Infrared Spectrum of Gaseous Methyl Radical by Rapid Scan Spectroscopy

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Abstract
Gaseous methyl radical was produced through flash photolysis of methyl iodide and of dimethyl mercury and a portion of its infrared spectrum was recorded (450–740 cm−1) using a rapid scan infrared spectrometer with scan rates up to 1 cm−1/μsec. Two absorptions were observed for CH3, at 607.0 and 603.3 cm−1, two for CH2D, at 561.2 and 558.0 cm−1, and one for CD3 at 460.6 cm−1, with a second possible feature at 457 cm−1. These frequencies are close to the corresponding matrix frequencies, which permits these bands to be assigned to ν2, the out‐of‐plane (umbrella) mode of CH3. The possibility that the doublets are due to inversion doubling of a pyramidal molecule is considered but evidence is presented to show that the lower frequency component is due, instead, to vibrationally excited methyl radical. The out‐of‐plane vibrational potential function is calculated assuming CH3 is planar. If the harmonic oscillator approximation is used, k2=0.174 mdyn/Å but the CD3 frequency shows that the quartic contribution is not negligible. If a quartic term is included, the force constants obtained are k2=0.137 mdyn/Å and a=0.098 mdyn/Å3.