Spectra and structure of small ring compounds. XXVIII. 2,3-Dihydrofuran and 2,3-dihydrothiophene

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Abstract
The low frequency Raman spectra for 2,3‐dihydrofuran and 2,3‐dihydrothiophene have been recorded in the region of the ring puckering vibrations. Only Δν = 2 transitions were observed beyond the region of the rotational envelope for both molecules. The observed transitions in the Raman effect for 2,3‐dihydrofuran are in good agreement with the frequencies inferred from the previously reported infrared data. The data indicate that the 0 → 1 transition lies at 19.5 ± 0.5 cm−1 and the molecule is puckered with a barrier of 83 cm−1 to planarity. The previously unreported ring puckering transitions of 2,3‐dihydrothiophene have been reasonably well reproduced by the potential V(X) = 6.40 × 105X4 − 2.89 × 104X2 giving a barrier to planarity of 325 cm−1. These direct observations are in disagreement with the midinfrared difference band series previously reported.