Theoretical Study of the Gas-Phase Structure, Thermochemistry, and

Novel Bimolecular Reactions between NH3 and HNO3 in the Gas Phase. R. N. Musin , M. C. Lin. The Journal of Physical Chemistry A 1998 102 (10), 1808-18...
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6842

J. Phys. Chem. 1995,99, 6842-6848

Theoretical Study of the Gas-Phase Structure, Thermochemistry, and Decomposition Mechanisms of NH4N02 and N&N(N02)2 A. M. Mebel, M. C. Lh,* and K. Morokuma* Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322

C. F. Melius Combustion Research Facility, Sandia National Laboratories, Livermore, Califomia 94550 Received: December 21, 1994@

The structures, energetics, and decomposition mechanisms of gaseous ammonium nitrite ( N h N 0 2 ) and ammonium dinitramide [ADN, WN(N02)2] have been studied theoretically by different ab initio molecular orbital approaches. In the gas phase, both species have the tructures of molecular complexes, [NH,]*[HX]. The ionic geometries, [Nh+][X-], are not local minima on the potential energy surface and would not be

0

stable after vaporization. For “02, [NH31.[frans-HONO] is the most stable isomer, and [NH3].[cis-HONO] and [NH+[HNO2] structures lie higher by 1.4 and 8.4 kcaymol at the G1 level of theory. For the gaseous ADN, [NH,]*[HN(N02)2 is the most stable structure, while the [NH3]*[HON(O)NNO2]isomer is 2.3 kcallmol less favorable. The calculated dissociation energies of the [NH31.[HX] complex to NH3 and HX are 8-9 and and ADN, respectively. The energies for elimination of the NO2 group from 12- 14 kcal/mol for “ 0 2 HN(N02)2 and HON(O)NN02 are found to be 38-40 kcal/mol, while the barrier for HON(O)NN02 dissociation in the gas phase: is about 42 kcaymol. We predict the following values of the heats of formation, A&’(O), -35.5 kcallmol for [NH3]*[trans-HONO] and 3.2 kcallmol for [NH3]*[HN(N02)2]. A realistic mechanism for the decomposition of ADN, which is fully consistent with the products measured by Brill et al. (ref 3), has been proposed on the basis of these ab initio MO results.

Introduction There has been intensive interest in the energetics and the burning characteristics of ammonium dinitramide (ADN), NH&(N02)2.I This C1-free high-energy material is a potential replacement for ammonium perchlorate (AP),possibly with reduced plume signature and stratospheric ozone destruction. Recently, the stability of ADN has been qualitatively characterized in several ~ t u d i e s ~with - ~ mass and Fourier transformed infrared (FTIR) spectrometries. The temperaturejump FTIR data for ADN heated on a Pt filament obtained by Brill et al., suggest that the decomposition of ADN may take place by two branches: ADN

-

ADN

NH, -I- HNO, NH,

+ N,O

+ HN(N02),

The stable products identified in this study include NH3, NO2, NO, N20, HNO3, and NH4N03. The mechanisms involved in the formation of these products, most likely dominated by the radical species H, OH, NO, and NH, (x I2), are not clear yet. The goal of this work is the elucidation of the reaction mechanism during the initial phase of the ADN fragmentation process using the ab initio molecular orbital calculation. Politzer and Seminario6calculated the geometries and relative energies of various isomers of dinitraminic acid (DN), HN(N02)2. and the energetics of several processes that may be involved in its decomposition, using the MP2/6-31G* method and density functional approaches. However, a theoretical study of the structure and stability of ADN has not yet been reported. We present here an ab initio molecular orbital (MO) study of @

Abstract published in Advance ACS Abstracts, April 15, 1995.

the structure, thermochemistry, and decomposition mechanism of NH$J(N02)2 in the gas phase. Another ammonium salt, “02, may have features similar to ADN. Therefore, we have also studied the structure, stability, and decomposition mechanism of N a N 0 2 in the gas phase. The latter molecule is smaller than ADN; we have carried out calculations for it at the higher levels of theory. This allows us to estimate the accuracy of our calculations for ADN. It should be mentioned that the structure and thermochemistry of the NH3-HON0 molecular complex have recently been studied experimentally and theoretically by Pagsberg and co-~orkers.~

Methods of Calculation

In this study, various theoretical ab initio MO methods have been used. The geometries of the different structures of NO2, ADN, and products of their decomposition have been optimized at the MP2 level with various basis sets, such as 6-31G(d), 6-311G(d,p), and 6-31 l+G(d,p).* Vibrational frequencies have been calculated at the MP2 or HF level for characterization of the nature of stationary points and zero-point energy (ZPE) correction (ZPC). All the stationary points have been positively identified for minimum (number of imaginary frequencies NIMAG = 0), transition-state (NIMAG = l), or higher order top (NIMAG > 1). For anharmonicity correction,* the calculated MP2 and HF ZPC’s were scaled by 0.95 and 0.893, respectively. In order to obtain more reliable energies of various structures and the heats of the reactions involved, we used higher levels of theory, such as MP4(SDQ), MP4(SDTQ), QCISD(T)? and restricted single- and double-excitation coupled cluster RCCSDThe (T) methods,I0 as well as the G1 and G2 conventional G1 methodI1*l2uses a series of calculations to

0022-3654/95/2099-6842$09.00/0 0 1995 American Chemical Society

m-

Decomposition Mechanisms of NH4N02 and NH4N(N02)2

c2v

(0.978) (1.383) (1.022)

cis-HONO, C,

[NH3]'[cis-HONOl, C, 1.195

(110.3) (0.980)

trans-HONO, C,

[NH3]'[trans-HONOl, C,

1.237

(115.8)

