Far-infrared and combination-band spectra of the ring-puckering and ring-flapping vibrations of phthalan: A failure of the one-dimensional model

The far-infrared spectra of phthalan showing bands resulting from the ring-puckering and ring-flapping vibrations have been recorded and analyzed. The observation and assignment of the puckering fundamental at 30.9 cm−1 and of numerous hot-band, double quantum jump, and triple quantum jump transitions have provided the basis for generating a detailed and self-consistent energy map for the vibrational states. In addition, sum, difference, and hot bands associated with the C–O symmetric stretching mode also provide confirmation of the assignments. Previous far-infrared, electronic, and dispersed fluorescence spectra assignments are shown to be erroneous whereas a previous microwave study is consistent with the present results. The ring-puckering energy spacings show considerable irregularities and these can be ascribed to the puckering/flapping interaction. Consequently, a one-dimensional potential energy function can only do a mediocre job of fitting the observed vibrational data.