Role of Singlet Oxygen in the Degradation of Selected Insensitive

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Role of Singlet Oxygen in the Degradation of Selected Insensitive Munitions Compounds: A Comprehensive, Quantum Chemical Investigation Liudmyla K. Sviatenko, Leonid Gorb, Danuta Leszczynska, Sergiy I. Okovytyy, Manoj K. Shukla, and Jerzy Leszczynski J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b01772 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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The Journal of Physical Chemistry

ROLE OF SINGLET OXYGEN IN THE DEGRADATION OF SELECTED INSENSITIVE MUNITIONS COMPOUNDS: A COMPREHENSIVE, QUANTUM CHEMICAL INVESTIGATION Liudmyla K. Sviatenko,† Leonid Gorb,‡ Danuta Leszczynska,§ Sergiy I. Okovytyy,║ Manoj K. Shukla,* and Jerzy Leszczynski,*, †Department

of General and Biological Chemistry N2, Donetsk National Medical University, 1

Velyka Perspectyvna Str., Kropyvnytskyi, 25015, Ukraine ‡

Department of Molecular Biophysics, Institute of Molecular Biology and Genetics, National

Academy of Sciences of Ukraine, Kyiv 03143, Ukraine §Interdisciplinary

Center for Nanotoxicity, Department of Civil and Environmental Engineering,

Jackson State University, Jackson, Mississippi, 39217, USA ║Department

of Organic Chemistry, Oles Honchar Dnipropetrovsk National University, Dnipro,

49000, Ukraine

ACS Paragon Plus Environment

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US

Page 2 of 33

Army Engineer Research and Development Center, Vicksburg, Mississippi, 39180, USA

Interdisciplinary

Center for Nanotoxicity, Department of Chemistry, Physics and Atmospheric

Sciences, Jackson State University, Jackson, Mississippi, 39217, USA


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ABSTRACT:

The

DNAN

(2,4-dinitroanisole),

NTO

(3-nitro-1,2,4-

triazol-5-one), and NQ (nitroguanidine) are important energetic materials used in military applications. They may find their way to

the

environment

storage,

training,

possible

mechanisms

singlet

oxygen,

degradation,

was

during

and

disposal.

for

one

manufacturing,

reactions

of

performed

the

A

detailed of

the

potential

by

approach.

investigation

nitrocompounds methods

computational

PCM(Pauling)/M06-2X/6-311++G(d,p)

transportation,

for

study

with their

using

Obtained

of

the

results

suggest that reactivity of the investigated munitions compounds toward singlet oxygen follows the order: DNAN > NTO(anion) > NQ >>

NTO. DNAN is

involved in

[4+2]-addition with oxygen, and

further formation of diepoxide or epoxyketone through diradical intermediates

have

been

predicted.

The

NTO

may

undergo

intramolecular rearrangement with formation of peroxide compound or nitrite radical elimination and NQ may be transformed into urea.


3


Page 4 of 33

INTRODUCTION The

2,4-dinitroanisole

(NTO),

and

munitions

(DNAN),

nitroguanidine

(IMs)

compounds

(NQ) that

3-nitro-1,2,4-triazol-5-one are

are

important

parts

of

new

insensitive insensitive

munitions formulations IMX-101 and IMX-104 (Scheme 1). It can be speculated that production, transportation, storage, application and disposal of

these compounds may cause their occurrence in

the environment. The aqueous solubility of NTO, NQ and DNAN are 16642 mg/L5 (130 mM/L), 5000 mg/L4 (48 mM/L) and 276 mg/L6 (1.39 mM/L),

respectively.

transport Several alkaline

and

effects

methods

such

hydrolysis,

Detailed

studies

data

these

as

of

on

molecules

solar-induced

metal-catalyzed

environmental are

fate,

on-going.

photo-transformation,

reduction7-13

have

been

investigated with regard to the degradation of these compounds in the environment. It is well known that ground state of oxygen molecule is triplet. But, the solar radiation reaching generates

singlet

oxygen

which

is

very

reactive.

earth

Therefore,

singlet oxygen can also lead to the degradation of IM compounds in

the aqueous environment

and

need

to be

investigated. The

amount of dissolved oxygen in natural water is 10 mg/L at 20

0C.