105.8 (112.5)

NH3, c 3 v

Figure 1. Optimized geometries of NH40NO isomers and their decomposition products at the MP2/6-3 11+G(d,p) and MP2/6-3 1G(d) (in parentheses) levels of theory.

approximate a QCISD(T)/6-311+G(2df,p)//MP2/6-3 1G(d) calculation with an additional "higher order correction" based on the number of paired and unpaired electrons. The G2 methodI3 uses an additional correction to obtain an estimate of the QCISD(T)/6-31l+G(3df,2p) energy. In our procedure, we used the MP2/6-3 1lG(d,p)-optimized geometries and scaled ZPC's, and for the radicals, projected UMP4 and UMP2 (PUMP4 and PUMP2) energies which give more reliable results for openshell systems than the regular (unprojected) MP4 and MP2 methods. For some structures, we performed density functional calculations using the B3LYP approach, Le., Becke's three-parameter nonlocal-exchange functional14 with the nonlocal correlation functional of Lee et al.,15 implemented16 in the GAUSSIAN 92/DFT program.l7 The use of one or another method of calculation depended on the size of the system and will be discussed explicitly. All the calculations have been performed using the GAUSSIAN 92/DFT17 and MOLPRO 9418programs.

Structure and Stability of Various Isomers of NH40NO The geometric structures of various isomers of NH40N0, as well as HONO, HN02, and NH3, are shown in Figure 1, and their energies are presented in Table 1. All three isomers found for this species have the structure of molecular complexes [NH3].[HONO] or [NH3]*[HN02]with hydrogen bonds to NH3. We located two stationary points with an ionic [NH4]+[N02]geometry within C2". The C2" structure shown in Figure 1, with the NO2 group on the H- *NHy *Hplane, is 1.7 kcaYmol more stable at the MP2/6-31G(d) level than the similar structure with the NO2 plane perpendicular to the H- *NHy OHplane. Hence, more detailed calculations have been carried out for the C2" conformation drawn in Figure 1. Vibrational frequency analysis shows that it has three imaginary frequencies of b2 (580i cm-I),

J. Phys. Chem., Vol. 99, No. 18, 1995 6843

b2 (205 cm-I), and bl (140i cm-') symmetries. Once the symmetry is reduced to C,, reoptimization of this structure converges to the [NH3]*[HN02]complex, even when we use the 6-3 1l+G(d,p) basis set including diffuse functions for a better description of the anion. No ionic structure having an equilibrium geometry with an oxygen atom of NO2 oriented toward H2NH2+ exists. Upon optimization at the MP2/6-31G(d) or MP2/6-311+G(d,p) levels, it converges to the [NH3]* [HONO] molecular complex. It is not surprising that the ionic salt structure for W O N 0 in the gas phase cannot exist, because only the molecular complex [NH3]*[HCl] has been found for the ionic structure exists the W C 1 species in the gas in the solid phase or in solution. Self-consistent reaction field (SCRF) calculations for NH4Cl in a nonpolar solvent ( E = 2.0) have shown that in solution, both [NH3]*[HCl]and [NH4]+[Cl]are local minima.21 This may be true for NH40NO as well. However, once this salt is vaporized, only the molecular complex can exist. We have located three different isomers of the molecular complex, with NH3 corresponding to the three most stable isomers of the free HNO2 molecule. The most favorable structure is the [NH3]*[truns-HONO]complex. At our best G1 level, the [NH3]*[cis-HONO]and [NH3]*[HN02]complexes lie 1.4 and 8.4 kcaYmol higher than [NH3]*[truns-HONO]. The ionic [NH#[N02]- conformation with three imaginary frequencies lies 40.2 kcaYmo1 higher at the G1 level. At the best level of geometry optimization, MP2/6-3 11+G(d,p), the H* *N distance, separating two fragments of the complexes, is 1.781.79 8, for the complexes of NH3 with HONO and 1.83 8, for that with HN02. N* *HOand Ne *HNangles are close to 180". The geometry of the NH3 molecule remains virtually unchanged in the [NH3]*[HONO] complexes. The OH and NH distances in the HONO and HN02 fragments are elongated by 0.020.03 8, as compared to the free molecules. The major change in the geometry of the trans-HONO fragment is the truncation of the longer ON(0) bond length in the complex by 0.04 8,. In [NH3]*[cis-HONO],this distance is shortened by 0.02 A, and the HON angle is enlarged by 4.6" relative to the corresponding geometric parameters in free cis-HONO. Otherwise, the changes of the HONO and HN02 geometries in the complexes are very small. Since other isomers of HN02 are much higher in energy than those shown in Figure 1,22we did not consider complexes of these isomers. The energies of the complex formation are similar for all three isomers. At the best G1 level with MP2/6-311+G(d,p)optimized geometry, this energy is maximal for [NH3]*[HN02], 9.3 kcaYmo1, slightly lower for [NH3]*[truns-HONO],8.8 kcaY mol, and minimal for [NH3]*[cis-HONO]. The order of NH4ON0 isomers on the energy scale reproduces the order of H N 0 2 isomers. The complexation energy for [NH3]*[HCl], 11.1 kcaY mol at the MP2/6-31lG(d,p)//MP2/6-3 1lG(d,p) level, is close to that for the [NH3]*[HN02]and [NH3]*[HONO]species, 11.311.7 kcdmol at the similar MP2/6-3lG(d)//MP2/6-31G(d) level of theory. This can be anticipated because both species are [NH3]*[HX]-type complexes, and their stability is defined by the strength of the N0.H hydrogen bond. Recently, the thermochemistry of the gas reaction trans-HONO NH3 [NH3]*[truns-HONO]was studied experimentally.7 As seen in Table 1, our calculated thermochemical parameters for this reaction, AH"298 = -8.6 kcaYmol and AS"298 = -25.7 tal/ (mokdeg), are in good agreement with the experimental data, AH"298 = -11.8 f 0.8 kcaYmo1 and AS"298 = -26.7 f 1.0 caY(molodeg). For the [NH3]*[cis-HONO] complex, we calculated the formation energy at the MP4(SDQ)/6-31+G(d,p) level with the MP2/6-3 lG(d)-optimized geometry. As can be seen in Table

+

-

Mebel et al.

