Nitrogen NMR assessment of interactions between nitroso, cyano and acetyl groups across aliphatic systems

Abstract

The nitrogen NMR shieldings (Aa, equivalent to -A6 on the frequency scale of chemical shifts) of the NO group in nitrosoalkanes are shown to be a sensitive probe to electron-withdrawing effects of substituents and to steric strain effects. The former produce high-field (low-frequency) shifts, which can amount to about 90 ppm, while the latter result in considerable low-field (high-frequency) shifts of the NO nitrogen resonance. The CN nitrogen shieldings show less pronounced low-field (high-frequency) shifts under the influence of electron acceptors. The signs and magnitudes of the observed effects are explained in terms of semi-empirical molecular orbital calculations of the shieldings and molecular mechanics evaluations of strain energies. The overall picture of relationships between the NO and CN nitrogen shieldings and electron charge migration within the molecules concerned is in accord with that obtained in earlier studies of solvent-induced effects on the nitrogen shieldings in alkyl cyanides and nitro compounds.

Solvent-Induced Effects on the Nitrogen NMR Shieldings of Some Nitrosobenzene Systems

“…A similar conclusion was reached in our earlier work on aliphatic nitroso compounds. 1 A general survey of the nitrogen shielding data presented in Table 1 shows that, if solutions in TFE are excluded from consideration, the range of nitrogen shielding variations as a function of solvent is modest, about 8 ppm. For 3 the range is slightly larger, about 15 ppm.…”

Solvent Effects on Nitrogen Chemical Shifts

“…It is independent of the identity of the alkyl group attached to the cyano function, as revealed by the data of [16][17][18][19] [27,36]. Their term a, which describes the response of the nitrogen shielding to the hydrogen bond donor strength of the solvent, is large and positive, yet less than half in magnitude as compared to those of sp 2 -hybridized compounds in Table 3.…”

“…For aromatic and aliphatic nitroso compounds, 14 N NMR data is available, with the chemical shifts varying between À600 and +400 ppm [5,25,27]. Aromatic nitroso compounds commonly exist in equilibria between their nitroso monomer and azodioxy dimer forms (Fig.…”