Geometric and electronic structures of benzoyl nitrite and benzoyl nitrate

Article abstract of DOI:10.1016/j.molstruc.2007.07.037

The electronic structures of benzoyl nitrite (C₆H₅C(O)ONO) and benzoyl nitrate (C₆H₅C(O)ONO₂) have been studied by HeI photoelectron spectroscopy (PES) and quantum chemical calculations. The photoelec

Full text of DOI:10.1016/j.molstruc.2007.07.037

Available online at www.sciencedirect.com
Journal of Molecular Structure 878 (2008) 26–31
www.elsevier.com/locate/molstruc
Geometric and electronic structures of benzoyl nitrite
and benzoyl nitrate
*
Lin Du, Xiao-Qing Zeng, Mao-Fa Ge , Zheng Sun, Dian-Xun Wang
Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
Received 19 March 2007; received in revised form 24 July 2007; accepted 26 July 2007
Available online 6 August 2007
Abstract
The electronic structures of benzoyl nitrite (C
6 5 6 5 2
H C(O)ONO) and benzoyl nitrate (C H C(O)ONO ) have been studied by HeI pho-
toelectron spectroscopy (PES) and quantum chemical calculations. The photoelectron spectra are assigned with the help of the outer
valence Green’s function (OVGF) calculations. The first vertical ionization energies of C H C(O)ONO and C H C(O)ONO are deter-
6
5
6
5
2
mined to be 9.20 and 9.54 eV, respectively. According to the results of theoretical calculations, it can be concluded that a planar
CAC(O)AOANAO skeleton in C
6
5 6 5 2
H C(O)ONO and a planar CAC(O)AOAN skeleton in C H C(O)ONO are the stable structures
in the gas phase.
ꢀ
2007 Elsevier B.V. All rights reserved.
Keywords: HeI photoelectron spectroscopy; Ionization energy; Benzoyl nitrite; Benzoyl nitrate
˚
1
. Introduction
length of HONO (1.397(6) A) is the shortest among known
nitrites (RONO) [6]. Organic nitrites RONO with
R = methyl [7,8], ethyl [9], isopropyl [10], t-butyl [11], are
thermally stable and their structures have been studied by
microwave spectroscopy. Our group has reported longer
Compounds containing the moiety of AONO or
AONO attract much attention because of their important
2
role in atmospheric chemistry. For example, the molecule
of FC(O)ONO, which may be regarded as the NO adduct
of the FC(O)O radical, is postulated to be an intermediate
in the atmospheric degradation of chlorofluorocarbons [1].
Chlorine and bromine nitrates (ClONO , BrONO ) can
˚
NAO bond length of CM C(O)ONO (M = H (1.560 A),
3
˚
˚
Cl (1.660 A), F (1.675 A)) by theoretical calculations
(B3LYP/6-311+G(d,p)) [12].
The number of stable nitrates is larger than that of
nitrites because of their chemical stability. The NAO single
bond length in gaseous nitrates RONO depends strongly
2
2
participate in the halogen and NO cycles of ozone deple-
tion in the stratosphere [2,3]. Peroxynitric acid (HOONO2)
2
and peroxynitrates (ROONO ) act as reservoir and trans-
on the substituent R. These compounds possess predomi-
nately covalent character with some polarity of the OAR
bond. In covalent nitrates, the NO moiety adopts C sym-
2
port species for NO in atmospheric chemistry [4].
2
As for the structure of nitrites and nitrates, the bonding
properties are of great interest. Recently, Oberhammer
reviewed the NAO bond property in covalent nitrates
and nitrites [5]. Because of the chemical instability of
nitrites, a small number of gas phase structural studies of
covalent nitrites have been performed [5]. The NAO bond
3
s
metry in comparison to the D3h symmetry for NO radical
3
[13]. Among all the known nitrates, the shortest OAN sin-
˚
˚
gle bond is in Me SiONO (1.383(5) A) [14], and the lon-
3
2
gest is in CF C(O)ONO (1.604 A, derived at the
3
2
B3LYP/6-311+G(d,p) level) [12].