6844 J. Phys. Chem., Vol. 99, No. 18, 1995

TABLE 1: Total and Relative Energies of Various Isomers of NH3oHONO and Heats of the NHs*HONO---, NH3 -I- HONO Reactions, AZ3 energiesc species

ZPE"

Sb

MP2d/ 6-3 1G(d)

MP2d/ MP4(SDQ)d/ 6-3 1+G(d,p) 6-3 1+G(d,p)

MP4(SDTQ)'/ 6-3 11+G(d,p)

QCISD(T)e/ 6-3 1 1G(d,p)

G1"

~~

NH3 HONO(cis) HONO(truns) HN02

21.0(0) 12.0(0) 11.9(0) 13.4(0)

-56.428 43 -56.486 07 45.9 -56.354 21 -56.391 86 -56.406 I I -53.434 28 59.0 -205.167 14[0.0] -205.192 8 7 -205.199 68 -205.321 18[1.1] -205.303 SO[O.O] -205.474 73[0.8] 59.3 [1.1] [O.OI [0.21 [O.OI 56.9 [5.2] [8.21 [10.9] P.21

NH3oHONO (cis) NH3oHONO (trans) NH3*HN02 NH4+N02AE(cis), kcaVmol AE(truns), kcal/mol AE(HN02), kcaVmol

35.1(0) 35.2(0) 36.2(0) 35.2(3)

77.7 -261.542 77[0.0] -261.603 22 -261.623 22 -261.772 31[1.7] -261.751 09[0.5] -261.97722[1.4] 79.5 [0.8] [O.OI [O.OI [O.OI 75.1 [5.1] ~7.21 [10.5] ~8.41 70.5 [42.8] [36.2] [41.O] [40.7] [50.0] [40.2] -27.2f 11.3 9.5 8.8 8.5 9.7 8.2 (7.8)g -25.7f 11.7 9.1 10.4 8.8 (8.6)g -27.7f 11.4 10.1 10.7 9.7 (9.7)g

" Zero-point energy corrections (kcaVmol), calculated at the MP2/6-31G(d) level and scaled by 0.95. In parentheses: number of imaginary frequencies. Entropies (cal/(mol*deg))calculated at the scaled MP2/6-31G(d) level for 298.15 K and 1 atm. The total energies (italic) are given in hartrees. Relative energies (kcal/mol, in brackets) with ZPE corrections are presented for different isomers of NHyHONO and HONO. With MP2/6-3lG(d)-optimized geometry. e With MP2/6-3 11+G(d,p)-optimized geometry. f ASO298. g AHO298. 1.270 1.240

mHd+m(N0,),1-, I"2- Cs, (NIMAG = 1); HF - C I ,(NIMAG = 0)

.

I .205

Dihedral angles: $N2N30' = 158.6 $N2N306= -20.8 HI2N2N3d = 37.5 HI2N2N3O6= -141.9 N3N2$07= -132.2 N3N2$08= 48.4 ~12~2~4 = 0-12.1 7 HI2N2$O7 = 168.5

117.2 1 . 3 f T $ 1 . 0 1 8

1 2 9 . 2 4

Figure 2. Optimized geometries of ADN at the MP2/6-31G(d) level of theory.

1, it appears to be close to the Gl//MP2/6-311+G(d,p) result. In general, the MP2/6-3 lG(d)-optimized geometries of the complexes also reproduce the geometries optimized at the higher MP2/6-3 1l+G(d,p) level well. Therefore, we use the MP4(SDQ)/6-31+G(d,p)//MP2/6-3 1G(d) approach for the ADN molecule, where the high-level calculations are not possible.

Structure and Stability of Various Structures of NJ"N02)2 As we did for NH40N0, we also first tried to locate an ionic [N&]+[N(N02)2]- structure for the ADN molecule. MP2/631G(d) optimization within C, symmetry results in the ionic structure shown in Figure 2. For a molecule of this size, Hessian calculations at the MP2 level are not feasible. Therefore, we

