Initial Decomposition Pathways of Aqueous Hydroxylamine Solutions

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Initial Decomposition Pathways of Aqueous Hydroxylamine Solutions Yu-ichiro Izato, Mitsuo Koshi, and Atsumi Miyake J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b10546 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 5, 2017

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The Journal of Physical Chemistry

Title: Initial Decomposition Pathways of Aqueous Hydroxylamine Solutions.

Authors

Yu-ichiro Izato†, Mitsuo Koshi, and Atsumi Miyake Institute

of

advanced

sciences,

Yokohama

National

University 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JAPAN Phone: +81-45-339-3981 † Corresponding address : [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT This work examined the reaction pathways involved in the initial decomposition of aqueous hydroxylamine solutions via the overall reaction 2NH2OH → NH3 + HNO + H2O, using quantum chemistry calculations incorporating solvent effects. Several possible

decomposition

mechanisms

were identified

and

investigated: three

neutral-neutral bimolecular, two water-catalyzed, one neutral trimolecular, two ion-neutral bimolecular and one cation-catalyzed. Optimized structures for the reactants, products

and

transition

states

were

obtained

at

the

ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level of theory, and the total electron energies of such structures were calculated at the CBS-QB3 level of theory. The cation-catalyzed reaction 2NH2OH + NH3OH+ → NH4+ + HNO + H2O + NH2OH (maximum energy barrier (∆‫ܧ‬଴‡ ) = 53.6 kJ/mol) and the anion-neutral bimolecular reaction NH2OH + NH2O- → NH3 + 1NO- + H2O (∆‫ܧ‬଴‡ = 79.0 kJ/mol) were both found to be plausible candidates for the dominant step in the initial decomposition. The results of this study indicate that both acidic and basic conditions can affect the thermal stability of hydroxylamine in water.

2


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1. Introduction Hydroxylamine (HA), an oxygenated derivative of ammonia having the chemical formula NH2OH, is an important reducing agent and antioxidant in the chemical and pharmaceutical industries1-5. HA solutions are also used to strip process residues from integrated circuit devices6. In addition to these applications, HA is the main component of hydroxylamine nitrate (HAN), a next-generation rocket propellant oxidizer7. Despite its usefulness, HA is a potentially hazardous material8, and there have been two well-known catastrophic industrial explosions involving HA to date: in Pennsylvania and in Gunnma, Japan, in 1999 and 2000, respectively9. To ensure that HA can be used safely, it is important to understand the reaction mechanisms of HA compositions in detail and to develop effective stabilizers based on this knowledge. Numerous studies have been conducted involving calorimetric and explosion hazard assessments of aqueous HA solutions10-20, and some reasonable reaction mechanisms have been developed21-26. HA in the free base form has been shown to decompose at high temperatures according to reactions R1 and R2, below, which respectively account for five and two sevenths of the total decomposition21,22. 3NH2OH → N2 + NH3 + 3H2O

(R1)

4NH2OH → N2O + 2NH3 +3H2O

(R2) 3



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In addition, the following reaction has been proposed as the initial step of HA decomposition23. 2NH2OH → NH3 + HNO + H2O

(R3)

The nitroxyl (HNO) generated in this reaction accelerates the HA decomposition to yield dinitrogen and dinitrogen oxide, as follows24. NH2OH + HNO → N2 + 2H2O

(R4)

2HNO → N2O + H2O

(R5)

These exothermic reactions generate heats of reaction of 451.6 and 366.9 kJ/mol, respectively25, and can trigger runaway reactions leading to hazardous explosions. To prevent the hazardous accelerated decomposition of HA, the initial decomposition and subsequent accumulation of HNO must be inhibited based on a detailed knowledge of the reaction mechanisms. However, to the best of our knowledge, these decomposition mechanisms remain unclear. Wang et al.27 reported that a transition state could not be determined for reaction R3, and implied that R3 may not be an elementary reaction. Wang et al.27,28 also suggested that NH3O, an isomer of HA, plays an important role in the initial decomposition process, and that the reaction NH3O + H2O → NH3 + H2O2 is one possible initial decomposition pathway. This same prior study calculated the dissociation enthalpy of NH3O (NH3O → NH3 + 3O) to be 146 kJ/mol at the G2 level

