Clar valence bond representation of pi-bonding in carbon nanotubes

The application of the Clar aromatic sextet valence bond (VB) model to extended, defect-free single-walled carbon nanotubes (CNTs) with roll-up vectors (m, n) provides a real space model of their electronic structure. If m - n = 3k, where k is an integer, then all pi-electrons can be represented by aromatic sextets, and the CNT is fully benzenoid; the converse is also true. Since m - n = 3k is known to be a necessary criterion for conductivity in CNTs, only fully benzenoid CNTs are metallic, and only potentially metallic CNTs are fully benzenoid. This behavior contrasts with that of planar polycyclic aromatic hydrocarbons, in which the fully benzenoid structures are known to have large HOMO-LUMO gaps. For CNTs that are not fully benzenoid, e.g., m - n = 3k + l, where l = 1 or 2 and k is an integer, a seam of double bonds wraps about an otherwise benzenoid CNT at the chiral angle - 60 degrees or the chiral angle, respectively. Nucleus-independent chemical shift calculations on hydrogen-terminated CNT segments support this, and show that the magnetic manifestation of aromatic sextets is not due to electron correlation. The resonance hybrid of the Clar VB structures corresponds to patterns occasionally observed in scanning tunneling microscopy images of CNTs.     

Low-temperature stability and high-temperature reactivity of iron-based core-shell nanoparticles

Iron-based core-shell nanoparticles have been prepared that exhibit low-temperature stability and high-temperature reactivity toward oxygen in solution. The concentration of oxygen was determined from the fluorescence decay of pyrene. At low temperatures (<110 degrees C), the decays are short, indicating oxygen in solution. At higher temperatures (>110 degrees C), the decays become long and are consistent with no oxygen in solution. The change is abrupt, occurring over a narrow temperature range, and reproducible.     

Electrochemical reactivity and stability of platinum nanoparticles in imidazolium-based ionic liquids

Journal of Solid State Electrochemistry - The electrocatalytic activity of synthesized quasi-spherical Pt nanoparticles (NPs) has been studied, taking as a model the COads electrooxidation reaction...     

Distinct pi-bonding capability between phosphinidene and phosphonium ion: a computational study

The heavy dipnictenes (RE=ER, where E=P, As, Sb, and Bi with the substituent R) have essentially planar geometry and appreciable strength in pi-bonding, unlike related heavier main group 14 analogues of alkenes as concluded recently by Power. This work demonstrated that the protonated pnictenes behave more like the heavy carbene for their weak pi-bonding character from the computational study with the B3LYP/6-311++G** method. For example, although both phosphinidene (HP) and the phosphonium ion (H2P+) are isoelectronic to silylenes, the pi-bonding tendency of the former is rather strong and it forms a planar adduct with both the stable carbene and stable silylene ((HCNH)2E, where E=C and Si). In contrast, the latter forms trans-bent adducts with the two species. These results can be interpreted in terms of the Carter-Goddard-Malrieu-Trinquier (CGMT) model, and the fact that the value of DeltaEST [E(triplet)-E(singlet)] of the HP fragment increases significantly after protonation. All other heavy pnictenes resemble the phosphinidene. In contrast, nitrene (HN) and nitrenium (H2N+) have a ground triplet state, thus both have strong pi-bonding character similar to that of carbene.     

Revisiting superdelocalizability. Mathematical stability of reactivity indices

Superdelocalizability, S _r, defined by Fukui et al. as a variant of reactivity indices in the Hückel molecular orbital scheme for conjugated hydrocarbon molecules is reinvestigated with particular emphasis on its behavior for infinitely large π-electronic systems. Surprisingly, almost all the S _r values of polyacetylene are found to diverge with the size of molecule, while all the S _r values of comb polyene converge to certain values. Similarly, the S _r values of linear polyacene diverge, while those of zigzag polyacene converge. Whether the superdelocalizability for a series of periodic polymers converges or diverges can be predicted, respectively, if the density of states of the infinitely large π-electron network is shown to have non-zero or zero gap at the Fermi level. The behavior of atom-atom polarizability defined by Coulson et al. is also checked and discussed. Received: 27 July 1998 / Accepted: 9 September 1998 / Published online: 23 February 1999     

Preparation,stability and reactivity of fluorinated vinyl organometallic compounds

In contrast to the low stability of the fluorinated vinylic lithium or magnesium reagents, the analogous vinylic cadmium, zinc, copper, and silver reagents have been found to exhibit excellent stability. These organometallic substrates are readily prepared from the corresponding vinylic iodides or $\text{via}$ methathesis reaction. Stereochemical integrity is preserved in all organometallic-forming reactions as well as in all

subsequent chemical transformations of these reagents. In some cases (M = Cd), the $\text{bis}$-reagent can be isolated as a stable solid.A variety of chain-extension and/or functionalization reactions of these stable organometallic compounds will be presented and discussed.     

