The C–H overtone spectra of acetylene: Bend/stretch interactions below 10 000 cm−1

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Abstract
High resolution FT‐IR spectra of C2H2 have been obtained from 1800 to 10 000 cm−1. Over 1600 rovibrational transitions belonging to 30 vibrational bands were assigned and analyzed yielding band origins and rotational constants. Band centers from this and other studies were used to calculate various stretch/stretch and bend/stretch anharmonic coupling constants. Several resonances of the type between ν3 and (ν2+ν4+ν5)0 were analyzed, and an unperturbed value of ν3 was determined. This was combined with other data to obtain ωm, xm, and λ in the local mode basis. x–K relations were imposed to yield the normal mode constants for ν1 and ν3. Treatment of Darling–Dennison resonances in the first two overtone manifolds led to reassignments of previously reported spectra and to the discovery of a Fermi resonance involving overtones of ν3 and states containing ν1, ν2, and 2ν04. Calculated rotational constants and intensity data confirm the assignments and interactions treated in the vibrational analysis. Treatment of these resonances significantly improved the agreement between calculated and observed transition wave numbers, providing a coherent picture of acetylene C–H stretch dynamics below 10 000 cm−1.