reoptimized the geometry at the RHF level and calculated RHF/ 6-3 1G(d) frequencies. The [N&]+[N(N02)2]- C,structure has one imaginary frequency of a" symmetry. RHF/6-31G(d) optimization, following the eigenvector corresponding to the imaginary frequency, leads to a distorted geometry of C1 symmetry which still has an ionic character (see Figure 2). In this structure, the NH4 cation is shifted toward one of the anion oxygen atoms. However, at the higher MP2/6-31G(d) level, optimization starting from the symmetric [N&]+[N(N02)2]geometry without symmetry constraints converges to the molecular complex [NH3]*[HON(O)NNO2]. It is well-known that the HF approximation, especially with poor basis sets, tends to overestimate the stability of ionic structures with respect to the stability of molecular c o m p l e ~ e s . ' ~ Thus, - ~ ~ the conclusion can be made that [NH4]+[N(N02)2]- is not a local minimum, and the ionic structure of ADN does not exist in the gas phase. We have calculated two different isomers of the ADN molecular complex. As seen in Figure 2, the first complex corresponds to the complex of NH3 with the HN(N02)2 isomer of dinitraminic acid; the second is that of NH3 with HON(0)"02. As shown in Table 2, [NH3]*[HN(NO2)2]is the most stable structure at all the levels of calculation, while [NH3]* [HON(O)NN02] and the unstable [NH4]+[N(N02)2]- ionic structure lie higher by 2.3 and 12.1 kcavmol, respectively, at the MP4(SDQ)/6-31+G(d,p) level. A comparison of HN(N02)2 geometry in the free molecule (Figure 3) and in the complex (Figure 2) shows only a small difference. The NH distance is elongated by 0.04 A, similarly to the [NH3]*[HN02]complexes, while the adjacent NN bond lengths are shortened by 0.02-0.03 A. Some of the NNO angles fluctuate as much as 8". In the complexes, the hydrogen H3N-HX bond angle is close to 180". It is worth noting that for HN(N02)2, the MP2/6-3 1G(d) and MP2/6-31lG(d,p) approximations give very similar geometries. Hence, our MP2/ 6-3 1G(d)-optimized geometries for the ADN complexes are expected to be reliable. Another isomer of the dinitraminic acid, HON(O)NN02, also only slightly changes its geometry with the formation of the molecular complex with NH3. The major changes due to the formation of the H. *Nhydrogen bond are elongation of the OH bond by 0.05 A, shortening of the NO(H) bond by 0.04 A, and increasing the NOH angle by 3". Similar perturbations were found for the HONO geometry in the [NH3]*[HONO] complexes. The bond lengths, 1.76 8, for N**HN in [NH3]*[HN(N02)2]and 1.63 8, for N **HOin [NH3]* [HON(O)NN02], are noticeably shorter than the corresponding

J. Phys. Chem., Vol. 99, No. 18, 1995 6845

Decomposition Mechanisms of NH4N02 and NH4N(N02)2

TABLE 2: Total and Relative Energies of Various Structures of N&N(N02)2 and Heat of the NHyHN(N02)2 HN(N02)2 Reaction

-

NH3 -I-

energiesc ~

species

ZPE"

[NH31'[HN"2)21 [NH3]*[HON(O)NNO2]

48.1(0) 48.2(0) 49.0( 1) 47.1(0) 45.2(0)

