). The theoretical H-C-C-H dihedral angles gave the corresponding 1H, 1H vicinal coupling constants using a generalized Karplus-type equation. In turn, the 3J(H,H) values were used as initial input data for the spectral simulation of 1, which after iteration provided an excellent correlation with the experimental 1H NMR spectrum. The calculated 3J(H,H) values closely predicted the experimental values, excepting the coupling constant between the axial hydrogen alpha to the carbonyl group and the equatorial hydrogen beta to the carbonyl group (J(2beta, 3beta)). The difference is explained in terms of the electron density distribution found in the highest occupied molecular orbital (HOMO) of 1. The simulated spectrum, together with 2D NMR experiments, allowed the total assignment of the 1H and 13C NMR spectra of 1.     

Molecular structure-activity relationships for the oxidation of organic compounds using mesoporous silica catalysts derivatised with bis(halogeno)dioxomolybdenum(VI) complexes

Mo K-edge XAFS spectra have been measured for ordered mesoporous silica MCM-41 grafted with the complexes [MoO2X2(thf)2] (X=Cl, Br). For grafting reactions in the absence of triethylamine, materials with 1 wt. % Mo are obtained; the Mo K-edge EXAFS results indicate the co-existence of isolated surface-fixed monomeric species [MoO2[(-O)3SiO]2(thf)(n)] and [MoO2[(-O)3SiO]X(thf)(n)]. When Et3N is used in the grafting reactions, materials with 4 wt. % Mo are obtained. The EXAFS data for the material prepared using [MoO2Cl2(thf)2] and Et3N indicate the presence of dinuclear species with two Mo(VI) centres, each with two Mo=O groups and each linked by one or two oxo bridges (Mo...Mo 3.27 A). The molybdenum centres in the material prepared using the dibromo complex comprise mainly isolated four-coordinate dioxomolybdenum(VI) and trioxomolybdenum(VI) monomeric species, with a small contribution from dimeric species. All materials were further characterised in the solid state by powder X-ray diffraction, N2 adsorption analysis, MAS NMR (13C, 29Si) and FTIR spectroscopy. The derivatised MCMs perform differently as catalysts in the liquid-phase oxidation of various olefins and alcohols with tert-butyl hydroperoxide. The highest alkene epoxidation activity was recorded for the catalysts with low metal loading, whereas the material containing oxo-bridged dimers had the highest activity for oxidation of alcohols. The recyclability of all the catalysts was tested: the catalytic activity of the derivatised materials tended to stabilize with ageing.     

Formation of a stable cyano-bridged dinuclear iron cluster following oxidation of the superoxide reductases from Treponema pallidum and Desulfovibrio vulgaris with K(3)Fe(CN)(6)

Superoxide reductases catalyze the monovalent reduction of superoxide anion to hydrogen peroxide. Spectroscopic evidence for the formation of a dinuclear cyano-bridged adduct after K(3)Fe(CN)(6) oxidation of the superoxide reductases neelaredoxin from Treponema pallidum and desulfoferrodoxin from Desulfovibrio vulgaris was reported. Oxidation with K(3)Fe(CN)(6) reveals a band in the near-IR with lambda(max) at 1020 nm, coupled with an increase of the iron content by almost 2-fold. Fourier transform infrared spectroscopy provided additional evidence with CN-stretching vibrations at 2095, 2025-2030, and 2047 cm(-)(1), assigned to a ferrocyanide adduct of the enzyme. Interestingly, the low-temperature electronic paramagnetic resonance (EPR) spectra of oxidized TpNlr reveal at least three different species indicating structural heterogeneity in the coordination environment of the active site Fe ion. Given the likely 6-coordinate geometry of the active site Fe(3+) ion in the ferrocyanide adduct, we propose that the rhombic EPR species can serve as a model of a hexacoordinate form of the active site.     

Stereodefined synthesis of O3'-labeled uracil nucleosides. 3'-[(17)O]-2'-Azido-2'-deoxyuridine 5'-diphosphate as a probe for the mechanism of inactivation of ribonucleotide reductases

Thermolysis of a 2'-[(16)O]-O-benzoyl-[(17)O]-5'-O-(tert-butyldimethylsilyl)-O(2),3'-cyclouridine derivative gave the more stable 3'-[(17)O]-O-benzoyl-[(16)O]- 5'-O-(tert-butyldimethylsilyl)-O(2),2'-cyclouridine isomer, which was converted into 3'-[(17)O]-2'-azido-2'-deoxyuridine by deprotection and nucleophilic ring opening at C2' with lithium azide. The 5'-diphosphate was prepared by nucleophilic displacement of the 5'-O-tosyl group with tris(tetrabutylammonium) hydrogen pyrophosphate. Model reactions gave (16)O and (18)O isotopomers, and base-promoted hydrolysis of an O(2),2'-cyclonucleoside gave stereodefined access to 3'-[(18)O]-1-(beta-D-arabinofuranosyl)uracil. Inactivation of ribonucleoside diphosphate reductase with 2'-azido-2'-deoxynucleotides results in appearance of EPR signals for a nitrogen-centered radical derived from azide, and 3'-[(17)O]-2'-azido-2'-deoxyuridine 5'-diphosphate provides an isotopomer to perturb EPR spectra in a predictable manner.     

Total syntheses of four stereoisomers of 4alpha-hydroxy-1beta,7beta-peroxy- 10betaH-guaia-5-ene

[reaction: see text]. The first total syntheses of four stereoisomers of 4alpha-hydroxy-1beta,7beta-peroxy-10betaH-guaia-5-ene are reported starting from the readily available (+)-dihydrocarvone. These compounds have been synthesized from dienes (-)-isoguaiene and (-)-10-epi-isoguaiene by tandem ene hydroperoxylation-[4 + 2] cycloaddition with O(2) followed by selective reduction. The structure of the natural 4alpha-hydroxy-1beta,7beta-peroxy-10betaH-guaia-5-ene isolated from Liabum floribundum has been confirmed.