On the excited electronic state dissociation of nitramine energetic materials and model systems

In order to elucidate the difference between nitramine energetic materials, such as RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), and their nonenergetic model systems, including 1,4-dinitropiperazine, nitropiperidine, nitropyrrolidine, and dimethylnitramine, both nanosecond mass resolved excitation spectroscopy and femtosecond pump-probe spectroscopy in the UV spectral region have been employed to investigate the mechanisms and dynamics of the excited electronic state photodissociation of these materials. The NO molecule is an initial decomposition product of all systems. The NO molecule from the decomposition of energetic materials displays cold rotational and hot vibrational spectral structures. Conversely, the NO molecule from the decomposition of model systems shows relatively hot rotational and cold vibrational spectra. In addition, the intensity of the NO ion signal from energetic materials is proportional to the number of nitramine functional groups in the molecule. Based upon experimental observations and theoretical calculations of the potential energy surface for these systems, we suggest that energetic materials dissociate from ground electronic states after internal conversion from their first excited states, and model systems dissociate from their first excited states. In both cases a nitro-nitrite isomerization is suggested to be part of the decomposition mechanism. Parent ions of dimethylnitramine and nitropyrrolidine are observed in femtosecond experiments. All the other molecules generate NO as a decomposition product even in the femtosecond time regime. The dynamics of the formation of the NO product is faster than 180 fs, which is equivalent to the time duration of our laser pulse.     

Decomposition of excited electronic state s-tetrazine and its energetic derivatives

Decomposition of excited electronic state s-tetrazine and its energetic derivatives, such as 3-amino-6-chloro-1,2,4,5-tetrazine-2,4-dioxide (ACTO), and 3,3(')-azobis (6-amino-1,2,4,5-tetrazine)-mixed N-oxides (DAATO(3.5)), is investigated through laser excitation and resonance enhanced multi photon ionization techniques. The N(2) molecule is detected as an initial product of the s-tetrazine decomposition reaction, through its two photon, resonance absorption transitions [a(") (1)Σ(g)(+) (v(') = 0) ← X (1)Σ(g)(+) (v(") = 0)]. The suggested mechanism for this reaction is a concerted triple dissociation yielding rotationally cold (～20 K) ground electronic state N(2) and 2 HCN molecules. The comparable decomposition of excited electronic state ACTO and DAATO(3.5) yields an NO product with a cold rotational (～20 K) but a hot vibrational (～1200 K) distribution. Thus, tetrazine and its substituted energetic materials ACTO and DAATO(3.5) evidence different decomposition mechanisms upon electronic excitation. N(2)O is excluded as a potential intermediate precursor of the NO product observed from these two s-tetrazine derivatives through direct determination of its decomposition behavior. Calculations at the CASMP2∕CASSCF level of theory predict a concerted triple dissociation mechanism for generation of the N(2) product from s-tetrazine, and a ring contraction mechanism for the generation of the NO product from the energetic s-tetrazine derivatives. Relaxation from S(n) evolves through a series of conical intersections to S(0), upon which surface the dissociation occurs in both mechanisms. This work demonstrates that the substituents on the tetrazine ring change the characteristics of the potential energy surfaces of the derivatives, and lead to a completely different decomposition pathway from s-tetrazine itself. Moreover, the N(2) molecule can be excluded as an initial product from decomposition of these excited electronic state energetic materials.     

Ultrafast photodissociation dynamics of HMX and RDX from their excited electronic states via femtosecond laser pump–probe techniques

Femtosecond laser pump–probe techniques are employed to investigate the mechanisms and dynamics of the photodissociation of HMX and RDX from their excited electronic states at three wavelengths (230 nm, 228 nm, and 226 nm). The only observed product is the NO molecule. Parent HMX and RDX ions are not observed. The NO molecule has a resonant A²Σ ← X²Π (0, 0) transition at 226 nm and off-resonance two-photon absorption at 228 nm and 230 nm. Pump–probe transients of the NO product at both off-resonance and resonance absorption wavelengths indicate the decomposition dynamics of HMX and RDX falls into the timescale of our laser pulse duration (180 fs).     

