Rotationally Resolved Infrared Spectroscopy of Supersonic Jet-Cooled Isoprene.

The high-resolution infrared spectrum of isoprene has been observed under supersonic jet-cooled conditions in the region of the ν vibrational band near 992 cm. The spectrum was assigned and fit using a standard asymmetric top Hamiltonian, and an acceptable fit was obtained for transitions to excited state energy levels with ≤ 6, with an error in the fit of 0.002 cm. For excited state energy levels with > 6, a perturbation was present that prevented fitting using the standard asymmetric top Hamiltonian. Based on previous anharmonic frequency calculations and observed vibrational bands of isoprene, the perturbation is most likely caused by Coriolis coupling between the ν and ν vibrations or a combination band that lies near the ν band. The excited state rotational constants from the fit show reasonable agreement with previous anharmonic calculations performed at the MP2/cc-pVTZ level of theory. The jet-cooled spectrum is compared with previous high-resolution measurements of this band at room temperature and shows that understanding the perturbation will be necessary to accurately model this vibrational band.