Infrared Spectrum of the Propargyl Peroxyl Radical, HC≡C—CH₂OO X̃²A′′^†

When the propargyl radical, HCCCH2, and O-2 are codeposited onto a cold argon matrix, a chemical reaction ensues; infrared absorption spectra reveal the formation of the propargyl peroxyl radical: HC = C - CH2 (H) over tilde B-2(1) + O-2 -> trans-HC=CH2OO (H) over tilde (2) A. We do not observe the isomeric adduct, CH2=C=CHOO (X) over tilde (2)A ". The propargyl radicals are produced by a hyperthermal nozzle while a second nozzle alternately fires bursts of O2/Ar at the 20 K matrix. The absorption spectra of the radicals are measured using a Fourier transform infrared spectrometer. We observe 13 of the 18 fundamental infrared bands of the propargyl peroxyl radical in an Ar matrix at 20 K. The experimental frequencies (cm(-1)) of trans-HC C-CH2OO (H) over tilde (2) A " are assigned. The a' modes are v(1) = 3326, v(2) = 2960, v(3) = 2149, v(4) = 1440, v(5) = 1338, v(6) = 1127. v(7) = 982, v(8) = 928, v(9) = 684, and v(10) = 499 cm(-1), while the a " modes are v(14) = 1218, v(15) = 972, and v(16) = 637 cm(-1). Linear dichroism spectra were measured with photo-oriented HCCCH2OO radical samples to establish the experimental polarizations of several vibrational hands. The experimental frequencies (v) for the propargyl peroxyl radical are compared to the anharmonic frequencies (v) resulting from electronic structure calculations. We have used CBS-QB3 electronic structure calculations to estimate the peroxyl bond energies: Delta H-298(trans-HC CCh(2) -> OO -> CH2CCH (H) over tilde B-2(1) + O-2) = 19 +/- 1 kcal mol(-1) and Delta H-298(trans-CH2=C=CH-OO -> CH2CCH (H) over tilde B-2(1) + O-2) = 21 +/- 1 kcal mol(-1). The experimental thermochemistry for C3H3 reacting with oxygen has been reanalyzed as Delta H-rxn(298)(HCCCH2 + O-2 -> CH2=C=O + HCO) = -83 +/- 3 kcal mol(-1); Delta H-rxn(298)(HCCCH2 + O-2 -> CH3CO + CO) = -111 +/- 3 kcal mol(-1); Delta H-rxn(298)(HCCCH2 + O-2 -> CH2CHO + CO) = -106 +/- 4 kcal mol(-1); Delta H-rxn(298)(HCCCH2 + O-2 -> HCHO + HCCO) = -67 +/- 4 kcal mol(-1); Delta H-rxn(298)(HCCCH2 + O-2 -> CH2CH + CO2) = -105 +/- 3 kcal mol(-1).