The microwave spectrum and equilibrium conformation of phthalan

The microwave spectrum of phthalan has been observed in the frequency range of 18–70 GHz. The spectra of the ground and ten excited vibrational states have been assigned and rotational and quartic centrifugal distortion constants have been obtained from semirigid rotor fits to the observed line frequencies. The microwave spectrum is not consistent with either the C2v equilibrium conformation deduced from the far infrared spectrum or the C2 equilibrium conformation obtained from the single vibronic level fluorescence spectra. The microwave data indicates that the bending of the five membered ring is a very anharmonic vibration with a double minimum potential function with a low barrier and that phthalan therefore has a Cs equilibrium conformation. The previous spectroscopic data can be reinterpreted in a manner consistent with this and gives a barrier height of 38 cm-1. The twisting of the five membered ring is a reasonably harmonic vibration and its fundamental wave-number is ∼ 180 cm-1. The microwave data is in agreement with the far infrared assignment of the butterfly vibration in the region 215–220 cm-1.