Solution and X-Ray Crystal Structures of the Di- and Tetra-allyl Ether of tert- utylcalix[4]arene

The di- and tetra-allyl ethers of tert-butylcalix[4]arene 1 and 2 have been prepared by alkylation of tert-butylcalix[4]arene with allyl bromide and K₂CO₃ using different reaction times. Solution ¹H NMR measurement of the di-allyl ether 1 and X-ray crystal structures of the complexes of 1 with chloroform (1a) or methanol (1b) indicate the cone conformation of 1 in which intramolecular hydrogen bonding can be maximized. The crystalline state conformers 1a and 1b are distorted in different grades depending on the solvent. While methanol is incorporated in the macrocycle, chloroform molecules do not occupy the cage. The solution ¹H NMR spectra of tetra-allyl ether 2 show the co-existence of the cone and partial cone conformation. The partial cone conformer of 2 was investigated by X-ray crystallography. In this compound hydrogen bonding is not existent. The conformer distribution is likely affected by steric and template effects.     

Stable gaseous aziridinium ions

Gaseous protonated aziridine ions are produced at the threshold from β-phenoxyethylamine molecular ions. The evidence for this is collisional activation spectra, using various precursors (including labelled analogues) under electron impact and field ionization conditions. Partial conversion to the acyclic \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} {\rm CH = }\mathop {\rm N}\limits^ + {\rm H}_{\rm 2} $\end{document} isomer occurs at higher electron energies and is rationalized by means of a potential energy surface constructed from energetic data.     

Untersuchungen über S?H …︁ S-Wasserstoffbrücken Die Kristallstruktur der Diphenyldithiophosphinsäure bei 140 und 293 K

Die Kristallstruktur der Diphenyldithiophosphinsäure (C₆H₅)₂P(S)SH wurde röntgenographisch bei tiefer Temperatur und Normaltemperatur aus Einkristalldiffraktometerdaten bestimmt und bis zu R-Werten von 0,037 (140 K, (sin Θ)/λ < 0,81 Å^?1) und 0,035 (293 K, (sin Θ)/λ < 0,64 Å^?1) verfeinert. Die Verbindung kristallisiert in der monoklinen Raumgruppe P2₁/c mit den bei 140 K (in Klammern: 293 K) gemessenen Gitterkonstanten a = 9,824(3) (9,887), b = 10,061(3) (10,175), c = 14,342(4) (14,433) Å, β = 122,08(3) (121,73)° und V = 1201,1 (1234,9) ų, Z - 4. Im Kristall sind individuelle Moleküle über fast lineare S? H…?S-Wasserstoffbrückenbindungen zu schraubenförmig gewundenen Ketten verknüpft. Bei 140 K beträgt der S…?S-Abstand innerhalb der Brücke 3,790(1) Å; die weiteren geometrischen Daten der Wasserstoffbrücke sind: d(S? H): 1,25(2), d(S…?H): 2,56(2), d(P? S): 2,077(1), d(P?S): 1,954(1) Å, ? (S? H…?S): 169,5(14), ? (P? S…?S): 98,87(2), ? (P?S…?S): 96,65(2)°. Investigations on Compounds Containing S? H…?S Hydrogen Bonds. Crystal Structure of Diphenyldithiophosphinic Acid at 140 and 293 K The crystal structure of diphenyldithiophosphinic acid (C₆H₅)₂P(S)SH was determined from X-ray diffraction data collected at 140 and 293 K and was refined to R factors of 0.037 (140 K, (sin Θ)/λ < 0.81 Å^?1) and 0.035 (293 K, (sin Θ)/λ < 0.64 Å^?1) respectively. The compound crystallizes in the monoclinic space group P2₁/c with unit cell parameters at 140 K (in parentheses: at 293 K): a = 9.824(3) (9.887), b = 10.061(3) (10.175), c = 14.342(4) (14.433) Å, β = 122.08(3) (121.73)° and V = 1201.1 (1234.9) ų, Z = 4. In the crystalline state individual molecules are linked together by nearly linear S? H…?S hydrogen bonds so that endless helical chains are formed. At 140 K the S…?S distance within the hydrogen bond is 3.790(1) Å; the other distances and angles associated with the bridge are: d(S? H): 1,25(2), d(S…?H): 2,56(2), d(P? S): 2,077(1), d(P?S): 1.954(1) Å, ? (S? H…?S): 169.5(14), ? (P? S…?S): 98.87(2), ? (P? S…?S): 96.65(2)°.     

