Interpretation of heteronuclear and multidimensional NMR spectroscopy of humic substances

Mapping the chemical structures and organization of humic substances is vit al for a fundamental understanding of their roles and interactions in the s oil. One-dimensional nuclear magnetic resonance (NMR) techniques have advan ced our awareness of the composition of humic materials, but modem developm ents in two-dimensional NMR are soon likely to make obsolete reliance on on e-dimensional spectra alone. The advantages of using heteronuclear two-dime nsional NMR spectroscopy are illustrated in this paper in studies of the st ructural units in a fulvic acid fraction isolated from the Bh horizon of a Podzol. The structures identified from the NMR (at 500 MHz) experiments can be summ arized as: mono- and dicarboxylic acids (in about equal amounts), with an a verage chain length of about 10 carbon atoms (these are easily the major co mponents); smaller amounts (about 10-20% of the acids) of esters and alcoho ls or ethers; some carbohydrate and amino acid residues (evidence from chem ical shift data would suggest that these were likely to be in the form of c hains); and very small amounts of 1,2-, 1,4-, and 1,3,4-substituted benzene s and of cinnamic acids. The results suggest that applications of heteronuc lear and multidimensional NMR spectroscopy will allow considerable progress to be made in understanding the nature of the structural units and their c onnectivities in humic molecules provided that the heterogeneity of the hum ic mixtures can first be decreased significantly.