XPS characterization of wood chemical composition after heat‐treatment

XPS was used to characterize the chemical changes occurring after drying or applying a heat‐treatment to beech wood samples. Our results indicate that the surface of this air‐exposed material could be strongly affected either by the ambient atmosphere during storage or by the complex atmosphere in the oven during drying or heat‐treatment. However, the O/C ratio measured after removal of a thin slice of a few millimetres of an untreated sample is in reasonable agreement with that calculated from the well‐established chemical composition of beech. Through this methodology (equivalent to scraping for hard materials) it is expected to get a realistic characterization of the wood. The reliability and repeatability of the XPS measurements have been checked and the method applied to the study of the chemical changes of the beech samples subjected to heat‐treatment. Heating at 240 °C induces a significant decrease of the O/C ratio from 0.55 before to 0.44 after the treatment. Heat‐treatment induces also a decrease of the C₂ carbon contribution (carbon atom bound to a single non‐carbonyl oxygen) associated with an increase of the C₁ carbon contribution (carbon atoms bound only to carbon or hydrogen atoms), in agreement with chemical modifications reported previously in the literature. Thanks to the small analysed area of the equipment used in this study, different spots were analysed to demonstrate the presence or absence of a gradient of chemical composition due to thermal degradation or migration of extractives from within the wood structure to its surface. At the scale of our observations, the different wood samples investigated (dried or heat treated) appear to be homogeneous. Copyright © 2006 John Wiley & Sons, Ltd.     

Spatial structure of β-substituted alkoxyvinyl trifluoromethyl ketones

Spatial structure of six β-substituted enones, with common structure R₁O–CR₂CH–COCF₃, were R₁ = C₂H₅, R₂ = H (ETBO); R₁ = R₂ = CH₃ (TMPO); R₁ = C₂H₅, R₂ = C₆H₅ (ETPO); R₁ = C₂H₅, R₂ = 4- O₂NC₆H₄ (ETNO); R₁ = C₂H₅, R₂ = C(CH₃)₃ (ETDO) were investigated by ¹H and ¹⁹F NMR, infrared spectroscopy and AM1 calculations. NMR spectra revealed that enones (MBO), (ETBO) and (TMPO) are exclusively (3E) isomers, whereas in (ETPO), (ETNO) and especially in (ETDO) the percentage of (3Z) isomers is significant and depends on the nature of solvents. Conformational behaviour of studied enones are determined by the rotation around of CC double bond, C–C and C–O single bonds (correspondingly trifluoroacetyl and alkoxy groups), and (EZZ) conformer being the most stable in all cases. IR spectra revealed that with the exception of (ETDO) (EZZ) conformer is most populated in all cases. Bulky substituents like phenyl or tert-butyl group at β-position of enone result in the equilibrium mainly between (EZZ) and (ZZZ) forms, whereas β-hydrogen and β-methyl substituents determine the equilibrium between (EZZ) and (EEZ) or (EZE) conformers.     

Investigation of nu(N-H) and nu(C-N) stretching modes of propylamine (C3H7NH2) in a binary system C3H7NH2 + CH3OH via concentration dependent Raman study and ab initio calculations

Raman spectra of propylamine (C3H7NH2) and its binary mixtures, C3H7NH2 + CH3OH with varying mole fractions of the reference system, C3H7NH2, C were recorded in two widely apart wavenumber regions, 3100-3600 cm(-1) and 1225-1325 cm(-1). In the former region, the two Raman bands at approximately 3305 and approximately 3326 cm(-1), obtained after the line shape analysis, which were assigned to symmetric nu(N-H) and anti-symmetric nu(N-H) stretching modes, respectively, show a downshift upon dilution. However, whereas the nu(N-H) anti-symmetric mode shows a shift of 18.6 cm(-1), the nu(N-H) symmetric mode shows a much smaller shift (5.7 cm(-1)) between neat liquid and high dilution, C = 0.1. This aspect has been explained using the optimized geometries calculated employing ab initio theory (MP2 level) for the neat C3H7NH2 and its different hydrogen-bonded complexes. The linewidth versus concentration plot for the nu(N-H) anti-symmetric stretching mode, however exhibits a distinct maxima at C = 0.4, which has been explained as a slight departure from the concentration fluctuation model. In the latter region, a symmetric peak is observed, which corresponds to nu(C-N) stretching mode, which shows an upshift upon dilution and an almost linear concentration dependence. This has also been explained in terms of the parameters obtained from the optimized geometries of the different hydrogen-bonded complexes.     