According to the experimental data, singlet oxygen with quantum yield varying from 1 to 3% appears in natural water containing high loading of humic substances.14 An addition of singlet oxygen to unsaturated and aromatic compounds is well documented and


4

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involves the formation of organic peroxides and hydroperoxides and plays an important role in degradation processes.15-19 There are

several

types

of

oxygen

addition

reactions

as

shown

in

Scheme 2. The 1,2-cycloaddition occurs to an isolated double bond, resulting in the formation of 1,2-peroxides.18 The 1,4cycloaddition

or

[4+2]-cycloaddition

is

typical

for

a

system

containing at least two conjugated double bonds and results in the

formation

of

1,4-peroxides.15-17

the

The

1,3-addition

is

defined as a double bond connected to a hydrogen-bearing group, resulting

in

the

formation

of

allylic

hydroperoxides.19 The

dominant reaction depends on different electronic and structural factors. OMe

NO2

HN

N O

O

DNAN

O

NTO

O N

N

N H

NO2

H2N H2N

N O

NQ

Scheme 1. Molecular structures of DNAN, NTO, and NQ.

1O

O

2

1,2-cycloaddition

O

1O

H

2

1O

O

1,4-cycloaddition

O

1,3-addition

2

O

O

H


5


Page 6 of 33

Scheme 2. Types of oxygen additions to unsaturated and aromatic compounds. Whereas singlet

DNAN,

NTO,

oxygen

may

and

NQ

are

contribute

unsaturated

in

compounds

photooxidation

of

and the

a IM

compounds. The lifetime of 4 µs for singlet oxygen makes the experimental determination of the reaction between DNAN, NTO, NQ and singlet oxygen a challenging task. Therefore, an in silico investigation

of

transformation

of

the

reaction

mechanisms

IM

compounds

by

of

dissolved

the

oxidative

singlet

oxygen

offers an attractive, efficient approach. We have used recently developed semi-empirical method for the analysis of multistep chemical reactions which allows to computationally generate the amount of all the intermediate steps that take place between decomposition of reagents and products formation, including the speed limited steps.9 It allows to generate the kinetic equations for

the

total

processes

and

contributing

chemical

steps

to

predict relative speed of a decay of reagents and accumulating products.

Such

predictions

provide

needed

data

necessary

to

completely understand considered processes and have been applied in the present investigation. THEORETICAL METHODS All calculations were performed using the Gaussian 09 suite of programs.20

The

M06-2X

functional

of

the

Density

Functional


6

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Theory (DFT) was chosen for the present study because it was recommended kinetics,

for and

applications

noncovalent

involving

interactions

thermochemistry,

on

the

basis

of

assessment of its performance over a broad range of data.21 Its estimated mean unsigned error, which includes the averages of the

absolute

deviations

of

calculated

values

from

database

reference values, is about 1.3 kcal/mol. The influence of bulk water was simulated within polarizable continuum application

model

(PCM)

approach.22

Our

of

different

solvation

models

earlier for

study

the

on

alkaline

hydrolysis of nitroaromatic compounds and comparison of obtained results with experimental data suggests to select PCM model in a combination

with

Pauling

radii

as

a

most

suitable

solvation

model for the present study.23 The relevant stationary points (intermediates, solution

were

311++G(d,p)

transition fully

states,

optimized

level.21,22,24

Since

at our

and

products)

the

PCM(Pauling)/M06-2X/6-

computational

in

aqueous

analysis

is

based on the values of Gibbs free energy, stationary points were further characterized by calculation of the analytic harmonic vibrational frequencies at the same theory level as geometry optimization. Uni- and bi-molecular rate constants were calculated according to equations:


7


Page 8 of 33

G 

k uni

k  T  RTT  e (s-1) h

(1)

GT

k  T  RT  1  k bi  e    (Lmol-1s-1) h c

(2)

Where, k is the Boltzmann constant, T – temperature, h – Planck constant, ∆GT≠ – Gibbs free activation energy, R – universal gas constant, c – transformation coefficient equal to one mol/L. The rate constants kuni and kbi calculated using equations (1) and (2) were used to predict the rate of decay of the reactants and the rate of accumulation of products and intermediates. For this purpose, the system of differential equations (3, 4) was solved by using a Mathcad 15 program.

dni  dt i



dni  dt i



 k ji n j  ni  k il

j ( j i )

(3)

l (l  i )

(4)

 k ji n j nm  ni  k il no

j ( j ,m i )

l ( l ,o  i )

