Infrared spectra and structure of carbanions: XVIII. A semiempirical study of carbanions of cyanoacetic acid derivatives

The IR spectra of cyanoacetic acid, ethyl cyanoacetate and cyanoacetamide as well as those of related mono- and, whenever possible, dianions have been studied in dimethyl-sulphoxide (DMSO) and DMSO-d₆.The observed nitrile and carbonyl absorption frequencies correlate linearly with the corresponding Wiberg bond indices given by CNDO/2 calculations with full geometry optimization. These calculations predict carbanionic structures throughout except in the case of the dinegative ion of cyanoacetamide, which could be considered as originating from the aminoacetylenic tautomer of NCCH₂CONH₂. Parallel MINDO/3 calculations, however, predict that the latter dianion is again a carbanion. This result is in reasonable agreement with normal coordinate calculations and the experimental isotopic shifts of vibrational frequencies of the dianion ¹⁵NCCH^?CONH^?.     

Infrared spectra and structure of carbanions—XIII: A study of the carbanions generated from acetonitrile,acetonitrile-d3 and acetonitrile-15n

The study of the IR spectral data for metallated acetonitrile (counter ions Li⁺, Na⁺, K⁺ in solvents tetrahydrofuran, THF, and hexamethylphosphotriamide, HMPT) and its D_3? and ¹⁵N- derivatives together with $\text{CNDO}\text{2}$ and normal coordinate calculations showed that the mesomeric ion H₂CCN has a favoured planar structure and the carbon-metal bond has a pronounced ionic character.     

A Complete Active Space Self‐Consistent Field (CASSCF) Study of the Reaction Mechanism of the α‐Alkynone Rearrangement

The pyrolytic rearrangement of α‐alkynones has been discovered by Karpf and Dreiding [1] in 1979. The mechanism of this reaction, which involves an acetylene‐vinylidene rearrangement followed by cyclization of the intermediate vinylidene carbene by insertion into a β‐C−H bond, has been debated in a couple of theoretical investigations. Restricted Hartree‐Fock (RHF) and single‐point Møller‐Plesset 2 (MP2) calculations at the RHF geometries apparently indicate the carbene cyclization to be the rate‐determining step, contrary to chemical intuition. However, larger‐scale correlated calculations with completely optimized molecular geometries ((8,8) CASSCF/6‐311G**), augmented with a perturbative account for the dynamic correlation contribution to the electronic energy, show vanishing energy barriers to the cyclization step and large activation energies for the acetylene‐vinylidene rearrangement, which is thus confirmed as the rate‐determining step of the title reaction.     

Pseudocapacitive Lithium Storage of Cauliflower-Like CoFe2O4 for Low-Temperature Battery Operation

Binary transition-metal oxides (BTMOs) with hierarchical micro–nano-structures have attracted great interest as potential anode materials for lithium-ion batteries (LIBs). Herein, we report the fabrication of hierarchical cauliflower-like CoFe₂O₄ (cl-CoFe₂O₄) via a facile room-temperature co-precipitation method followed by post-synthetic annealing. The obtained cauliflower structure is constructed by the assembly of microrods, which themselves are composed of small nanoparticles. Such hierarchical micro–nano-structure can promote fast ion transport and stable electrode–electrolyte interfaces. As a result, the cl-CoFe₂O₄ can deliver a high specific capacity (1019.9 mAh g⁻¹ at 0.1 A g⁻¹), excellent rate capability (626.0 mAh g⁻¹ at 5 A g⁻¹), and good cyclability (675.4 mAh g⁻¹ at 4 A g⁻¹ for over 400 cycles) as an anode material for LIBs. Even at low temperatures of 0 °C and −25 °C, the cl-CoFe₂O₄ anode can deliver high capacities of 907.5 and 664.5 mAh g⁻¹ at 100 mA g⁻¹, respectively, indicating its wide operating temperature. More importantly, the full-cell assembled with a commercial LiFePO₄ cathode exhibits a high rate performance (214.2 mAh g⁻¹ at 5000 mA g⁻¹) and an impressive cycling performance (612.7 mAh g⁻¹ over 140 cycles at 300 mA g⁻¹) in the voltage range of 0.5–3.6 V. Kinetic analysis reveals that the electrochemical performance of cl-CoFe₂O₄ is dominated by pseudocapacitive behavior, leading to fast Li⁺ insertion/extraction and good cycling life.     

