Density functional theory study of C_nF₃^- (n = 1-9) clusters

In this paper, we report the design of numerous C_nF₃^- (n = 1-9) models. By means of B3LYP density functional method, we carried out geometry optimization and calculation on the vibrational frequency. After comparison of structure stability, we found that the structures of ground-state (G-S) isomers of odd-n C_nF₃^- (i.e., n = 3, 5, 7 and 9) are with the three fluorine atoms located at one end of the linear C_n chain. The G-S isomers of C₂F₃^-, C₄F₃^-, and C₈F₃^- are with two fluorine atoms bonded to an end carbon of the C_n chain, and one fluorine atom bonded to the adjacent carbon atom. In other words, the two carbon atoms involved in bonding to the fluorine atoms are sp² hybridized and the C_n chain is not linear. In the case of C₆F₃^-, the G-S isomer is planar cyclic in structure, with each of the three carbon atoms at one side of the hexagonal C₆ ring bonded to a fluorine atom. The C_n chain of G-S C_nF₃^- (n = 3-9; C₆F₃^- being the exception) isomers are polyacetylene-like. It is found that the odd-n G-S C_nF₃^- (n = 1-9) are more stable than the adjacent even-n ones. The finding is in accord with the relative intensities of C_nF₃^-observed in mass spectrometric studies. We provide explanations for such trend of even/odd alternation based on concepts of the geometrical structure, bonding character, atomic charges, vertical electron detachment energy, and incremental binding energy.