Products and Mechanistic Investigations on the Reactions of Hydrazines with Ozone in Gas-Phase

S S symmetry

Article Products and Mechanistic Investigations on the Reactions of Hydrazines with Ozone in Gas-Phase

Dan Huang 1, Xiangxuan Liu 1, Zheng Xie 1,* , Xuanjun Wang 1, Xin Gao 2 and Yuxue Yang 1 1 High-Tech Institute of Xi’an, Xi’an 710025, China; [email protected] (D.H.); [email protected] (X.L.); [email protected] (X.W.); [email protected] (Y.Y.) 2 High-Tech Institute of Beijing, Beijing 10085, China; [email protected] * Correspondence: [email protected]

Received: 30 July 2018; Accepted: 6 September 2018; Published: 11 September 2018

Abstract: The toxic transformation products of hydrazines are of great concern. These products’ properties combined with their formation mechanisms are needed to assess their potential environmental and human impacts. In this study, the gas-phase reaction of hydrazine (N2H4), monomethyldrazine (MMH) and unsymmetrical dimethyhydrazine (UDMH) with O3 have been studied at varying reactant ratios, both in the presence and absence of a radical trap. Gas chromatography-mass spectroscopy (GC-MS) has been implied to follow reactant consumption and product formation. Apart from the reported products detected by Fourier transform infrared spectroscopy (FT-IR), the newly found compounds (hydrazones, formamides, dimethylamine, 1,1,4,4-tetramethyl-1,2-tetrazene,dimethylamino-acetonitrile, N2,H2O, et al.) are identiﬁed by GC-MS. The relative yields of the organic products vary considerably at different O3/MMH or UDMH ratios. UDMH and MMH are conﬁrmed as high potential precursors of N-nitrosodimethylamine (NDMA). The presence of hydroxyl radicals (HO·) hinders NDMA formation in MMH-O3 system. Meanwhile, it increases NDMA formation in UDMH-O3 system. The suggested reaction mechanisms which account for the observed products are discussed.

Keywords: hydrazines; ozone; products; mechanism

1. Introduction

Hydrazine (N2H4) and its alkyl derivatives, monomethyldrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH) have been the main high-energy fuels for rocket launching for decades. Considering these compounds are used in large quantity every year, the releases to the atmosphere stemming from storage, transportation and ﬁlling cannot be ignored [1]. The hydrazines themselves are hazards to humans. The American Conference of Governmental Industrial Hygienist (ACGIH) has recommended that the threshold limit values (TLVs) for N2H4, MMH, and UDMH be lowered from 100, 200 and 500 ppb, respectively, to 10 ppb in air [2,3]. A major concern is that the toxicity of considerable amounts of the huge list of transformation products are equal to or greater than that of the hydrazines themselves [4,5]. N-nitrosodimethylamine (NDMA) can be taken as an example, whose theoretical cancer risk level is 0.7 ng/m3 in air [6]. In order to explore the atmospheric fate of these chemicals, lots of studies have been done to obtain chemical properties and formation mechanism of transformation products. Since the hydrazines cannot be photolyzed in the solar actinic region (λ > 290 nm), their likely reaction pathways in the atmosphere are reaction with ozone and hydroxyl radical. In the 1980’s, The rate constants for the reactions of hydroxyl radical, ozone with all three of the hydrazines have been measured by the Statewide Air pollution Reasearch Center (SAPRC) of the University of California at Riverside [7]. The results show that all three of the hydrazines could be rapidly consumed by

Symmetry 2018, 10, 394; doi:10.3390/sym10090394 www.mdpi.com/journal/symmetry Symmetry 2018, 10, 394 2 of 13

Symmetry 2018, 10, x FOR PEER REVIEW 2 of 13 reactionsreactions with with ozone ozone as wellas well as by as reactions by reactions with hydroxylwith hydroxyl radicals radicals [8]. The [8]. specific The specific ozonation ozonation products observedproducts in observed this laboratory in this are laboratory H2O2 and are N2 OH2 fromO2 and N2 HN42;O methyl from N hydroperoxide,2H4; methyl hydroperoxide, methyldiazene, diazomethane,methyldiazene, HCHO, diazomethane, CH3OH fromHCHO, MMH; CH3OH NDMA from inMMH; large NDMA yields fromin large UDMH yields [9 from]. The UDMH reaction of[9].The UDMH reaction with ozoneof UDMH is of with great ozone interest is of great because interest itleads because to it a leads high to yields a high of yields NDMA of NDMA [10–12 ]. The[10–12]. NDMA The formation NDMA mechanismformation mechanism from UDMH from ozonation UDMH ozonation has been has investigated been investigated by theoretical by calculationstheoretical [ 13calculations,14]. The NDMA[13,14]. The formation NDMA pathways formation includepathways hydrogen include hydrogen atom abstraction atom abstraction and direct oxygenand direct atom additionoxygen atom [15]. Theaddition results [15]. also The indicate results that also HO indicate· plays that a key HO· role plays in promoting a key role NDMA in formation.promoting However, NDMA theformation. experiment However, shows the that experime the radicalnt shows traps that increase the radical the NDMA traps formationincrease the [11 ]. Furthermore,NDMA formation various [11]. analytical Furthermore, techniques various such analytical as high techniques performance such liquid as high chromatography-mass performance liquid spectroscopychromatography-mass (HPLC-MS) spectroscopy[16–18], gas chromatography-mass (HPLC-MS) [16–18], gas spectroscopy chromatography-mass (GC-MS) [3,19 spectroscopy,20], and solid phase(GC-MS) micro-extraction [3,19,20], and (SPME-GC/MS) solid phase micro-extraction [21] have been (SPME-GC/MS) used for the identification [21] have been and used quantitative for the determinationidentificationof and these quantitative transformation determination products, of these and moretransformation than 30 oxidative products, productsand morehave than been30 identified.oxidative However,products thehave information been identified. about Howeve the productsr, the ofinformation the hydrazines about reactionthe products with ozoneof the is scarce,hydrazines and the reaction HO· influence with ozone on is NDMAscarce, and formation the HO· mechanisminfluence on isNDMA still in formation discussion. mechanism In order is to still in discussion. In order to find more products and more deeply understand the mechanism of the find more products and more deeply understand the mechanism of the hydrazines reaction with hydrazines reaction with ozone, GC-MS is utilized to follow reactant consumption and product ozone, GC-MS is utilized to follow reactant consumption and product formation. The reactions of formation. The reactions of N2H4, MMH, and UDMH with O3 are investigated at varying reactant N H , MMH, and UDMH with O are investigated at varying reactant ratios, both in the presence 2ratios,4 both in the presence and absence3 of radical traps. Meanwhile, the information of the highest and absence of radical traps. Meanwhile, the information of the highest occupied molecular orbital occupied molecular orbital (HOMO) has been found to account for the possible attack sites of N2H4, (HOMO)MMH, UDMH has been by found ozone. to Moreover, account forthe theformation possible mech attackanisms sites of ofthe N major2H4, MMH,detected UDMH products by have ozone. Moreover,been discussed. the formation mechanisms of the major detected products have been discussed. 2. Experimental Section 2. Experimental Section 2.1. Reaction Chamber and GC-MS System 2.1. Reaction Chamber and GC-MS System TheThe experiments experiments werewere carried out out using using a chambe a chamberr (internal (internal volume volume of 500 of mL, 500 cylinder). mL, cylinder). The Thedetailed detailed schematic schematic diagram diagram of this of this apparatus apparatus is provided is provided in Figure in Figure 1. 1.

