Structures and conformational analysis of the fluorinated ethyl radicals of CF 3CH 2., CH 3CF 2., and CF 3CF 2.: an ab initio SCF MO study

Abstract

Fully optimized ab initio geometries are reported for the CF 3CH 2 ., CH 3,CF 2 ., and CF 3CF 2 . radicals using the 3–21G basis set. Only one stable conformation, corresponding to the staggered structure 1, is predicted for each radical. Other stationary points, corresponding to the eclipsed structures 2 and 5, were located but frequency calculations revealed these to be first order saddle points (transition structures) for the interconversion between the staggered conformations. Transition structure 2 corresponds to an inversion-rotation pathway, whereas transition structure 5 (which does not exist for the CF 3CH 2 . radical) corresponds to a pure rotation mechanism. It is predicted that interconversion between the staggered conformations of CH 3 CF 2 . and CF 3CF 2 . radicals takes place via the pure rotation mechanism, the calculated barrier to which (ca. 2 kcal mol −1) is in excellent agreement with the experimental value. Hyperfine coupling constants are also reported for these radicals and they are in reasonable accord with the experimental values. The overall performance of the 3–21G basis set is evaluated with regard to closed shell fluorinated compounds of known structures. The results of 3–21G basis set calculations on CF 3 . are compared with those from higher level calculations.