Nitro group effects on the1J(CC) couplings in nitroalkanes

Abstract

One-bond 13C–13C coupling constants across a single bond are reported for a number of nitroalkanes including cases where two or three nitro groups are attached to the same carbon atom. The nitro group effect on the 1J(CC) across the adjacent bond becomes significant when more than one nitro moiety is present at the same carbon, and it can amount to an increase in the coupling of 7 Hz, i.e. by about 20% with respect to a typical value of 35 Hz which is characteristic of unsubstituted alkanes. The effect is comparable to that exerted by chlorine substituents, but is smaller than those induced by hydroxy or alkoxy oxygen atoms and by fluorine. The results for nitroalkanes seem to suggest that a major factor in substituent-induced effects on 1J(CC)s across a single bond is the electronegativity of the atoms which are attached directly to the bond, but some effects induced by methyl substituents suggest a contribution from steric hindrance. Semi-empirical calculations by the TNDO/2 method reproduced satisfactorily the general trends observed in substituent effects on 1J(CC)s, including those exerted by nitro groups. © 1998 John Wiley & Sons, Ltd.