Knowledge Bank

The microwave spectrum of tetrahydrofuran has been assigned in the ground vibrational state and in eight excited states of the pseudorotation mode. The rotational constants exhibit an unusual dependence upon the pseudorotational quantum number. The separation of the first pairs of levels has been determined from the effects of a coupling term linear in the overall rotational momentum and in the pseudorotational momentum. These data are interpreted in terms of a quantum mechanical Hamiltonian which contains the inertial constant as a function of the pseudorotational angle and a small barrier to pseudoratation. This barrier is about 20 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ for the two-fold term and 2 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ for the ten-fold term.