A numerical and experimental study of the decompositionpathways of guanidinium nitrate

Anand Sankaranarayanan1 • Lovely Mallick2 • Neeraj R. Kumbhakarna1

Received: 1 November 2016 / Accepted: 12 September 2017

� Akademiai Kiado, Budapest, Hungary 2017

Abstract The thermal decomposition behaviour of guani-

dinium nitrate (GN), an energetic material, was analysed

using a combined experimental and computational

approach. Simultaneous thermogravimetric analysis,

Fourier transform infrared spectroscopy and mass spec-

trometry (TG-FTIR-MS) experiments were carried out at

three different heating rates under closed and open crucible

conditions. A two-stage decomposition process was

observed, and the major gases evolved were found to be

NH3, N2O, NO2 and CO2. Quantum mechanics based

ab initio computations were performed to evaluate the

possible decomposition pathways available for GN. Results

indicate that decomposition of GN is not initiated in the

condensed phase as the guanidinium cation and the nitrate

anion are highly stable. The most likely mechanism

involves isomerization of GN followed by a proton transfer

in the gas phase to yield nitric acid and guanidine. These

products then further react to form nitroguanidine (NQ)

and H2O. NQ dissociates via several competing pathways

to yield NH3, N2O, H2O and CO2. HNO3 decomposition

can help explain NO2 formation. The residue left towards

the end of TG can be attributed to dimerization and

trimerization reactions of cyanamide.

Keywords Guanidinium nitrate � Thermal decomposition �Energetic material � TG–FTIR–MS � Ab initio

computations

Introduction

Guanidinium nitrate (GN) is a very stable compound [1]

commonly used as a gas-generating agent [2, 3] and in

solid propellant formulations [4]. GN is used to produce

nitroguanidine (NQ) [5], also known as picrate, which is a

secondary explosive often used in triple-base formulations

as a gun propellant and in impact insensitive military

ammunition [6]. The molecular structure of GN is shown in

Fig. 1.

The decomposition study of any propellant can help us

formulate detailed chemical reaction mechanisms with

which the combustion behaviour of the same can be

modelled [7]. Such modelling can be helpful in controlling

emissions and also provide us valuable guidelines regard-

ing the best practices to be followed during handling,

storage, transportation and disposal of these compounds

[8]. These studies also provide information which could aid

the development of better propellants in the future. Meth-

ods like differential thermal analysis (DTA), thermo-

gravimetric (TG) analysis and differential scanning

calorimetry (DSC) are the most commonly used diagnostic

tools, deployed to gain insights on the decomposition

behaviour of any energetic material [9–12].

One of the earliest studies on the decomposition beha-

viour of guanidine salts, to assess their thermal stability,

was carried out by Fauth [13]. Investigations were done

using DTA and TG, under conditions involving heating

rates of 8 �C per minute. Decomposition of GN was found

to be initiated at temperatures of around 100 �C and was

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10973-017-6707-4) contains supplementarymaterial, which is available to authorized users.

& Neeraj R. Kumbhakarna

neeraj_k@iitb.ac.in

1 Department of Mechanical Engineering, Indian Institute of

Technology, Bombay, Mumbai 400076, India

2 Department of Aerospace Engineering, Indian Institute of

Technology, Bombay, Mumbai 400076, India

123

J Therm Anal Calorim

DOI 10.1007/s10973-017-6707-4

95% complete at 250 �C. Mass spectral study of GN, along

with simultaneous TG–DTA studies, was carried out by

Udupa [14]. The TG–DTA studies were performed in an

argon atmosphere, with a heating rate of 4 �C per minute.

The DTA curve was reported to have an endotherm at

210 �C and an exotherm at 305 �C. The endothermic and

exothermic peaks were attributed to the melting of GN and

to the oxidative decomposition of GN, respectively. The

decomposition of GN was found to commence at 230 �Cand achieved completion by 500 �C. It was observed to be

fast between 275 and 325 �C and sluggish in the temper-

ature range 325–500 �C. The mass spectral results at

230 �C suggested the initiation of decomposition through a

proton transfer mechanism. Wendlandt et al. [4] reported

TG and DSC curves for guanidinium salts in the temper-

ature range between 25 and 500 �C. The studies were done

in a dynamic N2 environment with heating rates of 10 �Cper minute. The authors report a gradual mass loss for GN

