Knowledge Bank

The far-infrared spectra of phthalan and 1,3-benzodioxole in the 30 to $300cm-1$ region show numerous transitions of the ring-puckering $(\Delta vp=1,2$, and 3) and ring-flapping vibrations. Detailed energy maps for the vibrational states have been obtained and used to determine two-dimensional potential energy surfaces for the electronic ground states. A one-dimensional ring-puckering potential energy function for phthalan, however, can not adequately fit the irregular ring-puckering sequence. The two-dimensional calculations show that the spectral irregularities arise primarily from kinetic energy interactions. Phthalan has a small barrier to planarity of $35cm-1$ while that of 1,3-benzodioxole is considerable larger due to the anomeric effect