Thermodynamic stability and reactivity of the antiaromatic heterocycle 1H-azirine

The thermodynamic characteristics of reactions resulting in the conversion of the antiaromatic three-membered aza heterocycle 1H-azirine into isomeric 2H-azirine have been calculated by the SCF-MO-LCAO method and perturbation theory. The proton affinity has been investigated, and it has been shown that protonation occurs at a carbon atom with the subsequent transition to 2H-azirine protonated at the nitrogen atom.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 4, pp. 481–486, Jnly–August, 1989.     

Thermal stability and reactivity in oxygen of potassium ferrites with spinel structure

The potassium content really incorporated in the spinel lattice of the magnetite determined by thermogravimetric analysis in vacuum or in oxygen was about 2.45 wt%. In spite of this low content, the potassium strongly stabilises the defect spinel structure resulted by the potassium-substituted magnetite oxidation and increases the transformation temperature of the defect phases into -Fe₂O₃ or KFe₁₁O₁₇.

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Zusammenfassung Der tatsächlich in das Spinellgitter von Magnetit eingebaute Kaliumgehalt beträgt 2.45 Gewichtsprozente. Dies wurde durch TG-Analyse in Vakuum bzw. Sauerstoff ermittelt. Trotz dieses geringen Gehaltes wird das defekte Spinell des durch Oxydation erhaltenen kaliumsubstituierten Magnetits durch Kalium stark stabilisiert, was ein Ansteigen der Temperatur für die Umwandlung der defekten Phase in Fe₂O₃ oder KFe₁₁O₁₇ nach sich zieht.

     

Substituted diaryldiazomethanes and diazofluorenes: structure, reactivity and stability

The synthesis of several substituted diaryldiazomethanes and diazofluorenes, and an assessment of their structure, reactivity and stability, is reported.     

Effects of template sequence and secondary structure on DNA-templated reactivity

DNA-templated organic synthesis enables the translation, selection, and amplification of DNA sequences encoding synthetic small-molecule libraries. As the size of DNA-templated libraries increases, the possibility of forming intramolecularly base-paired structures within templates that impede templated reactions increases as well. To achieve uniform reactivity across many template sequences and to computationally predict and remove any problematic sequences from DNA-templated libraries, we have systematically examined the effects of template sequence and secondary structure on DNA-templated reactivity. By testing a series of template sequences computationally designed to contain different degrees of internal secondary structure, we observed that high levels of predicted secondary structure involving the reagent binding site within a DNA template interfere with reagent hybridization and impair reactivity, as expected. Unexpectedly, we also discovered that templates containing virtually no predicted internal secondary structure also exhibit poor reaction efficiencies. Further studies revealed that a modest degree of internal secondary structure is required to maximize effective molarities between reactants, possibly by compacting intervening template nucleotides that separate the hybridized reactants. Therefore, ideal sequences for DNA-templated synthesis lie between two undesirable extremes of too much or too little internal secondary structure. The relationship between effective molarity and intervening nucleic acid secondary structure described in this work may also apply to nucleic acid sequences in living systems that separate interacting biological molecules.     

Ellagitannin chemistry. Studies on the stability and reactivity of 2,4-HHDP-containing glucopyranose systems

The synthesis of a 2,4-HHDP containing glucopyranose ellagitannin model system has been achieved. Attempts to prepare a related target bearing a 1,6 bridge led instead to the discovery of a likely strain-driven tautomerization to cyclohexadienone intermediates.     

