Vibration–rotation spectrum of the acetylene–carbon monoxide van der Waals molecule in the 3 μ region

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Abstract
The infrared absorption spectrum of the acetylene–carbon monoxide van der Waals molecule has been obtained in the 3 μ region. A color center laser was used to excite the vibration of the complex which corresponds to the antisymmetric hydrogen stretching mode in the monomer in a pulsed molecular jet. The form of the spectrum indicates that the molecule is a linear, hydrogen bonded complex, although excited state perturbations are present. The vibrational origin is at 3279.898(2) cm−1 and the ground state B value is 1397.4(6) MHz, corresponding to a separation of 5.011 Å for the monomer centers of mass. The spectrum cannot directly distinguish between the HCCH–CO and the HCCH–OC configurations, but the combined evidence of other hydrogen bonded complexes of carbon monoxide, the expectation that the observed hydrogen bond should be slightly shorter than the sum of van der Waals radii, and a distributed multipole analysis strongly favors the HCCH–CO configuration.