The initial concentration of nitro-compounds and O2 was chosen as 1∙10-5 mol/L, since in water concentration of oxygen when equilibrated with air is about 3∙10-4 M at 20 and

Hoigne14

have

selected

similar

0C.14

Moreover, Haag

concentration

of

furfuryl

alcohol as a singlet oxygen trapping agent in natural waters. The details of the system of differential equations applied in


8

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this

work

are

listed

in

Supporting

Information

(SI)

section

(Schemes S1-S3, Tables S4-S6). According to our study highly multistep processes for reaction of DNAN, NTO and NQ with

1O

2

could be presented by the Gibbs free

energy diagrams as shown in Figs.1-4,6,7 and 9. The relative Gibbs free energies are listed in Tables S1-S3. There is lack of the experimental data for the degradation of DNAN, NQ and NTO in water solution caused by the singlet oxygen. Experimental data are

needed

prediction

to of

generate

fitting

degradation

parameters

kinetics

of

for

these

reliable compounds.

Therefore, we have used the fitting constant equal to 0.68 that was

obtained

from

comparison

of

experimental

and

calculated

kinetics and used for alkaline hydrolysis of DNAN in the ground state9 also in the present work. We expect that such fitting parameter

would

provide

reliable

qualitative

prediction

for

singlet oxygen induced degradation of IM compounds investigated in

the

current

manuscript.

Applying

the

principles

kinetic

control (see eqs. 3,4) to species presented in this diagram the main

reaction

pathways

were

generated

(Figs.5,8,10).

The

Cartesian coordinates for optimized local minima and transition states species are available upon request.

RESULTS AND DISCUSSION


9


Different

pathways

for

reaction

of

Page 10 of 33

DNAN,

NTO,

and

NQ

with

singlet oxygen were examined to evaluate possibility of their oxidation in aqueous solution. Singlet oxygen induced degradation of DNAN Possible

pathways

for

the

reaction

of

DNAN

with

1O

2

are

presented in Fig.1. The syn-addition of singlet oxygen could follow a concerted Diels–Alder reaction mechanism as has been found in a number of aromatic hydrocarbons such as benzene, and naphthalene derivatives.15-18 The cycloaddition reactions between 1O

DNAN and

2

generate peroxides such as DNAN_INT1, DNAN_INT2, and

DNAN_INT3 (Fig.1). Oxygen DNAN_INT1

has

however it stable

the

addition into 1,4-positions forming

smallest

energy

barrier

(20.8

kcal/mol),

is slightly less stable than reactants. The most

intermediate

of

the

first

step

of

cycloaddition

reactions, DNAN_INT3, is formed as a result of addition into 3,6-positions.

Another

route

for

the

first

step

of

DNAN

degradation would be singlet oxygen attack onto each of the 2, 4,

and

6

position

of

DNAN

DNAN_INT4, DNAN_INT5, and

with

formation

DNAN_INT6,

of

zwitterions

respectively. This route

has higher energy barriers than peroxide formation and leads to unstable intermediates; however, they can be easily converted into stable ones. Intermediate DNAN_INT4 may be transformed into four-membered

peroxides

DNAN_INT7

and

DNAN_INT8.

Intermediate

DNAN_INT5 leads to the formation of DNAN_INT9, while DNAN_INT6


10

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forms hydroperoxide DNAN_INT10. Obtained results show that the energetically

most

favorable

first-step

reaction

is

[4+2]-

cycloaddition, therefore, further transformation of DNAN_INT1, DNAN_INT2 and DNAN_INT3 was only considered.

