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 D3h 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.