Exact state-to-state quantum dynamics of the F + HD --> HF(v' = 2) + D reaction on model potential energy surfaces

In this paper, we present the results of a theoretical investigation on the dynamics of the title reaction at collision energies below 1.2 kcal/mol using rigorous quantum reactive scattering calculations. Vibrationally resolved integral and differential cross sections, as well as product rotational distributions, have been calculated using two electronically adiabatic potential energy surfaces, developed by us on the basis of semiempirical modifications of the entrance channel. In particular, we focus our attention on the role of the exothermicity and of the exit channel region of the interaction on the experimental observables. From the comparison between the theoretical results, insight about the main mechanisms governing the reaction is extracted, especially regarding the bimodal structure of the HF(v = 2) nascent rotational state distributions. A good overall agreement with molecular beam scattering experiments has been obtained.     

Dry Reforming of Methane on a Highly‐Active Ni‐CeO2 Catalyst: Effects of Metal‐Support Interactions on C−H Bond Breaking

Ni‐CeO₂ is a highly efficient, stable and non‐expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO₂ at temperatures as low as 300 K, generating CH_x and CO_x species on the surface of the catalyst. Strong metal–support interactions activate Ni for the dissociation of methane. The results of density‐functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO_2?x(111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CH_x or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way.     

Synthesis and characterization of a Gd(III) based contrast agent responsive to thiol containing compounds

A novel Gd(III) complex with a modified DO3A-like chelating cage has been synthesized and characterized as a candidate contrast agent responsive to the concentration of free thiols in tissues (essentially represented by reduced glutathione, GSH). The novel compound (called Gd-DO3AS-Act) bears a flexible linker ending with a 2-pyridyl-dithio group, that can promptly react with free thiols (XSH) to form mixed disulfides of the form Gd-DO3AS-SX. Compound Gd-DO3AS-Act is characterized by a millimolar relaxivity as high as 8.1 mM(-1) s(-1) (at 20 MHz, 25 degrees C and pH 7.4). Upon reaction with GSH, the Gd-DO3AS-SG covalent adduct is formed and the millimolar relaxivity drops to 4.1 mM(-1) s(-1). Such a decrease in relaxivity is explained on the basis of the formation of an intramolecular coordinative bond between one of the glutathionyl carboxyl groups and the Gd(III) centre, lowering the hydration state of the paramagnetic centre. (1)H-NMR dispersion profiles together with (17)O-NMR transverse relaxation time versus temperature profiles confirm that the hydration of the Gd(III) centre is strongly reduced ongoing from Gd-DO3AS-Act to the Gd-DO3AS-SG adduct. The relaxivity difference brought about by the reaction of Gd-DO3AS-Act with GSH can be enhanced up to 60% in the presence of poly-beta-cyclodextrin.     

Polymorphism in crystalline cinchomeronic acid

The structural relationship between the two crystal forms of cinchomeronic acid (CA 3,4-dicarboxypyridine) has been investigated by single crystal X-ray diffraction, IR and Raman spectroscopy and solid state NMR spectroscopy, showing that the two polymorphs form a monotropic system, with the orthorhombic form I being the thermodynamically stable form, while the monoclinic form II is unstable. In both forms CA crystallizes as a zwitterion and decomposes before melting. The crystal structure and spectroscopic analysis indicate that the difference in stability can be ascribed to the strength of the hydrogen-bonding patterns established by the protonated N-atom and the carboxylic/carboxylate O-atoms.     

Assessing the mechanical properties of a molecular spring

We report on the dramatic effect of increasing helix diameter on the hybridization of oligopyridine-dicarboxamide strands into double helices. Upon replacing a single pyridine by a 1,8-diazaanthracene unit within an oligomeric strand, a 4.7 A enlargement of the helix diameter occurs parallel to the long anthracene axis. This structure change results in a spectacular stabilization of the double helical hybrids derived from these strands (factors of over 10(7)). Detailed investigations of the hybridization process using X-ray crystallography, NMR, fluorescence measurements and molecular mechanics calculations allowed us to assign the duplex stabilization to two enthalpic effects. First, the increase in diameter results in an augmented surface, involved in intermolecular pi-pi stacking. Second, the enlarged diameter leads to a lower tilt angle of the helical strand, with respect to the helix axis, which in turn results in smaller dihedral angles at the aryl-amide linkages and thus a considerably lowered enthalpic cost of the spring-like extension of the strands during the hybridization process. These results provide novel insights into how subtle tuning of molecular components may result in considerable and rationalizable changes in double helical supramolecular architectures.     

