Studies on energetic compounds. Part 32: crystal structure,

thermolysis and applications of NTO and its salts

Gurdip Singh*, S. Prem Felix

Department of Chemistry, DDU Gorakhpur University, Gorakhpur 273 009, India

Received 5 September 2002; revised 11 November 2002; accepted 3 December 2002

Abstract

The research reports dealing with crystal structure, thermolysis and applications of 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-

one (NTO) has been thoroughly reviewed here. Stress has been given to theoretical considerations of both, structure and

thermolysis of NTO. The experimental and theoretical aspects of crystal structure of various NTO salts have been discussed.

The works reported on solubility and crystallisation of NTO has also been mentioned. The various experimental methods and

reported mechanisms for initial thermolysis reactions has been briefly mentioned so that to have a background for the discussion

of theoretical considerations. The theoretical considerations on the decomposition of NTO have been critically reviewed and

the new insights that are obtained from these works have been highlighted. The reported parameters available in the literature on

sensitivity and performance of NTO have been summarised. An overview has been presented on the various studies dealing

with applications of NTO as a high-energetic material, which reveals the potential of this compound for various applications.

q 2003 Elsevier Science B.V. All rights reserved.

Keywords: NTO; Crystal structure; Salts of NTO; Thermolysis; Applications

1. Introduction

The research in the field of high-energy materials

(HEMs) is directed towards preparing thermally

stable, high-performance and insensitive compounds.

The popular HEMs such as 1,3,5-trinitro-1,3,5-

triazacyclohexane (RDX), 1,3,5,7-tetranitro-1,3,5,7-

tetraazacyclooctane (HMX), pentaerythritol trinitrate

(PETN), etc. are sensitive towards hazardous stimuli

such as shock, impact, friction, etc. 1,3,5-trinitro-

2,4,6-triamino benzene (TATB) is an insensitive

high-explosive (IHE), but it does not have the

energetic performance of either RDX or HMX. NTO

is a new compound with high-energy and less

sensitivity [1,2]. It possesses high-energy release on

decomposition and high-velocity of detonation

(VOD). In addition, NTO exhibits good thermal

stability [3,4] and low-sensitivity to radiation damage

[5] and it is relatively insensitive to impact and spark

than RDX [2].

A lot of applications have been proposed for NTO,

such as in melt-castable, general-purpose and IHE

formulations [6] and plastic bonded explosives [7].

This compound is highly useful in non-azide inflating

propellant compositions for automobile air bags [8,9].

Owing to the acidic nature of NTO (pKa 3.67) [2], it

0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved.

PII: S0 02 2 -2 86 0 (0 2) 00 7 17 -2

Journal of Molecular Structure 649 (2003) 71–83

www.elsevier.com/locate/molstruc

* Corresponding author. Tel.: þ91-551-2202856/2200745; fax:

þ91-551-340459.

E-mail address: gsingh4us@yahoo.com (G. Singh).

forms salts [10] with a large number of metals as well

as aromatic and aliphatic amines, which are also

highly useful. The ease of formation of a large number

of salts offer a chance of making an appreciable

variety of compounds which can be tailored for a wide

spectrum of applications.

Various aspects of research reports on NTO [1] and

its salts [10] have been reviewed earlier. Crystal

structure of any energetic compound is very import-

ant, as performance parameters such as VOD depends

largely on density of the material [11]. Moreover, the

sensitivity of an explosive depends on properties such

as shear strength and molecular orientations of the

crystalline material [12]. The crystal structure of NTO

as well as its salts was given inadequate attention in

our earlier reviews. Therefore, both experimental as

well as theoretical studies related to crystal structure,

solubility and crystallisation of NTO are reviewed

thoroughly in this article.

In our earlier review [1], the various experimental

results on thermal decomposition of NTO, using a

wide variety of techniques were described. Yet there

is no consensus on the mechanism of thermolysis of

NTO. Theoretical considerations on the structure and

decomposition pathways [13–22] using quantum

mechanical (QM) methods would provide more

insight into the decomposition of NTO. We have not

included the theoretical considerations, in our earlier

review [1]. Thus, the present review will concentrate

more on these theoretical considerations and the new

inputs obtained from these studies.

2. Structure of NTO

NTO was first reported in the year 1905 [23],

incorrectly as its hydroxy tautomer and some studies

appeared in mid late 1960s also Refs. [24–27].

However, the applications of NTO as a HEM have

been revealed very recently only. Lee et al. [2] reported

NTO as a HEM and a large number of papers related to

this subject have been published thereafter [1,10]. The

structure of NTO is as given below in Fig. 1.

2.1. Experimental results

Lee and Gilardi [28] have reported that crystalline

NTO exists in two polymorphic forms viz. a and b,

where a is the most stable one. a-form belongs to the

triclinic space group P1 and contains eight molecules

in the unit cell. However, attempts to determine the

coordinates of individual atoms for this structure have

not been successful due to some kind of twinning

about the crystal needle axis. However, the b-form

has been resolved by X-ray diffraction measurements

[28]. The unit cell of b-form is monoclinic of the

space group P21=c with four molecules in the unit cell

(Fig. 2).