4


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of theory27, although this dissociation reaction is spin forbidden and does not proceed directly. To obtain a better understanding of HA reactivity, the present work examined the initial decomposition pathways of this compound on the basis of ab initio calculations. The activation energy barriers for this decomposition were obtained by taking into account the effects of bulk water as the solvent. Ab initio quantum chemical calculations are helpful with regard to determining which reactions to exclude from the decomposition mechanism based on thermodynamic arguments. If a reaction is found to be highly endothermic, or considerably more endothermic than a competing pathway, then that reaction may be safely omitted from the mechanism. This paper presents a reaction scheme for the initial HA decomposition represented by reaction R3, as one step towards providing an improved understanding of HA chemistry.

2. Computational The geometries of the reactants, products and transition states (TSs) were optimized at the ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level29 of theory using the Gaussian 09 program package30. Gordon et al. developed the ωB97XD method, which includes empirical dispersion forces and is believed to be reliable when applied to systems with weak van der Waals forces29. Their group also reported that the ωB97XD 5



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method yields satisfactory accuracy for kinetics and non-covalent interactions29. During these computations, TSs were extensively searched for and, if found, an intrinsic reaction coordinate (IRC) calculation was conducted in order to assign reactants and products to the TS. The energies of corresponding molecules were evaluated at the CBS-QB3/SCRF=(solvent=water)31 level of theory, since this represents a reasonable time-expense complete basis method. In this study, geometries and frequencies were calculated at the ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level, the optimized geometries were fixed with no changes allowed and the energies were calculated using the CBS-QB3 method. Solvent effects were included for all calculation by applying the self-consistent reaction field (SCRF) and polarizable continuum model (PCM) options within the program.

3. Results and discussions The most important reactions associated with the initial HA decomposition, and their respective

energy

barriers

and

energy

changes

ωB97XD/6-311++G(d,p)/SCRF=(solvent=water)

calculated

at

the and

CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) levels of theory, are listed in Table 1. Details of these reaction pathways are discussed in the following sections, while all structures and calculated total electron energies of the chemical species and 6


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TSs presented in this paper are provided in the Supporting Information. In the case of each reaction addressed in this paper, the total energy change at 0 K (∆‫ܧ‬଴) between the transition state and the reactants (∆‫ܧ‬଴‡ ) is considered. It should be noted that any prediction of chemical reaction make use of free energy. We, however, conducted simple SCRF calculation without special treatment which allow to obtain accurate thermodynamic data for liquid species as showed in refs32-36. The contribution of solvent effects obtained using the simple SCRF is normally added to the electronic energy using standard quantum chemical programs. Thus, we used total energy change (in fact, the potential of mean force, which is the sum of the electronic energy with the solvation free energy) to discuss a reasonable reaction path in this study.

7



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Table 1 Important reactions in the decomposition of HA with thermodynamic parameters calculated at the CBS-QB3 level of theory for ωB97XD/6-311++G(d,p)/scrf=(solvent=water) geometries.

REACTION

ωB97XD

CBS-QB3//ωB97XD

/6-311++G(d,p)

/6-311++G(d,p)

/SCRF=(solvent=water) /SCRF=(solvent=water) ‡ ∆‫ܩ‬ଶଽ଼

1

∆௥ ‫ܩ‬ଶଽ଼

2

‡ ∆‫ܩ‬ଶଽ଼

1

∆௥ ‫ܩ‬ଶଽ଼

295

-61

232

-66

NH2NH2O ⇌ NH3 + HNO (TS2)

158.8

-24.2

141

-74

NH2NH2O + H2O ⇌ NH3 + HNO+ H2O (TS3)

95.9

-24.2

143

-74

NH2OH + NH2OH ⇌ H2O−NHNH2O−ΟΗ (TS4)

202

18

201

7

H2O−NHNH2O−ΟΗ ⇌ NH2NH2O + H2O (TS5A)

64

-64

70

-61

H2O−NHNH2O−ΟΗ ⇌ NH2NH2O + H2O (TS5B)

