Carbon-Carbon Spin-Spin Coupling Constants in Monosubstituted Benzenes

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 1JC1C2 shows a substantial substituent effect, which suggests that the use of 1JCC is an attractive method for determining substituent patterns in aromatic systems. 1JC1C2 values correlate with the limited number of 1JCC values determined in the literature for other systems. Although a correl ation with the electronegativity of the first atom of the substituent, EX, is found for 1JC1C2 (and 1JC2C5), all other couplings being insensitive to substituent change, the difference of 6.2 Hz between XNO2 and NH2 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 1JC1C2 and resolves the problems encountered in the simple correlation of 1JC1–C2 with EX. Previous theoretical calculations of JCC are reassessed in the light of the present experimental data and INDO MO calculations. Although the relative magnitudes of 1JCC and 3JCC as well as the general effects of substituents are correctly predicted, the magnitudes of 2JCC are overestimated and specific substituent effects upon 1JC1C2 and 1JC2C5 are poorly reproduced.