Why are the kinked polyacenes more stable than the straight ones? A topological study and introduction of a new topological index of aromaticity

A topological model for estimating the stability of benzenoid hydrocarbons (BHs) is presented showing an acceptable linear dependence on Hess-Schaad resonance energy per pi-electron values. The topological measure of stability is accessible by use of pencil calculation and is based on counting cis-type fragments of double bonds in all canonical structures of a given BH. Evidence is given that infinite chains of straight linear polyacenes are always less stable than the kinked ones.     

Y-aromaticity: why is the trimethylenemethane dication more stable than the butadienyl dication?

[reactions: see text] Resonance energies of the trimethylenemethane dication (1) and the butadienyl dication (4) were evaluated using two independent computational methodologies to provide insight into the validity of Y-aromaticity. One methodology employed density functional theory calculations and examined the resonance contribution of the C=C double bond toward the double hydride abstraction enthalpies of methylpropene (6) and 2-butene (8), yielding 1 and 4, respectively. These resonance contributions by the double bond were determined by calculating the double hydride abstraction enthalpies of both the parallel and perpendicular conformations of vinylogues of 6 and 8, in which n = 1-4 vinyl units were inserted between the central carbon-carbon double bond and each of the reaction centers. Extrapolation of the resonance contribution in each vinylogue to n = 0 yielded the resonance contribution in the respective parent molecules. The second methodology employed an orbital deletion procedure (ODP), which effectively allowed us to examine the energies of individual resonance structures. The resonance energy of each dication is computed as the difference between the most stable resonance structure and that of the delocalized species. The two methodologies are in agreement, suggesting that the resonance energy of the trimethylenemethane dication is substantially greater than that of the butadienyl dication. The origin of this difference in resonance stabilization is discussed.     

Why are quasicrystals quasiperiodic?

In the past two decades significant progress has been made in the search for stable quasicrystals, the determination of their structures and the understanding of their physical properties. Now, quasiperiodic ordering states are not only known for intermetallic compounds, but also for mesoscopic systems such as ABC-star terpolymers, liquid crystals or different kinds of colloids. However, in spite of all these achievements fundamental questions concerning quasicrystal formation, growth and stability are still not fully answered. This tutorial review introduces the research in these fields and addresses some of the open questions concerning the origin of quasiperiodicity.     

Why are water-hydrophobic interfaces charged?

We report ab initio molecular dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the molecular charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) molecules. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several experimental observations and explain why hydrophobic surfaces in contact with water acquire a net negative charge, a phenomenon that has important implications for biology and polymer science.     

Why are Cold Molecules so Hot?

Brrrr! Cold molecule and cold atom research are juxtaposed and the challenges in cooling and trapping molecules are recounted. Both indirect and direct techniques of producing cold and slow molecules (such as buffer‐gas cooling and magnetic trapping, see picture) are described. Advanced techniques of manipulating cold or slow molecules are reviewed and ongoing work with cold molecules is outlined.

     

Which nido:nido-macropolyhedral boranes are most stable?

A comparison of the relative stabilities computed at RB3LYP/6-311+G(d,p)//RB3LYP/6-31G(d)+ZPE of the neutral nido-single clusters and two vertex-sharing macropolyhedral nido:nido-clusters shows single-cluster nido-boranes with up to 11 vertexes to be energetically more favorable than isomeric macropolyhedral boranes. Extra hydrogen atoms at the open face have a significant influence on the relative stabilities of the single cluster nido-boranes vs nido:nido-macropolyhedral boranes. For anionic species, a clear-cut turning point for macropolyhedral preference is shifted to no less than 17 vertexes. Thermodynamically most stable neutral and anionic nido:nido-macropolyhedral boranes usually consist of a nido-10-vertex and a nido-11-vertex unit, respectively. The relative stabilities of isomeric neutral macropolyhedra reflect the patterns exhibited by the sum of thermodynamic stabilities of the individual clusters.     

Why Are B ions stable species in peptide spectra?

