The high-resolution visible overtone spectrum of acetylenea)

Abstract

Direct, gas‐phase overtone spectra of states corresponding to five and six quanta of C–H stretch have been obtained for four isotopically substituted acetylenes in the region 14 900–18 500 cm⁻¹. Peak positions of individual rotation–vibration lines were determined to a precision of about 0.003 cm⁻¹. A total of 39 bands are analyzed: 16 for ¹²C₂H₂; 14 for ¹³C₂H₂; five for ¹²C¹³CH₂; and four for ¹²C₂HD. The rotational structure of the bands is fit to a semirigid rotor Hamiltonian. However, the bands suffer from severe rotational perturbations, with typical matrix elements being on the order of 0.3 cm⁻¹ or less. The error in the determined band origins varies between 0.001 and 0.1 cm⁻¹, depending on the degree of perturbation. Precision rotational constants are also determined for the observed bands. The vibrational term values cannot be adequately explained in terms of the usual slightly anharmonic normal mode expansion. Even though acetylene represents an intermediate case between the local mode and normal mode limits, the simple local mode theory described by Child and Lawton [M. S. Child and R. T. Lawton, Faraday Discuss. 71, 273 (1981)] can be used to rationalize many aspects of the spectrum—e.g., the splitting of the states ‖005^g01¹〉 and ‖005^u01¹〉 is found to be only 0.44 cm⁻¹. The intermediate character of acetylene is seen upon single deuterium substitution. The state corresponding to five quanta of ¹²C–H stretch shifts about 100 cm⁻¹ to the blue, a result which is certainly anomalous from the point of view of intuitive local mode theory. The band origins for the observed Σ–Σ transitions for all four acetylenes studied are found to be in good agreement with the results of a recent variational calculation [L. Halonen, M.S. Child, and S. Carter, Mol. Phys. 47, 1097 (1982)]. It is also observed that the anharmonic shifts of the sequence band transitions are quite large—for the v=5 overtone band, the sequence bands are shifted 74 cm⁻¹ fo the Π_g bend and 39 cm⁻¹ for the Π_u bend.