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30. Theory for the anisotropic hyperfine coupling with fluorine: the ·CF3 radical

Author

Edlund, O., Lund, A., Shiotani, M., Sohma, J., and Thuomas, K.-Å.

Abstract

A method has been developed to estimate the dipolar hyperfine interaction for 19F in fluorinated radicals based on INDO theory. The dipolar integrals have been calculated using Slater-type orbitals. Careful selection of effective nuclear charges, different from those obtained by Slater's rules, and adjustment for the valence electron distribution proved necessary in order to obtain agreement between theory and experiment. The method has been applied to a prototype system, the ·CF3 radical. The results confirm a pyramidal structure with an F-C-F bond angle θ = 109·5° - 112·0° and a C-F bond length of 1·30-1·35 Å. With θ=111·1° the dipolar tensor is calculated to be (122·2, -55·5, -66·7) G, close to the experimental result (120·0, -56·5, -63·5) G.An ambiguity in the determination of the sign of the angle α' between the 120·0 G component and the C3v symmetry axis was found to occur experimentally but not theoretically.According to experiment α' = ± 17·8°, where the negative sign corresponds to an axis perpendicular to the C-F bond. INDO theory gives α' = 26·8°. Ab initio theory confirms the pyramidal structure with θ = 111° and α' = 19·7°. Thus the 120 G component is believed not to be perpendicular to the C-F bond, but to make an angle of about 54° with the bond axis. This result is explained by the contribution to the dipolar couplings from spin density in 19F 2p orbitals. In this instance bond directions cannot be obtained from E.S.R. data without detailed analysis of the electronic structure.

Published
1976
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