Six-way vavle GC/MS

Reaction Chamber

Ozonier Ethanol

FigureFigure 1. 1.Schematic Schematicdiagram diagram ofof the main experimental apparatus. apparatus.

2.2.2.2. Materials Materials AnhydrousAnhydrous hydrazine hydrazine (stated (stated purity purity 97+%,97+%, HenanHenan Liming Liming Chemical Chemical Reagent Reagent Co., Co., Zhengzhou, Zhengzhou, China),China), methylhydrazine methylhydrazine (98%, (98%, HenanHenan LimingLiming Chemical Reagent Reagent Co., Co., China.), China), and and unsymmetrical unsymmetrical dimethylhydrazinedimethylhydrazine (99+%, (99+%, Henan Henan LimingLiming Chemical Reagent Reagent Co., Co., China.) China) were were commercial commercial available. available. OzoneOzone was was produced produced in in a a laboratory laboratory ozonizer ozonizer withwith oxygen source source (Model (Model FL-810ET, FL-810ET, Shenzhen Shenzhen Feili Feili ElectricElectric Technology Technology Co., Co., Shenzhen, Shenzhen, China). China).

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2.3. Products Analysis 2.3. Products Analysis Before each runs, the reaction chamber was flushed with the pure N2. The blanked sample was Before each runs, the reaction chamber was flushed with the pure N . The blanked sample was analyzed to make sure that the gas in the reaction chamber was pure. Then2 the reaction chamber (500 analyzed to make sure that the gas in the reaction chamber was pure. Then the reaction chamber mL, cylinder) was evacuated. The samples (N2H4 10μL, MMH 10μL, UDMH 10μL) were injected (500 mL, cylinder) was evacuated. The samples (N H 10 µL, MMH 10 µL, UDMH 10 µL) were injected into it. After 5 min, the chamber was flushed with2 4 different concentration ozone until the pressure into it. After 5 min, the chamber was flushed with different concentration ozone until the pressure reaches 1.8 bar pressure. Then the reacted samples were detected by the GC-MS through the six-way reaches 1.8 bar pressure. Then the reacted samples were detected by the GC-MS through the six-way valve on line. To further investigate the intermediate reaction in the presence of O3, the chamber was valve on line. To further investigate the intermediate reaction in the presence of O3, the chamber flushed with O3 after the experiments of each sample. Each operation was repeated three times. The was flushed with O3 after the experiments of each sample. Each operation was repeated three times. reaction chamber was thoroughly flushed with the pure N2, and the exhaust gas was washed with The reaction chamber was thoroughly flushed with the pure N , and the exhaust gas was washed with ethanol solution to remove the toxic gas (NDMA, etc.). Each2 sample was detected through GC-MS ethanol solution to remove the toxic gas (NDMA, etc.). Each sample was detected through GC-MS (7890A-5795C, Agilent, Santa Clara, CA, USA), based on the calibrated internal standards. (7890A-5795C, Agilent, Santa Clara, CA, USA), based on the calibrated internal standards. Separation Separation was performed on a DB-225S (30 m × 250 μm × 0.25 μm) capillary column. The conditions was performed on a DB-225S (30 m × 250 µm × 0.25 µm) capillary column. The conditions of the of the GC-MS were as follows: helium as the carrier gas at a rate of 1.5 mL/min, the temperature of GC-MS were as follows: helium as the carrier gas at a rate of 1.5 mL/min, the temperature of the the injector 35 °С, column from 35 °С to 150 °С with a rate of 10 °С/min; the split ratio 1:1. injector 35 ◦C, column from 35 ◦C to 150 ◦C with a rate of 10 ◦C/min; the split ratio 1:1.

3.3. Results Results and and Discussion Discussion

3.1.3.1. The The Reactions Reactions between Between HH HH with with Ozone Ozone

FigureFigure2 shows2 shows the the ﬁnal final products products of of N N2H2H4 4during during ozonation ozonation detected detected by by GC-MS. GC-MS. The The major major productsproducts are are listed listed in in Table Table1 .1. The The products products detected detected inin thethe reactionreaction of O 33 withwith hydrazine hydrazine are are H H2O,2O, N2 Nand2 and N N2O.2O. Compared Compared to to the the product product observed observed by by FT-IR, HH22O and NN22 are newlynewly discovereddiscovered products products andand the the N N2O2O is is relativelyrelatively minorminor too.too. NO x (NO(NO or or NO NO2)2 )is is also also not not detected. detected. Based Based on on the the products, products, we wecan can propose propose that that the the oxidation oxidation of ofNH NH2 group2 group is initiated is initiated by hydrogen by hydrogen atom atom abstraction abstraction rather rather than thanby direct by direct oxygen oxygen atom atom addition. addition. Therefore, Therefore, the ultimate the ultimate fates fatesof N2 ofH4 N reaction2H4 reaction with ozone with ozone are N2 are and NH2 2andO. The H2O. release The release of N2H of4 Nwill2H 4probablywill probably have havelittle littleeffect effect on the on thelevels levels of NO of NOx, nitratesx, nitrates in inthe theatmosphere. atmosphere.

FigureFigure 2. 2.Gas Gas chromatogram chromatogram of of the the ozonation ozonation products products of of hydrazine. hydrazine.

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Symmetry 2018, 10, x FOR PEER REVIEW 4 of 13 Table 1. Products of the reaction of hydrazine with ozone. Table 1. Products of the reaction of hydrazine with ozone. Product Peak/min CAS Pro. Product Peak/min CAS Pro. N2 2.236 7727-37-9 60.4 N2 2.236 7727-37-9 60.4 H2O 2.571 7732-18-5 98.2 H2O 2.571 7732-18-5 98.2 N2O 3.657 10024-97-2 30.8 N2O 3.657 10024-97-2 30.8 N2H4 3.983 302-01-2 93.2 N2H4 3.983 302-01-2 93.2 3.2. The Reactions between MMH with Ozone 3.2. The Reactions between MMH with Ozone 3.2.1. Products 3.2.1. Products Figure3 shows that the ﬁnal products are detected at vary O 3/MMH ratios. The detailed Figure 3 shows that the final products are detected at vary O3/MMH ratios. The detailed information of all the products is listed in Table2. As shown in Table2, the mixture of products in the information of all the products is listed in Table 2. As shown in Table 2, the mixture of products in MMH + O3 system detected by GC-MS is more complex than that observed by FT-IR. The number of the MMH + O3 system detected by GC-MS is more complex than that observed by FT-IR. The the various oxidation products reaches 16. number of the various oxidation products reaches 16.

