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