Knowledge Bank

The 2 MHz resolution infrared spectrum of the asymmetric C-O stretching band of tetrahydrofuran (THF) at $1087cm-1$ has been observed using a microwave-sideband $CO2$-laser electric-resonance optothermal spectrometer. Transitions arising from ground state levels up to $V=3$ in the pseudorotation quantum number have been measured; definitive band assignments have been made for $V=0\to V=0,V=0\to V=1,V=1\to V=0$ and $V=1\to V=1$ transitions. Two additional bands are observed at higher energy which are associated with higher pseudorotation levels; although the J assignments for these bands are definitive, the excited state V assignments are not. All assignments are confirmed by microwave-infrared double resonance or precise ground-state combination differences. For all four bands, the upper-state term values are found to fit to a Watson Hamiltonian to better than 10 MHz, with a few levels not included in the fits perturbed by $>100$ MHz. Precise microwave $\Delta V$ gaps give equally precise $\Delta V$ gaps for the C-O stretching vibration. In the absence of a third perturbing state, these measurements will furnish information on the vibrational dependence of the pseudorotation portential.