The dynamics of NO radical formation in the UV 266 nm photodissociation of nitroethane

Article abstract of DOI:10.1016/j.cplett.2006.01.055

Photodissociation of gaseous nitroethane at 266 nm has been studied by monitoring the NO(X²Π) product using laser-induced fluorescence technique. Rotational state distributions of the NO(X²Π _1/2 and X²Π_3/2<

Full text of DOI:10.1016/j.cplett.2006.01.055

Chemical Physics Letters 421 (2006) 232–236
www.elsevier.com/locate/cplett
The dynamics of NO radical formation in the
UV 266nm photodissociation of nitroethane
a,c
a
a,
a
b
*
Yamin Li , Julong Sun , Keli Han , Guozhong He , Zhuangjie Li
a
c
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China
b
Department of Chemistry and Biochemistry, California State University Fullerton, Fullerton, CA 92834–6808, United States
College of Environmental Science and Engineering, Department of Applied Chemistry, Dalian Jiaotong University, Dalian, Liaoning 116028, China
Received 5 November 2005; in final form 12 January 2006
Available online 17 February 2006
Abstract
Photodissociation of gaseous nitroethane at 266 nm has been studied by monitoring the NO(X P) product using laser-induced fluo-
2
2
2
00
rescence technique. Rotational state distributions of the NO(X P1/2 and X P3/2, v = 0) photofragment have been measured and char-
00
acterized by Boltzmann temperature of 810 ± 100 K. Only the NO photoproduct in v = 0 state can be observed in the present work. The
geometries of the nitroethane, the ethyl nitrite and the transition state connecting the two isomeric structures have been investigated
using ab initio method. The photodissociation dynamics of nitroethane is discussed on the basis of experimental observation and calcu-
lation results.
Ó 2006 Elsevier B.V. All rights reserved.
1
. Introduction
nitromethane has been accomplished by several groups
2,3,6,14]. Excitation of nitromethane at 280 nm leads to
[
Studies of photodissociation dynamics of nitro com-
three dissociation channels: (1) the production of NO and
2
pounds [1–29] are of much interest because these molecules,
upon UV irradiation, can dissociate through different path-
ways including simple bond cleavage, concerted molecular
dissociation, and rearrangement or elimination through
cyclic transition states, which may compete with each other
methyl radical, (2) the formation of oxygen atoms in con-
junction with nitrosomethane, CH NO, and (3) the produc-
3
tion of NO and methoxy radicals.
CH
3
NO
2
þhv !CH
3
þNO
2
ð1Þ
ð2Þ
ð3Þ
!
CH NO þO
3
[
1]. There has been an extensive review on the chemistry of
nitrocompounds [16], and previous understanding of the
various primary and secondary reactions of nitroalkanes
has focused on acquiring knowledge of thermochemistry
and thermal decomposition of these compounds. The main
experimental efforts on photodissociation of these molecules
have concentrated on the determination of distributions of
internal-state distribution and the translational-energy dis-
tributions of the fragments [7,8]. Direct detection of the pri-
mary photodissociation fragments following excitation of
!CH O þNO
3
The dominant channel of nitromethane photolysis at
00 nm is the production of electronically excited NO2
2
and methyl radicals. For larger nitroalkanes, C H2n+1NO2
n
with n > 1, the most interesting feature of their chemistry is
the number of energetically possible dissociation pathways.
It is known that their lowest energy pathway is HONO
elimination to produce stable alkene [12]. Other possible
channels for nitroalkane photodissociation include NO2
and alkyl radical formation, as well as NO and alkoxy rad-
ical production.
*
Corresponding author.
Very limited information is available in literature
regarding the photodissociation of nitroalkanes with two
E-mail addresses: ymli@dicp.ac.cn (Y. Li), klhan@dicp.ac.cn (K.
Han).
0
009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.01.055

Y. Li et al. / Chemical Physics Letters 421 (2006) 232–236
233
or more carbon atoms. For nitroethane, C H NO , the
by a pressure gauge. Only the NO photoproduct in
2
5
2
00
infrared multiple photon dissociation (IRMPD) of nitroe-
thane at 9.6 lm had been researched by Wodtke et al. [3].
