期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2017
卷号：114
期号：24
页码：E4706-E4713
DOI：10.1073/pnas.1705089114
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：We report the vibrational spectra of the hydronium and methyl-ammonium ions captured in the C3 v binding pocket of the 18-crown-6 ether ionophore. Although the NH stretching bands of the CH3NH3+ ion are consistent with harmonic expectations, the OH stretching bands of H3O+ are surprisingly broad, appearing as a diffuse background absorption with little intensity modulation over 800 cm−1 with an onset ∼400 cm−1 below the harmonic prediction. This structure persists even when only a single OH group is present in the HD2O+ isotopologue, while the OD stretching region displays a regular progression involving a soft mode at about 85 cm−1. These results are rationalized in a vibrationally adiabatic (VA) model in which the motion of the H3O+ ion in the crown pocket is strongly coupled with its OH stretches. In this picture, H3O+ resides in the center of the crown in the vibrational zero-point level, while the minima in the VA potentials associated with the excited OH vibrational states are shifted away from the symmetrical configuration displayed by the ground state. Infrared excitation between these strongly H/D isotope-dependent VA potentials then accounts for most of the broadening in the OH stretching manifold. Specifically, low-frequency motions involving concerted motions of the crown scaffold and the H3O+ ion are driven by a Franck–Condon-like mechanism. In essence, vibrational spectroscopy of these systems can be viewed from the perspective of photochemical interconversion between transient, isomeric forms of the complexes corresponding to the initial stage of intermolecular proton transfer.
