Infrared absorption of gaseous benzoyl radical C ₆H ₅CO recorded with a step-scan Fourier-transform spectrometer

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the gaseous transient species benzoyl radical, C ₆H ₅CO. C ₆H ₅CO was produced either from photolysis of acetophenone, C ₆H ₅C(O)CH ₃, at 248 nm or in reactions of phenyl radical (C ₆H ₅) with CO; C ₆H ₅ was produced on photolysis of C ₆H ₅Br at 248 nm. One intense band at 1838 ± 1 cm ^-1, one weak band at 1131 ± 3 cm ^-1, and two extremely weak bands at 1438 ± 5 and 1590 ± 10 cm ^-1 are assigned to the C=O stretching (ν ₆), the C-C stretching mixed with C-H deformation (ν ₁₅), the out-of-phase C ₁C ₂C ₃/C ₅C ₆C ₁ symmetric stretching (ν ₁₀), and the in-phase C ₁C ₂C ₃/C ₄C ₅C ₆ antisymmetric stretching (ν ₇) modes of C ₆H ₅CO, respectively. These observed vibrational wavenumbers and relative IR intensities agree with those reported for C ₆H ₅CO isolated in solid Ar and with values predicted for C ₆H ₅CO with the B3LYP/aug-cc-pVDZ method. The rotational contours of the two bands near 1838 and 1131 cm ^-1 simulated according to rotational parameters predicted with the B3LYP/aug-cc-pVDZ method fit satisfactorily with the experimental results. Additional products BrCO, C ₆H ₅C(O)Br, and C ₆H ₅C(O)C ₆H ₅ were identified in the C ₆H ₅Br/CO/N ₂ experiments; the kinetics involving C ₆H ₅CO and C ₆H ₅C(O)Br are discussed.