Crown Ether Complexes with H3O+ and NH4+: Proton Localization and Proton Bridge Formation

The complexes formed by crown ethers with hydronium and ammonium cations are of key relevance for the understanding of their supramolecular behavior in protic solvents. In this work, the complexes of the 15-crown-5 (15c5) and 18-crown-6 (18c6) ethers with H3O+ and NH4+ and their deuterated variants are investigated under isolated conditions. The study employs infrared multiple photon dissociation (IRMPD) vibrational spectroscopy and DFT B3LYP/6-31++G(d,p) calculations for conformational assignment. The 18c6 ether provides two energetically nearby C-3 nu conformations with commensurate linear O-H center dot center dot center dot O and N-H center dot center dot center dot O bonds. The 15c5 ether ring adopts partially folded asymmetric pyramidal geometries, yielding one shorter linear H bond and two longer non-linear H bonds. Remarkably, an appreciable broadening of the IRMPD vibrational bands is observed for the 15c5-H3O+/D3O+ complexes. This can be interpreted as a signature for partial sharing of the proton (or deuteron) between the water and the crown ether along the linear O-H center dot center dot center dot O intermolecular H bond, which is indeed particularly short for this complex.