NMR of terminal oxygen. 1517O NMR spectra of the N+O− group: tert-Amine oxides R3N+O−, nitrones HC N+(O−), azoxy derivatives N N+(O−), nitrile oxides C N+O− and related compounds. Shielding anisotropy in linear heteroatomar π-systems

✍ Scribed by Hans Dahn; Péter Péchy; Vien Van Toan; Pierre-Alain Carrupt

• Publisher: John Wiley and Sons
• Year: 1995
• Tongue: English
• Weight: 608 KB
• Volume: 33
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260330408

✦ Synopsis

The "0 NMR signals of trimethylamine oxide and of six (para-substituted) N-aryldimethylamine oxides appear at 240-260 ppm, cu. 200 ppm more deshielded than hydroxylamines. The influence of the arene group and its substituents is small, confirming the absence of resonance interaction between the ring and the N*Ogroup.

X=N+(O-)Ar compounds (X

= R,C, nitrones; X = RN, azoxy compounds; X = ArN(O), azodioxy compounds = C-nitroso dimers) are more deshielded, 6,350-450 ppm, due to resonance participation of -N--Otype structures (true nitroso compounds X-N=O appear at 650-1500 ppm). Carbonitrile oxides R-C=N*-O-and N=N*-O-show high shielding, 6, eu. 140 and 115 ppm, respectively, higher than amiw oxides; their C and N atoms also are significantly shielded. The increase in shielding, relative to sp2-type Rsystems, is explained by the shielding anisotropy in linear (sptype) n-systems. The shift values observed for four classes of N -0 compounds are in excellent agreement with predictions by IGLO calculations. KEY WORDS "0 NMR tert-Amine oxides Nitrones Azoxy compounds Azodioxy compounds Nitrile oxides