OMe 1

NO2

O

OMe

O 3

5

1

O

NO2

5

DNAN_INT1 4.27 20.84

NO2

O

OMe

3

1

NO2

5

DNAN_INT2 -0.18

24.06

2 4

NO2

21.46

+

3

MeO

O

1

+

DNAN

5

+

29.19

3

DNAN_INT6 18.77

1

DNAN_INT4 14.82

NO2

NO2

OMe

NO2

OMe 1

3

no TS

29.11

O

3

NO2 DNAN_INT7 -22.40

O

NO2

5

4.57 OMe HO

1

O

NO2 3

5

NO2 DNAN_INT10

5

O 2N

5

OMe

OMe

1

1

+ O

NO2

O

DNAN_INT5 13.62

O 2N O

NO2 O

3

O

NO2 NO2

no TS 5

3

O

NO2

5

2.12

OMe

26.34

O 1

-7.05

O2

NO2

O

3

DNAN_INT3

1

5

NO2

NO2

OMe 6

O O

DNAN_INT8

3

-25.59

O

DNAN_INT9 -20.75

-60.92


11


G, kcal/mol

G, kcal/mol

30

30

20

20

10 0

Page 12 of 33

DNAN_INT5

10

DNAN_INT1 DNAN_INT2

DNAN + O2

DNAN_INT6 DNAN_INT4

0

DNAN_INT3

-10

DNAN + O2 DNAN_INT9 DNAN_INT7 DNAN_INT8

-10 -20 -30

DNAN_INT10

-40 -50

Figure 1. Computer generated pathways for reaction of DNAN with singlet oxygen along with the corresponding Gibbs free energy diagrams. In the top figure, numbers along with each of the arrow

indicate

the

kcal/mol while no located

and

the

corresponding

reaction

barrier

TS suggests that transition corresponding

reaction

height

state

proceeds

in

was not

without

any

energy barrier. The O-O bond in peroxide DNAN_INT1 cleaves with formation of diradical DNAN_INT11, which is by 8.40 kcal/mol less stable than the

peroxide

(Fig.2).

There

are

four

different

pathways

for

further intramolecular ring closure in DNAN_INT11 to form an epoxide.

The

DNAN_INT14

most

without

energetically any

energy

favorable barrier

pathway and

leads

partially

to to

DNAN_INT13 with energy barrier of 1.88 kcal/mol. Formation of a second epoxide ring occurs without a barrier yielding

the syn-

diepoxides DNAN_INT16 and DNAN_INT17.


12

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An attempt to cleave the O-O bond in DNAN_INT2 leds to nitrite radical elimination with formation of radical DNAN_INT18 which is by 22.20 kcal/mol more stable than DNAN_INT2 (Fig.3). Further hydrogen transfer from DNAN_INT18 to nitrite radical leads to the

formation

of

highly

stable

1,4-benzoquinone

(DNAN_INT19)

derivative, 2-nitro-5-methoxy-1,4-benzoquinone. The

O-O

bond

cleavage

in

peroxide

DNAN_INT3

may

give

a

diradical DNAN_INT20, which is by 14.08 kcal/mol less stable than the peroxide (Fig.4). From the diradical DNAN_INT20, two pathways exist, one leads to epoxide, and the other to ketone. Hydrogen shift with formation of ketone diradicals DNAN_INT25 and DNAN_INT26 requires higher activation energy, however leads to more stable intermediates than epoxide formation. Despite the lesser

stability

of

epoxidic

diradicals

(DNAN_INT21

to

DNAN_INT24) they easily transform to subsequent products without energy barriers. There are two routes for the transformation to epoxyketones DNAN_INT27, DNAN_INT29, DNAN_INT30, DNAN_INT32, and to the diepoxides DNAN_INT28, DNAN_INT31. Both types of these products are known to be formed experimentally from unsaturated bicyclic endoperoxides with a product ratio sensitive to the nature

of

substitutents.25

Our

calculations

show

that

epoxyketones are 25 kcal/mol more stable than diepoxides, and the most energetically favorable pathway is the formation of DNAN_INT29.


13


OMe O

..

Page 14 of 33

NO2

O NO2

DNAN_INT12 -3.84

.

OMe

O

.

12.88

.

OMe

NO2

O

O

NO2

no TS

O

.

NO2 DNAN_INT1 4.27

O

NO2

no TS

DNAN_INT11 8.40

O

. .

O

NO2

NO2

DNAN_INT13

DNAN_INT16

-7.93

-56.72

. .

O

OMe

no TS O NO2

O

7.77

OMe O

O NO2

1.88

OMe

NO2

OMe NO2

O

OMe NO2

no TS O

NO2

NO2

DNAN_INT17 -54.53

DNAN_INT14 -6.21

NO2

DNAN_INT15 -6.42

G, kcal/mol 30 20 DNAN_INT11

10 0

DNAN_INT14 DNAN + O2

DNAN_INT1

DNAN_INT12 DNAN_INT15 DNAN_INT13

-10 -20 -30 -40 DNAN_INT17

-50

DNAN_INT16

-60

Figure 2. Computer generated pathways of DNAN_INT1 degradation along with the corresponding Gibbs free energy diagrams. In the top figure, numbers along with each of the arrow indicate the corresponding reaction barrier height in kcal/mol while no TS


14

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suggests

that

transition

state

was

not

located

and

the

corresponding reaction proceeds without any energy barrier. OMe

OMe

NO2 O

O

no TS

.

