Direct Dynamics Study on the Reaction of N₂H₄ with F Atom:  A Hydrogen Abstraction Reaction?

Abstract

We present a systematic direct ab initio dynamics investigation of the reaction between N2H4 and F atom, which is predicted to have three possible reaction channels. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels were calculated at the UB3LYP/6-31+G(d,p) level of theory. Energetic information of stationary points and the points along the MEPs was further refined by means of the CCSD(T)/aug-cc-pVTZ method. The calculated results revealed that the first two primary channels (N2H4+F-->N2H3+HF) are equivalent and occur synchronously via the formation of a pre-reaction complex with Cs symmetry rather than via the direct H abstraction. The pre-reaction complex then evolves into a hydrogen-bonding intermediate through a transition state with nearly no barrier and a high exothermicity, which finally makes the intermediate further decompose into N2H3 and HF. Another reaction channel of minor role (N2H4+F-->NH2F+NH2) was also found during the calculations, which has the same Cs pre-reaction complex but forms NH2F and NH2 via another transition state with high-energy barrier and low exothermicity. The rate constants of these channels were calculated using the improved canonical variational transition state theory with the small-curvature tunneling correction (ICVT/SCT) method. The three-parameter ICVT/SCT rate constant expressions of k(ICVT/SCT) at the CCSD(T)/aug-cc-pVTZ//UB3LYP/6-31+G(d,p) level of theory within 220-3000 K were fitted as (7.64x10(-9))T (-0.87) exp(1180/T) cm3 mole-1 s-1 for N2H4+F-->N2H3+HF and 1.45x10(-12)(T/298)(2.17) exp(-1710/T) cm3 mole-1 s-1 for N2H4+F-->NH2F+NH2.

Ab Initio Study of the F + CH3NHNH2 Reaction Mechanism

“…In previous studies, CH 3 NHNH 2 has two kinds of potential binding sites in the course of reactant’s attacking: one is H atoms in amino group or imido group, and the other is N atoms . In the present research, the attempts of directly attacking H atoms in amino group or imido group with an F atom have been tried by various means, but all failed.…”

“…The most commonly used space-storable bipropellant is the mixture of fluorine and hydrazine, which can provide desirable propulsion for a wide variety of unmanned planetary explorations owing to its large specific impulse ( I sp ) and high condensed density . To accurately model the combustion of F 2 /N 2 H 4 bipropellants, extensive experimental and theoretical research has been performed to investigate the decomposition mechanisms and the kinetics of the involved reactions in the past several decades. − But N 2 H 4 has its own limitations, such as less thermodynamic stability and less remaining-liquid-state ability over a wide temperature range. With the substitution of methyl for one hydrogen in N 2 H 4 , the formed compound methyl hydrazine (CH 3 NHNH 2 ) is stable in thermodynamics and able to remain in the liquid state over a wide temperature range, thus, it can be used as an important space propellant.…”

The atmospheric relevance of primary alcohols and imidogen reactions

“…Therefore, hydroxyl radicals as a key atmospheric scavenger are the main sink for alcohols in the air. Also, it has been proved that other species such as HO 2 72 – 75 , O 76 – 81 , O 2 82 – 84 , O 3 85 – 88 , N 89 , 90 , H 91 – 93 , F 94 – 98 , Cl 95 , 99 – 102 , NH 103 – 106 , NH 2 107 , 108 , NO 109 , 110 , H 2 O 111 , 112 , NO 2 109 , 113 , 114 , NO 3 109 , 115 – 118 , CH 3 72 , 119 , 120 and Criegee intermediates 121 , 122 have good contributions in eliminating atmospheric pollutants. Therefore, we must remember this point in mind that the role of other oxidants like NH can not be ignored because, in the absence of hydroxyl radicals, the principal task of these species is the elimination of pollutants from the air of living environments.…”