π+–π Interactions between (Hetero) aromatic Amine Cations and the Graphitic Surfaces of PyrogenicCarbonaceous Materials

Environ. Sci. Technol., 2015, 49 (2),pp 906–914

http://pubs.acs.org/doi/abs/10.1021/es5043029

Many organic compounds of environmental concerncontain amine groups that are positively charged at environmental pH. Here we present evidence that (hetero)aromatic amine cations can act as π acceptors informing π⁺–π electron donor–acceptor(EDA) interactions with the π electron-rich, polyaromatic surface of pyrogenic carbonaceous materials (PCMs) (i.e., biochar, black carbon, and graphene). Theπ⁺–π EDA interactions combine a cation−π force with a π–π EDA force resulting from charge polarization of the ring’s quadrupole. Adsorption on a biochar and reference adsorbent graphite was conducted of triazine herbicides, substituted anilines, heterocyclic aromaticamines, and other amines whose charge is insulated from the aromatic ring. Whennormalized for the hydrophobic effect, the adsorption increased with decreasing pH as the amines became ionized, even on graphite that had no significant fixedor variable charge. The cationic π acceptor (quinolinium ion) was competitively displaced more effectively by the π acceptor 2,4-dinitrobenzene than by the π donor naphthalene. The maximum electrostatic potential of organocations computed with density functional theory was found to be a strong predictor ofthe π⁺–π EDA interaction. The π⁺–π EDA interaction was disfavored by electropositive alkyl substituents and by chargedelocalization into additional rings. Amines whose charge was insulated fromthe ring fell far out of the correlation (more positive free energy ofadsorption). Identifying and characterizing this novel π⁺–π EDA interaction on PCMs will help in predicting the fate of organocations in both natural and engineered systems.