Pathways to Triplet or Singlet Oxygen during the Dissociation of Alkali Metal Superoxides: Insights by Multireference Calculations of Molecular Model Systems

Recent experimental investigations demonstrated the generation of singlet oxygen during charging at high potentials in lithium/oxygen batteries. To contribute to the understanding of the underlying chemical reactions a key step in the mechanism of the charging process, namely, the dissociation of the intermediate lithium superoxide to oxygen and lithium, was investigated. Therefore, the corresponding dissociation paths of the molecular model system lithium superoxide (LiO₂) were studied by CASSCF/CASPT2 calculations. The obtained results indicate the presence of different dissociation paths over crossing points of different electronic states, which lead either to the energetically preferred generation of triplet oxygen or the energetically higher lying formation of singlet oxygen. The dissociation to the corresponding superoxide anion is energetically less preferred. The understanding of the detailed reaction mechanism allows the design of strategies to avoid the formation of singlet oxygen and thus to potentially minimize the degradation of materials in alkali metal/oxygen batteries. The calculations demonstrate a qualitatively similar but energetically shifted behavior for the homologous alkali metals sodium and potassium and their superoxide species. Fundamental differences were found for the covalently bound hydroperoxyl radical.     

Electrophilic attack on sulfur-sulfur bonds: coordination of lithium cations to sulfur-rich molecules studied by ab initio MO methods

Complex formation between gaseous Li+ ions and sulfur-containing neutral ligands, such as H2S, Me2Sn (n = 1-5; Me = CH3) and various isomers of hexasulfur (S6), has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Generally, the formation of LiS(n) heterocycles and clusters is preferred in these reactions. The binding energies of the cation in the 29 complexes investigated range from -88 kJ mol(-1) for [H2SLi]+ to -189 kJ mol(-1) for the most stable isomer of [Me2S5Li]+ which contains three-coordinate Li+. Of the various S6 ligands (chair, boat, prism, branched ring, and triplet chain structures), two isomeric complexes containing the S5==S ligand have the highest binding energies (-163+/-1 kJ mol(-1)). However, the global minimum structure of [LiS6]+ is of C(3v) symmetry with the six-membered S(6) homocycle in the well-known chair conformation and three Li--S bonds with a length of 256 pm (binding energy: -134 kJ mol(-1)). Relatively unstable isomers of S6 are stabilized by complex formation with Li+. The interaction between the cation and the S6 ligands is mainly attributed to ion-dipole attraction with a little charge transfer, except in cations containing the six sulfur atoms in the form of separated neutral S2, S3, or S4 units, as in [Li(S3)2]+ and [Li(S2)(S4)]+. In the two most stable isomers of the [LiS6]+ complexes, the number of S--S bonds is at maximum and the coordination number of Li+ is either 3 or 4. A topological analysis of all investigated complexes revealed that the Li--S bonds of lengths below 280 pm are characterized by a maximum electron-density path and closed-shell interaction.     

Intrinsic Dynamic and Static Nature of Halogen Bonding in Neutral Polybromine Clusters,with the Structural Feature Elucidated by QTAIM Dual-Functional Analysis and MO Calculations

The intrinsic dynamic and static nature of noncovalent Br-∗-Br interactions in neutral polybromine clusters is elucidated for Br₄–Br₁₂, applying QTAIM dual-functional analysis (QTAIM-DFA). The asterisk (∗) emphasizes the existence of the bond critical point (BCP) on the interaction in question. Data from the fully optimized structures correspond to the static nature of the interactions. The intrinsic dynamic nature originates from those of the perturbed structures generated using the coordinates derived from the compliance constants for the interactions and the fully optimized structures. The noncovalent Br-∗-Br interactions in the L-shaped clusters of the C_s symmetry are predicted to have the typical hydrogen bond nature without covalency, although the first ones in the sequences have the vdW nature. The L-shaped clusters are stabilized by the n(Br)→σ*(Br–Br) interactions. The compliance constants for the corresponding noncovalent interactions are strongly correlated to the E(2) values based on NBO. Indeed, the MO energies seem not to contribute to stabilizing Br₄ (C_2h) and Br₄ (D_2d), but the core potentials stabilize them, relative to the case of 2Br₂; this is possibly due to the reduced nuclear–electron distances, on average, for the dimers.     

Mechanism of Dissolution of a Lithium Salt in an Electrolytic Solvent in a Lithium Ion Secondary Battery: A Direct Ab Initio Molecular Dynamics (AIMD) Study

The mechanism of dissolution of the Li⁺ ion in an electrolytic solvent is investigated by the direct ab initio molecular dynamics (AIMD) method. Lithium fluoroborate (Li⁺BF₄^?) and ethylene carbonate (EC) are examined as the origin of the Li⁺ ion and the solvent molecule, respectively. This salt is widely utilized as the electrolyte in the lithium ion secondary battery. The binding of EC to the Li⁺ moiety of the Li⁺BF₄^? salt is exothermic, and the binding energies at the CAM–B3LYP/6‐311++G(d,p) level for n=1, 2, 3, and 4, where n is the number of EC molecules binding to the Li⁺ ion, (EC)_n(Li⁺BF₄^?), are calculated to be 91.5, 89.8, 87.2, and 84.0 kcal mol^?1 (per EC molecule), respectively. The intermolecular distances between Li⁺ and the F atom of BF₄^? are elongated: 1.773 Å (n=0), 1.820 Å (n=1), 1.974 Å (n=2), 1.942 Å (n=3), and 4.156 Å (n=4). The atomic bond populations between Li⁺ and the F atom for n=0, 1, 2, 3, and 4 are 0.202, 0.186, 0.150, 0.038, and 0.0, respectively. These results indicate that the interaction of Li⁺ with BF₄^? becomes weaker as the number of EC molecules is increased. The direct AIMD calculation for n=4 shows that EC reacts spontaneously with (EC)₃(Li⁺BF₄^?) and the Li⁺ ion is stripped from the salt. The following substitution reaction takes place: EC+(EC)₃(Li⁺BF₄^?)→(EC)₄Li⁺?(BF₄^?). The reaction mechanism is discussed on the basis of the theoretical results.     

