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

Free radicals C(60)F and C(70)F were generated in solid argon by means of chemical reaction of photogenerated fluorine atoms with isolated fullerene molecules (C(60) or C(70)). High resolution anisotropic electron paramagnetic resonance (EPR) spectra of C(60)F and C(70)F at low temperature have been obtained for the first time. The spectrum of C(60)F is characterized by an axially symmetric hyperfine interaction on (19)F nucleus. The hyperfine coupling constants A(iso)=202.8 MHz (Fermi contact interaction) and A(dip)=51.8 MHz (electron-nuclear magnetic-dipole interaction) have been measured for C(60)F 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 C(60)F is theoretically characterized using the Hirshfeld atomic partitioning scheme. Unlike C(60), five isomers of C(70)F can in principle be produced by the attachment of a fluorine atom to one of the five distinct carbon atoms of the C(70) molecule (denoted A, B, C, D, and E, from pole to equator). The measured high resolution EPR spectrum of the C(70)+F reaction products is interpreted to show the presence of only three regioisomers of C(70)F. 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 C(70) molecule, namely, C(A)[Double Bond]C(B) and C(D)=C(E), 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 C(70)F isomers is excluded by the high activation energy ( approximately 30 kcal/mol) for the F atom shifts. The explanation of the high selectivity presents a challenge for theoretical modeling.