A comparison of the decomposition of electronically excited nitro-containing molecules with energetic moieties C-NO2, N-NO2, and O-NO2.

Decomposition of electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) moieties has been intensively investigated over the past decades; however, their decomposition behavior has not previously been compared and contrasted. Comparison of their unimolecular decomposition behavior is important for the understanding of the reactivity differences among electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) bond connections. Nitromethane (NM), dimethylnitramine (DMNA), and isopropylnitrate (IPN) are used as model molecules for C-NO(2), N-NO(2), and O-NO(2) active moieties, respectively. Ultraviolet lasers at different wavelengths, such as 226, 236, and 193 nm, have been employed to prepare the excited states of these molecules. The decomposition products are then detected by resonance enhanced two photon ionization (R2PI), laser induced fluorescence (LIF) techniques, or single photon ionization at 10.5 eV. NO molecules are observed to be the major decomposition product from electronically excited NM, DMNA, IPN using R2PI techniques. The NO products from decomposition of electronically excited (226 and 236 nm) NM and IPN display similar rotational (600 K) and vibrational distributions [both (0-0) and (0-1) bands of the NO molecule are observed]. The NO product from DMNA shows rotational (120 K) and vibrational distributions (only (0-0) transition is observed) colder than those of NM and IPN. At the 193 nm excitation, electronically excited NO(2) products are observed from NM and IPN via fluorescence detection, while no electronically excited NO(2) products are observed from DMNA. Additionally, the OH radical is observed as a minor dissociation product from all three compounds. The major decomposition pathway of electronically excited NM and IPN involves fission of the X-NO(2) bond to form electronically excited NO(2) product, which further dissociates to generate NO. The production of NO molecules from electronically excited DMNA is proposed to go through a nitro-nitrite isomerization pathway. Theoretical calculations show that a nitro-nitrite isomerization for DMNA occurs on the S(1) surface following a (S(2)/S(1))(CI) conical intersection (CI), whereas NO(2) elimination occurs on the S(1) surface following the (S(2)/S(1))(CI) conical intersection for NM and IPN. The present work provides insights for the understanding of the initiation of the decomposition of electronically excited X-NO(2) energetic systems. The presence of conical intersections along the reaction coordinate plays an important role in the detailed mechanism for the decomposition of these energetic systems.