19F High-Resolution Nuclear Magnetic Resonance Study of Structure and Properties of Crystalline sym-C6Cl3F3

From the high-resolution 19F NMR spectrum of single crystal sym-C6Cl3F3 its chemical shift shielding tensor is determined. The most shielded direction lies perpendicular to the molecular plane and the least shielded one in the molecular plane perpendicular to the C-F bond. The most (σ33) and least (σ11) shielded components change significantly in going from sym-C6H3F3 and sym-C6Cl3F3 to C6F6 whereas the component in the C-F bond direction (σ22) changes little. It is concluded that the π-electron density distribution varies little but the σ-electron density is changed when other substituents are introduced. At 20 °C the sym-C6Cl3F3 molecule rotates about its figure axis and its motion averages out the shielding tensor components within the molecular plane. The mode of the molecular motion does not change at the phase transition point (296 K) and up to the melting point in the time scale of l9F high-resolution spectra.