Polarized matrix infrared spectra of cyclopentadienone: observations, calculations, and assignment for an important intermediate in combustion and biomass pyrolysis.

ABSTRACT

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 (C6H4O2SO) 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, Γvib = 9a1 ⊕ 3a2 ⊕ 4b1 ⊕ 8b2. 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 (1)A1 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.