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 V(Z)=0.551×104Z2+0.533×106Z4+0.554×103exp(−100Z2) with the central barrier of 244.6 cm−1, and that of 2,5-dihydrothiophene was V(Z)=0.1396×105Z2+0.6637×106Z4, 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.