MULTINUCLEAR MAGNETIC-RESONANCE SPECTROSCOPIC AND SEMIEMPIRICAL MOLECULAR-ORBITAL (AM1) STUDIES OF SUBSTITUTED ANISOLES

C-13, N-15, and O-17 NMR spectra have been recorded for 4-nitroanisole (1), its 2-methyl-, 2-chloro-, 2-bromo-, 2-iodo-, 2,6-dimethyl-, 2,6-dichloro-, 2,6-dibromo-, and 2,6-diiodo-derivatives 2-9, also nitrobenzene (1a), its 3-methyl-, 3-chloro-, 3-bromo-, and 3-iodo-derivatives 2a-5a and 3,5-dichloro- and 3,5-dibromo-derivatives 7a and 8a. Analysis of the chemical shifts of carbon bearing nitro group and nitro oxygens in these compounds suggests that presence of one substituent ortho- to the methoxyl group enhances its resonance interaction with the benzene ring whereas presence of two ortho-substituents inhibits this resonance. However, in no case the resonance is completely inhibited. The extent of enhancement or inhibition is almost independent of the nature of the ortho-substituent. This conclusion has also been arrived by analyzing the reported chemical shifts of the para-carbons in anisoles 1b-9b and the corresponding carbons in benzene derivatives 1c-9c. Though evidence could not be obtained for steric enhancement of resonance using methoxyl oxygen chemical shifts, analysis of these chemical shifts in di-ortho-substituted anisoles 6-9 and 6a furnishes evidence for steric inhibition of resonance. However, N-15 chemical shifts are of no use in studying these phenomena. Semiempirical molecular orbital calculations using AM1 Hamiltonian suggest that the methoxyl group is coplanar with the benzene ring in anisole, 4-nitroanisole and 2-substituted-4-nitroanisoles but is perpendicular to the benzene ring in 2,6-disubstituted-4-nitroanisoles. Moreover, in 2-substituted-4-nitroanisoles the O-methyl group is anti to the 2-substituent.