Near-Infrared Band Progressions of Ring Molecules and Ring-Puckering Motion

A series of bands found in the 2900–2600‐cm⁻¹ region of infrared spectra of 2,5‐dihydrofuran, trimethylene oxide, cyclopentene, 2,5‐dihydrothiophene, 2,3‐dihydrofuran and 2,3‐dihydrothiophene are assigned to the ring‐puckering difference spectra, of which the reference band is the CH₂ symmetrical stretching vibration. The assignments were confirmed for 2,5‐dihydrofuran and trimethylene oxide, for which the pure ring‐puckering bands had already been analyzed in detail. From the difference spectra the puckering potential of cyclopentene was determined as ${\text{V(Z)=0.551×10}}^4{Z}^2{\text{+0.533×10}}^6{Z}^4{\text{+0.554×10}}^3\exp {\text{(−100Z}}^2)$ with the central barrier of 244.6 cm⁻¹, and that of 2,5‐dihydrothiophene was ${\text{V(Z)=0.1396×10}}^5{Z}^2{\text{+0.6637×10}}^6{Z}^4$ , where V is in cm⁻¹ and Z is in angstroms. The spectrum showed that 2,3‐dihydrofuran has the nonplanar structure with the potential barrier height of about 86.8 cm⁻¹. For 2,3‐dihydrothiophene tentative assignments were given and the puckering potential was calculated.