Photo-induced intramolecular hydrogen transfer in ortho-nitroaniline: A matrix-isolation infrared spectroscopic and quantum-chemical study

Article abstract of DOI:10.1016/j.molstruc.2012.11.061

The photo-induced intramolecular hydrogen transfer in ortho-nitroaniline (ortho-NA) was investigated by matrix isolation infrared spectroscopy and quantum chemical calculations. Under UV irradiation of ortho-NA in solid argon, the hydrogen atom transferred isomer was formed. It was found that the isomer is stable and not rearranged to its nitro form upon diffusion annealing at 30 K. The infrared spectra and vibrational frequency assignments of the newly observed isomer were reported, which were supported by quantum chemical calculations.

Full text of DOI:10.1016/j.molstruc.2012.11.061

Accepted Manuscript
Photo-induced Intramolecular Hydrogen Transfer in Ortho-nitroaniline: A Ma‐
trix-isolation Infrared Spectroscopic and Quantum-chemical Study
Chaoyang Zhang, Mohua Chen
PII:
S0022-2860(12)01139-8
DOI:
http://dx.doi.org/10.1016/j.molstruc.2012.11.061
MOLSTR 19398
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
28 October 2012
24 November 2012
27 November 2012
Please cite this article as: C. Zhang, M. Chen, Photo-induced Intramolecular Hydrogen Transfer in Ortho-
nitroaniline: A Matrix-isolation Infrared Spectroscopic and Quantum-chemical Study, Journal of Molecular
Structure (2012), doi: http://dx.doi.org/10.1016/j.molstruc.2012.11.061
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Photo-induced Intramolecular Hydrogen Transfer in Ortho-nitroaniline: A
Matrix-isolation Infrared Spectroscopic and Quantum-chemical Study
Chaoyang Zhang 1* and Mohua Chen2
1Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box
919-327, Mianyang, Sichuan 621900, China.
2Department of Chemistry, Fudan University, Shanghai 200433, China.
Abstract The photo-induced intramolecular hydrogen transfer in ortho-nitroaniline
(ortho-NA) was investigated by matrix isolation infrared spectroscopy and quantum chemical
calculations. Under UV irradiation of ortho-NA in solid argon, the hydrogen atom transferred
isomer was formed. It was found that the isomer is stable and not rearranged to its nitro form
upon diffusion annealing at 30 K. The infrared spectra and vibrational frequency assignments
of the newly observed isomer were reported, which were supported by quantum chemical
calculations.
Keywords intramolecular hydrogen transfer, ortho-nitroaniline, matrix isolation infrared
spectroscopy, quantum chemical calculation.
1

Introduction
The decomposition of explosives is stressed for their safety during preparation,
processing, transportation, storage and usage. The external stimuli inducing the
decomposition may be shock, impact, heat, spark, irradiation, and so forth [1]. Accordingly,
some engineering-oriented standards have been established for detecting the thresholds to the
decomposition to assess the sensitivities of explosives against these external stimuli [1]. Up to
the present, it is still challengeable to reveal the complete decomposition mechanism of any
applied explosive, in particular, by means of experiments. This is attributed to the rapidness,
the complexity and the conditional changeability of the decomposition mechanism of
explosives. A feasible way to the detailed decomposition mechanism is to study that of the
simplest prototypes of explosives like nitromethane (NM), dimethylamine and nitrobenzene
(NB). Obviously, the results of these prototypes are useful to understand the more
complicated decomposition reactions of applied explosives.
The isomerization and dissociation reactions of nitro compounds have been the subjects
of various experimental and theoretical studies [2-27]. Isomerization reactions were assumed
to be important initial steps in decomposition. The explosive reactants may rearrange to their
isomers before they rupture into the finial small molecules, that is, these isomers are
intermediates in the decomposition reactions. For example, it was found that NM can be
converted into its nitrite isomer during decomposition, as shown in equation 1[2,3]. UV
photon induced isomerization and dissociation of NB in solid argon have been studied in our
group using infrared absorption spectroscopy [4]. The phenyl nitrite (C6H5ONO) was
produced upon UV irradiation of NB, as indicated in equation 2. The hydrogen atom
transferred isomers (HT) were observed in the UV photon induced reactions of
2

ortho-nitrotoluene (ortho-NT) and ortho-nitrophenol (ortho-NP) in solid argon, as shown in
equations 3 and 4, respectively [5,6].
CH3NO2ꢀCH3ONO
(1)
(2)
phNO2ꢀphONO
NO2
N(O)OH
N(O)OH
N(O)OH
H3C
H2C
(3)
(4)
(5)
NO2
HO
O
NO2
H2N
HN
The intramolecular HT can be occurred as equation 5 in ortho-nitroaniline (ortho-NA) in
terms of the structural similarity among ortho-NA, ortho-NT and ortho-NP. However, there is
no report on the HT of ortho-NA trapped experimentally according to our best knowledge.
Because ortho-NA is the prototype of the applied insensitive explosives such as
1,3,5-triamino-2,4,6-trinitrobenzene (TATB), 1,3-diamino-2,4,6-trinitrobenzene (DATB) and
1-amino-2,4,6-trinitrobenzene (TNA) [7], its photochemistry may be helpful to understand
that of those explosives. In this paper, the UV photon-induced intramolecular HT of ortho-NA
was investigated by matrix isolation infrared spectroscopy and quantum chemical
calculations.
Experimental and Computational Methods
The experimental setup for matrix isolation infrared absorption spectroscopic
investigation has been described in detail previously [28]. Different from previous
experiments of NB and nitrotoluene (NT) [29], the ortho-NA samples were vaporized at 35
3