beginning at around 50 �C with the main apparent disso-

ciation occurring at around 215 �C. The exothermic peak at

305 �C reported by Udupa [14] was not observed in this

study. The anomaly was thought to be caused due to dif-

ference in the sample size, design of sample container and

heating rates. Oxley et al. [15] reported the activation

energy (Ea) and pre-exponential factor (A) values for the

decomposition of GN under isothermal heating conditions

and proposed three possible decomposition routes of GN:

first involving deprotonation, second proceeding by dehy-

dration and the third resulting in ammonium nitrate as an

intermediate. Decomposition route via formation of urea

was ruled out as neither urea nor its decomposition prod-

ucts were observed. Damse [16] investigated the cause for

fast burning behaviour of triaminoguanidine azide

(TAGAZ). As a part of this study, TG, DTA and DSC

studies were performed to understand the thermal beha-

viour of TAGAZ and GN. In addition, hyphenated tech-

niques of thermogravimetry–Fourier transformer infrared

spectroscopy (TG–FTIR) and pyrolyser–gas chromatogra-

phy–mass spectroscopy (Py–GC–MS) were also used in

order to find out the decomposition gases. It was concluded

that the decomposition of GN proceeded through the dis-

sociation of relatively strong C–N bonds. FTIR results

indicated evolution of NH3 and HCN during the first and

second stages of decomposition, respectively. An exotherm

was reported between 290 and 325 �C with a peak at

303.17 �C. Mei et al. [3] investigated the thermal decom-

position of GN and basic cupric nitrate mixtures using

thermogravimetry–differential scanning calorimetry–mass

spectrometry–Fourier transform infrared spectroscopy

(TG–DSC–MS–FTIR) and automatic calorimeter. Results

of pure GN samples were also reported. The onset of GN

decomposition was reported to occur at 278 �C. 72.8% of

the sample was found to be dissociated by 320 �C. The

authors report no exothermic peaks for decomposition of

pure GN.

Studies on thermal decomposition behaviour of

nitroguanidine (NQ), a dehydration product of GN, can be

found in literature [17]. NQ was found to be relatively

stable below its melting point of approximately 230 �C,

above which it decomposed, evolving ammonia and water

vapour, and forming solid products. The major intermedi-

ates identified during the decomposition of NQ were

nitrous oxide, cyanamide, melamine, cyanic acid, cyanuric

acid, ammeline and ammelide. The compounds, on further

decomposition, yielded carbon dioxide, urea, nitrogen,

hydrogen cyanide, cyanogen and other compounds not yet

fully confirmed, such as melam, melem and mellon prob-

ably containing condensed triazine rings. Most of the

products reported during NQ decomposition have been

observed for GN decomposition as well, indicating that NQ

could be an intermediate during GN decomposition. There

is no available literature on the use of ab initio computa-

tional methods to assess the decomposition behaviour of

GN to the best of the authors’ knowledge. How-

ever, studies on guanidine derivatives [18–20] may provide

useful insights in this regard. These studies reveal that

guanidinium azotetrazolate (GzT) and guanidinium

5-amino tetrazolate (GA) do not allow for a proton transfer

in the condensed phase. Their condensed phase decompo-

sition proceeds though the dissociation of the anion frag-

ments. Similar results can be expected for GN. The

dissociation of the nitrate anion is, however, unlikely as it

is quite stable.

Although much work has been done in the past few dec-

ades on studying the thermal decomposition behaviour of

GN, they have been inconclusive on the exact decomposition

mechanism of GN. Inconsistency in experimental data

reported in separate studies has also been noted. Some of

these studies have been performed with equipment which are

nowadays considered obsolete. It is therefore imperative to

revisit the decomposition study of GN. Developing a

detailed chemical kinetic mechanism (DCKM) for GN

decomposition could also prove to be beneficial for future

studies in this area. The present work attempts to elucidate

the reaction mechanism for GN decomposition while vali-

dating experimental results from previous studies.

NC

+

N

N

H

H

H

H

HH

N+

O-

O-

O

Fig. 1 Molecular structure of guanidinium nitrate

A. Sankaranarayanan et al.

123

The technique chosen for this purpose was the

hyphenated TG–FTIR–MS method, supported by quantum

mechanics based ab initio calculations. GN was subjected

to testing under three distinct heating rates to extract the

variation of global activation energies and pre-exponential

factors with the reacted mass fraction using iso-conver-

sional methods and the kinetic compensation effect. The

TG and TDG curves were plotted to identify decomposi-

tion patterns and the temperature regimes of their occur-

rence. The gas species evolved during the process were

identified using FTIR spectroscopy. The mass spectra,

though uncalibrated, served as a valuable corroboration of

the data obtained from the FTIR spectra. Ab initio com-

putations were then performed to explain the formation of

product gases detected during experiments.

