The Bond Order of C2 from a Strictly N‐Representable Natural Orbital Energy Functional Perspective

The bond order of the ground electronic state of the carbon dimer has been analyzed in the light of natural orbital functional theory calculations carried out with an approximate, albeit strictly N‐representable, energy functional. Three distinct solutions have been found from the Euler equations of the minimization of the energy functional with respect to the natural orbitals and their occupation numbers, which expand upon increasing values of the internuclear coordinate. In the close vicinity of the minimum energy region, two of the solutions compete around a discontinuity point. The former, corresponding to the absolute minimum energy, features two valence natural orbitals of each of the following symmetries, σ, σ*, π and π*, and has three bonding interactions and one antibonding interaction, which is very suggestive of a bond order large than two but smaller than three. The latter, features one σ–σ* linked pair of natural orbitals and three degenerate pseudo‐bonding like orbitals, paired each with one triply degenerate pseudo‐antibonding orbital, which points to a bond order larger than three. When correlation effects, other than Hartree–Fock for example, between the paired natural orbitals are accounted for, this second solution vanishes yielding a smooth continuous dissociation curve. Comparison of the vibrational energies and electron ionization energies, calculated on this curve, with their corresponding experimental marks, lend further support to a bond order for C ₂ intermediate between acetylene and ethylene.     

Aufbau principle and singlet-triplet gap in spherical Hooke atoms

Singlet and triplet spin state energies for three-dimensional Hooke atoms, that is, electrons in a quadratic confinement, with even number of electrons (2, 4, 6, 8, 10) is discussed using Full-CI and CASSCF type wavefunctions with a variety of basis sets and considering perturbative corrections up to second order. The effect of the screening of the electron–electron interaction is also discussed by using a Yukawa-type potential with different values of the Yukawa screening parameter (λ_ee = 0.2, 0.4, 0.6, 0.8, 1.0). Our results show that the singlet state is the ground state for two and eight electron Hooke atoms, whereas the triplet is the ground spin state for 4-, 6-, and 10-electron systems. This suggests the following Aufbau structure 1s < 1p < 1d with singlet ground spin states for systems in which the generation of the triplet implies an inter-shell one-electron promotion, and triplet ground states in cases when there is a partial filling of electrons of a given shell. It is also observed that the screening of electron–electron interactions has a sizable quantitative effect on the relative energies of both spin states, specially in the case of two- and eight-electron systems, favoring the singlet state over the triplet. However, the screening of the electron–electron interaction does not provoke a change in the nature of the ground spin state of these systems. By analyzing the different components of the energy, we have gained a deeper understanding of the effects of the kinetic, confinement and electron–electron interaction components of the energy.     

Fluorene‐based rib waveguides with optimized geometry for long‐term amplified spontaneous emission stability

Amongst the different optoelectronic applications of conjugated polymers, waveguide amplifiers and optically pumped lasers are those requiring larger photochemical stability, owing to the large irradiation conditions under operation. In this context, suitable waveguide optimization enabling the reduction of amplified spontaneous emission (ASE) threshold values appears as important as synthetic chemistry protocols to promote polymer robustness against photo‐oxidation. In this work, we develop rib waveguides with different geometries based on four different fluorene‐based compounds and assess the influence of rib confinement on ASE properties. We observe ASE threshold values as low as 8.9 × 10^?4 mJ cm^?2, being among the lowest threshold values reported so far on blue emitting polymer/oligomer waveguides. We demonstrate that the enhanced ASE efficiency on some of these rib waveguides leads to a fivefold increase in operation lifetime respect to spin‐coated slab waveguides, thus confirming the impact of waveguide geometry on ASE operation stability. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1040–1045     

A comparison of scanning electron microscopy energy dispersive X-ray (SEM/EDX) and inductively coupled plasma optical emission spectrometry (ICP-OES) for provenance inferences of grog-tempered Bronze Age pottery

Pottery tempered with former pottery fragments (named grogs) is very common during the Bronze Age in the Cantabrian Cornice (North Spain). Grog tempered pottery from two coeval archaeological sites Santa Maria de Estarrona and Peracho are studied in order to establishes pottery provenance. Chemical analyses of pottery are used to discriminate locally from imported pottery under the assumption that different geological regions exhibit a distinctive chemical fingerprint. However, the pottery-maker usually transforms the original clay by tempering and prepares the material suitable for firing modifying the raw material chemical fingerprint. The problem increases when grogs are used as tempering agent. In these cases the chemical input is clearly contaminated with foreign materials making impossible to assign a source area. SEM-EDX has been applied to both the matrix and the different types of grog tempers and the results have been compared to the whole potsherd analysis performed by ICP-OES. Chemometric analysis of matrices enable to discriminate different raw materials that can be attributed to locally produced pottery. This chemical characterization of the matrices by SEM-EDX is more effective than the chemical composition determined for the whole potsherd by ICP-OES. The occurrence of grogs within grog tempers and various types of grogs indicates modes of production based on intensive recycling. The presence of some pottery vessels at the Estarrona site with chemical features similar to those of Peracho is indicative of the transport of pieces during the migrations of semi-sedentary groups and/or the exchange of pieces between different groups of humans.     