Figure 3. Gas chromatogram of the ozonation products of methylhydrazine: (a) O3/MMH ≈ 1:1; Figure 3. Gas chromatogram of the ozonation products of methylhydrazine: (a)O3/MMH ≈ 1:1; (b) O3/MMH ≈ 10:1. (b)O3/MMH ≈ 10:1.

TableTable 2. 2.Products Products ofof thethe reactionreaction of hydrazine with with ozone. ozone.

ProductProduct Peak/minPeak/min CASCAS No. No. Pro.

2 N2 N 2.2412.241 7727-37-97727-37-9 98.0 H2OH2O 2.7402.740 7732-18-57732-18-5 97.2 DimethylamineDimethylamine (DMA) (DMA) 2.438 2.438 124-40-3 124-40-3 70.5 FormaldehydeFormaldehyde dimethylhydrazone dimethylhydrazone (FDMH) (FDMH) 3.276 3.276 2035-89-4 2035-89-4 91.4 Formaldehyde monomethylhydrazone (FMH) 4.646 36214-48-9 30.5 Formaldehyde monomethylhydrazone (FMH) 4.646 36214-48-9 30.5 Acetadehyde methydrazone (AMH) 5.687 17167-73-6 35.5 N,NAcetadehyde-tetramethyl-Methanediamine methydrazone (AMH) 7.430 5.687 51-80-9 17167-73-6 35.5 10.8 1,2-dimethyldiaziridineN,N-tetramethyl-Methanediamine (DDZ) 8.8597.430 6794-95-251-80-9 10.8 20.1 N-nitrododimethylamine1,2-dimethyldiaziridine (NDMA) (DDZ) 9.430 8.859 62-75-9 6794-95-2 20.1 85.4 DimethylformamideN-nitrododimethylamine (DMF) (NDMA) 10.092 9.430 68-12-2 62-75-9 85.476.6 1-methyl-2-piperidinoneDimethylformamide (DMF) 11.047 10.092 931-20-4 68-12-2 76.617.3 1-methyl-1,2,4-triazole (MT) 11.638 6086-21-1 74.2 1-methyl-2-piperidinone 11.047 931-20-4 17.3 Acetaldehyde-N-formyl-N-methydrazone (AFMH) 12.171 16568-02-8 55.1 2-Nitroethanol1-methyl-1,2,4-triazole (MT) 12.956 11.638 625-48-9 6086-21-1 74.238.6 N-methyl-Acetaldehyde-N-formyl-N-methydrazoneN-nitro-ethanamine (AFMH) 13.060 12.171 10595-95-6 16568-02-8 55.1 26.0 1,3-dimethyl-Imidazolidinone2-Nitroethanol 13.08712.956 80-73-9625-48-9 38.6 28.1 N-methyl-N-nitro-ethanamine 13.060 10595-95-6 26.0 1,3-dimethyl-Imidazolidinone 13.087 80-73-9 28.1 The products are different in the excess of O3 and of MMH. In the excess of MMH, the major observed products are FDMH, FMH, DDZ, with minor NDMA and DMF. Meanwhile, in the excess of ozone, the major products are NDMA and DMF, with minor yields of CH3NO2. The NDMA and DMF

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SymmetryThe2018 products, 10, 394 are different in the excess of O3 and of MMH. In the excess of MMH, the major5 of 13 observed products are FDMH, FMH, DDZ, with minor NDMA and DMF. Meanwhile, in the excess of ozone, the major products are NDMA and DMF, with minor yields of CH3NO2. The NDMA and DMFare formed are formed in 58% in and 58% 17.95% and 17.95% yields withinyields within 5 min. From5 min. the From observed the observed products, products, the dominant the dominant reaction reactionmechanism mechanism is H-abstraction is H-abstraction in the excess in the of MMH,excess whichof MMH, can explainwhich can the explain high yields the ofhigh hydrazones. yields of hydrazones.Otherwise, in Otherwise, the excess ofin ozone,the excess the main of ozone, reaction the mechanism main reaction includes mechanism not only includes H-abstraction not only but H-abstractionalso O-addition, but which also O-addition, account for which NDMA account and DMF for formation.NDMA and DMF formation. Furthermore, thethe observedobserved products products (CH (CH3COOH,3COOH, CH CH3OH3OH and and HCHO) HCHO) by by FT-IR FT-IR yields yields [22 ][22] in the in thepresence presence of excess of excess O3 accountedO3 accounted for for 92% 92% ofthe of the initial initial carbon carbon in MMH. in MMH. However, However, these these products products are arenot detectednot detected by GC-MS by GC-MS in our experimentalin our experimental results, which results, probably which because probably that thesebecause compounds that these are compoundsdifﬁcult to be are separated difficult from to be O separated2. At least from 95% ofO the2. At initial least nitrogen95% of the in MMHinitial couldnitrogen not in be MMH observed could by notFT-IR. be Whereas,observed theby FT-IR. dimethyl-compounds Whereas, the dimethyl with nitrogen-compounds are detected with nitrogen by GC-MS are which detected account by GC-MS for the whichinitial nitrogen.account for the initial nitrogen.

3.2.2. The The Variation Variation of the Major Products Conversion Rates during OzonationOzonation The reaction between MMH and ozone is too fast to be measured. In order to investigate the major productsproducts variationvariation during during ozonation, ozonation, each each run run is ﬂushed is flushed with with ozone ozon in eache in 15each min. 15 The min. results The resultsare shown are shown in Figure in4 Figure. 4.

Figure 4. The curves of change in relative conversion rate of the products in MMH + O3 system Figure 4. The curves of change in relative conversion rate of the products in MMH + O3 system (ﬂushed (flushedwith ozone with continuously). ozone continuously).

