Dynamics of coordination of H₃O⁺ and NH₄⁺ in crown ether cavities

Crown ethers stand out for their ability to form inclusion complexes with metal cations and positively charged molecular moieties. Hydronium and ammonium interact strongly with crown ethers and potentially modulate their ionophoric activity in protic solvents and physiological environments commonly involved in (bio)technological applications. In this work, Born-Oppenheimer molecular dynamics (BOMD) computations are employed to gain insights into the coordination arrangements of H₃O⁺ and NH₄⁺ in the complexes with the native crown ethers 15-crown-5 (15c5) and 18-crown-6 (18c6). Both cations display dynamic changes in coordination inside the cavities of the crown ethers. On the one hand, hydronium explores different coordination arrangements, through rotation around its C₃ axis in the 15c5 complex, and through breathing motions, involving rapid inversions of the O atom along the C₃ axis in the 18c6 complex. On the other hand, ammonium undergoes a facile rotation in three dimensional space, leading to frequent changes in the NH bonds involved in the coordination with the crown ether. The reduced host-guest symmetry matching of the 15c5 macrocycle enhances the reorientation dynamics and, in the case of H₃O⁺, it promotes short H-bonding distances yielding events of proton transfer to the crown ether. The infrared vibrational spectra predicted by the BOMD computations within this dynamic framework reproduce with remarkable accuracy the action spectra of the isolated complexes obtained in previous infrared laser spectroscopy experiments. The experimentally observed band positions and broadening can then be rationalized in terms of orientational diffusion of the cations, changes in the coordinating H-bonding pairs sustaining the complex and eventual proton bridge formation.