[Radu]

Why do carboxylic acids (C¹³ at carbonyl group) resonate more downfield than, say, anhydrides? I know it is not a big difference( 180 vs 165-170), but I simply feel it should be the other way round, given the carbonyl group is linked in both cases to O, but in the case of anhydrides there is less conjugation( so, a greater e density for acids)?