Quantum Dynamics of Intramolecular Double Hydrogen Transfer in Porphycene

The making and breaking of H-bonds on highly anharmonic potential energy surfaces involved in proton and hydrogen transfer reactions require a full-dimensional quantum mechanical treatment of not only electrons, but also of nuclei. Here we demonstrate that dealing with this complexity is necessary for achieving predictive simulations that can solve puzzling properties of these reactions by addressing the intramolecular double hydrogen transfer (DHT) in porphycene[1]. Our theoretical treatment combines dispersion corrected hybrid density-functional theory calculations and path-integral ring-polymer methods. Our simulations predict the position and width of the N-H stretching band of porphycene and DHT rates in excellent agreement with experiments, thus confirming our determination of the tunneling pathways and the anharmonic mode couplings that play a role in this reaction. They also confirm the importance of the usually ignored competition between concerted and stepwise DHT pathways for this system. Our general theoretical approach provides a quantitative framework for a deeper physical understanding of hydrogen transfer dynamics in complex systems. [1] Y. Litman, J. O. Richardson, T. Kumagai, M. Rossi. arXiv:1810.05681 (2018)