Methodology for experimental and numericalwork

The decomposition of GN was studied using the hyphen-

ated TG–FTIR–MS method. FTIR and MS, when used

alongside TG, have been found to be synergistic in nature,

as the complete spectrum of the evolved gases is revealed

by either of the instruments. A detailed description of the

experimental set-up used for our present study is given in

Reference [21]. A brief description of the same along with

the experimental procedure employed for our present work

is given here. The particle size distribution of GN used for

our experiments along with digital microscopic images of

the same has been provided in Online Resource 1. Tests

were conducted under open and pierced lid conditions with

approximately 2 mg of the GN sample placed in an alu-

mina crucible. A constricting lid with 0.1-mm hole at the

centre was used to cover the crucible for the pierced lid

tests. The thermal decomposition process of GN was

studied by using Netzsch 209 thermogravimetry analyser,

with heating rates of 5, 10 and 15 K min-1.

The computational part of the present work was carried

out with the help of Gaussian 09 [22] suite of programs.

Preliminary investigations for ground-state and transition-

state optimizations were done using the B3LYP density

function theory with 6-31?? G(d,p) basis set [23, 24]. The

more accurate CBS-QB3 [25] compound method was used to

recalculate the values which have been reported for the

reaction pathways listed in this paper. The same has been

recognized to provide a good balance between accuracy of

results and computational resources required for calculations

[26, 27]. Further details of the methodology adopted for

numerical simulations can be found in Online Resource 2.

Results and discussion

TG Data and Kinetic Analysis

The TG and DTG plots of GN crystals subjected to a

heating rate of 15 K min-1 are presented in Fig. 2. The

samples were analysed under two crucible conditions: open

condition (oc) and pierced lid condition (cc). All experi-

ments were repeated thrice with similar test conditions, and

the average values were used for plotting the results.

The first stage of the decomposition process, between

260 and 336 �C, was observed to be fast with 75% mass

loss. The second stage was found to be sluggish in com-

parison to the first stage with a mass loss of approximately

10% and left 15% residues at the end temperature of

500 �C. The DTG peak was observed towards the end of

the first stage at a temperature of around 314 �C for GNoc

and GNcc. Similar trends have been reported by other

researchers [14, 16]. The onset and extent of decomposi-

tion, however, varied according to the experimental con-

ditions. The TG curves and the DTG peaks for closed and

open crucible tests were found to nearly overlap each other

with the rate of mass loss percentage of TGcc being higher

than that of TGoc. The occurrence of a two-stage decom-

position process can be understood by plotting the variation

of the global activation energy (Ea) with the reacted mass

fraction, a. The same has been shown in Fig. 3 for GNoc

and GNcc. Ea was calculated using the procedure outlined

in our previous work [21], with heating rates of 5, 10 and

15 K min-1. The curves were found to follow similar

trends, with the first four points constituting the fast

decomposition stage and fourth and fifth points

Temperature/°C

Mas

s/%

DTG

/% m

in–1

100 200 300 400 5000

20

40

60

80

100

–6

–5

–4

–3

–2

–1

0

TGOCDTGOCTGCCDTGCC

Fig. 2 TG–DTG of GNoc and GNcc at a heating rate of 15 K min-1

A numerical and experimental study of the decomposition pathways of guanidinium nitrate

123

representing the slow decomposition stage as apparent

from the sudden increase in Ea value.

The activation energies of GNoc and GNcc were found to

be approximately constant from a = 0.1 to 0.8 with aver-

age values of 139 and 156 kJ/mol, respectively. The pre-

exponential factors (A) for open and pierced lid conditions

were found to be 2.64 9 109 and 7.53 9 1010 min-1,

respectively. The Ea increased considerably, henceforth, till

it attained a maximum of 201 and 239 kJ/mol at a = 0.9

for GNoc and GNcc, respectively. The activation energy

(Ea) and pre-exponential factor (A) of recrystallized GN,

under isothermal heating conditions, reported in [15] were

199 kJ/mole and 1.94 9 1015 s-1, respectively. Purity of

the GN sample, particle size distribution, heating rates,

sample size, crucible conditions and nature and flow rate of

purge gas are some of the factors which can affect the

results.

