Theoretical calculations of infrared bands of CH₃⁺ and CH₅⁺

Existing theoretical calculations predict that infrared spectra of the two most fundamental reactive carbo-ions, methyl cation CH₃⁺ with D_3h symmetry and protonated methyl cation CH₅ ⁺ with C_s(I), C_s(II), and C_2v symmetries, appear together in the 7-μm region corresponding to the C-H bending modes. Vibrational band profiles of CH₃⁺ and CH₅⁺ have been compared by ab initio calculation methods that use the basis sets of MP2/ aug-cc-pVTZ and CCSD(T)/cc-pVTZ. Our results indicate that the bands of rotation-vibration transitions of CH₃ ⁺ and CH₅⁺ should overlap not only in the 3-μm region corresponding to the C-H stretching modes but also in the 7-μm region corresponding to the C-H bending modes. Five band intensities of CH ₅⁺ among fifteen vibrational transitions between 6 and 8 μm region are stronger than those of the ν₂ and ν₄ bands in CH₃⁺. Ultimate near degeneracy of the two bending vibrations ν₂ and ν₄ of CH ₃⁺ along with the stronger intensities of CH ₅⁺ in the three hydrogen scrambling structures may cause extreme complications in the analysis of the highresolution carbo-ion spectra in the 7-μm region.