The perpendicular CD stretching band ν8 of allene-d4

Abstract The perpendicular infrared band of the degenerate CD stretching vibration ν 8 (species E) in allene- d 4 with band origin at 2321.2473 cm −1 was measured on a high resolution Fourier transform spectrometer and deconvolved to a linewidth of ∼0.0028 cm −1 . A total of 23 subbands with K Δ K ranging from −10 to +12 have been assigned. An improved set of ground state constants B 0 , D J 0 , D JK 0 , and H JK 0 was obtained using ∼1300 combination differences from this band and from the parallel bands ν 5 and ν 6 measured on the same instrument. The ν 8 band exhibits many perturbations caused by interactions with various combinations of lower lying vibrations. The nature of the perturbing states is discussed and three of the strongest perturbations are quantitatively accounted for by constructing a Hamiltonian matrix which includes three different perturbing states and their Coriolis interactions with the upper state of ν 8 . The ν 2 + ν 7 parallel band, strongly enhanced by Fermi resonance with ν 5 , is observed just below the ν 8 band. Its upper state was also included in the H matrix, but its J xy -Coriolis interaction with ν 8 could not satisfactorily account for the perturbations seen in the K Δ K = −8 to −10 subbands of ν 8 . Using data for the subbands from K Δ K = −7 to +10, a set of spectroscopic constants was obtained for the ν 8 state (and for the three perturbing states) with the aid of our computer program system SYMTOP designed for the treatment of symmetric top bands whose upper states are involved in arbitrary interactions. The standard deviation of the fit, 0.0031 cm −1 , far exceeds the precision of the measurements (∼0.0004 cm −1 ) and is evidence for the presence of additional high-order perturbations.