Devis Di Tommaso

A Multilateral Mechanistic Study into Asymmetric Transfer Hydrogenation in Water

The mechanism of aqueous-phase asymmetric transfer hydrogenation (ATH) of acetophenone (acp) with HCOONa catalyzed by Ru-TsDPEN has been investigated by stoichiometric reactions, NMR probing, kinetic and isotope effect measurements, DFT modeling, and X-ray structure analysis. The chloride [RuCl(TsDPEN)(p-cymene)] (1), hydride [RuH(TsDPEN)(p-cymene)] (3), and the 16-electorn species [Ru(TsDPEN-H)(p-cymene)] (4) were shown to be involved in the aqueous ATH, with 1 being the precatalyst, and 3 as the active catalyst detectable by NMR in both stoichiometric and catalytic reactions. The formato complex [Ru(OCOH)(TsDPEN)(p-cymene)] (2) was not observed; its existence, however, was demonstrated by its reversible decarboxylation to form 3. Both 1 and 3 were protonated under acidic conditions, leading to ring opening of the TsDPEN ligand. 4 reacted with water, affording a hydroxyl species. In a homogeneous DMF/H(2)O solvent, the ATH was found to be first order in the concentration of catalyst and acp, and inhibited by CO(2). In conjunction with the NMR results, this suggests that hydrogen transfer to ketone is the rate-determining step. The addition of water stabilized the ruthenium catalyst and accelerated the ATH reaction; it does so by participating in the catalytic cycle. DFT calculations revealed that water hydrogen bonds to the ketone oxygen at the transition state of hydrogen transfer, lowering the energy barrier by about 4 kcal mol(-1). The calculations also suggested that the hydrogen transfer is more step-wise in nature rather than concerted. This is supported to some degree by the kinetic isotope effects, which were obscured by extensive H/D scrambling.

Structure and dynamics of the hydrated magnesium ion and of the solvated magnesium carbonates: insights from first principles simulations

We report first principles molecular dynamics simulations based on the density functional theory and the Car–Parrinello method to study the structures and dynamics of the hydrated Mg2+ ion and of the solvated MgHCO3+ and MgCO3 complexes in aqueous solution. According to these simulations, the first hydration shell of the hydrated magnesium ion consists of six water molecules, whereas in the solvated magnesium bicarbonate and magnesium carbonate complexes the Mg2+ is mostly five-coordinated, which indicates that when coordinated to magnesium the HCO3− and CO32− anions reduce its the coordination sphere. Our simulations show that the structures of the most stable monomers of magnesium bi-carbonate and magnesium carbonate in solution are Mg[η1-HCO3](H2O)4+ and Mg[η1-CO3](H2O)4, i.e. the preferred hydration number is four, while the (bi-)carbonate is coordinated to the magnesium in a monodentate mode. The analysis of the exchange processes of the water molecules in the first and second hydration shell of Mg2+ shows that the HCO3− or CO32− ligands affect the dynamics of the magnesium coordination spheres by making its hydration shell more “labile”. Furthermore, molecular dynamics simulations of the non-associated Mg2+/Cl− pair in water suggest that, despite negligible differences in the coordination spheres of Mg2+, the chloride anion has a significant influence on the water exchange rates in the second hydration shell of Mg2+.

Density functional study on the circular dichroism of photoelectron angular distribution from chiral derivatives of oxirane

The linear combination of atomic orbitals B-spline density functional method has been successfully applied to a series of four chiral derivatives of oxirane, to calculate the photoionization dynamical parameters, the circular dichroism in the angular distribution effect, and to identify trends along the series. The computational algorithm has proven numerically stable and computationally competitive. The photoionization cross section, asymmetry, and dichroic parameter profiles relative to valence orbitals have been systematically studied for the states which retain their nature along the series: the identified trends have been ascribed to the different electronic properties of the substituents. A rather unexpected sensitivity of the dichroic parameter to changes in the electronic structure has been found in many instances, making this dynamical property suitable to investigate the electronic structure of chiral compounds. The magnitude of the circular dichroism in the angular distribution effect does not seem to be associated with the initial state chirality, but rather to be governed by the ability of the delocalized photoelectron wave function to probe the asymmetry of the molecular effective potential.

