¹³C-C¹³ coupling constants in dimethyl phthalate and phthalic anhydride. Determination of signs—III

Relative signs of 13C-13C coupling constants between ring carbons and carbonyl carbons in dimethyl phthalate and phthalic anhydride, 13C‐labelled in both carbonyl groups, have been determined. The two‐and three‐bond coupling constants are shown to be positive, whereas four‐bond coupling constants are negative. Substituent effects on carbon‐carbon couplings due to a carboxyl group have been determined and effects of 5‐membered ring formation in phthalic anhydride have been observed.