Fabio Ramondo

Liquid structure of 1-alkyl-3-methylimidazolium-hexafluorophosphates by wide angle x-ray and neutron scattering and molecular dynamics

We report for the first time joined energy dispersed x-ray and neutron diffraction experiments on a series of (both protiated and selectively deuteriated) 1-alkyl-3-methylimidazolium hexafluorophosphate salts (alkyl = butyl, hexyl, octyl) at ambient conditions. The x-ray experimental data are used to optimize the interaction potential used for running molecular dynamics simulations on these systems. Such a potential leads to a good description of neutron scattering data from the samples without additional refinement, thus further validating the potential definition. The molecular dynamics simulations were used to access microscopic information on the morphology of the proposed systems, thus probing the role played by alkyl chain length on the structure. The comparison of x-ray weighted and neutron-weighted computed diffraction patterns allows the rationalization of several diffraction features. Further insight into cation-anion coordination and alkyl chain conformational equilibrium is provided on the basis of the MD-derived snapshots, confirming and extending previously obtained results on these issues.

Molecular structure of aniline in the gaseous phase: A concerted study by electron diffraction and ab initio molecular orbital calculations

The molecular structure of free aniline has been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF and MP2 levels of theory, using the 6-31G*(6D) basis set. Least-squares refinement of a model withCs symmetry, with constraints from MP2 calculations, has led to an accurate determination of the C-C-C angle at theipso position of the benzene ring, α=119.0±0.2‡ (where the uncertainty represents total error). This parameter provides information on the extent of the interaction between the nitrogen lone pair and the π system of the benzene ring, and could not be determined accurately by microwave spectroscopy. The angles at theortho, meta, andpara positions of the ring are 120.3±0.1‡, 120.7±0.1‡, and 119.0±0.3‡, respectively. Important bond distances are 〈rg(C-C)〉=1.398±0.003 å andrg(C-N) =1.407±0.003 å. The effective dihedral angle between the H-N-H plane and the ring plane, averaged over the large-amplitude inversion motion of the amino group, is 〈¦Τ¦〉=44±4‡. The equilibrium dihedral angle is calculated to be 41.8‡ at the HF level and 43.6‡ at the MP2 level, in agreement with far-infrared spectroscopic information. The MO calculations predict that the differencer(Cortho-Cmeta) -r(Cipso-Cortho) is 0.008–0.009 å. They also indicate that the nitrogen atom is displaced from the ring plane, on the side opposite to the amino hydrogens. The displacement is 0.049 å at the HF level and 0.072 å at the MP2 level. The two calculations, however, yield very different patterns for the minute deviations from planarity of the ring carbons.

Unravelling the structure of protic ionic liquids with theoretical and experimental methods: ethyl-, propyl- and butylammonium nitrate explored by Raman spectroscopy and DFT calculations.

We present an analysis of gas-phase structures of small clusters of n-alkylammonium nitrates (ethyl, propyl, and butyl) together with vibrational Raman spectroscopy of their respective liquid phases. The assignment and interpretation of the resonant frequencies have been performed by comparison with high-quality ab initio (DFT) computations. The theoretical spectra are in excellent agreement with the measured ones and allow the interpretation and assignment of almost all the spectral features. A careful analysis of the vibrational frequencies and of the electronic structure of the compounds has provided additional information on various structural features and on the rather complex hydrogen bonding network that exists in such compounds. A geometric structure of the short-range local arrangement in the bulk phases is also proposed.

Structure of the Molten Salt Methyl Ammonium Nitrate Explored by Experiments and Theory

We present an analysis of the structure of the monomethylammonium nitrate (MMAN) compound. Vibrational Raman spectroscopy and X-ray powder diffraction have been used to characterize the bulk phases of MMAN, and assignment of the resonant frequencies has been performed by ab initio (DFT) computations on small clusters of the compound. The theoretical spectra are in excellent agreement with the experimental ones and provide a means by which an interpretation of the hydrogen-bonding network that exists in such compound can be analyzed. In particular, we found that the spectrum of one of the solid phases is structurally very similar to that of the liquid. We present experimental evidence for the existence of such phase both from X-ray data and Raman spectra which, in turn, is easily interpreted with a one-to-one correspondence with the ab initio simulation of the small clusters. A geometric structure of the short-range local arrangement in these two bulk phases is therefore proposed.

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.

