Pinchas Aped

The behaviour of lithium electrodes in propylene and ethylene carbonate: Te major factors that influence Li cycling efficiency☆

Abstract The Li cycling efficiency surface chemistry and Li morphology in ethylene carbonate (EC) and propylene carbonate (PC) based electrolyte solutions were investigated and correlated. Surface sensitive ex situ FTIR spectroscopy, X-ray microanalysis and scanning electron microscopy were used in conjunction with standard electrochemical techniques. EC is more reactive than PC in electroreduction processes and is reduced on noble metals to ethylene dicarbonate. The difference in reactivity between the two solvents is discussed, based on MO ab initio calculations of their radical anions (and Li+ stabilized radical anions). In spite of the high reactivity of these systems to lithium, the Li cycling efficiency is strongly dependent on the presence of additives and contaminants at the ppm level that modify the Li surface chemistry in solutions. The two alkyl carbonate solvents decompose when stored over activated Al2O3 and CO2 is formed. The presence of CO2 in solutions increases the Li cycling efficiency considerably due to the formation of Li2CO3 on the Li surfaces.

Diagnostic structural criteria for the anomeric effect in carbohydrates and inferences of general significance on their scope and limitations

Abstract A systematic analysis of 529 carbohydrate structures that contain OCO units, retrieved from the Cambridge Structural Database, was performed with regard to the bond lengths (CO) and bond (COC and OCO) angles as they depend on the dihedral angles in the sequence COCOC. This dependence was then interpreted in terms of the anomeric effect. Known and new concepts that concern the manifestations of the anomeric and exo -anomeric effects were thus reassessed. A set of structural criteria of diagnostic value was defined which, together with qualifying arguments, allow evaluation of the scope and limitations of these stereoelectronic effects in carbohydrate systems.

Conformational Schemes: An Available Tool for the Assignment of NMR-Measured Barriers, Demonstrated with the Example of Crowded Piperidines

MM3-Derived conformational schemes for systems with complex conformational dynamics, such as the substituted piperidine depicted here, permit the assignment of NMR-measured barriers to the corresponding processes.

Computation of O-C-F and N-C-F systems: AB-initio calculations and a MM2 parameterization study. Theory vs. experiment☆

Abstract A parameterization scheme of Allingers MM2 force field for the anomeric effect in O-C-F and N-C-F systems is presented. The scarcity of experimental data, in particular for the N-C-F case, dictated the use of ab-initio calculations to account for the energetic and structural manifestations of the effect. The resulting modified force field was tested against available X-ray, microwave and NMR results leading to a very good agreement between calculations and experiment. In addition, ab-initio results were used to demonstrate the role of the anomeric effect in lowering barriers to N-inversion and elevating barriers for rotation around single CN bonds. The results for the fluoro compounds, when juxtaposed to other systems, provide a complete treatment of the anomeric effect for the first row elements.

Computation of N-C-N systems: Theory vs. experiment☆

Abstract In sequel to a previous recent study4, netgtkebeduanube (MDA) was calculated by higher level ab initio methods namely 6-31G* abd NO3//6-31G* levels. The influence of polarization functions on the calculated pyramidality of nitrogen and on the magnitude of the anomeric effect is discussed. At the same time, a structural-statistical study was performed on molecules containing the N-C-N moiety retrieved from the Cambridge Structural Database (CSD). Both methods complement and improve results from the previous study. Special attention was given to the tertiary amine case and the N-C-N bond angle for which new MM2 parameters are put forward. These are mainly based on ab initio calculations of N,N-dimethylmethylenediamine and tetramethylmethylenediamine and on a comparative study of CN bond lengths in primary, secondary and tertiary amines.

