Microwave Spectrum, Structure, and Barrier to Internal Rotation of Pentafluoroethane

The rotational spectrum of CF3CHF2 has been studied in the 8.4- to 26.5-Gc/sec region, with c-type, R-branch transitions being assigned for both the ground vibrational state and the first excited torsional state. With the assumptions that rC–H=1.10 Å, ∠CCH=110°, rF–F=2.162 Å for the CF3 group, that the CF3 group is symmetrical and its axis and the C–C bond are collinear, and that the CHF2 fluorines are equivalent, an approximate structure of rC–F=1.335 Å, ∠FCF=108.14°, rC–C=1.520 Å, rC–F′=1.345 Å, ∠F′CF′=109.06°, and ÅCCF′=109.58° was found to fit the observed moments of inertia and to be consistent with determinations in other molecules. The primed F's refer to fluorine atoms of the CHF2 group. Relative intensity measurements gave a torsional frequency of 74 cm−1 compared with the infrared observation of 73.4 cm−1. From these data, the barrier to internal rotation is 1223±34 cm−1 or 3.51±0.10 kcal/mole.