From the calculated 13C NMR spectrum, the carbon atom C4
appearing at very higher chemical shift value (175.9 ppm in B3LYP/
6-31G(d,p), 176.5 ppm in B3LYP/6e31þþG(d,p)) due to double
bond of oxygen atom and hence the shielding is very small and
appears up field (see Table 9)