Chemical polarisation of 15N and 13C nuclei in diazo coupling reactions

Chemical polarisation of nitrogen and carbon nuclei in the decomposition and diazo coupling reactions of benzenediazonium fluoborate has been investigated. If homo- and heterolytic processes run in parallel in a multistage reaction sequence, then the nuclear polarisation created in a free-radical reaction can be nearly quantitatively carried over to products of rapid ionic reactions such as azo dyes.     

CIDNP of 13C and 1H nuclei in the reactions of cyclohexadienone carbenes

Thermal decomposition of 4-diazo-2,6-di-tert-butyl-2, 5-cyclohexadien-1-one in perhalogenated solvents has been investigated. The decomposition reaction proceeds via a carbene intermediate first to a singlet and then to an effectively triplet free encounter radical pair formed in a halogen abstraction reaction. The polarisation of the stable reaction product is determined by competing processes in the primary cage, intersystem crossing and escape from the cage.     

Carbon-13 chemical shifts and conformations of dimethylcyclohexanols

Carbon-13 chemical shifts of all twenty-two dimethylcyclohexanols, formed by the hydrogenation of isomeric xylenols, have been measured and assigned. Conformational peculiarities of dimethylcyclohexanols are discussed on the basis of their carbon-13 chemical shifts.     

13C, 14N, 15N and 17O NMR spectra of nitropyrroles and nitroimidazoles

Carbon, nitrogen and oxygen NMR spectra of some nitro derivatives of pyrrole and imidazole have been investigated. The ¹³C chemical shifts of para-carbons and the ¹⁷O chemical shifts of the nitro group correlate qualitatively with the electron densities on these carbon and oxygen atoms, which in turn depend upon the degree of conjugation of the nitro groups with the heterocyclic ring. Conjugation of several nitro groups with the benzene ring is in most cases not impaired by mutual interactions and the ¹³C shifts show good additivity. Such additivity is much worse in pyrrole and imidazole derivatives. Taken together with the diamagnetic nature of these deviations from additivity, this leads to a possible conclusion about the less pronounced conjugation of the nitro groups with the heterocyclic ring in heterocyclic dinitro derivatives.     

High resolution 29Si NMR study of the structure and devitrification of lead-silicate glasses

High speed magic-angle rotation of glass samples in the strong polarizing field of a superconducting magnet yields high-resolution ²⁹Si NMR spectra. Using this technique glasses of various composition, PbO·SiO₂, (PbO)₂·SiO₂, and (PbO)₄·SiO₂ were studied and the influence of thermal treatment followed. Crystallization of PbO·SiO₂ glass has been found to be a complex process leading to a structure identical with that of the mineral alamosite. The ²⁹Si NMR spectrum of crystalline alamosite consists of three lines in agreement with the structure determined by X-ray diffraction.     

13C-NMR-chemische Verschiebungen von Polymethinen. Elektronenstruktur und Lösungsmitteleinfluss

The ¹³chemical shifts of simple polymethines (cyanines and merocyanines) show the very pronounced charge alternation in these compounds. The ¹³C shifts of polar merocyanines are also susceptible to solvent polarity, which in this case has a strong influence upon the electron structure.     

Calculations of pKa of superacids in 1,2-dichloroethane

Acidity calculations for some CH and NH superacids in 1,2-dichloroethane (DCE) were carried out using SMD and COSMO-RS continuum solvation models. After comparing the results of calculations with respective experimental pK(a) values it was found that the performance of SMD/M05-2X/6-31G* method is characterized by the mean unsigned error (MUE) of 0.5 pK(a) units and the slope of regression line of 0.915. The similar SMD/B3LYP/6-31G* approach was slightly less successful. The strong correlation over entire data set is confirmed by R(2) values of 0.990 and 0.984 for M05-2X and B3LYP functionals, respectively. The COSMO-RS method, while providing the value of the linear regression line slope similar to the corresponding values from SMD approach, characterized by rather loose correlation (R(2) = 0.823, MUE = 1.7 pK(a) units) between calculated and experimental pK(a) values in DCE solution.