The atom labelling for NTO is shown in Fig. 3

and the geometrical parameters are summarised in

Table 1.

Prabhakaran et al. [29] conducted XRD studies

using powder diffraction method and showed that

the sample belongs to the tetragonal crystal system

Fig. 1. Structure of NTO.

Fig. 2. Single crystal of b-NTO.

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–8372

with a c=a axial ratio of 0.329. Crystal structure of

NTO has been reported by Sanderson [30] also.

Electron density, Laplacian and electrostatic poten-

tial distributions of b-NTO have been determined

[31] from a low-temperature (100 K) X-ray diffrac-

tion experiment. The bonding (3, 21) critical

points and the molecular dipole moment of 3.2

have also been obtained. The higher performance

characteristics and exceptional insensitivity of NTO

is attributed to its special structure [4]. The lone

pair of electron on the nitrogen atom joins the

conjugation, which results in the enhancement of

aromaticity of the ring and thus the thermal

stability is increased. The melting point of NTO

is increased beyond its decomposition temperature

due to the presence of intermolecular hydrogen

bonding.

2.2. Theoretical studies on structure of NTO

Some of the geometric parameters for several

tautomers of NTO as well as some conjugate bases

of these compounds have been reported by Ritchie

[13] based on molecular orbital (MO) calculations

at the AM1, 3-21G//3-21G and 6-31G*//3-21G

levels. He found that the structure given in Fig. 3

is the most stable isomer at these levels of theory,

irrespective of the nitro group substitution. Harris

and Lammertsma [16] have studied tautomerisation

and ionisation of NTO using MO self-consistent

field (SCF) and Moller–Plesset (MP2) theories and

with the Becke’s three parameter hybrid method in

combination with the Lee, Yang and Parr co-

relational function (B3LYP) hybrid density func-

tional using the 6-31 þ G* and 6-31 þ G** basis

sets. The B3LYP and MP2/6-31 þ G* structures of

NTO anion compared well with the reported

structure of NTO diaminoguanidium salt

(DAGuNTO). The IR frequencies calculated with

B3LYP compared well with those observed for

crystalline NTO. NTO anion was found to be more

aromatic than NTO molecule. Later on Sorescu and

Thompson [18] have determined the structure and

vibrational spectra of NTO using MO calculations

at the Hartree–Fock (HF) and MP2 levels and by

density functional theory (DFT), B3LYP method.

The calculated values have been compared with

that of experimental frequencies for the molecule

Fig. 3. Molecular configuration of NTO.

Table 1

Geometrical parameters for NTO

r(N1–N2) 1.369 u(N1–N2–C3) 112.8

r(N2–C3) 1.367 u(N2–C3–N4) 103.8

r(C3–N4) 1.378 u(C5–N1–N2) 102.4

r(N4–C5) 1.349 u(H6–N2–N1) 115.7

r(C5–N1) 1.290 u(O7–C3–N2) 126.9

r(C3–O7) 1.226 u(O7–C3–N4) 129.3

r(C5–N9) 1.447 u(H6–N4–C3) 123.3

r(N9–O10) 1.217 u(N9–C5–N4) 123.2

r(N9–O11) 1.218 u(O10–N9–C5) 117.4

u(O11–N9–C5) 116.2

u(O10–N9–O11) 126.4

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–83 73

from IR spectra of pure NTO films and NTO

molecules isolated in an argon matrix at 21 K. The

experimental spectra recorded for isolated NTO

molecules and thin films have significant differ-

ences with large red or blue shifts of more than

100 cm21. The observed spectral changes are partly

due to extensive hydrogen bonding. Relatively

good agreement had been obtained between the

scaled ab initio frequencies at the MP2/6-311G**

level and the values measured for NTO isolated in

the argon matrix. Similar good agreement had been

obtained between experimental geometry of b-NTO

and the optimised geometry at MP2/6-311G**

level, except that for N–H bond lengths. It was

also found that the calculated values of the

geometric parameters and vibrational frequencies

at the MP2 and B3LYP levels are in fairly good

agreement with each other and with the corre-

sponding experimental values.

Sorescu and Thompson [18] have later on

developed an intermolecular potential to describe

the b-structure of NTO crystal in the approxi-

mation of rigid molecules. The potential was

composed by pair wise Lennard–Jones, hydrogen

bonding terms and Coulombic interactions. Crystal

packing calculations performed with the proposed

potential acceptably reproduced the main crystal-

lographic features of the crystal and yielded very

good agreement with the estimated lattice energy.

These findings were also supplemented by the

results of isothermal-isobaric molecular dynamics

(MD) simulations at zero pressure for the tempera-

ture range from 4.2 to 400 K. Throughout the MD

simulations, the average structure of the crystal

maintained the same space group symmetry as the

one determined experimentally. The thermal expan-

sion coefficients calculated for the model indicate

anisotropic behaviour.

