Non-planarity and barrier to ring puckering in phthalan

From the analysis of its rotational spectrum, phthalan appears to be non-planar with a barrier of about 40 cm-1 hindering the ring puckering of the five membered component. This result is in contrast to both a far infrared investigation (Smithson, T. L., Duckett, J. A., and Wieser, H., 1984, J. phys. Chem., 88, 1102) that suggested the molecule to be planar, and a single vibronic level fluorescence spectrum (Hassan, K. H., and Hollas, J. M., 1991, J. molec. Spectrosc., 147, 100) that suggested a barrier hindering a twisting motion.