17

-64

24

-61

H2O−NHNH2O−ΟΗ ⇌ tN2H2 + 2H2O (TS5C)

-4

-241

-1

-249

NH2OH + NH3O ⇌ NH2(O)OH + NH3 (TS6)

182

-63

182

-63

NH2(O)OH ⇌ NH3 + HNO (TS7)

82

-127

82

-126

NH2(O)OH + H2O ⇌ NH3 + HNO + H2O (TS8)

89

-127

89

-126

NH2(O)OH + NH3 ⇌ NH3 + HNO + NH3 (TS9)

56

-127

56

-126

NH2OH + NH2OH + H2O ⇌ NH2NH2O + 2H2O (TS10)

213

-60

209

-63

NH2OH + NH2OH + 2H2O ⇌ NH2NH2O + 3H2O (TS11)

203

-61

210

-63

NH2OH + NH2OH + NH2OH ⇌ NH2NH2O + NH3O + H2O (TS12)

NH2OH + NH2OH ⇌ NH2NH2O + H2O (TS1)

195

-7

192

-10

+

226

-91

224

-91

NH2NH2OH ⇌ NH3NH2O (TS14)

126

45

125

49

78

47.4

86

49

9

-120

3

-136

126

-62

121

-131

91

72

89

65

-7

-50

-3

-46

131

-91

139

-91

+

NH2OH + NH3OH ⇌ NH2NH2OH + H2O (TS13) +

+

+

+

NH2NH2OH + H2O ⇌ NH3NH2O + H2O (TS15) +

+

NH3NH2O ⇌ NH4 + HNO (TS16) NH2OH + NH2O－ ⇌ NH2NHO－ + Η2Ο (TS17) －

－

NH2NHO ⇌ NH2(H)NO (TS18) 1

－

－

NH2(H)NO ⇌ NH3 + NO (TS19) +

+

NH2OH + NH2OH + NH3OH ⇌ NH2NH2OH + NH2OH + H2O (TS20) 1

energy barrier in the forward direction [kJ/mol]

2

total energy change of reaction [kJ/mol]

8


2

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3.1 neutral-neutral bimolecular reaction of NH2OH We identified and investigated the following bimolecular reactions. NH2OH + NH2OH → H2O + NH2NH2O

(R6)

NH2NH2O → HNO + NH3

(R7)

NH2OH + NH2OH → 2H2O + tN2H2

(R8)

Figure 1 shows the potential energy profiles, including the optimized structures for the TSs and intermediate complexes of the reactants and products for these neutral-neutral bimolecular reactions of HA. In the mechanism that proceeds via TS1, cleavage of the N−O bond of an HA molecule triggers the decomposition, and the resulting NH2· group removes a H from an −OH group in a second HA molecule to produce NH3 and NH2O· simultaneously. Subsequently, the original dissociated OH· subtracts a H from the NH3 to generate NH2· and H2O, and the NH2· combines with NH2O· to form NH2NH2O. Figure 2 depicts the potential energy surface for the decomposition of this NH2NH2O. Here, H2O molecules assist in forming the intramolecular hydrogen transfer ring structure (TS5). The maximum energy barrier for this process was calculated to be 191.2 kJ/mol at the CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level of theory. Since R6 has a much higher energy barrier than the reactions discussed below, we conclude that 9



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this bimolecular reaction does not play an important role in the initial decomposition of HA. In the mechanism that proceeds via TS2, decomposition begins with cleavage of the N−O bond of one HA molecule, in the same manner as the mechanism that produces TS1. The NH2· dissociated from the HA then combines with the NH2− group in another HA molecule to generate both OH· and NH2NH2OH. The OH· resulting from the original bond cleavage subtracts a H from an NH2− group in the NH2NH2OH to produce the intermediate complex IM2P (H2O−NHNH2−OH). The subsequent step could potentially proceed via two pathways, producing NH2NH2O and H2O or trans-diazene (t-N2H2) and two H2O. The former pathway has two TSs in which intramolecular H transfer occurs from an −OH group to an NH− group in NHNH2−OH. TS5A represents intramolecular H transfer, while TS5B is associated with H transfer via a cyclic ring structure with the assistance of H2O. The energy barrier for this latter process was determined to be 14.1 kJ/mol, which is significantly lower than the value for direct hydrogen transfer (70.0 kJ/mol). The second potential pathway involves the dissociation of OH· from NHNH2−OH, after which this OH· removes a H from NHNH2 to yield t-N2H2 and H2O, as shown in the TS5C mechanism. The energy barrier of 1.7 kJ/mol for this intramolecular subtraction pathway is lower than those for the H transfer pathways,

