Millimetre-wave spectroscopy of HC4Cl in ground and excited vibrational states

The semi-stable HC4Cl molecule has been detected in the pyrolysis products of propyne and carbon tetrachloride mixtures. The rotational spectrum of the most abundant isotopologue HC435Cl has been investigated in the millimetre- and sub-millimetre-wave regions for the ground and 12 vibrationally excited states which approximately lie below 630 cm−1, namely (ν5ν6ν7ν8ν9)=(10000), (01000), (00100), (00010), (00001), (00020), (00002), (00003), (00004), (00101), (00011) and (00012). Transitions up to J=151←150 were measured for the ground state, allowing for a precise evaluation of the quartic and sextic centrifugal distortion constants D and H. The l-type resonances between the different sublevels of the bending states and the anharmonic resonance which couples the states ν5=1, ν8=2 and ν9=4 have been taken into account in the analysis of the spectra, which yielded precise determinations of the xL(99), xL(88), xL(89) and xL(79) anharmonicity constants and of the ϕ588 normal coordinate cubic force constant. Extensive measurements have also been performed for the HC437Cl isotopologue.