Within 6060 min,min, the the yields yields of FDMH,of FDMH, FMH FMH and DDZand DDZ increase increase at the earlyat the stage early and stage then and decrease. then decrease.Hence, the Hence, FDMH, the FMH FDMH, and DDZ FMH are and proposed DDZ toare be pr theoposed intermediates to be the during intermediates the ozonation during process. the ozonationMoreover, theprocess. amounts Moreover, of NDMA, the DMF amounts and AFMH of NDMA, increase DMF during and the AFMH ozonation increase process. during When the ozonationFDMH decreases process. sharply When within the FDMH 40–60 min,decreases the NDMA sharply increases within sharply.40–60 min, So the the FDMH NDMA may increases convert sharply.to the NDMA So the during FDMH ozonation. may convert This to is consistentthe NDMA with during the resultozonation. that relatively This is consistent high molar with NDMA the resultyields that 95% relatively during FDMH high molar ozonation NDMA in aqueous yields 95% [23 ].during FDMH ozonation in aqueous [23]. In conclusion, the the FMH and DDZ may convert to DMF and AFMH. The ultimate products of MMH reaction with ozone are NDMA, DMF and AFMH.

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Symmetry 2018, 10, x FOR PEER REVIEW 6 of 13 3.2.3.3.3.3. InfluenceInfluence of Radical TrapTrap (Propylene(Propylene Alcohol)Alcohol) onon NDMANDMA FormationFormation 3.3.3.TheThe Influence influenceinfluence of ofRadical of radical radical Trap trap trap (Propylene on on the the formation formationAlcohol) of on NDMA, of NDMA NDMA, as Formation a resultas a result of ozone of ozone reaction reaction with MMH, with isMMH, presentedThe is presentedinfluence in Figure ofin5 ,radicalFigure which 5, showstrap which on that the shows theformation radical that the of trap radicalNDMA, decreases trap as adecreases NDMA result of formation NDMAozone reaction formation and increases with and DMFincreasesMMH, formation is DMFpresented significantly.formation in Figure significantly. The 5, which yields shows ofThe NDMA thatyields the occupy of radical NDMA 58% trap in occupy thedecreases absence 58% NDMA ofin thethe formationradical absence trap andof andthe 30.7%radicalincreases in trap the DMF and presence 30.7%formation of in radical the significantly. presence trap. This of Theradical means yields thattrap. of the ThisNDMA radicals means occupy play that a the58% significant radicals in the roleplayabsence in a promotingsignificant of the theroleradical formation in promoting trap and of NDMA 30.7% the formation in in the the presence MMH-O of NDMA of3 system.radical in the trap. MMH-O This means3 system. that the radicals play a significant role in promoting the formation of NDMA in the MMH-O3 system.

3 FigureFigureFigure 5. 5.5.Inﬂuence InfluenceInfluence of of radical radical trap traptrap on onon NDMANDMA NDMA formation formation (O (O (O33/UDMH/UDMH/UDMH ratio ratio ≈ 10:1).≈ 10:1).10:1).

3.3.3.3.3.3. The The Reactions Reactions between between UDMHUDMHUDMH withwith OzoneOzone 3.3.1. Products 3.3.1.3.3.1. Products Products The runs are conducted in different O3/UDMH ratios (See Figure6). The observed reaction rate TheThe runs runs are are conductedconducted in different OO33/UDMH/UDMH ratios ratios (See (See Figure Figure 6). 6). The The observed observed reaction reaction rate rate of UDMH with O3 is as fast as that of MMH with O3. The relative yields of the organic products vary ofof UDMH UDMH with with O O33 isis asas fastfast asas that of MMHMMH withwith O O3.3 .The The relative relative yields yields of of the the organic organic products products vary vary considerably at different O3/UDMH ratios. The details of the total products observed by GC-MS are considerablyconsiderably at at different different OO33/UDMH/UDMH ratios.ratios. TheThe details details of of the the tota total productsl products observed observed by by GC-MS GC-MS are are listedlistedlisted inin in TableTable Table3 3.. 3.

FigureFigure 6. 6.Gas Gas chromatogramchromatogram of of the the ozonation ozonation products products of ofmethylhydrazine: methylhydrazine: (a) O (a3/UDMH)O3/UDMH ≈ 1:1; ≈ 1:1; Figure 6. Gas chromatogram of the ozonation products of methylhydrazine: (a) O3/UDMH ≈ 1:1; (b()Ob) O3/UDMH3/UDMH ≈ 10:1.10:1. (b) O3/UDMH ≈ 10:1.

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Symmetry 2018, 10, 394 Table 3. Products of the reaction of 1,1-dimethyldrazine with ozone. 7 of 13 Product Peak/min CAS No. Pro. N2 Table 3. Products of the reaction of 1,1-dimethyldrazine2.241 with7727-37-9 ozone. 98.0 H2O 2.740 7732-18-5 97.2 ProductDimethylamine (DMA) Peak/min 2.438 CAS 124-40-3 No. Pro. 70.5 NFormaldehyde2 dimethylhydrazone (FDMH) 2.241 3.276 7727-37-9 2035-89-4 98.091.4 HAcetadehyde2O dimethylhydrazone (ADMH) 2.740 4.363 7732-18-5 7422-90-4 97.285.5 Dimethylamine (DMA) 2.438 124-40-3 70.5 1,1,4,4-tetramethyl-1,2-tetrazene (TMT) 6.888 6130-87-6 44.0 Formaldehyde dimethylhydrazone (FDMH) 3.276 2035-89-4 91.4 AcetadehydeDimethylamino-Acetonitrile dimethylhydrazone (ADMH) 4.3638.013 7422-90-4924-64-7 85.597.4 1,1,4,4-tetramethyl-1,2-tetrazeneN-nitrododimethylamine (NDMA) (TMT) 6.888 9.430 6130-87-6 62-75-9 44.090.8 Dimethylamino-AcetonitrileDimethylformamide (DMF) 8.013 10.092 924-64-7 68-12-2 97.460.8 N1-methyl-1,2,4-triazole-nitrododimethylamine (NDMA)(MT) 9.430 11.633 62-75-9 6086-21-1 90.874.2 Dimethylformamide (DMF) 10.092 68-12-2 60.8 1-methyl-1,2,4-triazoleN-formyl-N-methyl-Formamide (MT) 11.63312.890 6086-21-118197-25-6 74.240.9 NN-methyl-N-nitro-ethanamine-formyl-N-methyl-Formamide 12.89013.060 18197-25-610595-95-6 40.926.0 N-methyl-N-nitro-ethanamine 13.060 10595-95-6 26.0

The major products observed in the UDMH-O3 system are FDMH and NDMA, with minor yields TMT, DMF, MT. In the excess of UDMH, the major product is FDMH, while, in the excess of The major products observed in the UDMH-O3 system are FDMH and NDMA, with minor yields ozone,TMT, DMF,the major MT. Inproduct the excess is NDMA, of UDMH, whose the molar major conv productersion is rate FDMH, reaches while, 88.1%. in theThe excess large yields of ozone, of FDMHthe major is identified product is in NDMA, the excess whose of UDMH molar conversion by GC-MS, rate which reaches may 88.1%. be the The unidentified large yields product of FDMH with is −1 itsidentified strongest in absorption the excess of [9] UDMH at ~976 by cm GC-MS,. The high which yields may beof theNDMA unidentified formation product is consistent with its with strongest that ofabsorption EC Tuzon’s [9] atstudy ~976 [7]. cm −1. The high yields of NDMA formation is consistent with that of EC Tuzon’s study [7]. 3.3.2. The Variation of the Major Products Conversion Rates during Ozonation 3.3.2. The Variation of the Major Products Conversion Rates during Ozonation Similar to the MMH + O3 system, the experiment is first conducted with low ozone concentration.Similar to After the MMH each +sample, O3 system, the thechamber experiment was injected is first conducted with ozone. with The low results ozone are concentration. shown in FigureAfter each 7. sample, the chamber was injected with ozone. The results are shown in Figure7.

