Oxidation of ethanol and its derivatives on well defined pt single crystal electrodes vicinal to pt(111): a comparative study

The oxidation of ethanol (EtOH) at Pt(111) electrodes is dominated by the 4e path leading to acetic acid. The inclusion of surface defects such as those present on stepped surfaces leads to an increase of the reactivity towards the most desirable 12e path leading to CO2 as final product. This path is also favored when the methyl group is more oxidized, as in the case of ethylene glycol (EG) that spontaneously decomposes to CO on Pt(111) electrodes, thus showing a more effective breaking of the C-C bond. Some trends in reactivity can be envisaged when other derivative molecules are compared at well-ordered electrodes. This strategy was used in the past, but the improvement in the electrode pretreatment and the overall information available on the subject suggest that relevant information is still missing.

Bioprocess and biotecnology: effect of xylanase from Aspergillus niger and Aspergillus flavus on pulp biobleaching and enzyme production using agroindustrial residues as substract

This study compares two xylanases produced by filamentous fungi such as A. niger and A. flavus using agroindustrial residues as substract and evaluated the effect of these enzymes on cellulose pulp biobleaching process. Wheat bran was the best carbon source for xylanase production by A. niger and A. flavus. The production of xylanase was 18 and 21% higher on wheat bran when we compare the xylanase production with xylan. At 50°C, the xylanase of A. niger retained over 85% activity with 2 h of incubation, and A. flavus had a half-life of more than 75 minutes. At 55°C, the xylanase produced by A. niger showed more stable than from A. flavus showing a half-life of more than 45 minutes. The xylanase activity of A. niger and A. flavus were somehow protected in the presence of glycerol 5% when compared to the control (without additives). On the biobleaching assay it was observed that the xylanase from A. flavus was more effective in comparison to A. niger. The kappa efficiency corresponded to 36.32 and 25.93, respectively. That is important to emphasize that the cellulase activity was either analyzed and significant levels were not detected, which explain why the viscosity was not significantly modified.

Remarkable coordination behavior of alkyl isocyanides toward unsaturated vicinal frustrated P/B Lewis pairs

The conjugated frustrated phosphane/borane Lewis pairs formed by 1,1-carboboration of a substituted diphenylphosphino acetylene, undergo a synergistic 1,1-addition reaction to n-butyl isocyanide with formation of new B-C and P-C bonds to the former isonitrile carbon atom. Using tert-butyl isocyanide dynamic behaviour between the isocyanide-[B] adduct and the 1,1-addition product formation was observed in solution. The different modes of isocyanide binding to the FLPs in the solid state were characterized using X-ray crystal structure analyses and comprehensive 11B and 31P solid-state magicangle- spinning (MAS-) NMR experiments. The free FLP, the Lewis adduct at the borane group, and the cyclic product resulting from isocyanide addition to both reaction centers, can be differentiated via 11B and 31P isotropic chemical shifts, 11B nuclear electric quadrupole coupling constants, isotropic indirect 11B-31P spin-spin coupling constants, and 11B...31P internuclear distances measured by rotational echo double resonance.

Intramolecular surfaces for vicinal proton–proton coupling constants 3JHH

Equations for the intramolecular surfaces of the 3JHH coupling constants in ethane, ethylene, and acetylene are formulated, and the corresponding coefficients are estimated from calculations at the DFT/B3LYP level. The chosen variables are changes in bond lengths, in the torsion angle φ between the coupled protons Ha and Hb, in bond angles, and in dihedral angles. The 3JHH surface of ethane is formulated as an extended Karplus equation with the coefficients of a truncated Fourier series on the torsion angle φ expanded as second-order Taylor series in the chosen variables taking into account the invariance of 3JHH under reflections and rotations of nuclear coordinates. Partial vibrational contributions from linear and square terms corresponding to changes in the geometry of the Ha − Ca − Cb − Hb fragment are important while those from cross terms are small with a few exceptions. The 3JHH surface of ethane is useful to predict contributions to 3JHH from changes in local geometry of derivatives but vibrational contributions are predicted less satisfactorily. The predicted values at the B3LYP/BS2 level of the 3JHH couplings (vibrational contributions at 300 K) from equilibrium geometries are 9.79 (−0.17) for acetylene, and 17.08 (1.93) and 10.73(0.93) for the trans and cis couplings of ethylene.

