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

Strong and Sensitive Coupling of the IPB with Flanking Group Motions

The development of infrared multiphoton dissociation (IRMPD) together with the method of messenger tagging infrared predissociation technique (IRPD) paved the way to probe the vibrational regime of ions. One of the blockbuster ions studied was H5O2+, where a doublet instead of a singlet was observed for H+ stretch parallel to the O-O axis at 1000 cm-1. The arduous task of unmasking the doublet’s identity ended when Meyer and co-workers performed a full 15-dimensional calculation. To date with, it is established that the doublet is due to the coupling of the combination band of the overtone of water wagging and O-O stretch with the H+ stretch. Meanwhile, IRPD measurements for (MeOH)2H+ reveals a triplet peak near the H+ stretching window. To date with, these bands were not yet assigned. The goal of this study is to understand the triplet signature’s origin. We have utilized a reduced-dimensional approach by considering four normal modes namely: O-O stretch (ν1), H+ stretch (ν2), out-of-phase C-O stretch (ν3) and out-of-phase CH3 twist coupled to out-of-phase COH bend (ν3). The potential energy along the key normal modes was constructed using MP2 and CCSD(T) with aug-cc-pVDZ as a basis set. On the other hand, dipole surfaces build from MP2 and CCSD with the same basis set were used for the evaluation of integrated absorption coefficients. We have found out that two out of the three doublets are due to the coupling of Q2 with Q3, while the third band is Q4.

Figure 7 shows our assignments for the experimental triplet signature of the IRPD spectrum measured by Johnson’s group from Yale University. It turns out that ν1+ ν2, ν3, and ν4 couples to each other and resulted to a unique triple signature. However, the coupling was so strong that the lowest two peaks are heavily mixed. At the MP2 level, the first peak is 66% of the IHB fundamental stretch and 31% of the C-O fundamental stretch. Meanwhile, the second peak is 66% of the C-O fundamental stretch and 29% of the IHB fundamental stretch.

One way to further understand the coupling is to examine the isotopologues of (CH3OH)2H+. Thru a H to D isotopic substitution, some of the fundamental bands would shift and hence, affect the overall coupling.