The synthesis and electronic structure of a novel [NiS4Fe2(CO)6] radical cluster: implications for the active site of the [NiFe] hydrogenases

A novel [NiS4Fe2(CO)6]cluster (1: 'S(4)'=(CH(3)C(6)H(3)S(2))(2)(CH(2))(3)) has been synthesised, structurally characterised and has been shown to undergo a chemically reversible reduction process at -1.31 V versus Fc(+)/Fc to generate the EPR-active monoanion 1(-). Multifrequency Q-, X- and S-band EPR spectra of (61)Ni-enriched 1(-) show a well-resolved quartet hyperfine splitting in the low-field region due to the interaction with a single (61)Ni (I=3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non-coincidence between g(1) and A(1), and g(3) and A(3) to reproduce all of the features in the S- and X-band spectra. This behaviour provides a rare example of the detection and measurement of non-coincidence effects from frozen-solution EPR spectra without the need for single-crystal measurements, and in which the S-band experiment is sensitive to the non-coincidence. An analysis of the EPR spectra of 1(-) reveals a 24 % Ni contribution to the SOMO in 1(-), supporting a delocalisation of the spin-density across the NiFe(2) cluster. This observation is supported by IR spectroscopic results which show that the CO stretching frequencies, nu(CO), shift to lower frequency by about 70 cm(-1) when 1 is reduced to 1(-). Density functional calculations provide a framework for the interpretation of the spectroscopic properties of 1(-) and suggest that the SOMO is delocalised over the whole cluster, but with little S-centre participation. This electronic structure contrasts with that of the Ni-A, -B, -C and -L forms of [NiFe] hydrogenase in which there is considerable S participation in the SOMO.     

Synthesis and Spectroscopic Studies of Chosen Heteropolytungstates and Their Ln(III) Complexes

The heteropolytungstates [(Na)P₅W₃₀O₁₁₀]^4– (I), [(Na)Sb₉W₂₁O₈₆]^18– (II) and [(Na)As₄W₄₀O₁₄₀]^27– (III) and the monovacant Keggin structure of the general formula [XW_11–xMo_xO₃₉]^n– (X-Si, P; n = 7 for P and 8 for Si) (IV) as well as their europium(III) complexes were studied. The structures of I–IV as well as the europium(III) encrypted [(Eu)P₅W₃₀O₁₁₀]^12– (VI), [(Eu)Sb₉W₂₁O₈₆]^16– (VII), [(Eu)As₄W₄₀O₁₄₀]^25– (VIII) and sandwiched [Eu(XW_11–xMo_xO₃₉)₂]^n– (n =11 for P and n = 13 for Si) (V) complexes were synthesized and spectroscopically characterized. The complexes were studied using UV-Vis absorption and luminescence, as well as the laser-induced europium ion luminescence spectroscopy. Absorption spectra of Nd(III) were used to characterize the complexes formed. Excitation and emission spectra of Eu(III) were obtained for solid complexes and their solutions. The relative luminescence intensities of the Eu(III) ion, expressed as the ratio of the two strongest lines at 594 nm and 615 nm, = I₆₁₅/I₅₉₄, which is sensitive to the environment of the primary coordination sphere about the Eu(III) ion, was calculated. In the case of the sandwiched [Eu(XW_11–xMo_xO₃₉)₂]^n– complexes a linear dependence of the luminescence quantum yield of Eu(III) ion, , (calculated using [Ru(bpy)₃]Cl₂ as a standard) on the content of Mo (number of atoms, x) in the [Eu(XW_11–xMo_xO₃₉)₂]^n– structure was observed.     