"I+

"0212-

NH3 NH3

+ HN(N02)2

+ HON(O)NN02

Sb 99.5 100.5 93.5 125.3 128.2

MP2/6-3 1G(d) -520.681 72 4.5 11.0 14.2 20.9

MP2/6-3 1+G(d,p) -520.758 88 2.4 11.6 14.0 17.2

~~~

MP4(SDQ)/6-3l+G(d,p) -520.759 36 2.3 12.1 12.4 16.4

Zero-point energy corrections (kcaVmol), calculated at the HF/6-31G(d) level and scaled by 0.893. In parentheses: number of imaginary frequencies. Entropies (cal/(mol*deg))calculated at the scaled HF/6-3 1G(d) level. The total energies for [NH3]*[HN(N02)2]are given in hartrees. For other species, relative energies (kcaVmol) with respect to [NH3]*[HN(N02)2],calculated with ZPC, are presented. Geometries have been optimized at the MP2/6-31G(d) level. a

tion, while participating in the [NH3]*[HON(O)NNO2]complex, the latter is only 2.3 kcal/mol higher than [NH3]*[HN(N02)2]. Hence, the complexation with NH3 stabilizes the H3N* *HO structures somewhat more strongly than the H3N *HN structures. This is in accordance with the shorter N *HO distance than the N *HN distance. Summarizing our findings, we may conclude that the mechanism of ADN decomposition in the gas phase is controlled by the decomposition mechanisms of the different isomers of DN. Once ADN is vaporized, the ionic structure no longer exists. A molecular complex is formed which is only stable at low temperatures and dissociates to NH3 and HN(N02)2 or HON(O)NO2 readily with an endothermicity of 12- 14 kcdmol. This is in agreemeent with the experimental observation that in ADN, pyrolysis decomposition into NH3 and HN(N02)2 is prevailing, with a maximum rate at 155 0C.4

129.8

\\

(112.4) 115.9 1.463 (1.460

/

105.3

(l.02@

(130.4

HNN02, C1

Decomposition of Dinitraminic Acid

8

(1.219)

NO29 C2v

%F

.967v

NH, C,

OH, , C

HON(O)NNOz,

TS for HN03+N20 diss., C1

n

@$

-+

1.207 131.3 1.203 114.3 1.393 104.4 o.973

HON(O)N, C,

Politzer and Seminario discussed various mechanisms of HN(N02)2 decomposition.6 Among the homolytic and heterolytic ruptures of the H-N and N-N bonds, the most favorable process is calculated to be the homolytic N-N bond cleavage to form HNNO2 and NO2 radicals. The DF-GGA/DZVPP// MP2/6-31G* energy of the HN(N02)2 HNN02 NO2 reaction is 44.0 kcal/mol.6 The value calculated earlier at the BAC-MP4 level of theory is 46.6 kcaVm01.~~On the other hand, our MP2 and MP4 calculations (Table 3) with various basis sets give the energy of the NN bond cleavage as much lower, in the range of 30-37 kcaymol. Also, from the results of our recent calculation^,^^ we conclude that the energy for N-N bond cleavage in HNN02 is 29.8 kcaVmol at the QCISD(T)//MP2/ 6-3 1lG(d,p) level of theory. In order to obtain more reliable values for the NN bond strengths, we recalculated the energies of elimination of the first and second NO2 groups from HN(NO& at higher levels of theory, up to G1 for the first N02, and up to G2 for the second one. The results are compiled in Table 4. At the G1 level, the energies of the N-N bond cleavage both in HN(N02)2 and HNNO;! are similar: 38.4 and 39.1 kcal/mol, respectively. The G2 method changes the G1 energy for HNN02 very little. To better understand the influence of spin contamination in the wave function for HNN02 and NO2 radicals on the energy of HN(N02)2 decomposition, we also carried out the restricted single- and double-excitation coupled cluster calculations, RCCSD(T).'O We used the MOLPRO 94 programI8 and the double-5 correlation consistent basis set of Dunning25 and polarization on all atoms (we call this VDZP, though it is called VDZ in the original25). As seen in Table 4, the RCCSD(T) and the unrestricted QCISD(T) methods give similar results. Therefore, one may say that the spin contamination is treated well at the QCISD(T) level, and the G1 and G2 methods, using the QCISD(T) energy as the reference, can be trusted. The HON(O)NN02 isomer of the dinitraminic acid can

113.7 1.202 1.163

0.969p -102.1 4 0 .115.7 6 1.213

'

1.179O

N20, CWv

v

HON02, C,

Figure 3. Optimized geometries of different structures of the DN acid and their decomposition products at the MP2/6-3 1lG(d,p) and MP2/ 6-3 1G(d) (in parentheses) levels of theory.

distances in the [NH3]*HN02]and [NH3]*[HONO]complexes, respectively. Politzer and Seminario6 have found several isomers of HN(N02)2 in a narrow energy range of 7 kcaymol, including four different conformations of HON(O)NN02 in the range of 3 kcal/ mol. We expect similar conformations to exist for the [NH3]* [HON(O)NN02] complex. Using the analogy with the [NH3]* [HONO] complexes, we speculate that the energy order of the isomers of dinitraminic acid would reproduce the energy order of the ADN molecular complex isomers. The complex formation energy for [NH3]*[HN(N02)2]calculated at our best MP4(SDQ)/6-3 l +G(d,p)//MP2/6-3 1G(d) level is 12.4 kcaymol. This is 3-4 kcaVmol higher than the complexation energies for [NH3]*[HN02]and [NH3]*[HONO], in correspondence with the shorter bonding N* *H distance in [NH3]*[HN(N02)2]. Reflecting its more electronegative NO2 groups, HN(N02)2 is presumably a better proton acceptor than HN02. As shown in Table 3, according to the MP4(SDQ)/63 1+G(d,p)//MP2/6-3 1G(d) calculations, the HN(N02)2 isomer is 4.0 kcal/mol more stable than the HON(O)NNO:! conforma-

+