Liquid structure of 1-ethyl-3-methylimidazolium alkyl sulfates by x-ray scattering and molecular dynamics

We report a joined X-ray diffraction and molecular dynamics study on the series of 1-ethyl-3-methylimidazolium alkyl sulfates (alkyl = ethyl, butyl, hexyl, octyl) ionic liquids. A general good agreement between experimental and theoretical structure functions has been found for each term of the series in all ranges of q values. Once the quality of the employed force field in reproducing structural data was tested, we used dynamics simulations to access information on morphology and properties of these systems. The series of ionic liquids presents nanoscale structural heterogeneity, whose size depends on the anion alkyl chain size. Analyzing our simulation data on the basis of alkyl chain length, we propose a structural model consistent with the presence of low q peaks.

Toward a more accurate silicon stereochemistry: An electron diffraction study of the molecular structure of tetramethylsilane

The present electron diffraction study of the molecular structure of tetramethylsilane, augmented with theoretical calculations, answers the need for accurate and detailed information on the most fundamental molecules containing silicon. The Si—C bond length is rg = 1.877 ± 0.004 Å, in perfect agreement with a previous study (Beagley, B.; Monaghan, J. J.; Hewitt, T. G. J. Mol. Struct.19718 401). The C—H bond length is rg= 1.110 ± 0.003 Å and the Si—C—H angle is 111.0 ± 0.2°. The experimental data are consistent with a model of Td symmetry and staggered methyl conformation. The barrier to methyl rotation is estimated to be 5.7 ± 2.0 kJ mol−1 on the basis of the experimentally observed average torsion of the methyl groups.

Ab initio study on the radical anions SO−3 and CO−2 and on the charge-transfer complexes MSO3 and MCO2 (M = Li, Na)

Abstract The structures of the MSO 3 ion pairs (M = Li, Na) and the effects of coordination on the vibrational modes of the radical anion SO − 3 were examined by means of ab initio molecular orbital calculations. Both the bidentate and tridentate structures of LiSO 3 and NaSO 3 were predicted to be stable isomers and bidentate binding was inferred as the preferred coordination model. The stability of the lowest energy structures was tested against frozen-core UMP2, UMP3 and UMP4 calculations. Harmonic frequencies were calculated for the bidentate and tridentate LiSO 3 and NaSO 3 complexes and 34 S and 18 O frequency shifts reported for the two stable structures of NaSO 3 . The study was extended to the radical anion CO − 2 and to the bidentate and monodentate isomers LiCO 2 and NaCO 2 . The geometry, stability and vibrational spectra of the stable bidentate and monodentate isomers of LiCO 2 and NaCO 2 are discussed.

FTIR matrix isolation study on gaseous calcium dihalide molecules

Abstract FTIR measurements were carried out on matrix-isolated calcium dihalide vapours. CaF 2 , CaCl 2 , CaBr 2 and CaI 2 samples were vaporized at temperatures ranging from 1100 K to 1700 K and the vapours were trapped in argon and krypton matrices. Bands of monomers, dimers and in one case of a more complex aggregate were observed. The spectra were interpreted on the basis of polarizable ion model calculations. Regarding the structure of the dimer molecule, D 2h symmetry was established from the experimental measurements and theoretical results. A trimer molecule was found for CaF 2 and a D 2d symmetry structure is suggested for it.

Ab initio study on the coordination structures of AsO3−, VO3− and CO32− with alkali cations

Abstract This study reports the results of an ab initio Hartree-Fock - Self-Consistent-Field study carried out on D3h symmetry AsO3−, VO3− and CO3·- and on the coordination structures of these anions with alkali cations. Geometry optimizations and vibrational frequencies were calculated from polarized split-valence basis sets. Bidentate, monodentate and tridentate coordination structures were considered for LiAsO3, LiVO3, Li2CO3, Na2CO3 and LiNaCO3. Bidentate binding is the most stable coordination structure for all the molecules, whereas the monodentate structures of Li2CO3, Na2CO3 and LiNaCO3 are first-order saddle points with regard to the in-plane migration of the metal atom and are the transition states between equivalent bidentate structures. Monodentate LiAsO3 and LiVO3 are second-order saddle points with respect to the in-plane and out-of-plane motion of the alkali metal. The trigonal pyramidal structures of all the molecules are the highest energy configurations and are higher-order saddle points. This work includes MP2/6-31G∗ geometry optimizations of the bidentate coordination structures of HCO2Li, HCO2Na, LiNO2, NaNO2, LiPO2, LiClO2, LiSO2, NaSO2, LiBO2, NaBO2 and of LiNO3 and LiPO3.