Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA.
J Chem Phys. 2012 Jan 14;136(2):024321. doi: 10.1063/1.3668139.
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.
过去几十年，人们对具有不同 X-NO₂（X = C、N、O）基团的电子激发含硝基分子的分解进行了深入研究；然而，它们的分解行为此前并未进行过比较和对比。比较它们的单分子分解行为对于理解具有不同 X-NO₂（X = C、N、O）键连接的电子激发含硝基分子之间的反应性差异很重要。硝基甲烷（NM）、二甲基硝胺（DMNA）和异丙基硝酸酯（IPN）分别用作 C-NO₂、N-NO₂和 O-NO₂活性基团的模型分子。使用不同波长的紫外激光，如 226、236 和 193nm，来制备这些分子的激发态。然后通过共振增强双光子电离（R2PI）、激光诱导荧光（LIF）技术或 10.5eV 的单光子电离来检测分解产物。通过 R2PI 技术观察到电子激发 NM、DMNA、IPN 中的主要分解产物是 NO 分子。电子激发（226 和 236nm）NM 和 IPN 分解产生的 NO 产物显示出相似的旋转（600K）和振动分布[观察到 NO 分子的（0-0）和（0-1）带]。DMNA 分解产生的 NO 产物的旋转（120K）和振动分布（仅观察到（0-0）跃迁）比 NM 和 IPN 的更冷。在 193nm 激发下，通过荧光检测观察到 NM 和 IPN 中的电子激发 NO₂产物，但 DMNA 中未观察到电子激发的 NO₂产物。此外，还观察到 OH 自由基是这三种化合物的少量离解产物。电子激发 NM 和 IPN 的主要分解途径涉及 X-NO₂键的断裂，形成电子激发的 NO₂产物，进一步分解生成 NO。电子激发 DMNA 中 NO 分子的生成被认为是通过硝基亚硝酸盐异构化途径进行的。理论计算表明，DMNA 的硝基亚硝酸盐异构化发生在 S₁ 表面上，遵循（S₂/S₁）（CI）锥形交叉（CI），而 NM 和 IPN 的 NO₂消除则发生在 S₁ 表面上，遵循（S₂/S₁）（CI）锥形交叉。本工作为理解电子激发 X-NO₂能系统分解的引发提供了见解。反应坐标上存在锥形交叉在这些能系统分解的详细机制中起着重要作用。
