Why is the “Donor-Acceptor” Stretching a sine qua non in Understanding the Vibrational Signatures of Ionic Hydrogen Bonds?

Abstract

Vibrational coupling between O–O stretch and ionic hydrogen bond (IHB) stretch were carefully examined for the following proton-bound Zundel dimers: H5O2+, (MeOH)2H+, and (Me2O)2H+. Two-dimensional vibrational calculations were performed at MP2/aug-cc-pVDZ level using the method of discrete variable representation (DVR). It was found that in bare Zundel dimers, the first overtone of O–O stretch does not couple with IHB fundamental, ruling out the possibility of Fermi resonance. Meanwhile, the O–O stretch combination tone with IHB stretch strongly couples with the IHB fundamental, resulting in an observable combination band and a red shifted intense band in comparison with the zero-order picture. Breaking the inherent symmetry of these Zundel dimers by Ar tagging relaxes the selection rule against Fermi resonance which is dramatically exhibited by ArH5O2+. These results demonstrate the critical role of the hydrogen bond “donor-acceptor” coordinate in modulating the IHB stretch frequency and intensity distribution. Such role was found to be sensitive to the presence of the messenger atom.