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Abstract

The 13C resonance of the carbonyl and carboxyl carbons in the compounds listed in the title has been studied by Fourier transform NMR at 61 MHz. in benzoic acid the complete shielding tensor σ was determined from a single crystal study. The shielding is largest when the magnetic field is perpendicular to the molecular plane and smallest when it is in the plane bisecting the OCO angle. The principal elements of the σ tensor of silver benzoate and benzoic acid anhydride obtained from powder spectra show relatively small deviations from the values found in benzoic acid, contrary to the situation in the corresponding acetic acid series. The mean shielding anisotropy δAσ = σzz - 12(σxx+σyy) for the central carbon increases from 151 ppm in benzophenone to 280 ppm in thiobenzophenone. In terms of an MO description it is shown that for the carbonyl carbon the dominating contributions to the principal elements of σ arise from very few single electron excitations. One of these is the n → π* transition, for which the excitation energy can be obtained from the optical spectra.