Vibrational Spectroscopy of Intermediates and C-H Activation Products of Sequential Zr⁺ Reactions with CH₄

Vibrational spectra of the entrance channel complexes Zr⁺(CH₄)_n(Ar) (n=1-2), Zr⁺(CH₄)_n (n=3-4), and the reaction products ZrC₃H₁₀⁺, ZrC₄H₁₃⁺, ZrC₄H₁₄⁺, and ZrC₄H₁₅⁺ in the C-H stretching region (2550-3100 cm^-1) are obtained using photofragment spectroscopy. The experimental spectra and simulations based on calculations at the B3LYP/aug-cc-pVTZ level of theory work together to identify the structures of the ions. The n=1-3 entrance channel complexes show peaks around 2800 and 3000 cm^-1, while the n=4 complex has two peaks around 2800 cm^-1 which indicates different hydrogen coordination. Observation of the low-frequency C-H stretch of an agostic carbene group, as well as the high-frequency H-H stretch, also confirms production of (H₂)ZrCH₂⁺(CH₄)_n-1 (n=1-2) exit channel complexes. The observed C-H activation products correspond to loss of H₂ from Zr⁺(CH₄)₃ and loss of H, H₂, and H₂ + H from Zr⁺(CH₄)₄. Comparison of experiment and simulations indicate the activation products are Zr(CH₃)₂⁺(CH₄), Zr(CH₃)₃⁺(CH₄), Zr(CH₃)₂⁺(CH₄)₂, and HZrCH₂⁺(CH₄)₃ and/or ZrCH₃⁺(CH₄)₃.