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NMR of Terminal Oxygen. Part 13 . 17 O‐NMR spectra of C ‐nitroso compounds, thionitrites and NO + Ion: Resonance effects in ON—X compounds and correlation with CD spectra
Author(s) -
Dahn Hans,
Péchy Peter,
Flögel Rainer
Publication year - 1994
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19940770130
Subject(s) - chemistry , resonance (particle physics) , nmr spectra database , analytical chemistry (journal) , crystallography , bond length , anisotropy , spectral line , stereochemistry , nuclear magnetic resonance , atomic physics , crystal structure , physics , chromatography , astronomy , quantum mechanics
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.