Meredith et al. [19] have used ab initio QM

methods to study the energetics associated with

several proposed initiation routes of NTO. Geometries

for all stationary points were computed using

restricted HF SCF analytical gradient methods. The

theoretical harmonic vibrational frequencies for NTO

were scaled by a factor of 0.91 to account for the

effects of anharmonicity and electron correlation.

Their theoretical gas phase values for the bond lengths

compared well with that of experimental values

except that for C5–N1 bond.

3. Solubility and crystallisation of NTO

Since crystal structure of energetic materials

affects the performance parameters, studies on

solubility and crystallisation are highly important.

Spear et al. [32] have conducted detailed study on the

solubility and recrystallisation of NTO. They have

reported that solubility of NTO in either water or

acetone is less than 2% at ambient temperatures,

which on heating to 100 8C rises up to 10% in water.

Solubility of NTO in ethyl acetate or dichloromethane

is low, a property, which will facilitate formulation of

NTO based plastic bonded explosives, using solvent

processes. They have attempted [32] to recrystallise

NTO from water in presence of a number of neutral,

anionic and cationic surfactants, so that to enhance

flow and bulk density. But all the surfactants favoured

acicular growth of NTO. However, NTO with

desirable handling properties could be obtained from

aqueous crystallisation with stirring.

Some preliminary studies on recrystallisation of

NTO have been done by Koo et al. [33] and they

obtained desirable particle size and morphology for

the compound, using recrystallisation techniques.

Spherical or cubic shaped crystals were obtained on

cooling of aqueous solution of NTO, by adjusting

operating conditions. The size of NTO particles was

in the range 5–1000 mm, which depended mainly on

supersaturation. On evaporation of aq. NTO solutions,

hexagonal and cubic crystals in the particle size range

of 20–30 mm were obtained. Cubic particles with a

size range of 0.5–2 mm were obtained by recrystalli-

sation using the gas-antisolvent method with NTO/

DMF and NTO/DMSO solutions.

Solid–liquid equilibrium for NTO þ C1 to C7 1-

alkanols has been studied by Kim et al. [34]. The

solubility of NTO in alcohols increases with increas-

ing polarity of the solvent. The enthalpy of mixing for

NTO þ C1 to C7 1-alkanols decreases with an

increase in the number of carbon atoms in the alcohol.

Kim et al. [35] have studied the kinetics of crystal

growth and nucleation in dependence on the super-

saturation of an aq. solution of NTO, by a draft tube-

baffle (DTB) crystalliser. The crystal growth rate is

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–8374

proportional to the supersaturation to the 2.9 power

and nucleation rate to the 4.2 power. The nucleation in

the crystalliser was found to act with heterogeneous

nucleation and surface nucleation simultaneously.

Kim and Mersman [36] have developed models based

on the fact that metastable zone width is decided by

the nucleation processes acted in the crystalliser for

NTO along with a number of other organic systems

and aq. inorganic systems at various cooling rates and

compositions. Van der Heijden [37] has published a

review on crystallisation and characterisation of

energetic materials, in which general aspects of

crystallisation of NTO have been discussed along

with other energetic compounds such as TNT, RDX,

HMX, hexanitro hexaaza isowurtzitane (HNIW/CL-

20), etc.

Lier [38] measured crystallisation kinetics par-

ameters, the primary and secondary (seeded) meta-

stable zone widths and growth rates for NTO and

benzil. The results showed generally poor correlation

between the solubility and the kinetic parameters as

well as mean crystal sizes. Good correlation was only

obtained for groups of similar solvents, i.e. a series of

alcohols. The best correlation was observed for the

growth rates measured at a fixed super saturation.

Poor correlation was observed between any meta-

stable zone width and the mean crystal sizes.

Donnio and Spyckerelle [39] have patented a

method for obtaining small-diameter spherical par-

ticles of NTO by atomisation of its solution and

drying its aerosol suspension with heated inert gases.

This process produces particles of NTO, which have

increased explosive power per unit volume.

4. Crystal structure of NTO salts

4.1. Experimental results

Owing to the easiness in preparation, a very large

number of salts of NTO have been reported in Ref.

[10]. XRD analysis on single crystals of some NTO

salts is also available in Refs. [40–52]. Of the various

salts of NTO with ethylene diamine (ENTO) [40],

DAGuNTO [41], ammonia (ANTO) [42], picryl

derivatives [43] and various metal complexes

[44–52], the structure of ANTO has been highlighted

here as a representative compound.

Jiarong et al. [42] have reported the crystal

structure of ANTO. Bright yellow and long needle

crystals of ANTO were grown by slow evaporation of

water solution. The crystal structure is

orthorhombic with space group P222: Crystal par-

ameters were a ¼ 6:282ð2Þ �A; b ¼ 8:423ð2Þ �A; c ¼

12:693ð5Þ �A; a ¼ b ¼ g ¼ 908; V ¼ 663:6 �A3;

dcalc: ¼ 1:65 g=cm3; Z ¼ 4: Final R ¼ 0:042: The

bond distances and angles were as given in Table

2.The molecular structure and atom labelling scheme

were as given in Fig. 4 and the packing of molecules in

the crystal lattice are illustrated in Fig. 5.

