Reinvestigation of the acetylenic C–H stretching fundamental of propyne via high resolution, optothermal infrared spectroscopy: Nonresonant perturbations to ν₁

We present the high resolution spectrum of the ν₁ fundamental of propyne near 3335 cm⁻¹ obtained using a very warm free jet expansion in our optothermal detection spectrometer. By using a high concentration sample expanded at low backing pressures we have been able to observe transitions for K values up to K=6. The additional data available allow us to reinvestigate this vibrational band. We find an unusual perturbation pattern in this band where the individual subbands (rovibrational transitions for a single K value) appear to be completely unperturbed at the level of precision of our data (7.5 MHz), but the subband origin orderings are perturbed through nonresonant interactions. Attempts to account for the subband ordering using a two‐state anharmonic interaction are unsuccessful indicating that the perturbations are of multistate origin. This type of nonresonant perturbation to the subband origins of symmetric top molecules should be a common feature of symmetric tops with large A rotational constants. As a result of this investigation we conclude that the previously reported value of α^A, determined from a very cold expansion where only K=0 and K=1 were observed, is not a measure of the true (unperturbed) value of this constant. This conclusion is also supported by force field calculations presented here that use an empirical harmonic force field augmented by diagonal anharmonicities for the hydride stretches. These calculations, which reproduce measured values of α^A and α^B for lower energy bands quite successfully, also show that the previous determination of α^A is too large and must be dominated by perturbation contributions. We have also measured the weak Fermi resonant band ν₃+2ν⁰₉ which acquires its intensity through interaction with ν₁. Again we find an anomalous subband ordering like that observed in ν₁.