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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 126; no. 6; pp. 879 - 888
• Main Authors: Word, Mi’Kayla D, López Peña, Hugo A, Ampadu Boateng, Derrick, McPherson, Shane L, Gutsev, Gennady L, Gutsev, Lavrenty G, Lao, Ka Un, Tibbetts, Katharine Moore
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 17.02.2022
• Subjects: A: Structure, Spectroscopy, and Reactivity of Molecules and Clusters
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.1c10288

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.