The chemical structure of highly aromatic humic acids in three volcanic ash soils as determined by dipolar dephasing NMR studies

Dipolar dephasing ¹³C NMR studies of three highly aromatic humic acids, one from a modern soil and two from paleosols, have permitted the determination of the degree of aromatic substitution. From these data and the normal solid-state ¹³C NMR data we have been able to develop a model for the average chemical structure of these humic acids that generally correlates well with permanganate oxidation data. The models depict these humic acids as benzene di- and tricarboxylic acids interconnected by biphenyl linkages. An increasing degree of substitution is observed with increasing geologic age. These structures may be characteristic of the resistant aromatic part of the “core” of humic substances that survives degradation.