L. Bencivenni

Thermodynamic study of the vaporization of uracil

Abstract The vapour pressure of uracil was measured in the temperature range 452–587 K using different techniques and the pressure—temperature equation log P (kPa) = 12.13 ± 0.50 — (6823 ± 210)/ T was derived. The thermodynamic functions of gaseous and solid uracil were also evaluated through spectroscopic and calorimetric measurements. The sublimation enthalpy of uracil, Δ H 0 298 = 131 ± 5 kJ mole −1 , was derived from second and third law treatment of the vapour data.

Hydrogen bonding in barbituric and 2-thiobarbituric acids: a theoretical and FT-IR study

Abstract The effects of intermolecular hydrogen bonding on the molecular properties of barbituric acid (BA) and thiobarbituric acid are discussed on the basis of density functional theory calculations. B3LYP methods were applied to monomers and cyclic dimers. Trimer and hexamer of BA were studied as examples where several CO and NH groups are involved in hydrogen bonding. The theoretical IR spectra of monomers and all oligomers here considered are compared with the FT-IR spectra measured in Ar and nitrogen matrices at different concentrations.

Matrix and ab initio infrared spectra of thiourea and thiourea-d4

Abstract The FTIR spectrum of thiourea and thiourea-d 4 isolated in argon and nitrogen matrices are presented for the first time and discussed in terms of normal modes predicted by ab initio calculations. Vibrational frequencies, IR intensities and isotopic shifts obtained from the density functional B3-LYP/6-31+G(2d,p) calculations show the most satisfactory agreement with experiments. In contrast with previous results, the present theoretical study indicates non-planarity of the molecule. The lowest energy structure has C 2 symmetry and the less stable isomers has C s symmetry. Experimental data clearly indicate the presence of a single isomer in the vapor. The decomposition of thiourea for vaporization temperature over 410 K is also discussed.

A thermodynamic study of the sublimation processes of aluminium and copper acetylacetonates

Abstract The vapor pressures of aluminium and copper acetylacetonates were measured in the temperature ranges 337–405 K and 316–445 K, respectively, by using the torsion effusion method. The corresponding sublimation enthalpies, ΔH2980 = 47 ± 1 kJ mole−1 for aluminium acetylacetonate and ΔH2980 = 57 ± 1 kJ mole−1 for copper acetylacetonate, were derived by treating the vapor pressure data by the so-called second- and third-law methods of thermodynamics. Heat capacity measurements were carried out for the solid complexes from 4.2 to 450 K. The thermodynamic functions of the solid phase were derived from the measured heat capacity values and those of the vapor phase from spectroscopic and structural data.

MOLECULAR CONFORMATIONS AND HARMONIC FORCE FIELD OF 1,3,5-BENZENETRIOL MOLECULE FROM AB INITIO AND DENSITY FUNCTIONAL THEORY INVESTIGATIONS

Abstract The molecular conformers of the molecule 1,3,5-benzenetriol have been studied by ab initio and density functional methods to determine optimized equilibrium geometries, harmonic vibrational frequencies and relative stability. The results of the quantum-chemical calculations have been used to investigate the functional theory-infrared (FT-IR) spectrum of the 1,3,5-benzenetriol vapors trapped in Ar matrix at 12 K.

A new technique for the study of phase transitions by means of energy dispersive x-ray diffraction. Application to polymeric samples

A new application of energy dispersive x-ray diffraction to the kinetics of phase transitions is reported. The aim of this work is to provide a new investigation tool to follow the evolution of polymeric systems from the initial to the final phase. The theoretical treatment is developed both for constant density transitions and for the variable density ones. The experimental route is illustrated by an example.

Ab initio SCF study on LiClO4 and Li2SO4 molecules: Geometries and vibrational frequencies

Abstract The coordination structures of the molecules LiClO4 and Li2SO4 were studied by ab initio SCF calculations carried out at the 3-21G*, 6-31G* and DZP basis sets level; the geometries of ClO4− and SO42− were determined at the 3-21G*, 6-31G*, 6-31 + G*, DZP and TZP levels. LiClO4 and Li2SO4 prefer at all the levels of the HF-SCF theory bidentate structures of C2v and D2d symmetries, respectively. The tridentate and monodentate C3v, symmetry structures of LiClO4, and the C2v and tridentate C3v ones of Li2SO4 were also considered in detail. Harmonic frequencies of vibration of the anions and of each coordination model of LiClO4 and Li2SO4 as well as the 18O spectra of LiClO4 (C2v symmetry) and Li2SO4 (D2d symmetry) are reported and commented on using the experimental IR spectroscopy results.

Two Different Models to Predict Ionic-Liquid Diffraction Patterns: Fixed-Charge versus Polarizable Potentials

This study reports the performance of classical molecular dynamics (MD) in predicting the X-ray diffraction patterns of butylammonium nitrate (BAN) and two derivatives, 4-hydroxybutan-1-ammonium nitrate (4-HOBAN) and 4-methoxybutan-1-ammonium nitrate (4-MeOBAN). The structure functions and radial distribution functions obtained from energy-dispersive X-ray diffraction spectra, recorded newly for BAN and for the first time for 4-MeOBAN and 4-HOBAN, are compared with the corresponding quantities calculated from MD trajectories, to access information on the morphology of these liquids. The different behavior of two force fields, a polarizable multipole force field and a fixed-charge one supplemented by an explicit three-body term, is shown. The three-body force field proves to be superior in reproducing the intermediate q range, for which the polarizable force field gives the wrong peak position and intensities. In addition, both models can correctly account for the presence or absence of a low q peak in the scattering patterns.

Structural and spectroscopical study of glutamic acid in the nonzwitterionic form

Abstract The molecular structure of glutamic acid in the non-zwitterionic form has been optimized by using the semiempirical MNDO and AMI methods and an ab initio calculation at the 4–31G level. The results were compared with previous reported data for other amino acids. The ab initio optimized structure was used as the starting point for a further force field calculation, and vibrational frequencies and theoretical assignments were thus obtained. In order to compare these results with experiments, the FT-IR spectrum of glutamic acid in an argon matrix was recorded and the measured frequencies were successfully compared with theoretical values, which were previously scaled by using a set of scaling factors.

Molecular aggregation phenomena in solution: an energy dispersive X-ray diffraction study of concentrated imidazole water solutions

Abstract An energy dispersive X-ray diffraction study of concentrated imidazole water solutions is reported. It reveals the presence of extended solute–solute aggregates. Such results can be interpreted in terms of a branch of a pseudo-helix of imidazole dimers interacting with water molecules. The mean distance between molecules in the dimer is ∼3.7 A, while the mean distance between dimers is 4.75–4.95 A depending on concentration. The model structure functions and theoretical radial distribution functions fit the experimental data.