6846 J. Phys. Chem., Vol. 99, No. 18, 1995

Mebel et al.

TABLE 3: Total and Relative Energies of Various Structures of HN(N02)z and Its Decomposition Products energies' species HN(N02):, HON(0)NNOz TS, HNO, N20 diss. HNNO:, NO:, HON(0)N NO:, HN03 f N20

+

+ +

ZPE"

MP2/ 6-31G(d)

Sb

MP4(SDTQ)/ ME/ MP4(SDQ)/ MP2/ RCCSD(T)/ B3LYP/ 6-31G(d,p) 6-31+G(d,p) 6-31+G(d,p) 6-311G(d,p) VDZP 6-311+G(3df,2p)

26.1(0) 79.4 -464.303 29 -464.371 23 -464.343 09 -464.331 88 -464.526 71 -464.425 10 24.2(0) 82.3 6.7 4.6 3.2 4.0 6.1 4.7 23.5(1) 75.1 49.7 44.9 45.2 46.2 49.2 21.2(0) 124.0 36.7 32.1 34.3 29.5 35.5 31.2 21.9(0) 122.1 49.2 42.3 45.6 42.1 46.4 37.6 22.8(0) 116.2 -44.0 -44.0 -45.3 -42.3 -46.8 -42.4

-465.671 54 5.6 44.8 28.7 32.2 -39.4

Zero-point energy corrections (kcaVmol), calculated at the MP2/6-31G(d) level and scaled by 0.95. In parentheses: number of imaginary frequencies. Entropies (caU(moldeg)), calculated at the scaled MP2/6-31G(d) level. The total energies for HN(N02)2 are given in hartrees. For other species, relative energies (kcdmol) with respect to HN(NO&, calculated with ZPC, are presented. Geometries have been optimized at the MP2/6-3 lG(d) level. Projected UMP2 (PUMP2) and UMP4(SDTQ) [PUMP4(SDTQ)] energies are used for relative energy calculations of the open-shell species

TABLE 4: Energies (kcdmol) of Elimination of the First and Second NO2 Groups from HN(N02)z methods

1St NO:,

2nd NO1

MP2/6-31 lG(d,p) MP4/6-31 lG(d,p) MP2/6-3 1l+G(d,p) MP4/6-31 lfG(d,p) MP2/6-31 1G(2df,p) MP4/6-3 11G(2df,p) QCISD(T)/6-31lG(d,p) RCCSD(T)NDZP G1 G2 B3LYP16-311+G(3df,2p) DF-GGAIDZVPPb

35.4 32.0 34.6 31.1 40.6 37.0 30.7 31.2 38.4

27.5 25.8 27.9 26.1 33.1 35.2 29.8 28.0 39.1 38.9 36.7

28.7 44.0

a Geometries of HN(NO:,):,, HNN02, NO2, and NH are optimized at the MP2/6-31 lG(d,p) level. Projected UMP2 (PUMP2) and UMP4 (PUMP4) energies of the open-shell species are used for elimination energy calculations. From ref 6.

decompose in two different ways. The first channel is the cleavage of the single NN bond to form HON(0)N and NO2. The endothermicity of the decomposition process calculated at the MP4(SDQ)/6-31+G(d,p)//MP2/6-31G(d) level is 38.1 kcal/ mol. However, this level is not reliable enough; it underestimated the energy of the NN bond rupture in HN(N02)2 by 8.9 kcal/mol as compared to the G1 result. Also, at the MP4(SDQ)/ 6-3 1+G(d,p) level, the relative energy of the HON(0)N radical with respect to H " 0 2 is 12.6 kcdmol, while this energy at the higher QCISD(T)/6-31lG(d,p) and RCCSD(T)NDZP levels is 6.124and 6.4 kcaymol, respectively. On the other hand, the relative energies of the HN(N02)2 and HON(O)NN02 isomers of DN are not very sensitive to the approximation used; the MP4(SDQ)/6-31+G(d,p) and RCCSD(T)NDZP methods give similar results. A better estimate for the NN bond rupture energy in HON(O)NN02 can be obtained on the basis of the G1 endoergicity of the HN(N02)2 H " 0 2 NO2 reaction and the best relative energies of HN(N02)2 and HON(O)NN02, as well as H " 0 2 and HON(0)N radicals, computed at the RCCSD(T)NDZP level. This calculation gives the value of 40.1 kcal/mol. The other mechanism of HON(O)NN02 decomposition produces the nitric acid HON02 and N20. The transition state (TS) for this dissociation is shown in Figure 3. One of the oxygen atoms of the NO2 fragments moves toward the HONO fragment, and a four-membered ring N'N203N5 is formed in the transition state. After this TS is cleared, N203 and the double N'N5 bonds are broken and a new N503bond is formed, leading to the dissociation of HON(O)NN02 onto HON02 and N20. The decomposition process has a high exoergicity, 38.3 kcal/mol at the MP4(SDQ)/6-3l+G(d,p)//MP2/6-3 1G(d) level, and the transition state processesses an early character. The breaking N203 and N'N5 bonds in the TS are longer only by

-

+

0.12 and 0.15 8, than those in HON(O)NNO2, while the forming N503bond is still -0.6 8, longer than the NO bonds in HON02. The N'N204 angle in TS increases by 14.3", but it is still very far from 180" in the N20 molecule. On the other hand, the N'N2 bond length diminishes by 0.17 8, from HON(O)NN02 to TS and by 0.15 8, from TS to N20. The activation energy for the HON(O)NNO2 HON02 N20 decomposition, 42.2 kcaVmo1 at MP4/6-3 l+G(d,p)//MP2/ 6-31G(d) ZPC[MP2/6-31G(d)], is somewhat higher than the endothermicity of the HON(O)NNO2 HON(0)N NO2 process. At high temperatures, the entropy factor would make the latter mechanism more preferable than the former. Our conclusion is that the two mechanisms can compete with each other, but as the temperature increases, the elimination of the NO2 group becomes more and more dominant. Politzer and Seminario6 have shown that the HON(O)NN02 HON02 N20 channel can be acid-catalyzed; in the presence of H+, it occurs without activation energy. For different structures of the DN acid and for the products of their decomposition, we have carried out density functional B3LYP/6-3 11+G(3df,2p) calculations with the large basis set including diffuse functions, three d and one f polarization functions on heavy atoms and two p polarization functions on hydrogens. The results are shown in Tables 3 and 4. One can see that the B3LYP approach reproduces fairly well the relative energies of the