Infrared spectroscopic investigation of photoionization-induced acidic C-H bonds in cyclic ethers

Infrared (IR) predissociation spectroscopy based on vacuum-ultraviolet photoionization detection is performed for the neutral and cationic tetrahydrofuran (THF) and tetrahydropyran (THP). The CH bonds in neutral THF and THP are regarded as aprotic, even though the CH bonds are weakened by the negative hyperconjugation. After 118 nm photoionization, however, the negative hyperconjugation changes to the positive hyperconjugation and their CH bond acidities remarkably increase. In the IR spectrum of the THF cation, an intense band is observed at ca. 2700 cm(-1). This band is assigned to the antisymmetric stretch vibration of the two CαH bonds next to the oxygen atom. The high intensity and low frequency of this band are due to the delocalization of the σ electrons of the two CαH bonds to the singly occupied molecular orbital (SOMO) through the hyperconjugation. In the IR spectrum of the THP cation, on the other hand, the stretch bands of the CαH bonds do not show obvious low-frequency shift and intensity enhancement, while the stretch band of the equatorial CγH bond, at the para-position to the oxygen atom, appears at 2855 cm(-1) with high intensity. This acidity enhancement of the equatorial CγH bond is attributed to the mutiple hyperconjugation among the CγH bond, two CαCβ bonds, and SOMO of the oxygen atom. These results suggest that the difference of the hyperconjugation mechanism between the THF and THP cations arises from their preferable conformations.