Dependence of 19F chemical shielding on internal coordinates in CF4, SiF4, and BF3

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Abstract
The thermal average of chemical shielding for CF4‐, BF3‐, SF6‐, HCN‐, CO2‐, and NH3‐type molecules is explicitly expressed in terms of molecular constants such as fundamental frequencies and elements of the L matrix for the totally symmetric vibration, Coriolis constants, moments of inertia, and cubic force constants kss′s′. Expressions for the klss cubic force constants for a BF3‐type molecule are derived and values of klss are calculated for CF4, SiF4, and BF3. The thermal average of chemical shielding calculated using these cubic force constants is compared with the experimental temperature dependence of the 19F NMR signal, yielding values of (∂σ/∂R) for CF4, SiF4, and BF3. Despite the large differences in chemical shifts the values of (∂σ/∂R) obtained are nearly identical: 1115, 1170, and 1115 ppm/Å, respectively.