Ligand-modification effects on the reactivity, solubility, and stability of organometallic tantalum complexes in water

Tantalum complexes [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NMe(2))=CH)py}] (4) and [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NH(2))=CH)py}] (5), which contain modified alkoxide pincer ligands, were synthesized from the reactions of [TaCp*Me{κ(3)-N,O,O-(OCH(2))(OCH)py}] (Cp* = η(5)-C(5)Me(5)) with HC≡CCH(2)NMe(2) and HC≡CCH(2)NH(2), respectively. The reactions of [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(Ph)=CH)py}] (2) and [TaCp*Me{κ(4)-C,N,O,O-(OCH(2))(OCHC(SiMe(3))=CH)py}] (3) with triflic acid (1:2 molar ratio) rendered the corresponding bis-triflate derivatives [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(OCHC(Ph)=CH(2))py}] (6) and [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(OCHC(SiMe(3))=CH(2))py}] (7), respectively. Complex 4 reacted with triflic acid in a 1:2 molar ratio to selectively yield the water-soluble cationic complex [TaCp*(OTf){κ(4)-C,N,O,O-(OCH(2))(OCHC(CH(2)NHMe(2))=CH)py}]OTf (8). Compound 8 reacted with water to afford the hydrolyzed complex [TaCp*(OH)(H(2)O){κ(3)-N,O,O-(OCH(2))(OCHC(CH(2)NHMe(2))=CH(2))py}](OTf)(2) (9). Protonation of compound 8 with triflic acid gave the new tantalum compound [TaCp*(OTf){κ(4)-C,N,O,O-(OCH(2))(HOCHC(CH(2)NHMe(2))=CH)py}](OTf)(2) (10), which afforded the corresponding protonolysis derivative [TaCp*(OTf)(2){κ(3)-N,O,O-(OCH(2))(HOCHC(CH(2)NHMe(2))=CH(2))py}](OTf) (11) in solution. Complex 8 reacted with CNtBu and potassium 2-isocyanoacetate to give the corresponding iminoacyl derivatives 12 and 13, respectively. The molecular structures of complexes 5, 7, and 10 were established by single-crystal X-ray diffraction studies.     

Thermal stability and reactivity of metal halide filled single-walled carbon nanotubes

Thermal stability and reactivity to oxidation of several nanocomposite systems obtained by encapsulation of metal halides in single-walled carbon nanotubes are studied. Thermogravimetric analysis coupled with Raman spectroscopy allows insight into the various contributing factors, such as charge transfer, strain, and defect formation, and establishing a hierarchy of reactivity for the systems studied (AgX@SWCNTs, with X = Br, I; SWCNTs = arc discharge and HiPCO). The activation energy for oxidation decreases considerably after filling, indicating that filled nanotubes are more amenable to controlled modifications based on chemical reactivity than the originating empty nanotubes. The complete removal of the carbon shell at high temperatures does not preserve the nanowire morphology of the encapsulated halides; these are freed on surfaces in the form of nanoparticles arranged in 1D patterns. Metallic nanoparticles were obtained after hydrogen reduction of the halides, and growth of silicon nanowires in the footprint of the originating nanocomposites was demonstrated from such Co seeds. MX@SWCNTs (M = Ag, Co) can thus be used as environmentally stable nanoscale containers that allow the deliverance of catalytic nanoparticles in a prepatterned and aligned way.     

N-Heterocyclic carbene complexes of rhodium: structure, stability and reactivity

The organometallic chemistry of N-heterocyclic carbene complexes of the transition metals has received significant attention over the past ten years, especially with respect to complexes of Pd and Ru. The present Perspective highlights the chemistry of NHC complexes of another important transition metal, Rh. The application of Rh-NHC complexes in the industrially significant reactions of hydrogenation and hydroformylation is described. In order to accurately assess the activity of these complexes, their stability must be carefully determined. Various synthetic routes to Rh-NHC complexes are also described.     

Complexation of lappaconitine with glycyrrhizic acid: stability and reactivity studies

NMR and UV-vis spectroscopy have been used to study the complexation of antiarrhythmic alkaloid lappaconitine with an efficient complexing agent from licorice, glycyrrhizic acid, which is known to profoundly influence the therapeutic activity of the alkaloid in the complex. In MeOH, DMSO, or aqueous solutions, lappaconitine has been shown to form a stable complex with glycyrrhizic acid with 1:1 stoichiometry over a broad concentration range from 1 microM to 300 microM. The stability constant K(11) equals 2.0 x 10(5) M(-1) in aqueous solution. A similar complex of lappaconitine hydrobromide--the pharmaceutical formulation used in the treatment of arrhythmia--is 2 orders of magnitude less stable than pure lappaconitine. A notable decrease in the rate of the photoinduced electron-transfer reaction between lappaconitine in a complex with glycyrrhizic acid and tyrosine allows the suggestion of an explicit interrelation between the suppressed chemical reactivity of the bound alkaloid and the changes of its therapeutic efficiency.