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 ν26 vibrational band near 992 cm–1. 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 J ≤ 6, with an error in
the fit of 0.002 cm–1. For excited state energy
levels with J > 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 ν26 and ν17 vibrations or a combination band that lies near the ν26 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.