Vincenzo Schettino

Lattice dynamics of molecular crystals

1 Lattice Dynamics.- 1.1 Introduction.- 1.2 The Dynamical Equations in Cartesian Coordinates.- 1.3 Dispersion Curves. Acoustic and Optical Modes.- 1.4 Invariance Conditions.- 1.5 Molecular Coordinates.- 1.6 The Dynamical Equations in Molecular Coordinates.- 2 Symmetry.- 2.1 Space Group Symmetry.- 2.2 Irreducible Representations of the Translational Group.- 2.3 Irreducible Representations of the Space Groups.- 2.4 Time Reversal.- 2.5 Symmetry of the Dynamical Matrix.- 2.6 Symmetry Properties of Vibrational States.- 2.7 Selection Rules.- 3 Intermolecular Potentials.- 3.1 The Crystal Potential.- 3.2 The Intramolecular Potential.- 3.3 The Intermolecular Potential.- 3.4 Intermolecular Force Constants.- 3.5 Lattice Sums and Ewalds Method.- 2.6 Calculation of Phonon Frequencies.- 4 Anharmonic Interactions.- 4.1 Introduction.- 4.2 The Crystal Hamiltonian.- 4.3 Quantum Field Treatment of Phonons.- 4.4 The Hamiltonian Renormalization Procedure.- 4.5 The Self-Consistent Phonon Method.- 4.6 The Method of the Greens Functions.- 4.7 Thermal Strain.- 4.8 Anharmonic Calculations.- 5 Two-Phonon Spectra of Molecular Crystals.- 5.1 Introduction.- 5.2 General Considerations.- 5.3 Two-phonon Density of States.- 5.4 Two-phonon Absorption Coefficients.- 6 Infrared and Raman Intensities in Molecular Crystals.- 6.1 Introduction.- 6.2 Historical Survey.- 6.3 Electrostatic Model of Infrared and Raman Intensities of Molecular Crystals.

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.

High pressure reactivity of solid benzene probed by infrared spectroscopy

The chemical transformation of benzene under pressure is investigated, at room temperature and at 100 K, by means of infrared spectroscopy. Pressurization-decompression cycles in the 0–50 GPa pressure range have been performed to achieve the complete transformation of the monomer. The yellow-brownish recovered sample has been identified as an amorphous hydrogenated carbon (a-C:H). A correlation has been established between the pressure behavior of the frequencies of both Raman and infrared internal modes, and the corresponding vibrational energies in the S1 excited state (1B2u). From this comparison we conclude that pressure induces a mixing between the ground and the S1 electronic states. The increased ring flexibility enhances the interactions among nearest-neighbor molecules inducing the formation of a network of interconnected benzene units where the aromatic character is lost. The bond breaking mainly occurs during the decompression cycle favored by the density decrease. Radical species form in this ...

Structure and dynamics of carbon dioxide clusters: A molecular dynamics study

Molecular dynamics calculations have been used to explore the structure and dynamics of clusters of carbon dioxide, ranging in size up to 164 molecules. The most detailed calculations have been carried out around T=90 K with a view to interpreting the results of infrared data on clusters produced in a seeded argon beam. Under appropriate conditions, in addition to liquid‐like clusters, we have been able to produce clusters with the following structures: (i) an ordered solid—analogous to the bulk crystal, (ii) a bulk–solid core with liquid‐like outer layers, (iii) an amorphous solid, and (iv) an amorphous solid core with liquid‐like outer layers. We have calculated the dispersion of the infrared active intramolecular Q3 mode for these clusters using the transition‐dipole transition‐dipole interaction that is usually invoked to explain the infrared spectrum of the bulk crystal. The resulting spectrum for the Q3 mode of each type of cluster is quite distinct in appearance. This observation suggests that mole...

Molecules under extreme conditions: Chemical reactions at high pressure

The basic features of diamond anvil cells and of the available techniques to study chemical reactions at ultrahigh pressures are reviewed. A number of study cases including reactions of simple molecular systems (like nitrogen, carbon dioxide and monoxide, nitrous oxide), of unsaturated compounds (like acetylene, butadiene, propylene, cyano derivatives) and of aromatics are discussed to illustrate the perspectives of chemical reactivity at ultrahigh pressures.

Fourier transform infrared study of the pressure and laser induced polymerization of solid acetylene

The polymerization of solid acetylene under pressure has been studied by Fourier transform infrared (FTIR) spectroscopy. Controlled laser irradiation cycles and the employment of infrared sensors to measure the sample pressure, allowed to separate the photochemical and the pressure effect on the injection and on the evolution of the reaction. The careful assignment of all the spectral features and analysis of their relative intensities and frequencies gave evidence to the specific effect of pressure and laser irradiation on the reaction products. Pressure induces an ordered growth of trans-polyenic species, while irradiation produces the opening of the double bonds and a consequent branching of the chains. Constant pressure measurements allowed to obtain precise information on the kinetics of the reaction. A monodimensional growth geometry, involving the molecules on the bc plane, agrees with the parameters extracted by the kinetic curves. Comparison between experiments at different temperatures suggests ...

The nature of intermolecular interactions between aromatic amino acid residues

The nature of intermolecular interactions between aromatic amino acid residues has been investigated by a combination of molecular dynamics and ab initio methods. The potential energy surface of various interacting pairs, including tryptophan, phenilalanine, and tyrosine, was scanned for determining all the relevant local minima by a combined molecular dynamics and conjugate gradient methodology with the AMBER force field. For each of these minima, single‐point correlated ab initio calculations of the binding energy were performed. The agreement between empirical force field and ab initio binding energies of the minimum energy structures is excellent. Aromatic–aromatic interactions can be rationalized on the basis of electrostatic and van der Waals interactions, whereas charge transfer or polarization phenomena are small for all intermolecular complexes and, particularly, for stacked structures. Proteins 2002;48:117–125.

Vibrational relaxation in molecular crystals

Abstract The relaxation processes responsible for the finite lifetime of excited vibrational states in molecular crystals are discussed on the basis of recent experimental and theoretical developments. Experimental information on the lifetime of optically active phonons is obtained in the time domain by time-resolved CARS spectroscopy or in the frequency domain by high-resolution infrared and Raman spectroscopy. The experimental results are then interpreted in terms of many-body perturbation techniques or of molecular dynamics simulation. The elementary mechanisms involved in vibrational energy relaxation processes are discussed in detail. These can be summarized as: (a) depopulation processes of phonon states via energy transfer to other phonon states; (b) pure dephasing processes due to interaction with thermal bath phonons; and (c) depopulation or scattering processes due to impurities and crystal defects. The contributions to the phonon lifetime and band profile of these different mechanisms are analy...

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.