We derive a consistent approach for predicting the solvation free energies of charged solutes in the presence of implicit and explicit solvents. We find that some published methodologies make systematic errors in the computed free energies because of the incorrect accounting of the standard state corrections for water molecules or water clusters present in the thermodynamic cycle. This problem can be avoided by using the same standard state for each species involved in the reaction under consideration. We analyze two different thermodynamic cycles for calculating the solvation free energies of ionic solutes: (1) the cluster cycle with an n water cluster as a reagent and (2) the monomer cycle with n distinct water molecules as reagents. The use of the cluster cycle gives solvation free energies that are in excellent agreement with the experimental values obtained from studies of ion-water clusters. The mean absolute errors are 0.8 kcal/mol for H(+) and 2.0 kcal/mol for Cu(2+). Conversely, calculations using the monomer cycle lead to mean absolute errors that are >10 kcal/mol for H(+) and >30 kcal/mol for Cu(2+). The presence of hydrogen-bonded clusters of similar size on the left- and right-hand sides of the reaction cycle results in the cancellation of the systematic errors in the calculated free energies. Using the cluster cycle with 1 solvation shell leads to errors of 5 kcal/mol for H(+) (6 waters) and 27 kcal/mol for Cu(2+) (6 waters), whereas using 2 solvation shells leads to accuracies of 2 kcal/mol for Cu(2+) (18 waters) and 1 kcal/mol for H(+) (10 waters).

There is increasing evidence that cyclic and linear carbonates, commonly used solvents in Li ion battery electrolytes, are unstable in the presence of superoxide and thus are not suitable for use in rechargeable Li-air batteries employing aprotic electrolytes. A detailed understanding of related decomposition mechanisms provides an important basis for the selection and design of stable electrolyte materials. In this article, we use density functional theory calculations with a Poisson-Boltzmann continuum solvent model to investigate the reactivity of several classes of aprotic solvents in nucleophilic substitution reactions with superoxide. We find that nucleophilic attack by O(2)(•-) at the O-alkyl carbon is a common mechanism of decomposition of organic carbonates, sulfonates, aliphatic carboxylic esters, lactones, phosphinates, phosphonates, phosphates, and sulfones. In contrast, nucleophilic reactions of O(2)(•-) with phenol esters of carboxylic acids and O-alkyl fluorinated aliphatic lactones proceed via attack at the carbonyl carbon. Chemical functionalities stable against nucleophilic substitution by superoxide include N-alkyl substituted amides, lactams, nitriles, and ethers. The results establish that solvent reactivity is strongly related to the basicity of the organic anion displaced in the reaction with superoxide. Theoretical calculations are complemented by cyclic voltammetry to study the electrochemical reversibility of the O(2)/O(2)(•-) couple containing tetrabutylammonium salt and GCMS measurements to monitor solvent stability in the presence of KO(2)(•) and a Li salt. These experimental methods provide efficient means for qualitatively screening solvent stability in Li-air batteries. A clear correlation between the computational and experimental results is established. The combination of theoretical and experimental techniques provides a powerful means for identifying and designing stable solvents for rechargeable Li-air batteries.

This article summarizes experimental and theoretical evidence for the existence of four distinct binding modes for complexes of anions with charge-neutral arenes. These include C-H hydrogen bonding and three motifs involving the arene-pi system-the noncovalent anion-pi interaction, weakly covalent sigma interaction, and strongly covalent sigma interaction.

Supporting Information Available. Cartesian coordinates and energies for optimized clusters at the B3LYP/6-311++G(2d,2p) level of theory, and Table 1S comparing the performance of the various DFT methods employing the 6-311++G** basis set. This material is available free of charge via the Internet at http://pubs.acs.org.