Beta-sheet preferences from first principles

The natural amino acids have different preferences of occurring in specific types of secondary protein structure. Simulations are performed on periodic model beta-sheets of 14 different amino acids, at the level of density functional theory, employing the generalized gradient approximation. We find that the statistically observed beta-sheet propensities correlate very well with the calculated binding energies. Analysis of the calculations shows that the beta-sheet propensities are determined by the local flexibility of the individual polypeptide strands.     

Total synthesis of (+)-xyloketal D, a secondary metabolite from the mangrove fungus Xylaria sp.

(+)-Xyloketal D was prepared in a one-pot multistep domino reaction by heating optically active 5-hydroxy-4-methyl-3-methylenepentan-2-one (R) in toluene with 2,4-dihydroxyacetophenone. The absolute configuration of the natural product was confirmed by preparation of the starting enone from a lactone of established absolute configuration.     

Isomerization of hydrocarbon ions. VIII—The electron impact induced decomposition of n-dodecane

The literature on the mass spectrometry of ²H and ¹³C labelled higher alkanes is reviewed and the decomposition behaviour of both the molecular and the fragment ions of n-dodecane, n-dodecane-1, 12-[¹³C₂] and n-dodecane-1,1,1,12,12,12-[²H₆] studied with special emphasis on metastable decompositions. It is shown that the elimination of alkane molecules and alkyl radicals from the n-dodecane molecular ion occurs primarily by simple splitting of the C? C bond. In addition, both small alkane molecule and alkyl radicals are eliminated with low probability from centreal parts of the molecular ion. The alkane elimination is less specific than the alkyl elimination. The methyl elimination shows an exceptionally high non-specificity, but is of negligible abundance in the 70 e V electron impact spectrum. The metastable ion spectra suggest, but do not prove unambiguously, that those small alkyl ions (with up to four carbon atoms) originating directly from the molecular ion, may be formed both by direct cleavage of the terminal groups and from central parts of the molecular ion. However, the majority of the small alkyl fragment ions in the 70 eV spectrum are formed by secondary decomposition explaining their apparent non-specific formation. The strikingly different fragmentation behaviour of even electron, [C_nH_2n+1]⁺, and odd electron fragment ions, results from differences in the product stabilities. Using collisional activation and metastable ion spectra it is shown that the odd electron fragments have the structure of the linear alkene (most probably the 1-alkene) molecular ion. In contrast to the molecular ions, alkyl fragment ions decompose with complicated skeletal rearrangements, which lead to substantial, but not complete, carbon randomization. The terminal hydrogen atoms, however, show little scrambling.     

Freie Auflösungen äußerer Potenzen

Let R be a commutative noetherian ring with identity. To a finite free resolution F of an R-module M and every integer p1 we associate finite free complexes C _* ^p F. C _* ^p F is a free resolution of PM-the p-th exterior power of M if the degree of torsionfreeness of M is large enough. Using C _* ^p F we further study the degree of torsionfreeness of PM. As an application we prove a special case of a conjecture of P. Hackman concerning a connection between rank and torsionfreeness of a module.     

Complex forming and extraction properties of oligo benzo-fused crown ethers

The complex forming properties with alkali metal and ammonium ions of a series of oligo benzo-condensed 18-crown-6 ethers1–8 having a different gradation of lipophilicity and of molecular rigidity are investigated by voltammetry at the interface of two immiscible electrolyte solutions (ITIES) and by a liquid-liquid extraction technique. The experimental results obtained in the two phase system H₂O/nitrobenzene are discussed in relation to the structure of the crown and the cation type. The stability constants for the 1 : 1 complexes of Na⁺, K⁺, Rb⁺, Cs⁺ and NH ₄ ⁺ in nitrobenzene have been determined and compared with the extraction constants for the 1: 1 complexes of Na⁺ and K⁺ and for the 1 : 1 and 1 : 2 complexes of Cs⁺, showing the effect of oligo benzo condensation for the 18-crown-6 system.     

On the formation of protonated acetic acid from ionized n-alkanoic acids in the gas phase

By means of collisional activation mass spectrometry and semi-empirical molecular orbital calculations (MNDO) it is shown that the [C₂H₅O₂]⁺ ions from metastable molecular ions of n-pentanoic and hexanoic acid have the structure of carbonyl protonated acetic acid [CH₃C(OH)₂]⁺. The MNDO calculated geometries of this ion and some other isomeric forms are given together with the ΔH_f⁰ values.