Stefania Di Tommaso

Computational Study of Alkynes Insertion into Metal-Hydride Bonds Catalyzed by Bimetallic Complexes

Density Functional Theory investigations on the insertion mechanism of phenylacetylene into metal-hydride bonds in bimetallic (Pt,Os) catalysts have been carried out. The results obtained have been also compared with the non-reactive monometallic (Os-based) system, to elucidate the cooperative effects and to explain the observed absence of reactivity. The identified reaction path involves phenylacetylene coordination followed by the insertion into the metal-hydride bond, leading to the formation of the experimentally observed products. Both steps do not require large energies compatible with the experimental conditions. The comparison with the reaction path for the monometallic species gives some hints on the cooperative effects due to the presence of the second metal which is related to its role in the CO release for creating a coordination site for phenylacetylene and not in the insertion energetics. The calculations provide a detailed analysis of the reaction complexity and provide a rationale for the efficiency of the process.

Structure of genipin in solution: a combined experimental and theoretical study

A combined experimental and theoretical study is here presented on the structure of genipin, a widely used molecule, whose applications range from natural cross-linker for proteins to regulating agent for drug delivery to precursor of natural dye. In particular the tridimensional structure of the genipin in solution has been studied by NMR spectroscopy and the experimental outcomes have been then supported by Density Functional Theory (DFT) calculations on the electronic structure and spectroscopic parameters. The equilibrium between open and closed-ring tautomers of genipin in solution has been also theoretically investigated. The mechanism of ring opening has been elucidated and the catalytic role of water has been clearly shown. Beyond the excellent agreement between experiments and theoretical calculations (in particular concerning 1H-NMR data), our results unravel the nature of the most stable isomer of genipin and related reactive species in solution.

Oxidation Mechanism of Aliphatic Ethers: Theoretical Insights on the Main Reaction Channels

This paper presents a quantum chemical study on oxidation process of a series of aliphatic ethers. On the basis of a detailed theoretical work on diethyl ether oxidation, the mechanism has been reduced at three competing reactions: the β-scission of the alkyl radical (R(I)OR(II)(•)) issued from the initiation step, the isomerization of the peroxy radical (R(I)OR(II)OO(•)) produced by reaction of the alkyl radical with molecular oxygen, and the hydroperoxide production, a bimolecular reaction between the peroxy radical and an ether molecule that also regenerates a R(I)OR(II)(•) radical. Results obtained from DFT calculations, including thermochemistry and rate constant evaluations, have been reported and discussed. The influence of the presence of the oxygen atom in the ether skeleton has been evaluated by making a comparison between some ethers and parent hydrocarbons. In particular, it has been found that oxygen increases the reactivity of vicinal sites by lowering activation barriers and favors the stabilization of radicals. Direct proportionality relationships have been searched between activation and reaction enthalpies of each class of competing reactions, but one has been found only for the isomerization reaction.

A mechanistic and experimental study on the diethyl ether oxidation

This work presents the results of the theoretical investigations on autoxidation process of diethyl ether (DEE), a chemical largely used as solvent in laboratories and considered to be responsible for various accidents. Based on Density Functional Theory calculations, the aims of this study were the identification of all the most probable reaction paths involved in DEE oxidation (at ambient temperature and under conditions that reflect normal storage conditions) and the characterization of products and all potential hazardous intermediates, such as peroxides. Results indicate that industrial hazards could be related to hydroperoxide formation and accumulation during the chain propagation step. A detailed kinetics model of DEE oxidation in the gas phase was then developed from all energetic and kinetics parameters collected during the mechanistic study. Outputs of the kinetics model, in terms of time of evolution of product concentrations, have been then compared with the experimentally measured concentration of products (notably hydroperoxides) issued from tests on DEE oxidation conducted under accelerated conditions with autoclaves.

From iridoids to dyes: a theoretical study on genipin reactivity

A mechanistic theoretical study of the reaction between methylamine and genipin, an iridoid obtained from some natural sources, is here presented. Starting from the opening of the genipin six-membered ring experimentally hypothesized and proved in our previous theoretical work, methylamine reaction with the three different forms of genipin in equilibrium has been studied in terms of activation enthalpies and product stabilization. The kinetically favored reaction pathways, initiated by the attack of the amine on the carbonyl groups of the dialdehydic open-ring form of genipin, have been then studied until the formation of the first orange/red product. The catalytic role of a few water molecules has been clearly shown for many reactions in the process and a fundamental redox autocatalytic cycle has been proposed for the formation of the orange/red product. This mechanism explains well the experimental evidence previously reported in literature. 1H-NMR and UV-visible spectra have been reproduced for each stable intermediate of the process.

Toward tailorable surfaces: A combined theoretical and experimental study of lanthanum niobate layered perovskites

A comprehensive theoretical investigation of the MLaNb2O7 (M = H, Li, Na, K, Rb, and Cs) series of ion-exchangeable layered perovskite is presented. These perovskites are in particular interesting in view of their potential applications as inorganic supports for the design of new hybrid inorganic-organic proton conductors. In particular, their structural and electronic properties have been investigated by periodic calculations in the framework of Density Functional Theory, using different exchange-correlation functionals. A general very good agreement with the available experimental (XRD, NPD, and EXAFS) data has been found. The structure of the protonated HLaNb2O7 form has also been further clarified and a new tetragonal space group is proposed for this compound, better reproducing the experimental cell parameters and yielding to a more realistic picture of the system. The electronic investigation highlighted that all the compounds considered are very similar to each other and that the interaction between interlayer cations and perovskite slabs is purely ionic, except for the proton that is, instead, covalently bound.

Theoretical approaches for predicting the color of rigid dyes in solution

Aiming at developing an affordable and easily implementable computational protocol for routine prediction of spectral properties of rigid molecular dyes, density functional theory, and time‐dependent density functional theory were used in conjunction with a vibronic coupling scheme for band shape estimate. To predict the perceived color of molecules in solution, a model has been setup linking the UV‐vis spectra predicted at ab initio level to the L*a*b* colorimetric parameters. The results show that a mixed protocol, implying the use of a global hybrid functional for the prediction of adiabatic energy differences and a range separated hybrid for the prediction of potential energy curvature, allows perceived colors to be quantitatively predicted, as demonstrated by the comparison of L*a*b* colorimetric parameters obtained from computed and experimental spectra.

Defect interaction and local structural distortions in Mg-doped LaGaO3: A combined experimental and theoretical study

A combined experimental and theoretical study of Mg-doped LaGaO3 electrolyte was carried out, with the aim to unveil the interaction between oxygen vacancy (Vo) and perovskite B site cations. LaGaO3 (LG) and LaGa0.875Mg0.125O2.938 (LGM0125) samples were comprehensively characterized by X-ray absorption spectroscopy (XAS) and X-ray diffraction, in order to investigate short- and long-range structures of both undoped and Mg-doped materials. XAS analysis evidenced a preferential Ga-Vo interaction in LGM0125, confirmed by periodic hybrid density functional theory calculations, which were combined with a symmetry-independent classes (SICs) approach in order to (a) obtain a detailed picture of the different Mg and Vo configurations in the doped material and (b) characterize the structural features of the conducting sites. Among the 28 structures of LGM0125 considered in the SIC approach, the Ga-Vo-Ga and Ga-Vo-Mg axial configurations (oriented along the b crystallographic axis) were found to be the most stable. The relative stability of all vacancy configurations considered could be related to geometric distortions of the B-sites, possibly significantly affecting the oxygen-ion diffusion process in such electrolytes.