Christian Silvio Pomelli

Recent Advances in the Description of Solvent Effects with the Polarizable Continuum Model

Publisher Summary This chapter presents a set of methods addressed to the study of solvation problems at the quantum mechanical level with the use of polarizable continuum model (PCM). The chapter describes the evolution of continuous methods since the first proposals or the first formulation at a QM level, semiempirical, or ab initio. PCM belongs to the family of methods in which focuses on a limited portion of matter, the ‘solute’ (one or more molecules) while the remaining, and larger, portion of the solution, called here the ‘solvent’, is treated at a lower level of accuracy. PCM is a quantum mechanical (QM) method in which use is made of an effective Hamiltonian for the solute M , and the corresponding Schrodinger equation is generally (but not compulsory) treated at the ab initio level. PCM makes use of continuous solvent distributions to describe the solute-solvent interaction potential. The chapter demonstrates that PCM manages to treat not only the well known model based on a uniform isotropic dielectric description of the solvent, whose interactions are limited to the electrostatic terms, but also more complex models including interactions of different physical origin and other solvent distribution functions.

A rationalization of the solvent effect on the Diels–Alder reaction in ionic liquids using multiparameter linear solvation energy relationships

The Diels-Alder reaction between cyclopentadiene and three dienophiles (acrolein, methyl acrylate and acrylonitrile) having different hydrogen bond acceptor abilities has been carried out in several ionic liquids and molecular solvents in order to obtain information about the factors affecting reactivity and selectivity. The solvent effects on these reactions are examined using multiparameter linear solvation energy relationships. The collected data provide evidence that the solvent effects are a function of both the solvent and the solute. For a solvent effect to be seen, the solute must have a complimentary character; selectivities and rates are determined by the solvent hydrogen bond donation ability (alpha) in the reactions of acrolein and methyl acrylate, but not of acrylonitrile.

Influence of Structural Variations in Cationic and Anionic Moieties on the Polarity of Ionic Liquids

The polarity of a series of ionic liquids (ILs) arising from the quaternarization of N-methylmorpholine, N-methylpyrrolidine, N-methylpiperidine, N-methylazepane, 4-hydroxy-1-methylpiperidine, 1,2-dimethylimidazole, and 1-methylimidazole with simple alkyl chains and/or hydroxyl (mono- or dihydroxyl) functionalized alkyl chains and having bistriflimide, dicyanamide, or nitrate as counteranions has been investigated using solvatochromic dyes and expressed in terms of E(T)(N) and Kamlet-Taft parameters (dipolarity/polarizability (π*), hydrogen bond donor acidity (α), and hydrogen bond basicity (β)). Significant variations of polarity were observed on changing the anion and cation combination. The resulting E(T)(N) and α values were strongly anion dependent; on going from bistriflimide to dicyanamide, a significant decrease in E(T)(N) and α values was observed. On the other hand, the alkyl chain length has only a moderate effect on these parameters; either an increase or decrease in E(T)(N) and α values was observed on increasing the alkyl chain length, depending on the cation core. In the case of cyclic onium salts, the size of the cation ring affected the α parameter; the ILs based on the seven-membered ring system N-methyl-N-butylazepanium (also named N-methyl-N-butylhexamethyleneiminium, [HME(1,4)](+)) have high polarity values, comparatively to the ILs based on analogous five- and six-membered cyclic cations (pyrrolidinium and piperidinium). The introduction of the OH groups on the cation alkyl chain increases the polarity; the effect is substantial for the first OH group and more moderate for the second. Also, the thermosolvatochromism (changes in solvatochromic properties with the change in temperature) was studied for four dihydroxyl functionalized ILs. Finally, the principal component analysis (PCA) carried out on 67 ILs has shown that there are only two statistically relevant parameters: PC1, a weighted sum of E(T)(N) and α, which is able to discern between the cation core structure, functionalization, and cation-anion association, and PC2, very close to β, which is related principally to the anion nature.

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.

Ecotoxicity of pristine graphene to marine organisms

The ecotoxicity of pristine graphene nanoparticles (GNC1, PGMF) in model marine organisms was investigated. PGMF resulted more toxic than GNC1 to the bioluminescent bacterium Vibrio fischeri and the unicellular alga Dunaliella tertiolecta on the basis of EC50 values (end-points: inhibition of bioluminescence and growth, respectively). No acute toxicity was demonstrated with respect to the crustacean Artemia salina although light microscope images showed the presence of PGMF and GNC1 aggregates into the gut; a 48-h exposure experiment revealed an altered pattern of oxidative stress biomarkers, resulting in a significant increase of catalase activities in both PGMF and GNC1 1mg/L treated A. salina and a significant increase of glutathione peroxidase activities in PGMF (0.1 and 1mg/L) treated A. salina. Increased levels of lipid peroxidation of membranes was also observed in PGMF 1mg/L exposed A. salina.

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.

Effect of ionic liquids on the Menschutkin reaction: an experimental and theoretical study.

The effect of ionic liquids (ILs) on the Menschutkin reaction between N-methylimidazole and benzyl halides has been investigated. Hammett correlations have been used to obtain information on the reaction mechanism of benzyl chlorides in ionic and molecular solvents. The solvent effects on this reaction have been examined using both multiparameter linear solvation energy relationships and theoretical calculations. These latter have been performed using a supermolecular approach, i.e., the IL is represented by an explicit ionic pair, IP. Two possible different reaction pathways have been obtained changing the position of the IL-IP with respect to the reagents. It is hypothesized that inside the IL the Menschutkin reaction of benzyl chloride with N-methylimidazole can follow different reaction pathways by considering the influence of dynamic heterogeneity in ionic media on the time scale of the lifetime of a transition state.

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.

DefPol: New procedure to build molecular surfaces and its use in continuum solvation methods

We present a method to define van der Waals, solvent‐accessible, and solvent‐excluding molecular surfaces with their partition in nonoverlapping surface portions (tesserae). The procedure is more efficient than those available in the literature to describe solvent effects on molecular systems of large size, and it can also be applied to solutes of small size without reducing the accuracy of the output and without increasing computational times. All the tesserae are expressed in terms of spherical triangles, having all the characterizing elements (vertices, centers, etc.) analytically defined. The method was tested by comparing the results for the surface area and the solvation free energy (decomposed in electrostatic, dispersion, and steric contributions) obtained using the GEPOL procedure within the framework of the polarizable continuum model solvation method. These comparisons regard 87 molecules at the molecular mechanics level and 28 molecules at the ab initio Hartree–Fock level: the results are quite satisfactory. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1758–1776, 1998

Dissolution of Metal Salts in Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids: Studying the Affinity of Metal Cations Toward a “Weakly Coordinating” Anion

Despite the weakly coordinating ability of the bis(trifluoromethylsulfonyl)imide anion ([Tf2N](-)) the corresponding ionic liquids (ILs) are able to dissolve relevant amounts of metal salts having the same anion, M[Tf2N]x. To better understand the metal dissolution process we evaluated the interaction ability of a set of metal cations (Y(III), Al(III), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Li(I), and Na(I)) toward the [Tf2N](-) anion measuring the relative aptitude to give the corresponding anionic monocharged complex, [M(Tf2N)x+1](-) using the ESI-MS technique. UV-vis and NMR measurements were carried out to verify the consistence between the liquid and the gas phase. Density functional theory calculations have been used to identify the metal-containing species and determine their relative stability. An interesting correlation between interaction ability and chemical properties (Lewis acidity) was found.