Abstract
Helquats are a recently introduced class of compounds, which can be perceived as a combination of helicenes with the skeleton of bispyridinium ions such as paraquat. In addition to their helical shape, some of these compounds can also adopt saddle-like conformations.
The focus here is on a specific [5]helquat dication HQ2+, which is investigated by various experimental techniques in condensed media as well as in the gas phase in combination with parallel computational studies. For this compound, the helical form is strongly favored in the free dication HQ2+ in the gas phase as well as in the condensed-phase salts [HQ2(+) . 2X-], whereas in the singly charged ion pairs in the gas phase, the saddle conformer [SQ2(+).
X-] is significantly more stable than the helical form [HQ2(+). X-] for a number of anions X-.
The reason for this inversion of stability in the binary ion pairs is that the saddle conformer has a central binding pocket, in which the anion can interact with both pyridinium centers, whereas the more compact helix conformer permits the anion to approach only a single cationic center efficiently.