Fascinating interaction of the ammonium cation with [2.2.2]paracyclophane: experimental and theoretical study

<p>By means of electrospray ionisation mass spectrometry, it was evidenced experimentally that the ammonium cation (NH<sub>4</sub><sup>+</sup>) reacts with the electroneutral [2.2.2]paracyclophane ligand (C<sub>24</sub>H<sub>24</sub>) to form the cationic complex [NH<sub>4</sub>(C<sub>24</sub>H<sub>24</sub>)]<sup>+</sup>. Moreover, applying quantum chemical calculations, the most probable conformation of the proven [NH<sub>4</sub>(C<sub>24</sub>H<sub>24</sub>)]<sup>+</sup> complex was solved. In the complex [NH<sub>4</sub>(C<sub>24</sub>H<sub>24</sub>)]<sup>+</sup> having a symmetry very close to <i>C</i><sub>3</sub>, the ‘central’ cation NH<sub>4</sub><sup>+</sup> is coordinated by three strong bifurcated intramolecular hydrogen bonds to the corresponding six carbon atoms from the three benzene rings of [2.2.2]paracyclophane via cation–π interaction. Finally, the interaction energy, <i>E</i>(int), of the considered complex [NH<sub>4</sub>(C<sub>24</sub>H<sub>24</sub>)]<sup>+</sup> was evaluated as  −625.8 kJ/mol, confirming the formation of this fascinating complex species as well. It means that the [2.2.2]paracyclophane ligand can be considered as an effective receptor for the ammonium cation in the gas phase.</p>