The millimeter-wave spectra of 2.5 -dihydrofuran in the ground state, the first five excited states of the ring-puckering vibration, and the first excited state of the ring-twisting vibration have been measured in the frequency range (100-250 \mathrm{GHz}). The main features of the observed ring-puckering dependence of the quartic centrifugal distortion constants have been accounted for on the basis of the potential energy function for this vibration and the vibrational dependence of the inverse inerial tensor. Ab initio computations using STO-3G, 4-31G, and 6-31G* basis sets and full geometry optimization have been performed for the planar ring and several puckered conformations of 2.5-dihydrofuran. All basis sets predict the planar ring to be the equilibrium conformation and structural relaxation including rocking, wagging, and twisting of the methylene hydrogen atoms during the ring-puckering motion. The hest agreement with the observed potential energy function for ring puckering is obtained with the STO-3G basis set. However. the comparison between the observed and calculated vibrational dependence of the rotational constants allow the conclusion that only the (6-31 \mathrm{G}^{*}) basis set provides an acceptable description of ring puckering in this molecule.
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