This paper refines the nature of the interactions between electron-deficient arenes and halide anions. Conclusions are based on (i) new crystal structures containing alkali halide salts with 1,2,4,5-tetracyanobenzene (TCB) and 18-crown-6, (ii) evaluation of crystal structures found in the Cambridge Structural Database, and (iii) MP2/aug-cc-pVDZ calculations of F-, Cl-, and Br- complexes with TCB, 1,3,5-tricyanobenzene, triazine, and hexafluorobenzene. When the halide lies above the plane of the π system, the results establish that three distinctly different types of complexes are possible:  strongly covalent σ complexes, weakly covalent donor−π-acceptor complexes, and noncovalent anion−π complexes. When aryl C−H groups are present, a fourth type of interaction leads to C−H · · · X- hydrogen bonding. Characterization of the different geometries encountered with the four possible binding motifs provides criteria needed to design host architectures containing electron-deficient arenes.

A major challenge in the development of rechargeable Li-O(2) batteries is the identification of electrolyte materials that are stable in the operating environment of the O(2) electrode. Straight-chain alkyl amides are one of the few classes of polar, aprotic solvents that resist chemical degradation in the O(2) electrode, but these solvents do not form a stable solid-electrolyte interphase (SEI) on the Li anode. The lack of a persistent SEI leads to rapid and sustained solvent decomposition in the presence of Li metal. In this work, we demonstrate for the first time successful cycling of a Li anode in the presence of the solvent, N,N-dimethylacetamide (DMA), by employing a salt, lithium nitrate (LiNO(3)), that stabilizes the SEI. A Li-O(2) cell containing this electrolyte composition is shown to cycle for more than 2000 h (>80 cycles) at a current density of 0.1 mA/cm(2) with a consistent charging profile, good capacity retention, and O(2) detected as the primary gaseous product formed during charging. The discovery of an electrolyte system that is compatible with both electrodes in a Li-O(2) cell may eliminate the need for protecting the anode with a ceramic membrane.

Atomistic molecular dynamics (MD) simulations of a G4-NH2 PAMAM dendrimer were carried out in aqueous solution using explicit water molecules and counterions (with the Dreiding III force field optimized using quantum mechanics). Our simulations predict that the radius of gyration (R g) of the dendrimer changes little with pH from 21.1 Å at pH ∼10 (uncharged PAMAM) to 22.1 Å at pH ∼5 (charged with 126 protons), which agrees quantitatively with recent small angle neutron scattering (SANS) experiments (from 21.4 Å at pH 10 to 21.5 Å at pH 5). Even so we predict a dramatic change in the conformation. The ion pairing in the low pH form leads to a locally compact dense shell with an internal surface area only 37% of the high pH form with a dense core. This transformation from “dense core” at high pH to “dense shell” at low pH could facilitate the encapsulation and release of guest molecules (e.g., drugs) using pH as the trigger, making dendrimers a unique drug delivery vehicle.

Solvent plays a major role in determining the nature of discharge products and the extent of rechargeability of the nonaqueous lithium-air (oxygen) battery. Here we investigate chemical stability for a number of aprotic solvents against superoxide, including N,N-dialkyl amides, aliphatic and aromatic nitriles, oxygenated phosphorus (V) compounds, substituted 2-oxazolidinones, and fluorinated ethers. The free energy barriers for nucleophilic attack by superoxide and the C-H acidity constants in dimethyl sulfoxide are reported, which provide a theoretical framework for computational screening of stable solvents for Li-air batteries. Theoretical results are complemented by cyclic voltammetry to study the electrochemical reversibility of the O2/O2− couple containing tetrabutylammonium salt and GCMS measurements to monitor solvent stability in the presence of KO2 and a Li salt. Excellent agreement among all quantum chemical, electrochemical, and chemical methods has been obtained in evaluating solvent stability against superoxide. The combined theoretical and experimental methodology provides a comprehensive testing ground to identify electrolyte solvents stable in the air cathode. Based upon this knowledge we report on the use of an amide-based electrolyte for rechargeable oxygen electrodes in Li-O2 secondary cells.