Microwave rotation-tunneling

In dimers composed of equal molecules the dimer components can replace each other through tunneling. This effect has been discovered by Dyke et al. [1972] as interconversion splitting of rotational levels of (HF)2 in molecular beam electric resonance spectra. This dimer has been studied in many papers by microwave and far infrared tunable difference-frequency laser spectroscopy (see review papers by Truhlar [1990] and by Quack and Suhm [1991]). The dimer consists of two inequivalent HE molecules, the H atom of one of them participating in the hydrogen bond between the fluorine atoms (fig. 60). PES is a function of six variables indicated in this figure. 

The microwave spectram exhibits a tunnelling sphtting due to a large amphtude internal rotation of the H2O subunit that exchanges bonded and non-bonded atoms. The internal rotor states are labelled with the asymmetric rotor quantum number for the rotational levels of free water jkak. ... 

Baughcum SL, Smith Z, Wilson EB, Duerst RW (1984) Microwave spectroscopic study of malonaldehyde. 3. Vibration-rotation interaction and one-dimensional model for proton tunneling. J Am Chem Soc 106 2260... 

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