ARTICLE pubs.acs.org/JPCA
Ab Initio Study of the F þ CH3NHNH2 Reaction Mechanism Nana Ding,† Qiong Luo,*,† and Qian Shu Li†,‡ † ‡
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, China Center for Computational Quantum Chemistry, South China Normal University, Guangzhou, 510631, China
bS Supporting Information ABSTRACT: The F þ CH3NHNH2 reaction mechanism is studied based on ab initio quantum chemistry methods as follows: the minimum energy paths (MEPs) are computed at the UMP2/6-311þþG(d,p) level; the geometries, harmonic vibrational frequencies, and energies of all stationary points are predicted at the same level of theory; further, the energies of stationary points and the points along the MEPs are refined by UCCSD(T)/6-311þþg(3df,2p). The ab initio study shows that, when the F atom approaches CH3NHNH2, the heavy atoms, namely N and C atoms, are the favorable combining points. For the two N atoms, two prereaction complexes with Cs symmetry are generated and there exists seven possible subsequent reaction routes, of which routes 1, 2, 5, and 7 are the main channels. Routes 1, 2, and 5 are associated with HF elimination, with H from the amino group or imido group, and route 7 involves the N-N bond break. Routes 3 and 6 with relation to HF elimination with H from methyl, and route 4 involved the C-N bond break, are all energetically disfavored. For the C atom, the attack of F results in the break of the C-N bond and the products are CH3F þ NHNH2. This route is very competitive.
investigated.11-22 Chemical kinetics mechanisms for modeling the gas-phase combustion processes of CH3NHNH2/(NO2)2 and CH3NHNH2/RFNA have been developed by the U.S. Army Research Laboratory.11-15 One concern about the full mechanism was the abstraction of the H atom in the imido group from CH3NHNH2 by NO2 to form CH3NNH2 and HONO. According to Ishikawa and co-workers' report in 2006,21 the H-atom abstraction from CH3NHNH2 by NO2 took place via H atoms in amino group or imido group. In all systems, H-abstractions route of the reaction CH3NHNH2 þ NO2 were identified as playing a critical role in the preignition phase of the combustion process.15-21 In Ishikawa's research,21 two low-energy conformational isomers of CH3NHNH2 were observed and characterized. The higherenergy conformer was 0.7 kcal mol-1 above the ground state. The two low-energy conformational isomers of CH3NHNH2 were also investigated in the present study. Our theoretical study reveals that these two isomers reacting with F atom at the N atoms produce the same prereaction complexes. Therefore, only the ground state will be discussed for the sake of simplicity and clarity.
1. INTRODUCTION Hydrazine (N2H4), methylhydrazine (CH3NHNH2), and unsymmetrical dimethylhydrazine ((CH3)2NNH2) have attracted much attention for their important applications in rocket fuels.1 Those diamine-based fuels are favorite deoxidizers for bipropellant rockets due to their notable advantages of highreaction capabilities with various oxidizers, high energy density, low molecular weight of the combustion products, and no special requirement for the production bases. The most commonly used space-storable bipropellant is the mixture of fluorine and hydrazine,2 which can provide desirable propulsion for a wide variety of unmanned planetary explorations owing to its large specific impulse (Isp) and high condensed density.3 To accurately model the combustion of F2/N2H4 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.4-10 But N2H4 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 N2H4, the formed compound methyl hydrazine (CH3NHNH2) 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. However, as far as we know, the combustion of fluorine and methyl hydrazine (CH3NHNH2) has not drawn much attention. On the other hand, the mixture of N2O4/CH3NHNH2, the conventional earth-storable bipropellant,2 has been widely r 2010 American Chemical Society
2. AB INITIO CALCULATION METHODS The geometries of the stationary points were fully optimized using traditional UMP223 method with the 6-311þþG(d,p) Received: July 11, 2010 Revised: December 7, 2010 Published: December 30, 2010 805
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Figure 1. Schematic potential energy profile for the reaction of CH3NHNH2 þ F along with the relative energies (kcal mol-1, without zero-point energy correction) of all stationary points at the UCCSD(T)//UMP2 and CR-CC (2,3)//UMP2 levels of theory. The results in parentheses are calculated by CR-CC (2,3) method. The solid lines represent the main reaction routes.