The study of electronic structures of carboxylic acids
and their derivatives reveals the interactions between car-
*
Corresponding author. Tel./fax: +86 10 62554518.
E-mail address: gemaofa@iccas.ac.cn (M.-F. Ge).
bonyl oxygen lone pair (n ) and the approximately non-
O
0
022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.07.037

L. Du et al. / Journal of Molecular Structure 878 (2008) 26–31
27
bonding anti-symmetric p orbital [15]. Photoelectron spec-
troscopy has been proved to be an effective method for
studying the electronic structures of nitrites and nitrates
bond syn or anti with respect to the OAN bond (Chart 1).
Furthermore, benzoyl nitrites may adopt a syn- or antiperi-
planar conformation between the benzoyl moiety and the
N@O bond (Chart 2) [5]. So all the four possible conform-
ers of C H C(O)ONO and two possible conformers of
2
[
12,13]. No work has been reported when the substituent
R of RONO and RONO is a conjugated aryl group. In
2
6
5
the present study, we report the photoelectron spectra of
benzoyl nitrite and benzoyl nitrate. The molecular struc-
tures have been investigated by quantum chemical calcula-
tions. Their electronic structures are also analyzed
according to the experimental and theoretical results.
C H C(O)ONO were optimized. However, only the two
6 5 2
conformers with C@O bond syn with respect to the OAN
bond (syn structure in Chart 1) were found to be
stable conformers after vibrational analysis. As for
C H C(O)ONO, the conformer with benzoyl moiety anti
6
5
to the N@O bond (anti structure in Chart 2) is the stable
one. The other conformers are not local minima by vibra-
tional analysis. The spatial interactions between the ben-
2
. Experimental and computational methods
Benzoyl nitrite (C H C(O)ONO) and benzoyl nitrate
C H C(O)ONO ) were generated at room temperature
6 5 2
zoyl group and the ONO or ONO
stability of the other conformers.
moiety decrease the
2
6
5
(
of 20 ꢁC through gas–solid reactions by passing ClNO or
In order to search for other possible conformers with
different dihedral angle of dCCON, relax scans of the poten-
tial energy surface were performed by rotating the torsional
angle dCCON in steps of 10ꢁ with HF/6-31G(d) and B3LYP/
6-31G(d) approximation. The potential curves for
ClNO vapor over finely powdered C H C(O)OAg, and
2
6
5
in situ photoelectron spectra of the gas phase products
were recorded. This procedure is similar to the description
in Ref. [12] in detail. The reaction route is illustrated as
follows:
C
1
H
C(O)ONO and C
H C(O)ONO are shown in Figs.
6 5 2
6
5
and 2, respectively. At both levels, there are three minima
ClNOðgÞþC
6
H
5
CðOÞOAgðsÞ ! C
CðOÞOAgðsÞ ! C
6
H
5
CðOÞONOðgÞþAgClðsÞ
ClNO
2
ðgÞþC
6
H
5
6
H
5
CðOÞONO ðgÞþAgClðsÞ
2
O
NO
x
O
NOx
ClNO and ClNO were prepared according to Ref. [16],
C H
6 5
2
C
O
C
and the photoelectron spectra of both compounds were the
same as that in Refs. [17,18]. C H C(O)OAg were pur-
O
C H
6
5
6
5
chased from Alfa Aesar, and had been dried in vacuum
syn
anti
x
C(O)ONO .
ꢀ
4
(
1 · 10 Torr) for 3 h at 60 ꢁC before use.
Chart 1. Possible conformers of C
6
H
5
Photoelectron spectrum was recorded on a double-
chamber UPS-II instrument [19], which was specially
designed for detecting unstable species. The spectral resolu-
tion of HeI spectra is about 30 meV, when measured as the
O
N
ꢀ
1 2
N
O
full width at half-maximum (FWHM) of the 3p
P2/3
+
1
C H C(O)
O
C
6
H
5
C(O)
O
anti
6
5
Ar ‹ Ar ( S ) line. During the experiments, small amount
0
syn
of Ar gas and CH I was added to the sample flow to cali-
3
brate the experimental vertical ionization potentials.
