NMR of Terminal Oxygen. Part 13 . 17 O‐NMR spectra of C ‐nitroso compounds, thionitrites and NO + Ion: Resonance effects in ON—X compounds and correlation with CD spectra

The 17 O‐NMR signals of four true C‐nitroso compounds 1–4 appear at particularly low field (1550–1265 ppm), whereas the dimers (azodioxy type) resonate at ca . 400 ppm and the ‘isonitroso compounds’ ( quinone‐oximes; 5 and 6 ) at ca . 250 ppm. S ‐Nitroso compounds ( thionitrites; 8 and 9 ) show shift values of ca . 1300 ppm, not far from C—NO; the NO + ion is much stronger shielded (474 ppm). The results, together with those for higher‐shielded nitroso compounds X—NO (X RO, R 2 N, Cl, O − ) are discussed in terms of ( a ) resonance stabilization through n‐donation from X(π‐bond order, approximated by the known barriers of rotation around the X—N bond) and of ( b ) electronic excitation energies Δ E . The latter are approximated by long‐wave (symmetry‐forbidden) UV/VIS absorptions and confirmed, where available, by the maxima of the curves of circular dichroism (CD); the CD curve of thionitrite 9 has been measured. It is found that the δ( 17 O) values of X—NO depend both on bond order and on Δ E , which could not be separated. The higher shielding of NO + compared with X—NO is explained on the basis of anisotropy effects, which differ between sp and sp 2 systems.