Pseudorotation pathway and equilibrium structure from the rotational spectrum of jet-cooled tetrahydrofuran

The rotational spectrum of jet-cooled tetrahydrofuran has been investigated in the 8-18 and 60-78 GHz frequency ranges. Only the four lowest vibration al levels remain populated in the jet conditions. Two pseudorotation vibrat ional spacings have been measured directly to be Delta E-01=21 307.71(3) MH z and Delta E-23=61 205.69(3) MHz. They have been used, together with the l owest 10 measured far infrared transition frequencies and with the effectiv e moments of inertia of the ground state and their shifts upon excitation o bserved in this work, to determine the potential energy function for pseudo rotation and the associated structural relaxations. The potential energy ba rriers are 45 cm(-1) at the envelope conformation of the oxygen and 16 cm(- 1) at the twisted conformation of the C-beta-C-beta' bond relative to the C alphaOCalpha' plane. The four symmetrically equivalent equilibrium structur es are close to the envelope conformations of the CH2 groups next to the ox ygen atom. While the C-O bond lengths and the local CH2 angles appeared to remain unaffected by pseudorotation, significant changes found for the diag onal O ... C distances as well as for the C-C and C-H bond lengths are of i nterest as possible effects of hyperconjugation. (C) 1999 American Institut e of Physics. [S0021-9606(99)01841-3].