TG–FTIR analysis

The FTIR spectra of the evolved gases during the decom-

position process of GNoc and GNcc for a heating rate of

15 K min-1, at various junctures, are shown in Figs. 4 and

5, respectively. The quantity of the evolved gases was too

minute to be detected at the decomposition onset temper-

ature of 285 �C. The second spectrum was taken at a

temperature of 314 �C, corresponding to the maximum of

the DTG signal. The major species detected were CO2,

HNCO, N2O, HNO3, NO2 and NH3. In this stage, the

intensities of N2O and NH3 were high, while that of HNO3

was low. The intensities of CO2, HNCO and N2O were

found to be interdependent and varying. The end of the fast

decomposition phase was marked by the high transmittance

values of the third spectrum, taken at a temperature of

333 �C. The fourth spectrum, at 409 �C, corresponds to the

Reacted fraction

E a/k

Jmol

–1

0 0.2 0.4 0.6 0.8 1

140

160

180

200

220

240GNOCGNCC

Fig. 3 Comparison of activation energies with reacted fraction

during decomposition of GNoc and GNcc

100015002000250030003500

0.97

0.98

0.99

1

1

1

1

1

(a)

0.97

0.98

0.99

(b)HNO3

N2ONO2N2O NH3

CO2

Tran

smitt

ance

0.97

0.98

0.99

(c)

0.97

0.98

0.99

(d)NH3

NO2

N2ON2O

CO2 HNCO

Wavenumber/cm–1

1000150020002500300035000.97

0.98

0.99

(e)

Fig. 4 FTIR spectra of evolved gases during decomposition of GNoc at 15 K min–1; (a) T = 285 �C, (b) T = 314 �C, (c) T = 333 �C, (d)T = 409 �C and (e) T = 500 �C

A. Sankaranarayanan et al.

123

middle of the second stage of decomposition, and the last

spectrum shown in Figs. 4(e) and 5(e) represents the end of

the programmed temperature, at 500 �C.

In order to further understand the kinetics of the reac-

tions governing the decomposition of GN, the FTIR spectra

of the evolved gases were quantified using the iterative

procedure described in our previous work [21]. The results

for open and pierced lid conditions obtained using the same

are respectively displayed in Figs. 6 and 7.

The figures show the variation of relative mole fractions

of various species with temperature, which appear in the

ppm range, owing to the overabundance of nitrogen as the

purge gas. It is clear from both the figures that NH3 and

N2O are the dominant species. H2O was detected only for

open condition. The ratio of the mole fractions of various

species with respect to that of NO2, at the temperature

where the FTIR signals were the strongest, is shown in

Table 1. The mole fraction of CO2 was comparable to that

100015002000250030003500

0.97

0.98

0.99

1

1

1

1

1

(a)

0.97

0.98

0.99

(b) N2ONO2N2O NH3

CO2

Tran

smitt

ance

0.97

0.98

0.99

(c)HNCO

0.97

0.98

0.99

(d)NH3

NO2

N2ON2O

CO2

Wavenumber/cm–1

1000150020002500300035000.97

0.98

0.99

(e)

Fig. 5 FTIR spectra of evolved gases during decomposition of GNcc at 15 K min–1; (a) T = 285 �C, (b) T = 314 �C, (c) T = 333 �C, (d)T = 409 �C and (e) T = 500 �C

Temperature/°C

Rel

ativ

e m

ole

frac

tion/

%

Mas

s los

s/%

250 300 350 400 450 500

0.02

0.04

0.06

0.08

0.1

0.12

20

40

60

80

100NH3N2OH2ONO2CO2HNO3TGOC

Fig. 6 Relative mole fraction of evolved gases quantified from FTIR

spectra during decomposition of GNoc at 15 K min-1

Temperature/°C

Rel

ativ

e m

ole

frac

tion/

%

Mas

s los

s/%

250 300 350 400 450 5000

0.02

0.04

0.06

0.08

0.1

0.12

20

40

60

80

100NH3N2OH2ONO2CO2HNO3TGCC

Fig. 7 Relative mole fraction of evolved gases quantified from FTIR

spectra during decomposition of GNcc at 15 K min-1

A numerical and experimental study of the decomposition pathways of guanidinium nitrate

123

of NO2, while that of HNO3 found to be considerably lower

than NO2.

TG–MS analysis

The mass spectra revealed a wider array of species as

compared to the FTIR spectra. Major peaks were obtained

corresponding to m/z = 16, 17, 18 and 44. These values

can be assigned to O/NH2, NH3/OH, H2O and CO2/N2O,

respectively. In addition to this, smaller signals were also

detected at m/z = 12, 14, 20, 22, 23, 26, 28, 32, 34, 36, 40,

41, 42, 43, 45 and 46. Important among these corresponds

to CO/N2 (m/z = 28), H2NCN/NHCNH (m/z = 42),

HNCO (m/z = 43) and NO2 (m/z = 46). The result is

somewhat consistent with those obtained in previous

studies [14, 16]. The signals corresponding to the species

NH3, N2O, NO2, CO2, H2O and HNCO, identified in FTIR,

were confirmed through MS.

