J Mol Model (2015) 21:145 DOI 10.1007/s00894-015-2699-9

ORIGINAL PAPER

A theoretical prediction of the possible trigger linkage of CH3NO2 and NH2NO2 in an external electric field Fu-de Ren 1 & Duan-lin Cao 1 & Wen-jing Shi 2 & Min You 1 & Man Li 1

Received: 24 February 2015 / Accepted: 4 May 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract The effects of an external electric field on the C/N– NO2 bond with C/N–H and N–O bonds in CH3NO2 or NH2NO2 were compared using the DFT-B3LYP and MP2 methods with the 6-311++G(2d,p) and aug-cc-pVTZ basis sets. The results show that such fields have a minor effect on the C–N or C–H bond but a major effect on the N–O bond in CH3NO2, while in NH2NO2 electric fields affect the N–N bond greatly but the N–O or N–H bond only slightly. Thus, in CH3NO2, oxygen transfer or unimolecular isomerization to methyl nitrite might precede breaking of the C–N bond in the initial stages of decomposition, and the N–O bond could be the trigger bond in electric fields. In NH2NO2, however, N–N bond rupture may be preferential in an electric field and, consequently, the N–N bond might always be the real trigger bond. Atoms in molecules and natural bond orbital delocalization analyses, together with examination of shifts in electron density and frequencies support the above viewpoints. Forty-eight good linear correlations were found along the different field orientations at different levels of theory, including those between field strength (E) and changes in N−O/N−N bond length (ΔRN−O/N−N), ρ(N−O/N−N) values [Δρ(N−O/N−N), or stretching frequencies of the N−O/N−N bond (ΔυN−O/N−N). Electronic supplementary material The online version of this article (doi:10.1007/s00894-015-2699-9) contains supplementary material, which is available to authorized users. * Fu-de Ren [email protected] 1

College of Chemical Engineering and Environment, North University of China, Taiyuan 030051, China

2

The Third Hospital of Shanxi Medical University, Taiyuan 030053, China

Keywords Trigger linkage . External electric field . AIM . Shifts of electron density . MP2

Introduction It has long been anticipated that the introduction of an external electric field into an energetic material will increase the energy content of the conduction gases in the vicinity of the detonation front, consequently accelerating the detonation velocity and leading to an increase in detonation pressure [1–3]. However, it is well known that explosive molecules cannot possess performance and stability simultaneously, i.e., high performance generally results in high sensitivity, and thus high energy molecules might be more unstable in the presence of external electric fields. Therefore, there is an extremely urgent need to predict the sensitivity of explosives in external electric fields. In some cases, the explosive sensitivity of nitro explosive exhibits a good linear relationship with the bond dissociation energy (BDE) of the C–NO2 or N–NO2 “trigger linkages”, the breaking of which is a key factor in detonation initiation [4–10]. Therefore, the effects of external electric fields upon trigger linkages have received much attention recently in theoretical studies [3, 11–13]. It has been found that external electric fields have a significant effect on the bond length and stretching frequency of the C–NO2 and N–NO2 trigger bonds, suggesting that one could use changes in a trigger linkage to infer whether it was made stronger or weaker by the imposition of a field [12]. Thus, changes in trigger linkage strengths could be used to judge whether sensitivities are reduced or increased in the presence of external electric fields. However, Politzer et al. [13] found that, although the external electric fields that reinforce a molecules’ intrinsic polarities increase C–NO2 stretching frequencies, the C–NO2