instead of room temperature, due to its low vapor pressure. A gas stream of argon carrying the
ortho-NA vapor was deposited onto a CsI window cooled to 6 K by means of a closed-cycle
helium refrigerator (ARS, 202N). The matrix samples were deposited for 60 minutes at a rate
of about 10 mmol/h. The as-deposited samples were subjected to UV irradiation through a
CaF2 window using a low-pressure mercury arc lamp with dominant irradiation at 185 nm.
After a period of irradiation, the matrix samples were annealed at 30 K. Infrared spectra were
1
1
recorded on a Bruker IFS 66v/s spectrometer with 0.5 cm- resolution and 0.1 cm- accuracy
1
between 4000 and 450 cm- using a liquid nitrogen cooled HgCdTe (MCT) detector.
The Becke’s three-parameter hybrid functional with additional correlation corrections
and the 6-311++G** basis set were adopted to fully optimize the interested structures [30-33].
The reliability of optimized structures was verified by frequency analyses: there is no
imaginary frequency for each stable structure and only one for each transition state (TS)
structure. Zero point vibrational energies (ZPVE) were considered in the energy calculations.
All the calculations were carried out using the Gaussian 03 program package [34].
Results and Discussion
Sample was first made by condensation of ortho-NA and argon mixture at 6 K. The
spectrum taken directly after deposition showed the strong absorptions of ortho-NA as well as
some weak impurity absorptions due to H2O and CO2 that are common in matrix isolation
experiments. When the as-deposited sample was subjected to 120 min of 185 nm UV
irradiation, leading to a decrease of the ortho-NA absorptions, some new absorptions were
produced as illustrated in the difference spectra in Figures 1 and 2. After UV irradiation, a
diffusion experiment was also performed, raising the sample temperature to 30 K. But it did
not lead to obvious changes of IR intensities of the product absorptions, showing no chemical
4

rearrangement.
As shown in Figures 1 and 2 and Table 1, the two strongest absorptions of the HT of
ortho-NA are sited at 3515.4 and 1272.6 cm-1 due to the OH stretching and bending modes,
respectively. These two absorptions are close to those in the HT of ortho-NT, 3524 and 1270
cm-1 [5,29], and to those in the HT of ortho-NP, 3512 and 1292 cm-1 [6], respectively. The
sub-strongest absorptions are at 1083.7 and 948.7 cm-1, belonging to a CH bending mode and
a mixed vibrational mode, respectively.
As for no conversion back to ortho-NA of its HT at 30 K, it is much different from the
case of ortho-NT. That is, the HT of ortho-NT can back readily to ortho-NT when heated to
30 K [29]. DFT calculations on the thermodynamics of these two conversions can give a
good explanation on the difference. For ortho-NA, we were failed to find a ground state TS
structure, while succeeded in finding an excited one. Therefore, it is thought that the HT to
ortho-NA proceeds via an excited TS structure. As shown in Figure 3, we obtained four stable
HT conformations after full optimization from all six possible ones, including HTa, HTb,
HTc and HTd. As illustrated in Figure 4, the barrier of HTd to ortho-NA is predicted to be
84 kJ/mol, much higher than that of HT to ortho-NT, 31 kJ/mol [29]. This barrier will
increase to 108 kJ/mol if HTa is considered which possesses the lowest total energy among
the four stable conformations. At the same time, the barriers of ortho-NA and ortho-NT to
their HT are predicted to be above 200 kJ/mol, suggesting that these reactions cannot be
occurred at 30 K. It is just the reason for no IR adsorption reduction of the HT of ortho-NA
when it is subjected to the diffusion annealing at 30 K.
In summary, the HT of ortho-NA in Ar matrix was trapped upon 185 nm UV irradiation,
and it cannot be converted back to ortho-NA when annealed at 30 K, due to a high barrier to
5