SEM analysis of GN

In our previous work [21], the nature of ammonium per-

chlorate (AP) particles was studied under various condi-

tions. The porosity observed in the microstructure under

specific conditions helped to understand the complexity of

AP decomposition to a certain extent. Similar studies on

Table 1 Ratio of relative mole fractions (%) of various species obtained during decomposition of GN at 15 K min-1

Temperature (314 �C) N2O: NO2 NH3: NO2 CO2: NO2 HNO3: NO2

GNoc 4.68 8.01 1.40 0.44

GNcc 2.88 4.59 1.10 0.17

Fig. 8 SEM images of the decomposition of GNoc (a, b, c) at 313 �C and GNcc (d, e, f) at 317 �C for a heating rate of 15 K min-1

A. Sankaranarayanan et al.

123

GN particles at various temperatures and crucible condi-

tions are crucial in understanding its decomposition

mechanism. The GN samples were subject to TG tests

under open and pierced lid conditions, at a heating rate of

15 K min-1, as explained in the ‘‘TG Data and Kinetic

Analysis’’ section. The end of programmed temperature for

GNoc and GNcc was chosen as 313 and 317 �C, respec-

tively, based on the DTG peaks of the compound. The

samples were removed from the crucible on completion of

the tests, with the help of a spatula and stored in a dry

place. Morphologies of the samples thus obtained were

analysed using a JSM–7000F field emission gun scanning

electron microscope. Figure 8 shows SEM images of GN

samples, visualized using varying scales, for open and

pierced crucible tests. The particles from open crucible

tests were found to be larger in size than those from pierced

lid tests, where they were observed to be fragile and had

fragmented into smaller pieces. Anomalies in

Table 2 Condensed phase decomposition pathways for guanidinium nitrate with thermodynamic parameters computed at CBS-QB3 level of

theory

No Reaction DHRa D�Hf

b D�Hbc DGR

d D�Gfe D�Gb

f

R1GN INT 1_c

TS1_c 90.4 211.3 120.9 109.5 264.7 138.7

R2TS2_c

INT1_c + N

O

OOH

NH2NH2

NH

2.6 5.7 3.1 -3.0 3.4 6.9

R3 NH2NH2

NH

+ N

O

OOH

TS3_c

NH

NH2 NHN

O

O

+ OH2

7.4 106.5 99.1 0.3 239.9 239.6

R4 TS4_cONO2

–NO3

–230.6 432.5 201.9 222.6 490.9 234.8

R5TS5_c

ONO2–G+ + +

NH2NH2

NH

N

OO

O

H

1.3 -3.2 -4.4 2.5 8.0 5.0

R6TS6_c

N

O

OOH

N

OO

O

H

-144.3 137.2 281.5 -132.4 156.8 309.1

R7 TS7_cG+ INT2_c

111.9 211.2 99.3 124.9 265.8 122.1

R8 TS8_cINT2_c INT3_c

11.0 82.4 71.5 10.3 78.1 66.3

R9 TS9_cINT3_c NH4

+ + C NNH

H 7.3 86.1 78.8 -85.8 77.0 175.8

R4 has been computed at B3LYP level of theory using 6-31??G(d,p) basis set

_c has been used to denote condensed phase reactionsa Enthalpy of reaction (kJ/mol)b Activation enthalpy in the forward direction (kJ/mol)c Activation enthalpy in the backward direction (kJ/mol)d Gibbs free energy of reaction (kJ/mol)e Gibbs free energy of activation in the forward direction (kJ/mol)f Gibbs free energy of activation in the backward direction (kJ/mol)

A numerical and experimental study of the decomposition pathways of guanidinium nitrate

123

microstructure or nuclei formation on the surface were not

found, which may be indicative of decomposition process

dominating over sublimation.

