Jake Tan's Project in Kuo's Group: (MeOH)2H+ Isotopologues

Investigating the Isotope effects on the vibrational signature of IPB in (MeOH)2H+

We performed vibrational calculations on four experimentally accessible isotopologues namely: (CH3OH)2H+, (CD3OH)2H+, (CH3OD)2D+, and (CD3OD)2D+. These isotopologues are predicted to exhibit two stable conformers: MA and MB. Across all isotopologues, harmonic zero-point energy corrections show that conformer MA is more stable than MB. Furthermore, under Boltzmann distribution at 50K, the population of conformer MA is very dominant (> 98%). The previously reported strong quantum coupling in (CH3OH)2H+ was revisited by representing the vibrational Hamiltonian in “pure state” basis. From such representation, the coupling nature between IHB and O-O stretches is not between the IHB fundamental and O-O stretch overtone, but rather between the IHB stretch and its combination band with O-O stretch. In general, this type of coupling is expected to be present in all Zundel IHBs, making the stretching along the “donor-acceptor” coordinate an essential ingredient in simulating IHBs. Such coupling scheme accounts for the intensities of the combination bands. In addition to this, the weak peak at 1160 cm-1 is tentatively assigned as the out-of-phase out-plane CH3 rock. Lastly, we found that the IHB isotopologues {(CH3OH)2H+ and (CD3OH)2H+} are heavily mixed and has a rich vibrational signature at 850-1100 cm-1. It was found out that the coupling strengths between IHB stretch and out-of-phase in-plane CH3 rock are stronger than that with an out-of-phase out-plane CH3 rock in (CH3OH)2H+. However, for (CH3OH)2H+ IHB stretch was found to couple stronger with out-of-phase out-plane CH3 rock than the corresponding in-plane mode. Upon deuteration to (CH3OD)2D+, and (CD3OD)2D+. A cleaner and less coupled spectrum is predicted to be observed. Unfortunately, there were no current experimental measurements for the deuterated isotopologues. We do hope that experimentalist would be motivated to verify our findings shortly.