The structure of diethylaminosulfur trifluoride (DAST) - A combined theoretical and nuclear magnetic resonance study

The structure of diethylaminosulfur trifluoride (DAST) was computed and fully optimized in C-1 symmetry at the RHF/6-31G** level of theory. In agreement with VSEPR considerations, the isolated molecule adapts the structure of a pseudo trigonal bipyramid (psi-tbp) with one fluorine atom, the nitrogen atom and the lone pair in the equatorial position; the other two fluorine atoms are in the axial positions. In agreement with earlier work by other authors a psi-tbp structure was also postulated for the DAST molecule dissolved in non-polar solvents from temperature-dependent solution F-19 NMR spectroscopy. The free activation enthalpy for the interconversion was estimated from experimental data to be Delta G(#) = 8.5 +/- 0.5 kcal mol(-1). In the N-14 NMR spectrum at low temperatures only a significant Line broadening but no splitting of the signals was observed. The molecule (H2N)SF3 was calculated as a model compound for DAST at the same level of theory, and in addition at electron correlated MP2 level. The global minimum (ATM, absolute true minimum) for (H2N)SF3 is also represented by a psi-tbp structure with the nitrogen atom and the lone pair in the equatorial position. In addition, a second true minimum (TM) was found with two fluorine atoms and the lone pair in equatorial positions and the nitrogen and one fluorine atom occupying the axial positions. Three transition states (TS) were found for the interconversion of the model compound (H2N)SF3.