Figure 7. The curves of change in relative conversion rate of the products in MMH + O3 system Figure 7. The curves of change in relative conversion rate of the products in MMH + O3 system (ﬂushed (flushedwith ozone with continuously). ozone continuously).

WithWith the the consumption consumption of of UDMH, UDMH, the the main main products products FDMH, FDMH, NDMA, NDMA, DMA DMA and and TMT TMT are are generating.generating. The The concentration concentration of of FDMH FDMH (47.5%) (47.5%) increases increases quickly quickly at atfirst ﬁrst 10 10min min and and reaches reaches to toa peaka peak at at40 40min min through through a a20 20 min min terrace, terrace, after after which which it itdecreases decreases and and is is consumed consumed over over 50 50 min. min. NDMANDMA generates generates slowly slowly at at the the first ﬁrst 40 40 min min and and then then increases increases faster. faster. The The sharp sharp increase increase of of NDMA NDMA maymay be be due due to to the the FDMH consuming. consuming. The The concentrations concentrations of of TMT TMT and and DMA DMA are are almost almost unchanged unchanged

Symmetry 2018, 10, 394 8 of 13 Symmetry 2018, 10, x FOR PEER REVIEW 8 of 13 with the ozone injection,injection, indicating that they areare inin lowlow reactivityreactivity withwith ozone.ozone. The molar NDMA conversion rate of UDMH is 57.4% after 70 min.

3.3.3. Influence Influence of Radical Trap (Propylene(Propylene Alcohol)Alcohol) onon NDMANDMA FormationFormation The influenceinfluence of of radical radical trap trap on on the the formation formation of NDMA, of NDMA, as a resultas a result of ozone of reactionozone reaction with UDMH, with isUDMH, presented is presented in Figure in8. Figure 8. The influenceinfluence of radical trap on NDMA formatio formationn from UDMH is is opposite opposite to to that that from from MMH. MMH. The radical traps increase NDMA formation. The yi yieldselds of the other products are suppressed by the presence of the radical trap.

Figure 8. Influence of radical trap on the major products formation (O3/UDMH ratio 5:1). Figure 8. Inﬂuence of radical trap on the major products formation (O3/UDMH ratio 5:1).

3.4. Mechanism of the Reactions between Hydrazines with Ozone

3.4.1. Frontier Molecular Orbitals The frontierfrontier molecular molecular orbitals orbitals [24 ][24] are oftenare often used toused reveal to thereveal relationship the relationship between thebetween electronic the andelectronic geometric and geometric structures structures and to obtain and qualitativeto obtain qualitative information informatio about then about optical the and optical electrical and propertieselectrical properties of molecules. of molecules. The charges The are charges computed are computed by ﬁnding by the finding electron the population electron population of each atom of (Multiwfn)each atom [(Multiwfn)25]. The result [25]. is The that theresult energy is that gaps the between energy the gaps lowest between unoccupied the lowest molecular unoccupied orbital (LUMO)molecular of orbital O3 and (LUMO) the highest of O occupied3 and the molecular highest occupied orbital (HOMO) molecular of orbital H2H4, MMH,(HOMO) UDMH of H2H are4, smallerMMH, UDMH than that are between smaller than the LUMO that between of HH, th MMH,e LUMO UDMH of HH, and MMH, the HOMOUDMH ofand O 3the, respectively. HOMO of Thus,O3, respectively. the reactions Thus, occur the through reactions the electronoccur throug transferh the from electron the HOMO transfer of H from2H4, the MMH HOMO and UDMH of H2H to4, theMMH LUMO and UDMH of O3, suggesting to the LUMO an electrophilic of O3, suggesting attack an of electrophilic O3 to the hydrazines. attack of TheO3 to plots the hydrazines. of energy levels, The HOMOplots of energy and LUMO levels, are HOMO presented and in LUMO Figure are9. presented in Figure 9. To evaluate the the reactive reactive site, site, we we calculated calculated the the atom atom contribution contribution by Hirshfeld by Hirshfeld method method [26]. [For26]. ForH2H H4, 2theH4 ,contributions the contributions of N1 of and N1 andN2 atom N2 atom are similar, are similar, 40.65%. 40.65%. For MMH, For MMH, the contributions the contributions of N1, of N1,N3, N3,C6 are C6 are59.6%, 59.6%, 10.1%, 10.1%, 8.7%, 8.7%, respectively. respectively. This This indicates indicates that that the the reactions reactions based based on on attack attack of of a electrophilic are favored on N1 atom. For UDMH, the contributions of N1, N2, C3, C3, and and C4 C4 are are 55.1%, 55.1%, 8.6%, 7.7%, 7.4%, respectively. Even though the N1 makes the highest contribution, it is difficult difﬁcult to attack duedue toto spaces spaces steric steric effect. effect. The The more more the contributionthe contribution to the to HOMO, the HOMO, the easier the iteasier is to be it oxidized.is to be Therefore,oxidized. Therefore, the most reactive the most site reactive may be site the may N-H be site. the N-H site. Overall, the possible initial step proceeds via H-abstraction of methyl group or amino group. In addition, the H-abstraction of N-H bond is prior to that of C-H bond.

Symmetry 2018, 10, 394 9 of 13

Overall, the possible initial step proceeds via H-abstraction of methyl group or amino group. InSymmetry addition,Symmetry 2018 the, 10 , H-abstraction2018 x FOR, 10 PEER, x FOR REVIEW PEER of N-H REVIEW bond is prior to that of C-H bond. 9 of 13 9 of 13

FigureFigure 9. Figure9.Frontier Frontier 9. Frontier molecular molecular molecular orbital orbital energies orbitalenergies energies levels levels and anlevelsd the the an electronic electronicd the electronic distribution distribution distribution of of hydrazines hydrazines of hydrazines (HOMO)(HOMO)(HOMO) and and ozone ozone and (LUMO) (LUMO) ozone (LUMO) (energy (energy unit (energy unit is is eV). eV).unit is eV).