Numerical simulations

Condensed phase reactions

As the objective of the numerical simulation was to explain

the species detected in the experimental studies, the con-

densed phase mechanisms were explored first. The path-

ways involving proton transfer, dehydration of GN and

formation of ammonium nitrate as an intermediate during

GN decomposition, as suggested by Oxley et al. [15], were

explored. A mechanism involving a single-step proton

transfer to form HNO3 could not be found in the condensed

phase. The result was similar to those obtained for guani-

dine derivatives GA and GzT [19, 20], where the decom-

position was initiated through the rupture of the anionic

fragments. Two mechanisms, one involving isomerization

of GN and another involving isomerization of NO3- to form

ONO2-, were hence explored but found to have high acti-

vation energies and low rate constants. The possibility of

guanidinium ion (G?) decomposing to form NH4? ion was

also investigated and found to be improbable. Condensed

phase reactions obtained, pertaining to initiation of GN

decomposition, are presented in Table 2 along with rele-

vant thermodynamic parameters. The CBS-QB3-optimized

structures of some of the compounds of interest in the

condensed phase are shown in Fig. 9.

Gas-phase initiation reactions

Since the condensed phase reactions did not yield any

promising decomposition pathways, the gas-phase decom-

position mechanism of GN was also explored. Transition

states were obtained for different reactions corresponding

to the stable isomer of GN. The major reaction pathways

identified were (1) isomerization of GN to yield the inter-

mediate INT1, (2) decomposition of GN to guanidine and

HNO3 and (3) decomposition of GN to nitroguanidine and

H2O. Although a reaction involving a single-step proton

was found to occur for GN, the more likely pathway on

account of lower activation energies and higher rate con-

stant is via the formation of INT1. Reaction corresponding

to dehydration of INT1 to yield nitroguanidine was also

found. The structures corresponding to the lowest energy

state for GN and INT1, after ground-state optimization at

CBS-QB3 level of theory in gas phase, are presented in

Fig. 10, and a summary of the explored initiation reactions

involving gas-phase decomposition of GN is presented in

Table 3.

Decomposition of GN was found to be initiated by the

reaction involving isomerization of GN to INT1, as the

same had the lowest activation energy and the highest rate

constants among the explored pathways. INT1 would then

decompose to form guanidine and HNO3. These products

then react to form NQ and H2O, once sufficient concen-

tration of Gu and HNO3 is built-up. The reaction involving

GN

INT2_c

INT1_c

INT3_c

Fig. 9 Optimized structure of GN and intermediates in condensed

phase

GN INT1

Fig. 10 Optimized structures for guanidinium nitrate and intermedi-

ate 1 in gas phase

A. Sankaranarayanan et al.

123

Table 3 Decomposition pathways for guanidinium nitrate in gas phase with thermodynamic parameters computed at CBS-QB3 level of theory

No Reaction DHRa D�Hf

b D�Hbc DGR

d D�Gfe D�Gb

f

(R10)GN

TS1INT 1

21.0 15.5 -5.6 19.6 26.6 7.0

(R11)

GN TS2 NH2

NH

NH2

+OH

N

O

O

86.3 189.7 103.4 10.0 208.0 198.0

(R12)

GN TS3

NH

NH H

NN

O

O

H

NQ

+ OH2

36.5 228.8 192.3 -32.4 249.9 282.3

(R13)

GN TS4

NH2

NH2

NQ0

NN

O

O

+ OH2

2.0 258.6 256.6 -67.9 274.5 342.3

(R14)

INT 1TS5

NH2

N

NH2

H

+H

N

O

OO

1.7 -2.6 -4.3 -1.1 5.2 6.2

(R15)

INT 1 + OH2

TS6N

H

NH H

NN

O

O

H

NQ

15.4 196.8 181.4 -52.0 218.4 270.4

(R16) NH2NH2

NH

+ N

O

OOH

N

O

OOH+ NH3 + N C NH2

TS766.6 226.7 160.1 -32.7 308.5 341.2

(R17) NH2NH2

NH

NH3N C NH

HTS8

+

77.3 213.9 136.6 -20.6 216.2 236.7

A numerical and experimental study of the decomposition pathways of guanidinium nitrate

123

formation of NQ was found to be exothermic which is

typical of nitration reactions.

Reactions involving nitroguanidine

Two configurations of nitroguanidine have been reported in

the literature [5]. These have the formula (H2N)2–CN–NO2

and (H2N)–C(NH)NH–NO2. Several reactions involving

nitroguanidine and its isomers were explored. The

nomenclature adopted for these compounds in this paper,

along with their optimized structures, is presented in

Fig. 11. The energy values calculated at CBS-QB3 level of

theory, at a temperature of 587 K, are also reported.

NQ0 and NQ have relatively stable structures and do not

dissociate directly. In fact, the most likely pathway found

for the dissociation of NQ0 was via the formation of NQ.

