Multinuclear Magnetic Resonance Spectroscopic and Semiempirical Molecular Orbital (AM1) Studies of Substituted Anisoles

Abstract

Abstract 13C, 15N, and 17O 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, 15N 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.