Nitrogen NMR shieldings of nitroalkanes as a structural and conformational probe

✍ Scribed by Michal Witanowski; Zenobia Biedrzycka; Karol Grela; Krystyna Wejroch

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 259 KB
• Volume: 36
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199806)36:13<s85::aid-omr303>3.0.co;2-i

✦ Synopsis

Nitrogen NMR shieldings (*p \ [d on the frequency scale of chemical shifts), measured by highprecision 14N NMR and bulk susceptibility corrected, of 20 nitroalkanes in dilute solutions in cyclohexane and in acetone are reported, including vic-dinitro, gem-dinitro and trinitro structures and tetranitromethane. The data obtained for cyclohexane solutions yield an excellent linear correlation with theoretical shieldings calculated for the corresponding isolated molecules by the semi-empirical TNDO/2 method. The nitrogen shieldings of nitro-NO 2 alkanes are shown to depend linearly on the net charges of the corresponding nitro groups, each taken as a whole, but there is no correlation with the atomic charges of the nitrogen atoms concerned. Thus, the nitrogen shieldings seem to provide a measure of electron-acceptor or electron-donor strength of the substituted alkyl system bound directly to the nitro group. Solvent polarity e †ects on the shieldings are discussed. The nitro group nitrogen shieldings are shown to be generally sensitive to conformational e †ects, particularly in sterically hindered molecules, and to provide insight into conformational or rotamer equilibria in nitroalkane systems. Ab initio calculations of the shieldings of some simple nitroalkanes, using the HartreeÈFock/GIAO approach, are reported. They indicate that the semi-empirical method employed, combined with the experimental data, yields results which are comparable to those obtained by the ab initio method, as far as relative shieldings are concerned.