Benzhydroximic acids - NMR study of trimethylsilyl derivatives

NMR spectra (H-1 C-13, N-15, and Si-29) of trimethylsilylated para and meta substituted benzhydroxamic acids were studied in chloroform solutions. The silylation products have the structure of Z-O,O'-bis(trimethylsilyl) deriv atives of benzhydroximic acid, independently of the ring substituent. Accor ding to aromatic proton chemical shifts, the geometry of the hydroximic gro up and its torsion angle with the ring plane are not affected by the para s ubstituent. The chemical shifts of the nuclei in the hydroximic part of the molecule show surprisingly strong dependence on the remote ring substituen t. The two Si-29 chemical shifts exhibit essentially the same sensitivity t o substitution despite the fact that the Si(O-1) is one bond closer to the substituent than the Si(O-4) silicon. It is suggested that while electron d onor substituents increase the shielding of the silicon atoms they also inc rease the basicity of the oxygen in the Si-O moiety, thus leading to the st ronger hydrogen bending with the solvent. Association with chloroform parti ally compensates the direct substituent effect on the shielding in the case of Si(O-1) silicon. The influence of other factors not covered by substitu ent constants is demonstrated by excellent correlations with the chemical s hifts in analogous tert-butyldimethylsilyl derivatives. (C) 2000 Elsevier S cience S.A. All rights reserved.