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distances in CH3NO2 do not show unambiguous trends with changes in field strength. Li et al. [14] also found that changes in most structural parameters of crystalline lead styphnate were not monotonically dependent on the increased electric field. These investigations indicated that changes in C–NO2 bond strength might be not synchronous with those of electric field strength. A question arises: can changes in strength of the conventional C/N–NO2 trigger bond be use to predict changes in explosive sensitivity in the presence of external electric fields? In fact, breaking of the C/N–NO2 bond is not the only mechanism for initiating detonation, and consequently the C/N–NO2 bond is not the only trigger linkage. Proton dissociation and oxygen transfer from the nitro group might also play an important role in the decomposition process of nitro explosive [15]. For example, for nitromethane, due to its dissociation energy being the lowest, C−N bond scission is regarded as the first step [16]. However, the view that proton decomposition occurs prior to C−N bond scission has been supported recently based on first principles [17–19]. Guo et al. [20] found that proton dissociation is the first step in the pyrolysis of nitromethane, and that C−N bond dissociation takes place at a later time in the nonimpact simulation. Although the possibility of O atom elimination (i.e., unimolecular N−O cleavage [21]) was excluded in view of the estimated N−O bond energy of more than 73 kcal [22], a Car-Parrinello molecular dynamics simulation has shown that the fast vibrations of the nitro group induced an inversion-type motion along with a stretch in the N–O bond, which led to oxygen transfer from the nitro to the methyl group of nitromethane [23]. At the G2MP2//B3LYP/6-311++G(2d,2p) level of theory, the C–N BDE for nitromethane is lower than the nitromethane→methyl nitrite and nitromethane→aci-nitromethane isomerization barriers by only 2.7 and 2.1 kcal mol−1, respectively [24], suggesting that three different reaction paths might occur in competition at the initial stage. Bardo [25] proposed a few reaction models involving the bimolecular exchange of an oxygen atom to form nitrosomethane and nitromethanol, and the kinetic parameters were consistent with the time scales of shock initiation [26]. The NH2NO2 molecule decomposed in four kinds of ways (NH2NO+O, NH2N+2O, NH2 +NO2, and NH2 +NO +O) [27], with N–NO2 bond scission and rearrangement (NH2NO2 →NH2ONO) competing quite evenly [28]. For nitrotriazole molecules, formation of nitrosoaromatic intermediates, reactions of the –NO2 group with an ortho substituent, etc., have also been found as mechanisms for initiating detonation [29–32]. These results suggest that the proton dissociation or oxygen transfer from the nitro group might be related to the initiation of energetic material. In other words, in addition to the C/N–NO2 bond, the C/N–H bond of the –CH– NO2 or –NH–NO2 moiety and the N–O bond of the nitro group might also become trigger linkages. Therefore, in order to reveal the nature of sensitivity, it is crucial to be able to predict changes in N–O and C/N–H bonds in the presence of

J Mol Model (2015) 21:145 Fig. 1 Changes in N−O or N−N bond lengths versus field strengths in„ the different field orientations at the different levels of theory

an external electric field. However, to our knowledge, few theoretical investigations into the effects of external electric fields on C/N–H and N–O bonds have been presented for nitro explosives, although there have been computational studies into the effects of external electric fields on the polarities, bond distances and stretching vibration frequencies of C/N– NO2 bonds [12, 13]. In this paper, we present a detailed and extensive comparison of the effects of external electric fields on the C/N–NO2 bond with the C/N–H or N–O bond in CH3NO2 (nitromethane, NM) and NH2NO2 (nitramide, NA) using the DFTB3LYP and MP2 methods. Our goal was to predict theoretically which chemical bond is affected most notably and even ruptured more easily in detonation initiation in the presence of an external electric field. The results of this investigation will help us understand the initiation mechanism of more complex energetic materials (such as RDX, HMX and CL-20, etc.) in the presence of external electric fields and thus will be very useful in informing the safe use of explosives and avoiding catastrophic explosion in external electric fields.

Computational details All calculations were performed with Gaussian 03 programs [33]. Each molecular geometry was fully optimized and properties, such as atoms in molecules (AIM) [34, 35] results and natural bond orbital (NBO) delocalization interactions [36], were computed using the DFT-B3LYP and MP2 methods with the 6-311++G(2d,p) and aug-cc-pVTZ basis sets in the presence of external electric fields (as well as no fields). Energy minima were confirmed by the absence of imaginary frequencies. The shifts in electron densities [37] were displayed at the MP2/6-311++G(2d,p) level. Three kinds of field directions, i.e., parallel to the C/N–N and N–O bond axes as well as perpendicular to the C/N−NO2 plane, were considered, with field strengths of ±0.002, ±0.004, ±0.006, ±0.008, ±0.010 a.u. The sign of the electric field was taken to be positive in the direction from the electron-donating to the electron-attracting group. Thus, for fields along the C/N−NO2 bond axes, the positive direction means from the C/N atom to the NO2 group, and for those parallel to the N–O bonds, the positive direction denotes from the N to the O atom.

