Enrico Bodo

Structural properties of 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ionic liquids: X-ray diffraction data and molecular dynamics simulations.

X-ray diffraction data for 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amides are reported as a function of the length of the alkyl chain on the imidazolium ring. The measured diffraction patterns have been compared with the theoretical patterns calculated (from the geometries obtained) with molecular dynamics simulations. This provides a detailed description (at the atomistic level) of the morphology in the liquid state of these salts, highlighting the role played by the alkyl chain length. An analysis of the behavior of the hydrogen bonds that are formed between the imidazolium acidic protons and the anion is presented.

F+ D2 reaction at ultracold temperatures

We present a quantum mechanical study of the reaction F+D2 at ultracold temperatures based on the potential energy surface of Stark and Werner. The reaction cross section at low energies is controlled by the tunneling through the activation barrier, a mechanism that is favored by the long duration of the collision at those energies. Differences are found in the behavior of the reactive cross section compared to that for F+H2, due to the changed mass and zero point energy. A new feature is detected in the reaction probabilities and is attributed to a Feshbach resonance corresponding to a metastable state in the exit channel.

The gas-phase lithium chemistry in the early universe: elementary processes, interaction forces and quantum dynamics

Abstract In this work we review the efforts made in the last 10 years to understand the neutral and ionic chemistry of LiH. The relevance of most of the studies on this subject is due to the possible importance of the LiH molecules and relative ions in the primordial universe chemistry. Although it is still not clear what could be the role of LiH in the early universe chemistry, since experimentally important data are indeed still missing and its relevance may be limited by the small abundance of Li molecular species that is thought to exist at the recombination era, it is already important from a fundamental point of view to gather the various results obtained up to now since, in our opinion, they are already able to shed light on a large portion of the gas-phase chemistry of LiH and of its positive ion. Most of the results that will be summarized here are theoretical and computational, intending to provide the present state of our knowledge on the relevant potential energy surfaces and dynamical processes which ensue from their features.

Accurate potential energy surfaces for the study of lithium–hydrogen ionic reactions

Three-dimensional potential energy surfaces (PESs) have been computed, and numerically fitted, for the two lowest electronic states of the LiH2+ system, which are of importance for the astrophysically relevant LiH++H→Li++H2 and LiH+H+→Li+H2+ exoergic reactions. We extend the recently computed 11 000 multi reference valence bond ab initio energy values [Martinazzo et al., Chem. Phys. 287, 335 (2003)] with 600 multireference configuration interaction calculations with complete active self-consistent field reference functions and a large Li(12s10p4d1f)/H(8s6p3d1f) basis set. We have fitted the full set of energy values with a modified Aguado–Paniagua ansatz that correctly takes into account in this ionic system the important long-range contributions to the potential. Calibration calculations on the three-body potential term and the use of essentially exact results for the two-body contributions allow us to estimate the overall accuracy of the analytic PESs to be within that required for accurate quantum scat...

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.

The interpretation of diffraction patterns of two prototypical protic ionic liquids: a challenging task for classical molecular dynamics simulations.

In this study, we discuss the performance of classical molecular dynamics in predicting the experimental X-ray diffraction patterns of liquid ethylammonium nitrate (one of the simplest protic room-temperature ionic liquid showing amphiphilic behavior) and of its hydroxy derivative (2-ethanolammonium nitrate, 2-HOEAN). Newly recorded energy-dispersive X-ray diffraction structure factors are compared with the corresponding quantities extracted from molecular dynamics simulations. Other useful theoretical and experimental indicators are used as a probe of the local structure of the title ionic liquids. We shall show that the use of a general purpose, two-body terms only, force field, such as OPLS/AA is able to describe most of the structural experimental data. However, we shall also point out that an improved description of some key structural features observed in the X-ray radial distribution function, can be obtained very easily by adding a general three-body potential energy term instead of changing the two-body potential parameters, in order to optimize the agreement with experimental data. This three-body term turns out to be naturally able to describe the complex polarization effects due to hydrogen bonding without requiring a quanto-mechanical treatment or a polarizable force field. In addition the present model turns out to be able to account for the presence of a low-Q peak in the scattering patterns of EAN, which has been commonly interpreted as a manifestation of the amphiphilic nature of this compound.

Chemical reactions in the limit of zero kinetic energy: virtual states and Ramsauer minima in F + H2 → HF + H

The behaviour of reactive scattering at ultracold temperatures is explored by calculating the real and imaginary parts of the scattering length for the reaction of F with a molecule composed of a pair of pseudo-hydrogen atoms of arbitrary mass. The origin of a low energy feature in the cross section for the reaction of F with H2 and its absence for the reaction with D2 is investigated. Close-coupling calculations of the scattering matrix show that the F–H2 feature arises from the presence of a virtual state associated with the van der Waals well in the entrance channel and that the virtual state is responsible for the enhanced zero temperature rate coefficient of the F–H2 reaction. For a mass of about 1.12 hydrogen masses the virtual state turns into a zero energy resonance and the corresponding zero temperature rate coefficient is 1 × 10−9 cm3 s−1 despite an energy barrier of 300 K. Evidence in support of the virtual state is also provided by the detection of a deep Ramsauer–Townsend minimum in the elastic component of the total cross section for F–H2 which the present calculations predict to occur at low energies.

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.

Bosonic helium droplets with cationic impurities : Onset of electrostriction and snowball effects from quantum calculations

Variational Monte Carlo and diffusion Monte Carlo calculations have been carried out for cations such as Li(+), Na(+), and K(+) as dopants of small helium clusters over a range of cluster sizes up to about 12 solvent atoms. The interaction has been modeled through a sum-of-potential picture that disregards higher order effects beyond atom-atom and atom-ion contributions. The latter were obtained from highly correlated ab initio calculations over a broad range of interatomic distances. This study focuses on two of the most striking features of the microsolvation in a quantum solvent of a cationic dopant: electrostriction and snowball effects. They are discussed here in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the cluster features.

Possible reaction paths in the LiH+2 chemistry: a computational analysis of the interaction forces

Abstract The present study addresses the problem of establishing from fully ab initio quantum methods some quantitative features of the chemical interactions which play an important role in the ionic lithium chemistry of astrophysical relevance. In particular, the LiH + 2 energetics is examined by looking at the various possible chemical channels producing LiH, LiH + , H 2 and H + 2 . An accurate evaluation of the relative energy landscapes as the complex breaks up into its asymptotic partners is presented for the first time. It allows us to clearly select those reactive pathways which can be excluded when setting up a kinetic modeling of the lithium chemistry network in early universe processes.