Oxidation of Ethanol and Its Derivatives on Well Defined Pt Single Crystal Electrodes Vicinal to Pt(111): A Comparative Study

The oxidation of ethanol (EtOH) at Pt(111) electrodes is dominated by the 4e path leading to acetic acid. The inclusion of surface defects such as those present on stepped surfaces leads to an increase of the reactivity towards the most desirable 12e path leading to CO2 as final product. This path is also favored when the methyl group is more oxidized, as in the case of ethylene glycol (EG) that spontaneously decomposes to CO on Pt(111) electrodes, thus showing a more effective breaking of the C-C bond. Some trends in reactivity can be envisaged when other derivative molecules are compared at well-ordered electrodes. This strategy was used in the past, but the improvement in the electrode pretreatment and the overall information available on the subject suggest that relevant information is still missing.

On the behavior of the Pt(1 0 0) and vicinal surfaces in alkaline media

We address in this paper a voltammetric study of the charge transfer processes characteristic of Pt(1 0 0) and vicinal surfaces in alkaline media. The electrochemical behavior of a series of stepped surfaces of the type Pt(S)[n(1 0 0) × (1 1 1)] has been characterized using cyclic voltammetry at different pHs, charge displacement measurements and FTIR experiments for adsorbed CO. The results from these techniques allow assigning the different peaks appearing in the voltammogram to hydrogen and/or OH adsorption on the different sites of these surfaces, namely, terrace and step sites. Additionally, the potential of zero total charge (pztc) of the electrodes was determined. The resulting pztc values shift to more negative values when the step density increases on the surface up to n = 5. FTIR spectroscopy experiments have been used to monitor the adsorption of CO on the different surfaces as well as the consequent CO oxidation, accompanying a positive potential sweep. The oxidation of adsorbed CO on (1 0 0) terraces is catalyzed by the presence of the (1 1 1) steps. The FTIR spectra revealed that CO is mostly bonded in bridge configuration at low potentials interconverting to on-top when the electrode potential is increased.

Traité pratique de voirie vicinale : présentant dans un ordre méthodique les dispositions législatives qui régissent cette branche de l'administration, ainsi que les ordonnances réglementaires, instructions ministérielles, avis du Conseil d'État, décisions en la forme contentieuse et arrêts de la Cour de cassation intervenus depuis 1824 jusqu'à ce jour, sur les différentes parties du service vicinal /

Indices.

Nanolines on silicon surfaces

Although a wide range of periodic surface nets can be grown on low index silicon surfaces, only a few of these have quasi-one dimensional symmetry. If high index silicon surfaces, such as (553) and (557), are used instead, the surface unit cell contains steps. It is possible to fabricate a number of quasi-one dimensional nanoline systems on the terraces and some of these have nested energy bands near the Fermi level. These nano-scale systems may support exotic many-electron states produced by enhanced electron correlations and a reduction in electron screening in one spatial dimension. In this paper, our groups' experimental and theoretical studies of nanolines phases, grown on both low index and vicinal silicon surfaces are reviewed. These studies give us insight into the electronic properties of artificial nanoline structures.

Bridged systems via radical cyclization reactions. Stereospecific synthesis of chiral tricyclo[6.2.1.0(1,5)]undecanes

The application of radical-mediated cyclizations and annulations in organic synthesis has grown in importance steadily over the years to reach the present status where they are now routinely used in the strategy-level planning.2 The presence of a quaternary carbon atom is frequently encountered in terpenoid natural products, and it often creates a synthetic challenge when two or more quaternary carbon atoms are present in a contiguous manner.3 Even though creation of a quaternary carbon atom by employing a tertiary radical is very facile, creation of a quaternary carbon atom (or a spiro carbon atom) via radical addition onto a fully substituted olefinic carbon atom is not that common but of synthetic importance. For example, the primary radical derived from the bromide 1 failed to cyclize to generate the two vicinal quaternary carbon atoms and resulted in only the reduced product 2.4 The tricyclic carbon framework tricyclo[6.2.1.01,5]undecane (3) is present in a number of sesquiterpenoids e.g. zizzanes, prelacinanes, etc.5

