Correlation between the¹⁹F absolute nuclear magnetic shielding and its temperature dependence in the fluoromethanes

Absolute nuclear shieldings for 19F are compared with the temperature dependence in the zero-pressure limit obtained from N.M.R. studies of fluoromethanes in the gas phase. From the correlation between these quantities and between absolute shielding and N.M.R. isotope shifts, it is concluded that σe, the 19F shielding at the equilibrium configuration and (δσ/δΔrCF)e, the change in nuclear shielding with CF bond extension, are correlated, (δσ/δΔrCF)e being more negative for smaller σe. A similar correlation is deduced for (δσ/δΔrCO)e and 13C shielding.