Ultrafast Dynamics of Nitro–Nitrite Rearrangement and Dissociation in Nitromethane Cation
Mi’Kayla Word, Hugo A. López Peña, Derrick Ampadu Boateng, Shane L. McPherson, G. L. Gutsev, Lavrenty G. Gutsev, Ka Un Lao, Katharine Moore Tibbetts
Abstract
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.