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Propargyl (HCCCH2) radicals have been generated by expanding precursors through a supersonic pyrolysis nozzle, and their infrared (IR) absorption spectra have been recorded in a rare gas matrix. Besides the 10 vibrational modes observed in our previous studies, this investigation detected the HCCCH2 X̃ 2B1 in-plane bending mode (ν12) at 344.2 (±0.8) cm−1 in a cryogenic argon matrix. This is the first experimental observation of ν12 for the propargyl radical. In the previous study, ν12 had been fixed at 333 (±10) cm−1 based on detection of its overtone (2ν12, 667.7 ± 1.0 cm−1) and a possible combination band (ν10 + ν12, 1339.0 ± 0.8 cm−1). The new observation is in reasonable agreement with this previous experimental estimation of ν12. Ab initio coupled cluster anharmonic force field calculations were used to guide the analysis. Our finding completes the fundamental frequency database for propargyl.