Amine and nitramine functionalization of imidazole-triazine and triazole-triazine skeletons: Exploring for potential multipurpose energetic materials

A series of five and six-membered C-C bonded energetic materials ( 2 – 7 ) based on a combination of imidazole-triazine and triazole-triazine backbones were designed, synthesized, and characterized using NMR, IR, Mass spectrometry, and TGA-DSC studies. Further, the structure of compound 4 was supported by single-crystal X-ray analysis. All the newly synthesized energetic compounds exhibit good density, excellent thermal stability, good detonation performance, and low mechanical sensitivity toward impact and friction. Among all, the nitrate salt 4 exhibits balanced properties, including high density (1.80 g cm⁻³), excellent thermal stability (254°C), good detonation velocity (8178 m s⁻¹), and low sensitivity towards impact and friction. The facile synthetic feasibility, thermal stability, energetic performance, and insensitivity of all the molecules suggest they can be used as an insensitive secondary explosive in various defense and civilian applications.     

面向有机光电材料的电子激发态结构与过程的计算方法

共轭聚合物与有机分子材料中的电子激发结构与过程决定了材料的光电功能：根据Kasha规则,低能级激发态的排序决定能否发光;最低激发态至基态的辐射跃迁与无辐射跃迁之间的竞争决定了发光效率,后者主要由非绝热耦合（声子作用）决定;电荷激发态载体的传输由电子分布与振动耦合或杂质和无序的散射弛豫过程决定．本文针对有机功能材料的发光性能,介绍两种理论方法的研究进展,即可用于计算共轭聚合物激发态结构的量子化学密度矩阵重整化群方法和计算发光效率的多模耦合无辐射跃迁速率方法．这些方法被应用于有机功能材料的性能预测和分子设计中．     

Cyclodextrin-assisted capillary electrophoresis for determination of the cyclic nitramine explosives RDX,HMX and CL-20 comparison with high-performance liquid chromatography

A sulfobutyl ether-beta-cyclodextrin-assisted electrokinetic chromatographic method was developed to rapidly resolve and detect the cyclic nitramine explosives 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-isowurtzitane (CL-20), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and their related degradation intermediates in environmental samples. Development of the electrophoretic method required the measurement of the aqueous solubility of CL-20 which was determined to be 3.59 +/- 0.74 mg/l at 25 degrees C (95% confidence interval, n=3). The performance of the method was then compared to results obtained from existing high-performance liquid chromatography methods including US Environmental Protection Agency method 8330.     

The structures of molecules in excited electronic states

A rule originally due to Walsh for predicting the shapes of electronically excited molecules can also be derived from structure theory based on the second-order Jahn-Teller effect. The first excited state of a molecule containing n electrons should belong to the same point group as the ground state of a similar molecule containing n + 1 or n + 2 electrons. The rule is tested by examining recent information on structures of excited states.     

The low-lying electronic excited states of NiCO

Highly correlated coupled cluster methods with single and double excitations (CSSD) and CCSD with perturbative triple excitations were used to predict molecular structures and harmonic vibrational frequencies for the electronic ground state X 1Sigma+, and for the 3Delta, 3Sigma+, 3Phi, 1 3Pi, 2 3Pi, 1Sigma+, 1Delta, and 1Pi excited states of NiCO. The X 1Sigma+ ground state's geometry is for the first time compared with the recently determined experimental structure. The adiabatic excitation energies, vertical excitation energies, and dissociation energies of these excited states are predicted. The importance of pi and sigma bonding for the Ni-C bond is discussed based on the structures of excited states.     

Lifetime measurements of CuBr and CuI excited states: identification of the electronic states

Radiative decay of the fluorescence following resonant excitation by a pulsed dye laser has been used to measure the radiative lifetimes of excited electronic states of molecular CuBr and CuI. Comparisons are made with radiative lifetimes for CuF and CuCl, and symmetries for the (3d⁹'4s) Cu⁺ excited states assigned for the heavier CuBr and CuI copper halides.     

Dinitromethyl,fluorodinitromethyl derivatives of RDX and HMX as high energy density materials: a computational study

Structural Chemistry - The development of high energy density materials (HEDMs) with balanced detonation energy and sensitivity is an urgent task in the current energetic material field. Here, by...     

Theoretical study on initial decomposition paths of energetic materials featuring RDX ring

The molecular properties of RDX are affected by the introduction of different functional groups, and the decomposition process of these analogues is studied in this paper. DFT method is used to study the initial decomposition reaction paths of 30 high energy materials based RDX skeleton. In the nitro cleavage reaction, the energy barrier become relatively low by introducing CH(NO₂)₂ or  C(NO₂)₃ groups on the C site of the six membered ring. In the ring opening reaction, the ring opening process is easier to proceed by introducing  NH₂ or  NHNH₂ groups on the C site of the six membered ring.     

