Multidimensional Quantum Dynamics of Double Hydrogen Transfer in Porphycene: Effects of Anharmonic Vibrational Mode Couplings

Porphycene is an important prototype for the study of double hydrogen transfer (DHT) tunneling and has attracted great attention. However, a multidimensional quantum dynamics (QD) study based upon a global ab initio potential energy surface (PES) is still unavailable, which is imperative for the study of anharmonic vibrational mode couplings. Here, we perform the first multidimensional QD calculations on the global HDNNP-G PES (Litman et al. Faraday Discuss. 2020, 221, 526) using a QD scheme developed in our group to study DHT tunneling in porphycene. Various anharmonic vibrational couplings are revealed, and a systematic analysis uncovers those modes strongly coupled to trans–trans DHT, including the Q 56 mode that was unrecognized before. Our QD results of tunneling splittings agree well with experiment. Moreover, we find that isotope effects have a significant impact on anharmonic vibrational couplings. In particular, in singly deuterated porphycene, Q 2 strongly couples with Q 1, whereas this coupling is negligible for the original porphycene.