Benzhydroximic acids — NMR study of trimethylsilyl derivatives

NMR spectra (1H 13C, 15N, and 29Si) 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) derivatives of benzhydroximic acid, independently of the ring substituent. According to aromatic proton chemical shifts, the geometry of the hydroximic group and its torsion angle with the ring plane are not affected by the para substituent. The chemical shifts of the nuclei in the hydroximic part of the molecule show surprisingly strong dependence on the remote ring substituent. The two 29Si chemical shifts exhibit essentially the same sensitivity to substitution despite the fact that the Si(O1) is one bond closer to the substituent than the Si(O4) silicon. It is suggested that while electron donor substituents increase the shielding of the silicon atoms they also increase the basicity of the oxygen in the SiO moiety, thus leading to the stronger hydrogen bonding with the solvent. Association with chloroform partially compensates the direct substituent effect on the shielding in the case of Si(O1) silicon. The influence of other factors not covered by substituent constants is demonstrated by excellent correlations with the chemical shifts in analogous tert-butyldimethylsilyl derivatives.