The Langevin dynamics of vibrated powders

We present a microscopic theory of the relaxational behaviour of a granular pile submitted to vibration, elucidating the different roles of collective and independent-particle relaxation. We write down and solve Langevin equations for these processes, which have an explicit coupling. The analysis of the solution in terms of independent-particle and collective relaxations provides a consistent framework for the interpretation of experimental results.     

Chiral induction effects in ruthenium(II) amino alcohol catalysed asymmetric transfer hydrogenation of ketones: an experimental and theoretical approach

The enantioselective outcome of transfer hydrogenation reactions that are catalysed by ruthenium(II) amino alcohol complexes was studied by means of a systematically varied series of ligands. It was found that both the substituent at the 1-position in the 2-amino-1-alcohol ligand and the substituent at the amine functionality influence the enantioselectivity of the reaction to a large extent: enantioselectivities (ee values) of up to 95% were obtained for the reduction of acetophenone. The catalytic cycle of ruthenium(II) amino alcohol catalysed transfer hydrogenation was examined at the density functional theory level. The formation of a hydrogen bond between the carbonyl functionality of the substrate and the amine proton of the ligand, as well as the formation of an intramolecular H...H bond and a planar H-Ru-N-H moiety are crucially important for the reaction mechanism. The enantioselective outcome of the reaction can be illustrated with the aid of molecular modelling by the visualisation of the steric interactions between the ketone and the ligand backbone in the ruthenium(II) catalysts.     

Quantum Monte Carlo calculations of the dissociation energy of the water dimer

We report diffusion quantum Monte Carlo (DMC) calculations of the equilibrium dissociation energy D(e) of the water dimer. The dissociation energy measured experimentally, D(0), can be estimated from D(e) by adding a correction for vibrational effects. Using the measured dissociation energy and the modern value of the vibrational energy Mas et al., [J. Chem. Phys. 113, 6687 (2000)] leads to D(e)=5.00+/-0.7 kcal mol(-1), although the result Curtiss et al., [J. Chem. Phys. 71, 2703 (1979)] D(e)=5.44+/-0.7 kcal mol(-1), which uses an earlier estimate of the vibrational energy, has been widely quoted. High-level coupled cluster calculations Klopper et al., [Phys. Chem. Chem. Phys. 2, 2227 (2000)] have yielded D(e)=5.02+/-0.05 kcal mol(-1). In an attempt to shed new light on this old problem, we have performed all-electron DMC calculations on the water monomer and dimer using Slater-Jastrow wave functions with both Hartree-Fock approximation (HF) and B3LYP density functional theory single-particle orbitals. We obtain equilibrium dissociation energies for the dimer of 5.02+/-0.18 kcal mol(-1) (HF orbitals) and 5.21+/-0.18 kcal mol(-1) (B3LYP orbitals), in good agreement with the coupled cluster results.     

Identification of isomeric flavonoid glucuronides in urine and plasma by metal complexation and LC-ESI-MS/MS

Noncovalent complexes were used for structural determination and isomer differentiation of flavonoid glucuronides. Several flavonoid glucuronides including naringenin-7-O-glucuronide, synthesized here for the first time, were used as test compounds. Electrospray ionization quadrupole ion trap mass spectrometry with collision-induced dissociation (CID) was used to analyze complexes of the form [Co(II) (L-H) (Aux)]+ and [Co(II) (L-H) (Aux)2]+, in which L is the flavonoid glucuronide and Aux is a phenanthroline-based ligand. These complexes yielded characteristic fragmentation patterns that facilitated assignment of the substitution position of the glucuronides. The methods were adapted to liquid chromatography/tandem mass spectrometry (LC-MS/MS) with postcolumn cobalt complexation and were tested on extracts from biological fluids. The metabolites naringenin-7-O-glucuronide and naringenin-4'-O-glucuronide were detected in human urine following the consumption of grapefruit juice. Isomeric quercetin glucuronides were identified and differentiated after spiking rat plasma at the 1 microM level, proving that the new methods are effective at biologically relevant concentrations.     

Quantum Monte Carlo study of the first-row atoms and ions

Quantum Monte Carlo calculations of the first-row atoms Li-Ne and their singly positively charged ions are reported. Multideterminant-Jastrow-backflow trial wave functions are used which recover more than 98% of the correlation energy at the variational Monte Carlo level and more than 99% of the correlation energy at the diffusion Monte Carlo level for both the atoms and ions. We obtain the first ionization potentials to chemical accuracy. We also report scalar relativistic corrections to the energies, mass-polarization terms, and one- and two-electron expectation values.     

Electron emission from diamondoids: a diffusion quantum Monte Carlo study

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.     

Equation of state and Raman frequency of diamond from quantum Monte Carlo simulations

We report variational and diffusion quantum Monte Carlo (VMC and DMC) calculations of the equation of state and Raman frequency of diamond. The pressure derivative of the bulk modulus, which has not been accurately determined experimentally, is calculated to be 3.8 (VMC) and 3.7 (DMC). The values of the Raman frequency calculated at the experimental volume are 1373 cm-1(VMC) and 1359 cm-1 (DMC), in good agreement with the experimental value of 1333 cm-1.     

High-Pressure Behaviour of Si AND Ge: A Theoretical Study

In this work we summarize the results of first-principles theoretical studies of the sequence of pressure-driven phase-transitions followed by the group-IVa elements Si and Ge. The agreement with the available experimental data is very good.