Guo Y Q, Bhattacharya A, Bernstein E R
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA.
J Chem Phys. 2008 Jan 21;128(3):034303. doi: 10.1063/1.2822283.
We report the first experimental and theoretical study of gas phase excited electronic state decomposition of a furazan based, high nitrogen content energetic material, 3,3'-diamino-4,4'-azoxyfurazan (DAAF), and its model systems, diaminofurazan (DAF) and furazan (C2H2N2O). DAAF has received major attention as an insensitive high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. In order to understand the initial decomposition mechanism of DAAF and those of its model systems, nanosecond energy resolved and femtosecond time resolved spectroscopies and complete active space self-consistent field (CASSCF) calculations have been employed to investigate the excited electronic state decomposition of these materials. The NO molecule is observed as an initial decomposition product from DAAF and its model systems at three UV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of the three excitation wavelengths coincide with the (0-0), (0-1), and (0-2) vibronic bands of the NO A 2Sigma+<--X 2Pi electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for DAAF, which generates the NO product with a rotationally cold (20 K) and a vibrationally hot (1265 K) distribution. On the contrary, excitation wavelength dependent dissociation channels are observed for the model systems, which generate the NO product with both rotationally cold and hot distributions depending on the excitation wavelengths. Potential energy surface calculations at the CASSCF level of theory illustrates that two conical intersections between the excited and ground electronic states are involved in two different excitation wavelength dependent dissociation channels for the model systems. Femtosecond pump-probe experiments at 226 nm reveal that the NO molecule is still the main observed decomposition product from the materials of interest and that the formation dynamics of the NO product is faster than 180 fs. Two additional fragments are observed from furazan with mass of 40 amu (C2H2N) and 28 amu (CH2N) employing femtosecond laser ionization. This observation suggests a five-membered heterocyclic furazan ring opening mechanism with rupture of a CN and a NO bond, yielding NO as a major decomposition product. NH2 is not observed as a secondary decomposition product of DAAF and DAF.
我们报道了对一种基于呋咱的高氮含量含能材料3,3'-二氨基-4,4'-氧化偶氮呋咱（DAAF）及其模型体系二氨基呋咱（DAF）和呋咱（C₂H₂N₂O）的气相激发电子态分解的首次实验和理论研究。DAAF作为一种钝感高能炸药受到了广泛关注；然而，这种材料分解的机理和动力学尚不清楚。为了理解DAAF及其模型体系的初始分解机理，我们采用了纳秒能量分辨和飞秒时间分辨光谱以及完全活性空间自洽场（CASSCF）计算来研究这些材料的激发电子态分解。在三个紫外激发波长（226、236和248 nm）、脉冲持续时间为8 ns的条件下，观察到NO分子是DAAF及其模型体系的初始分解产物。这三个激发波长的能量分别与NO的A²Σ⁺←X²Π电子跃迁的（0-0）、（0-1）和（0-2）振动带重合。对于DAAF，观察到一个独特的与激发波长无关的解离通道，该通道产生具有旋转冷态（20 K）和振动热态（1265 K）分布的NO产物。相反，对于模型体系，观察到与激发波长相关的解离通道，根据激发波长产生具有旋转冷态和热态分布的NO产物。在CASSCF理论水平上的势能面计算表明，对于模型体系，激发态和基态电子态之间的两个锥形交叉点参与了两个不同的与激发波长相关的解离通道。在226 nm处的飞秒泵浦-探测实验表明，NO分子仍然是所关注材料的主要观察到的分解产物，并且NO产物的形成动力学快于180 fs。采用飞秒激光电离从呋咱中观察到另外两个质量分别为40 amu（C₂H₂N）和28 amu（CH₂N）的碎片。这一观察结果表明了一种五元杂环呋咱环开裂机理，伴随着一个C-N键和一个N-O键的断裂，产生NO作为主要分解产物。未观察到NH₂作为DAAF和DAF的二次分解产物。