Doubling the Capacity of Lithium Manganese Oxide Spinel by a Flexible Skinny Graphitic Layer

By coating nanoparticular lithium manganese oxide (LMO) spinel with a few layers of graphitic basal planes, the capacity of the material reached up to 220 mA h g^?1 at a cutoff voltage of 2.5 V. The graphitic layers 1) provided a facile electron‐transfer highway without hindering ion access and, more interestingly, 2) stabilized the structural distortion at the 3 V region reaction. The gain was won by a simple method in which microsized LMO was ball‐milled in the presence of graphite with high energy. Vibratory ball milling pulverized the LMO into the nanoscale, exfoliated graphite of less than 10 layers and combined them together with an extremely intimate contact. Ab initio calculations show that the intrinsically very low electrical conductivity of the tetragonal phase of the LMO is responsible for the poor electrochemical performance in the 3 V region and could be overcome by the graphitic skin strategy proposed.     

Ab Initio Theoretical Investigation on the Geometrical and Electronic Structures of AlAun-/0 (n = 2-4) Clusters

A systematic density functional theory and wave function theory investigation on the geometrical and electronic properties of AlAu n 0/-(n=2-4) clusters has been performed in this work. AlAu n-anions prove to possess ground states of the V-shaped C2v AlAu2 - , umbrella-shaped C3v AlAu3- , and perfect tetrahedral T d AlAu4- , while their neutrals favor the V-shaped C2v AlAu2 , perfect planar triangular D3h AlAu3 , and severely distorted C s AlAu4 , respectively. Aluminum aurides appear to be analogous to the corresponding aluminum hydrides, expect C s AlAu4 . Molecular orbitals (MOs) analyses also support this conclusion. Detailed orbital analyses indicate that Au 6s makes 94-96% and Au 5d makes 6-4% contribution to the Au-based orbitals in Al-Au bonds, which is smaller than the BAu n0/- series, partially reflecting the relativistic effect of gold. The one-electron detachment energies of the anions and characteristic stretching vibrational frequencies of Al-Au bonds between 100-400 cm -1 have been calculated to facilitate future experimental characterization of these clusters.     

S‐Trifluoromethylation of Thiols by Hypervalent Iodine Reagents: A Joint Experimental and Computational Study

The radical trifluoromethylation of thiophenol in condensed phase applying reagent 1 (3,3‐dimethyl‐1‐(trifluoromethyl)‐1λ³,2‐benziodoxol) has been examined by both theoretical and experimental methodologies. On the basis of ab initio molecular dynamics and metadynamics we show that radical reaction mechanisms favourably compete with polar ones involving the S‐centred nucleophile thiophenol, their free energies of activation, ΔF^≠, lying between 9 and 15 kcal mol^?1. We further show that the origin of the proton activating the reagent is important. Hammett plot analysis reveals intramolecular protonation of 1 , thus generating negative charge on the sulfur atom in the rate‐determining step. The formation of a CF₃ radical can be thermally induced by internal dissociative electron transfer, its activation energy, ΔF^≠, amounting to as little as 10.8 and 2.8 kcal mol^?1 for reagent 1 and its protonated form 2 , respectively. The reduction of the iodine atom by thiophenol occurs either subsequently or in a concerted fashion.     

Spectroscopic and theoretical studies of optically active porphyrin dimers: a system uninterpretable by exciton coupling theory.

The electronic excited states of a meso-meso beta-beta doubly linked bis-porphyrin are comprehensively investigated by measuring its circular dichroism (CD) and magnetic circular dichroism (MCD) spectra. The observed spectroscopic properties are rationalized by DFT calculations. The frontier molecular orbitals (MOs) are constructed by the linear combinations of the constituent monomers' four MOs. Comparison of a theoretical CD spectrum based on time-dependent DFT (TDDFT) with the experimental spectra resulted in the assignment of the helical conformation of the dimer. This assignment is contrary to the previous assignment based on the point-dipole approximation (exciton coupling theory).     

Gaseous H5P2O8- ions: a theoretical and experimental study on the hydrolysis and synthesis of diphosphate ion

The structure and reactivity of gaseous H5P2O8- ions obtained from the chemical ionization (CI) of an H4P2O7/H2O mixture and from electrospray ionization (ESI) of CH3CN/H2O/H4P2O7 solutions were investigated by Fourier transform ion cyclotron (FTICR) and triple quadrupole mass spectrometry. Theoretical calculations performed at the B3LYP/6-31+G* level of theory and collisionally activated dissociation (CAD) mass spectrometric results allowed the ionic population obtained in the CI conditions to be structurally characterized as a mixture of gaseous [H3P2O7...H2O]-, [H3PO4...H2PO4]-, and [PO3...H3PO4...H2O]- clusters. The energy profile emerging from theoretical calculations affords insight into the mechanism of diphosphate ion hydrolysis and synthesis.