# Why do carboxylic acid derivatives show a more upfield shift than carbonyls in 13C-NMR?

The prototypical 13C NMR shifts of carbonyl carbons is in the range 190–200 ppm, however, carboxylic acid derivatives have a 13C NMR shift in the range 160–180 ppm instead. What structural factors cause this shift upfield? Shouldn't the carbon in the carboxylic acid be more deshielded than the analogous carbonyl, since there are two oxygens instead of one?

• No time for a full answer now but this mostly has to do with the $\Delta E$ term in the paramagnetic shift, cf. Gunther NMR Spectroscopy 3ed p410 and pubs.acs.org/doi/pdf/10.1021/ed077p905 May 9 '18 at 17:20