Electron‐Proton Transfer Mechanism of Excited‐State Hydrogen Transfer in Phenol‐(NH3)n (n=3 and 5)

• Published in: Chemistry : a European journal Vol. 24; no. 4; pp. 881 - 890
• Main Authors: Miyazaki, Mitsuhiko, Ohara, Ryuhei, Dedonder, Claude, Jouvet, Christophe, Fujii, Masaaki
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 19.01.2018; Wiley-VCH Verlag
• Subjects: Absorption; Ammonia; Aromatic compounds; Atomic and Molecular Clusters; Chemistry; Clusters; Electron transfer; Electrons; hydrogen transfer; I.R. radiation; Infrared spectroscopy; Internal conversion; Migration; Near infrared radiation; Phenols; Photochemicals; Physics; Potential energy; Protons; reaction mechanisms; solvated clusters; time-resolved spectroscopy; Vibration; time-resolved spectroscopy; reaction mechanisms; electron transfer; solvated clusters; hydrogen transfer
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201704129

Excited‐state hydrogen transfer (ESHT) is responsible for various photochemical processes of aromatics, including photoprotection of nuclear basis. Its mechanism is explained by internal conversion from the aromatic ππ* to πσ* states via conical intersection. This means that the electron is transferred to a diffuse Rydberg‐like σ* orbital apart from proton migration. This picture means the electron and the proton do not move together and the dynamics are different in principle. Here, we have applied picosecond time‐resolved near‐infrared (NIR) and infrared (IR) spectroscopy to the phenol‐(NH3)5 cluster, the benchmark system of ESHT, and monitored the electron transfer and proton motion independently. The electron transfer monitored by the NIR transition rises within 3 ps, while the overall H transfer detected by the IR absorption of NH vibration appears with a lifetime of about 20 ps. This clearly proves that the electron motion and proton migration are decoupled. Such a difference of the time‐evolutions between the NIR absorption and the IR transition has not been detected in a cluster with three ammonia molecules. We will report our full observation together with theoretical calculations of the potential energy surfaces of the ππ* and πσ* states, and will discuss the ESHT mechanism and its cluster size‐dependence between n=3 and 5. It is suggested that the presence and absence of a barrier in the proton transfer coordinate cause the different dynamics. Going different ways: Time‐resolved NIR and IR spectroscopy coupled with ab initio calculations on solvated clusters of phenol by ammonia established that the initial ultrafast delocalization of an electron over the solvents by photoexcitation next eliminates the proton from phenol to retrieve the hydrogen atom in the solvents moiety (see graphic). The results support the conclusion that decoupling of the electron and proton of a hydrogen atom occurs in the excited state hydrogen transfer (ESHT).