Substituent effects on15N NMR chemical shifts in selectedN-alkylthiohydroxamic acids. A comparative study

The N-15 NMR spectra of three N-alkyl-delta-carbomethoxyvalerothiohydroxamic acids (2) and six synthesized N-isopropylbenzothiohydroxamic acids (3) were measured and compared with appropriate spectra of structurally similar hydroxylamines (1), benzohydroxamic acids (4), benzamides (5) and thiobenzamides (6). The analysis of the chemical shifts of the thiohydroxamic acids under investigation indicates that the inductive effect of the hydroxyl group rather than steric hindrance is responsible for non-additivity of the effect of substituents. Additionally, N-hydroxyl diminishes the effect of aromatic ring substituents on the N-15 chemical shifts in the thiohydroxamic acids 3 which is approximately half that in the respective thiobenzamides 6. The chemical shift values correlate best with Brown's sigma(+) parameter. Copyright (C) 2004 John Wiley Sons, Ltd.