Environ Sci Technol. 2015 Jan 20;49(2):906-14. doi: 10.1021/es5043029.
Many organic compounds of environmental concern contain amine groups that are positively charged at environmental pH. Here we present evidence that (hetero)aromatic amine cations can act as π acceptors in forming 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). The π(+)–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 aromatic amines, and other amines whose charge is insulated from the aromatic ring. When normalized for the hydrophobic effect, the adsorption increased with decreasing pH as the amines became ionized, even on graphite that had no significant fixed or 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 of the π(+)–π EDA interaction. The π(+)–π EDA interaction was disfavored by electropositive alkyl substituents and by charge delocalization into additional rings. Amines whose charge was insulated from the ring fell far out of the correlation (more positive free energy of adsorption). 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.
许多环境关注的有机化合物含有在环境 pH 下带正电荷的胺基。在这里，我们提供了证据表明（杂）芳族胺阳离子可以作为π受体，与热解碳质材料（PCM）（即生物炭、黑碳和石墨烯）的富π电子多环芳烃表面形成π(+)–π电子给体-受体（EDA）相互作用。π(+)–π EDA 相互作用结合了阳离子-π 力和由于环的四极矩的电荷极化而产生的 π–π EDA 力。三嗪除草剂、取代苯胺、杂环芳香胺和其他电荷与芳环隔离的胺在生物炭和参考吸附剂石墨上的吸附，当归一化为疏水性效应时，随着胺的离解，吸附随着 pH 的降低而增加，即使在没有显著固定或可变电荷的石墨上也是如此。阳离子π受体（喹啉鎓离子）比π受体 2,4-二硝基苯更有效地被π受体取代。用密度泛函理论计算的有机阳离子的最大静电势被发现是π(+)–π EDA 相互作用的强有力预测指标。正烷基取代基和电荷离域到其他环中不利于π(+)–π EDA 相互作用。电荷与环隔离的胺远远超出了相关性（吸附自由能更正）。在 PCM 上识别和表征这种新型的π(+)–π EDA 相互作用将有助于预测有机阳离子在自然和工程系统中的命运。
