An unusual `ring-puckeringʹ: tunneling of Ar in difluoromethane–Ar

The rotational spectrum of the difluoromethane–argon van der Waals complex has been investigated by free-jet absorption millimeter-wave spectroscopy. More than 60 μc-type transitions have been measured. All of them are split into two component lines due to the `ring-puckeringʹ of the H2CFFAr `four-membered ringʹ and the corresponding vibrational splitting is 193.51(7) MHz. The barrier to `ring-puckeringʹ has been calculated to be 100(4) cm−1. From centrifugal distortion effects the dissociation energy of the complex has been estimated to be ∼1.5 kJ mol−1.