Knowledge Bank

Ethyl trifluoroacetate is a nearly prolate symmetric top, $\kappa$ = -0.95, which displays three series of a-type R-branch bands under low resolution conditions in a Stark-modulated spectrometer.} Since the molecule exists in only two stable conformers, the origin of the third band series has remained a puzzle for thirty years. We show that the third band series arises from rotational spectra of species undergoing exchange between the two stable forms. This is a purely microwave analog of conformational relaxation which has been observed in NMR spectra under different time, potential energy, and rate regimes.