| Abstract: | Nitromethane, the smallest organic-nitro compound, is commonly studied to model ignition and detonation of energetic materials. Using pump-probe femtosecond laser photoionization mass spectroscopy, coupled-cluster theory, and ab initio molecular dynamics, we study nitromethane cation (\chem{NM}$^+$) fragmentation into \chem{CH_3}$^+$, \chem{NO_2}$^+$, and \chem{NO}$^+$. From theoretical analysis, \chem{NO_2}$^+$ and \chem{CH_3}$^+$ are formed through direct cleavage of the C-N bond, whereas \chem{NO}$^+$ forms spontaneously upon nitro-nitrite rearrangement (NNR) of the \chem{NM}$^+$ cation. Direct ionization into the electronically excited D$_1$ or D$_2$ states by the pump pulse provides sufficient excess energy to initiate the NNR pathway. With excess energy stored in the NNR transition state, molecular dynamics simulations indicate that NNR requires $660\pm230$ fs and is typically followed by rapid \chem{NO}$^+$ loss ~100-200 fs later. Experimentally, the fragmentation pathways to \chem{NO}$^+$ and \chem{CH_3}$^+$ are in competition, with associated decay timescale of $\sim480\pm200$ fs that is similar to the computed NNR timescale. This result suggests that \chem{CH_3}$^+$ is formed by further excitation of the \chem{NM}$^+$ initially ionized into the D$_1$ or D$_2$ states before it undergoes NNR. Finally, the dissociation to \chem{NO_2}$^+$ from \chem{NM}$^+$ can be assigned to a D$_0$ $\to$ D$_2$ transition. |
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