1H nuclear magnetic resonance and molecular orbital studies of the conformations of 3-fluoroanisole

Abstract

The proximate spin-spin coupling constant between the methyl protons and the ring protons, 5J(H,OCH3), is extracted from a full analysis of the 1H and 19F nuclear magnetic resonance spectra of 3-fluoroanisole in CS2 and acetone-d6 solutions. The values of 5J(H,OCH3) imply that the less polar cis conformer is slightly more stable at 300 K than the more polar trans conformer in both solvents, in agreement with geometry-optimized STO-3G MO computations for the free molecule. The latter also find a higher barrier to internal rotation of the methoxy group for 3-fluoroanisole than for the parent molecule. The present results are compared with other measurements of the conformer ratio for the vapor and for solutions. The STO-3G and 6-3 1G structures of the cis and trans conformers are compared. The C-F bond length is computed more reliably with the minimal basis set, as is the COC bond angle. The internal angles of the benzene moiety are, of course, found more accurately with the 6-3 1G basis. The computations indicate additivity of the substituent effects on the internal angle, as found experimentally for a variety of benzene derivatives.