Protonated amino acids and derivatives RCH(NH₂)C(+O)X · H⁺ (X = OH, NH₂, OCH₃) do not form stable acylium ions on loss of HX, but rather the acylium ion eliminates CO to form the immonium ion RCH = NH ₂ ⁺ . By contrast, protonated dipeptide derivatives H₂NCH(R)C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B₂ ions by elimination of HX. These B₂ ions fragment on the metastable ion time scale by elimination of CO with substantial kinetic energy release (T _1/2 = 0.3–0.5 eV). Similarly, protonated N-acetyl amino acid derivatives CH₃C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B ions by loss of HX. These B ions also fragment unimolecularly by loss of CO with T _1/2 values of ～ 0.5 eV. These large kinetic energy releases indicate that a stable configuration of the B ions fragments by way of activation to a reacting configuration that is higher in energy than the products, and some of the fragmentation exothermicity of the final step is partitioned into kinetic energy of the separating fragments. We conclude that the stable configuration is a protonated oxazolone, which is formed by interaction of the developing charge (as HX is lost) with the N-terminus carbonyl group and that the reacting configuration is the acyclic acylium ion. This conclusion is supported by the similar fragmentation behavior of protonated 2-phenyl-5-oxazolone and the B ion derived by loss of H-Gly-OH from protonated C₆H₅C(+O)-Gly-Gly-OH. In addition, ab initio calculations on the simplest B ion, nominally HC(+O)NHCH₂CO⁺, show that the lowest energy structure is the protonated oxazolone. The acyclic acylium isomer is 1.49 eV higher in energy than the protonated oxazolone and 0.88 eV higher in energy than the fragmentation products, HC(+O)N⁺H = CH₂ + CO, which is consistent with the kinetic energy releases measured.     

1,2-Dimorpholyl-?than als Reagens in der quantitativen Analyse

Zusammenfassung 1,2-Dimorpholyl-äthan gibt in Gegenwart von Jodid, Bromid oder Chlorid mit Quecksilber eine Anzahl schwer löslicher Verbindungen, deren Zusammensetzung und Eigenschaften untersucht werden. Sie eignen sich vorzüglich zur quantitativen gravimetrischen Quecksilberbestimmung auch in Gegenwart mehrerer anderen Elemente.Dem Verband der Chemischen Industrie und der Deutschen Forschungsgemeinschaft danken wir für die Unterstützung dieser Arbeit.     

1,2-Dimorpholyl-?than als Reagens in der quantitativen Analyse

Zusammenfassung Es wird über eine gravimetrische bzw. (bei Mengen unter 50 mg) fällungsphotometrische Wismutbestimmung mit 1,2-Dim.orpholyläthan berichtet.

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Summary A new procedure for the determination of bismuth is based on its precipitation with 1,2-dimorpholinoethane. The precipitate is dried and weighed or (in the case of Bi quantities <50 mg) dissolved and submitted to photometric measurement.

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II. Mitt.: Asmus, E., u. J. Peters: diese Z.195, 86 (1963).

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Für die Durchführung dieser Arbeit wurden dankenswerterweise Forschungsmittel aus dem ERP-Sondervermögen bereitgestellt.Dem Verband der Chemischen Industrie und der Deutschen Forschungs-gemeinschaft danken wir für die Unterstützung dieser Arbeit.     

1,2-Dimorpliolyl-?than als Reagens in der quantitativen Analyse

Zusammenfassung Es wird über eine potentiometrische Quecksilberbestimmung mit 1,2-Dimorpholyl-äthan berichtet.1. Mitteilung: Asmus, E., u. K. Ohls: diese Z. 177, 100 (1960).Für die Durchführung dieser Arbeit wurden dankenswerterweise Forschungsmittel aus dem ERP-Sondervermögen bereitgestellt.Dem Verband der chemischen Industrie und der Deutschen Forschungs-gemeinschaft, die diese Arbeit unterstützten, danken wir bestens.     

1,2-Dimorpholyl-?than als Reagens in der quantitativen Analyse

Zusammenfassung 1,2-Dimorpholyl-äthan gibt in saurer Lösung mit Phosphat und Molybdat eine schwerlösliche Verbindung, die sich vorzüglich zur gravimetrischen Phosphatbestimmung eignet. Es wird die Zusammensetzung der Verbindung ermittelt und eine Vorschrift zur Bestimmung von Phosphor gegeben.