Results and discussion The fields parallel to the C/N–N and N–O bond axes affected the structures and properties considerably more than the fields

J Mol Model (2015) 21:145

0.002

Page 3 of 9 145

Y = 0.20X - 1.45 × 10-4 R2 = 0.9819

0.002

Δ RN⎯ O (Å)

0.001

Δ RN⎯ O (Å)

Y = -0.42X - 1.36 ×10-4 R2 = 0.9901

0.004

0.000

0.000

-0.001

-0.002

-0.002

-0.004 -0.010

-0.005

0.000

0.005

0.010

-0.010

-0.005

MP2/aug-cc-pVTZ (NM) 0.010 0.008

Y = 0.76X + 2.55 × 10

-4

0.000

0.005

0.010

EN⎯O (a.u.)

EC⎯N (a.u.)

MP2/aug-cc-pVTZ (NM)

2

R = 0.9978

0.010

Y = 0.84X + 3.0 × 10

-4

2

R = 0.9944

0.008

0.006

ΔRN⎯O (Å)

Δ RN⎯ O (Å)

0.006

0.004 0.002 0.000

-0.002

0.004 0.002 0.000 -0.002

-0.004

-0.004

-0.006

-0.006 -0.008

-0.008 -0.010

-0.005

0.000

0.005

-0.01

0.010

0.00

EN⎯N (a.u.)

B3LYP/6-311++G(2d,p) (NA) 0.04

-4

Y = -2.94X - 2.45×10

MP2/aug-cc-pVTZ (NA)

2

0.020

R = 0.9981

0.03

-4

2

Y = -1.40X + 7.46 × 10 R = 0.9935

0.015

0.02

0.010

Δ RN⎯ N(Å)

Δ RN⎯ N (Å)

0.01

EN⎯ O (a.u.)

0.01 0.00

0.005 0.000

-0.01 -0.005

-0.02 -0.010

-0.03 -0.015

-0.010

-0.005

0.000

EN⎯⋅N (a.u.)

0.005

0.010

Δ RN⎯ N (Å)

0.005 0.000

-0.005 -0.010 -0.015 -0.020 -0.005

0.000

0.000

0.005

MP2/aug-cc-pVTZ (NA)

Y =1.11X - 9.64 ×10-4 R2 = 0.9407

-0.010

-0.005

EN⎯O (a.u.)

MP2/6-311++G(2d,p) (NA) 0.010

-0.010

0.005

EY (a.u.) MP2/6-311++G(2d,p) (NA)

0.010

0.010

J Mol Model (2015) 21:145

Page 4 of 9 0.003

Y = -0.25X + 1.82× 10

-5

-4

0.008

R

2

0.006

Δ ρN⎯ O (a.u.)

Δ ρN⎯ O (a.u.)

Y = -0.90X - 3.27×10

2

R = 0.9808

0.002

0.001

0.000

-0.001

0.004 0.002 0.000 -0.002 -0.004 -0.006 -0.008

-0.002

-0.010 -0.003

-0.012 -0.010

-0.005

0.000

0.005

0.010

-0.010

EC⎯ N (a.u.)

MP2/aug-cc-pVTZ (NM) Y = -0.94X - 3.18 × 10

-4

-0.005

0.000

EN⎯ O (a.u.)

0.005

0.010

MP2/aug-cc-pVTZ (NM)

2

R = 0.9977

Y = -1.12X - 4.00× 10

0.010

Δ ρN⎯O (a.u.)

Δ ρN⎯ O (a.u.)

0.010

0.005

0.000

-4

2

R =

0.005

0.000

-0.005

-0.005

-0.010 -0.010

-0.015 -0.010

-0.005

0.000

EN⎯ N (a.u.)

0.005

0.010

-0.010

B3LYP/6-311++G(2d,p) (NA) 0.0015

-5

0.03

2

Y = 0.01X - 7.27×10 R = 0.9841

0.000

EN⎯ O (a.u.)

0.005

0.010

Y = 2.18X - 6.73 ×10-4 R2= 0.9976

0.02

Δ ρN⎯ N(a.u.)