Structure, conformation, and stereoelectronics of 1,4,5,8-tetraazadecalins. Chemical, multinuclear NMR and molecular mechanics studies

Abstract We present a multidisciplinary investigation of some known and new 1,4,5,8-tetraazadecalin (TAD) derivatives. Their structure, static and dynamic conformational analysis, isomerization pathways and relative stabilities were studied using 1H-, 13C-, and 15N-NMR techniques. Molecular mechanics calculations were carried out using the MM2 force field and an earlier modified version for NCN containing systems. The peculiar stereoelectronic features of the CNCNC moieties in the TAD systems are emphasized.

Modified MM2 scheme for computation of O-C-N-containing systems

A modification of Allingers MM2 force field for the anomeric effect in OCN systems is presented. For optimal consistency, it was parameterized by alternate use of ab initio (3‐21G level) and X‐ray results to account for the energetic and structural manifestations of the effect in the gas or condensed phase, respectively. The results obtained with the modified force field are in good agreement with those from both theoretical and experimental methods. The parameterization scheme explicitly treats all structural parameters of the COCNC moiety as well as CN bond lengths in tertiary amines contained within an anomeric unit. In addition, it includes directional H‐bond type interactions. A limited number of parameters is put forward, in accord with the general MM2 force‐field approach.

Intramolecular dynamics in 4- to 6-membered saturated azacycles: a MM3 study☆

Abstract We show that the MM3 force field provides a reasonable modeling of the transition states of dynamic processes related to ring inversion for azetidines, pyrrolidines and piperidines. These involve isolated ring inversion (RI) and concerted ring inversion-nitrogen inversion-CN rotation (RINIR). Pseudorotation and concerted pseudorotation-nitrogen inversion are additional processes for piperidines. The schemes of conformational transformations (including a relationship of conformers with corresponding transition states) were established for these amines using a normal mode vibrational analysis. Pyramidal nitrogen inversion was found to occur only as the RINIR process for azetidine and pyrrolidine compounds, while nitrogen inversion as well as RINIR are inherent to NMe piperidine. Only the C s mode was attributed to interconversion of the piperidine ring (differing from the known cyclohexane case). The designed conformational schemes allowed the reassignment of the previously measured barriers (NMR) for these cyclic amines.

A Valence bond description of bond dissociation energy curves

Bond dissociation energy curves have been calculated ab initio using the valence bond self-consistent field (VBSCF) method for CH3-Y molecules, with Y = F, OH, NH2, CH3, BH2, CN and NO. Basis sets of at least double-zeta polarization quality for first row atoms have been used. The VB wavefunction is expanded in terms of one covalent (CH3:Y) and two ionic (CH3 +Y- and CH3 -Y+) structures. All results were referenced to the perfect pairing generalized valence bond method. The VB curves for CH3-F were calculated using both the same orbitals for the different structures (SODS) and tailored (different) orbitals for the three different structures (TODS). Each of these model levels was examined further using purely localized fragment (CH3 and Y) orbitals (L-SODS and L-TODS) and allowing delocalization between the CH3 and Y fragments among the passive orbitals (D-SODS and D-TODS), where only the electron pair bond undergoing homolytic dissociation uses purely localized fragment orbitals. Only the two SODS model l...

MM3 force field as a tool in mechanistic studies of nitrogen inversion processes for alkylamines

Abstract We show that the MM3 force field provides a reasonable modelling of the transition state of nitrogen inversion-rotation (NIR) for alkylamines and thus a quantitative estimate of nitrogen inversion barriers for amines of different structures. In general, the NIR barriers for most open-chain, monocyclic and polycyclic amines are now available within 0.7 kcal mol−1 (standard deviation) of the experimental values. However, for isopropyl- or isobutyl-type amines and pyrrolidine compounds this correlation between the calculated and experimental barriers drops down to 2.7 kcal mol−1. An effective study of the mechanism of the NIR process, and thus the assignment of NMR-measured barriers, proved possible using MM3-calculated barriers for the isolated rotation (ISR), as well as the concerted NIR. A simple MM3-based test was developed to identify the amines for which an NIR process excludes an isolated rotation process.