Far-infrared spectra, ab initio calculations, and the ring-puckering potential energy function of 2,3-dihydrofuran

The far-infrared spectrum resulting from the ring-puckering vibration of 2, 3-dihydrofuran (23DHF) has been re-examined, and I I single-quantum jump tr ansitions and nine triple-quantum jump transitions were observed. High-leve l ab initio calculations were used to predict the, structure which was then used to calculate the kinetic energy (reciprocal reduced mass) as a functi on of the puckering coordinate. The experimental data along with the kineti c energy expression produced the potential energy function V(x) = 1.019 x 1 0(6)x(4) -1.946 x 10(4)x(2) which has a barrier to planarity of 93 cm(-1) a nd energy minima at dihedral angles of +/- 22 degrees. The values agree wel l with the ab initio values of 96 cm(-1) and +/- 23 degrees. Quantum transi tions to the 11th and 12th puckering states are fit less well with the pote ntial function, and this apparently reflects the fact that higher-order (x( 6)) potential energy terms become more important due to steric effects at l arger values of x. The interaction between the ring-puckering and ling-twis ting modes was analyzed by examining the hot bands near 446 cm(-1). A two-d imensional potential energy surface, which fits the puckering data well in both the twisting ground and excited states, was determined. The interactio n cross term was somewhat smaller than those for cyclopentene and related m olecules.