Knowledge Bank

We have studied the relaxation of conformers and the formation/relaxation of isomeric, weakly bonded dimers in pulsed supersonic expansions of seeded rare gases (He, Ne, Ar, Kr). The relaxation was determined from the intensity of a rotational transition for the higher energy species as a function of carrier gas composition, using the Balle/Flygare Fourier transform microwave spectrometer. Of thirteen molecules with rotational conformers which we examined, those with barriers to internal rotation greater than $400cm-1$ did not relax significantly in any of the carriers. The higher energy forms of ethyl formate, ethanol, and isopropanol, with smaller barriers, were not relaxed by He, somewhat relaxed by Ne, and completely relaxed by as little as 5 to 20 mole percent of Ar or Kr in He or Ne. Similar results have been obtained for the formation/relaxation of the weakly bonded dimer pairs: linear OCO-HCN, T-shaped $HCN-CO2$; linear FH-NNO and bent HF-DF and DF-HF. The case of the $HCN/CO2$ dimers is particularly striking. The T-shaped dimer was found first, using Ar as the carrier gas. Five years later the linear form was found with first run neon as carrier, but it could not be detected at all with Ar as the carrier.