High-resolution rovibrational study of the Coriolis-coupled ν12 and ν15 modes of [1.1.1]propellane

Infrared spectra of the small strained cage molecule [1.1.1]propellane have been obtained at high resolution (0.0015 cm−1) and the J, K, and l rovibrational structure has been resolved for the first time. We recently used these spectra to obtain combination-differences to deduce ground state parameters for propellane; over 4100 differences from five fundamental and four combination bands were used in the fitting process. The combination-difference approach eliminated potential errors caused by localized perturbations in the upper states and gave well-determined ground state parameters. In the current work, these ground state constants were fixed when fitting the upper state parameters for the ν12 (e′) perpendicular and ν15(a2″) parallel bands. Over 4000 infrared transitions were fitted for each band, with J, K values ranging up to 71, 51 and 92, 90, respectively. While the transition wavenumbers for both bands can be fit nicely using separate analyses for each band, the strong intensity perturbations observed in the weaker ν12 band indicated that Coriolis coupling between the two modes was significant and should be included. Due to correlations with other parameters, the Coriolis coupling parameter ζ15,12ay for the ν15 and ν12 interaction is poorly determined by a transition wavenumber fit alone but, fortunately, the intensity perturbations gave useful added constraints on ζ15,12ay. By combining the wavenumber fit with a fit of experimental intensities, a value of −0.42 was obtained, quite close to the value of −0.44 predicted by Gaussian ab initio density functional calculations using a cc-pVTZ basis. This intensity fit also yielded a (∂μz/∂Q15)/(∂μx/∂Q12a) dipole derivative ratio of 36.5, in reasonable agreement with a value of 29.2 predicted by the ab initio calculations. This ratio is unusually high due to large charge movement as the novel central Caxial-Caxial bond is displaced along the symmetry axis of the molecule for the ν15 mode.