Asymmetric Synthesis of Tetrasubstituted α-Aminophosphonic Acid Derivatives

Due to their structural similarity with natural α-amino acids, α-aminophosphonic acid derivatives are known biologically active molecules. In view of the relevance of tetrasubstituted carbons in nature and medicine and the strong dependence of the biological activity of chiral molecules into their absolute configuration, the synthesis of α-aminophosphonates bearing tetrasubstituted carbons in an asymmetric fashion has grown in interest in the past few decades. In the following lines, the existing literatures for the synthesis of optically active tetrasubstituted α-aminophosphonates are summarized, comprising diastereoselective and enantioselective approaches.     

Nucleophilic attack on phosphate diesters: a density functional study of in-line reactivity in dianionic, monoanionic, and neutral systems

A density functional study of the hydrolysis reaction of phosphodiesters with a series of attacking nucleophiles in the gas phase and in solution is presented. The nucleophiles HOH, HO-, CH3OH, and CH3O- were studied in reactions with ethylene phosphate, 2'3'-ribose cyclic phosphate and in their neutral (protonated) and monoanionic forms. Stationary-point geometries for the reactions were determined at the density functional B3LYP/6-31++G(d,p) level followed by energy refinement at the B3LYP/6-311++G(3df,2p) level. Solvation effects were estimated by using a dielectric approximation with the polarizable continuum model (PCM) at the gas-phase optimized geometries. This series of reactions characterizes factors that influence the intrinsic reactivity of the model phosphate compounds, including the effect of nucleophile, protonation state, cyclic structure, and solvent. The present study of the in-line mechanism for phosphodiester hydrolysis, a reaction of considerable biological importance, has implications for enzymatic mechanisms. The analysis generally supports the associative mechanism for phosphate ester hydrolysis. The results highlight the importance for the reaction barrier of charge neutralization resulting from the protonation of the nonbridging phosphoryl oxygens and the role of internal hydrogen transfer in the gas-phase mechanism. It also shows that solvent stabilization has a profound influence on the relative barrier heights for the dianionic, monoanionic, and neutral reactions. The calculations provide a comprehensive data set for the in-line hydrolysis mechanisms that can be used for the development of improved semiempirical quantum models for phosphate hydrolysis reactions.     

The Nature of Chemical Bonds from PNOF5 Calculations

Natural orbital functional theory (NOFT) is used for the first time in the analysis of different types of chemical bonds. Concretely, the Piris natural orbital functional PNOF5 is used. It provides a localization scheme that yields an orbital picture which agrees very well with the empirical valence shell electron pair repulsion theory (VSEPR) and Bent’s rule, as well as with other theoretical pictures provided by valence bond (VB) or linear combination of atomic orbitals–molecular orbital (LCAO‐MO) methods. In this context, PNOF5 provides a novel tool for chemical bond analysis. In this work, PNOF5 is applied to selected molecules that have ionic, polar covalent, covalent, multiple (σ and π), 3c–2e, and 3c–4e bonds.     

Enhanced Lithium‐Ion Conductivity of Polymer Electrolytes by Selective Introduction of Hydrogen into the Anion

The anion chemistry of lithium salts plays a pivotal role in dictating the physicochemical and electrochemical performance of solid polymer electrolytes (SPEs), thus affecting the cyclability of all‐solid‐state lithium metal batteries (ASSLMBs). The bis(trifluoromethanesulfonyl)imide anion (TFSI^?) has long been studied as the most promising candidate for SPEs; however, the Li‐ion conductivities of the TFSI‐based SPEs still remain low (Li‐ion transference number: ca. 0.2). In this work, we report new hydrogen‐containing anions, conceived based on theoretical considerations, as an electrolyte salt for SPEs. SPEs comprising hydrogen‐containing anions achieve higher Li‐ion conductivities than TFSI‐based ones, and those anions are electrochemically stable for various kinds of ASSLMBs (Li–LiFePO₄, Li–S, and Li–O₂ batteries). This opens up a new avenue for designing safe and high‐performance ASSLMBs in the future.     

Solvent and Substituent Effects in the Periselectivity of the Staudinger Reaction between Ketenes and alpha,beta-Unsaturated Imines. A Theoretical and Experimental Study

The outcome of the cycloaddition between activated ketenes and alpha,beta-unsaturated imines has been investigated both experimentally and theoretically. Our results indicate that activated monosubstituted ketenes yield exclusively [2 + 2] cycloadducts. Disubstituted activated ketenes yield [2 + 2] and/or [4 + 2] cycloadducts. In one case, an unexpected piperidin-2-one has been obtained, although its relative abundance with respect to the corresponding [2 + 2] or [4 + 2] cycloadducts can be minimized by the proper choice of experimental conditions. The ability of different ab initio and semiempirical methods to account for these results has been tested. The best agreement between theory and experiment is achieved at the MP2/6-31G level of theory, with solvent effects taken into account. The semiempirical hamiltonian AM1, at the RHF level, tends to overestimate the stability of the transition structures leading to six-membered cycloadducts, whereas 3 x 3CI-HE/AM1 and CASSCF(2,2)/6-31G methods tend to overestimate the stability and the biradical character of the transition structures leading to [2 + 2] cycloadducts.