High resolution EPR spectroscopy of C60F and C70F in solid argon: Reassignment of C70F regioisomers

• Published in: The Journal of chemical physics 2007-08, Vol.127 (8)
• Main Authors: Misochko, E. Ya, Akimov, A. V., Belov, V. A., Tyurin, D. A., Laikov, D. N.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.2768350

Free radicals C60F and C70F were generated in solid argon by means of chemical reaction of photogenerated fluorine atoms with isolated fullerene molecules (C60 or C70). High resolution anisotropic electron paramagnetic resonance (EPR) spectra of C60F and C70F at low temperature have been obtained for the first time. The spectrum of C60F is characterized by an axially symmetric hyperfine interaction on F19 nucleus. The hyperfine coupling constants Aiso=202.8MHz (Fermi contact interaction) and Adip=51.8MHz (electron-nuclear magnetic-dipole interaction) have been measured for C60F in solid argon. Quantum chemical calculations using hybrid density-functional models (either PBE0 or B3LYP) with high-quality basis sets give a theoretical estimate of the hyperfine coupling constants in good agreement with the measurements. The electron spin density distribution in C60F is theoretically characterized using the Hirshfeld atomic partitioning scheme. Unlike C60, five isomers of C70F can in principle be produced by the attachment of a fluorine atom to one of the five distinct carbon atoms of the C70 molecule (denoted A, B, C, D, and E, from pole to equator). The measured high resolution EPR spectrum of the C70+F reaction products is interpreted to show the presence of only three regioisomers of C70F. Based on the comparison of the measured hyperfine constants with those estimated by the quantum chemical calculation, an assignment of the spectra to the isomers (A, C, and D) is made, which differs strongly from the previous one [J. R. Morton, K. F. Preston, and F. Negri, Chem. Phys. Lett. 221, 59 (1994)]. The new assignment would allow the conclusion that the low-temperature attachment of F atom to the asymmetric CC bonds of C70 molecule, namely, CACB and CDCE, shows remarkably high selectivity, producing only one of the two isomers in each case, A and D, respectively. Theoretical investigation of the reaction mechanism is made, and it shows that the attachment reaction should have no barrier in the gas phase. The thermodynamic equilibration of the C70F isomers is excluded by the high activation energy (∼30kcal∕mol) for the F atom shifts. The explanation of the high selectivity presents a challenge for theoretical modeling.