Infrared Spectrum of the Methyl Radical in Solid Argon

Abstract

Reaction between lithium atoms and methyl halides during condensation in solid argon at 15°K produces absorptions at 730.3 and 1383 cm⁻¹ that can be attributed to the methyl radical CH₃. Isotopic substitutions (CH₃, CH₂D, CHD₂, CD₃, ¹³CH₃) and behavior on warming, during which ethane absorptions replace the methyl features, furnish convincing evidence for the assignment. Normal‐coordinate analysis shows that the 730‐cm⁻¹ band is due to ν₂, the out‐of‐plane bending mode of CH₃ with a force constant of 0.25 mdyn/Å. These calculations favor the planar structure and they exclude an equilibrium deviation from planarity in excess of 5°. The absorption at 1383 cm⁻¹ is due to ν₄, the in‐plane bending mode, with a force constant of 0.34 mdyn/Å. The asymmetric stretching mode, ν₃, was not detected. The out‐of‐plane bending force constants of D_3h molecules can be correlated by separating the force constant into sigma‐ and pi‐bonding parts. This correlation leads to a prediction of the out‐of‐plane frequency of BH₃, 590 ±50 cm⁻¹. Thermodynamic functions for both CH₃ and BH₃ are presented.