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

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.