Far-Infrared Spectra, ab Initio Calculations, and the Ring-Puckering Potential Energy Function of 2,3-Dihydrofuran
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The far-infrared spectrum resulting from the ring-puckering vibration of 2,3-dihydrofuran (23DHF) has been re-examined, and 11 single-quantum jump transitions and nine triple-quantum jump transitions were observed. High-level ab initio calculations were used to predict the structure which was then used to calculate the kinetic energy (reciprocal reduced mass) as a function of the puckering coordinate. The experimental data along with the kinetic energy expression produced the potential energy function V(x) = 1.019 × 106x4 - 1.946 × 104x2 which has a barrier to planarity of 93 cm-1 and energy minima at dihedral angles of ± 22°. The values agree well with the ab initio values of 96 cm-1 and ±23°. Quantum transitions to the 11th and 12th puckering states are fit less well with the potential function, and this apparently reflects the fact that higher-order (x6) potential energy terms become more important due to steric effects at larger values of x. The interaction between the ring-puckering and ring-twisting modes was analyzed by examining the hot bands near 446 cm-1. A two-dimensional potential energy surface, which fits the puckering data well in both the twisting ground and excited states, was determined. The interaction cross term was somewhat smaller than those for cyclopentene and related molecules.
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