.

NO2

O

NO2

DNAN_INT2

DNAN_INT18 -22.20

O

26.86 - HNO2

NO2

G, kcal/mol

OMe

O

20

O

10


0

DNAN + O2

DNAN_INT2

-10 -20

DNAN_INT18 -30 -40 -50 -60 -70 -80 -90

DNAN_INT19 -100

Figure 3. Computer generated pathways of DNAN_INT2 degradation along with the corresponding Gibbs free energy diagrams. In the left figure, numbers along with each of the arrow indicate the corresponding reaction barrier height in kcal/mol while no TS suggests

that

transition

state

was

not

located

and

the

corresponding reaction proceeds without any energy barrier.


15


O

OMe NO2

O

OMe

O

.

no TS

OMe O

.

O

NO2

O

NO2

NO2

DNAN_INT27 -88.97

DNAN_INT28 -64.24 from INT21 -53.89 from INT22

no TS

DNAN_INT21 6.90

NO2

no TS

NO2

.

O

OMe

.

OMe

NO2

NO2

O

no TS

O

13.97

O

NO2

.

OMe

NO2

O O

DNAN_INT22 -2.62 9.45

OMe NO2

O

no TS

.

NO2 DNAN_INT29

NO2

DNAN_INT3

DNAN_INT20 14.08

OMe O

.

.

DNAN_INT23 3.77

.

O

O

NO2

no TS

.

O

O


20 DNAN_INT24

DNAN_INT22

10

10

DNAN_INT20 DNAN + O2

0

DNAN_INT21

-20

-30

-30

DNAN + O2

DNAN_INT23 DNAN_INT3 DNAN_INT26 DNAN_INT25

-40

-40 DNAN_INT28

-50

DNAN_INT31

-60

-60

-80

DNAN_INT20

-10

DNAN_INT3

-20

-70

DNAN_INT31 -61.11 from INT23 -57.43 from INT24

30

20

-50

NO2 O

G, kcal/mol

30

-10

OMe

O NO2

OMe

DNAN_INT24 0.09

DNAN_INT25 -27.15

G, kcal/mol

0

no TS

NO2

NO2

DNAN_INT26 -26.78

DNAN_INT30 -86.55

NO2

OMe NO2

.

NO2

no TS

O

OMe

O

O

O NO2

19.59

.

NO2

no TS

O NO2 12.80

18.97

NO2

.

NO2

O

OMe

O

11.04

OMe

-76.40

.

O

NO2

Page 16 of 33

DNAN_INT29 DNAN_INT27

-70 -80

DNAN_INT30 DNAN_INT32

Figure 4. Computer generated pathways of DNAN_INT3 degradation along with the corresponding Gibbs free energy diagrams. In the


16

Page 17 of 33

top figure, numbers along with each of the arrow indicate the corresponding reaction barrier height in kcal/mol while no TS suggests

that

transition

state

was

not

located

and

the

corresponding reaction proceeds without any energy barrier. Computed respective Gibbs free energy diagrams (Figs.1-4) were used for the calculations of rate constants, construction of the corresponding

kinetic

equations

as

described

in

Theoretical

Methods (Scheme S1, Table S4, SI) and subsequent solution of the resulting equations to produce kinetic plots for the process. Our calculation suggests that main products of reaction of DNAN with

singlet

oxygen

would

be

diepoxide

DNAN_INT17

and

epoxyketone DNAN_INT29 with ratio of 3:1 (Fig.5). Moreover, it is

also

expected

that

an

estimated

half-life

time

for

DNAN

decomposition under singlet oxygen environment would be about 5 min (Fig. 5). 10

Concentration, M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8

DNAN

6

DNAN_INT17

4

DNAN_INT16

2

DNAN_INT29

0

DNAN_INT30 0

20

time, min

40

60

Figure 5. Plots of the concentration vs. time for reaction of DNAN

with

singlet

oxygen

calculated

at

298.3

K

at

the

PCM(Pauling)/M06-2X/6-311++G(d,p) level.


