Infrared spectroscopy of carbo-ions. III. ν3 band of methyl cation CH+3

Abstract

The infrared spectrum of the degenerate ν₃ vibration–rotation band of methyl cation CH⁺₃, one of the most fundamental carbo‐ions, has been observed and analyzed. The spectrum was observed in the frequency range of 3240–2960 cm⁻¹ using the difference frequency laser system as the tunable coherent infrared source. A gas mixture of He:H₂:CH₄∼700:20:1 with a total pressure of ∼7 Torr was used for the ac discharge in an air‐cooled and a water‐cooled multiple inlet–outlet discharge tube. The velocity modulation method together with noise subtraction and the unidirectional multiple passing of the beam was used for high sensitivity. The spectral lines of CH⁺₃ appeared amid many other lines of C₂H⁺₂ and C₂H⁺₃ but their widths and their response to chemical conditions gave a good initial clue for the identification. Over 200 absorption lines have been assigned to CH⁺₃ and the isotopic species ¹³CH⁺₃. The spectral pattern clearly shows that the equilibrium molecular structure is a planar, equilateral triangle as expected from Herzberg’s Rydberg spectrum of CH₃ and from ab initio theory. The spectrum has been analyzed by using the symmetric rotor Hamiltonian in a degenerate vibrational state with various A₁–A₂ splittings and l resonance. The spectral lines have been least‐squares fit with a standard deviation of 0.0058 cm⁻¹, and led to 21 vibration–rotation constants of CH⁺₃. Several analytical expressions have been derived among these constants to determine the structure and potential constants. Some perturbation has been noticed and discussed. Also quite a few lines which are definitely due to carbocations with one carbon atom have been left unassigned. The prominence of the CH⁺₃ spectrum in hydrocarbon discharges confirms the importance of this species in the ion–molecule reaction scheme in the laboratory and in space which has already been noted by many people in the field. A simple discussion is given about the formation and destruction of CH⁺₃ as well as other carbocations.