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 nu(1) + nu(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 nu(1) + nu(5) - nu(5) spectrum, the energy of the lowest excited state with angular momentum about the a axis, nu(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 nu(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)]. Copyright 1999 Academic Press.