The OðH2OÞ· · ·HðH2OÞ· · ·NðNHþ4Þ intermolecular

hydrogen bonding can be seen from Fig. 5. The N1

proton is more acidic than the one at N2 and thus

deprotonation is easier from N1 than from N2. The

charge of anion is mainly concentrated at oxygen

atom (CyO). The crystal structures of a number of

metal complexes have been reported by a group of

Chinese scientists [44–52]. A compilation of the

structure and crystallographic parameters of all these

salts are given in Table 3.

Table 2

Geometrical parameters for ANTO

r(C1–N1) 1.355(7) u(N1–C1–N2) 108.2(4)

r(C1–N2) 1.371(7) u(Nl–Cl–Ol) 127.1(5)

r(Cl–Ol) 1.266(6) u(N2–C1–O1) 124.7(5)

r(C2–N1) 1.343(7) u(N1–C2–N3) 119.3(5)

r(C2–N3) 1.305(7) u(N1–C2–N4) 121.0(5)

r(C2–N4) 1.457(7) u(N3–C2–N4) 119,6(5)

r(N2–N3) 1.367(6) u(C1–N1–C2) 101.3(4)

r(N4–O2) 1.299(6) u(C1–N2–N3) 111.0(4)

r(N4–O3) 1.221(6) u(C2–N3–N2) 100.1(4)

r(O–Ha) 0.961(75) u(C2–N4–O2) 118.4(4)

r(O–Hb) 0.983(80) u(C2–N4–O3) 118.1(4)

u(O2–N4–O3) 123.5(4)

u(Ha–O–Hb) 105.3(76)

Fig. 4. Molecular structure and atom labelling scheme of ANTO.

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–83 75

4.2. Theoretical studies on the structure of NTO salts

Relatively less number of theoretical studies are

reported on salts of NTO [41,48,50,52] than its parent

compound. In the case of DAGuNTO [41], AM1

calculations were performed to explain difference in

conformation of the cation in its salts with nitrate ion

and NTO. In both cases, the carbon atom and all five

nitrogen atoms are coplanar. In the nitrate salt an ‘S’

configuration is found where one of the terminal NH2

groups is cis to the central CyNH2 bond and the other

is trans. Whereas in the NTO salt a ‘W’ configuration

is found where both of terminal NH2 are positioned in

such a fashion that the hydrogens appear above and

below the plane of the rest of the molecule and the

lone pairs are directed toward the hydrogens of

the CyNH2 moiety. The calculations suggested that

the nitrate salt contains DAGu in its lower energy

form, while favourable electrostatic interactions lead

to selective binding of the less stable W form of

DAGu with NTO.

In the case of [Li(NTO)(H2O)2] [48], the MNDO

MO calculations shows that the coordinate bonds of

the complex possess a certain extent of covalent

character. The Li atom is tetrahedral with bonds to the

N (between the nitro group and the adjacent ring N) of

NTO anion and three water molecules. One of the

water molecules acts as a bridging ligand, which leads

to the formation of infinite Li–H2O–Li chains. Jirong

et al. [50] have obtained optimised geometry for

[Pb(NTO)2·H2O] by using SCF-PM3-MO method.

They have particularly optimised the positions of

hydrogen atoms. It has been found that Pb atom bind

the ligands mainly with 6Pz and 6Py atomic orbitals.

Chang et al. [53] have conducted AM1 semi-

empirical MO calculations on the geometrical struc-

ture of ANTO. They found that there are four

distinguished intermolecular H-bonding in the

ANTO molecule/ionic system. The binding energies

for NTO2/NH4þ, NTO2/H2O and NH4

þ/H2O were

calculated to be 2230.516, 2131.671 and

214.664 kcal/mol, respectively.

5. Thermolysis of NTO

Of the various parameters, thermal stability is the

most important safety aspect of an energetic material,

since any hazardous stimuli, eventually triggers off a

thermal event, which is the main cause of initiation of

any explosive [54]. The early thermal decomposition

reactions are very much important from the viewpoint

of understanding the mechanism of explosion. During

thermolysis of NTO, sublimation process competes

with decomposition in the condensed phase [55,56].

Since the decomposition reaction steps depend upon

temperature, pressure and possible phases, studies

carried out under different experimental conditions

have led to strong disagreement about the initial

pathways. As our aim is to present theoretical studies

on the thermolysis of NTO, only a short description of

Fig. 5. Packing of molecules in the single crystal of ANTO.

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–8376

the various experimental results on decomposition

mechanism of NTO is given here.

