Direct Microscopic Analysis of Individual C60 Dimerization Events: Kinetics and Mechanisms

• Published in: Journal of the American Chemical Society Vol. 139; no. 50; pp. 18281 - 18287
• Main Authors: Okada, Satoshi, Kowashi, Satori, Schweighauser, Luca, Yamanouchi, Kaoru, Harano, Koji, Nakamura, Eiichi
• Format: Journal Article
• Language: English
• Published: American Chemical Society 20-12-2017
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.7b09776

Modern transition state theory states that the statistical behavior of a chemical reaction is the sum of individual chemical events that occur randomly. Statistical analysis of each event for individual molecules in a three-dimensional space however is practically impossible. We report here that kinetics and mechanisms of chemical reactions can be investigated by using a one-dimensional system where reaction events can be observed in situ and counted one by one using variable-temperature (VT) atomic-resolution transmission electron microscopy (TEM). We thereby provide direct proof that the ensemble behavior of random events conforms to the Rice–Ramsperger–Kassel–Marcus theory, as illustrated for [2 + 2] cycloaddition of C60 molecules in carbon nanotubes (CNTs). This method gives kinetic and structural information for different types of reactions occurring simultaneously in the microscopic view field, suggesting that the VT-TEM opens a new dimension of chemical kinetics research on molecules and their assemblies in their excited and ionized states. The study carried out at 393–493 K showed that pristine CNT primarily acts as a singlet sensitizer of the cycloaddition reaction that takes place with an activation energy of 33.5 ± 6.8 kJ/mol. On the other hand, CNT suffers electron damage of the conjugated system at 103–203 K and promotes a reactive radical cation path that takes place with an activation energy of only 1.9 ± 0.7 kJ/mol. The pre-exponential factor of the Arrhenius plot gave us further mechanistic insights.