Solvation of Carbanions in Organic Solvents:  A Test of the Polarizable Continuum Model

Abstract

The solvation of carbanions in the solvents N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) has been analyzed on the basis of experimental and theoretical data. Experimental solvation energies are obtained from present and previously reported electrochemical measurements of reduction potentials of the corresponding radicals. Theoretical solvation energies are obtained from quantum chemical calculations using the polarizable continuum model (PCM). It is found that the solvation energy is relatively independent of molecular size and structure for the saturated carbanions. This indicates that the negative charge is strongly localized to the anionic carbon. The conjugated carbanions have considerably lower absolute solvation energies ( |) than the saturated carbanions. This is a consequence of the strong delocalization of the negative charge in the former group. The propargyl anion is also found to have a surprisingly low absolute solvation energy. However, high-level quantum chemical calculations show that the electronic structure has large contributions from two different resonance structures, CH⋮CCH2 - and -CHCCH2, which results in a significant charge delocalization. There is good agreement between calculated and experimental solvation energies for both the conjugated and nonconjugated primary anions. However, the PCM method consistently underestimates the absolute solvation energies of the secondary and tertiary carbanions. This is attributed to an insufficient treatment of first-layer solvation effects in the method. According to the experimental measurements, the absolute solvation energies are on average 2−3 kcal mol-1 lower in THF than in DMF. The theoretical data indicate a considerably larger solvent effect, 7−10 kcal mol-1. The discrepancy between theory and experiment may partly be attributed to the use of a supporting electrolyte in the measurements, but the main cause seems to be that the short-range interaction tendencies of the solvent cannot be fully characterized by its dielectric constant.

On the nature of tetraalkylammonium ions in common electrochemical solvents: General and specific solvation – Quantitative aspects

“…This is reasonable because the dielectric constants of pentane and heptane are similar (1.8 and 1.9, respectively) [35]. PCM parameters were estimated for DMF: the solvent radius (2.18 Å) was that suggested by Brinck et al [36] other necessary parameters assigned were dielectric constant (38.3), density in particles per Å [3] (0.007788), and the dielectric constant at infinite frequency (1.9). In Section 3.2 we discuss our procedure for extracting the respective contributions of specific and general solvation from the computational data, together with our criterion for deciding whether a given ion is specifically solvated.…”

Accurate prediction of basicity in aqueous solution with COSMO‐RS

“…Despite the well‐known deficits of DCSM methods, that is, the inadequate treatment of the short‐range electrostatics of polar solutes and ions58–63 and the neglect of hydrogen bonding, it is possible to correlate the quantum chemical dissociation free energy of the solvated molecule Δ G diss with its pK a via a linear free energy relationship (LFER):58 From basic thermodynamics c 1 is expected to be unity if Δ G diss would be calculated without systematic error. Looking in detail into the DCSM studies,3–57 in the regression of pK a vs. the calculated dissociation free energy Δ G diss , the studies report a slope that is significantly lower than the theoretically expected value of 1/ RT ln(10) unless large efforts have been taken in the often physically hardly justifiable adjustment of numerous additional and often physically doubtful parameters of the DCSM method, such as atom type specific cavity radii and cavity definitions that depend on the charge of the molecule. Such a behavior has been reported for aqueous3–58 and nonaqueous acids3, 4, 58 as well as for bases (in ref.…”

“…The rapid development of efficient quantum chemical (QC) methods in the last years has opened new perspectives for the rigorous prediction of liquid phase pK a . Of the different quantum chemical methodologies available for the computation of pK a dielectric continuum solvation methods (DCSMs2) have become quite popular in the recent years,3–57 because they are able to accurately describe long‐range electrostatic interactions of solutes at moderate computational cost in the context of quantum chemical programs. Despite the well‐known deficits of DCSM methods, that is, the inadequate treatment of the short‐range electrostatics of polar solutes and ions58–63 and the neglect of hydrogen bonding, it is possible to correlate the quantum chemical dissociation free energy of the solvated molecule Δ G diss with its pK a via a linear free energy relationship (LFER):58 From basic thermodynamics c 1 is expected to be unity if Δ G diss would be calculated without systematic error.…”

A Correlation for Establishing Solvolysis Rates of Aqueous Al(III) Complexes: A Possible Strategy for Colloids and Nanoparticles

“…It is expressed in terms of the sum of charge densities at the surfaces of the tesserae. The PCM model has proven to be highly successful at estimating the effects of solvation on various thermodynamic and molecular properties (32)(33)(34)(35)(36)(37)(38).…”