3.4.2.3.4.2. The The3.4.2. Products Products The Products Formation Formation Formation Mechanism Mechanism Mechanism Initialed Initialed Initialed by by H-Abstraction H-Abstraction by H-Abstraction of of N-H N-H Bondof Bond N-H Bond Based on the frontier molecular orbitals results, the reactions between H H , MMH and UDMH Based Basedon the onfrontier the frontier molecular molecular orbitals orbitals results, results, the reactions the reactions between between H2 2H4 4, MMHH2H4, MMHand UDMH and UDMH withwith ozone ozonewith are ozoneare suggested suggested are suggested to to be be initialed initialed to be initialed by by H-abstraction. H-abstraction. by H-abstraction. The The reaction reaction The reaction of of ozone ozone of with ozonewith N-H N-H with bond bond N-H is isbond is preference to the reaction of ozone with C-H bond. The oxidation of N-H bond in UDMH-O system preferencepreference to the reactionto the reaction of ozone of ozonewith C-H with bond. C-H Thebond. oxidation The oxidation of N-H of bond N-H in bond UDMH-O in UDMH-O3 3 system3 system leadsleads to toleads NDMA NDMA to NDMA formation. formation. formation. The The NDMA NDMA The NDMA formation formation formation is is mainly mainly is throughmainly through through H-abstraction H-abstraction H-abstraction and and O-addition. O-addition. and O-addition. TheThe detail detailThe NDMA NDMAdetail NDMA formation formation formation mechanism mechanism mechanism is is as as follow: follow: is as follow: H C 3 H3C O O O O N+ N- N+ N- 3 3 H3C H3C O H CO H C 3 3 H C H C 3 3 3 3 H3C H C 3 3 N NH N NH N NH N NH 3 N N O N N O 2 H2 C H C H C H C (1) H C H C 3 3 O O 3 3 (1) (1) 3 3 H3C H3C N NH N NH H3C H3C

In the MMH-O3 system, the formation way of methyl radicals is probably an important reaction In the MMH-OIn the MMH-O3 system,3 system, the formation the formation way of way methyl of methyl radicals radicals is probably is probably an important an important reaction reaction pathway [27,28]. The atom contribution results indicate that the possible reaction site is at N1-H bond. pathwaypathway [27,28]. [27,28]. The atom The contributionatom contribution results results indicate indicate that the that possible the possible reaction reaction site is siteat N is1-H at N1-H The H-abstraction of N1-H bond leads to the H3C-N=N formation, which cleave to CH3 and N2 (2). bond. Thebond. H-abstraction The H-abstraction of N1-H of bond N1-H leads bond to leads the Hto3 C-N=Nthe H3C-N=N formation, formation, which whichcleave cleaveto CH 3to and CH N3 2and N2 With the subsequent reactions (2), the methyl radicals formed which account for the high yields of the (2). With(2). the With subsequent the subsequent reactions reactions (2), the (2), methyl the methyl radicals radicals formed formed which whichaccount account for the for high the yields high yields observed dimethylamine groups. of the observedof the observed dimethylamine dimethylamine groups. groups.

OOOOO OOOO  O O O   H C-NHNH⎯⎯3333→⎯⎯ H⎯3 ⎯ C-N3333→⎯⎯ -NH· →⎯⎯3 H C-N=NH→⎯⎯3 →⎯⎯ H C-N=N-→⎯⎯3 →+≡· CH→+≡ N N 32323H3C-NHNHH32323 C-NHNH2 → H3C-N H C-N-NH -NH2 → H3C-N=NH H C-N=NH→ H3C-N=N- 3 H 3 C-N=N-→ CH3 + N 3≡ CHN 3 N N (2)(2) (2) It is noticeableIt is noticeable that MMH that MMHhas also has high also potentia high potential for NDMAl for NDMA formation. formation. The main The reason main reason is the is the methylmethyl radical radical (CH3·) (CHformation3·) formation in MMH-O in MMH-O3 system.3 system. The methyl The methyl process process can promote can promote more stablemore stable compoundscompounds with dimethylaminewith dimethylamine groups gr formationoups formation which whichfinally finallygenerate generate NDMA, NDMA, so that so the that the productsproducts in MMH-O in MMH-O3 system3 system are similar are similar to those to in those the UDMH-Oin the UDMH-O3 system.3 system.

Symmetry 2018, 10, 394 10 of 13

It is noticeable that MMH has also high potential for NDMA formation. The main reason is the methyl radical (CH3·) formation in MMH-O3 system. The methyl process can promote more stable compounds with dimethylamine groups formation which ﬁnally generate NDMA, so that the products in MMH-O3 system are similar to those in the UDMH-O3 system.

3.4.3. TheSymmetry Products 2018, 10 Formation, x FOR PEER REVIEW Mechanism Initialed by H-Abstraction of C-H Bond 10 of 13

