Carboncarbon spin-spin coupling constants in monosubstituted benzenes

Abstract

One-bond and several long-range 13C- 13C spin-spin coupling constants have been determined in natural abundance for about 30 monosubstituted benzenes. Of the one-bond couplings, only 1 J C1C2 shows a substantial substituent effect, which suggests that the use of 1 JCC is an attractive method for determining substituent patterns in aromatic systems. 1 J C1C2 values correlate with the limited number of 1 JCC values determined in the literature for other systems. Although a correl ation with the electronegativity of the first atom of the substituent, E X, is found for 1 J C1C2 (and 1 J C2C5), all other couplings being insensitive to substituent change, the difference of 6.2 Hz between XNO 2 and NH 2 indicates the presence of further interactions. Consequently, a new substituent parameter, group electron-withdrawing power, is defined and calculated by INDO MO theory; it gives a better correlation with 1 J C1C2 and resolves the problems encountered in the simple correlation of 1 J C1–C2 with E X. Previous theoretical calculations of J CC are reassessed in the light of the present experimental data and INDO MO calculations. Although the relative magnitudes of 1 J CC and 3 J CC as well as the general effects of substituents are correctly predicted, the magnitudes of 2 J CC are overestimated and specific substituent effects upon 1 J C1C2 and 1 J C2C5 are poorly reproduced.