Infrared (IR) absorption spectra of matrix-isolated HCCCH2 have been measured. Propargyl radicals were generated in a supersonic pyrolysis nozzle, using a method similar to that described in a previous study (Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Varner, M. E.; Stanton, J. F.; Ellison, G. B. J. Phys. Chem. A 2005, 109, 3812-3821). Besides the nine vibrational modes observed in the previous study, this investigation detected the HCCCH2 X 2B1 out-of-plane bending mode (ν8) at 378.0 ((1.9) cm-1 in a cryogenic argon
˜ matrix. This is the first experimental observation of ν8 for the propargyl radical. In addition, seven overtone and combination bands have also been detected and assigned. Ab initio coupled-cluster anharmonic force field calculations were used to guide the analysis. Furthermore, ν12, the HCCCH2 in-plane bending mode, has been assigned to 333 ((10) cm-1 based on the detection of its overtone (2ν12, 667.7 ( 1.0 cm-1) and a possible combination band (ν10 + ν12, 1339.0 ( 0.8 cm-1). This is the first experimental estimation of ν12 for the propargyl radical.