Comparative study of water dissociation on Rh(111) and Ni(111) studied with first principles calculations

The dissociation and formation of water on the Rh(111) and Ni(111) surfaces have been studied using density functional theory with generalized gradient approximation and ultrasoft pseudopotentials. Calculations have been performed on 2x2 surface unit cells, corresponding to coverages of 0.25 ML, with spot checks on 3x3 surface unit cells (0.11 ML). On both surfaces, the authors find that water adsorbs flat on top of a surface atom, with binding energies of 0.35 and 0.25 eV, respectively, on Rh(111) and Ni(111), and is free to rotate in the surface plane. Barriers of 0.92 and 0.89 eV have to be overcome to dissociate the molecule into OH and H on the Rh(111) and Ni(111) surfaces, respectively. Further barriers of 1.03 and 0.97 eV need to be overcome to dissociate OH into O and H. The barriers for the formation of the OH molecule from isolated adsorbed O and H are found to be 1.1 and 1.3 eV, and the barriers for the formation of the water molecule from isolated adsorbed OH and H are 0.82 and 1.05 eV on the two surfaces. These barriers are found to vary very little as coverage is changed from 0.25 to 0.11 ML. The authors have also studied the dissociation of OH in the presence of coadsorbed H or O. The presence of a coadsorbed H atom only weakly affects the energy barriers, but the effect of O is significant, changing the dissociation barrier from 1.03 to 1.37 and 1.15 eV at 0.25 or 0.11 ML coverage on the Rh(111) surface. Finally, the authors have studied the dissociation of water in the presence of one O atom on Rh(111), at 0.11 ML coverage, and the authors find a barrier of 0.56 eV to dissociate the molecule into OH+OH.     

Investigating the temperature dependence of the viscosity of a non-Newtonian fluid within lithographically defined microchannels

We present a study of the rheological phenomenology of a non-Newtonian glass former within hybrid microchannels above the vitrification region. We determined the temperature behavior of the viscosity, which is well fitted by a Vogel-Fulcher-Tamman law for shear rates between 4 x 10(-2) and 9 x 10(-1) s(-1). The microflow viscosity was compared with previously reported conductivity data of the investigated molecular system. Our findings provide an insight into the coupling between the structural dynamics in the bulk and that within the microchannels, suggesting lithographically defined microfluidic systems as promising tools for the investigation of the rheological properties of complex liquids.     

Origin of enantioselectivity in palladium-catalyzed asymmetric allylic alkylation reactions using chiral N,N-ligands with different rigidity and flexibility

The chiral bidentate-N,N ligands, (S(a))-1, (S(a))-2, (S,S)-3 and (S,S)-4, were synthesized. They were shown to contain rigid 2-pyridinyl or 8-quinolinyl building blocks and the C(2)-symmetric chiral frameworks trans-2,5-dimethylpyrrolidinyl or (S)-(+)-2,2'-(2-azapropane-1,3-diyl)-1,1'-binaphthalene. In the (S(a))-2, and (S,S)-4 ligands pair, the 8-quinolinyl skeleton is directly bonded to the C(2)-symmetric chiral frameworks (S)-(+)-2,2'-(2-azapropane-1,3-diyl)-1,1'-binaphthalene or trans-2,5-dimethylpyrrolidinyl. This feature induces rigidity in this pair of ligands upon the N,N-framework. However, this does not occur for the (S(a))-1 and (S,S)-3 ligands, in which the presence of the -CH(2)- spacer between the frameworks bearing the nitrogen atom donors gives greater flexibility to the ligand. A further difference between the pairs of ligands is significant from the electronic properties of the chiral framework N-donor atom. The coordinating properties and the specific steric structural features of the (S(a))-1, (S(a))-2, (S,S)-3, and (S,S)-4 ligands are explained by their reactions with the [Pd(PhCN)(2)Cl(2)] and [Pd(eta(3)-PhCHCHCHPh)(mu-Cl)](2) substrates, in which the reported ligands form chelate complexes, with the exception of (S(a))-2, which failed to react with [Pd(eta(3)-PhCHCHCHPh)(mu-Cl)](2). The ligands were used in the palladium-allyl catalyzed substitution reaction of 1,3-diphenylallyl acetate with dimethylmalonate, with the best result being obtained using the (S(a))-1 ligand, giving the substitution product 2-(1,3-diphenylallyl)dimethylmalonate with an enantiomeric excess of 82% in the S form and a yield of 96%. The work demonstrates that in the presence of a steric ligand control, the electronic properties of the ligand donor atoms play a role though not significant in determining the enantioselectivity of palladium(II) catalyzed allylic substitution reactions. The results of the catalytic reaction do not provide a convincing explanation considering the coordinated chiral ligand features, as rigidity or flexibility and electronic properties of the N-donor atoms. A rationalization of the results is proposed on the basis of NMR studies and DFT calculation on the cationic complexes [Pd(eta(3)-PhCHCHCHPh)(N-N*)]CF(3)SO(3), (N-N* = (S(a))-1, 9; (S,S)-3, 10; (S,S)-4, 11).