Anomalies in the Structural Chemistry of Silicon

In contrast to carbon, silicon fails to form multiple bonds that are stable at room temperature. Consequently molecules in which silicon exhibits coordination numbers (CN) of 1, 2, and 3 may only be obtained at very high or low temperatures. Under these conditions their structural features, including multiple bonds, resemble those of carbon. On the other hand, silicon is capable of forming various hexacoordinated compounds making use of its d orbitals. Nitrogen and oxygen bonded to silicon develop an unusual stereochemistry: planar nitrogen, nearly or completely linear oxygen, and considerable shortening of SiN, SiO, and SiF bonds are specific examples. N(SiR₃)₂ and CH₂SiR₃ ligands permit stabilization of unusually low CNs of many metals and give rise to amino and alkyl derivatives of unexpectedly high stability due to the particular electronic, the R₃Si group.     

15N-NMR spectroscopy. III. Neighboring residue effects in sequence polymers containing glycylglycine units

The tripeptides Phe-Gly-Gly, β-Ala-Gly-Gly, and ?-Aca-Gly-Gly as well as the peptide derivatives δ-isothiocyanatovaleroyl-Gly-Gly and ?-isothiocyanatocaproyl-Gly-Gly were synthesized by using known methods so that the peptide nitrogen between the two glycyl residues was isotopically enriched in ¹⁵N to a level of 0.8–0.9%. These monomer units were then used to produce the sequence polymers (Phe-Gly-Gly)_n, (β-Ala-Gly-Gly)_n, (δ-Ava-Gly-Gly)_n, and (?-Aca-Gly-Gly)_n. The 18.24 MHz ¹⁵N-NMR spectra of the oligo- and polypeptides were obtained by using trifluoroacetic acid as solvent, since the solutions have relatively low viscosity and exhibit a strong negative nuclear Overhauser enhancement of the ¹⁵N signals. For comparison, ¹⁵N-NMR spectra of the homopolymers (Gly)_n, (β-Ala)_n, (γ-Abu)_n, (δ-Ava)_n, and (?-Aca)_n were also recorded. The ¹⁵N signals from the ω-aminoacyl residues in the sequence polymers appear up to 11 ppm upfield of the signals observed for the homopolyamides. The ¹⁵N signals from the two glycyl residues are separated by 3–7 ppm. Comparison with the ¹³C-NMR spectra of the same polymers indicates that ¹⁵N-NMR is better suited for the characterization and sequence analysis of these types of polymers.     

L'acide hydroxylamine-O-sulfonique,réactif de choix pour la conversion d'aldéhydes en nitriles

Hydroxylamine-O-sulfonic acid: a convenient reagent for the conversion of aldehydes into nitriles . Hydroxylamine-O-sulfonic acid (HAS) proved to be an excellent reagent for the conversion of aliphatic, aromatic and heteroatomic aldehydes into nitriles. There, aldehydes in the form of suspension in HAS water solution, react rapidely to give the corresponding nitriles in high yield. Only with formylpyridines could the intermediate oxime-O-sulfonic acids be isolated.     

Fluorescence from the second excited singlet state of [18] annulenes

The emission spectra of the second excited singlet state of [18] annulene and of monofluoro [18] annulene (in a 3-methylpentane glass at 4 K) are reported. The large energy gap between the first and second excited singlet states inhibits fast internal conversion and favours the appearance of S₂ → S₀ emission. In addition, fluorescence from the S₁ state can be observed in monofluoro [13] annulene by exciting into the S₂ or directly into the S₁ absorption.     

Chlor-dimethylaminosulfonium-Salze Darstellung und IR-Spektren

Chloro-dimethylaminosulfonium Salts. Preparation and I. R. Spectra By reaction of bis-dimethylamino-mono- resp. -disulfane with a mixture of chlorine/antimony(V) chloride the chloro dimethylaminosulfonium hexachloroantimonates(V) [(CH₃)₂N]₂SCI^⊕SbCl₆^? (I) and (CH₃)₂NSCI₂^⊕SbCl₆^? (II) are yielded. The i. r. spectra of these compounds were measured and assigned.     

Chemische Konstitution und Zellstreckungswirkung verschiedener Stoffe

Zusammenfassung Die Pastenmethode gestattet als einziger Zellstreckungstest fördernde und hemmende Wirkungen von außen zugeführter Stoffe am normal gesteuerten pflanzlichen Streckungswachstum gleichzeitig und gleichwertig festzustellen und zu messen. Ihre Beziehung zu anderen Testmethoden wird diskutiert.Auf Grund einer einfachen Überlegung über die allgemeinen Wirkungsmöglichkeiten von Wirkstoffen innerhalb der von ihnen beeinflußten Systeme wird eine allemeine Wirkstoffhypothese entwickelt, welche annimmt, daß ein Wirkstoff entweder eine Wirkgruppe und eine Affinität zu einer Wirklücke oder eine Hemmgruppe und eine Affinität zu einer Hemmlücke oder aber beides haben muß, wenn er Wirkstoffeigenschaften besitzen soll. Die im speziellen Fall als Gruppen oder Affinitäten bezeichneten Strukturbereiche können sich auch überschneiden oder untereinander identisch sein, doch ist ihre anfänglich getrennte Betrachtung aus heuristischen Grüden in jedem Falle zweckmäßig. Beliebige Stoffe können jeweils eine oder mehrere der genannten Gruppen oder Affinitäten in verschiedenen Stärken aufweisen und sich im Wirkstoffsystem dementsprechend verhalten. Es wird eine Deutung verschiedener Konzentrations-Wirkungskurven auf Grund dieser Hypothese gegeben.Die Kennzahlen der Konzentrations-Wirkungskurven von über 50 verschiedenen Stoffen im Pastentest werden mitgeteilt und vergleichend diskutiert.Mit 5 Abbildungen.Die hier veröffentlichte Wirkstoffhypothese wurde zusammen mit einem Teil des Materials dieser Mitteilung am 13. 8. 1953 beim Wuchsstoff-symposium der Universität Lund/Schweden erstmals vorgetragen.     

Aminyloxide (nitroxide)-XXVI: ESR-spektroskopische untersuchung von hydrazonyloxiden. Ermittlung der spindichteverteilung

The so-called “Bergmann oxide” 4a and the related compounds 4b-i dissociate reversibly to the corresponding radicals 5 at elevated temperatures. Analysis of the ESR spectra reveals that in 5a-f the unpaired electron is delocalized over the entire molecule. On the other hand the strongly reduced spin density at the β-carbon atom in 5h-i as well as in 7 indicates a twisting about the N,N-bond in these radicals, whereas m 5g the bond between the β-carbon atom and the group Ar¹ is twisted. The results of spin density calculations for radicals 5 are in agreement with the experimentally estimated spin densities. In spin trap experiments with nitrosobenzene or nitrones respectively, 5a reacts at the β-carbon atom, indicating this position as the most reactive one.     

Neue Verbindungen mit Granatstruktur. III. Arsenate des Typs {NaCa2} [M] (As3)O12

Attempts to prepare phosphate granates were without success. However, three arsenate granates of the type {NaCa₂}[M](As₃)O₁₂ with M^II ? Mg, Co, Ni have been prepared by solid state reaction and their lattice constants determined. All these arsenates are characterised by a thermal transformation into a high-temperature form, proceeding monotropically for the Mg compound, and reversibly for the Ni and Co compound.