Evidence for π-π electron donor-acceptor interactions between π-donor aromatic compounds and π-acceptor sites in soil organic matter through pH effects on sorption

Elucidation of molecular-level interactions controlling the sorption of organic compounds in soils is of major theoretical and practical interest. Sorption of π-electron donor compounds, pentamethylbenzene (PMB), naphthalene (NAPH), and phenanthrene (PHEN), in a number of soils was found to increase with decreasing pH in the range of approximately pH 2.5-7. This behavior could not be attributed to pH-dependent alteration of the hydrophobic character of humic substances, π-H-bonding, interaction with mineral surfaces, interaction with black carbons, solute coplanarity, or pH effects on solute activity coefficient. No significant effect of pH was observed for non-π-donor hydrophobic compounds, whether planar or not: trans-1,2-dichlorocyclohexane (DCCH), hexachloro-1,3-butadiene (HCBD), 1,2,4-trichlorobenzene (TCB), 2,2′,5,5′-tetrachlorobiphenyl, and 3,3′.4,4′- tetrachlorobiphenyl. The opposite pH effect was observed for 2-nonanol and 2-nonanone, which are non-π-donors, but capable of H-bonding. Also, no pH-dependent sorption was observed between the π-donor PHEN and alumina, a model inorganic surface. We propose that the π-donor solutes interact with π-acceptor sites in soil organic matter (SOM), including aromatic rings with multiple carboxyl groups, aromatic amines, or heteroaromatic amines. The π-acceptor ability of such aromatic moieties would increase with protonation, π-π Interactions between PMB, NAPH, and PHEN, and model SOM acceptors, 1,3,5-benzenetricarboxylic acid (BTA), 1,4,5,8-naphthalenetetracarboxylic acid (NTA), and pyridine (PY) in methanol and methanol-water, were verified by the appearance of pH-dependent upfield ¹H NMR chemical shifts induced by ring current effects. UV/vis spectra showed pH-dependent charge-transfer bands for various donors with NTA. No NMR shifts or charge-transfer bands were found for nondonor compounds paired with the model acceptors.