Infrared spectroscopy and ab initio potential energy surface for Ne–C2H2 and Ne–C2HD complexes

The rotationally resolved spectra of Ne–C2H2 and Ne–C2HD were measured in the region of the asymmetric C–H stretch (ν3) band of the acetylene monomer. The transitions in the Ne–C2H2 spectrum are substantially broadened by vibrational predissociation, while those of Ne–C2HD are quite narrow. This difference is attributed to the fact that in the former dissociation proceeds through a “doorway” state, related to a Fermi resonance involving the bending vibrations of C2H2. In C2HD this Fermi resonance is absent. The potential energy surface (PES) for the Ne–acetylene complex has been computed using symmetry-adapted perturbation theory. This PES has been fit to an analytic form and applied in calculations of the rovibrational energy levels of Ne–C2H2 and Ne–C2HD. From these levels and calculated transition intensities we generated the near-infrared spectra of these complexes in the region of the ν3 band. These complexes may be considered as nearly free internal rotors. For Ne–C2H2 the results obtained from the ground state PES gave semiquantitative agreement with the measured spectrum. For Ne–C2HD we could assign all of the (much sharper) lines in the experimental spectrum and obtain the ν3 excited state interaction potential from a fit of the calculated spectrum to the experimental one. The ground state ab initio potential was not altered in this fit; the excellent agreement between the calculated and measured infrared spectrum for Ne–C2HD demonstrates that our Ne–acetylene potential is quite accurate.