Gaussian 03 programs [20] were used for the theoretical
calculations of the two molecules, C H C(O)ONO and
6 5
Chart 2. Possible conformers of C H C(O)ONO.
6
5
C H C(O)ONO . These two molecules were optimized
6
5
2
with three different methods MP2/6-311++G(d,p),
B3PW91/6-311++G(d,p), and B3LYP/6-311++G(d,p),
respectively. The vertical ionization energies were calcu-
lated at the ab initio level according to Cederbaum’s outer
valence Green’s function (OVGF) [21] method. Compared
with the experimental determined structures of covalent
nitrates and nitrites, the B3LYP method reproduces all
bond lengths very well [5]. So the structures derived from
B3LYP/6-311++G(d,p) are used for OVGF calculations.
3
. Results and discussion
3
.1. Molecular structures
According to Ref. [12], benzoyl nitrites and nitrates may
exist in the gas phase of two conformers with the C@O
Fig. 1. Calculated potential curves for internal rotation around the CAO
bond in C C(O)ONO at the HF/6-31G(d) and B3LYP/6-31G(d) levels.
6 5
H

28
L. Du et al. / Journal of Molecular Structure 878 (2008) 26–31
ent R, RONO tends to show an anti structure. This is con-
sistent with the theoretically determined structure of
C H C(O)ONO. The optimized structures of
C H C(O)ONO with three different methods of MP2/6-
6
5
6
5
3
3
11++G(d,p), B3PW91/6-311++G(d,p), and B3LYP/6-
11++G(d,p) are given in Table 1. The B3LYP method
reproduces all bond lengths very well for nitrites and
nitrates [5], so our discussion of the structural parameters
for them are all based on the results of B3LYP/6-
3
(
11++G(d,p). The NAO bond length in C H C(O)ONO
6
5
˚ ˚
1.555 A) is close to that of CH C(O)ONO (1.560 A,
3
B3LYP/6-311+G(d,p)) [12], and much longer than that in
˚
their parent compound, HONO (1.397(6) A, microwave
spectroscopy) [6]. The corresponding bond length in
˚
(
CF ) NONO (1.572(21) A) is slightly longer as determined
3 2
Fig. 2. Calculated potential curves for internal rotation around the CAO
by gas electron diffraction (GED) [22]. The gas phase struc-
bond in C
levels.
6 5 2
H C(O)ONO at the HF/6-31G(d) and B3LYP/6-31G(d)
tures of the parent compound of nitrates, HONO [23,24]
2
and of CH ONO [25] were determined by microwave spec-
3
2
˚
troscopy. Their NAO single bond lengths are 1.410(2) A
˚
and 1.402(5) A, respectively, which are much shorter
in the curves for C H C(O)ONO, which implies that
6
5
˚
than that of C H C(O)ONO (1.523 A, B3LYP/6-
besides the planar structure with dCCON = 180ꢁ, two
gauche structures may exist. The two gauche conformers
with dCCON = 60ꢁ or 300ꢁ (HF/6-31G(d)) are enantiomers,
while the dihedral angles dCCON are predicted to be 50ꢁ or
6
5
2
3
11++G(d,p)) (Table 1). An even shorter NAO bond has
˚
been determined in Me SiONO (1.383(5) A) by GED [14].
3
2
3
10ꢁ at the B3LYP/6-31G(d) level. The enantiomers have
3.2. Photoelectron spectroscopy
the same energy and vibrational frequency. The energy dif-
ference between the gauche and the planar conformers is
calculated to be 6.99 (HF/6-31G(d)) and 4.64 kcal/mol
The HeI photoelectron spectra of C H C(O)ONO and
6
5
C H C(O)ONO are shown in Figs. 4 and 5, respectively.
6
5
2
(
B3LYP/6-31G(d)), which demonstrates that the planar
conformer is the most stable conformer of C H C(O)ONO.