Vibrational relaxtion in excited electronic states of aromatic hydrocarbons

Time-resolved experiments are reported Showing kinetic evidence for vibrational relaxation of electronically excited molecules in solution at room temperature. The experiments involve higher electronic states of 3,4,9,10-dibenzpyrene. Data are consistent with slow vibrational relaxation (≈ 15 ps), similar to that for ground state species.     

Calculation of some electronic excited states of formaldehyde

The optimized MO's of several excited states of formaldehyde have been calculated by means of a large basis set of modified Gaussian functions; particular attention has been paid to the * transition. The total energy of the various states has been obtained as the sum of the SCF and correlation energies; the last one has been calculated as a functional of the electronic density. The calculated values for the transition energies are in good agreement with the experiment. A strong interaction of the * state with the continuum is evidentiated; this fact can justify the absence of the * band in the absorption spectrum.     

On the lower excited electronic states of biacetyl

An ab initio SCF and CI study has been carried out for the ground and electronically excited states of biacetyl (CH₃COCOCH₃). The second absorption band in the 4.40 eV region has been assigned to a ¹A_g → ¹B_g nπ^* transition The character of the lower-lying states has been analyzed in terms of the CI wavefunctions.     

Electron-phonon interactions in photoinduced excited electronic states in fluoroacenes

The electron-phonon coupling constants [l(B1u(HOMO-->LUMO))] in the photoinduced excited electronic states in fluoroacenes are estimated and compared with those in the monoanions (l(LUMO)) and cations (l(HOMO)). The l(B1u(HOMO-->LUMO)) values are much larger than the l(LUMO) and l(HOMO) values in fluoroacenes. Furthermore, the Coulomb pseudopotential mu* values for the excited electronic states are estimated to be smaller than those for the monoanions and cations. The complete phase patterns difference between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) is the main reason why the electron-phonon coupling constants and the mu* values are larger and smaller, respectively, in the photoinduced excited electronic states than in the monoanions and cations. The possible electron pairing and Bose-Einstein condensation in the excited electronic states of fluoroacenes are discussed. Because of larger electron-phonon coupling constants and smaller mu* values in the excited electronic states than in the charged states, the conditions under which the electron-electron interactions become attractive can be more easily realized, in principle, in the excited electronic states than in the charged states in fluoroacenes. The l(B1u(HOMO-->LUMO)) values hardly change by H-F substitution, even though the l(LUMO) and l(HOMO) values significantly increase by H-F substitution in acenes. Antibonding interactions between carbon and fluorine atoms in the HOMO and LUMO are the main reason why the l(B1u(HOMO-->LUMO)) values hardly change by H-F substitution in acenes.     

On the lower excited electronic states of glyoxal

The polarization of both nπ* absorption bands of glyoxal has been measured in a glass matrix of 2-methyltetrahydrofuran by the photoselection method. The second absorption band in the 30 000 cm^?1 region has been assigned to a ¹A_g → ¹B_g nπ* transition. Its intensity is mainly induced by interaction with the solvent. An absorption band at about 43 000 cm^?1 has been ascribed to a charge transfer transition in complexes of glyoxal and 2-MTHF.     

The excited electronic states of all-trans-1,3,5-hexatriene

Extensive configuration interaction calculations based on ab initio wavefunctions including diffuse basis functions are reported for all-trans-1,3,5-hexatriene. Using these results we have assigned the one-photon spectra of Gavin and Rice and the electron-impact spectra of Kuppermann, and we have confirmed the assignment of the two-photon spectra of El-Sayed. The valence 2 ¹A_g state is found to lie above the strongly allowed valence 1 ¹B_u state.     

Second-derivative method: Application to vibrational spectroscopy in excited electronic states

The second-derivative method, which has been known to be useful for finding peaks overlapped in a broad band, is applied to u.v. and visible absorption bands to derive vibrational frequencies in excited electronic states of molecules in solution. By using this method, much better estimates of vibrational frequencies are possible for Franck—Condon active modes than can be made from the absorption spectrum itself. Computational procedure and the results obtained for trans-1,3,5-hexatriene, azulene, pyrazine and anthracene are described with remarks about the effectiveness and limitations of the second-derivative method.