Knowledge Bank

The far-infrared spectra of cyclopentanone has been reexamined. In addition to the ring bending series previously observed in the $80-100cm-1$ region, ring-twisting bands ($200-240cm-1$), bend-twist sum bands ($305-335cm-1)$, and bend-twist difference bands were also detected for the first time. From this data a two dimensional potential energy surface with a barrier to planarity of $1408cm-1$ and a barrier to pseudorotation of 1$358cm-1$ was determined. This same surface yields calculated frequencies in good agreement for not only the undeuterated cyclopentanone, but also for four isotopomers. In addition to determining the potential energy surface for the $S0$ electronic ground state, we have also utilized our previously reported fluorescence excitation spectra to determine the corresponding potential energy surface of the $S1(n,\Pi \ast )$ electronic excited state. The barrier to planarity in the $S1$ stated is $1273cm-1$ while the barrier to pseudorotation has been reduced substantially to $850cm-1$.