The Onset of Calcium Carbonate Nucleation: A Density Functional Theory Molecular Dynamics and Hybrid Microsolvation/Continuum Study

Density functional theory (Perdew-Burke-Ernzerhof) based methods have been used to study the structure and hydration environment of the building blocks of CaCO 3 in aqueous solutions of calcium bicarbonate and calcium carbonate. Car-Parrinello molecular dynamics simulations of Ca(2+)/CO3(2-) and Ca (2+)/HCO3(-) in explicit water were performed to investigate the formation of CaCO3 and the hydration shell of the solvated hetero-ion pair. Our simulations show that the formation of the monomer of CaCO3 occurs with an associative mechanism and that the dominant building block of calcium (bi)carbonate in aqueous solution is Ca[eta(1)-(H)CO3](H2O)5, i.e., the preferred hydration number is five, while the (bi)carbonate is coordinated to the calcium in a monodentate mode. This result agrees with static calculations, where a hybrid approach using a combination of explicit solvent molecules and a polarizable continuum model has been applied to compute the solvation free energies of calcium bicarbonate species. Furthermore, the discrete-continuum calculations predict that the Ca(HCO3)2 and Ca(HCO3)3(-) species are stable in an aqueous environment preferentially as Ca(HCO3)2(H2O)4 and Ca(HCO3)3(H2O)2(-), respectively.

Calcite surface structure and reactivity: molecular dynamics simulations and macroscopic surface modelling of the calcite-water interface.

Calcite-water interactions are important not only in carbon sequestration and the global carbon cycle, but also in contaminant behaviour in calcite-bearing host rock and in many industrial applications. Here we quantify the effect of variations in surface structure on calcite surface reactivity. Firstly, we employ classical Molecular Dynamics simulations of calcite surfaces containing an etch pit and a growth terrace, to show that the local environment in water around structurally different surface sites is distinct. In addition to observing the expected formation of more calcium-water interactions and hydrogen-bonds at lower-coordinated sites, we also observed subtle differences in hydrogen bonding around acute versus obtuse edges and corners. We subsequently used this information to refine the protonation constants for the calcite surface sites, according to the Charge Distribution MUltiSite Ion Complexation (CD-MUSIC) approach. The subtle differences in hydrogen bonding translate into markedly different charging behaviour versus pH, in particular for acute versus obtuse corner sites. The results show quantitatively that calcite surface reactivity is directly related to surface topography. The information obtained in this study is not only crucial for the improvement of existing macroscopic surface models of the reactivity of calcite towards contaminants, but also improves our atomic-level understanding of mineral-water interactions.

Computational study of the factors controlling enantioselectivity in ruthenium(II) hydrogenation catalysts

The reduction of prochiral ketones catalyzed by Ru(diphosphine)(diamine) complexes has been studied at the DFT-PBE level of theory. Calculations have been conducted on real size systems [trans-Ru(H)2(S, S-dpen)(S-xylbinap) + acetophenone], [trans-Ru(H)2(S, S-dpen)(S-tolbinap) + acetophenone] and [trans-Ru(H)2(S, S-dpen)(S-xylbinap) + cyclohexyl methyl ketone] with the aim of identifying the factors controlling the enantioselectivity in Ru(diphosphine)(diamine) catalysts. The high enantiomeric excess (99%) in the hydrogenation of acetophenone catalyzed by trans-Ru(H)2(S, S-dpen)(S-xylbinap) has been explained in terms of the existence of a stable intermediate along the reaction pathway associated with the (R)-alcohol. The formation of this intermediate is hindered with the competitive pathways, which consequently increases the activation energy for the hydrogen transfer acetophenone/(S)-phenylethanol reaction. For the [trans-Ru(H)2(S, S-dpen)(S-tolbinap) + acetophenone] system, the lower enantioselectivity (i.e. 80%) is rationalized by the smaller differences in the activation energy between the competitive pathways which differentiate between the two diastereomeric approaches of the prochiral ketone. The DFT-PBE results suggest that this reaction is driven to the (R)-product only by the process of binding the acetophenone to the active site of the trans-Ru(H) 2(S, S-dpen)(S-tolbinap) catalyst. For the hydrogenation of cyclohexyl methyl ketone catalyzed by trans-Ru(H)2(S, S-dpen)(S-xylbinap), the low performance in the enantioselective hydrogenation of the dialkyl ketone (i.e. 37%) is again explained by the small differences in the activation and binding energies which are the factors which could effectively differentiate between the two alkyl groups.