However, the UV photodecomposition of nitroethane is
less studied. This may be due to the fact that the UV pho-
toexcitation dynamics of nitroethane is very complicated.
On the other hand, the competition among various energet-
ically allowed channels makes it an interesting system for
experimental and theoretical studies. The UV absorption
spectrum of nitroethane consists of two smooth, broad
bands centered roughly at 280 and 200 nm [12]. The band
v = 0 state can be observed in this work. Experiments were
carried out at 298 ± 3 K. The polarization effects were not
taken into consideration.
In an effort to unravel the possible photodissociation
mechanism of nitroethane, we also employed ab initio
molecular orbital theory to investigate the isomerization
process of nitroethane. All calculations were carried out
using the GAUSSIAN98 program package [30]. The geome-
tries of the nitroethane, the ethyl nitrite and the transition
state between the two isomeric structures were fully opti-
mized using Møller–Plesset correlation energy correction
truncated at second-order (MP2) theory in conjunction
with 6-31G** basis set (MP2/6-31G ). Vibrational fre-
quencies were calculated at the same level of theory for ver-
ification of structural optimization and for computation of
zero point energy. Single point energies of nitroethane the
at longer wavelength has been assigned to a p*
tion that is located on the NO functional group, while the
n transi-
2
feature at 200 nm is assigned to a p*
p transition also
**
located on the NO chromophore [12]. In this Letter, we
2
report our investigation of photolysis of CH CH NO at
3
2
2
2
66 nm
ethyl nitrite and the transition state were calculated at
MP4/6-311G** level, and the best estimate of energetics
C
2
H
5
NO
2
þhv !C
2
H
5
þNO
2
ð4Þ
!
!
!
C H NO O
ð5Þ
ð6Þ
ð7Þ
was made by adding zero point energy correction to the
MP4/6-311G** single point result (MP4/6-311G //MP2/
2
2
2
5
5
4
þ
**
C
C
H
H
O þNO
þHONO
6
-31G** + DZPE).
with focus on the detection of NO from the photolysis. We
also performed the ab initio study on the reaction dynamics
for reaction (6). We will discuss the photodissociation
dynamics of nitroethane leading to NO observation based
on our ab initio computational results.
3. Results and analysis
Investigation of the internal-state distribution of the NO
X P fragment was carried out via one-photon laser-
2
induced fluorescence (LIF) technique monitoring the NO
2
+
0
2
00
+
A R (v = 0)
X P(v = 0) transition. Fig. 1 shows the
2
0
2
00
2
. Experimental and theoretical approaches
region of the NO A R (v = 0) X P(v = 0)) excitation
0
0
spectrum at 223.75–227.15 nm. In order to assign the J
2
The dissociation pulsed Nd: YAG laser (Quanta–Ray
values of NO (X P) populated in the dissociation process
precisely the experimental spectra were compared with
the simulated NO excitation spectra calculated from the
known spectroscopic constants and H o¨ nl–London factors
[31]. Since the selection rules for one-photon absorption
allow transitions with DJ = 0, ± 1, only P, Q, and R
DCR–3G) beam and the probe laser beam were propa-
gated collinearly and were focused on the center of a fluo-
rescence cell, and the probe pulse was delayed by ca. 20 ns
optically with respect to the dissociation pulse. The light
from the fourth harmonic of a Nd: YAG laser (Quanta–
Ray DCR–3G) at 266 nm provided the photolysis pulses
at a repetition rate of 10 Hz. The probe beam was gener-
ated by frequency doubling a YAG (Quanta–Ray GCR–
2
2 +
branches are observed. Furthermore, the I and R states
are split by spin–orbit and spin–rotation interactions caus-
ing each band to consist of 12 rotational sub-branches [31].
