Microwave spectroscopy of FCCCN in the ground and excited vibrational states

Abstract

Microwave spectrum of fluorocyanoacetylene (FCCCN) produced by a glow discharge in pentafluorobenzonitrile (C 6F 5CN) was observed using a source-modulated spectrometer with a free-space absorption cell. Rotational transitions of FCCCN in the range of J = 9-8 to 20-19 were measured with an accuracy of 30 kHz for the ground state as well as for the vibrationally excited states, ν 5 (<CCN bend), ν 6 (<FCC bend), ν 7 (skeletal bend), 2 ν 6, ν 5 + ν 7, ν 6 + ν 7, 2 ν 7, 3 ν 7, and 4 ν 7. The rotational and centrifugal distortion constants obtained for the ground state are 2070.9571 ± 0.0006 MHz and 90.32 ± 0.95 Hz, respectively, where the uncertainties correspond to one standard deviation. The observed l-type doubling constants for the ν 5 (0.7718 ± 0.0002 MHz), ν 6 (0.9017 ± 0.0002 MHz), and ν 7 (2.0264 ± 0.0003 MHz) states agree with the values calculated from the harmonic force field. Anomalous behavior of the centrifugal distortion constants observed for some vibrational states was interpreted as due to the l-type resonance between sublevels with different l values. The unperturbed centrifugal distortion and l-type doubling constants and the intervals between sublevels were determined. Rotational transitions in the Σ + sublevel of the 2 ν 6 as well as the ν 5 + ν 7 state are perturbed by the Fermi resonance with the ν 4 (CC stretch) vibrational state.