5.1. Experimental methods and proposed initial steps

5.1.1. Homolysis of one or more bonds to ring

substituents

Many of the studies suggested that the first step in

the thermal decomposition of NTO is the scission of

C–NO2 bond, either by direct thermal activation or

catalysed by H atom transfer. Beard and Sharma [5]

have subjected NTO to X-ray and UV radiation

damage and X-ray photoelectron spectroscopy (XPS)

detected the damage induced. X-ray damaged residue

suggested the loss of nitro functionality and/or its

transition to a more reduced chemical state. They have

also studied [57] the early reaction chemistry of NTO,

induced by drop weight, impact, shock, heat and

radiation, using XPS and chemical ionisation mass

spectrometry (CIMS). XPS spectra indicated the loss

of NO2 concentration in response to damage in all

cases. Ostmark and co-workers [14,15,58] have

studied the decomposition of NTO using MS [both

electron impact (EI) and CI], laser induced-MS

(LIMS), chemiluminescence and DSC. They have

concluded that the probable thermal decomposition

mechanism is the elimination of NO2 followed by a

breakdown of azole ring. Prabhakaran et al. [29] have

studied the kinetics and mechanism of thermolysis of

NTO using TG, DTA, IR, DSC, XRD and hot stage

microscopy. Their results show that cleavage of C–

NO2, with rupture of adjacent C–N bond, appears

to be the probable mechanism in thermolysis of

NTO and that this step is likely to be the rate

controlling step.

It was Williams et al. [59], who first refuted the

hypothesis that C–NO2 homolysis is the initial step in

thermolysis of NTO. They investigated the product

gases of NTO decomposition under fast heating rates,

using IR spectroscopy, as a function of pressure and

found neither NO2 nor HONO initially. Oxley et al.

[60] studied the decomposition of NTO, over a wide

range of temperature (220–280 8C). They have

proposed a high-temperature mechanism, which is

initiated by loss of HONO or NO2. They have also

reported [60] both primary and secondary deuterium

kinetic isotope effects (DKIE) and concluded that

hydrogen transfer is involved as a rate-limitingTab

le3

Aco

mp

ilat

ion

of

cry

stal

stru

ctu

rean

dp

aram

eter

so

fso

me

met

alco

mp

lex

eso

fN

TO

Nam

eS

pac

eg

rou

pa

(nm

)b

(nm

)c

(nm

)b

(deg

ree)

Zm

(cm

21)

Fð0

00Þ

r(g

/cm

3)

Fin

alR

Ref

.

[Dy(N

TO

) 2(H

2O

) 6]·

NT

O·4

H2O

P2

1=n

1.0

09

4(1

)1

.258

6(2

)1

.884

5(2

)1

06

.31

(1)

43

3.8

91

44

4–

0.0

41

[44

]

[Co

(H2O

)](N

TO

) 2·2

H2O

P2

1=c

0.9

28

4(1

)1

.423

9(1

)0

.650

6(1

)9

1.6

5(1

)2

––

1.7

89

0.0

39

[45

]

[Yb(N

TO

) 3(H

2O

) 4]·

6H

2O

C2=c

3.6

93

1(9

)0

.668

3(1

0)

2.5

65

6(3

)1

30

.97

4(5

)8

40

.17

28

50

2.0

13

0.0

25

8[4

6]

[Sr(

NT

O) 2

(H2O

) 4]·

6H

2O

P2

1=c

1.1

03

4(1

)2

.274

2(2

)0

.633

98

(9)

10

1.7

98

(13

)4

34

.45

91

21

.936

0.0

34

7[4

6]

[Cd

(NT

O) 4

·4C

d(H

2O

) 6]·

4H

2O

C2=c

2.1

22

9(3

)0

.626

1(8

)2

.116

5(3

)9

0.6

02

(7)

4–

–2

.055

0.0

28

2[4

7]

[Li(

NT

O)(

H2O

) 2]

P2

1=n

0.7

42

0(2

)0

.344

9(1

)2

.490

6(3

)9

4.8

9(1

)4

1.5

91

39

21

.799

0.0

51

[48

]

H[P

r(N

TO

) 4(H

2O

) 4]·

2H

2O

P1

1.0

46

3(1

0)

1.0

48

4(1

0)

1.1

47

4(1

0)

–2

–5

40

2.0

88

0.0

20

6[4

9]

[Pb

(NT

O) 2

·H2O

]P

21=n

0.7

26

2(1

)1

.212

9(2

)1

.226

8(3

)9

0.3

8(2

)4

15

7.8

38

88

2.9

70

.027

[50

]

[Y(N

TO

) 2N

O3(H

2O

) 5]·

2H

2O

Cm

0.6

77

3(2

)2

.086

6(2

)0

.655

1(1

)–

2–

54

01

.970

0.0

32

[51

]

Rb(N

TO

)·H

2O

P2

1=n

0.6

33

0(1

)0

.824

1(2

)1

.296

4(3

)9

7.9

(1)

4–

44

82

.306

–[5

2]

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–83 77

process at lower temperatures. A subsequent study by

Oxley et al. [61] suggested two distinct mechanisms.

One pathway proceeds by means of CO2 and N2

evolution, a route that would account for the observed

DKIE [60,62]. They have also speculated that, nitro

group migration from the ring carbon (C5) to an

adjacent ring nitrogen (N4) may occur. Oxley et al.

[63] tracked NTO decomposition products, using 15N

labels and this study also asserted the mechanism-

involving homolysis of nitro group from the NTO

ring.

