Propargyl Radical:  Ab Initio Anharmonic Modes and the Polarized Infrared Absorption Spectra of Matrix-Isolated HCCCH2

The propargyl radical has twelve fundamental vibrational modes, Γvib(HCCCH2) = 5a1 ⊕ 3b1 ⊕ 4b2, and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide some of the assignments. The experimental HCC CH2 matrix frequencies (cm-1) and polarizations are a1 modes3308.5 ± 0.5, 3028.3 ± 0.6, 1935.4 ± 0.4, 1440.4 ± 0.5, 1061.6 ± 0.8; b1 modes686.6 ± 0.4, 483.6 ± 0.5; b2 modes1016.7 ± 0.4, 620 ± 2. We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HCC CH2 X̃ 2B1. From an analysis of the vibrational spectra, the small electric dipole moment, μD(HCCCH2) = 0.150 D, and the large resonance energy (HCCCH2), roughly 11 kcal mol-1, we conclude that propargyl is a completely delocalized hydrocarbon radical and is best written as HCC CH2.