Δ ρN⎯ O (a.u.)

-0.005

MP2/aug-cc-pVTZ (NA)

0.0010 0.0005 0.0000 -0.0005

0.01 0.00 -0.01

-0.0010

-0.02 -0.0015

-0.03 -0.0020 -0.010

-0.005

0.000

0.005

0.010

-0.010

B3LYP/6-311++G(2d,p) (NA) -4

0.010

2

Y = 1.09X - 5.64×10 R = 0.9945

0.005

0.000

-0.005

-0.010

-0.015 -0.010

-0.005

-0.005

0.000

EN⎯ N (a.u.)

EY (a.u.)

Δ ρN⎯ N (a.u.)

145

0.000

EN⎯ O (a.u.)

0.005

0.010

0.005

0.010

J Mol Model (2015) 21:145

ƒFig. 2

Changes in ρ(N−O) or ρ(N−N) values versus field strengths in different field orientations at different levels of theory

perpendicular to the C/N−NO2 plane. Thus, emphasis was placed on the geometric and property changes accompanying fields parallel to the C/N–N and N–O bond axes. The calculated geometrical parameters and properties as well as their change trends at four levels of theory were almost in agreement with each other. Structure, AIM analysis and electron density shifts Tables S1–S4 summarize the selected geometric parameters calculated using the B3LYP and MP2 methods with the 6311++G(2d,p) and aug-cc-pVTZ basis sets. At all four levels of theory, the fields in the positive direction along the C/N–N bond axis shortened the N−N and C/N−H bonds (except for the C−H bond at the B3LYP/aug-cc-pVTZ level) and lengthened the N−O bond, while those in the negative direction did the opposite. The fields in the positive direction parallel to the N–O bond axis shortened the C/N−N bond, but lengthened one of the N−O bonds while shortening the other N−O bond. The C–N distances of CH3NO2 were elongated in the fields of the negative direction along the C–N and N–O bond axes but showed no unambiguous trends with changes in field strength in the positive direction, in accordance with previous investigations [11]. For CH3NO2, the changes in C−N and C−H bond lengths (in comparison with those in no field) were less than those of the N−O bond, while in NH2NO2 the changes in N−N bond lengths were larger than those of the N−O and N−H distances. The C−N bond length in CH3NO2 was little affected by even the stronger electric fields, as evidenced by the fact that the change was only 0.0008 Å in a field of +0.010 a.u. parallel to the N–O bond axis at the MP2/aug-cc-pVTZ level, while the corresponding change in the N−O bond length was up to 0.0041 Å. The change in N−O bond length was about five times larger than that of the C−N bond length, as was also seen in the fields parallel to the C–N bond axes in the positive direction. For NH2NO2, the field of +0.010 a.u. along the N–N bond axis causes the N–N bond to be shortened by 0.030 Å; while N−O or N−H is stretched by only 0.006 or 0.002 Å at the MP2/aug-cc-pVTZ level. These results show that fields oriented parallel to the C–N and N–O bond axes have a minor effect on C–N or C–H bond length but a major effect on N–O bond length in CH3NO2, while in NH2NO2 external fields greatly affect N–N bond length but the N–O or N–H bond is only slightly affected. There were 12 good linear correlations between the changes in the N−O bond length (ΔRN−O) and field strengths (E) with the linear correlation coefficients equal to the range of 0.9819– 0.9993 along the different field orientations at the four levels of theory. Six good (R2 =0.9407–0.9981) linear correlations

