The C-F small middle dot small middle dot small middle dotH-C "Anti-Hydrogen Bond" in the Gas Phase: Microwave Structure of the Difluoromethane Dimer.

A shortening of the C-H bond lengths and a blue shift of the C-H stretching frequencies for the C-F small middle dot small middle dot small middle dotH-C groups indicates that anti-hydrogen bonds are present the difluoromethane dimer. The most stable conformer has three such interactions (shown schematically).