Indirect carbon-carbon spin-spin couplings in nitrosobenzenes and benzene. Experiment and theory

Dynamically averaged (1)J(CC) spin-spin coupling constants in nitrosobenzene,p-substituted (F, Cl, Br, I, Me, NO2, OMe, NMe2) nitrosobenzenes, and o-nitrosotoluene as well as (3)J(CC)'s in nitrosobenzene are reported and compared with the literature data on (1)J(CC), (2)J(CC) and (3)J(CC) in benzene. The (1)J(CC)'s are shown to span a range of 55.5 through 70.5 Hz, and the substituent effects on the constants turn out to be significant, but largely local and almost additive. The effects on the constants seem to augment the coupling with the increasing Pauling's electronegativity of the first atom of the substituent concerned, but complications arise if nitrogenous substituents, NMe2, NO and NO2 are considered. For the first time it is shown that quantum mechanical DFT calculations of aromatic carbon-carbon couplings can yield, within a small and random spread, the simplest relationship possible, J(CC)(exptl) = J(CC)(calcd.), over a broad range of the couplings, starting from -2.5 Hz for (2)J(CC) in benzene, through that of about +8 to +10 Hz for (3)J(CC)'s in benzene and nitrosobenzene, up to the span of +55 to +70 Hz for (1)J(CC)'s including that of benzene, for a total of 34 individual couplings. This has been attained using the B3PW91/6-311++G(d,p)//B3PW91/6-311++G(d,p) approach, where the same functional-basis set combination was employed for geometry optimizations and for subsequent computations of the couplings. These computations revealed significant effects on the couplings of spatial arrangement of angular substituents with respect to the carbon-carbon bonds within a benzene ring. Attractive potential applications of this combination of experiment and theory are indicated in assessing syn-anti equilibria in disubstituted benzenes, by means of aromatic (1)J(CC) couplings.