5.1.2. Rupture of triazole ring

Beardell et al. [64] have studied the thermolysis of

NTO using pulsed IR laser pyrolysis of thin films and

Fourier-transform IR spectroscopy, for product anal-

ysis. Unlike previous investigators, they observed

appreciable quantities of CO2, and was suggested to

be formed by intramolecular oxidation by the nitro

group at the keto carbon atom. Moreover, they

detected neither NO2 nor HONO as primary

decomposition species. They suggested a ring opening

mechanism.

5.1.3. Exchange of the nitro substituent with vicinal

hydrogen substituent

Oxley et al. [61] have proposed that the exchange

of the nitro group with vicinal hydrogen substituent,

yielding higher energy intermediate as an initial step.

This exchange is followed by a unimolecular break-

down of triazole ring, yielding CO2 and other

products.

5.1.4. Nitro-nitrite rearrangement

McMillan et al. [65] obtained real time photo-

ionisation of NTO, using surface analysis by laser

ionisation (SALI) apparatus. Their result suggested

several decomposition pathways, all of which feature

a nitro-nitrite rearrangement followed by NO loss.

5.1.5. Bimolecular routes

Many bimolecular decomposition routes have also

been suggested for the thermolysis of NTO. One of

the bimolecular mechanisms was reported by Mena-

pace et al. [62], who suggested the initial step to be a

hydrogen abstraction from either a neighboring NTO

molecule or a solvent molecule, yielding HONO and a

cyclic open-shell intermediate. Botcher et al. [66]

have confirmed the previous identification [64] of

CO2 as the initial gas-phase product. The kinetic data

in this study suggested an initial bimolecular step.

Thus, oxygenation of carbonyl group to CO2 was

found to be occurring by reaction with NO2 functional

group on a neighboring molecule.

Garland et al. [67] studied the decomposition of

NTO using heating solid NTO samples by irradiating

with pulsed laser light and the gaseous products were

detected using single-photon ionisation on a time of

flight MS. They suggested the initial reaction to be

bimolecular with a net loss of O atom to yield nitroso-

TO. However, they also noted that fragments

produced by loss of NO and by scission of the C–

NO2 bond are early, but not the initial products.

5.2. Theoretical studies on thermal decomposition

of NTO

Although considerably large number of studies

have been reported on the thermal decomposition of

NTO, there is no assertive study regarding the

mechanism of the process. Theoretical studies based

on various QM calculations may be of help, in further

understanding of the mechanism. Ostmark and Aqvist

[15] have done some empirical, QM calculations, in

order to explain the high-stability of NTO and verify

the initial step in the decomposition path. They have

used the semi-empirical modified neglect of diatomic

overlap (MNDO) method with the parametric method

3 (PM3), at the unrestricted Hartree–Fock (UHF)

level. The bond scission energy, DEp for C–NO2 and

the two N–H bonds were found to be 255, 328 and

345 kJ/mol, respectively. Thus, these calculations

verified that the week bond in NTO is the nitro C–

N bond and not the N–H bond.

Harris et al. [16] have studied bond dissociation of

NTO also along with their theoretical studies on

tautomerism and ionisation. Their method has been

described in Section 2.2. They have estimated N–H

and C–NO2 bond dissociation energies for the planar

keto tauomer as 389 and 293 kJ/mol, respectively.

Thus, they proposed that C–NO2 bond cleavage is the

likely initial decomposition step at high-temperatures,

whereas hydrogen atom transfer may play a key role

in the condensed phase. Moreover, since several

tautomers of NTO are energetically accessible, these

may prove to be significant in the condensed phase,

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–8378

where they can be formed by a base catalysed

mechanism.

Meredith et al. [19] have used ab initio QM

methods to study the energetics associated with

several proposed initiation routes of NTO. Analytical

second derivative methods were applied to determine,

which structure correspond to potential minima.

Single point energies for all proposed decomposition

product species were evaluated using the configur-

ation interaction with single and double excitation

(CISD) and the coupled cluster with single and double

substitutions (CCSD) methods. A comparison of the

energies computed for NTO and other molecules that

may be formed during its decomposition provided

considerable insight into the feasibility of the

proposed unimolecular decomposition mechanisms

for NTO. The results also indicated that homolysis of

C–NO2 bond (energy ¼ 280.3 kJ/mol), is the unim-

olecular pathway, which require lowest initial energy.

For the two-step mechanism involving loss of HONO,

313.8 kJ/mol of energy is required. Thus, on the basis

of the energetics of the various schemes, C–NO2

bond homolysis was found to be the most plausible

initial step for unimolecular decomposition of NTO.

Wang et al. [20] have considered 39 decomposition

paths among 18 intermediates and 14 transition states.

They have suggested different mechanisms for low-

temperature and high-temperature decomposition of

NTO. At lower temperatures, two reaction pathways,

which involve proton transfer and internal rotation

prior to C–NO2H cleavage, may be predominant.