Page 5 of 9 145

between the changes in N−N bond length (ΔRN−N) and field strengths (E) were also observed (see Figs. 1, S2). However, no correlation between changes in C−N bond length and field strengths was found. These results show that the external electric field has an important effect on the strength of the N−O bond in CH3NO2 and the N−N bond of NH2NO2 but that the C−N bond length is little affected. The AIM results show that, at the four levels of theory, as the external electric field becomes stronger in the positive direction along the C/N–N bond, there is an increase in the values of ρ(N−N) and ρ(C/N−H), with a decrease in ρ(N−O) (see Tables S1–S4). Fields in the negative direction correspondingly show the opposite trends. The values of ρ(C−N) decreased in the negative C–N bond axis direction while no unambiguous trends were seen with changes in field strengths in the positive direction. The changes in ρ(C−N) values (in comparison with those in the absence of field) were very small in CH3NO2, being near zero even if the external field was up to ±0.010 a.u. along the C–N or N–O bond axis. However, changes in ρ(N−O) were marked, especially in external fields parallel to the N–O bond axis. According to the AIM theory of Bader [38], the larger the change in electron density ρ at the bond critical point (BCP), the more notable the change in bond strength. This explains the fact that changes in C−N bond length were far lower than those of N–O bond length in the presence of external electric fields. The reverse occurs for NH2NO2: the change in ρ(N−N) is far larger than that of ρ(N−O). For example, the change in ρ(N−N) reached 0.0225 a.u. in a field of +0.010 a.u. along the N–N bond axis at the MP2/6311++G(2d,p) level, while that of ρ(O−N) was only 0.0074 a.u. Thus, the change in N−N bond length was greater than that of N–O bond length. In previous investigations, although no comparison of the C/N−NO2 with N–O bond lengths in

Fig. 3 Electron density shifts when electric field strengths reach + 0.008 a.u. Purple Accumulation of additional electron density, yellow loss of density

145

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CH3NO2 or NH2NO2 was carried out, the fact that N−NO2 bond tends to be weaker than C−NO2 in the same electric fields along the C/N–N bond axis positive direction was confirmed [39, 40]. Except for the ρ(C−H) values at the MP2/aug-cc-pVTZ level, the changes of the ρ(C/N−H) values were all slight. There were 13 linear correlations between the changes in the ρ(N−O) values [Δρ(N−O)] and field strengths (E) with linear correlation coefficients in the range of 0.9808 – 0.9989 along different field orientations at the four levels of theory. Five linear correlations between the changes of ρ(N−N) [Δρ(N−N)] and field strengths (E) were also found with good linear correlation coefficients (R2 =0.9801 – 0.9981) (see Figs. 2, S3). However, no correlation between Δρ(C−N) and E was observed. In order to obtain deeper insight into the origin of the electric field effects on C/N–NO 2 , C/N–H and N–O bond strengths, an analysis of the electron density shifts accompanying the introduction of external electric fields into CH3NO2 or NH2NO2 along the positive C/N–N bond axes direction was carried out. For this purpose, the electron density shift was calculated by evaluating the difference between the electron densities of compounds in electric fields and those in the absence of external electric fields. ρshift ¼ ρðelectric

fieldÞ −ρðnofieldÞ

The shifts in electron density are illustrated in Fig. 3. Purple regions represented the accumulation of additional electron density; yellow regions indicated loss of density. The obvious purple area is around the O atom of the NO2 group, showing that the electron density shifts toward the O atom. It is interesting that the notable yellow region is around the N–O bond, suggesting that it loses density. This loss of density weakens the N–O bond. Furthermore, with the increase of the electric field strength in the positive C/N–N bond axes direction, the yellow region around the N–O bond becomes more marked, indicating that the N–O bond becomes weaker, and thus, oxygen transfer from the nitro or nitromethane isomerizing into methyl nitrite might become easier. Most important for our present consideration is the region along the C–N and C–H bonds in CH3NO2. When the electric field strength reaches +0.008 a.u., a notable purple region appears on one side of the C–N and C–H bond axes, showing accumulation of additional electron density. The accumulation of density strengthens C–N and C–H bonds. However, the notable yellow region near the other side of the C–N and C– H bond axes makes it apparent that there is much charge loss. This loss of density weakens C–N and C–H bonds. Indeed, the delocalization interactions, E(2) [36], σ(C–H) →σ(C–N)* were found to be 0.21 and 0.23 kJ mol−1 in the electric fields of + 0.008 and +0.010 a.u., respectively. The C–H bond offers electrons of the σ-orbital to the contacting σ(C–N)* antibonding

