F-19 NUCLEAR MAGNETIC SHIELDING TENSOR OF CH3P

Calculations are presented of the F-19 nuclear magnetic shielding cons tant and anisotropy in CH3F as well as the first and second derivative s of the shielding constant with respect to variations in C-F and C-H bond lengths evaluated at equilibrium geometry. Calculations are repor ted also of the temperature dependence of the F-19 shielding. The gaug e origin independent CTOCD method and Geertsen's method as modified by Smith, Amos and Handy were used and compared with results obtained us ing the common origin, LORG/SOLO and CIAO methods. The calculations we re carried out at the SCF level as well as at the correlated level wit hin the SOPPA approximation, and a large enough basis set was used to ensure that the same results were obtained in all these methods. It wa s found that correlation is most important for the first derivative of the fluorine shielding with respect to C-F bond length change. We als o find that the anisotropy converges faster as a function of the basis sets in the CTOCD method than in Geertsen's method. It is demonstrate d that the apparent disagreement between computed and experimentally i nferred first derivatives of shielding constants is a consequence of t he assumption made when extracting this quantity from the temperature dependence of the shielding.