Abstract
The CH···π hydrogen bonds (HBs) between trimethylamine (TMA) and an assortment of π-systems are generally weaker than those in which CF3H serves as a proton donor, despite the larger number of CH groups available to serve as donors in the amine. The added positive charge of tetramethylammonium (TMA+) enhances the binding energy by a factor between 4 and 7. The strongest such interaction for TMA+ occurs with indole, bound by 15.5 kcal/mol. Changing from ionic CH···π to NH···π further strengthens the interaction. Conjugation of the π-system improves its proton-accepting capacity, which is further enhanced by aromaticity. Dispersion plays a major role in CH···π HBs: It is the prime contributor in the neutral HBs of TMA, and comparable to Coulombic forces for CF3H and even in ionic CH···π HBs of TMA+. Many of the results can be understood on the basis of a combination of electrostatic potentials and charge transfers.
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