10


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and so the TS5C mechanism is thermally preferable. The maximum energy barrier for reaction R7 was calculated as 159.5 kJ/mol at the CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level of theory. Although this value is lower than that for R6, it is markedly higher than the barriers for the reactions discussed below. Therefore, although we successfully identified several neutral-neutral bimolecular elementary mechanisms for the initial HA decomposition, we concluded that these reactions are not dominant.

Figure 1. Potential energy profiles for the bimolecular reaction of HA. The energy profiles

have

been

calculated

11


at

the


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Page 12 of 37

CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level of theory.

Figure 2. Potential energy profiles for the decomposition of the intermediate NH2NH2O. The

energy

profiles

have

been

calculated

at

the


Some theoretical calculations have predicted that the NH3O isomer is sufficiently stable to be kinetically relevant and therefore may play a key role in many reactions of HA in aqueous solution27,37,38. Felnadez et al.38 calculated the value for the tautomeric equilibrium between NH2OH and NH3O in aqueous solution, and reported an equilibrium constant, K (equal to [NH3O]/[NH2OH]), of 2.6 × 10-2. In this study, the 12


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following reactions involving NH3O were therefore identified and investigated. NH2OH + NH3O → NH2(O)OH + NH3

(R9)

NH2(O)OH → HNO + H2O

(R10)

Potential energy surfaces and optimized structures for this pathway are presented in Figures 3 (R9) and 4 (R10). The decomposition starts with cleavage of the N−O bond in NH3O, following which the dissociated O combines with the NH2− group in NH2OH to form NH2(O)OH as TS 6, as shown in Figure 3. The energy barrier for R9 was determined

to

be

143.5

kJ/mol

at

the

CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level of theory. The NH2(O)OH subsequently decomposes to HNO and H2O through an intramolecular H transfer. Here, H2O and NH3 molecules can assist in the intramolecular H transfer of NH2(O)OH by transporting the H such that a ring structure is formed, as shown in TS7 and TS8 in Figure 4. The ammonia molecules that participate in this process are generated by the preceding reaction R9. Although the unimolecular decomposition has an energy barrier of 83.7 kJ/mol, the H2O- and NH3-catalysis effects decrease this value to 51.6 and 20.3 kJ/mol, respectively. Because the energy barrier values of these reactions are markedly less than that of R9, R9 is the rate-determining step in this series of reactions.

13



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The energy barrier height of 143.5 kJ/mol identified in the case of reaction R9 is the lowest among the neutral-neutral bimolecular pathways investigated in this study. However, this value is still much higher than those for the water-catalyzed, trimolecular and ionic mechanisms discussed further on. For this reason, although we examined this neutral-neutral bimolecular mechanism via the NH3O isomer, we concluded that this reaction is not predominant during HA decomposition.

Figure 3. Potential energy profile for the bimolecular reaction of HA and NH3O. The energy

profile

has

been

calculated

at


14


the

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Figure 4. Potential energy profiles for NH2(O)OH decomposition. The energy profiles have

been

calculated

at

the


3.2 water catalyzed bimolecular reaction It was determined that H2O molecules could assist the decomposition of HA, as shown in the following reactions. NH2OH + NH2OH + H2O → 2H2O + NH2NH2O

(R10)

NH2OH + NH2OH + 2H2O → 3H2O + NH2NH2O

(R11)

NH2NH2O → NH3 + HNO

(R7)

In the present study, we identified and investigated both a one-H2O ring TS (TS10) and a two-H2O ring TS (TS11), and the potential energy surfaces of reactions R10 and R11 15



