High-Resolution Infrared Spectroscopy of Diacetylene below 1000 cm-1

The infrared spectrum of diacetylene has been recorded at high resolution between 500 and 1000 cm(-1) by Fourier transform spectroscopy. More than 1200 transitions were assigned to five bands: the nu(8) fundamental, the nu(7) + nu(9) combination, and the nu(3) - nu(9) difference bands as well as the most intense nu(8) + nu(9) - nu(9) and nu(7) + 2 nu(9) - nu(9) hot bands. The data were analysed together with the previously recorded millimetre-wave lines for the nu(8) - nu(6) and nu(8) + nu(9) - (nu(6) + nu(9)) difference bands. Rotational and vibrational l-type resonances, together with the cubic anharmonic interactions which couple the nu(3) = 1 stretching state with the nu(8) = nu(9) = 1 combination and nu(7) = 2 overtone states, have been considered in the least-squares fits to the observed wavenumbers in order to derive reliable spectroscopic parameters. The spectral analyses deliver very precise B-0 and D-0 parameters and experimental values of the anharmonicity constants for the bending-bending combination states of diacetylene below 1000 cm(-1)