The ν1 + ν5 Band of HCCN: Determination of the ν5 Vibrational Energy

Abstract The high-resolution infrared spectrum of HCCN in the region 3323–3383 cm −1 was observed by infrared kinetic spectroscopy. The band observed is assigned as ν 1 + ν 5 of the quasilinear molecule HCCN with its origin at 3355.500(2) cm −1 . Combining this number with the band origin of the previously investigated ν 1 + ν 5 − ν 5 spectrum, the energy of the lowest excited state with angular momentum about the a axis, ν 5 , is determined to be 128.907(2) cm −1 . This value is lower than the results obtained by means of relative intensity measurements on the millimeter-wave spectra (145 ± 15 cm −1 ) or from similar relative intensity measurements on the IR spectra (187 ± 20 cm −1 ). The present value of the energy of ν 5 predicts a barrier to linearity of 300 cm −1 similar to 280 cm −1 obtained from the corresponding band of DCCN and comparable to 277 cm −1 from the quantum chemistry calculation by Seidl and Schaefer [ J. Chem. Phys. 96, 4449–4452 (1992)].