NQ undergoes further reactions involving formation of

NQ1, NQ2 and NQ3. NQ1 and NQ2 follow pathways

which ultimately result in the formation of cyanamide, H2O

and N2O. The pathway involving NQ3 decomposition

yields NH3, N2O and HNCO. The reaction pathways

explored are presented in Table 4. Structures of reaction

intermediates INT2, INT3 and INT4 are provided in

Fig. 12. Several exothermic reaction steps can be traced,

which can justify the exothermic peaks reported in previ-

ous studies [14–16].

Table 3 continued

No Reaction DHRa D�Hf

b D�Hbc DGR

d D�Gfe D�Gb

f

(R18)

NH2NH2

NH

+ N

O

OOH

+

TS9

NH

NH H

NN

O

O

H

OH2

NQ

-49.8 131.6 181.4 -42.4 227.9 270.3

(R19)

NH2NH2

NH

+ N

O

OOH

TS10

N

N NN

O

O

H

H

H

H

+ OH2

NQ1

-42.1 182.1 224.1 -35.5 271.6 307.1

a Enthalpy of reaction (kJ mol-1)b Activation enthalpy in the forward direction (kJ mol-1)c Activation enthalpy in the backward direction (kJ mol-1)d Gibbs free energy of reaction (kJ mol-1)e Gibbs free energy of activation in the forward direction (kJ mol-1)f Gibbs free energy of activation in the backward direction (kJ mol-1)

A. Sankaranarayanan et al.

123

Reactions involving formation of NO2, CO2 and solid

residues

The mechanism for decomposition of HNO3 to NO2, N2O

and H2O above temperatures of 290 �C has been reported

by Oxley et al. [28]. The extent of dissociation depends on

the pressure and thus can help partially explain the varia-

tion in HNO3, NO2, N2O and H2O concentrations seen in

open and closed crucible tests. In closed crucible tests,

where some of the HNO3 remained undissociated, no traces

of H2O could be detected.

The parent molecule, GN, has only one carbon atom

which can be traced to cyanamide and HNCO as the reaction

proceeds. HNCO is known to hydrolyse to CO2 and NH3

[29]. A single-step oxidation reaction from cyanamide to

CO2 is not possible as it is strongly bound to N atoms on both

sides. Cynamide, once formed, undergoes dimerization and

trimerization reactions to form melamine and other products

which form the solid residues that remain towards the end of

the TG process. These species have been confirmed in M/S

results in previous studies [14]. The intermediate steps

involve formation of NH3 [19, 20].

Summary of GN decomposition

Based on the reactions explored, a schematic of the

important reaction pathways in the decomposition of GN is

shown in Fig. 13.

The activation energy values reported in Fig. 13 have

been computed using transition-state theory [30]. Equa-

tions for calculating reaction rate parameters from infor-

mation provided in Tables 2–4 have been provided in

Online Resource 3. The transition-state structures of all

reactions explored in this work have been provided in

Online Resource 4.

NQ0

NQ1

NQ3

NQ2

NQIsomer E (hartree)NQ0 –409.303096NQ –409.289978NQ1 –409.287017NQ2 –409.287116NQ3 –409.250647

Fig. 11 Optimized structure and electronic energy values of nitroguanidine and its isomers, computed at CBS-QB3 level of theory

A numerical and experimental study of the decomposition pathways of guanidinium nitrate

123

Table 4 Decomposition pathways of nitroguanidine and its isomers

No Reaction DHRa D�Hf

b D�Hbc DGR

d D�Gfe D�Gb

f

(R20)

NQ0TS1

NQ11 34.4 163.6 129.1 35.5 168.2 132.7

(R21)

NQ0TS1

NQ32 137.7 199.2 61.5 135.2 206.8 71.6

(R22)

NQ0TS13

N C NH2 + N

H

HN

O

O

93.8 343.6 249.9 -6.4 339.7 346.1

(R23)

NQTS1

NQ24 7.5 79.5 72.0 4.4 78.7 74.3

(R24)

NQTS15

N

H

C N

H

+ N

H

HN

O

O

70.1 239.5 169.4 -29.8 236.9 266.7

(R25)

NQ2TS16

N

H

C N

H

+ INT2

102.7 141.4 38.6 5.8 135.9 130.2

(R26)

H

N C N

H

+ INT2TS17

N C NH2 + N

H

HN

O

O

-50.9 33.8 84.7 -52.1 133.6 185.7

(R27)N C + INT2NH2

TS18NQ1

-91.7 50.3 142.0 8.9 146.3 137.4

(R28)

NQ1TS1

NQ29 -0.3 179.4 179.6 -2.5 177.6 180.1

(R29)