6
5
The situation of C H C(O)ONO is a little different. The
potential curves predict only one minimum of
6
5
2
Table 1
Optimized
geometric
a
parameters
for
C
6
H
5
C(O)ONO
and
dCCON = 180ꢁ at each level. The planar conformer of
6 5 2
C H C(O)ONO
C H C(O)ONO is also the most stable one. The molecular
6
5
2
6 5
C H C(O)ONO
C H C(O)ONO
6
5
2
models of the stable conformers of C H C(O)ONO and
b
b
b
b
b
b
6
5
MP2
B3PW91 B3LYP MP2 B3PW91 B3LYP
C H C(O)ONO are shown in Fig. 3. Both the two stable
6
5
2
r
r
C1C2
1.499
1.214
1.357
1.674
1.140
123.8
111.5
111.3
107.8
180.0
180.0
1.487
1.200
1.375
1.522
1.146
125.1
111.2
113.5
108.6
180.0
180.0
1.491
1.203
1.377
1.555
1.145
124.9
111.4
113.5
108.4
180.0
180.0
1.491 1.481
1.212 1.199
1.364 1.378
1.595 1.503
1.187 1.183
125.5 126.5
111.3 111.1
107.4 111.0
112.0 113.4
180.0 180.0
1.485
1.201
1.384
1.523
1.187
126.5
111.2
111.1
113.3
180.0
91.7
conformers have the Cs overall symmetry, a planar
CAC(O)AOANAO skeleton in C H C(O)ONO and a pla-
C1O1
6
5
rC1O2
rO2N
nar CAC(O)AOAN skeleton in C H C(O)ONO ,
6
5
2
respectively.
r
NO3
a
a
a
C2C1O1
C2C1O2
C1O2N
Organic nitrites are chemically more stable than halogen
nitrites and microwave spectroscopy investigations for
CH ONO [7,8], CH CH ONO [9], (CH ) CHONO [10],
3
3
2
3 2
aO2NO3
dC2C1O2N
d
and (CH ) CONO [11] could be performed. In CH ONO
3
3
3
and CH CH ONO, the syn form (Chart 2) is favored,
C1O2NO3
91.5
91.7
3
2
a
˚
whereas only the anti conformer was detected for
CH ) CHONO and (CH ) CONO. With bigger substitu-
Distances in A, angles in degree. As for atom numbering, see Fig. 3.
At the 6-311++G(d,p) level.
b
(
3
2
3 3
6 5 6 5 2
Fig. 3. Molecular models of C H C(O)ONO and C H C(O)ONO .

L. Du et al. / Journal of Molecular Structure 878 (2008) 26–31
29
Table 2
HeI experimental vertical ionization energies (eV), calculated vertical
ionization energies (eV), and molecular orbital characters of C C(O)O-
NO and C C(O)ONO
6
H
5
6
H
5
2
C
6
H
5
C(O)ONO
C
6
H
5
C(O)ONO
2
exp
cal
exp
cal
IP
I
v
Orbital Character
IP
I
v
Orbital Character
0
0
0
0
00
9
9
1
.20
.40
9.24 6a
9.27 5a
p
p
n
1
9.54 9.61 10a
9.68 9a
10.98 11.13 33a
p
p
n
1
00
2
2
0
0
0
0
0
0.12 10.24 33a
O(C@O),
O(C@O),
r
ONO, rCAC
r
CAC, n
O
00
1
1
1.18 11.09 32a
n
O(C@O)
,
11.95 12.34 8a
nOðNO2Þ
r
ONO
0
0
00
1.78 11.63 4a
p
O(C@O),
12.12 12.37 7a
p
3
, pO(C@O)
n
r
O(CAO)
0
0
0
1
1
1
1
1
1
1
1
2.14 31a
2.39 3a
12.49 12.42 32a
12.42 31a
r
r
0
0
ONO
0
0
0
00
2.63 30a
4.54 29a
4.67 28a
12.54 6a
13.06 30a’
00
13.27 5a
0
0
0
0
0
4.94 2a
14.95 29a
15.29 28a
15.36 27a
0
0
5.35 27a
5.45 26a
6 5
Fig. 4. HeI photoelectron spectrum of C H C(O)ONO.
