Nitrogen-15 Magnetic Resonance Spectroscopy XVI—Natural-Abundance Spectra of Aromatic Amines

Abstract

The ^(15)N chemical shifts of aniline, the toluidines, xylidines, and several halogen and oxygen substituted anilines have been measured at the natural abundance level of ^(15)N. Substituent parameters obtained by multiple regression analysis show that the methyl group induces comparable upfield shifts at the ortho and para positions (2·37 and 2·55 ppm/methyl, respectively) and a small (0·77 ppm/methyl) upfield shift at the meta position. The chemical shifts correlate reasonably well with ^(19)F shifts of similarly substituted fluorobenzenes, with C-1 of the anilines themselves and with Hammett sigma values. While the shifts of C-methyl substituted anilines do not correlate with the methyl resonances of corresponding polymethylbenzenes, those of the halo- and alkoxyanilines show a reasonable parallelism with corresponding ^(13)C-methyl shifts. The results are interpreted in terms of possible modes of transmission of electron density in an alternating and additive manner through the sigma framework.