Dissociation dynamics of 3- and 4-nitrotoluene radical cations: Coherently driven C-NO 2 bond homolysis

• Published in: The Journal of chemical physics Vol. 148; no. 13; p. 134305
• Main Authors: Ampadu Boateng, Derrick, Gutsev, Gennady L, Jena, Puru, Tibbetts, Katharine Moore
• Format: Journal Article
• Language: English
• Published: United States 07.04.2018
• ISSN: 1089-7690
• DOI: 10.1063/1.5024892

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 NO loss to form C H , 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 NO group based on theoretical results showing that the dominant relaxation pathway in 4-nitrotoluene radical cations involves the rotation of the NO 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 NO and CH moieties based on analysis of the normal modes. These results demonstrate that coherent nuclear motions determine the probability of C-NO homolysis in the nitrotoluene radical cations upon optical excitation within several hundred femtoseconds of the initial ionization event.