MESOIONIC COMPOUNDS - STRUCTURE AND NMR PARAMETERS

It has been shown, that H-1, C-13, N-14, N-15, O-17 and Se-77 NMR are very successful techniques for the identification and investigation of potential valence tautomerism in all types of mesoionic compounds. Es pecially the combined use of N-14 and N-15 NMR studies has been shown to be very effective in solving structural problems. From N-15 NMR it is possible to obtain accurate chemical shifts and spin-spin couplings , whereas the N-14 NMR spectra provide us with nuclear relaxation and some chemical shift data. In particular N-14 NMR Is very useful for th e identification of charged nitrogen atoms in molecules containing non equivalent nitrogen positions. This information is available from the relative N-14 signal widths, which depend upon the rates of N-14 rela xation, positively charged nitrogen atoms usually have relatively slow N-14 relaxation rates thus giving rather sharp NMR signals. Some soli d state, CP MAS, C-13 and N-15 NMR results are available for compariso n with the solution NMR data.