17


Page 18 of 33

Singlet oxygen induced degradation of NTO Interaction of singlet oxygen with NTO was studied for neutral as well as for deprotonated forms because in water, unless pH is low, NTO exists mainly in the anionic form. Calculations were performed for NTO-anion deprotonated at N4 position which is 5.48 kcal/mol more stable than NTO deprotonated at N1 position. Schemes

for

different

mechanistic

pathways

of

reaction

of

singlet oxygen with NTO and its anion are provided in Figs.6,7 along with respective Gibbs free energy diagrams. Values for activation Gibbs free energy and Gibbs free energy of reaction are listed in Table S2. Addition of oxygen to carbon atom of C=N double bond of NTO requires high activation energy of 29.47 kcal/mol and may yield two instable intermediates zwitterionic NTO_INT1 and diradical NTO_INT2. Further transformation of NTO_INT2 occurs easily and leads

to

nitrite

a

stable

radical

transform

to

radical

NTO_INT7,

elimination.

neutral

NTO_INT8

which

Moreover, species

is

formed

NTO_INT2

through

can

after also

intramolecular

rearrangement. Other pathways, such as formation of hydroperoxide NTO_INT3 through an addition of oxygen to nitrogen atom of C=N double bond,

to

C=O

double

bond

forming

NTO_INT5

require

higher

activation energies than the addition of oxygen to carbon atom of C=N double bond and will not occur. An attempt to abstract


18

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hydrogen

from

NTO

to

form

NTO_INT9

and

NTO_INT10

led

to

rearrangement into the reactants. Interaction of singlet oxygen with NTO anion requires lower activation energy as compare to the neutral NTO (Fig.7). Oxygen attack on C3 leads to intermediate NTOa_INT1, which is slightly more stable than NTO anion. Further transformation of NTOa_INT1 may occur by different pathways: the formation of 1,2-addition product

NTOa_INT6,

elimination

of

nitrite

with

formation

of

NTOa_INT7, and formation of NTOa_INT8 as a result of oxygen atom attack of carbonyl atom C5 with simultaneous cleavage of N1-C5 bond.

The

last

pathway

is

the

most

energetically

favorable

because it occurs without activation energy and leads to stable intermediate. Barrierless elimination of nitrite from NTOa_INT8 yields NTOa_INT9.


19


O

1

HN O

N

5

O

3

4

+

N

N H

N

O

O

N

O

N H

NO2

NO2

NTO_INT1

NTO_INT6

9.30

2.65

. .

HN

O

29.47

O2

HN

7.44

O

N H

29.47

2

O

+

HN

Page 20 of 33

HN

O

N

O

.

O

N H

NTO

O O

NO2

NTO_INT7

4.65

.

34.51

N H

N O

.

O

N

N

O

NTO_INT10

.

.

O

N O

.

N H

.

HN

O

O

N

NO2

NTO_INT8 -27.95

N

N

N H

N

N

O HO

O

NTO_INT5

NTO_INT9

N

O

O HN

O

N

N H

O

30.82

40.59

O

O N

O

N

O

NH O

NTO_INT3

62.10

- OOH

-16.46

14.58 O

N

+

O HN

O

HN

O

N H

- NO2

NTO_INT2

- OOH

.

N

12.30

O

O

NTO_INT4 -3.67

28.53

G, kcal/mol

G, kcal/mol

60

30 50

20

NTO_INT1

10 0

NTO + O2

40

NTO_INT6

NTO_INT3

30

NTO_INT2

NTO_INT5

20

-10

NTO_INT7

-20

NTO_INT8

10 0

-30

NTO + O2

NTO_INT4

-10

Figure 6. Computer generated pathways for reaction of NTO with singlet oxygen along with the corresponding Gibbs free energy diagrams. In the top figure, numbers along with each of the arrow

indicate

the

corresponding

reaction

barrier

height

in

kcal/mol.


