Knowledge Bank

The far-infrared spectrum of $\Delta 3,4$ dihydropyran has been obtained with a resolution of 0.2-0.5 $cm-1$ with the Jarrell-Ash 78-900 far infrared spectrometer at $MIT1$ The spectrum consists of 3 main groups of Q” branch transitions. Those between 275-300 $cm^{-1}$ are assigned to the ring twisting vibration and from 160-180 $cm^{-1}$ to the bending vibration. Much weaker Q” branch transitions [FIGURE] between 100 and 130 $cm-1$ ate assigned as difference tones between the two. The data are analyzed as arising from two highly coupled anharmonic vibrations. The resulting potential surface yields an estimate of the barrier to interconversion of the stable twisted forms and an estimate of the barrier to planarity of the ring. The far $infrared2$ and $Raman3$ data for 1,4 dioxene and $\Delta 2,3$ dihydropyran have been reanalyzed.