Aromatic Molecules in Anion Recognition:  Electrostatics versus H-Bonding

Mass-selected complexes A-·C6F n H6- n (A = Cl, I, SF6; n = 0−5) were studied by infrared photodissociation spectroscopy and computational chemistry methods to investigate the interaction of negative ions and aromatic molecules, in which the charge distribution can be tuned by fluorination. Surprisingly, we find that, despite positive partial charges on the carbon atoms at high levels of fluorination, all anions under study prefer hydrogen bonding to the remaining H atoms in the ligand rather than binding to the positively charged ring. Moreover, bifurcated hydrogen bonds to two neighboring CH groups are energetically favored over linear hydrogen bonds to a single CH group.