Ultrafast Dynamics of Nitro-Nitrite Rearrangement and Dissociation in Nitromethane Cation

We report new insights into the ultrafast rearrangement and dissociation dynamics of nitromethane cation (NM⁺) using pump-probe measurements, electronic structure calculations, and ab initio molecular dynamics simulations. The "roaming" nitro-nitrite rearrangement (NNR) pathway involving large-amplitude atomic motion, which has been previously described for neutral nitromethane, is demonstrated for NM⁺. Excess energy resulting from initial population of the electronically excited D₂ state of NM⁺ upon strong-field ionization provides the necessary energy to initiate NNR and subsequent dissociation into NO⁺. Both pump-probe measurements and molecular dynamics simulations are consistent with the completion of NNR within 500 fs of ionization with dissociation into NO⁺ and OCH₃ occurring ∼30 fs later. Pump-probe measurements indicate that NO⁺ formation is in competition with the direct dissociation of NM⁺ to CH₃⁺ and NO₂. Electronic structure calculations indicate that a strong D₀ → D₁ transition can be excited at 650 nm when the C-N bond is stretched from its equilibrium value (1.48 Å) to 1.88 Å. On the other hand, relaxation of the NM⁺ cation after ionization into D₀ occurs in less than 50 fs and results in observation of intact NM⁺. Direct dissociation of the equilibrium NM⁺ to produce NO₂⁺ and CH₃ can be induced with 650 nm excitation via a weakly allowed D₀ → D₂ transition.