3
G) laser pumped dye laser (Lumonics HD–500). The
photolyzing beam and probe beam, which were slightly
focused by a lens of f = 0.75 m, had a diameter of ꢀ3
and ꢀ1 mm at the observation point, respectively. Satura-
tion effect was avoided by keeping low laser pulse energies
of 1.4 mJ for the photolyzing beam and <0.1 mJ for the
probe beam. The NO LIF signal was found to be linearly
proportional to the laser power, and did not exhibit a lev-
eling off at high laser powers. The fluorescence resulted
2
+
from NO at the A R state was detected by a photomulti-
plier tube (PMT, Hammamatsu R 1460) at a right angle to
both the gas beam and the two counter propagating laser
beams. The PMT signal was passed through a band–pass
filter (centered at k = 236.5 nm, bandwidth ꢁ10 nm), and
then averaged with a boxcar integrator (Stanford SR250).
About 100 mtorr of gaseous nitroethane was flowed
through a reaction cell, which was constantly monitored
0
0
2
+
0
2
Fig. 1. Portion of the R (v = 0)
LIF spectrum of the NO fragment from C
266 nm. Upper spectrum, experiment; lower spectrum, simulation.
P(v = 0) band of the one-photon
2
H
5
NO photodissociation at
2

234
Y. Li et al. / Chemical Physics Letters 421 (2006) 232–236
The intensity of the spectra can be described by the equa-
tion as follows:
followed by cleavage of the O–NO bond of the alkyl nitrite.
Experimental evidence for unimolecular nitro-nitrite
0
0
00
X
rearrangement, R–NO
2
!R–O–NO, as an initial step in
NðJ ; v Þ
IJ
0
/
S
0 00
J J
f ð~t
0
~t
Dv;v ;J00!J0
00
Þ
thermal decomposition had been reported in several
nitro-compounds [2,3,7,8]. In general, nitro compounds
photodissociation process that produces NO radical can
be described as
00
2
J þ1
00
00
where IJ
0
is the intensity of fluorescence; N(J ,v ) is the quan-
tum number, S
0
00 is the H o¨ nl–London factor; f(m ,m) =
J
J
0
2
q(v )exp(ꢂa(v ꢂv) ) is the laser intensity distribution.
0
0
ꢃꢄ!½RONOꢃꢄ
0
0
R ꢂNO
2
þhv !½R ꢂNO
!RO þNO
Thus, for nitroethane,
C H NO þhv !½C H ꢂNO ꢃ !½C H ONOꢃ
2
The rotational distribution of NO (v = 0) fragment from
photodissociation of nitroethane can be described by Boltz-
mann distribution. The corresponding rotational tempera-
ture was obtained by fitting a straight line to a plot of
ðIÞ
0
0
00
00
00
00
ꢄ
ꢄ
ln[P(v ,J )/(2J + 1)] vs Bv00J ðJ þ1Þ, where Bv00 is the rota-
2
5
2
2
5
2
2
5
tional constant. Fig. 2 shows the distribution of the rota-
!
C
2
H
5
O þNO
ð6aÞ
2
2
tional state populations of NO (X P and X P3/2) as a
1
/2
function of rotational energy (in the form of a Boltzmann
We also used ab initio method to study the energetics of the
nitroethane isomerization into ethyl nitrite. Fig. 3 shows
the optimized geometry of nitroethane, ethyl nitrite and
the transition state at MP2/6-31G** level of theory. In order
to verify the transition state, we have performed the intrinsic
reaction coordinate (IRC) for the isomerization process at
00
plot) in the vibrational level of v = 0 following dissociation
of nitroethane at 266 nm. From the relative intensity of sev-
eral branches belonging to different spin components, we
deduced the rotational-state distribution temperature for
0
NO of v = 0 to be 810 ± 100 K within experimental uncer-
00
tainty. Attempts to observe transitions due to v = 1 at 233–
37 nm and higher vibrational levels of the NO fragment
were unsuccessful. This suggests that majority of the NO
2
00
fragment was formed in v = 0, indicating that the vibra-
tional distribution did not have an anomalous maximum
at some higher vibrational level. It had been found that in
the photodissociation of other nitro-compound, such as
nitrobenzene at 280–220 nm [8] and nitromethane at
1
93 nm [13], the NO fragments were also produced mainly
0
0
in v = 0 state. The fact that NO product distributions are
very similar for the different nitroalkanes shows that the
rotational state distribution of NO is not strongly dependent
on the structure of the nitroalkane. This is compatible with
the participation of a common–lived intermediate state,
rather than with a repulsive process.
