R. J. Bemish^a, L. Oudejans^a, R. E. Miller^a, Robert Moszynski^b, Tino G. A. Heijmen^c, Tatiana Korona^b, Paul E. S. Wormer^c, and Ad van der Avoird^c

^a Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599, USA
^b Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
^c Institute of Theoretical Chemistry, NSR Center, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands

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

Abstract

The rotationally resolved spectra of Ne-C₂H₂ and Ne-C₂HD were measured in the region of the asymmetric C-H stretch (3) band of the acetylene monomer. The transitions in the Ne-C₂H₂ spectrum are substantially broadened by vibrational predissociation, while those of Ne-₂HD 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 3 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 3 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.

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