basis set,24 as well as UMPWB1K25-27 and UCCSD26-31 methods with the 6-31þG(d,p) basis set. Frequencies of all stationary points were also predicted by the Hessian computed numerically at the same levels of theory. Single-point calculations were refined for all stationary points by UCCSD(T)/6-311þþg(3df,2p)32,33 method based on the optimized geometries at the UMP2/6-311þþG(d,p) and UMPWB1K/6-31þG(d,p) levels, denoted as UCCSD(T)//UMP2 and UCCSD(T)//UMPWB1K, respectively. In addition, for the purpose of comparison, the energies of stationary points were also refined by single-point multilevel energy calculation (UHL)34 and CR-CC(2,3)35 methods, namely, UHL//UMPWB1K, UHL//UMP2, and CR-CC(2,3)//UMP2. The information on the geometries and frequencies obtained by UMP2, UMPWB1K, and UCCSD methods are in good agreements. Considering efficiency and authenticity synthetically, the minimum energy paths (MEPs) of this reaction were only predicted at the UMP2/6-311þþG(d,p) level of theory using the intrinsic reaction coordinate (IRC) theory36 in mass-weighted Cartesian coordinates with a step size of 0.08 (amu)1/2 bohr. At the selected points along the MEPs, the harmonic vibrational frequencies were calculated using UMP2/6-311þþG(d,p) method. Further, UCCSD(T) method was employed to recalculate the energies of the selected points along the MEPs. All the above calculations were performed using the Gaussian 03 program suite,37 except for the CR-CC(2,3) calculations which were carried out by the GAMESS package.38-40
potential combining sites for F atom. When the F atom approaches the N atom of the imido or amino group in CH3NHNH2, the prereaction complex CMR1 or CMR2 (Figures 1 and 2) is formed, respectively. An elementary reaction, relative to the break of C-N bond in CH3NHNH2 to produce CH3F þ NHNH2 via TS3-1 (Figures 1 and 3) appears while the F atom nears the C atom in CH3NHNH2. There are 8 channels in the reaction of F þ CH3NHNH2, shown as follows:
Routes 1-4 are developed by the complex CMR1, while routes 5-7 are developed by the complex CMR2. Route 8 is an elementary reaction via the transition state, TS3-1. The optimized geometries of the reactants, complexes, and products involved in the reaction FþCH3NHNH2 at the UMP2/6311þþG(d,p) level of theory are depicted in Figure 2. Likewise, the optimized geometry and harmonic vibrational frequency with negative eigenvalue of the transition states at the UMP2/6-311þþG(d,p) level of theory are shown in Figure 3. The reaction energies (ΔV), classical potential barriers (VMEP), vibrationally adiabatic ground-state potentials (VaG), and reaction enthalpies (ΔH298K) of routes 1-8 by UMP2 and UCCSD(T)//UMP2 are listed in Table 1. The energetic profile for the potential energy surface of the reaction of CH3NHNH2 þ F at the UCCSD(T)//UMP2 level of theory is shown in Figure 1 with the ground-state energy of CH3NHNH2 plus that of the F atom arbitrarily taken as zero. For comparison, the corresponding
3. RESULTS AND DISCUSSION In previous studies, CH3NHNH2 has two kinds of potential binding sites in the course of reactant's attacking: one is H atoms in amino group or imido group,21 and the other is N atoms.10 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. However, in reaction F þ CH3NHNH2, the three heavy atoms in CH3NHNH2 are the 806
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Figure 2. Optimized structures for reactants, complexes and products for the reaction of CH3NHNH2 þ F system at the UMP2/6-311þþG(d,p) level of theory. P2-2 and P1-3 are the optical isomers. The bond lengths shown are given in Å.
relative energies predicted by the CR-CC(2,3)//UMP2 method are also listed in Figure 1. It is clear that the difference between the results from the UCCSD(T)//UMP2 method and the CR-CC(2,3)//UMP2 method is very little.
It should be noted that the following discussion is based on the UMP2/6-311þþG(d,p) and UCCSD(T)//UMP2 levels of theory. Those predicted by UMPWB1K/6-31þG(d,p), UCCSD(T)//UMPWB1K, UHL//UMPWB1K, and 807
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Figure 3. Optimized structures and the lowest vibrational frequencies (νmin) of transition states for the reaction of CH3NHNH2 þ F system at the UMP2/6-311þþG(d,p) level of theory. The bond lengths and frequencies shown are given in Å and cm-1, respectively.