Fig. 4 Changes in stretching vibration frequencies of N−O or N−N„ bonds versus field strengths in different field orientations at different levels of theory

orbital, leading to C–N bond weakening and elongation. Thus, we conclude that, on the one hand, the electron density shifts toward the C–NO2 bond accompanied by possible bond contract; on the other hand, the loss of density weakens the C–N bond, leading to possible bond elongation. Therefore, when the electric field strength reaches +0.008 a.u., the C–NO2 distance in CH3NO2 does not show any unambiguous trends with further changes in field strength. In general, notable changes in bond length and electron density can lead to marked changes in bond strength, even leading to breaking of the chemical bond. In CH3NO2, the greater elongation of the N–O bond compared to that of the C–N bond implies that, in the presence of “certain” (maybe strong enough, after all the very large BDE of N–O bond) external electric fields of the positive direction along the C– N or N–O bond axis, oxygen transfer from the nitro to the methyl group of nitromethane or unimolecular isomerization to methyl nitrite might occur more easily than breaking of the C–N bond in the initial stages of decomposition. In this case, the N–O bond might become the trigger bond in the presence of an electric field. However, for NH2NO2, on the one hand, the N–N bond is more easily ruptured than the N–H or N–O bond in the absence of a field [15, 39]; on the other hand, the changes in bond length and electron density of the N–N bond are larger than those of the N–H or N–O bond in the presence of electric fields. Thus, the N–N bond might be always the real trigger bond. It is worth mentioning that the strong external fields imposed are likely to exceed the dielectric strengths of these materials, and cause their breakdown. In this case, oxygen transfer or unimolecular isomerization to methyl nitrite for CH3NO2 and breaking of the N–N bond of NH2NO2 may not be the mechanism for initiating detonations, and consequently the N–O and N–N bonds would not be the trigger linkages. Frequency analysis According to Politzer et al. [13], changes in the trigger linkage frequency might enhance or inhibit the vibrational excitation that is thought to precede its rupture in detonation initiation. From Tables S5 and S6, at three levels of theory, the external electric fields in the positive direction along the C–N and N−O bond axes increase greatly the asymmetrical/ symmetrical stretching vibration frequencies υ of the N−O bond of CH3NO2, while those in the negative direction do the opposite. The significant changes in υ suggest a notable enhancing or inhibiting vibrational excitation. Thus, oxygen transfer from the nitro group or unimolecular isomerization to methyl nitrite will occur easily in detonation initiation under

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Y = -1063.41X + 0.62 R = 0.9989

2

Y = -739.05X + 0.96 R = 0.9720

10 8 6

-1

Δ υΝ⎯ΟΟ (cm )

-1

Δ υN⎯ O (cm )

10

5

0

4 2 0 -2

-5

-4 -6

-10

-8 -0.010

-0.005

0.000

0.005

0.010

-0.010

EC⎯N (a.u.)

MP2/6-311++G(2d,p) (NM)

0.005

0.010

MP2/6-311++G(2d,p) (NM)

Y = -3787.36X - 1.04 R = 0.9935

2

Y = -1746.41X + 0.36 R = 0.9951

15

30

10

Δ υΝ⎯ O (cm )

20 10

-1

-1

0.000

EN⎯O (a.u.)

2

40

Δ υN⎯ O (cm )

-0.005

0 -10 -20 -30

5 0 -5

-10

-40

-15

-50 -0.010

-0.005

0.000

0.005

0.010

-0.010

-0.005

B3LYP/6-311++G(2d,p) (NA) Y = 563.23X - 0.46 R = 0.9635

30

-1

Δ υΝ⎯ N ( cm )

-1

0.010

2

Y = 2079.50X + 0.82 R = 0.9866

20

2

Δ υN⎯ΟΟ (cm )

0.005

MP2/6-311++G(2d,p) (NA)

2

4

0.000

EN⎯ O (a.u.)

EN⎯N (a.u.)

0 -2 -4

10

0

-10

-6 -20

-8 -0.010

-0.005

0.000

EY (a.u.)

0.005

-0.010

0.010

ΔυN⎯N (cm-1)

5

0

-5

-10

-15 -0.005

0.000

0.005

MP2/6-311++G(2d,p) (NA)

Y = 1164.59X - 1.26 R2 = 0.9807

-0.010

0.000

EN⎯N (a.u.)

