Nitrogen NMR studies on some fused ring N-heterocycles

¹⁵N and¹⁴N NMR data are reported for a number of fused ring N-heterocycles.¹⁴N line widths,¹⁵N-¹H couplings and nitrogen shielding additivity rales are used for¹⁵N signal assignments. The results obtained lead to some progress in the generalization of the effects of nitrogen-nitrogen interactions on nitrogen shieldings.     

A quantitative N NMR investigation of tautomeric equilibria of two 1-hydroxybenzimidazoles

¹⁵N nuclear shielding data are used in a study of the tautomeric equilibria of two 1-hydroxybenzimidazoles. The nitrogen signal assignments are supported by INDO-S/SOS shielding calculations. The observation of the N-3 signal is shown to give a quantitatively reliable estimate of the position of equilibrium. In the unsubstituted 1-hydroxybenzimidazole the amount of the hydroxy from present in the equilibrium mixture is found to be proportional to the pK_a value of the solvent employed.     

A H and C NMR study of the prototropic equilibria of 2-indolinone

¹³C NMR data are reported for 0.5 M solutions in CDCl₃ of 2-indolinone and six related compounds. The question of prototropic equilibria is discussed involving three possible tautomers, A, B, C. Form A is found to predominate in the dynamic equilibria established for three of the compounds studied. In the case of a fourth compound slow exchange between forms B and C results in the finding of both of these forms in the approximate ratio 1:1.8. These findings are supported by ¹H NMR measurements. Corresponding ¹H and ¹³H C NMR measurements on methanol solutions are essentially the same.     

14N and 13C NMR of tautomeric systems of hydroxy-pyridines

¹⁴N NMR is shown to be an efficient method of investigation of the tautomeric equilibria in hydroxy-and related substituted pyridines. Large relative ¹⁴N chemical shifts within tautomeric pairs estimated from N-Me and O-Me derivatives make possible quantitative determinations of the equilibria. Qualitative conclusions may be drawn from ¹⁴N line widths as well as from ¹³C and proton chemical shifts.     

A 13C, 15N and 77Se NMR Study of Some Diseleno Sulfonamides

¹³C, ¹⁵N and ⁷⁷Se NMR data are reported for ten title compounds. Some linear correlations of selenium, nitrogen and carbon chemical shifts values are described. A number of one- and two- bond ⁷⁷Se-¹³C coupling constants values are also given.     

Solid-phase approach to tetrahydroquinolones using a sulfur linker cleaved by SmI2

[reaction: see text] A sulfur alpha-heteroatom-substituted carbonyl (HASC) linker has been utilized in a solid-phase approach to tetrahydroquinolones. The route illustrates the compatibility of the linker system with palladium-catalyzed transformations and its utility for library synthesis. The linker is cleaved by electron transfer from samarium(II) iodide.     

Mesoionic compounds — structure and NMR parameters

It has been shown that¹H,¹³C,¹⁴N,¹⁵N,¹⁷O, and⁷⁷Se NMR are very successful techniques for the identification and investigation of potential valence tautomerism in all types of mesoionic compounds. Especially the combined use of¹⁴N and¹⁵N NMR studies has been shown to be very effective in solving structural problems. From¹⁵N NMR it is possible to obtain accurate chemical shifts and spin-spin couplings, whereas the¹⁴N NMR spectra provide us with nuclear relaxation and some chemical shift data. In particular¹⁴N NMR is very useful for the identification of charged nitrogen atoms in molecules containing non equivalent nitrogen positions. This information is available from the relative¹⁴N signal widths, which depend upon the rates of¹⁴N relaxation, positively charged nitrogen atoms usually have relatively slow¹⁴N relaxation rates thus giving rather sharp NMR signals. Some solid state, CP MAS,¹³C and¹⁵N NMR results are available for comparison with the solution NMR data.Published in Khimiya Geterotsiklischeskikh Soedinenii, No. 9, pp. 1180–1199. September 1995.     

An X-ray study of some 2,3-diphenyltetrazolium salts

An X-ray diffraction study is reported for three molecules of salts containing a cation with a positive charged 2,3-diphenyltetrazolium ring. The results confirm aromatic structure of the tetrazole ring and the delocalization of charge on the whole molecules of the cations. The molecular geometry obtained is compared with that for free mesoionic 2,3-diphenyltetrazoles which have already been studied and the ring parameters do not reveal significant differences. But is shows that the geometry of the (NNO)^– exocyclic group is changed as a result of alkylation. Compound I C₁₃H₁₂N₅Cl, P2₁/a,a=16.934(4),b=6.575(1),c=26.121(4) Å, =108.15(2)°, II C₁₄H₁₃N₆OI, P3₁2,a=10.774(3),c=12.821(4) Å, IV C₂₆H₂₀N₈Cl₂, C2/c,a=22.316(4),b=6.6903(7),c=20.437(3) Å, =92.51(1)°.     

An NMR study of hydrogen bonding in some azo dyestuffs

The ¹H, ¹³C, and ¹⁵N NMR data reported for compounds 1–4 show that in DMSO solutions all of them exist in the azo form only and do not participate in the azo–hydrazoimine equilibrium. The NMR data for compounds 1 and 2 show the presence of a weak hydrogen bond for the non-protonated forms, between N10 and the 2-NHCH₃ proton. All compounds have also been studied in TFA solutions in which they are protonated. The site of protonation of 1, 2 and 3 is determined to be at N10 in TFA solutions. These results are supported by some ab initio GIAO-CHF molecular orbital calculations.