The photolabile 2-nitrotoluyl group undergoes rapid intramolecular hydrogen transfer upon optical excitation and ionization, which contributes to initial dissociation processes in nitroaromatic explosives such as TNT. I will present results of our group’s recent pump-probe spectroscopy measurements on the 2-nitrotoluene (2-NT) radical cation, a model system for TNT that contains the 2-nitrotoluyl moiety. Our experiments use strong-field 1300 nm “pump” pulses to induce adiabatic tunneling ionization, which prepares ground electronic state radical cations in a coherent superposition of highly excited vibrational states, i.e., as a nuclear “wave packet”. Excitation of the oscillating wave packet by a time-delayed weak-field 650 nm “probe” pulse accesses one or more electronic excited states in the cation that lead to dissociation, thereby resulting in oscillations in the ion yields as a function of pump-probe delay. These coherent vibrational dynamics drive multiple bond-dissociation pathways in 2-NT. In addition to coherent vibrational dynamics in the parent 2-NT ion, our pump-probe measurements reveal the coherent vibrational signature of the hydrogen transfer product aci-2-NT isomer in the fragment C7H6NO+ ion. DFT and molecular dynamics calculations indicate that the hydrogen transfer reaction in the 2-NT cation has a low activation barrier of ~0.7 eV, proceeds within 20-60 fs of ionization, and results in spontaneous loss of OH to produce C7H6NO+. These results demonstrate that coherent vibrational dynamics can be preserved following an intramolecular rearrangement reaction.

Capturing coherent vibrational dynamics following molecular rearrangement with ultrafast pump-probe spectroscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.