The free jet millimeter wave spectrum of 2,4'-dipyridyl has been inves
tigated in the frequency range 60-78 GHz. The two pyridine rings are t
ilted with respect to each other by 22.7 degrees. Each rotational tran
sition is split into two component lines due to the torsional motion c
onnecting 4 equivalent minima. The lower of the two barriers to invers
ion, corresponding to the planar configuration, has been calculated to
be 45 cm(-1). (C) 1997 Elsevier Science B.V.