¹⁹F nuclear magnetic shielding tensor of CH₃F

Calculations are presented of the 19F nuclear magnetic shielding constant and anisotropy in CH3F as well as the first and second derivatives of the shielding constant with respect to variations in C-F and C-H bond lengths evaluated at equilibrium geometry. Calculations are reported also of the temperature dependence of the 19F shielding. The gauge origin independent CTOCD method and Geertsen's method as modified by Smith, Amos and Handy were used and compared with results obtained using the common origin, LORG/SOLO and GIAO methods. The calculations were carried out at the SCF level as well as at the correlated level within the SOPPA approximation, and a large enough basis set was used to ensure that the same results were obtained in all these methods. It was found that correlation is most important for the first derivative of the fluorine shielding with respect to C-F bond length change. We also find that the anisotropy converges faster as a function of the basis sets in the CTOCD method than in Geertsen's method. It is demonstrated that the apparent disagreement between computed and experimentally inferred first derivatives of shielding constants is a consequence of the assumption made when extracting this quantity from the temperature dependence of the shielding.