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

We report new insights into the ultrafast rearrange-ment 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 OCH3 occurring similar to 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 D-0 -> D-1 transition can be excited at 650 nm when the C-N bond is stretched from its equilibrium value (1.48 angstrom) to 1.88 angstrom. 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 NO2+ and CH3 can be induced with 650 nm excitation via a weakly allowed D-0 -> D-2 transition.