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

Abstract

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 2 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 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 Hatom 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 C 7 H 6 NO + , 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 −1) than the 2-NT + ion (47.9 cm −1). Additional DFT calculations identify reaction pathways leading to the formation of the dissociation products C 7 H 6 NO + , C 7 H + 7 , and C 6 H 6 N +. Collectively, these results reveal a rich picture of coherently-and incoherently-driven dissociation pathways in 2-NT + .

Coherent Vibrational and Dissociation Dynamics of Polyatomic Radical Cations

“…This structure forms through “H‐atom attack” of a methyl hydrogen to a nitro oxygen and has been a known reaction of 2‐NT cation for decades . DFT calculations at the B97XD/CBSB7 level with ADMP molecular dynamics trajectories indicate a transition state barrier of 0.76 eV above the vertical IP of 2‐NT and an expected reaction of 20–60 fs after ionization . The aci‐nitro tautomer spontaneously loses OH to form C 7 H 6 NO + , consistent with our computations showing that C 7 H 6 NO + +OH lies 0.59 eV below the initial transition state energy .…”

“…Additional coherent vibrational dynamics become visible in 2‐NT when the probe wavelength is shifted from 800 to 650 nm, resonant with a transition predicted by time‐dependent DFT (TDDFT) . The transient parent and fragment ions signals exhibit oscillations in the first 1000 fs after ionization, followed by slow decay dynamics visible over the 8000 fs measurement window (Figure a).…”

“…The distinct oscillation frequencies of C 7 H 7 NO 2 + and C 7 H 6 NO + are attributed to the coherent excitation of two structures of 2‐NT cation . The C 7 H 7 NO 2 + oscillations arise from the coherent torsional motion of the parent 2‐NT ion structure with a computed frequency of 48 cm −1 (Figure d), similar to 4‐NT.…”

“…A schematic FTRMS pump‐probe setup for measuring radical cation dynamics is shown in Figure . Concepts underlying the technique are described here, and details on constructing FTRMS setups are reported elsewhere . To create a pump‐probe pulse pair, the initial laser pulse is first split into two replicates with a beam splitter (BS).…”

“…The probe pulse is taken from a portion of the laser output split either before (shown) or after the OPA. Splitting prior to the OPA results in 800 nm probe pulses, whereas splitting after the OPA and frequency doubling produces tunable probe wavelengths in the region of 600–800 nm . Because the pump and probe beams both travel at the speed of light along their respective paths, the time‐delay between them can be controlled by specifying the distance that each beam travels.…”