High-resolution infrared spectroscopic determination of the nν5±n vibrational energies of HCCN and DCCN

The high-resolution infrared kinetic spectra of HCCN and DCCN have been recorded, the former between 3395 and 3481 cm−1 and the latter in the 2517–2593 cm−1 region. A total of four dominant Q branches were observed and have been subsequently assigned to the ν1+2ν5±2←ν5±1, ν1+3ν5±3←2ν5±2 bands of the two isotopomers. The band origins have been determined to be 3420.666(1), 2544.743(2), 3460.912(2), and 2573.480(1) cm−1 respectively. These new values have been used in conjunction with the previously obtained ν1+2ν5±2←2ν5±2 band origins of HCCN and DCCN to obtain 2ν5±2←ν5±1 vibrational energies of 212.822(1) and 133.106(1) cm−1 for HCCN and DCCN, respectively. Semirigid bender calculations of the energy levels of these molecules have been carried out. By adjusting the height of the barrier to linearity, excellent agreement with experimental data has been attained.