Combining rare events techniques: phase change in Si nanoparticles

We introduce a combined Restrained MD/Parallel Tempering approach to study the difference in free energy as a function of a set of collective variables between two states in presence of unknown slow degrees of freedom. We applied this method to study the relative stability of the amorphous vs crystalline nanoparticles of size ranging between 0.8 and 1.8 nm as a function of the temperature. We found that, at variance with bulk systems, at low T small nanoparticles are amorphous and undergo an amorphous-to-crystalline phase transition at higher T. On the contrary, large nanoparticles recover the bulk-like behavior: crystalline at low $T$ and amorphous at high T

Criminalità e violenza in Sardegna: una interpretazione

All’inizio degli anni ’40 del secolo XIX, nell’ambito di una mirabile sintesi di storia sarda, Carlo Cattaneo descrive la Sardegna di fine ‘700 come terra isolata e sempre più desolata, a causa della “recrudescenza dell’anarchia feudale e della pastorale licenza”. A tale situazione connette un clima di violenza talmente diffuso che “sopra una popolazione di 360.000 abitanti incirca si contano sino a mille omicidj in un anno”

Omental well-differentiated liposarcoma: US, CT and MR findings

Liposarcomas are the most common of sarcoma tumours, they are usually located in the lower limbs, retroperitoneum, or abdominal cavity; up to date, only a few cases of omental liposarcoma with different histotype have been described. We present a case of omental well-differentiated liposarcoma and discuss imaging findings on ultrasound, computed tomography, and magnetic resonance to differentiate omental liposarcomas from other abdominal tumour entities

Investigation of 2,5-dimethylfuran oxidation reaction initiated by O(3P) atoms via synchrotron photoionization

xidation of 2,5-dimethylfuran (2,5-DMF), a biofuel candidate, initiated by atomic oxygen (O(3P)) is analyzed at temperatures of 550 and 700 K employing vacuum-ultraviolet synchrotron radiation from the Advanced Light Source (ALS) located at the Lawrence Berkeley National Laboratory. Collected photoionization spectra from products are used for their identification by comparison to literature and simulated spectra. The simulation for photoionization spectra is obtained by approximating Frank-Condon factors based on the vibronic transitions from neutral to cation, calculated at the B3LYP/CBSB7 level of theory. The CBS-QB3 composite method is used to probe the thermodynamics of the proposed reaction pathways, which are.O(3P) addition to and hydrogen abstraction from 2,5-DMF

Experimental and Theoretical Investigations of the Structure and the Stability of the BNSi Molecule

Theoretical computations were carried out to determine the structure and molecular parameters of the BNSi molecule. The most stable isomer is found to have a BNSi linear geometry. Thermal functions as derived from the theoretical computed molecular parameters were used in the evaluation of the thermodynamic properties of BNSi from high-temperature Knudsen effusion mass spectrometric equilibrium data. From the reactions analyzed by the second-law and third-law methods, the enthalpy of formation,ΔfHo0, and of atomization, ΔaHo0, in kJ mol−1, for BNSi, were obtained as 398±16 and 1078±17, respectively

An observable for vacancy characterization and diffusion in crystals

To locate the position and characterize the dynamics of a vacancy in a crystal, we propose to represent it by the ground state density of a quantum probe quasi-particle for the Hamiltonian associated to the potential energy field generated by the atoms in the sample. In this description, the h^2/2mu coefficient of the kinetic energy term is a tunable parameter controlling the density localization in the regions of relevant minima of the potential energy field. Based on this description, we derive a set of collective variables that we use in rare event simulations to identify some of the vacancy diffusion paths in a 2D crystal. Our simulations reveal, in addition to the simple and expected nearest neighbor hopping path, a collective migration mechanism of the vacancy. This mechanism involves several lattice sites and produces a long range migration of the vacancy. Finally, we also observed a vacancy induced crystal reorientation process

Thermodynamic Investigation of the Si7 and Si8 Clusters by Knudsen Cell Mass Spectrometry

The Knudsen cell mass spectrometric method has been employed to measure the partial pressures of the Si[sub 7] and Si[sub 8] clusters under equilibrium conditions above liquid silicon, contained in a boron nitride liner inside a graphite Knudsen cell. Gaussian 2 (G2) theory and B3LYP density functional method were employed to determine the geometry, the vibrational frequencies, and the binding energy of the Si[sub 8] cluster. From the all-gas analyzed equilibria the following atomization enthalpies, Δ[sub a]H[sub 0][sup o](Si[sub n]), and enthalpies of formation, Δ[sub f]H[sub 298.15][sup o](Si[sub n]), in kJ mol-1, have been obtained: Si[sub 7], 2381±36 and 743±36; Si[sub 8], 2735±65 and 837±65. Experimental literature values for the electron affinities of Si[sub n](n=3–8) have been combined with present and previous results to obtain the bonding energies for the Si[sub n][sup -](n=3–8) cluster anions. The experimental atomization energies are compared with available theoretical values

Thermodynamic Study of the Gaseous Molecules Al2N, AlN, and Al2N2 by Knudsen Cell Mass Spectrometry

The Knudsen effusion mass spectrometric method has been employed to measure the equilibrium partial pressures of the Al2N molecule over the AlN–Au–graphite system. Theoretical computations were carried out to determine the structure, molecular parameters, and thermodynamic properties of Al2N. The partial pressures have been combined with the calculated thermal functions to determine the atomization enthalpy,ΔaHo0, and enthalpy of formation,ΔfHo298.15, in kJ mol−1, of 783.2±15 and 342.7±15 for Al2N, respectively. Upper values for the dissociation energy of AlN, Do0(AlN,g)⩽368±15 kJ mol−1, and for the atomization enthalpy of Al2N2, ΔaHo0(Al2N2,g)⩽1402 kJ mol−1 have been obtained. These results are discussed and compared with recent theoretical literature values

Response to “Comment on ‘Atomization energies and enthalpies of formation of the SnBi[sub n] (n=1–3) gaseous molecules by Knudsen cell mass spectrometry’ ” [J. Chem. Phys. 118, 4766 (2003)]

Discusses atomization energies and enthalpies of formation of gaseous molecules of compounds containing tin and bismuth by Knudsen cell mass spectrometry. Procedure followed to evaluate the mass spectrometric equilibrium data regarding these molecules; Statistics of atomization energies and enthalpies calculated; Factors contributing to the necessity of high level of calculations to obtain these figures

Thermodynamic Investigation of the AINC and AICN Isomers by Knudsen Cell Mass Spectrometry

Equilibria involving the isomers AlNC and AlCN above a mixture of aluminum nitride, graphite, and gold contained in a graphite Knudsen cell were investigated with a mass spectrometer. The enthalpies of formation,ΔfHo0, and of atomization, ΔaHo0, in kJ mol−1, for AlNC and AlCN, were derived as 281.3±14 and 303.8±14, and as 1228.1±15 and 1205.6±15, respectively