One possible NO formation is through the isomerization
of the nitroalkane (R–NO ) into alkyl nitrite (R–O–NO)
2
-
-
-
-
6
7
8
9
π
π
1
3
/2
/2
Linear Fit
-
10
11
-
0
500
1000
1500
2000
2500
-1
Rotational energy(cm )
00
2
Fig. 2. NO(X P) v = 0 rotational state population as a function of
rotational energy for photodissociation of C NO at 266 nm.
2
H
5
2
Fig. 3. Geometries optimized at the MP2/6-31G** level.

Y. Li et al. / Chemical Physics Letters 421 (2006) 232–236
235
ꢂ1
0
ꢂ1
ꢂ1
the MP2/6-31G** level and found that the transition state
correlated to CH CH NO and CH CH ONO as reactant
H O) = ꢂ4.1 kcal mol
,
D H (C H ) = 28.4 kcal mol
,
,
5
f
2
5
0
ꢂ1
0
D H (NO) = 21.6 kcal mol , D H (NO ) = 7.9 kcal mol
and D H (O) = 59.6 kcal mol . This leads to D H (reac-
f r
tion (6a)) = 42.0 kcal mol , and D H (reaction (4a)) =
134.1 kcal mol , respectively. Reaction (6a) is then less
3
2
2
3
2
f
f
2
0
ꢂ1
0
and product. We were unable to locate the transition state
for ethyl nitrite dissociation into C H O + NO. It is not unu-
ꢂ1
0
2
5
r
ꢂ1
sual that a closed shell molecule dissociates without involv-
ing a well defined transition state. The best estimate
endothermic than reaction (4a) and should be more ther-
modynamically favored than reaction (6a). However,
assuming reaction (4a) has no activation energy barrier
[35], whether the reaction (6a) is kinetically favored or
not will depend on the activation energy barrier associated
with the isomerization of nitroethane into ethyl nitrite. On
the basis of our calculation results, the photon energy at
*
*
relative energy computed at MP4/6-311G //MP2/6-
1G** + DZPE level of theory for reaction (6a) is summa-
3
rized in Fig. 4. Based upon our ab initio computational
0
results, the heat of reaction (6a), D H (reaction (6a)), is pre-
r
ꢂ1
dicted to be 46.0 kcal mol , which is about 10% higher than
ꢂ1
the experimental value of 42.0 kcal mol . An activation
ꢂ1
ꢂ1
energy barrier of 68.4 ± 4.0 kcal mol is then estimated
266 nm (107.4 kcal mol ) absorbed by the nitroethane will
for the isomerization of nitroethane into ethyl nitrite based
be sufficient to allow reaction (6a) to occur. The photon en-
ergy is also sufficient for production of C H and NO , but
on our computational results. This value is higher by
2
5
2
ꢂ1
8
.6 kcal mol than that calculated by Denis et al. [35] using
not sufficient for production of C H + O + NO, which re-
2 5
ꢂ1
density functional methods. Since the C–N bond dissocia-
quires 134.1 kcal mol . Therefore, reaction (6a) is consid-
ered to be both thermodynamically and kinetically more
favored than reaction (4a) for production of NO. Addition-
ally, the NO might also be produced through reaction (7)
and (8). Although the activation energy of the
(C H + HONO) elimination channel via an intramolecu-
ꢂ1
tion energy of CH CH NO is ca. 61 kcal mol [10,32,34],
3
2
2
and the activation energy of molecular elimination decom-
position (reaction (7), CH CH NO !C H + HONO)
3
2
2
2
4
ꢂ1
has been reported to be 43 ± 2 kcal mol [22], the nitroe-
thane isomerization energy barrier of 68.4 kcal mol is
ꢂ1
2
4
ꢂ1
higher than both the C–N bond dissociation energy and
lar rearrangement is relatively lower (43 ± 2 kcal mol ),
the subsequent HONO dissociation energy to produce the
NO product is 48 kcal mol , while the dissociation energy
of [C H ONO] !C H O + NO is 43.3 kcal mol
the molecular elimination (CH CH NO !C H + HO-
3
2
2
2
4
ꢂ1
NO) activation energy. Therefore, the production of NO
and ethoxy radicals from the photodissociation of nitroe-
thane is estimated to be a minor channel comparing to the
other two dissociation channels.