TheSymmetry3.4.3. observed The 2018 Products, 10 non-negligible, x FOR Formation PEER REVIEW yields Mechanism of HCHO Initialed indicate by H-Abstraction that the reactions of C-H initialed Bond by H-abstraction10 of 13 of N-H bond cannot be the only process. The formation of these products can be attributed to the SymmetryThe 2018 , 10observed, x FOR PEER non-negligible REVIEW yields of HCHO indicate that the reactions initialed10 of 13 by occurrence3.4.3. of The an Products alternate Formation reaction Mechanism route initialed Initialed by H-abstraction by H-Abstraction of C-Hof C-H bond. Bond H-abstraction of N-H bond cannot be the only process. The formation of these products can be Many researches [29–31] report that the atmospheric element of the -CH oxidation is HCHO, 3.4.3.attributed TheThe Products observedto the occurrenceFormation non-negligible Mechanismof an alternate yields Initialed reactionof HCHO by routeH-Abstraction indicate initialed bythatof H-abstractionC-H the Bond reactions3 of C-Hinitialed bond. by which isH-abstraction detected by of FT-IR N-H inbond MMH-O cannot3 andbe the UDMH-O only process.3 system. The Theformation HCHO of formationthese products pathway can be is: TheMany observed researches non-negligible [29–31] report yields that theof HCHOatmospheric indicate element that of thethe -CHreactions3 oxidation initialed is HCHO, by attributedwhich is detected to the occurrence by FT-IR in of MMH-O an alternate3 and reaction UDMH-O route3 system. initialed The by HCHO H-abstraction formation of C-Hpathway bond. is: H-abstraction of N-H bond cannotO be theO only/O process. The formation of these products can be Many researches [29–31] 3report that· 3the 2atmospheric element of the -CH3 oxidation is HCHO, R-CH3 → R-CHOO/O→ R-(O)CH2 → R + HCHO (3) attributed to the occurrenceR-CH of an⎯⎯ alternate332→⎯⎯⎯ R-CH2 reaction →→route R-() Oinitialed CH by R+HCHO H-abstraction of C-H bond. which is detected by FT-IR in32 MMH-O3 and UDMH-O3 system. 2 The HCHO formation pathway is:(3) Many researches [29–31] report that the atmospheric element of the -CH3 oxidation is HCHO, OO/O As soon as HCHO formed [32⎯⎯,33332→⎯⎯⎯],3 MMH and→→ UDMH3 () would immediately react with HCHO, which isAs detected soon as by HCHO FT-IRR-CH formedin MMH-O32 [32,33], R-CH and MMH UDMH-O and R- UDMH system. O CH would2The R+HCHOHCHO immediately formation react pathway with HCHO, is: (3) finally leadfinally to lead thehydrazones to the hydrazones formation. formation. This can This explain can explain for the for high the yields high ofyields hydrazones of hydrazones observed R-CH⎯⎯OO/O332→⎯⎯⎯ R-CH →→ R-() O CH R+HCHO by GC-MS.observedAs soon by GC-MS. as HCHO formed32 [32,33], MMH and UDMH 2 would immediately react with HCHO,(3) finally lead to the hydrazones formation. This can explain for the high yields of hydrazones As soon as HCHO formedR-NH [32,33],2R-NH+ HCHO MMH +HCHO an→d→+ R-N=CHUDMH R-N=CH would2 + HH Oimmediately2O react with HCHO, (4) observed by GC-MS. 222 (4) finally lead to the hydrazones formation. This can explain for the high yields of hydrazones Furthermore,Furthermore, UDMH UDMH removes removes one one methyl methyl group→+ group to to form form MMH.MMH. MMH after after removing removing one one observed by GC-MS. R-NH222 +HCHO R-N=CH H O (4) methylmethyl group group converts converts to N toH N.2 InH4 the. In NtheH N2-OH4-Osystem,3 system, the the major major productsproducts are H 2OO2, N,N2 andand H2 HO O 2 4 2 4 3→+ 2 2 2 2 Furthermore, UDMH removesR-NH 222one +HCHO methyl R-N=CHgroup to form H O MMH. MMH after removing(4) one observedobserved in experiments. in experiments. The The major major ultimate ultimate fate fate of of nitrogen nitrogen inin thethe hydrazines-Ohydrazines-O3 3systemsystem is N is2. N 2. methyl group converts to N2H4. In the N2H4-O3 system, the major products are H2O2, N2 and H2O Hence,Hence, theFurthermore, demethylation the demethylation UDMH process removes process is an oneis important an methyl important group step step for to theforform the conversion MMH. conversion MMH of of UDMH afterUDMH removing and and MMH MMH one to to N 2. observedN2. in experiments. The major ultimate fate of nitrogen in the hydrazines-O3 system is N2. methyl group converts to N2H4. In the N2H4-O3 system, the major products are H2O2, N2 and H2O Hence, the demethylation process is an important step for the conversion of UDMH and MMH to observed in experiments.H C The major ultimate fate of nitrogen in the hydrazines-O3 system is N2. N2. 3 H3C Hence, the demethylation process is an important step for the conversion of UDMH and MMH to(5) (5) N NH N NH H2N NH2 NN 2 H C 2 2 N . H33C H C H3 N NH H N NH NN (5) H3C 2 N NH2 2 2 Moreover,H3C the formation pathwayH3C of another main products (DMF) would include N-N bond H (5) Moreover,cleavage thestep. formation BasedN NH on pathwaythe Sun’s ofNreports, another the main C·HH2 productsNHNH2N NH2 radical (DMF) dissociatesNN would include quickly N-N to the bond H C 2 NH2 2 Moreover,3 the formation pathwayH of another main products (DMF) would include N-N bond cleavageproducts step. Based H2C=NH on the + NH Sun’s2 with reports, a low barrier the C· ofH 212.0NHNH kcal/mol2 radical [34,35]. dissociates quickly to the products cleavage step. Based on the Sun’s reports, the C·H2NHNH2 radical dissociates quickly to the H2C=NHMoreover, + NH2 with theH aCformation low barrier pathway of 12.0 of kcal/molanother main [34 ,35products]. (DMF) wouldO include N-N bond products H2C=NH3 + NH2 with a lowH2C barrier of 12.0 kcal/mol [34,35]. cleavage step. Based on the Sun’s reports,N NH the C·H2NHNHN2 Rradical dissociates quickly to the(6) N NH2 2 H2C R-N-CH products H2C=NHHR +C NH2 with a low Rbarrier of 12.0 kcal/mol [34,35]. O 3 H2C N NH N R (6) (6) N NH2 2 H2C R-N-CH Thus, HH2C=NH3C is an important intermediate for DMF formation. DMFO formation proceeds via R H2CR (6) H-abstraction of C-HN NH bond. N NH2 H C N R 2 R 2 R-N-CH Thus, HR2C=NH is an important intermediate for DMF formation. DMF formation proceeds via Thus,H-abstraction3.4.4. H Influence2C=NH of isof C-H anRadicals importantbond. on NDMA intermediate Mechanism for DMF formation. DMF formation proceeds via H-abstractionThus, ofH2 C-HC=NH bond. is an important intermediate for DMF formation. DMF formation proceeds via H-abstraction of C-H bond. 3.4.4.The Influence heterolytic of Radicals cleavage on NDMApathways Mechanism explain the significant HO· formation during ozonation of 3.4.4. InfluenceMMH and of UDMH Radicals [36,37]. on NDMA Mechanism 3.4.4. InfluenceThe heterolytic of Radicals cleavage on NDMA pathways Mechanism explain the significant HO· formation during ozonation of R-H+O→⋅→ HOOO HO +O TheMMH heterolytic and UDMH cleavage [36,37]. pathways explain32 the significant HO· formation during ozonation(7) of The heterolytic cleavage pathways explain the significant HO· formation during ozonation of MMHMMH and and UDMHIn the UDMH presence [36 ,[36,37].37]. of excess ozone,R-H+O the radical→⋅→ HOOO trap would HO increase +O the NDMA yields from UDMH, 32· (7) and decrease the NDMA yieldsR-H +fromO3 →MMH.HOOO This· → mayHO indicate+O2 that HO· plays a key role in (7) R-H+O→⋅→ HOOO HO +O destructionIn the presence of the C-H of excessand N-H ozone, bonds. the 32radical trap would increase the NDMA yields from UDMH,(7) Inand the presencedecrease the of excess NDMA ozone, yields the from radical MMH. trap This would mayincrease indicate thethat NDMA HO· plays yields a key from role UDMH, in In the presence of excess ozone, the radical trap would increase the NDMA yields from UDMH, and decreasedestruction the NDMA of the C-H yields andH3C fromN-H bonds. MMH. This may indicateH C that HO· plays a key role in destruction and decrease the NDMA yields from MMH. This may2 indicate that HO· plays a key role in N NH + HO + (8) of the C-H and N-H bonds. 2 N NH2 H2O destruction of the C-H andHH N-H3CC bonds. H C 3 H32C N NH + HO + (8) H C 2 N NH2 H2O H3 C H2C 3 H3C H3CN NH + HO + (8) 2 N NH2 H2O (8) H3C (9) N NH2 + HO H3CH C N NH + H O H C 3 2 2 H3 N NH HO (9) 2 + H3C N NH + H O The differenceH between3C UDMH and MMH is the (CH3)2N group2 and2 (CH3)HN group. As H shown in (5), UDMH couldN convertNH + to HNDMAO directly by the oxidation the amino groups, with(9) high 2 H3C N NH2 + H2O yieldsThe (88%). difference The reaction betweenH between UDMH HO· and and MMH the C-His the bonds (CH 3of)2N UDMH group leadsand (CHto HCHO3)HN group.and other As shown in (5), UDMH could convert to NDMA directly by the oxidation the amino groups, with high The difference between UDMH and MMH is the (CH3)2N group and (CH3)HN group. As yields (88%). The reaction between HO· and the C-H bonds of UDMH leads to HCHO and other shown in (5), UDMH could convert to NDMA directly by the oxidation the amino groups, with high yields (88%). The reaction between HO· and the C-H bonds of UDMH leads to HCHO and other