Table 1. Reaction Energetic Parameters (kcal mol-1) at UMP2 and UCCSD(T)//MP2 Levels of Theory in Reaction Routes ΔVa
VMEPb
UCCSD(T) // UMP2
UMP2
CH3NHNH2 þ F f CMR1
-24.8
-26.0
CMR1 f CMP1
-41.5
-40.7
CMP1 f P1-1 þ HF
13.5
12.9
CMP1 f CM1-2
1.5
1.9
UCCSD(T) // UMP2
3.0
UMP2
0.0
ΔH298Kd
VaGc UCCSD(T) UMP2
0.4
//UMP2 -2.6
20.5
21.0
19.3
19.8
47.7
45.8
45.7
43.8
CM1-2 f P1-2 þ HF
13.8
13.1
CMP1 f CM1-3
17.9
19.1
CM1-3 f P1-3 þ HF
7.5
7.7
CMR1 f P1-4 þ P1-5 CH3NHNH2 þ F f CMR2
34.3 -19.3
31.9 -21.4
55.6
45.5
51.5
UCCSD(T) UMP2
//UMP2
-22.5
-23.7
-44.3
-43.5
11.7
11.2
1.9
2.2
12.1
11.3
17.8
18.9
6.3
6.1
41.5
28.4 -15.5
26.0 -17.6 -47.4
CMR2 f CM1-2
-45.2
-43.5
1.5
-0.3
-2.6
-4.5
-49.1
CM1-2 f CM2-2
16.8
17.2
46.9
42.5
44.3
39.9
16.3
16.7
CM2-2 f P2-2 þ HF
7.1
7.7
5.7
6.3
CMR2 f CMP2-2
17.7
14.1
CMP2-2 f P2-3 þ P2-4
5.6
5.0
CH3NHNH2 þ F f P3-1 þ P3-2
-38.1
-42.5
38.1
27.9
9.2
5.3
33.9 8.3
23.7 4.4
12.5
9.0
4.2
3.6
-39.5
-43.8
a
Reaction energy without zero-point energy correction. b Classical potential barrier calculated at corresponding saddle points. c Vibrationally adiabatic ground-state potentials at corresponding saddle points. d Reaction enthalpies at 298 K.
A. Stationary Points. In Figure 2, the distance of F10-N2 in CMR1 is 1.987 Å, a little longer than the normal F-N covalent bond length (1.51 Å) but much shorter than the sum of the van der Waals radii of F and N atoms (2.85 Å), which indicates that F is bound by weak interaction that would be overcome easily. The structure of TS1-11 is very similar to that of the complex CMR1, only with a bit turning of the F atom from the N2 atom to the H6 atom for the convenience of forming the F-H bond.
UHL//UMP2 methods are given in the Supporting Information (SI). 3.1. Route 1. Route 1 consists of three steps: (i) CH3NHNH2 þ F f CMR1, which leads to a complex between methylhydrazine and the fluorine atom; (ii) CMR1 f CMP1, in which F atom turns toward the H6 atom with the forming of the F-H6 bond and the breaking of the F-N2 bond; (iii) CMP1 f P1-1 þ HF, where the bond of N2-H6 is split directly (Figures 2 and 3). 808
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As to the complex CMP1, the bond length of F10-H6 (0.950 Å) is close to the normal F-H covalent bond length (0.92 Å), while the H6-N2 distance (1.666 Å) is much longer than the H6-N2 bond length (1.014 Å) in the CH3NHNH2. It is suggested that CMP1 is a hydrogen-bonding complex. The optimized geometrical parameters of P1-1 are much similar to the corresponding parameters of CMP1. Step 1 is exothermic and has no potential barrier, with ΔH298K of -23.7 kcal mol-1 (Table 1). Why is the N atom in CH3NHNH2 the favorable combination point for the F atom? The reason may be the N atom has a lone pair of electrons and the F atom has strong electronegativity, which leads the spontaneous formation of the CMR1 when the F atom and CH3NHNH2 approach mutually. For CMR1 f CMP1, the conversion experience a transition state TS1-11 of 773i cm-1. However, there is nearly no barrier. In the decomposition process CMP1 f P1-1 þ HF, there also do not find barrier. The ΔH298K is 11.2 kcal mol-1 (Table 1), which are consistent with the energy of N-HF hydrogen bonding.41 B. Reaction Path Properties. In route 1, the step CH3NHNH2 þ F f CMR1 is a spontaneous formation process, and CMP1 f P1-1 þ HF is a direct decomposition process. Here, the discussion of reaction path properties focuses on the process of CMR1 f CMP1. The step (ii) CMR1 f CMP1 is a transfer process of F atom from N2 atom to H6 atom, in which the N2-F10 distance will increase continually along the reaction. Thus, the distance of N2-F10 can be taken as a distinguished reaction coordinate. In addition, this reaction has no potential barrier at the UCCSD(T)// UMP2 level. So a function of the N2-F10 