The motivation for this theoretical study is the experimental spectrum of this anion from Mark Johnson’s group in 2015 and 2025. Joint theoretical work, including recent VPT2 calculations, using a machine-learned potential (MLP) from the Meuwly group, failed to reproduce the highly fractionated band in the region of the OH-stretch fundamental.1 However, large-scale VSCF/VCI calculations, using MULTIMODE(MM) software with a permutationally invariant polynomial MLP, do so, as shown in the figure.2 Several metrics rom the chaos literature applied to the quantum wavefunction suggest that chaos is induced by excitation of the OH-fundamental; however, not among all the vibrational modes.
The talk will begin with a brief history of quantum chaos in vibrational motion, from the 1980s to the present, where eigenstate spreading, random matrix theory, projection analysis, etc. have been applied. The onset of fractionation is examined by varying the number of modes coupled to the OH-stretch. Classical treatments and analysis, including Poincare surface-of section plots, of the vibrational dynamics have also been done and provide some useful qualitative insights. A key feature in this system is the incipient proton transfer between two O atoms as depicted in the figure via saddle point of energy roughly 4 kcal/mol. This transfer is a major source of the strong coupling in the OH-stretch and other modes of the molecule. This will be discussed in detail in the talk. An extension of VPT2 theory to describe major fractionation will be briefly discussed.
Refs.
1.Andreichev, V.; K¨aser, S.; Bocanegra, E. L.; Salik, M.; Johnson, M. A.; Meuwly, M. Dynamics of protonated oxalate from machine-learned simulations and experiment: infrared signatures, proton transfer dynamics and tunneling splittings. Phys. Chem. Chem. Phys. 2025, 27, 23288–23300.
2. Qu, C.; Houston, P. L.; Bowman, J. M. Does the diUuse OH stretch band in the IR spectrum of protonated oxalate exhibit quantum chaos? J. Chem. Phys. 2026, 164, 061103.
3. Quantum and Classical and Analyses of the Fractionated OH Stretch Band of Protonated Oxalate Anion and an Extended VPT2 Proposal, Chen Qu, Paul L. Houston, Saikiran Kotaru, and Joel M Bowman, submitted
