A closer examination of the coupling between ionic hydrogen bond (IHB) stretching and flanking group motions in (CH3OH)2H+: the strong isotope effects

Abstract

The intermode coupling between shared proton (O–H⁺–O) fundamental stretching and flanking modes in (CH₃OH)₂H⁺ was revisited in the following contexts: (1) evaluation of Hamiltonian matrix elements represented in a “pure state” (PS) basis and (2) tuning of coupling strengths using H/D isotopic substitution. We considered four experimentally accessible isotopologues for this study. These are: (CH₃OH)₂H⁺, (CD₃OH)₂H⁺, (CH₃OD)₂D⁺, and (CD₃OD)₂D⁺. Potential energy surfaces (PESs), as well as dipole moment surfaces (DMSs), were constructed at the MP2/aug-cc-pVDZ level. Multidimensional vibrational calculations were conducted by solving a reduced dimensional Schrödinger equation using a discrete variable representation (DVR). We found that vibrational states in (CH₃OH)₂H⁺ and (CD₃OH)₂H⁺ are much more heavily mixed than those in (CH₃OD)₂D⁺ and (CD₃OD)₂D⁺. Furthermore, each isotopologue chooses to strongly couple between out-of-phase in-plane CH₃ rocking and its out-of-plane counterpart. Lastly, the interaction between O–O stretching and O–H⁺–O stretching was explored. We found that between the first overtone of O–O stretching and its combination tone with O–H⁺–O fundamental stretching, only the second couples with O–H⁺–O fundamental stretching. We hope that our isotopologue calculations would motivate experimentalists to measure them in the future.