High resolution infrared spectroscopy of [1.1.1]propellane: The region of the ν9 band

The region of the infrared-active band of the ν9 CH2 bending mode [1.1.1]propellane has been recorded at a resolution (0.0025 cm−1) sufficient to distinguish individual rovibrational lines. This region includes the partially overlapping bands ν9 (e′) = 1459 cm−1, 2ν18 (l = 2, E′) = 1430 cm−1, ν6 + ν12 (E′) = 1489 cm−1, and ν4 + ν15 (A2″) = 1518 cm−1. In addition, the difference band ν4 − ν15 (A2″) was observed in the far infrared near 295 cm−1 and analyzed to give good constants for the upper ν4 levels. The close proximities of the four bands in the ν9 region suggest that Coriolis and Fermi resonance couplings could be significant and theoretical band parameters obtained from Gaussian ab initio calculations were helpful in guiding the band analyses. The analyses of all four bands were accomplished, based on our earlier report of ground state constants determined from combination differences involving more than 4000 pairs of transitions from five fundamental and four combination bands. This paper presents the analyses and the determination of the upper state constants of all four bands in the region of the ν9 band. Complications were most evident in the 2ν18 (l = 2, E′) band, which showed significant perturbations due to mixing with the nearby 2ν18 (l = 0, A1′) and ν4 + ν12 (E′) levels which are either infrared inactive as transitions from the ground state, or, in the latter case, too weak to observe. These complications are discussed and a comparison of all molecular constants with those available from the ab initio calculations at the anharmonic level is presented.