Evaluation of the reactivity of new activated difluoroaromatic compounds

To evaluate the reactivity of new difluroroaromatic compounds in nucleophilic substitution, the positive charges on carbon atoms of C−F bonds were calculated using the quantum-chemical semiempirical PM3 method. A correlation between the charges calculated and the chemical shifts in the¹⁹F NMR spectra was established. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 623–625, April, 1998.     

Experimental and Theoretical Studies of a Pulsed Microwave Excited Ar/CF4 Plasma

The present work deals with a pulsed microwave discharge in an Ar/CF ₄ gas mixture under a low pressure (1–10 mbar). The discharge chamber developed has a cylindrical geometry with a coupling window alternatively made of quartz or alumina. The setup allows one to investigate the plasma–wall interactions (here etching of the quartz window) and the ignition process of the pulsed microwave plasma. Microwave pulses with a duration of 50–200 s and repetition rate between 1 and 10 kHz are typical for the experiments. The space-time behavior of the fluorine number density in the discharge has been investigated experimentally by optical actinometry. The discharge kinetics is modeled using electron-transport parameters and rate coefficients derived from solutions of the Boltzmann equation. Together with the solution of the continuity and electron balance equations and the rate equations describing the production of CF _x (x=2, 3, 4) radicals and F atoms, a good agreement between experimental and theoretical data can be achieved.     

Can triorganoboroxins exist in a “monomeric” R---B=O form? MNDO calculations and ebulliometric molecular weight determination

MNDO calculations were made for triethylboroxin (EtBO)₃ and triphenylboroxin (PhBO)₃ using both X-ray determined and optimized geometry of these molecules. The results were compared with hypothetical “monomeric” molecules R---B=O. Calculated energies of trimerization are about −200 kJ mol⁻¹ for both compounds and confirm the much higher stability of the “trimer”. Ebulliometric determination of molecular weight of triphenylboroxin in 2-pentanone confirms its trimeric character.     

Proton transfer in nafion membrane by quantum chemistry calculation

We estimated the energy barriers of proton transfers in the systems of (CF₃SO₃/H/SO₃CF₃)⁻ and (CF₃SO₃/H/H₂O/SO₃CF₃)⁻ as models of a water-swollen Nafion membrane by an ab initio density functional quantum calculation method with consideration of the hydration effect. As a result, the proton transfer between the SO sites, which is accompanied by one water molecule, was found to be one of the proton-transfer mechanisms in the water-swollen Nafion membrane; that is, the surface diffusion mechanism was found to be important for the proton transfer in that membrane. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1905–1914, 2004     

The steric and electronic effects of aliphatic fluoroalkyl groups

Ab initio calculations were performed on 18 fluorinated and unfluorinated alcohols at the B3LYP and HF levels with the 6-311G^∗∗ basis set. Molar volumes of the alcohols were computed at each level and averaged to produce a scale of relative size. From this, various isosteric replacements of potential use in drug design were suggested: ethyl by FCH₂CH₂ or HCF₂CH₂, propyl by CF₃CH₂, isopropyl by CF₃(CH₃)CH or (FCH₂)₂CH, isobutyl or t-butyl by (CF₃)₂CH, and 3-methyl-2-butyl by CF₃(CH₃)₂C. Calculation of the charge on oxygen and the Wiberg index of the CO bond allowed an electronegativity scale to be constructed for the fluoroalkyl groups. Electronegativity decreased in the order: (CF₃)₃C>(CF₃)₂CH>C₂F₅CH₂>CF₃CH₂>CH₃(CF₃)₂C>HCF₂CH₂>CF₃(CH₃)CH>(FCH₂)₂CH>FCH₂CH₂>CF₃(CH₃)₂C. This ranking agreed with literature acid dissociation data for the alcohols studied.