Correlations between CNDO/2 charge distribution and 13C NMR chemical shift in 7-acylamino side chains of cephalosporins.

Molecular orbital calculations by the CNDO/2D method yield charge distributions which correlate well with the observed 13C NMR chemical shift for the amide carbon of acylamino side chains of cephalosporins. Acyl groups that withdraw electrons from the amide C-N bond and concomitantly make the amide nitrogen more negatively charged increase the chemical shift. The trends are related to the degree of amide resonance. No direct correlation was found between the chemical shift of the amide carbon and the antibacterial activity of the cephalosporins.