distance, instead of the reaction coordinate, is adopted for the reactive moiety of the corresponding minimum energy paths (MEPs).42 Therefore, the bond distance variations, the generalized normal-mode vibrational frequency changes, and the variations of both the classical potential energy (VMEP) and the ground-state vibrationally adiabatic potential energy (VaG) in CMR1 f CMP1 as a function of the N2-F10 bond distance are depicted in Figure 4a, b, and c, respectively. As is clearly seen in Figure 4a, when the N2-F10 distance increases from 2.10 to 2.30 Å, the bond lengths of the forming H6-F10 bond and the breaking N2-H6 bond change most intensively. While among all the vibration frequencies, the vibration mode 1 represented by a solid line (Figure 4b) also changes most intensively in the same range, which drops and increases corresponding to a combination vibration of the H6-N2 stretching and F10-H6 stretching. Meanwhile, the curves of both potential energies are strictly downhill shaped at the range from 2.10 to 2.30 Å. It can be confirmed that, the step CMR1 f CMP1 is a transfer process of F atom from N2 atom to H6 atom and the reaction site in the step CMR1 f CMP1 is from 2.10 to 2.30 Å of the N2-F10 distance. 3.2. Route 2. Reaction route 2 consists of four steps: (i) CH3NHNH2 þ F f CMR1; (ii) CMR1 f CMP1; (iii) CMP1 f CM1-2; (iv) CM1-2 f P1-2 þ HF. The first two steps are the same as those of route 1. The third step experiences a 1,2 shift of F across H6 and H4 atoms in CH3NHNH2, and the fourth step resembles the process of CMP1 f P1-1 þ HF (Figures 2 and 3). Therefore, only the third step will be discussed in detail. A. Stationary Points. As shown in Figure 3, the active bonds of the transition state TS1-21 for the transformation CMP1 f CM1-2 are F10-H6 and F10-H4 bonds with bond length of 1.480 and 1.510 Å, respectively. The geometric parameters of the rest part of TS1-21 are much similar to those of the CH3NHNH2. The conversion from CMP1 to CM1-2 undergoes a classical potential
Figure 4. Variations of the bond lengths (Å), the generalized normalmode vibrational frequencies (cm-1) and both the VMEP and VaG (kcal mol-1) in the reaction CMR1 f CMP1 as a function of the N2-F10 bond distance (Å): (a) the bond lengths (Å), (b) the vibrational frequencies (cm-1, excluding the imaginary one), (c) the VMEP and the VaG (kcal mol-1).
barrier (VMEP) of 21.0 kcal mol-1 at both UCCSD(T)//UMP2 and CR-CC(2,3)//UMP2 levels of theory (Table 1 and Figure 1). B. Reaction Path Properties. As mentioned above, only the reaction path properties of CMP1 f CM1-2 in this reaction route is discussed. Figure 5 depicts the bond lengths variations (Å), generalized normal-mode vibrational frequencies (cm-1) 809
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changing most strongly among all the vibration frequencies. The frequency of the vibrational mode 1 drops at s = -4.0 (amu)1/2 bohr and then increases from s = -0.5 to 1.5 (amu)1/2 bohr, corresponding to a combination vibration of the H6-F10 and H6-N2 stretching along the IRC path. The vibration of the mode 2 in the range of s = -1.0 to 3.0 (amu)1/2 bohr represents a combination vibration of the H4-N3 and H4-F10 stretching. Moreover, the curved shapes of both VMEP and VaG are very close to each other, and both of them change intensively in the range of s = -4.0 to 3.0 (amu)1/2 bohr. 3.3. Route 3. The reaction route 3 consists of four steps, the first two of which are common; the third step, CMP1 f CM1-3, is the characteristic reaction; the fourth step is also a no transition state process to obtain HF þ P1-3 (Figure 2). The characteristic step of route 3, CMP1 f CM1-3, is an isomerization, in which F atom shift from H4 to H9, which belongs to methyl. The isomerization from CMP1 to CM1-3 has to overcome a high barrier (VMEP) of 45.8 kcal mol-1 (Table 1). In addition, the conversion is endothermicity with ΔH298K of 18.9 kcal mol-1 (Table 1). This energy information indicates that the CMP1 f CM1-3 reaction is obviously far from competitive. 