Ring-Polymer Instanton Tunneling Splittings of Tropolone and Isotopomers using a ?-Machine Learned CCSD(T) Potential: Theory and Experiment Shake Hands

Tropolone, a 15-atom cyclic molecule, has received much interest both experimentally and theoretically due to its H-transfer tunneling dynamics. An accurate theoretical description is challenging owing to the need to develop a high-level potential energy surface (PES) and then to simulate quantum-mechanical tunneling on this PES in full dimensionality. Here, we tackle both aspects of this challenge and make detailed comparisons with experiments for numerous isotopomers. The PES, of near CCSD(T)-quality, is obtained using a Delta-machine learning approach starting from a pre-existing low-level DFT PES and corrected by a small number of approximate CCSD(T) energies obtained using the fragmentation-based molecular tailoring approach. The resulting PES is benchmarked against DF-FNO-CCSD(T) and CCSD(T)-F12 calculations. Ring-polymer instanton calcu-lations of the splittings, obtained with the Delta-corrected PES are in good agreement with previously reported experiments and a significant improvement over those obtained using the low-level DFT PES. The instanton path includes heavy-atom tunneling effects and cuts the corner, thereby avoiding passing through the conventional saddle-point transition state. This is in contradistinction with typical approaches based on the minimum-energy reaction path. Finally, the subtle changes in the splittings for some of the heavy-atom isotopomers seen experimentally are reproduced and explained.

Automated summary

In this study, we successfully describe the H-transfer tunneling dynamics of the Tropolone molecule by developing a high-level potential energy surface and simulating quantum-mechanical tunneling effects. The calculations on the corrected potential energy surface are in good agreement with experimental results, and show significant improvement compared to calculations based on a low-level DFT surface. The subtle changes in the splittings for some heavy-atom isotopomers observed experimentally are also reproduced and explained.