聯氨與四氧化二氮自燃反應機制的研究

Abstract

本研究討論聯氨與四氧化二氮的自燃反應機制,此反應物常用於火箭推進的燃料,至目前為止,仍無實驗或是理論計算的期刊解釋此反應機制. 因為四氧化二氮具有多種同份異構物,並且可能解離成二氧化氮分子,因此在此研究中利用量子化學理論於Gaussian03軟體計算N2O4(D2h),trans-ONONO2,cis-ONONO2,以及NO2與聯氨的雙分子反應. 藉由計算結果,我們預測此自燃反應是經由trans-ONONO2+N2H4的無能障反應所引發,此反應路徑提供快速的反應速率,並且其反應產物H2NN(H)NO分子可以分離成N2H3與NO高活性分子以進行後續的反應,此分離能為27.83 kcal/mol. 在所有的雙分子反應中,均以氫原子轉移反應為主要可能發生的途徑,其所需要跨越的能障較低.

Hydrazine and dinitrogen tetroxide reaction is a spontaneously ignited combustion reaction, and the propellants are practically used in the space shuttle of NASA. However, the mechanism of this hypergolic combustion system is still unknown theoretically and experimentally. In the practical system, the reactants are not only composed of N2H4(C2) and N2O4(D2h) molecules, and they also contain trans-ONONO2(Cs), cis-ONONO2(Cs) and NO2(C2v) molecules which can react with hydrazine when the liquid N2O4 oxidizer is ejected into reaction chamber with high velocity. We consider these four bimolecular reactions by using ab-initio calculations with the Gaussian03 code. The geometries of all stationary points on the potential energy surfaces (PESs) are optimized by B3LYP/6-311++G(3df,2p). Moreover, the single-point calculations, including G3B3, CCSD(T), and G2M methods, correct the relative energies to give better values for the kinetic calculations. The G2M method provides the good prediction as CCSD(T)/6-311++G(3df,2p), and it also has smaller differences comparing with experimental data. The G2M(CC1) and G2M(CC3) schemes are chosen for the smaller and larger reaction system, respectively. The geometries of cis- and trans-ONONO2 molecules are unknown experimentally. The PES of N2O4 isomerization has been studied; the energy barrier between the cis-isomer and N2O4(D2h) is lower than that between the trans-isomer and N2O4(D2h) by 20.15 kcal/mol. The energy of transition state (TS) between cis- and trans-ONONO2 is only 1.71 kcal/mol; thus cis-trans isomerization can occur rapidly. In the bimolecular reactions, the major and lower-energy channels are the hydrogen abstraction reactions, and the mechanism for breaking the N-O bond of NO2 or N2O4 molecule needs much higher energies. In the N2H4 + NO2 reaction, the lowest-energy channel produces the N2H3 and cis-HONO via the TS with a 7.57 kcal/mol barrier. The rate constant predicted by transition state theory is 3.20×10-25T3.74exp(-1662.5/T) cm3/(molecule．sec) at 100K - 4000K. In the N2H4 + N2O4 reaction, the hydrogen transfer reaction accompanies with bonds breaking and forming via a ring-like TS. The energy barriers of transition state in most of the reaction channels are higher than 10 kcal/mo; however, the N2H4 + trans-ONONO2 reaction occurs by hydrogen transfer via a six-member ring TS without an intrinsic barrier. In this lowest-energy channel, the energy of transition state is predicted to be lower than intermediate by 2.66 kcal/mol at the G2M(CC3) level. However the difference is only 0.16 kcal/mol at the B3LYP/6-311++G(3df,2p) level, which is confirmed by IRC calculation; therefore, it may be caused by the zero-point energy correction and computational errors. This lowest-energy channel produces HONO2 + H2NN(H)NO with 15.57 kcal/mol exothermicity. Moreover, another lower-energy channel of N2H4 + N2O4(D2h) reaction is also exothermic, and the products are trans-HONO and H2NN(H)NO2. Furthermore, the decomposition reactions of H2NN(H)NO and H2NN(H)NO2 molecules provide another possibility producing the active species. The direct dissociation reaction to form N2H3 and NO radicals is the most possible path for H2NN(H)NO molecule, where it needs to overcome a small rotation TS with 6.28 kcal/mol barrier height and undergoes the dissociation reaction with 27.83 kcal/mol energy. On the other hand, the H2NN(H)NO2 can overcome a five-member ring TS, with a barrier is 26.6 kcal/mol barrier, and decompose to trans-N2H2 and trans-HONO. Besides, it is also possible to directly dissociate to N2H3 and NO2 with 38.23 kcal/mol energy. The hypergolic reaction of N2O4 with N2H4 is therefore believed to be initiated by the reaction of trans-ONONO2 with N2H4 which is shown to occur barrierlessly.