be overcome. And the results of this work are hopefully helpful to understand the
photochemistry of applied explosives like TATB, DATB and DNA, as they are the cousins of
nitroaniline compounds. Maybe, the reactions shown in eqs (6)-(8) will take place when they
are suffered from UV radiation. The comment feature of these reactions is due to just the
hydrogen transfer initiating the subsequent processes. It will be confirmed by the further
experiments.
O
NO2
N
O
O
O
N
N(O)OH
N
N
H2N
O2N
NH2
NO2
N
NH2
NO2
NH2
NO2
HN
N
N
O
H O
2
3H O
+
2
+
+
+
(6)
O2N
O2N
NH2
NO2
N
N
NH2
O
O
NH2
N
N(O)OH
N
N
H2N
O2N
N
HN
H O
2
2H O
2
+
(7)
(8)
NO2
NO2
O2N
NO2
NO2
O2N
NH2
NO2
N
NH2
O
NH2
N
N(O)OH
H2N
O2N
N
HN
H O
2
NO2
O2N
NO2
NO2
O2N
Acknowledgment Financial support from the fund of China Academy of Engineering
Physics (2011A0302014) and the National Natural Science Foundation of China (21173199)
is greatly appreciated.
References and Notes
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7

Captions of figures
Figure 1. Difference infrared spectrum in the 3550-3500 cm-1 and 1700-1100 cm-1 regions of
o-NA/Ar. Spectrum taken after 120 min of 185 nm irradiation minus spectrum taken after sample
deposition.
Figure 2. Difference infrared spectrum in the 1100-600 cm-1 region of o-NA/Ar. Spectrum taken
after 120 min of 185 nm irradiation minus spectrum taken after sample deposition.
Figure 3. Structural parameters related to the HT of ortho-NA and the relative energies (in kJ/mol).
Figure 4. Thermodynamic comparison of the conversion of HT back to ortho-NA or ortho-NT
(energies in kJ/mol).
9

Table 1. Comparison between observed and calculated vibrational frequencies (cm-1)
and intensities (in parentheses in km/mol) for the HT of ortho-NA.
Calcd.
Obs.
mode
HTa
HTb
HTc
HTd
3515.4 3729.3(172) 3710.4(180) 3735.1(160) 3694.5(166) OH stretching
3501.8(18)
3231.6(2)
3203.6(4)
3188.8(7)
3173.3(10)
3562.5(24)
3222.4(4)
3204.4(4)
3189.0(7)
3173.2(11)
3442.8(0)
3237.9(2)
3192.9(7)
3178.2(17)
3160.6(2)
3447.9(1)
3224.4(4)
3192.7(6)
3177.9(18)
3159.7(2)
NH stretching
CH stretching
CH stretching
CH stretching
CH stretching
mixed
1640.5
1688.5(139) 1681.1(7)
1687.0(314) 1676.7(14)
1655.5(340) 1671.3(16) 1651.2(315) mixed
1591.8(143) mixed
1668.0(76)
1588.6(13)
1555.1(47)
1468.5(9)
1596.1(99)
1581.4(71)
1463.6(2)
1424.8(7)
1361.4(12)
1580.2(23)
1564.6(105) 1579.7(37)
mixed
1473.7(3)
1428.7(15)
1393.3(5)
1469.7(1)
1426.2(51)
1382.9(44)
CC stretching
mixed
1407.3 1429.5(38)
1374.5(61)
mixed
1289.7 1343.9(26)
1309.6(179) 1317.9(204) 1312.9(107) mixed
1272.9 1296.2(248) 1290.1(91)
1307.3(128) 1291.2(375) OH bending
1272.1(168) 1263.2(147) 1278.4(7) mixed
1237.6
1258.3(71)
1198.7(33)
1229.4
1198.3(44)
1174.6(10)
1081.9(8)
1012.3(24)
1009.7(0)
990.8(0)
1203.2(3)
1192.7(32)
1100.5(27)
1011.7(3)
998.8(0)
1204.2(11)
1176.2(1)
1084.0(72)
mixed
1178.7 1188.8(31)
CH bending
mixed
1083.7
1101.5(59)
1008.8(0)
1002.6(23)
985.5(0)
1030.3(122) CH wagging
986.2
1000.1(62)
998.3(0)
988.7(1)
865.9(0)
861.3(69)
825.5(28)
744.9(15)
715.1(1)
706.9(6)
614.2(23)
613.4(18)
CC stretching
CH wagging
mixed
981.4(1)
948.7
930.1
962.9(181)
915.5(36)
842.5(24)
839.7(8)
977.6(145)
885.3(26)
865.3(10)
847.0(9)
940.1(176)
873.7(78)
825.6(13)
824.5(52)
749.8(17)
718.0(9)
900.1
856.8
829.6
711.5
NH bending
mixed
CH bending
CH bending
CC stretching
mixed
755.5(90)
701.5(17)
698.2(12)
611.2(4)
761.8(77)
716.4(27)
684.1(15)
608.5(11)
588.2(10)
634.2
698.8(26)
606.5(11)
602.2(7)
mixed
575.4(7)
mixed
10

Figure1

Figure2

Figure3

Figure4

ꢀ
ꢀ
ꢀ
The hydrogen transfer product (HT) of ortho-nitroaniline (o-NA) was trapped
firstly.
No conversion of the HT back to o-NA after annealing at 30 K, different from that
of ortho-nitrotoluene.
An excited TS structure between HT and o-NA.
11