title: Evidence for pi-pi electron donor-acceptor interactions between pi-donor aromatic compounds and pi-acceptor sites in soil organic matter through pH effects on sorption.
abstract: Elucidation of molecular-level interactions controlling the sorption of organic compounds in soils is of major theoretical and practical interest. Sorption of pi-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, pi-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-pi-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-pi-donors, but capable of H-bonding. Also, no pH-dependent sorption was observed between the pi-donor PHEN and alumina, a model inorganic surface. We propose that the pi-donor solutes interact with pi-acceptor sites in soil organic matter (SOM), including aromatic rings with multiple carboxyl groups, aromatic amines, or heteroaromatic amines. The pi-acceptor ability of such aromatic moieties would increase with protonation. pi-pi 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.
authors.affiliation: Department of Soil and Water, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504, USA.
authors.forename: Dongqiang
authors.initials: D
authors.lastname: Zhu
authors.forename: Seunghun
authors.initials: S
authors.lastname: Hyun
authors.forename: Joseph J
authors.initials: JJ
authors.lastname: Pignatello
authors.forename: Linda S
authors.initials: LS
authors.lastname: Lee
mesh_terms: D000327:Adsorption; D002244:Carbon / Q000737:chemistry; D004583:Electrons; D006841:Hydrocarbons, Aromatic / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D009930:Organic Chemicals; D012989:Soil Pollutants / Q000032:analysis*
chemical_list: D006841:Hydrocarbons, Aromatic; D009930:Organic Chemicals; D012989:Soil Pollutants; D002244:Carbon