The SCS values used for the calculations are taken from .
Better estimations are available for the chemical shifts of carboxylic acids and esters and amides.
Literature "Parameter set for the prediction of the 13C-NMR chemical shifts of sp2- and sp-hybridized carbon atoms in organic compounds"
Pretsch E, Fürst A, Robien W Anal. Chim. Acta 248 (1991) 415-428