Whereas at high-temperatures, the shortest mechan-

ism, which involve four steps and goes through

homolysis of C–NO2 bond should be the dominant

path. Leung et al. [21] have reported very recently a

theoretical study of unimolecular decomposition of

NTO, by combining ab initio MD and ab initio MO

methods. The various proposed reaction channels

were sampled theoretically by simulating a molecule

at high-temperature in a number of trajectories, using

DFT based abMD method with a plane wave basis set

and pseudo-potentials. Each of these channels was

then further examined by abMO method to locate the

energy barrier and transition structure and to verify

the abMD results. The C–NO2 homolysis was again

found to be the dominant channel at high-tempera-

tures. The departing NO2 could capture a H atom from

the NTO ring to form HONO, by either a concerted

bond breaking mechanism or by a bimolecular

reaction between the NO2 group and the triazole

ring. At lower temperature, the dissociation channels

initiated by hydrogen migration should be activated

first. The channel with hydrogen migration followed

by ring opening and then by HONO loss has an energy

barrier of 159 kJ/mol, at the rate determining step and

it is the path requiring lowest energy among the

various paths that have been studied. This study has

considered the path involving nitro-nitrite rearrange-

ment [67] also, and was found to have a lower energy

barrier than that for C–NO2 homolysis, but it would

make only a minor contribution, due to the entropy

factor.

A theoretical study (ab initio DFT) of initial

decomposition process of NTO dimer has also been

reported very recently [22]. At the B3LYP/6-31G(d,p)

levels of theory, it has been found that there is a

reaction path for the production of CO2 through the

dimer reaction with a potential energy barrier of

367.4 kJ/mol. This study has revealed a series of NTO

dimer reaction paths through four intermediates and

five transition states, with the evolution of nitro-TO,

CO2, N2, HONO and HCN in that order. Thus, the

study concluded that CO2 might be produced through

a cluster of NTO in the gas phase.

6. Performance parameters and applications

of NTO

The major attraction of NTO as an energetic

material is its insensitivity and good explosive

performance. Although many of its performance

parameters are available in literature, most of the

studies on the applications of this compound are

hidden in classified literatures. We have collected the

reports available in open literature and an overview is

presented here.

There are many detailed studies available on

various performance parameters and sensitivity

characterisation of NTO [2,30,68–75]. The sensitivity

data from various references are summarised in

Table 4. It can be seen from the table that the same

properties reported by different research groups differ

considerably, which may be attributed to the differ-

ence in experimental methods. The data for RDX

are also presented alongside to that of NTO for

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–83 79

comparison. Some experimental results for explosive

performance of NTO have also been summarised in

Table 5. Recently, Agrawal et al. [76] have conducted

a high-speed photographic study of the impact

initiation mechanisms of hexanitro-hexaaza-isowurt-

zitane (CL-20) and NTO by drop weight impact. NTO

was found to be less sensitive than b-HMX and CL-

20. But it was also found to be sensitised by both hard

high-melting grits and brittle polymers.

Spear et al. [32] have formulated plastic bonded

explosives with four different polymers (three ethyl-

ene vinyl acetate–EVA copolymers and a polypho-

sphazine) in a NTO: polymer ratio of 95:05. The

moulding granules of NTO formed with EVA

polymers were found to be highly insensitive to

impact. In contrast NTO/polyphosphazine was

slightly desensitised relative to NTO, but propagation

ability was slightly decreased. Huang and Wu [77]

described a method for estimating the autoignition

temperature of NTO and its salts from DSC data. Volk

and Bathelt [78] have calculated the energy output of

NTO in gun propellants using ICT-thermodynamic

code. Fouche et al. [79] have reported synthesis,

Table 4

Sensitivity of NTO and RDX to various hazardous stimuli

Test NTO RDX Ref.

Impact sensitivity

Rotter figure of insensitivity (mean gas evolved, ml) 90(3), 80(5) 80(17) [32]

h50% (cm) 291 22 [2]

Energy (J) 22 4.5 [71]

Thermal sensitivity

Vacuum thermal stability

ml/g/120 8C/40 h 0.0 0.2–0.6 [32]

ml/g/120 8C/40 h 0.3 0.12–0.9 [2]

Ignition temperature

Temperature of ignition (8C) 258 216 ^ 3 [32]

Henkin critical temperature (8C) 237 219.6 [2]

Temperature of ignition (8C) 280 220 [71]

DSC/DTA exotherm

DSC (8C) 273 212 [32]

DTA (8C) .236 210 [2]

Spark sensitivity

Energy (J) .4.5 (no ignition) 4.5 (ignition) [32]

Energy (10 mil, J) 3.40 0.55 [2]

Table 5

Performance parameters reported for NTO

Charge density

(g/cm3)

% TMD Charge diameter

(cm)

VOD

(m/s)

PCJ

(GPa)

Ref.