Hydrogen transfer and hydration properties of HnPO43−n (n=0–3) in water studied by first principles molecular dynamics simulations

Density functional theory Perdew-Burke-Ernzerhof [Perdew et al., Phys. Rev. Lett. 77, 3865 (1996)] molecular dynamics simulations of aqueous solutions of orthophosphate species H(n)PO(4)(3-n) (n=0-3) provide new insights into hydrogen transfer and intermolecular and hydration properties of these important aqueous species. Extensive Car-Parrinello molecular dynamics simulations of the orthophosphate ion PO(4)(3-), of the hydrogen phosphate anions, HPO(4)(2-) and H(2)PO(4)(-), and of the orthophosphoric acid, H(3)PO(4), in explicit water show that the process of proton transfer from H(n)PO(4)(3-n) to the surrounding water molecules is very fast, less than 1 ps, and indicate that the dehydrogenation occurs through a concerted proton hopping mechanism, which involves H(n)PO(4)(3-n) and three water molecules. Analysis of the intermolecular H(n)PO(4)(3-n)-water structure shows that the PO(4)(3-) anions have a significant effect on the H-bonding network of bulk water and the presence of P-O(-) moieties induce the formation of new types of H-H interactions around this orthophosphate. Calculated probability distributions of the coordination numbers of the first hydration shell of PO(4)(3-), HPO(4)(2-), and H(2)PO(4)(-) show that these phosphate species display a flexible first coordination shell (between 7 and 13 water molecules) and that the flexibility increases on going from PO(4)(3-) to H(2)PO(4)(-). The strength and number of hydrogen bonds of PO(4)(3-), HPO(4)(2-), and H(2)PO(4)(-) are determined through a detailed analysis of the structural correlation functions. In particular, the H-bond interactions between the oxygen atoms of the phosphates and the surrounding water molecules, which decrease on going from PO(4)(3-) to the hydrogenated H(2)PO(4)(-) species, explain the diminished effect on the structure of water with the increasing hydrogenation of the orthophosphate anions.

Theoretical study on the circular dichroism in core and valence photoelectron angular distributions of camphor enantiomers

In the present work the photoelectron circular dichroism of camphor has been theoretically studied using B-spline and continuum multiple scattering-Xalpha methods, and comparisons are made with available experimental data. In general, rather large dichroism effects have been found for both valence and core (O 1s, C 1s) photoionizations. The agreement between the two calculations reported here and previous experimental measurements for core C 1s data is essentially quantitative. For valence ionization satisfactory agreement between theory and experiment has been obtained and the discrepancies have been attributed to both exchange-correlation potential limitations and the absence of response effects in the adopted formalism. The calculations predict, moreover, important features in the cross-section profiles, which have been discussed in terms of dipole-prepared continuum orbitals.

Conformational Effects in Photoelectron Circular Dichroism of Alaninol

A photoelectron circular dichroism (CD) study of the valence states of 2-amino-1-propanol (alaninol) in the gas phase is presented. The aim of the investigation is to reveal conformer population effects in the valence-state photoelectron spectrum. The experimental dispersion of the dichroic D parameter of valence states as a function of the photon excitation energy is compared with its theoretical value calculated by employing a multicentric basis set of B-spline functions and a Kohn-Sham Hamiltonian. The theoretical values are in very good agreement with the experimental data when the conformer population distribution is taken into account. Moreover, thanks to a comparison between experiment and theory, a clear assignment of the molecular orbital character and conformer geometry is given to the features of the photoelectron spectrum. This work indicates in a detailed experimental analysis that CD in photoelectron spectroscopy is an effective technique to disentangle the conformer assignment in photoelectron spectra.

Modelling the effects of salt solutions on the hydration of calcium ions

Classical molecular dynamics simulations of several aqueous alkali halide salt solutions have been used to determine the effect of electrolytes on the structure of water and the hydration properties of calcium ions. Compared with the simulations of Ca(2+) ions in pure liquid water, the frequency of water exchange in the first hydration shell of calcium, which is a fundamental process in controlling the reactivity of calcium(ii) aqua-ions, is drastically reduced in the presence of other electrolytes in solution. The strong stabilization of the hydration shell of Ca(2+) occurs not only when the halide anions are directly coordinated to calcium, but also when the alkali and halide ions are placed at or outside the second coordination shell of Ca(2+), suggesting that the reactivity of the first solvation shell of the calcium ion can be influenced by the specific affinity of other ions in solution for the water molecules coordinated to Ca(2+). Analysis of the hydrogen-bonded structure of water in the vicinity of the calcium ion shows that the average number of hydrogen bonds per water molecules, which is 1.8 in pure liquid water, decreases as the concentration of alkali-halide salts in solution increases, and that the temporal fluctuations of hydrogen bonds are significantly larger than those obtained for Ca(2+) in pure liquid water. This effect has been explained in terms of the dynamics of reorganization of the O-H···X(-) (X = F, Cl and Br) hydrogen bond. This work shows the importance of solution composition in determining the hydrogen-bonding network and ligand-exchange dynamics around metal ions, both in solution and at the mineral-water interfaces, which in turn has implications for interactions occurring at the mineral-water interface, ultimately controlling the mobilization of ions in the environment as well as in industrial processes.