Marco Pagliai

Hydrogen bond dynamics in liquid methanol

A Car–Parrinello molecular dynamics simulation has been performed on fully deuterated liquid methanol. The results are compared with the latest available experimental and theoretical data. It is shown that the liquid is aggregated in chains of hydrogen bonded molecules. The structure of the aggregates is characterized and it is found that the dynamics includes a fast and a slow regime. The weak H bond formed by the methyl group hydrogens and oxygen atom of surrounding molecules has been characterized. The importance of inductive effects is shown and discussed in terms of maximally localized Wannier function centers. Special attention is devoted to clarify how the molecular dipole moment depends on the number of H bonds formed by each molecule. The IR spectrum is computed and analyzed in terms of H-bond interactions. Insights on the short time dynamics and on the H-bond network are illustrated.

SERS, XPS, and DFT Study of Adenine Adsorption on Silver and Gold Surfaces

The adsorption of adenine on silver and gold surfaces has been investigated combining density functional theory calculations with surface-enhanced Raman scattering and angle-resolved X-ray photoelectron spectroscopy measurements, obtaining useful insight into the orientation and interaction of the nucleobase with the metal surfaces.

Nitromethane decomposition under high static pressure.

The room-temperature pressure-induced reaction of nitromethane has been studied by means of infrared spectroscopy in conjunction with ab initio molecular dynamics simulations. The evolution of the IR spectrum during the reaction has been monitored at 32.2 and 35.5 GPa performing the measurements in a diamond anvil cell. The simulations allowed the characterization of the onset of the high-pressure reaction, showing that its mechanism has a complex bimolecular character and involves the formation of the aci-ion of nitromethane. The growth of a three-dimensional disordered polymer has been evidenced both in the experiments and in the simulations. On decompression of the sample, after the reaction, a continuous evolution of the product is observed with a decomposition into smaller molecules. This behavior has been confirmed by the simulations and represents an important novelty in the scene of the known high-pressure reactions of molecular systems. The major reaction product on decompression is N-methylformamide, the smallest molecule containing the peptide bond. The high-pressure reaction of crystalline nitromethane under irradiation at 458 nm was also experimentally studied. The reaction threshold pressure is significantly lowered by the electronic excitation through two-photon absorption, and methanol, not detected in the purely pressure-induced reaction, is formed. The presence of ammonium carbonate is also observed.

Anharmonic infrared and Raman spectra in Car-Parrinello molecular dynamics simulations

The infrared and Raman spectra of naphthalene crystal with inclusion of anharmonic effects have been calculated by adopting the generalized variational density functional perturbation theory in the framework of Car-Parrinello molecular dynamics simulations. The computational approach has been generalized for cells of arbitrary shape. The intermolecular interactions have been analyzed with and without the van der Waals corrections, showing the importance of such interactions in the naphthalene crystal to reproduce the structural, dynamical, and spectroscopic properties.

Raman and computational study of solvation and chemisorption of thiazole in silver hydrosol

A SERS investigation combined with ab initio computational analysis involving Car-Parrinello molecular dynamics simulations and Density Functional Theory approach allows fundamental information to be obtained on the behaviour of thiazole in silver aqueous suspension where solvation and chemisorption processes competitively occur.

A combined Raman, DFT and MD study of the solvation dynamics and the adsorption process of pyridine in silver hydrosols

The adsorption of pyridine onto silver colloids has been investigated by Raman spectroscopy experiments and by ab initio DFT and MP2 calculations. The solvation dynamics of the pyridine in water has been studied by a molecular dynamics simulation. The results are compared with the latest available experimental and theoretical data. It is found that the pyridine is essentially hydrogen bonded to one solvent molecule. Calculations based on pyridine-water and pyridine-Ag(+) complexes allow the reproduction of the experimentally observed Raman features and explain the adsorption process of the ligand in silver hydrosols.

Structure and Dynamics of Low-Density and High-Density Liquid Water at High Pressure

Liquid water has a primary role in ruling life on Earth in a wide temperature and pressure range as well as a plethora of chemical, physical, geological, and environmental processes. Nevertheless, a full understanding of its dynamical and structural properties is still lacking. Water molecules are associated through hydrogen bonds, with the resulting extended network characterized by a local tetrahedral arrangement. Two different local structures of the liquid, called low-density (LDW) and high-density (HDW) water, have been identified to potentially affect many different chemical, biological, and physical processes. By combining diamond anvil cell technology, ultrafast pump-probe infrared spectroscopy, and classical molecular dynamics simulations, we show that the liquid structure and orientational dynamics are intimately connected, identifying the P-T range of the LDW and HDW regimes. The latter are defined in terms of the speeding up of the orientational dynamics, caused by the increasing probability of breaking and reforming the hydrogen bonds.

Chemical Equilibrium Probed by Two-Dimensional IR Spectroscopy: Hydrogen Bond Dynamics of Methyl Acetate in Water

The solvation dynamics of methyl acetate in heavy water are analyzed by means of two-dimensional infrared spectroscopy, in conjunction with Car-Parrinello molecular dynamics simulations. The C horizontal lineO stretching infrared band of methyl acetate in water splits into a doublet as a consequence of the hydrogen bond interaction with the solvent, which leads to the equilibrium between two solvated species, consisting of one methyl acetate molecule bonded to one and two water molecules. The structure and dynamics of the water molecules bound to methyl acetate are characterized by means of experiments and simulations, allowing an accurate description of the kinetics of the exchange process and the lifetime of the hydrogen bond.

Vibrational Frequencies of Fullerenes C60 and C70 under Pressure Studied with a Quantum Chemical Model Including Spatial Confinement Effects

The equilibrium geometry structural and vibrational spectroscopic properties of fullerenes C60 and C70 under high pressure have been studied with a quantum-chemical computational approach in which ab initio calculations on a single fullerene molecule have been carried out within the polarizable continuum model framework to mimic pressure effects. The adopted approach has been revealed effective to explain the geometry variations and the frequency shifts observed experimentally.

Mechanism of the Ethylene Polymerization at Very High Pressure.

The reaction of ethylene in condensed phases under high pressure has been investigated by ab initio molecular dynamics. Both disordered and crystalline samples have been simulated, and some insights on the reaction mechanism have been obtained. System size effects have been investigated for the disordered samples. A polymerization reaction occurs by an ionic mechanism. In both the disordered and the crystal phases, the reaction products obtained (linear chains in the disordered systems and branched chains in the crystal) are in qualitative agreement with the experiments.