3.4. Route 4. In route 4, there is just one sequent step after CMR1, namely, CMP1 f P1-4 þ P1-5, which is associated with the C1-N2 bond split. The barriers (VMEP) and reaction enthalpy (ΔH298K) of the decomposition reaction are 45.5 and 26.0 kcal mol-1, respectively, indicating that it is rather energetically undesirable (Table 1). As mentioned above, the consecutive reaction routes 1-3 have the common two first steps. From CMP1, all three processes could be considered as the HF elimination reactions, while the difference is the origin of the H atom is different. Obviously, route 1 is the most competitive, while route 3, the elimination with H atom from methyl, has little opportunity to occur. Route 4 involved the C1-N2 bond rupture in CMR1, which is also not favored because of the high barriers more than 40 kcal/mol. 3.5. Routes 5-7. These three reactions are developed from CMR2, produced by the F atom attacking the N atom of the amino group in CH3NHNH2. For routes 5 and 6, the subsequent reaction is the same rearrangement reaction, CMR2 f CM1-2, with the shift of F atom across the N3-H4 bond. From CM1-2, the two routes are also HF elimination reactions. The direct HF elimination occurred in route 5 is the same as the last step in route 2, whereas in route 6, CM1-2 experiences another rearrangement reaction, CM1-2 f CM2-2, with a transformation of the F atom from H4 to the methyl hydrogen H8, which is followed by a direct HF elimination similar to the fourth step in route 3. Moreover, the product P2-2 (CH2NHNH2) in route 6 and the product P1-3 (CH2NHNH2) in route 3 are the optical isomers. In route 7, the reaction CMR2 f CMP2-2 can be described as a conversion from the complex between F and CH3NHNH2 to the complex between NH2F and CH3NH (Figures 2 and 3). The CH3NHNH2 þ F f CMR2 is a spontaneous reaction with the ΔH298K of -17.6 kcal mol-1, which is less than that of CH3NHNH2 þ F f CMR1 by about 6.0 kcal mol-1 (Table 1). In the endothermic process of CMP2-2 f NHCH3 þ NH2F, the N2-N3 is straightly broken with ΔH298K of 3.6 kcal mol-1 (Table 1). The classical potential barrier (VMEP) of CMR2 f CM1-2, CM1-2 f CM2-2, and CMR2 f CMP2-2 are -0.3, 42.5, and 27.9 kcal mol-1, respectively. Therefore, the elementary reaction CM1-2 f CM2-2 involved in route 6 is not favorable. Thus, this section discusses only the processes of CMR2 f CM1-2 and CMR2 f CMP2-2.
Figure 5. Variations of the bond lengths (Å), the generalized normalmode vibrational frequencies (cm-1), and both the VMEP and VaG (kcal mol-1) in the reaction CMP1 f CM1-2 along the IRC coordinate s ((amu)1/2 bohr): (a) the bond lengths (Å), (b) the vibrational frequencies (cm-1, excluding the imaginary one), (c) the VMEP and the VaG (kcal mol-1).
changes and the variations of VMEP and VaG along the IRC coordinate s ((amu)1/2 bohr) in the reaction CMP1 f CM1-2. As is shown, the bond lengths of the forming bond F10-H4 and N2-H6 and the breaking bond F10-H6 and H4-N3 change most intensively in the range of s = -4.0 to 3.0 (amu)1/2 bohr. Meanwhile, as the function of s increases from -4.0 to 3.0 (amu)1/2 bohr, the vibration modes 1 and 2 represented by solid lines are also 810
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Figure 7. Variations of the bond lengths (Å), the generalized normalmode vibrational frequencies (cm-1) and both the VMEP and VaG (kcal mol-1) in the reaction CMR2 f CMP2-2 along the IRC coordinate s ((amu)1/2 bohr): (a) the bond lengths (Å), (b) the vibrational frequencies (cm-1, excluding the imaginary one), (c) the VMEP and the VaG (kcal mol-1).
