Marco Malvaldi

The solvent effect on the Diels–Alder reaction in ionic liquids: multiparameter linear solvation energy relationships and theoretical analysis

This review illustrates how ionic liquid properties can affect Diels–Alder reactions. The mechanisms by which ionic solvents enhance the rate and selectivity of the reaction are discussed on the basis of correlation studies using empirical parameters and theoretical calculations.

Ionic liquids: Solvation ability and polarity

The role of ionic liquids (ILs) as solvents in chemistry is limited by the poor understanding of the solvation phenomenon in these media. The usual classification criteria used for molecular solvents through various experimental measurements fail to insert ILs into a univocal classification for ILs. Here, we first discuss the unsuitability of the usual interpretative scheme for molecular liquids and elucidate schematically the mechanism of solvation in ILs, pointing out the peculiarities that differentiate them with respect to molecular liquids. Second, we focus on reactivity and reaction kinetics in ILs, underlining the many problems that the complexity of these media reflects on the interpretation of kinetic data and some possible approaches to understand qualitatively the (often not trivial) kinetic problems for reactions performed in ILs.

An unusual common ion effect promotes dissolution of metal salts in room-temperature ionic liquids: a strategy to obtain ionic liquids having organic–inorganic mixed cations

A simple strategy has been reported to prepare new ionic liquids with binary systems of organic–inorganic cations exploiting the common ion effect, i.e. dissolving metal salts with organic or inorganic anions (bistriflylimide or nitrate) in ionic liquids bearing the same anions. The resulting concentrated solutions of metal cations in ionic environments, which may have great potential for electrochemical processes, have been characterized by X-Ray photoelectron spectroscopy (XPS) and electrospray ionization mass spectrometry (ESI-MS)

Ab initio study of ionic liquids by KS-DFT/3D-RISM-KH theory.

Properties of molecules solvated in ionic liquids (ILs) are strongly affected by solvent environment. For this reason, to give reliable results, ab initio calculations on solutes in ILs, including ions constituting ionic liquid itself, have to self-consistently account for the change of both electronic and classical solvation structure in ILs. Here, we study the electronic structure of the methyl-methylimidazolium ion in the bulk liquid of [mmim][Cl] by using the self-consistent field coupling of Kohn-Sham density functional theory and three-dimensional molecular theory of solvation (aka 3D-RISM) with the closure approximation of Kovalenko and Hirata. The KS-DFT/3D-RISM-KH method yields the 3D distribution of the IL solvent species around the [mmim] solute, underlying the most important peculiarities of this kind of systems such as the stacking interaction between neighboring cations, and reproduces the enhancement of the dipole moment resulting from the polarization of the cation by the solvent in a very good agreement with the results of an ab initio MD calculation. The KS-DFT/3D-RISM-KH method offers an accurate and computationally efficient procedure to perform ab initio calculations on species solvated in ionic liquids.

Solvation thermodynamics of alkali and halide ions in ionic liquids through integral equations

In this work, we study the solvation thermodynamics and other solvation properties of small ions in two room-temperature ionic liquids, dimethyl imidazolium hexafluorophosphate [mmim] [pf6] and dimethyl imidazolium chloride [mmim][cl] with the reference interaction site model (RISM). The nature of the charge affects several aspects of solvation, from electrostriction to the mutual disposition of cations around the solute; nevertheless, the long-range screening behavior of the liquid appears to be insensitive to both charge and dimensions of the solute. The ion solvation is energy driven, as expected for the nature of the solvent, and displays a marked asymmetry between cation and anion solvation chemical potential. Such asymmetry is dependent, even qualitatively, on the ionic liquid chosen as solvent. Partial molar volumes of ions in solution are found to follow the nature of ion-solvent interaction.

