NMR chemical shift prediction of carbonyl carbons in carboxylic acids and esters

The chemical shifts of carbonyl carbons in carboxylic acids and esters can be estimated using substituent chemical shifts (SCS) for the substituents.
The SCS values used for the calculations are taken from [1].

Literature

[1] "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
picture of ester

R1=
R2=