Deposition of gold nanoparticles on β-FeOOH nanorods for detecting melamine in aqueous solution

This study demonstrates a facile but efficient approach to deposit metallic (gold) nanoparticles on β-FeOOH nanorods to obtain Au/β-FeOOH nanocomposites without the assistance of any polymers or surfactants at ambient conditions. In this method, a strong reducing agent (NaBH(4)) can be used to extensively produce Au nanoparticles, converting β-FeOOH into Fe(3)O(4) and depositing gold particles onto magnetic Fe(3)O(4) simultaneously. The microstructure, composition, and chemical properties of the obtained nanocomposites are characterized by various advanced techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-vis spectroscopy. Moreover, the Au/β-FeOOH nanocomposite can be used to detect trace melamine using UV spectrum in the ultraviolet wavelength range (190-260 nm), in which the nanocomposites show a higher sensitivity toward melamine due to the promotion of symmetry-forbidden bands (n→π(*)) of melamine molecules and also avoid the disturbance of commercial products containing solid colloids or food colorings that distort visual spectrum during the detection of chemical sensing. The deposition mechanisms and their sensing detection toward melamine are discussed.     

Sur la contractibilité d'un graphe orienté en K4

We prove that every directed graph with n vertices at least 5n–8 arcs admits the complete symmetric digraph of order 4 as a minor. The result is sharp. This answers a question raised by Meyniel (1983). We conclude with some open problems.     

Correlated MO study of the low-barrier intramolecular motions in donor-acceptor ethenes

Correlated MP2 and MCSCF MO calculations of several model push-pull ethenes in most cases indicate no great participation of excited singlet and triplet electronic configurations in either the minima or the transition structures for the suggested facilitated intramolecular rotation about the polarized C=C bond. This situation changes significantly only in molecules with sulfur atoms in the molecule as either donors or acceptors. The outstanding contribution of sulfur atoms as either donors or acceptors is a significant increase of push-pull ethene molecular polarizabilities. Thus, within the studied small series of mostly planar push-pull ethenes, polarizability appears a better indicator of rapid intramolecular motions about the C=C bond than straight polarity. Substituents with larger steric demands around the C=C bond are shown to likely preclude its complete turnaround, thus prompting a ramification of the interpretations of dynamic NMR phenomena in sterically constrained push-pull ethenes as large-amplitude librations resulting from strong rovibrational and relatively weak electronic coupling. These librations, as shown by complete vibrational mode analysis of corresponding rotational transition structures, cover in fact certain sectors of the intuitively suggested full rotations similar to those about C-C single bonds.     

Theoretical study of ketenimine: Geometry,electronic properties,force constants and barriers to inversion and rotation

Non-empirical calculations of the structure and properties of ketenimine have been performed using nine Gaussian basis sets. Values for the bond lengths and angles, HOMO and LUMO energies, atomic charges, overlap populations, dipole moments, bond energies, force constants and barriers to nitrogen inversion and internal rotation are predicted.     

Thorpe-Ingold effects in cyclizations to five-membered and six-membered rings containing planar segments. The rearrangement of N(1)-alkyl-substituted dihydroorotic acids to hydantoinacetic acids in base

While the gem-dimethyl effect (GDME) is quantitatively similar for cyclizations to cyclopentane and cyclohexane rings and their homomorphs, in systems containing planar segments the GDME is stronger for the formation of five-membered rings. Planar pentagons have smaller angles than planar hexagons and their formation is helped by the decrease in the potential internal bond angle caused by substituents, as suggested by Thorpe and Ingold for small rings. The phenomenon is illustrated with crystal structure data on five-membered hydantoins and six-membered dihydrouracils containing four-atom planar segments. Such a Thorpe-Ingold effect explains the rearrangement in base of N-alkyl substituted dihydroorotic acids 1 to hydantoinacetic acids 3. The reaction involves initial hydrolysis to N-(N-alkylcarbamoyl)aspartic acids 2 and their subsequent cyclization. The unsubstituted N-carbamoylaspartic acid 2a is stable in 1 M KOH, the N(1)-methyl and ethyl compounds 2b and 2c are in equilibrium with the hydantoinacetic acids 3, while the cyclization of the N(1)-isopropyl and cyclohexyl derivatives 2d and 2e is irreversible. Experimental data on equilibria and pK(a)s for ionization of the carboxy and NH groups allow equilibria and rates involving the N-unsubstituted compounds to be estimated and compared with those for the N-alkyl derivatives. The strongest effect is observed on the equilibrium [3(2-)]/2[(2-)], where substitution of H by methyl increases K 600-fold. In vitro the kinetic regioselectivity for acid catalyzed cyclization of N-carbamoylaspartic to hydantoinacetic acid against dihydroorotic acid is only 10:1. This, together with the weaker acidity of the remote carboxyl group, favours cyclization to dihydroorotic acid under biological conditions.