Ab Initio Study of the Diels−Alder Reaction of Cyclopentadiene with Acrolein in a Ionic Liquid by KS-DFT/3D-RISM-KH Theory

We study the Diels-Alder reaction between cyclopentadiene and acrolein in a model room-temperature ionic liquid ([mmim][PF6]) as a solvent. The calculations have been performed with the KS-DFT/3D-RISM-SCF theory, where the reactants and transition state (TS) have been represented at a QM level, while the solvent is represented by a 3D distribution of classical (charge + LJ) sites obtained by solving the 3D-RISM integral equation. We show that this method, being computationally efficient, is able to reproduce the main experimental features displayed by the experiments, concerning the reaction rate enhancement and augmentation of the endo/exo ratio in ionic liquids (ILs). We find that the IL distorts noticeably the transition state geometry, inverting the order of the frontier orbitals and leading to an enhancement of the asynchronicity of the reaction. Finally, we find, in agreement with recent work, that formation of the hydrogen bond between the unique C2 hydrogen of the imidazolium ring is not essential to explain the peculiar features of these reactions in ILs.

Excess entropy scaling of diffusion in room-temperature ionic liquids

Excess entropy scaling relationships for diffusivity of ions in room-temperature ionic liquids are tested using molecular dynamics simulations for a model ionic liquid, dimethyl imidazolium chloride. The thermodynamic excess entropy of the single ions (estimated from the ion-ion pair correlation functions) is shown to be very strongly correlated with the diffusivity. An essential feature of these systems, the fact that the heavier and larger cation has a greater diffusivity with respect to the anion, is correctly captured by the excess entropy calculations, which estimates the diffusivity ratio between the two ions with noticeable precision.

From molten salts to ionic liquids: effect of ion asymmetry and charge distribution

We studied the influence of ion shape asymmetry and charge distribution on the liquid structure and transport properties of ionic liquids by the molecular dynamics of schematic models. The ion structure asymmetry results in a less compact packing, while the charge distribution gives potential wells with reduced depth with respect to the single-site centred charges. Both these aspects contribute to accelerate the dynamics of the melt. The liquids display a clearly detectable supercooled region, in which two different liquid structures appear to be present contemporarily. The diffusion process, as usual in supercooled liquids, proceeds through a cage-escaping process; the diffusion constants nevertheless show an opposite behaviour with respect to expectations from the cavity size distribution. The observed transient liquid structure can help to explain some surprising features of the diffusive behaviour of these systems.

Infrared multiphoton absorption and alignment of diatomic molecules in a continuous wave field

We compute multiphoton absorption spectra of HF and LiH in a continuous wave field, as generic examples of diatomic vib–rotors. Absorbed energy and other observables are determined by numerical integration of the time-dependent Schrodinger equation, taking into account the coupling of both permanent dipole and electronic polarizability with radiation. Three peculiar effects of strong laser fields are investigated: first, the frequency shift of the absorption peaks and its dependence on laser intensity, vibrational and rotational quantum numbers of the final state, and molecular parameters; second, the existence of subharmonic resonances, strongly influenced by the electronic polarizability; third, the considerable degree of alignment (with negligible orientation) along the laser polarization axis.

Dynamics of relaxation of entangled polymers in shear flow

The application of shear flow to entangled polymer melts can strongly modify its rheological and physicochemical behaviors, giving rise to an acceleration of several chemical processes such as diffusion-controlled reactions. In the present work, we investigate the modification of conformational and diffusive properties of an entangled polymer in shear flow by numerical methods. The flow affects both the conformational and diffusive properties of the system, giving rise to a quasinematic ordering of the macromolecules which take prolate spheroid shape with the main axis aligned to the shear direction. The shear flow is found to accelerate the overall diffusion of the chains in all directions at times longer than the polymer relaxation time. The polymer chains display a quite peculiar displacement behavior in direction parallel to the flow. At the same conditions, the linear relation between the diffusion constant in direction perpendicular to the flow and the inverse of the relaxation time, usually adopted in equilibrium regimes, is shown to hold even in the presence of flow.