Analysis of rovibrational resonances observed in the microwave spectrum of FCCCN

Microwave spectrum of fluorocyanoacetylene (FCCCN) produced by a glow discharge in pentafluorobenzonitrile (C6F5CN) was observed using a source modulation spectrometer with a free-space absorption cell. Rotational transitions in the range from J = 9-8 to 53-52 were observed for the vibrationally excited states of nu(4) (C-C stretch), nu(5) (CCN bend), nu(6) (FCC bend), nu(7) (CCC bend), and their associated overtone and combination states up to about 1000 cm(-1). Most of the vibrational states above 500 cm(-1) are perturbed by rovibrational resonances. The effective vibration-rotation constant of the nu(4) state has a negative value (-0.4 MHz), although a vibration-rotation constant generally has a positive value in the excited state of the stretching vibrational mode in a linear molecule. This anomalous behavior is interpreted as due to the rovibrational resonances between the nu(4) and several nearby states. By the simultaneous analysis of the states concerned, the unperturbed vibrational energy and rotational constant of the nu(4) state are obtained to be 686.50(76) cm(-1) and 2068.2387(21) MHz, respectively, where the uncertainties correspond to one standard deviation. (C) 1999 Academic Press.