Symmetry 2018, 10, x FOR PEER REVIEW 10 of 13

3.4.3. The Products Formation Mechanism Initialed by H-Abstraction of C-H Bond The observed non-negligible yields of HCHO indicate that the reactions initialed by H-abstraction of N-H bond cannot be the only process. The formation of these products can be attributed to the occurrence of an alternate reaction route initialed by H-abstraction of C-H bond. Many researches [29–31] report that the atmospheric element of the -CH3 oxidation is HCHO, which is detected by FT-IR in MMH-O3 and UDMH-O3 system. The HCHO formation pathway is:

⎯⎯OO/O332→⎯⎯⎯ →→() R-CH32 R-CH R- O CH 2 R+HCHO (3)

As soon as HCHO formed [32,33], MMH and UDMH would immediately react with HCHO, finally lead to the hydrazones formation. This can explain for the high yields of hydrazones observed by GC-MS. →+ R-NH222 +HCHO R-N=CH H O (4)

Furthermore, UDMH removes one methyl group to form MMH. MMH after removing one methyl group converts to N2H4. In the N2H4-O3 system, the major products are H2O2, N2 and H2O observed in experiments. The major ultimate fate of nitrogen in the hydrazines-O3 system is N2. Hence, the demethylation process is an important step for the conversion of UDMH and MMH to N2. H C 3 H3C N NH H N NH NN (5) 2 N NH2 2 2 H3C H Moreover, the formation pathway of another main products (DMF) would include N-N bond cleavage step. Based on the Sun’s reports, the C·H2NHNH2 radical dissociates quickly to the products H2C=NH + NH2 with a low barrier of 12.0 kcal/mol [34,35]. H C O 3 H2C (6) N NH N NH2 H C N R 2 R 2 R-N-CH R

Thus, H2C=NH is an important intermediate for DMF formation. DMF formation proceeds via H-abstraction of C-H bond.

3.4.4. Influence of Radicals on NDMA Mechanism The heterolytic cleavage pathways explain the significant HO· formation during ozonation of MMH and UDMH [36,37]. →⋅→ R-H+O32 HOOO HO +O (7)

In the presence of excess ozone, the radical trap would increase the NDMA yields from UDMH, and decrease the NDMA yields from MMH. This may indicate that HO· plays a key role in destruction of the C-H and N-H bonds. H C 3 H2C Symmetry 2018, 10, 394 N NH + HO + (8)11 of 13 2 N NH2 H2O H3C H C 3

H3C N NH + HO (9) (9) 2 H3C N NH2 + H2O H

The difference between UDMH and MMH is the (CH3)2N group and (CH3)HN group. As The difference between UDMH and MMH is the (CH ) N group and (CH )HN group. As shown shown in (5), UDMH could convert to NDMA directly by 3the2 oxidation the amino3 groups, with high in (5), UDMH could convert to NDMA directly by the oxidation the amino groups, with high yields yields (88%). The reaction between HO· and the C-H bonds of UDMH leads to HCHO and other (88%). The reaction between HO· and the C-H bonds of UDMH leads to HCHO and other products

(amines and CH3NO2 et al.) formation. Meanwhile, for MMH, the reaction between HO· and N-H bond of MMH can promote the methyl radical formation, ﬁnally leads to the increase of NDMA yields.

4. Conclusions

The products of N2H4, MMH and UDMH reaction with ozone are detected by GC-MS. The major products in the presence of excess hydrazines or excess ozone, the radical influence on NDMA formation are investigated. The reaction mechanisms which account for the major products (especially NDMA) are discussed. More specific conclusions are follows: Apart from the products (NDMA, N2O et al.) detected by FT-IR, the new compounds (hydrazones, formamides, dimethylamine, 1,1,4,4-tetramethyl-1,2-tetrazene, dimethylamino-acetonitrile, N2, H2O, et al.) are identified by GC-MS. And the relative yields of the organic products vary considerably depending on the O3/MMH or UDMH ratios. The ozonation of UDMH yields high amounts of NDMA (>50%). It is noticeable that MMH also has potential for NDMA formation in the presence of excess ozone. Radicals play an important role in NDMA formation. It can hinder NDMA formation from UDMH and promote NDMA formation from MMH. It may be due to that HO· makes many contributions to destructing C-H and N-H bonds. An analysis of the energy gap between the frontier molecular orbitals of the reactants indicates that the oxidations of the hydrazines as well as N-H and C-H bonds are initiated by hydrogen atom abstraction. The oxidation of N-H bond is the most favorable. The major ultimate fate of nitrogen (N2H4, MMH, UDMH) may be N2 through the demethylation process. However, the reaction of ozone with N-H bonds is preference to that with C-H bonds. NDMA and methyl radical formation proceeds via H-abstraction from N-H bonds. The N2, formaldehyde, amines, formamide and hydrazones formation proceeds via H-abstraction from C-H bonds.

Author Contributions: Formal Analysis, D.H.; Investigation, D.H.; Data Curation, Y.Y.; Writing-Original Draft Preparation, D.H.; Supervision, X.G., X.L. and X.W.; Funding Acquisition, Z.X. Acknowledgments: Authors thank for Mis Yang for the English checking. Funding: This research was financially supported by China Postdoctoral Science Foundation (grant No. 2016M600084). Conﬂicts of Interest: The authors declare no competing ﬁnancial interests.

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