RING PUCKERING MOTION IN INDAN - A MICROWAVE SPECTROSCOPY STUDY

O+ O- doubling due to the double minimum potential of the ring puckering has been observed in the rotational spectrum of indan. The barrier to inversion has been determined. The value of 433-5(1)cm-~ has been obtained, extremely low with respect to the value of 1979cm -~ from the far infrared (Smithson, T. L., Duckett, J. A., and Wieser, H., 1984, J. phys. Chem., 88, 1102) and single vibronic level fluorescence spectra (Hassan, K. H., and Hollas, J. M., 1991, J. molec. Spectrosc., 147, 100).