Knowledge Bank

The microwave spectrum of tetrahydrofuran $(C4H8O)$ was first observed by Engerholm, {et al.}$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ $\Delta V=0$ transitions from levels up to $V=8$ in the psuedrorotation quantum number were reported. The $V=0$ and 1 and $V=2$ and 3 levels are strongly Coriolis perturbed and were estimated to be separated by 19.5(1.5)GHz and 45(15) GHz respectively. We have extended the microwave measurements for the $V=0,1,2$ and 3 pseudorotation states in the 10 MHz to 40 GHz region using an electric resonance optothermal spectrometer with a microwave synthesizer. The extremely weak c-type transitions between the $V=0\to 1$ and $V = 2 \rightarrow 3 $ levels have been measured for the first time. Their assignments are verified by double resonance with 10 $\mu m$ infrared transitions arising from common around states. The $V=0$ and 1, and $V=2$ and 3 data sets are fit separately to a Watson Hamiltonian supplemented by a {b}-type Coriolis term. The standard deviations of the fits are = 1MHz and 3 MHz respectively, and the $V=0\to 1$ and $V = 2 \rightarrow 3 $ gaps are now precisely determined as 21301(1) MHz and 4668(2) MHz. The splittings together with the precise Coriolis constants will allow a refinement of the pseudorotation potential.