Overtone spectra and anharmonic resonances in haloacetylenes

In this work preliminary results are reported on an extensive vibrational analysis of the molecules HCCX and DCCX with X = F and Cl, in which a number of anharmonic resonances are analysed. The importance of quartic anharmonic resonances in these molecular types is reported involving the effective constants K₁₂₄₄ and K₁₂₅₅, and these are related to the corresponding resonances in acetylene and its isotopomers. The correct analysis of Fermi resonances and quartic anharmonic resonances is important not only in reproducing the high overtone energy levels, but also in fitting the observed rotational constants, and in determining the _r constants and hence the equilibrium rotational constants. In this paper we revise our recent analysis of the equilibrium structure of HCCF in the light of these effects.