20

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HN O

O

+ N

HN 11.28

O

N

O

NO2

-1.75 HN

O

3

N

N 4

+

O

O2

N

NO2

22.40

HN

O

N

N

N

O 41.25 O

HN

N O N

O

N

3.75

O

O

N

N

O O

N

O NTOa_INT5

O

O

O

O

NO2

- NO2

N N

-34.51 from NTOa_INT7 -8.26 from NTOa_INT8

-24.62

O

O

NH

NTOa_INT9

NTOa_INT8

N

O

N

14.40 HN

O

NH

NTOa_INT3

91.83

O

NTOa_INT7 1.63

O O

N

O O

N

O

NTOa

+

HN

- NO2

N

5

O

23.03

2

1

O

NTOa_INT6 8.96

5.70

NTOa_INT1

O

N

NTOa_INT4 6.07

G, kcal/mol 90 80 70 60 50 40 30 20 NTOa_INT3

10 NTOa+O2

0

NTOa_INT4

NTOa_INT6

NTOa_INT5

NTOa_INT7

NTOa_INT1

-10 -20

NTOa_INT8

-30

NTOa_INT9

-40

Figure 7. Computer generated pathways for reaction of NTO anion with

singlet

oxygen

along

with

the

corresponding

Gibbs

free

energy diagrams. In the top figure, numbers along with each of


21


Page 22 of 33

the arrow indicate the corresponding reaction barrier height in kcal/mol. Initial oxygen attacks on N4 and C5 of NTO anion require high activation energies and such reaction would not proceed (Fig.7). Addition of oxygen to N2 has lower activation barrier, however, it leads to unstable intermediate NTOa_INT3. Therefore, the most energy favorable process for NTO anion decomposition would be the formation of NTOa_INT9 as shown in Fig.7. Obtained Gibbs free energy diagrams (Figs.6,7) were used to calculate section

rate

constants

(Scheme

S2,

as

described

Table

S5,

SI).

in

Theoretical

Developed

Methods

systems

of

differential equations were solved and resulted kinetics plots are presented in Fig.8. We predict that at the beginning of the reaction

NTO_INT7

NTO_INT8

is

a

will

be

formed

thermodynamic

(kinetic

product

with

control). slower

While

rate

of

formation. After decomposition of 90% NTO the concentrations of both products will be the same. NTO anion is more reactive as compare

with

neutral

form,

and

degrades

with

formation

of

NTOa_INT9 (Fig.8).


22

Page 23 of 33

10

10

NTO NTO_INT8 NTO_INT7

8 6

Concentration, M

Concentration, M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 2 0 0

10

20

30

40

50

8 6

NTOa NTOa_INT9

4 2 0

60

0

time, months

5

10

15

20

time, hours

a

b

Figure 8. Plots of the concentration vs. time for reaction of NTO (a) and NTO anion (b) with singlet oxygen calculated at 298.3 K at the PCM(Pauling)/M06-2X/6-311++G(d,p) level. Based upon our calculations, it appears that the reaction of NTO with singlet oxygen is unlikely to take place in aqueous solution

due

to

high

activation

energy.

Therefore,

it

is

expected that such reaction represents very slow process. As follows from the graphs presented in Figures 5 and 8, the rate of NTO decomposition will be several order of magnitude lower as compared with the rate of DNAN decomposition. In contrast to the neutral towards

form

of

singlet

NTO,

its

oxygen

anion

would

induced

be

much

degradation

more

reactive

(Fig.8b).

The

estimated half-life in singlet oxygen environment would be 3 months for NTO and 1.5 hours for NTO anion. Singlet oxygen induced degradation of NQ Scheme oxygen

for is

different

provided

in

pathways

of

Fig.9

along

NQ

reaction

with

Gibbs

with free

singlet energy


23


Page 24 of 33

diagrams. Values for activation Gibbs free energy and Gibbs free energy of reaction are listed in Table S6 (SI). Addition of oxygen to nitrogen atom of C=N double bond may proceed with energy NQ_INT1 stable

barrier and than

of

25.31

diradical the

kcal/mol

NQ_INT4

reactants.

and

lead

to

intermediates;

Further

both

transformation

zwitterionic being of

less

NQ_INT1

will lead to the ring closure and formation of four-membered peroxide NQ_INT2; subsequent elimination

of N2O3 will produce

urea.

by

Intermediate

NQ_INT4

degrades

nitrite

radical

elimination forming NQ_INT5. The ring closure in NQ_INT5 leads to

the

formation

of

four-membered

peroxide

NTO_INT6,

and

subsequent elimination of NO would lead to the formation of urea. An attempt of addition of oxygen to carbon atom of C=N double bond also led to the formation of NQ_INT1. An abstraction of hydrogen from NQ to get NQ_INT8 does not occur.


