Rotational spectrum of 1,1-difluoroethane-argon: influence of the interaction with the Ar atom on the V-3 barrier to internal rotation of the methyl group

The free-jet millimeter-wave absorption spectrum of 1,1-difluoroethane-Ar i s reported. Most of the measured lines an split due to internal rotation of the methyl group and the tunnelling motion of Ar connecting two equivalent potential energy minima. The Ar atom, close to the CHF, group, eclipses on e of the methylic hydrogens in the symmetryless geometry of the complex, re ducing in this way the barrier to the internal rotation of the methyl group with respect to isolated 1,1-difluoroethane. For high J levels the distanc e of Ar from the molecule increases, however, due to the centrifugal distor tion, and the barrier increases towards the value for 1,1-difluoroethane. ( C) 2000 Elsevier Science B.V. All rights reserved.