Fluorine Spin—Rotation Interaction and Magnetic Shielding in Fluorobenzene

Abstract

The radio‐frequency spectrum corresponding to the reorientation of the ¹⁹F nuclear moment in fluorobenzene has been studied by the molecular‐beam magnetic resonance method. A molecular‐beam apparatus with an electron‐bombardment detector was used in the experiments. The ¹⁹F resonance is a composite spectrum with contributions from many rotational states and is not resolved. A detailed analysis of the resonance line shape and width led to the following diagonal components of the fluorine spin—rotational tensor in the principal inertial axis system of the molecule: C_aa^F = −1.0±0.5 kc/sec, C_bb^F = −2.7±0.2 kc/sec, and C_cc^F = −1.9±0.1 kc/sec. From these interaction constants, the paramagnetic contribution to the ¹⁹F nuclear shielding in C₆H₅F was determined to be −284±10 ppm. It was further concluded that the ¹⁹F nucleus in this molecule is more shielded when the applied magnetic field is directed along the C–F bond axis. The anisotropy of the magnetic shielding tensor, σ_∥—σ_⊥, is +160±30 ppm.