Conserved Vibrational Coherence in the Ultrafast Rearrangement of 2-Nitrotoluene Radical Cation.

2-Nitrotoluene (2-NT) is a good model for both photolabile protecting groups for organic synthesis and the military explosive 2,4,6-trinitrotoluene (TNT). In addition to the direct C-NO bond-cleavage reaction that initiates detonation in TNT, 2-NT undergoes an H atom attack reaction common to the photolabile 2-nitrobenzyl group, which forms the aci-nitro tautomer. In this work, femtosecond pump-probe measurements with mass spectrometric detection and density functional theory (DFT) calculations demonstrate that the initially prepared vibrational coherence in the 2-NT radical cation (2-NT) is preserved following H atom attack. Strong-field adiabatic ionization is used to prepare 2-NT, which can overcome a modest 0.76 eV energy barrier to H atom attack to form the aci-nitro tautomer as soon as ∼20-60 fs after ionization. Once formed, the aci-nitro tautomer spontaneously loses -OH to form CHNO, which exhibits distinctly faster oscillations in its ion yield (290 fs period) as compared to the 2-NT ion (380 fs period). The fast oscillations are attributed to the coherent torsional motion of the aci-nitro tautomer, which has a significantly faster computed torsional frequency (86.9 cm) than the 2-NT ion (47.9 cm). Additional DFT calculations identify reaction pathways leading to the formation of the dissociation products CHNO, CH, and CHN. Collectively, these results reveal a rich picture of coherently and incoherently driven dissociation pathways in 2-NT.