Figure 6. Variations of the bond lengths (Å), the generalized normalmode vibrational frequencies (cm-1) and both the VMEP and VaG (kcal mol-1) in the reaction CMR2 f CM1-2 as a function of the N3-F10 bond distance (Å): (a) the bond lengths (Å), (b) the vibrational frequencies (cm-1, excluding the imaginary one), (c) the VMEP and the VaG (kcal mol-1).
is very similar to that of the complex CMR2, except for a bit turning of the F atom from the N3 atom to the H4 atom. The F10-N3-H4 angle changes from 90.1 to 75.7° for the convenience of forming the F10-H4 bond. The transition state TS2-12 for CMR2 f CMP2-2 has the optimized active bond length of N2-N3 of 1.791 Å, which is longer than the normal N-N covalent bond length in complex CMR2 by
A. Stationary Points. As shown in Figure 2, the optimized N3-F10 distance in the complex CMR2 is 2.028 Å, in the range of the normal F-N covalent bond length (1.51 Å) and the sum of the van der Waals radii (2.85 Å), implying that the interaction between N3 and F10 atoms is not as strong as a normal covalent bond. For the reaction CMR2 f CM1-2, the optimized geometry of TS2-11 811
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This indicates that when the N atom in amino group is attacked by an atom with high electronegativity, such as F atom, the strength of chemical bond N-N in CH3NHNH2 might be reduced. B. Reaction Path Properties. As Figure 6 shown, during the transformation process of the F atom from N3 atom to H4 atom, CMR2 f CM1-2, when the N3-F10 bond distance increases from 2.05 to 2.25 Å, the bond lengths of the forming bond H4-F10 and the breaking bond N3-H4 change most intensively. While, at the same range, the vibration mode 1 represented by a solid line is also changing most intensively, which drops and increases corresponding to a combination vibration of the H4-N3 stretching and F10-H4 stretching. Meanwhile, the curves of both the classical potential energy and the vibrationally adiabatic ground-state potential energy are strictly downhill shaped. It is clear that the reaction site in CMR2 f CM1-2 is from 2.05 to 2.25 Å of the N3-F10 distance. The reaction path properties of the homolytic dissociation reaction, CMR2 f CMP2-2, is shown in Figure 7. The reactant complex between F and CH3NHNH2 undergoes the N2-N3 bond breaking and the N3-F10 bond forming via TS2-12 leading to another complex between NH2F and CH3NH. The effective reaction region for CMR2 f CMP2-2 is s = -2.0 to 1.5 (amu)1/2 bohr along the IRC path. 3.6. Route 8. When F approaches C atom in CH3NHNH2, the forming of the C1-F10 bond results in the breaking of the C1-N2 bond in CH3NHNH2. The elementary reaction takes place via TS3-1 (Figures 1 and 3). The geometrical parameters of CH3F predicted at the UMP2/6311þþG(d,p) level of theory is in good agreement with the corresponding experimental values,43 with the difference of 0.01 Å for C-F bond and 0.02 Å for C-H bond, respectively. The conversion in CH3NHNH2 þ F f P3-1 þ P3-2 experiences a transition state TS3-1 with a small potential barrier (VMEP) of 5.3 kcal mol-1 and is exothermic highly with ΔH298K of -43.8 kcal mol-1 (Table 1). This energy information indicates that the CH3NHNH2 þ F f P3-1 þ P3-2 route would be one of the main reaction channel. Two reasons could be contributed to the energetical favorite of route 8. First, the NBO analysis44 reveals that C atom in CH3NHNH2 is negatively charged. It is attracted to the atom with strong electronegativity such as F atom. The electron-attractive ability of F would lead to a reduction in the strength of the C1-N2 in CH3NHNH2. Second, previous study showed that CH3F is very kinetically stable upon decomposition to CH3 þ F, CHF þ H2, or CH2 þ HF with berries over 90 kcal mol-1.45 The extraordinary stability of CH3F might be beneficial to the forward reaction of CH3NHNH2 þ F f P3-1 þ P3-2 to form CH3F. As shown in Figure 8, the function of s from -1.5 to 4.0 (amu)1/2 bohr is the effective reaction region.