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are shown in Figure 5. These water-catalyzed mechanisms were found to have energy barriers of 93.5 and 127.7 kJ/mol, both of which are lower than those of the simple neutral-neutral bimolecular reactions presented above (159.5 and 191.2 kJ/mol). After initial cleavage of the N−O bond of one HA molecule, H2O molecule(s) act to transport H from the OH− group in NH2NH2-OH to the dissociated OH· to generate TS10 and 11, as shown in Figure 4. Reactions such as these, in which H2O molecules serve as H transfer agents, are known to be common. The TSs proposed in the absence of explicit solvent molecule effects typically involve highly strained rings since these are necessary to transfer the H from one position in the molecule to another, resulting in a high energy barrier to the reaction. The inclusion of solvent molecules allows larger, less strained ring structures, reducing the TS energy. In the present mechanisms, one or more H2O molecules simultaneously accept a H at a lone pair site and give up one of their original H atoms to another molecule.

16


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Figure 5. Potential energy profiles for the water catalysis reaction of HA. The energy profiles

have

been

calculated

at

the


3.3 trimolecular reaction of NH2OH + NH2OH + NH2OH This work also examined a trimolecular reaction involving a third NH2OH molecule, as shown below. NH2OH + NH2OH + NH2OH → H2O + NH2NH2O + NH3O

(12)

The potential energy surfaces associated with this reaction are shown in Figure 6. This trimolecular mechanism was found to have a lower energy height (103.9 kJ/mol) compared to the values of 143.6, 159.5 and 191.2 kJ/mol determined for the bimolecular reactions. 17



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Page 18 of 37

Optimized structures for the TSs and intermediate complexes are also depicted in Figure 6. In this mechanism, the third HA plays the role of a H transfer agent, similar to the H2O-catalyzed mechanism. After the initial cleavage of the N−O bond of an HA molecule, a H is transferred from the OH− group of the third HA to the dissociated OH·, following which a H transfers from the OH− in NH2NH2OH to the NH2− in the third HA to generate TS12, as shown in Figure 5. This series of reactions eventually yields NH2NH2O, H2O and NH3O. This is a self-catalytic mechanism, because NH3O can isomerize and regenerate NH2OH exothermically. This isomerization mechanism has been previously investigated on the basis of ab initio calculations27,28,37,38.

Figure 6. Potential energy profiles for the trimolecular reaction of HA. The energy profiles

have

been

calculated 18


at

the

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3.4 ion-neutral bimolecular reaction In aqueous solution, HA can protonate or deprotonate to form NH3OH+ or NH2O－, and so we identified and investigated reactions including these two species. Here we first address the cation-neutral reaction sequence below. NH2OH + NH3OH+ → NH2NH2OH+ + H2O

(R13)

NH2NH2OH+ → NH3NH2O+ →NH4+ + HNO

(R14)

The potential energy surface and optimized structures for this pathway are summarized in Figures 7 and 8. The energy barrier for R13 was calculated to be 180.5 kJ/mol at the CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level of theory. The decomposition begins with cleavage of the N−O bond of NH3OH+, after which the resulting NH3+· combines with NH2OH to form NH3NH2OH+· as TS13 (Figure 7). The OH group from the initial cleavage subtracts H from the NH3− group in NH3NH2OH+· and this series of reactions eventually yields H2O and NH2NH2OH+. H2O molecules assist in the decomposition of NH2NH2OH+ by forming a ring structure and acting as proton carriers to give TS15, as shown in Figure 8. Although the unimolecular decomposition has a relatively high energy barrier of 124.6 kJ/mol, the water-catalysis effect decreases this value to 52.8 kJ/mol. The NH3NH2O+ subsequently decomposes to 19



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Page 20 of 37

NH4+ and HNO via an intramolecular proton transfer. The maximum energy barrier of 180.5 kJ/mol associated with this ionic pathway is higher than that calculated for one of the neutral-neutral bimolecular pathways. This result indicates that the cationic species NH3OH+ is more stable in water than neutral HA and that the decomposition of NH3OH+ can therefore be omitted from further consideration.

Figure 7. Potential energy profile for the ion-neutral bimolecular reaction of HA and NH3OH+.

