Electron-spin-resonance studies of 12CH3F+, 13CH3F+, and 12CH2DF+ in neon matrices at 4 K: Comparison with theoretical calculations

Abstract

Various isotopic forms of the methyl fluoride cation ¹²CH₃F⁺, ¹³CH₃F⁺, and ¹²CH₂DF⁺ have been generated by photoionization at 16.8 eV and separately by electron bombardment at 50 eV. The first electron‐spin‐resonance (ESR) results are reported for this radical cation which was isolated in neon matrices at 4 K. The measured A tensors or nuclear hyperfine parameters were compared with the results obtained from various computational approaches. Surprising observations were the large amounts of spin density on the methyl group, especially the hydrogen atoms, and the extreme differences in the deuterated spectra compared to the nondeuterated case. The presence of a single D atom apparently acts to prevent dynamic Jahn–Teller averaging which makes the methyl hydrogens equivalent on the ESR time scale. Such a dramatic Jahn–Teller effect has been previously observed for the similar methane cations CH⁺₄ and CH₂D⁺₂. The magnetic parameters for CH₂DF⁺ in neon at 4 K are g_X=2.0032(5), g_Y=2.0106(8), and g_Z=2.0120(5); for H: A_X = 483(1), A_Y=476(1), and A_Z=483(1) MHz; for D: ‖A_X‖=5.0(3), ‖A_Y‖A_Z‖=7.1(3) MHz; for ¹⁹F : A_X=965(1), A_Y=−130(2), and A_Z=−166(1) MHz. For CH₃F⁺, the g tensor and ¹⁹F A tensor were similar to those above but the H atoms were equivalent with values of A_X=317(1), A_Y=323(2), and A_Z=312 MHz.