Electron spin resonance of the C6, C8, and C1 molecules

Abstract

Electron-spin-resonance (ESR) signals attributed to the linear C6, C8, and C10 molecules in their lowest 3Σ states, presumably their ground states, have been observed in solid neon and argon matrices at 4 K. There is evidence of two forms of the C10 molecule, perhaps indicating two slightly bent structural isomers. Laser vaporization of graphite and 13C-enriched graphite produced a high proportion of these larger molecules. Hyperfine interaction in the 13Cn molecules was small and resolved only for C6, indicating cumulene-type bonding with the unpaired spins in pπ orbitals, as in C4. The zero-field-splitting parameters ‖D‖ were found to be 0.363, 0.783, and 0.190 cm−1, respectively, in solid neon. The increase in ‖D‖ through C8 is attributed to a corresponding variation in the spin–orbit coupling with low-lying states, principally the 1Σ+g, as the chains lengthen. Gross orbital spin populations and 1Σ+g– X 3Σ−g energy differences were obtained from Hartree–Fock calculations in order to interpret the hfs and ‖D‖ data, respectively. Electron correlation was included via second and third order Mo/ller–Plesset perturbation theory. The possibility of quasilinear or nonlinear character in these chains is briefly considered. Relative concentrations of the linear and cyclic forms of these molecules in the vapor and in matrices were estimated from thermodynamics using their theoretically derived properties.