into two components (p and p ) in the benzoic acid and its
1
2
derivatives. And the pO molecular orbital is a somewhat
localized p orbital which has dominant amplitude on the
O atom of the CAO single bond. The effect of the interac-
tion varies among different substituent of C H C(O)O moi-
6
5
ety. The splitting of the p1,2 molecular orbital is 0.20 eV in
C H C(O)ONO, as derived from the photoelectron spec-
6
5
trum. The splitting in aminobenzoic acid derivatives is
much larger (1.2–1.8 eV) [27].
The peak centered at 10.12 eV is mainly caused by the
electron ionization from nO(C@O) orbital, which is the non-
bonding orbital on the carbonyl oxygen, and a rONO orbi-
tal, which is r orbital concentrated on ONO moiety. And
partly contribution also comes from the rCAC between
the benzyl ring and the C@O moiety. The next peak
at 11.18 eV could also be attributed to the ionization
È
À
ÁÉ
0
ꢀ1
process of 32a nOðC@OÞ; rONO
. The ionization from
p bonding orbital on the carbonyl group and the nonbond-
ing nO at CAO single bond causes the spectral band at
11.78 eV. At higher energy region, the bands are mainly
caused by the electron ionization from more complicated
orbitals of benzyl ring.
6 5 2
Fig. 5. HeI photoelectron spectrum of C H C(O)ONO .
Each of the two spectra shows three broad bands in the
ionization energy range of 7.0–17.0 eV. The assignments
of the spectra have been performed with the help of OVGF
calculations. The experimental and calculated (OVGF/6-
As for the photoelectron spectrum of C H C(O)ONO
2
6
5
(Fig. 5), its overall shape is similar to that of
C H C(O)ONO. The first two occupied molecule orbitals
6
5
3
1G) vertical ionization potentials are listed in Table 2.
are the p and p orbitals of the benzyl ring. However, the
1 2
Selected occupied molecule orbitals of C H C(O)ONO
splitting of the p1,2 molecular orbital could not be observed
6
5
and C H C(O)ONO are shown in Fig. 6.
in the photoelectron spectrum of C H C(O)ONO . The first
6
5
2
6
5
2
Similar to the photoelectron spectrum of benzoic acid,
C H C(O)OH, the first ionization process happens on the
vertical ionization energy of C H C(O)ONO (9.54 eV) is
6 5 2
6
5
0.34 eV higher than C H C(O)ONO, which is also affected
6
5
benzyl ring in C H C(O)ONO. The first vertical ionization
by the p /p interaction.
O 1,2
6
5
0
energy of C H C(O)ONO (9.20 eV) is close to that of ben-
The composition of the molecular orbital 33a is similar
6
5
0
zene molecule (9.25 eV) [26], but lower than C H C(O)OH
to the 33a MO of C H C(O)ONO, but the r orbital con-
6
5
6
5
(
9.6 eV) because of the p /p1,2 interaction [27]. The p1,2
centrated on ONO moiety is changed to be nO in
C H C(O)ONO . The corresponding ionization energy
O
molecular orbital is the e1g p orbital of benzene which splits
6
5
2

30
L. Du et al. / Journal of Molecular Structure 878 (2008) 26–31
6 5 6 5 2
Fig. 6. Selected occupied molecule orbitals of C H C(O)ONO and C H C(O)ONO .
increases 0.86 eV due to the change of the molecular orbi-
tal. The peak at 11.95 eV comes mainly from the ionization
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È
À
ÁÉ
00
ꢀ1
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2
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5
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2
[
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5
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5
2
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[
(
6
5
6
5
2
orbital on benzyl ring. Meanwhile, the geometries of these
two compounds are optimized at different levels of theory.
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symmetry, a planar CAC(O)AOANAO skeleton in
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