Conformational analysis of hydroxyproline and its protected derivatives by 1H NMR

From a computer simulation of the 270 MHz 1H NMR spectra of hydroxyproline (Hyp) and its protected derivatives, precise values of ring vicinal coupling constants were obtained. These couplings were related to ring torsional angles, using a Karplus type analysis. From the NMR analysis it was observed that the pyrrolidine ring possesses a unique and highly homogeneous conformation (Cγ-exo form). Temperature dependence studies on protected dipeptides suggest that the pyrrolidine ring conformation is independent of backbone conformation. An unusual X-Hyp, β-turn was observed for Boc-Aib-Hyp-NHMe.

Conformational analysis of small disulfide loops. Spectroscopic and theoretical studies on a synthetic cyclic tetrapeptide containing cystine

The conformational analysis of the synthetic peptide Boc-Cys-Pro-Val-Cys-NHMe has been carried out, as a model for small disulfide loops, in biologically active polypeptides. 'H NMR studies (270 MHz) establish that the Val(3) and Cys(4) NH groups are solvent shielded, while 13C studies establish an all-trans peptide backbone. Circular dichroism and Raman spectroscopy provide evidence for a right-handed twist of the disulfide bond. Analysis of the vicinal (JaB)c oupling constants for the two Cys residues establishes that XI - *60° for Cys(4), while some flexibility is suggested at Cys( 1). Conformational energy calculations, imposing intramolecular hydrogen bonding constraints, favor a P-turn (type I) structure with Pro(2)-Va1(3) as the corner residues. Theoretical and spectroscopic results are consistent with the presence of a transannular 4 - 1 hydrogen bond between Cys( 1) CO and Cys(4) NH groups, with the Val NH being sterically shielded from the solvent environment.

Studies in terpenoids-XIII. Magnetic anisotropy2 in and stereochemistry of some gem-dimethylcyclobutanoids related to pinonic acid

The chemical shifts of “axial” vs “equatorial” Me protons of some gem-dimethylcyclobutanoids derived from α-pinene, arising from magnetic anisotropy of the ring and as influenced by vicinal substituents, are discussed. Conformational aspects of some cis- and trans-pinonic, pinononic and pinic acids have been elucidated on the basis of NMR evidence.

An enantiospecific approach to pinguisanes from (R)-carvone. Total synthesis of (+)-pinguisenol

Enantiospecific total synthesis of (+)-pinguisenol 1, a sesquiterpene containing a cis-1,2,6,7-tetramethylbicyclo[4.3.0]nonane carbon framework incorporating two vicinal quaternary carbon atoms and four cis-oriented methyl groups on four contiguous carbon atoms, isolated from a liverwort, is described. The orthoester Claisen rearrangement of the allyl alcohol 9, obtained from (R)-carvone, generates the ester 12. Intramolecular cyclopropanation of the diazo ketone 13, derived from the ester 12, furnishes the tricyclic ketone 7. Degradation of the isopropenyl group followed by regioselective reductive cyclopropane ring cleavage transforms compound 7 into the hydroxy ketone 21. Wolff–Kishner reduction of the hydroxy ketone 21 followed by oxidation and Grignard reaction furnishes pinguisenol (+)- 1.

Total synthesis of (+/-)-herbertenediol

A formal total synthesis of the sesquiterpene (+/-)-herbertenediol and its dimers mastigophorenes A-D has been accomplished, starting from vanillin via 2,3-dimethoxy-5-methylbenzaldehyde. A combination of Claisen rearrangement and ring-closing metathesis reactions were employed for the generation of the two vicinal quaternary carbons on a cyclopentane ring.

The first total synthesis of (+/-)-lagopodin A

The first total synthesis of (±)-lagopodin A and a formal total synthesis of enokipodins A and B is described. The requisite precursors containing two vicinal quaternary carbon atoms were assembled employing Claisen rearrangement and an RCM reaction as key steps starting from 2,5-dimethoxy-4-methylacetophenone.