different structures of HN(N02)2. For the HN202 radical, the relative energy of HON(0)N with respect to H " 0 2 is underestimated by 2.6 kcal/mol at the B3LYP level as compared to the QCISD(T) result. However, for the bond dissociation energies, the DIT approach does not work well. For instance, the bond energy of the first NO2 group from HN(NO& at the B3LYP level is underestimated by about 10 kcaV mol as compared to G1, while this energy calculated by another DW, DF-GGA, method is overestimated by 5.6 kcal/mol. Thus, the energetic parameters obtained by the DFT methods still should be considered with caution. The overall mechanism of the early stage of ADN decomposition can be summarized as shown in Figure 4. After vaporization of the solid ADN, [NH3l*[HN(NO2)21and [NH31* [HON(O)NN02] molecular complexes are formed. The complexation energies are low, 12-14 kcal/mol, and the complexes readily dissociate, producing NH3 and various isomers of the DN acid. Then, HN(N02)z can decompose onto HNNOz and NO2, while HON(O)NNO2 would either eliminate the NO2 group or produce the nitric acid and N20, overcoming a high barrier.

-

+

-

-

-

Decomposition of

H " 0 2

+

+

and HON(0)N

The mechanism of H " 0 2 decomposition can be understood on the basis of the mechanism of the NH NO2 reaction, recently studied by us.24 The profile of the potential energy

+

J. Phys. Chem., Vol. 99, No. 18, 1995 6847

Decomposition Mechanisms of NH4N02 and NH4N(N02)2

TABLE 5: Experimental and Calculated Heats of Formation of Various Species species

AHf"(O),kcal/mol

NH NH3 NO2 HONO (cis) HONO (trans) [NH+[HONO] (cis) [NH3].[HONO] (trans) HNNO2 HN(N02)z [NH~[HN(NO~)ZI (ADN)

85.2" (87.0)b -9.3" (-6.8)b 8.6" (9.4)b -16.9" (-17.6)b -17.4"(-18.4)b -34.46 -35.9 54.7b 24.gb 3.2b

a Experimental value from the JANAF tables (ref 24). Calculated values at the GI level of theory, except for ADN, for which this is an estimated G1 value from the MP4(SDQ)/6-31+G(d,p) calculation.

Figure 4. Schematic profile of the potential energy surface for the early stage of ADN decomposition, calculated at the MP4(SDQ)/631+G(d,p)//MP2/6-31G(d) and G1 (in bold) levels of theory. The G1 energy of the HON(O)NN02 HON(0)N NO2 decomposition is estimated as discussed in the text.

-

+

f"\

\

-24

-

-18.0

k

O

-30.7 H -35.5

Figure 5. Profile of the potential energy surface of the reaction of NHH02 decomposition, calculated at the QCISD(T)/MP2/6-3 11G(d,p) ZPE and G2 (in bold) levels of theory.

+

surface for this reaction calculated at the QCISD(T) and G2 levels of theory is shown in Figure 5. HNN02 can either decompose onto NH NO2 with energy loss of 38.9 kcaYmol or undergo a hydrogen shift with activation energy of 38.7 k c d mol (both at the G2 level). After the shift, an HON(0)N intermediate is formed, which is 6.1 kcal/mol higher than HNNO;?at the QCISD(T)/AJMPU6-31lG(d,p)+ZPE level. After clearing a barrier of 10.4 kcaYmo1, HON(0)N decomposes to N20 and OH, which are 35.5 kcdmol lower than HNNO2 in the G2 approximation. Note here that [NH3]*[HON(O)NN02] would produce the HON(O)N intermediate after the breaking the hydrogen bond and eliminating NO2. HON(0)N can rearrange to HNN02 by the hydrogen shift with a barrier of 34.1 kcdmol at the QCISD(T)//UMP2/6-311G(d,p)+ZPE level. NH and NO, can recombine to form the HNONO intermediate without a barrier. Upon overcoming a low barrier of 3.5 kcaYmo1, HNONO can dissociate to give HNO NO, which lies 18.0 kcaYmol below HNN02 at the G2 level. The other pathway of HNO NO formation is oxygen shift in HNN02, but the barrier for this process is very high, 56.5 kcaYmol (QCISD(T)//UMP2/6-31lG(d,p)+ZPE). The isomerization reaction results in the formation of the HN(0)NO intermediate. Earlier, we found that the barrier for dissociation of the latter onto HNO NO is 13.0 kcaYmo1 at the QCISD(T)

+

+

+

+

+

+

+

+

Calculation of the Heats of Formation of Gaseous NILzONO and ADN

TS 0 shift

50

Recently, however, while studying potential energy surface of the reverse HNO NO reaction,26we found a lower barrier with a nonplanar transition state, 8.8 and 9.8 kcdmol at the QCISD and G2 levels, respectively. Overall, the decomposition of HNN02 and HON(0)N can lead to the following products: NH NO2, N20 OH, and HNO NO.

We can predict the heats of formation of m 0 N O and ADN in the gas phase on the basis of the heats of the reactions calculated in the previous sections and experimental M f " ( 0 ) for NH, NH3, NOz, and HON0.27 The experimental and calculated heats of formation for various species are presented in Table 5 . For the complexation reaction, NH3

+ HONO (trans) - [NH3]*[HONO](trans)

(1)

the exothemicity calculated at the G1 level is 8.8 kcdmol. Therefore, AHf"(0) for [NH3]*[HONO] (trans) is -35.5 kcaY mol. For the complexation reaction, NH,

+ HONO (cis)- [NH,]*[HONO] (cis)

(2)

the enthalpy change at the G1 level is 8.2 kcallmol, and our prediction for the heat of formation of [NH3]*[HONO] (cis) is -34.4 kcaYmo1. The differences between trans- and cis-HONO and between trans- and cis-[NH3].[HONO] at the G1 level are 0.8 (OS-e~periment)~~ and 1.4 kcdmol, respectively. For the association of NH with N02,

NH

+ NO2 - HNNO,

(3)

