The tetrazole–azide tautomerism of some nitrotetrazolo [1,5-a]pyridines studied by NMR, IR spectroscopy and molecular orbital calculations

The 6-nitro- and 8-nitro- groups in the tetrazolo[1,5-a]pyridine molecule exhibit completely different influences on the tetrazole–azide equilibrium. Introduction of the methyl-, nitro-, azido groups or a bromine atom in positions 5, 6, 7 or 8 of the nitrotetrazolopyridine produce changes in the equilibrium constants. Based on the IR spectra taken in the solid state, the tetrazole structure was assigned for almost all the compounds studied. Only one of them, 2,6-diazido-3-nitropyridine, exists in the diazido-form in the solid. The ¹H, ¹³C, ¹⁵N, and ¹⁷O NMR spectral parameters (coupling constants, chemical shifts) as well as ab initio molecular orbital calculations were used to describe the tetrazole–azide tautomerism in solutions. The differences in the NMR parameters between the neutral compound (6,8-dinitrotetrazolo[1,5-a]pyridine) and its σ-adducts are also included as data for distinguishing between both molecules.     

Nitrogen NMR sudies of some 15N labelled sydnones and related stuctures

¹⁵N NMR studies on some signly labelled sydnones, N-acetylsydonimines and their hydrochlorides, as well as those on some sydnonimine hydrochlorides, show that in each case protonation takes place at the exocyclic moiety, \documentclass{article}\pagestyle{empty}\begin{document}$ - {\rm O}^ \ominus - \mathop {\rm N}\limits^ \ominus {\rm COCH}_3 \,{\rm or}\, - \mathop {{\rm NH}}\limits^ \ominus $\end{document}, respectively. Complete assignments of the nitrogen chemical shifts are possible for the labelled compounds, including the isomeric structures of N-alkyl–N-cyanomethyl–N–nitrosoamines to which, unstable, free sydnonimines are converted.     

A 15N NMR investigation of some azolopyridines

¹⁵N NMR chemical shift data are presented for 14 azolopyridines, together with the results of INDO/S-SOS calculations of nitrogen shieldings. Previous ¹⁴N NMR results for some of these compounds are reinterpreted. The ¹⁴N data and their assignments are shown to be reliable for the indolizine nitrogen atom from arguments based on relative line widths. The pyridine-type nitrogens are more reliably assigned from the ¹⁵N spectra combined with the results of the INDO/S-SOS calculations for individual molecules. A combination of ¹⁴N and ¹⁵N NMR spectra, together with the shielding calculations, provides a basis for unambiguous assignments of all the various nitrogen environments considered.     

14N NMR study of some sydnones,sydnonimines and related structures

¹⁴N NMR spectra indicate that the sydnones, their hydrochlorides, the N-acetylsydnonimines, their hydrochlorides and the sydnonimine hydrochlorides studied exist as such in solution. The sydnonimines themselves are involved in a more complicated equilibrium; neutral solutions contain only the corresponding isomeric alkyl-cyanomethyl-nitrosoamine molecules which upon acidification are turned into cyclic cations, representing true sydnonimine structure, protonated at the exocyclic N atom.     

A 15N NMR Study of Protonation and Deprotonation of 4,5-Dicyanoimidazole and Its 2-Amino Analogue

Protonation and deprotonation of the title compounds, was studied by means of ¹⁵N NMR. The shieldings of the ring nitrogen atoms are found to be very sensitive to changes in the amount of protonation. In contrast the ¹⁵N shieldings of the cyano and amino groups are found to be relatively insensitive to protonation effects and are unsuitable for estimating the degree of protonation occurring.     

Lipophilic and Physicochemical Properties of Metalloporphyrins Separated by RP-TLC

JPC – Journal of Planar Chromatography – Modern TLC - The main objective of the study was to find a relationship between the structure and lipophilicity of metalloporphyrins with...     

Localization of N-oxide groups by means of nitrogen chemical shifts

The applicability of nitrogen chemical shifts to the localization of N-oxide groups in azine ring systems is exemplified by 1,2,4-triazine systems. The average excitation energy approximation in the theory of magnetic shielding gives a good linear correlation with the nitrogen shieldings in azine N-oxides.     

A13C and15N NMR study of some furoxans and related compounds

Carbon and nitrogen NMR results are reported for a collection of furoxans and related compounds, taken on DMSO solutions. The¹⁵N chemical shifts and those of neighboring¹³C nuclei have characteristic values for the structures studied. Relative¹⁴N signal widths are used to identify the nitro group resonances. Valence tautomerism at room temperature is reported for one compound. The INADEQUATE technique is used to determine some¹J(¹³C-¹³C) data.     

The investigation of the pore structure of silica gels with long-chain alkyl grafts by thermal analysis

The total pore volumes and the core size distributions were calculated for the silica gel Si-100 and chemically modified silica gels RP-8 and RP-18 on the basis of the thermal desorption data of various liquids. The results obtained are compared with those from the nitrogen adsorption/desorption method. Good consistency between the parameters obtained by using different methods is observed in the case of unmodified silica gel. The presence of long alkyl chains on the silica surface strongly influences the pore diameter calculated from thermogravimetric data.