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Summary 1,2-Dimorpholyl-ethane forms with phosphate and molybdate a compound of low solubility in acid solution, which can be used for the gravimetric determination of phosphate. The composition of this compound has ben analyzed and a method for the determination of phosphorus is described.

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Für die Durchführnng dieser Arbeit wurden dankenswerterweise Forschungsmittel aus dem ERP-Sondervermögen bereitgestellt.

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Dem Verbande der Chemischen Industrie und der Deutschen Forschungs-gemeinschaft, die diese Arbeit unterstützt haben, danken wir bestens.

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III. Mitt.: Asmus, E., H. Hähne u. K. Ohls: diese Z. 196, 161 (1963)     

1,2-Dimorpholyl-?than als Reagens in der quantitativen Analyse

Zusammenfassung Es wird eine gravimetrische Zinkbestimmung über die Verbindungen [C₁₀H₂₀O₂N₂H₂][Zn(SCN)₄] und C₁₀H₂₀N₂O₂ · Zn(SCN)₂ berichtet.

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Summary A method is described for the gravimetric determination of zinc via the compounds [C₁₀H₂₀O₂N₂H₂][Zn(SCN)₄] and C₁₀H₂₀N₂O₂ · Zn(SCN)₂.

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Mit 7 Textabbildungen

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IV. Mitt.: Asmus, E., u. P. Sarfert: diese Z. 198, 412 (1963).

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Für die Durchführung dieser Arbeit wurden dankenswerterweise Forschungsmittel aus dem ERP-Sondervermögen bereitgestellt.Dem Verband der Chemischen Industrie und der Deutschen Forschungsgemeinschaft danken wir für die Unterstützung dieser Arbeit.     

Why are a 3 ions rarely observed?

It has been determined experimentally that a(3) ions are generally not observed in the tandem mass spectroscopic (MS/MS) spectra of b(3) ions. This is in contrast to other b(n) ions, which often have the corresponding a(n) ion as the base peak in their MS/MS spectra. Although this might suggest a different structure for b(3) ions compared to that of other b(n) ions, theoretical calculations indicate the conventional oxazolone structure to be the lowest energy structure for the b(3) ion of AAAAR, as it is for other b(n) ions of this peptide. However, it has been determined theoretically that the a(3) ion is lower in energy than other a(n) ions, relative to the corresponding b ions. Furthermore, the a(3) --> b(2) transition structure (TS) is lower in energy than other a(n) --> b(n-1) TSs of AAAAR, compared with the corresponding b ions. Consequently, it is suggested that the b(3) ion does fragment to the a(3) ion, but that the a(3) ion then immediately fragments (to b(2) and a(3)) because of the excess internal energy arising from its relatively low energy and the facile a(3) --> b(2) reaction. That is why a(3) ions are not observed in the MS/MS spectra of b(3) ions.     

Why are glycoproteins modified by poly-N-acetyllactosamine glyco-conjugates?

Poly-N-acetyllactosamine structures occur in mammalian glycoproteins in both N- and O-linked glycans. They represent a backbone for additional modifications by fucosyltransferases, sialyltransferases and sulfotransferases. These glycans have been suggested to be involved in biospecific interactions with selectins and other glycan-binding proteins. Moreover, the poly-N-acetyllactosamine chains in N-glycans have been found to promote tumor progression and metastasis. Poly-N-acetyllactosamine chains are synthesized by repeated alternating additions of N-acetylglucosamine and galactose, catalyzed by beta-1,3-N-acetylglucosaminyltransferases (poly-N-acetyllactosamine synthase) and beta-1,4-galactosyltransferases. This review describes the poly-N-acetyllactosamine assembling machinery and focuses on recent advances in the molecular cloning and characterization of poly-N-acetyllactosamine synthase gene families. Recent progress in revealing the biological functions of poly-N-acetyllactosamine structures by various approaches in vitro and in vivo using different model systems has also been summarized.