24

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O

O

H2N

+

25.31 H2N

+

N

H2N

H2N O

14.25

N

H2N

1.18

O

O H2N

- N2 O 3

NQ_INT2

NQ_INT3 -95.18

4.85

11.00

O

O N

H2N

O NQ_INT1

O2

O

N

H2N

O N

N

O

25.31

.

NQ

- NO

.

O

.

O N

O H2N O

N

H2N

+

N

O

N

no TS

.

- NO2

H2N

O

24.93

H2N O

O

.

.

no TS

H2N

H2N NQ_INT5 -7.85

H2N O

N

N

O

O N

H2N

NQ_INT6 24.00

NQ_INT7 -29.26

N

H2N

NQ_INT8

O

O

NQ_INT4 16.74

- OOH

HN

N

O

H2N

.

.

.

O

H2N

O NQ_INT9

G, kcal/mol

NQ_INT6

30 20

NQ_INT4

10 0

NQ_INT5

NQ_INT1 NQ + O2

NQ_INT2 NQ_INT7

-10 -20 -30 -40 -50 -60 -70 -80

NQ_INT3

Figure 9. Computer generated pathways for reaction of NQ with singlet oxygen along with the corresponding Gibbs free energy diagrams. In the top figure, numbers along with each of the arrow

indicate

the

corresponding

reaction

barrier

height

in


25


kcal/mol while no located

and

the

TS suggests that transition corresponding

reaction

state

proceeds

was not

without

any

energy barrier. Obtained Gibbs free energy diagram (Fig.9) was used to predict kinetic behavior in the same way as it was performed above for DNAN and NTO (Scheme S3, Table S6, SI). Developed system of differential equations was solved and resulted kinetics plot is presented in Fig.10. We predict that urea is the only product that is formed for reaction of NQ with singlet oxygen. Half time of NQ decomposition would be 2 days when fitting coefficient of 0.68, as used in DNAN and NTO, was applied for prediction. 10

Concentration, M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

8 6

NQ

4

NQ_INT3

2 0 0

5

10

time, days

15

20

Figure 10. Plots of the concentration vs. time for reaction of NQ

with

singlet

oxygen

calculated

at

298.3

K

at

the

PCM(Pauling)/M06-2X/6-311++G(d,p) level. CONCLUSIONS Different possible mechanistic pathways were investigated for the reaction of singlet oxygen with DNAN, NTO and NQ species. The

main

pathway

for

DNAN

decomposition

is

predicted

to

be


26

Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[4+2]-cycloaddition with subsequent formation of diepoxide and epoxyketone. NTO may undergo oxygen addition to carbon atom of C=N double bond with subsequent nitrite radical elimination or intramolecular rearrangement with formation of cyclic peroxide. It

has

been

further

predicted

that

oxidation

of

NQ

would

initiate from oxygen addition to nitrogen atom of C=N double bond

with

subsequent

cycle

formation

and

may

lead

to

urea

formation after N2O3 elimination. Results

of

our

calculations

reveal

new

insight

into

environmental fate of nitroaromatic and high nitrogen explosives during

dissolution

in

water.

Our

calculations

predict

that

singlet oxygen would lead to degradation of DNAN, NTO, and NQ in the aqueous environment. Moreover, it has been predicted that DNAN could be oxidized by singlet oxygen compared to NTO and NQ. Our

theoretical

study

predicts

that

reactivity

of

energetic

compounds with singlet oxygen would follow the order: DNAN > NTO(anion) > NQ >> NTO.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The PCM(Pauling)/M06-2X/6311++G(d,p) calculated activation Gibbs free energy, Gibbs free energy, and rate constants for reactions of DNAN, NTO, and NQ with singlet oxygen,


27


Page 28 of 33

PCM(Pauling)/MP2/6-311++G(d,p)//PCM(Pauling)M06-2X/6-311++G(d,p) calculated activation energy and energy for reaction of DNAN with singlet oxygen in aqueous solution (PDF). AUTHOR INFORMATION Corresponding Author *Jerzy Leszczynski, e-mail: [email protected]. *Manoj K. Shukla, e-mail: [email protected]. Notes The authors declare no competing financial interest. Acknowledgement The use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government. Results in this study were funded and obtained from

research

conducted

under

the

Environmental

Quality

Technology Program of the United States Army Corps of Engineers by the USAERDC. Permission was granted by the Chief of Engineers to publish this information. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. REFERENCES


28

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33

Role of Singlet Oxygen in the Degradation of Selected Insensitive

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