Therefore, the carbon 4 of the structure F could best be assumed as a meta-carbon. In fact molecular orbital calculations and dipole moment data of 2-X-thiophene carboxylic acids suggested that the para, meta and ortho carbons of benzene correspond to 5, 4 and 3 carbons of the 5 membered heterocycle . From the correlation of carbon-13 chemical shifts versus 1H-proton chemical shifts, in which the striking parallelism of the resonance shifts of the benzene and five membered heterocycle nuclei, implies that the resonances of both nuclei respond in a parallel way to the local π-electron density on the carbon atom .