Section of the rotational spectrum of 1, 2-propanediol near 333.8 GHz. Transitions assignable to the three lowest energy conformers are marked. The 53_{8 45} − 52_{8 44} transition of the aG′g conformer occurs at 333 812.0 MHz, but it is overlapped by a stronger, unidentified line. All other unmarked lines belong to higher energy conformers or to excited states of the three low-energy conformers. The transitions appear as approximate second derivatives of a Gaussian line shape because of the 2f-modulation.