1.781 92.2 4.13 – 27.8 [2]

1.853 96.0 4.13 – 26.0 [2]

1.782 92.3 2.54 – 24.0 [2]

1.855 96.1 2.54 – Failed [2]

1.759 91.1 1.27 – 25.0 [2]

1.824 94.5 1.27 – Failed [2]

– 89.61 1.27 6040 – [32]

– 88.65 1.59 7440 – [32]

1.69 – – 7400 – [69]

1.71 – – 7770 – [69]

1.91 – – 8590 – [70]

1.91 – – 8200 33.8 [72]

1.91 – – 8590 – [73]

1.81 – 1.45 No go – [75]

1.80 – 1.60 7650 – [75]

1.81 – 2.00 7800 – [75]

1.80 – 2.50 7820 – [75]

1.80 – 3.00 7860 – [75]

1.78 – 2.36 7800 24.6 [75]

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–8380

recrystallisation, chemical and physical characteris-

ation of NTO. They have studied various RDX–TNT,

NTO–TNT and RDX–NTO–TNT formulations.

These studies proved that, incorporation of NTO

and/or fine RDX in TNT containing munitions,

showed significant improvements in sensitivity,

mechanical properties and structural integrity of

explosive columns. They have concluded that 40:60

NTO:TNT and 25:25:50 RDX:NTO:TNT can be

considered safe and reliable charge for various caliber

artillery shells. Kong et al. [80] have studied the

temperature dependence on composition and con-

structed phase diagram for 85:15 NH4NO3–KNO3/

ethylenediamine dinitrate/NTO.

Sunrall [6,81] has developed a melt-castable

general-purpose IHE formulation of NTO with a

thermoplastic binder (TTB-531). The formulation

also contained aluminium powder, ammonium per-

chlorate (AP) and RDX. The sensitivity and perform-

ance tests on this formulation demonstrated that it is

possible to formulate a high-performance melt-

castable explosive that does not detonate during

either fast-cook off or slow-cook off situations. Ellis

and Benzuidenhout [82] have determined detonation

energy, ballistic and Gurney energies of various NTO

based formulations. Fouche et al. [7] have developed

NTO based pressed PBX-formulations. They have

found that HNS/Kel F could be used as a suitable

booster formulation for the reliable initiation of a

pressable main charge filling consisting of NTO/

RDX/EVA. Moreover, NTO/RDX/EVA is suitable

for initiation of NTO/TNT melt-castable main charge

filling. Thus, they have constructed a reliable

explosive train.

Williams et al. [59] have shown that NTO on

thermolysis leaves a melon type cyclic azine poly-

meric residue, which is thermally stable up to 700 8C.

Therefore, they have suggested that NTO could be

used as a potential burning rate additive in solid

propellants to suppress the burning rate and enhance

combustion stability. Dalin et al. [83] have shown that

pyrolysis characteristics of NTO and its salts could be

used as a criterion to evaluate their catalytic activity,

when they are used as ballistic modifiers. However, a

recent study by Singh and Felix [84] showed that NTO

affects the processing parameters by building up

viscosity when it is used as a ballistic additive in

hydroxyl terminated polybutadiene (HTPB)-AP based

composite solid propellants. However, it has also been

shown that NTO slightly increases the burning rate of

the said propellant at 2% by weight concentration.

Another potential application suggested and tested for

NTO is in propellant compositions for automobile

restraint air bag systems [8,9,85]. NTO and its salts

are used in these systems in combination with other

energetic additives.

7. Conclusions

Crystalline NTO exists in two polymeric forms.

Theoretical studies show that many tautomeric

structures are feasible for this compound. NTO

crystals with different morphology and particle size

could be easily obtained by various recrystallisation

techniques. Very high-density crystals are obtained

for various metal complexes of NTO. The lack of

consensus in the thermolysis mechanism of NTO is

due to the dependence of the mechanism on the

experimental configurations. Thus, the thermolysis of

NTO is highly complex and the mechanism varies

according to the conditions of experiment. Theoretical

studies on the thermolysis of NTO also confirmed this

apprehension. There exists many decomposition

pathways with minor differences in energy and thus

feasible under different conditions. It is very difficult

to ascertain a single mechanism for decomposition of

NTO. Most of the experiments gave CO2 as the initial

product gas, which can only be explained by

bimolecular routes. However, almost all the theoreti-

cal studies proves that C–NO2 homolysis is the initial

step, which requires lowest energy. This is due to the

fact that most of the theoretical studies considered

unimolecular decomposition pathways. Studies con-

cerning bimolecular routes or NTO clusters indeed

show the feasibility of CO2 production. As the

experimental studies have been made on bulk

samples, the interaction between NTO molecules are

possible and CO2 is produced initially rather than

NO2. The sensitivity data available on NTO proves

the extreme insensitivity of this compound. Perform-

ance parameters also show the potential of NTO for

various applications. Thus, NTO could be used for

applications, which requires insensitivity and high-

performance.

G. Singh, S.P. Felix / Journal of Molecular Structure 649 (2003) 71–83 81

Acknowledgements

Head, Department of Chemistry, is thanked for

library facilities. Authors are grateful to the financial

support from DRDO, New Delhi and CSIR, New

Delhi to SPF. Dr Jaspreet Kaur, NCL, Pune and

INSDOC, Bangalore are also thanked for making

available some of the literature.

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