4. SUMMARY The ab initio quantum chemistry method is adopted to study the reaction mechanism of CH3NHNH2 with F. For the F atom with high electron affinity, the negatively charged heavy atoms, C and N in CH3NHNH2 are the favorable combination positions. For N atoms, the F atom is attracted to produce the complex CMR1 or CMR2 spontaneously and exothermally. Then the complexes CMR1 and CMR2 develop seven reaction routes via different transition states, where route 4 involves the C-N bond breaking and route 7 is associated with the N-N bond breaking; all other routes relate to HF elimination, as concluded as follows: routes 1, 2,
Figure 8. Variations of the bond lengths (Å), the generalized normalmode vibrational frequencies (cm-1) and both the VMEP and VaG (kcal mol-1) in the reaction CH3NHNH2 þ F f P3-1 þ P3-2 along the IRC coordinate s ((amu)1/2 bohr): (a) the bond lengths (Å), (b) the vibrational frequencies (cm-1, excluding the imaginary one), (c) the VMEP and the VaG (kcal mol-1).
about 0.4 Å, while the bond length of F10-N3 decreases to 1.547 Å, shorter than that in CMR2 by about 0.5 Å. For the complex CMP2-2, the N2-N3 distance is 2.897 Å, much longer than that in the transition state TS2-12; while the F10-N3 bond length is 1.425 Å, a little shorter than that in the transition state TS2-12. 812
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and 5 involve H atoms bonding to N atoms, and routes 3 and 6 are concerned with H atom from methyl. The combination between F and C in CH3NHNH2 produces CH3F and NHNH2 via a transition state TS3-1. The main routes of the reaction of CH3NHNH2 with F are routes 1, 2, 5, 7, and 8. This theoretical research would shed light to the combustion of fluorine and methyl hydrazine and would be beneficial for designing rocket motors fueled with hypergolic bipropellants.
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’ ASSOCIATED CONTENT
bS
Supporting Information. The relative energies without zero-point energy correction for various species at the UMPWB1K, UMP2, UCCSD, UCCSD(T)//UMPWB1K, UHL//UMPWB1K, and UHL//UMP2 levels of theory are summarized in Table S1. Tables S2-S4 list the optimized geometries of reactants, complexes, transition states, and products for the reaction routes 1-8 at the UMPWB1K/6-31þG(d, p), UMP2/6-311þþG(d,p), and UCCSD/6-31þG(d,p) levels of theory. Tables S5-S7 list the harmonic vibrational frequencies and zero point energies (ZPE) for the reaction routes 1-8 at the UMP2/6-311þþG(d,p) and UMPWB1K/6-31þG(d,p) levels of theory. The energies predicted by UMPWB1K, UCCSD(T)//UMPWB1K, UHL//UMPWB1K, and UHL// UMP2 methods are gathered in Tables S8-S10. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We are indebted to the Research Fund for the Doctoral Program of Higher Education of China (No. 20070533142), the National Natural Science Foundation (20802093), the scientific research fund of state key laboratory of explosion science and technology (QNKT10-11), and Excellent Young Scholars Research Fund of Beijing Institute of Technology (2008Y0206) for supporting this research. ’ REFERENCES (1) Sutton, G. P. Rocket Propulsion Elements. An Introduction to the Engineering of Rockets; Wiley: New York, 1992. (2) Bond, D. L. J. Spacecr. Rockets 1980, 17, 342. (3) Thunnissen, D. P.; Guernsey, C. S.; Baker, R. S.; Miyake, R. N. AIAA-2004-3488, 2004. (4) Lindsay, D. M.; Gole, J. L.; Lombardi, J. R. Chem. Phys. 1979, 37, 333. (5) Konnov, A. A.; Ruyck, J. D. Combust. Flame 2001, 124, 106. (6) Vaghjiani, G. L. Int. J. Chem. Kinet. 1995, 101, 4167. (7) Duewer, W. H.; Setser, D. W. J. Chem. Phys. 1973, 58, 2310. (8) Douglas, D. J.; Sloan, J. J. Chem. Phys. 1980, 46, 307. (9) Wategaonkar, S.; Setser, D. W. J. Chem. Phys. 1987, 86, 4477. (10) Zhang, X.; Li, Q. S. J. Phys. Chem. A 2006, 110, 11636. (11) Vanderhoff, J. A.; Anderson, W. R.; Kotlar, A. J. Proceedings of the 29th JANNAF Combustion Subcommittee Meeting, CPIA Publication 593; Chemical Propulsion Information Agency (CPIA): Columbia, MD, 1992; Vol. II, p 225. Anderson, W. R.; Ilincic, N. Meagher, N. E.; Seshadri, K.; Vanderhoff, J. A. Proceedings of the 32nd JANNAF Combustion Subcommittee Meeting and 1995 Propulsion Systems Hazards Subcommittee Meeting, CPIA Publication 638; Chemical 813
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