Orbiting resonances in formaldehyde reveal coupling of roaming, radical, and molecular channels

• Published in: Science (American Association for the Advancement of Science) Vol. 374; no. 6571; pp. 1122 - 1127
• Main Authors: Foley, Casey D., Xie, Changjian, Guo, Hua, Suits, Arthur G.
• Format: Journal Article
• Language: English
• Published: United States The American Association for the Advancement of Science 26.11.2021
• Subjects: Chemical reactions; Chemistry; Coupling (molecular); Dynamics; Ergodic processes; Formaldehyde; Photodissociation; Radicals; Reaction mechanisms; Rotational states
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.abk0634

Despite intense study, roaming systems have been generally treated classically and their dynamics have been assumed to be ergodic. Foley et al . provide a combined experimental and theoretical study of the photodissociation of formaldehyde near the threshold for radical production. Their observations revealed a strong dependence of the degree of roaming on the rotational state of the molecule that is excited, which is indicative of an important role for quantum orbiting resonances. This study demonstrates unexpected, non-ergodic roaming dynamics in the prototypical roaming system that does not fit into the conventional classical picture, subject to further studies in other systems. —YS Quantum resonance in the dissociation of H 2 CO affects the H 2 +CO products formed by the roaming and transition state pathways. The roaming chemical reaction mechanism involves near-dissociation of an energized molecule to radicals that leads instead to intramolecular reaction after reorientation at long range. Surprisingly, no clear quantum signatures of roaming have been observed to date, despite the quantum nature of the roaming event. We found evidence of quantum dynamics in the photodissociation of formaldehyde near the roaming threshold. This is ascribed to resonances associated to H+HCO( K a = 1) that have a profound impact on the CO rotational and translational energy distributions and cause the roaming fraction to vary by a factor of 2 over an energy range of 10 cm –1 . The roaming pathway serves both to modulate and report on the complex vibrational dynamics and coupling among the three dissociation pathways in the excited molecule as it decays to products.