Dissociation dynamics of 3- and 4-nitrotoluene radical cations: Coherently driven C$-$NO$_2$ bond homolysis

TL;DR

This study investigates the ultrafast nuclear dynamics of 3- and 4-nitrotoluene radical cations, revealing how coherent nuclear motions influence C-NO2 bond homolysis within hundreds of femtoseconds after ionization.

Contribution

It provides new insights into the specific vibrational motions driving bond dissociation in nitrotoluene radical cations using femtosecond spectroscopy and theoretical calculations.

Findings

Distinct coherent vibrational modes identified for each isomer

Faster in-plane bending motion in 3-nitrotoluene influences homolysis

Torsional motion in 4-nitrotoluene affects bond dissociation probability

Abstract

Monosubstituted nitrotoluenes serve as important model compounds for nitroaromatic energetic molecules such as trinitrotoluene. This work investigates the ultrafast nuclear dynamics of 3- and 4-nitrotoluene radical cations using femtosecond pump-probe measurements and the results of density functional theory calculations. Strong-field adiabatic ionization of 3- and 4-nitrotoluene using 1500 nm, 18 fs pulses produces radical cations in the ground electronic state with distinct coherent vibrational excitations. In both nitrotoluene isomers, a one-photon excitation with the probe pulse results in \ce{NO2} loss to form \ce{C7H7+}, which exhibits out-of-phase oscillations in yield with the parent molecular ion. The oscillations in 4-nitrotoluene with a period of 470 fs are attributed to the torsional motion of the \ce{NO2} group based on theoretical results showing that the dominant relaxation pathway in 4-nitrotoluene radical cation involves the rotation of the \ce{NO2} group away from the planar geometry. The distinctly faster oscillation period of 216 fs in 3-nitrotoluene is attributed to an in-plane bending motion of the \ce{NO2} and \ce{CH3} moieties based on analysis of the normal modes. These results demonstrate that coherent nuclear motions determine the probability of \ce{C-NO2} homolysis in the nitrotoluene radical cations upon optical excitation within several hundred femtoseconds of the initial ionization event.