*
ꢂ1
.
2
5
2
5
C
2
H
5
NO
0
2
þhv !C
2
H
4
þHONOꢄ
ꢂ1
However, the NO might also be produced from further
D H ¼18:6 kcal mol [24]
r
ð7Þ
ð8Þ
dissociation of NO that is generated from reaction,
2
ꢄ
0
ꢂ1
HONO !OH þNO; D H ¼48 kcal mol [28]
r
C
2
H
5
NO
2
þhv !C
2
H
5
þNO
2
!C
2
H
5
þO þNO ð4aÞ
Reaction (8) only produces NO when the nascent HONO
and it is difficult to experimentally distinguish the NO
product from these two different reaction pathways. On
the basis of available thermodynamic data [32–35], it has
produced via reaction (7) contains energy in excess of
ꢂ1
4
8 kcal mol . In the photodissociation of 1-nitropropane
0
at 282 nm, Haas and coworkers [24] found the quantum
yield of NO from HONO elimination and dissociation is
small by a factor of 5 than the OH yield.
been documented that D H (C H NO ) = ꢂ24.5 kcal
f
2
5
2
ꢂ1
0
ꢂ1
0
mol
,
D H (C H ONO) = ꢂ25.8 kcal mol
,
D H (C -
f 2
f
2
5
Thus, the photodissociation mechanism resulting in NO
production of nitroethane is then proposed as follows. In a
minor reaction pathway, after the parent nitroethane mol-
2
66nm
hυ=107.5
1
10
00
ꢂ1
ecule absorbed a photon at 266 nm (107.5 kcal mol ),
1
ꢂ1
6
8.4 kcal mol of the energy was used to cross the barrier
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
of the transition for isomerization to ethyl nitrite, the
remaining energy was then contributed to CH CH ONO
6
8.4
3
2
decomposition to yield C H O and NO radicals. Although
2
5
TS
the isomerization process is not a favored dissociation
channel of nitroethane, most of the NO product is pro-
duced from this channel. Our observation of the vibration-
ally cold, Boltzmann rotational distribution of the NO
fragment suggests that a relatively high degree of internal
4
6.0
C H O+NO
2
5
excitation for the C H O fragment is anticipated.
2
5
1
.9
C H NO
C H ONO
2
5
2
5
2
-10
4. Conclusion
Fig. 4. Relative energy diagram for the nitroethane isomerization to ethyl
nitrite calculated at the MP4/6-311G**//MP2/6-31G** + DZPE level of
theory.
We have probed the internal-state distribution of the
NO(X P) fragment after photodissociation from nitro-
2

236
Y. Li et al. / Chemical Physics Letters 421 (2006) 232–236
ethane at 266 nm using the one-photon laser-induced fluo-
[13] D.B. Moss, K.A. Trentelman, P.L. Houston, J. Chem. Phys. 96 (1992)
237.
00
rescence (LIF) technique,. The observed NO v = 0 state
fragment is vibrationally cold and the rotational distribu-
tion can be characterized by Boltzmann temperature of
[
[
[
14] N.C. Blais, J. Chem. Phys. 79 (1983) 1723.
15] J.C. Mialocq, J.C. Stephenson, Chem. Phys. 106 (1986) 281.
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Nitrocompounds and their Derivatives, Suppl. F, Part 1, Wiley, New
York, 1982.
8
10 ± 100 K. Ab initio calculation results suggest that the
NO may be produced from a two-step process in the pho-
todissociation of nitroethane, in which the nitroethane is
first isomerized into ethyl nitrite followed by dissociation
of the ethyl nitrite. This is based on the prediction of a bar-
[
[
17] G.H. Penner, J. Mol. Struct. (Theochem) 137 (1986) 121.
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(
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ꢂ1
rier height of 68.4 kcal mol for isomerization of nitroe-
5968.
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031.
1
[
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[
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Acknowledgements
This work was supported by NKBRSF (Grant
999075302) and NSFC (Grants 20373071, 20333050).
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