releases 39.1 kcaYmo1 of energy at the G1 level of theory. Hence, the calculated value of AiYf"(0) for the H " 0 2 radical is 54.7 kcallmol. For the association of H " 0 2 with NO2, HNNO,

+ NO, - HN(N02),

(4)

has the G1-calculated exothermicity of 38.4 kcaYmol, and thus the heat of formation of the gaseous dinitraminic acid is estimated to be 24.9 kcaYmol. For the formation of the NH3 complex, HN(NOJ2 -I- NH3

-

[NH31WN(NO2)21

(5)

we have calculated the exothermicity of 12.4 kcdmol only at the MP4 (SDQ)/6-31+G(d,p)//MP2/6-3 lG(d)+ZPE level. However, on the basis of the result for the related reaction (2), the energy calculated at this level of theory is close to that calculated by the GI method; hence, we expect that 12.4 kcal/mol is a

Mebel et al.

6848 J. Phys. Chem., Vol. 99, No. 18, 1995

good estimate for the exothermicity of reaction 5. Then, the heat of formation of ADN in the gas phase is calculated to be 3.2 kcdmol. In order to estimate A W A O ) for ADN in the solid phase, one has to add the sublimation energy to this value.

Acknowledgment. We gratefully acknowledge the support of this work by the Ofice of Naval Research (Contract N0001489-1-19490). We thank the Cherry L. Emerson Center for Scientific Computation for the use of computing facilities and various programs.

Concluding Remarks Both the NH4N02 and ADN species in the gas phase have the structures of molecular complexes [NH3].[HX]. The ionic, salt-like geometries are not local minima on the potential energy surfaces and, therefore, are not expected to be stable after vaporization. For NH4N02, the [NH3]*[HONO](trans) isomer is the most stable one, while [NH3f[HONO] (cis) and [NH3][HNO,] lie higher by 1.4 and 8.4kcdmol. For gaseous ADN, [NH3]*[HN(NO2)2]is calculated to be the most stable structure, but the [NH3]*[HON(O)NN02]isomer is only 2.3 kcdmol less favorable. The energies of dissociation of the [NH3]-[HX] complex to NH3 and HX are 8-9 kcdmol for NWONO and 12-14 k c d mol for ADN. The energies for the elimination of the first and second NO2 group from HN(N02)2 are calculated to be similar, in the 38-39 kcdmol range. We predict the following values of the heats of formation (AHP(0))for the most stable isomers of N h N 0 2 and ADN in the gas phase: -35.5 kcaYmol for [NH3].[HONO] (trans) and 3.2 kcdmol for [NH31.[HN(N02)2]. Our results suggest that the mechanism for ADN decomposition in the early stage is dominated by ADN(s) ADN(s)

-

-

[NH31*[HN(N0,),14 NH3 + HN(NO,),

[NH3]*[HON(O)NNO,] HN(NO,),

-

-NH, +

HNNO,

HON(O)NNO,

+ NO,

HON(0)N f NO,

HON(O)NNO,

HON(O)NNO,

N,O

+ HONO,

+ NO, HNNO, N,O + OH HON(0)N N,O + OH NH + NO2 -.HNO + NO NH3 + NO, - NH, 4-HONO HONO, - NO, + OH HNNO,

HONO

NH

-

NO

+ OH

followed by various reactions involving H, OH, NH, ( x 5 3), NO, (x 5 2), and HNO, (x 5 3) species.

References and Notes (1) Borman, S. Chem. Eng. News 1994, 72, 18. (2) Schmitt, R. J.; Krempp, M.; Bierbaum, V. M. Int. J. Mass Spectrom. Ion Processes 1992, 117, 621. (3) Brill, T. B.; Brush, P. J.; Patil, D. G. Combust. Flame 1993, 92, 7788. (4) Rossi, M. J.; Bottaro, J. C.; McMillen, D. F. Int. J. Chem. Kinet. 1993, 25, 549. (5) Doyle, R. J. Org. Mass Spectrom. 1993, 28, 83. (6) Politzer, P.; Seminario, J. M. Chem. Phys. Lett. 1993, 216, 348. (7) Pagsberg, P.; Ratajczak, E.; Sillesen, A.; Latajka, Z. Chem. Phys. Lett. 1994, 227, 6. ( 8 ) Hehre, W.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (9) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5768. (10) (a) Watts, J. D.; Gauss, J.; Bartlett, R. J. J. Chem. Phys. 1993, 98, 8718. (b) Hampel, C.; Peterson, K.; Werner, H.-J. Chem. Phys. Lett. 1992, 190, 1. (11) Pople, J. A,; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtiss, L. A. J. Chem. Phys. 1989, 90, 5622. (12) Curtiss, L. A.; Jones, C.; Trucks, G. W.; Raghavachari, K.; Pople, J. A. J . Chem. Phys. 1990, 93, 2537. (13) Curtiss, L. A,; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J . Chem. Phys. 1991, 94, 7221. (14) (a) Becke, A. D. J. Chem. Phys. 1993, 98,5648. (b) Becke, A. D. J . Chem. Phys. 1992,96,2155. (c) Becke, A. D. J. Chem. Phys. 1992, 97, 9173. (15) Lee, C.; Yang, W.; Parr,R. G. Phys. Rev. 1988, B37, 785. (16) Johnson, B. G.; Gill, P. M. W.; Pople, J. A. J . Chem. Phys. 1993, 98, 5612. (17) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Reglogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart. J. J. P.: PoDle. J. A. GAUSSIAN 92": Gaussian, Inc.: Pittsburgh, PA, 1993. (18) Werner. H.-J.: Knowles. P. J. MOLPRO 94. Universitv of Sussex, Falmer, Brighton, 1994. (19) Latajka, Z.; Scheiner, S. J. Chem. Phys. 1985, 82, 4131. (20) Vibok, A.; Mayer, I. Int. J. Quantum Chem. 1992, 43, 801. (21) Chipot, C.; Rinaldi, D.; Rivail, J.-L. Chem. Phys. Lett. 1992, 191, 287. (22) Nakamura, S.; Takahashi, M.; Okazaki, R.; Morokuma, K. J. Am. Chem. SOC. 1987, 109, 4142. (23) Melius, C. F. Unpublished results. (24) Mebel, A. M.; Morokuma, K.; Lin, M. C. J . Chem. Phys. 1994, 101, 3916. (25) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007. (26) Mebel, A. M.; Morokuma, K.; Lin, M. C.; Melius, C. F. J. Phys. Chem. 1995, 99, 1900. (27) Chase, M. W., Jr.; Davles, C. A,; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables. J. Phys. Chem. Ref. Data 1985, 14, Suppl. 1. JP943373T