The

energy

profile

has

been

calculated


20


at

the

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Figure 8. Potential energy profiles for the decomposition of the intermediate NH2NH2OH+.

The

energy

profiles

have

been

calculated

at

the


We next consider the anion-neutral bimolecular reaction between NH2O－ and HA, as shown below. NH2OH + NH2O－ → NH2NHO－ + H2O

(R15)

NH2NHO－ → 1NO－ + NH3

(R16)

Figures 9 and 10 present the potential energy surfaces and optimized structures for reactions R15 and R16, respectively. The decomposition starts with the cleavage of the N−O bond of a HA molecule to generate TS17 (Figure 9). The NH2· dissociated from the HA simultaneously combines with NH2O－ to generate [OH-NH2NH2O]－ (IM17P). In addition, the dissociated OH· removes a H from the −NH2− group of NH2NH2O to yield H2O and NH2NHO－. The energy barrier for this reaction was calculated to be 84.1

21



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kJ/mol at the CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level of theory. The NH2NHO－ subsequently decomposes to generate both NH3 and 1NO－ via an intramolecular H transfer. It should be noted that 3NO－ is more thermodynamically stable than 1NO－, but is spin forbidden and so is not directly produced in this reaction. The maximum relative energy barrier for the intermediate decomposition reaction R9 was determined to be less than the heat generated from the previous decomposition reaction R8, and so the latter is evidently the rate-determining step in this series of reactions. The maximum energy barrier for this series was calculated as 84.1 kJ/mol, a value that is considerably less than those determined for the neutral-neutral bimolecular, water-catalyzed and trimolecular reactions. Based on this low energy barrier and the exothermic nature of the anion-neutral bimolecular reaction, this mechanism is thought to play an important role, especially under basic conditions that elevate the concentration of NH2O－.

22


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Figure 9. Potential energy profile for the ion-neutral bimolecular reaction of HA and NH2O

－

.

The

energy

profile

has

been

calculated

at

the


Figure 10. Potential energy profile for the decomposition of the intermediate NH2NHO－. The

energy

profile

has

been

calculated


23


at

the


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Page 24 of 37

3.5 cation catalyzed trimolecular reaction In addition to the above deliberations, a cation-catalyzed trimolecular reaction including NH3OH+ was examined, as follows. NH2OH + NH2OH + NH3OH+ → NH2NH2OH+ + NH2OH + H2O

(R13)

The possible potential energy surfaces for this reaction are depicted in Figure 11. A proton transfer from the NH3− group of NH3OH+ to NH2OH initiates the reaction, following which the protonated HA, NH2(H2O)+, dissociates to H2O and NH2+ to give TS20, as in Figure 11. The NH2+ subsequently combines with another NH2OH to form NH2NH2OH+. This series of reactions eventually yields NH2NH2OH+, H2O and NH2OH. The

energy

barrier

height

was

calculated

as

53.6

kJ/mol

at

the

CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) level of theory. This is the lowest barrier of all the initial pathways investigated in this study and we therefore conclude that the ionic reaction 2NH2OH + NH3OH+ → NH2NH2OH+ + NH2OH + H2O is a plausible candidate for the dominant step in the initial HA decomposition process in aqueous solution.

24


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Figure 11. Potential energy profiles for the ion-catalyzed reaction in the presence of NH3OH+.

The

energy

profiles

have

been

calculated

at

the


3.6 initial decomposition scheme of HA Out of all the reaction pathways considered, the anion-neutral bimolecular and cation-catalyzed reactions had the lowest energy barriers, and so these were used to construct the following reaction scheme for the initial decomposition of HA as a solution in water. This initial decomposition is summarized by Schemes 1 and 2, both of 25



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which involve the reaction of two molecules of HA to yield NH3, HNO and H2O.

