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 D 2 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 OCH 3 occurring ∼30 fs later. Pump-probe measurements indicate that NO + formation is in competition with the direct dissociation of NM + to CH 3 + and NO 2 . Electronic structure calculations indicate that a strong D 0 → D 1 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 D 0 occurs in less than 50 fs and results in observation of intact NM + . Direct dissociation of the equilibrium NM + to produce NO 2 + and CH 3 can be induced with 650 nm excitation via a weakly allowed D 0 → D 2 transition.