B3LYP/6-311++G(2d,p) (NA) 10

-0.005

0.005

EN⎯O (a.u.)

MP2/6-311++G(2d,p) (NA)

0.010

0.010

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the influence of external electric fields. However, the influence of such fields on the vibration frequency of the C−N bond can be negligible. For example, the change in υ is no more than 1.0 cm−1 in even the strongest fields (0.010 a.u.) at the MP2/6-311++G(2d,p) level. The frequency shifts of the N −O bond are much greater than those of the C−N bond, possibly leading to more significant changes in vibrational excitation in the N−O bond than in the C−N bond. Thereby, the rupture of the C−N bond might not be influenced by external electric fields, and oxygen transfer from the nitro or unimolecular isomerization to methyl nitrite might play an important role in the decomposition process when an electric field is imposed. Thus, the N−O bond might become the trigger linkage of CH3NO2 in the presence of external electric fields, in accordance with the results obtained from the structures. For NH2NO2, at three levels of theory, the changes in stretching vibration frequencies υ of the N−N bond are close to those of the N−O bond in external electric fields. However, the BDE of the N−O bond in the nitro group is far larger than that of the N−N bond [28]. Thus, the N−N bond will rupture more easily than the N−O bond in detonation initiation in the presence of an external electric field, in agreement with the results of previous works in the absence of electric field [28]. Ten good (R2 =0.9635−0.9993) linear correlations between changes in stretching vibration frequencies of the N−O bond (ΔυN−O) and field strengths (E) were observed along the different field orientations at the different levels of theory (see Figs. 4 and S4). Two strong (R2 =0.9807 and 0.9866) linear correlations between changes in stretching vibration frequencies of the N−N bond (ΔυN−N) and field strengths (E) are also shown in Figs. 4 and S4. These results again demonstrate that the external electric field has a large effect on the N−O bond of CH3NO2 and N−N bond in NH2NO2. The frequency shifts of two N−H bonds are smaller than those of the N−N bonds, and those of three C−H bonds are small, at no more than 10 cm−1 at the MP2/6-311++G(2d,p) level, suggesting that the frequency shifts of the C/N−H bonds are also little affected by stronger electric fields.

and the N–O bond may become the trigger bond in electric fields. In NH2NO2, however, N–N bond rupture may be preferential in electric fields and, consequently, the N–N bond might always be the real trigger bond. Forty-eight good linear correlations were found along the different field orientations at the different levels of theory, including those between the field strengths (E) and the changes of the N−O/N−N bond length (ΔRN−O/N−N), ρ(N−O/N−N) values [Δρ(N−O/N−N)], or stretching vibration frequencies υ of the N−O/N−N bond (ΔυN−O/N−N).This theoretical investigation can help us understand the initiation mechanism of more complex energetic materials in external electric field, and will certainly be very useful in determining the safe use of explosives and avoiding catastrophic explosions in external electric fields. Ethical Statement We have full control of all primary data and we agree to allow the journal to review all the data if requested. We confirm the validity of the results from this manuscript. We have no financial relationship with the organization that sponsored the research, or authorships. The manuscript has not been submitted to more than one journal for simultaneous consideration, and it has not been published previously (partly or in full). This study is not split up into several parts to increase the quantity of submissions and submitted to various journals or to one journal over time. No data have been fabricated or manipulated (including images).

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Conclusions We carried out a comparison of the effect of an external electric field on the C/N–NO2 bond with that on C/N–H and N–O bonds in CH3NO2 or NH2NO2 using the DFT-B3LYP and MP2 methods. The results showed that the fields have a minor effect on the C–N or C–H bond but a major effect on the N–O bond in CH3NO2, while in NH2NO2 such fields greatly affect the N–N bond but the N–O or N–H bond are affected only slightly. Thus, in CH3NO2, oxygen transfer or unimolecular isomerization to methyl nitrite might occur more easily than breaking of the C–N bond in the initial stage of decomposition,

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A theoretical prediction of the possible trigger linkage of CH3NO2 and NH2NO2 in an external electric field.

The effects of an external electric field on the C/N-NO2 bond with C/N-H and N-O bonds in CH3NO2 or NH2NO2 were compared using the DFT-B3LYP and MP2 m...
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