SCHEME 1 Cation-catalyzed reaction NH2OH + NH2OH + NH3OH+ → NH2NH2OH+ + NH2OH + H2O NH2NH2OH+ → NH3NH2O+ NH3NH2O+ → NH4+ + HNO NH4++ NH2OH → NH3OH+ + NH3 NH2OH + NH2OH → NH3 + HNO + H2O

SCHEME 2 anion-neutral bimolecular reaction NH2OH + NH2O－ → NH2NHO－ + H2O NH2NHO－ → NH3 + 1NO－ NH2OH + 1NO－ → HNO + NH2O－ NH2OH + NH2OH → NH3 + HNO + H2O

These schemes allow us to discuss the safe handling of HA. As an example, Scheme 1 indicates that the balance between neutral NH2OH and the NH3OH+ cation is important. It is already well known that the free base form of HA is susceptible to explosive

26


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decomposition, and so HA is typically handled in an aqueous solution or as a salt, in conjunction with an appropriate stabilizer. In general, ionic species are stable, and the present work demonstrated that the cation-neutral bimolecular reaction has a high energy barrier, meaning that the direct decomposition of NH3OH+ need not be considered. However, Wei et al.12 has reported that the thermal decomposition behavior of HA is affected by the presence of acids (and bases) and that protons can increase the maximum self-heating and pressure rise rate. NH2OH cannot be completely removed from HA solutions due to the chemical equilibrium in solution. Thus, although the cation can potentially act as an acid catalyst to promote the decomposition of HA, generating the cation form of NH2OH is still an effective means of reducing its concentration and preventing HA decomposition. Scheme 2 suggests that the coexistence of NH2OH and NH2O- also affects the thermal stability of HA solutions, as the presence of NH2O- decreases the reaction energy barrier to 79.0 kJ/mol. Wei et al.12 also reported that the thermal decomposition of HA is affected by the presence of a base and that hydroxide ions can decrease the onset temperature and increase the generation of gaseous products. The results of this work demonstrate that both acidic and basic conditions affect the thermal stability of HA, and so aqueous solutions of HA must be handled and stored so

27



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Page 28 of 37

as to avoid accidental mixing with substances that can affect their pH. In the future, it would be helpful to construct quantitative models to predict the thermal behavior of HA under specific acid-base conditions.

4. Conclusion The initial decomposition pathway of HA in aqueous solution was investigated on the basis

of

ab

initio

calculations

performed

ωB97XD/6-311++G(d,p)/SCRF=(solvent=water)

at

the and

CBS-QB3//ωB97XD/6-311++G(d,p)/SCRF=(solvent=water) levels of theories. As such, a mechanism for the overall reaction 2NH2OH →NH3 + HNO + H2O was developed. Reactions involving H2O, a third HA molecule, the HA cation (NH3OH+) or the HA anion (NH2O－) were all found to promote HA decomposition, and decreased the energy barrier (∆‫ܧ‬଴‡ ) to 127.7 kJ/mol (one H2O molecule catalyzed decomposition), 93.5 kJ/mol (two H2O molecules catalyzed decomposition), 103.9 kJ/mol (neutral trimolecular reaction), 79.0 kJ/mol (anion-neutral bimolecular reaction) and 53.6 kJ/mol (cation-catalyzed reaction) at the same level of theory. Because the energy barrier of 53.6 kJ/mol is the lowest, we conclude that the cation-catalyzed reaction 2NH2OH + NH3OH+ → NH4+ + HNO + H2O + NH2OH is the dominant step during the initial HA decomposition in aqueous solution. However, under basic conditions, the anion-neutral 28


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bimolecular reaction NH2OH + NH2O－ → NH3 + 1NO- + H2O is also plausible. Both highly acidic and highly basic conditions are predicted to affect the thermal stability of HA and so aqueous solutions of this compound must be handled and stored so as to ‡ ensure that these conditions are avoided. The ∆‫ܩ‬ଶଽ଼ is also calculated and listed

in supporting information. The cation catalyzed reaction and anion-neutral ‡ bimolecular reaction has ∆‫ܩ‬ଶଽ଼ of 131 and 126 kJ/mol, respectively. Those

barriers are also much lower than ones of other initial reactions.

5. Acknowledgement This research was supported by the JSPS KAKENHI Grant Number JP 15J04632.

Supporting Information. all structures and calculated total electron energies of the chemical species and TSs presented in this paper are provided in the Supporting Information.

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TOC Graphic

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Initial Decomposition Pathways of Aqueous Hydroxylamine Solutions

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