Nuclear magnetic resonance study of charge-transfer complexes of 1,3,5-trinitrobenzene, picric acid, and fluoranil with methoxy- and methyl-substituted benzenes and biphenyls. Indication of the structure of the complexes in solution

Abstract

Association constants for the charge-transfer complexes formed between 1,3,5-trinitrobenzene, fluoranil, or picric acid and methoxy- and methyl-substituted benzenes and biphenyls have been determined from n.m.r. chemical shift measurements. With some exceptions the log K values of these complexes vary linearly with the energy of the charge-transfer transition.

For complexes of non-hindered biphenyl donors a larger shift of the acceptor protons in pure complex (Δ₀) is found than for the complexes of the corresponding substituted benzenes. It is shown that this increase in Δ₀ is only due to a ring-current effect of the second ring of the biphenyl donor. For several model structures of the complex this effect has been calculated. The agreement for the model with a coaxial arrangement of donor and acceptor is very satisfactory. The conclusion is that Mulliken's principle of maximum overlap does indeed apply for these complexes.