NQ1 NQ3TS20 95.5 156.9 61.4 92.8 163.1 70.3

(R30)

NQ3 INT3TS21 -46.4 43.3 89.7 -38.4 52.9 91.4

(R31)

NQ3TS22

+ NH3N C N

N

O

O

H

13.0 56.8 43.8 -88.6 35.5 124.1

(R32) TS23INT2N

H

HN

O

O

40.2 159.4 119.2 39.9 159.5 119.5

(R33) TS24H2O + N2OINT2

-200.0 131.1 331.1 -284.4 124.9 409.3

(R34)TS25

N C N

H

H

N C NH2

-10.8 322.9 333.6 -12.1 321.1 333.2

(R35)

INT3TS26

+ NH3N C N

N

O

O

H

59.4 86.8 27.4 -50.2 71.0 121.2

A. Sankaranarayanan et al.

123

Conclusions

GN is a very stable compound and does not decompose easily.

In this work, the experimental studies revealed that at 1

atmospheric pressure, the decomposition of GN commences

around 260 �C. Two distinct regimes were identified—fast

decomposition occurs at temperature ranges between 260 and

336 �C with 75% mass loss and slow decomposition occurs

from 336 to 500 �C with another 10% mass loss. These values

were in close agreement with those obtained by Udupa [14]

and Damse [16]. The major species identified from TG–

FTIR–MS were NH3, N2O, NO2, H2O and CO2.

The mechanisms for GN decomposition as proposed in

previous studies [14–16] were explored in addition to

several other pathways. The results from ab initio com-

putations indicate that decomposition of GN is unlikely in

the condensed phase on account of high activation ener-

gies and low rate constants for the initiation reactions.

The G? cation and NO3- anion being highly stable do not

decompose easily. The expected single-step proton

transfer reaction to form Gu and HNO3 was not found to

occur either in the condensed or in the gas phase. The

reactions involving direct cleavage of CN bond were also

revealed to be highly improbable. Single-step dehydration

Table 4 continued

No Reaction DHRa D�Hf

b D�Hbc DGR

d D�Gfe D�Gb

f

(R36)

TS27N C N

N

O

O

H

INT4

19.2 60.1 41.0 33.6 74.6 41.0

(R37) TS28INT4 N2O + N C O

H

-296.8 43.8 340.6 -392.5 39.6 432.1

INT2 INT3 INT4

Fig. 12 Structures of important reaction intermediates given in Table 4

A numerical and experimental study of the decomposition pathways of guanidinium nitrate

123

of GN to yield NQ and H2O or the formation of ammo-

nium nitrate intermediate can also be effectively ruled

out. The most probable decomposition mechanism

involves isomerization of GN to INT1 in the gas phase,

followed by a proton transfer from INT1 to yield HNO3

and guanidine. Gu and HNO3 further react, but rather

slowly to form NQ and H2O in an exothermic step. NQ

dissociates via several competing pathways to yield NH3,

N2O, H2O and CO2. HNO3 decomposition can help

explain NO2 formation. The residue left towards the end

of TG can be attributed to dimerization and trimerization

reactions of cyanamide. Thus, most of the products

observed during experiments can be explained by the

proposed reaction pathways.

Ea = 20.37 kJ/mol

GN

INT1

Ea = 2.24 kJ/mol

Ea = 141.37 kJ/mol

NO2

DECOMPOSITIONHNO3 Gu+

NQ H2O

NQ2

Ea = 84.36 kJ/mol

INT2 HNCNH

Ea = 146.24 kJ/mol

Ea = 61.64 kJ/mol

H2O

N2O

+

NH2NO2 INT2 NCNH2+

NQ1NQ3Ea = 161.81 kJ/mol

Ea = 136.03 kJ/mol

NH3

NO2NCNH

INT4Ea = 48.68 kJ/mol

HNCO

NH3

HYDROLYSIS

N2O

CO2

NH3

DIMERIZATION, TRIMERIZATION

MELAMINE

Ea = 60.07 kJ/mol

Ea = 43.53 kJ/mol

Ea = 164.26 kJ/mol

Ea = 65.02 kJ/mol

Fig. 13 Important reaction pathways in the decomposition of GN

A. Sankaranarayanan et al.

123

Acknowledgements The authors would like to acknowledge the

grants from Industrial Research & Consultancy Centre (IRCC), IIT

Bombay for supporting their project (Grant No. 14IRCCSG021). The

technical support provided by the Sophisticated Analytical Instrument

Facility (SAIF) IIT Bombay, towards conducting the SEM experi-

ments is also acknowledged.

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