Polarized Matrix Infrared Spectra of Cyclopentadienone: Observations, Calculations, and Assignment for an Important Intermediate in Combustion and Biomass Pyrolysis

A detailed vibrational analysis of the infrared spectra of cyclopentadienone (C₅H₄=O) in rare gas matrices has been carried out. Ab initio coupled-cluster anharmonic force field calculations were used to guide the assignments. Flash pyrolysis of o-phenylene sulfite (C ₆H₄O₂SO) was used to provide a molecular beam of C₅H₄=O entrained in a rare gas carrier. The beam was interrogated with time-of-flight photoionization mass spectrometry (PIMS), confirming the clean, intense production of C₅H₄=O. Matrix isolation infrared spectroscopy coupled with 355 nm polarized UV for photoorientation and linear dichroism experiments was used to determine the symmetries of the vibrations. Cyclopentadienone has 24 fundamental vibrational modes, Γ_vib = 9a₁3a₂4b₁8b ₂. Using vibrational perturbation theory and a deperturbation- diagonalization method, we report assignments of the following fundamental modes (cm^-1) in a 4 K neon matrix: the a₁ modes of X̃ ¹A₁ C₅H₄=O are found to be ν₁ = 3107, ν₂ = (3100, 3099), ν₃ = 1735, ν₅ = 1333, ν₇ = 952, ν₈ = 843, and ν₉ = 651; the inferred a₂ modes are ν₁₀ = 933, and ν₁₁ = 722; the b₁ modes are ν₁₃ = 932, ν₁₄ = 822, and ν₁₅ = 629; the b₂ fundamentals are ν₁₇ = 3143, ν₁₈ = (3078, 3076) ν₁₉ = (1601 or 1595), ν₂₀ = 1283, ν₂₁ = 1138, ν₂₂ = 1066, ν₂₃ = 738, and ν₂₄ = 458. The modes ν₄ and ν₆ were too weak to be detected, ν₁₂ is dipole-forbidden and its position cannot be inferred from combination and overtone bands, and ν₁₆ is below our detection range (<400 cm^-1). Additional features were observed and compared to anharmonic calculations, assigned as two quantum transitions, and used to assign some of the weak and infrared inactive fundamental vibrations.