Experimental and theoretical studies of 3-benzyloxy-2-nitropyridine

Article abstract of DOI:10.1016/j.molstruc.2012.05.064

The structure of 3-benzyloxy-2-nitropyridine has been investigated both experimentally and theoretically. The X-ray crystallography results show that the nitro group is tilted out of the pyridine ring plane by 66.4(4)°, which is mainly attributed to the e

Full text of DOI:10.1016/j.molstruc.2012.05.064

Journal of Molecular Structure 1026 (2012) 133–139
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Experimental and theoretical studies of 3-benzyloxy-2-nitropyridine
⇑
⇑
Wenting Sun, Yu Cui, Huimin Liu, Haitao Zhao , Wenqin Zhang
Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China
h i g h l i g h t s
"
"
"
"
Synthesis of 3-benzyloxy-2-nitropyridine.
The compound was characterized by IR, Raman and X-ray crystallography.
DFT studies on the structures of the compound.
DFT studies on the vibrational frequencies.
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure of 3-benzyloxy-2-nitropyridine has been investigated both experimentally and theoreti-
cally. The X-ray crystallography results show that the nitro group is tilted out of the pyridine ring plane
by 66.4(4)°, which is mainly attributed to the electron–electron repulsions of the lone pairs in O atom of
Received 20 March 2012
Received in revised form 23 May 2012
Accepted 23 May 2012
the 3-benzyloxy moiety with O atom in nitro group. An interesting centrosymmetric p-stacking molec-
Available online 6 June 2012
ular pair has been found in the crystalline state, which results in the approximate coplanarity of the pyr-
idine ring with the benzene ring. The calculated results show that the dihedral angle between the nitro
group and pyridine ring from the X3LYP method is much closer to the experimental data than that from
the M06-2X one. The existing two conformational isomers of 3-benzyloxy-2-nitropyridine with equal
energy explain well the disorder of the nitro group at room temperature. In addition, the vibrational fre-
quencies are also calculated by the X3LYP and M06-2X methods and compared with the experimental
results. The prediction from the X3LYP method coincides with the locations of the experimental frequen-
cies well.
Keywords:
3-Benzyloxy-2-nitropyridine
Crystal structure
Theoretical studies
Rotational barrier
Vibrational spectra
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
The molecular structures of 3- and 4-nitropyridine derivatives
have been extensively reported [10,11]. However, the molecular
In the aromatic nitro compounds without o-substituent group,
the nitro groups are usually coplanar with the aromatic ring planes,
in which the dihedral angles between the nitro groups and the aro-
matic ring planes are less than 5° [1–3]. In these cases, the molecules
structures of 2-nitropyridine derivatives are seldom investigated.
The title compound, 3-benzyloxy-2-nitropyridine (BNP, Scheme 1),
is an important intermediate for the synthesis of asymmetric cya-
nine dyes for the fluorescence detection of nucleic acids [12]. In this
paper, the molecular structure of BNP was determined by X-ray
single crystallography. The nitro group is disordered, and a very
large dihedral angle (66.4°) between the nitro group and the pyri-
dine ring plane was observed. In order to explain these observa-
tions, the molecular structure and the rotational barrier of the
nitro group were studied theoretically. Moreover, the vibrational
spectra of BNP were calculated by the X3LYP [13,14] and M06-2X
[15] methods.
are stabilized by the p–p conjugation. However, the dihedral angles
between the nitro group and the aromatic ring planes are varied
from 10° to 70° in some o-substituted aromatic nitro compounds
[4–6], and even to about 90° in some o,o0-disubstituted ones such
as 84(1)° in 1,3,5-triisopropyl-2-nitrobenzene [7]. The large
dihedral angle is generally attributed to the steric effect of
the o-substituent [8]. For example, in 2-[(4-methylphenylimi-
no)methyl]-1-nitrobenzene, the nitro plane is tilted out of the
benzene ring plane by 27.9(5)° to minimize the steric hindrance
between the ortho atoms of the nitro group and their vicinal H
atoms [9].
2. Experimental and theoretical methods
2.1. Materials and general methods
⇑
All reagents are commercially available and purified by stan-
dard methods prior to use. The melting points were measured on
Corresponding authors. Tel.: +86 22 58269011; fax: +86 22 27403475.
E-mail addresses: htzhao@tju.edu.cn (H. Zhao), wqzhang@tju.edu.cn (W. Zhang).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.05.064

134
W. Sun et al. / Journal of Molecular Structure 1026 (2012) 133–139
O N
2
N
O
Scheme 1.
Table 1
Crystal data and structure refinement summary for BNP.
Fig. 1. The crystal structure of BNP with atom labels and 30% probability ellipsoids
level.
Empirical formula
C12H10N2O3
Formula weight
230.22
Crystal system tetragonal
Space group
a (Å)
Monoclinic
P21/n
7.112(3)
b (Å)
c (Å)
b (°)
13.030(6)
12.407(5)
92.004(7)
1149.0(9)
1.331
V (Å3)
Dc (g/cm3)
F(000)
Z
480
4
l(MoK
a)/mm
0.098
h Range (°)
Range of h, k, l
Reflections collected/unique
Data/restraints/parameters
2.27–25.00
ꢀ7/8, ꢀ15/13, ꢀ14/13
5774/2029
2029/36/172
0.0510 and 0.1522
1.022
R and
xR
Goodness-of-fit on F2
Max. res. peak and hole (e Åꢀ3
)
0.131 and ꢀ0.185
Fig. 2. The crystal packing, viewed along the b axis.
Table 2
Selected bond lengths (Å) and bond angles (°) for BNP from the experiment and
theoretical calculations.
Parameters
M06-2X
1.212
X3LYP
1.221
Experiment
O(2)AN(2)
O(3)AN(2)
N(2)AC(12)
N(1)AC(12)
O(1)AC(8)
O(1)-C(7)
C(6)AC(7)
O(2)AN(2)AC(12)
O(3)AN(2)AC(12)
O(2)AN(2)AO(3)
C(11)AN(1)AC(12)
C(8)AO(1)AC(7)
O(1)AC(7)AC(6)
O(2)AN(2)AC(12)AN(1)
O(3)AN(2)AC(12)AN(1)
O(2)AN(2)AC(12)AC(8)
O(3)AN(2)AC(12)AC(8)
N(2)AC(12)AC(8)AO(1)
C(1)AC(6)AC(7)AO(1)
1.205
1.220
1.494
1.315
1.357
1.431
1.512
118.2
116.3
125.5
115.1
119.5
107.8
ꢀ114.8
66.2
1.204
1.489
1.304
1.337
1.421
1.506
1.214
1.493
1.309
1.343
1.432
1.507
116.5
116.5
117.1
126.4
117.7
118.7
108.3
ꢀ130.1
48.6
117.2
126.3
118.0
119.4
108.8
ꢀ125.7
53.0
Fig. 3. The centrosymmetric p-stacking molecular pairs.
1 h at room temperature. The formed sodium salt was collected
and air-dried.
49.4
54.0
65.6
A solution of benzyl bromide (2.15 ml, 18 mmol) in 18 ml DMF
was added dropwise to the mixture of the above sodium salt
(2.93 g, 18 mmol) in 18 ml DMF, and was stirred for 10 h at room
temperature. Then the reaction mixture was added 100 ml water,
extracted with ethyl ether (3 ꢁ 100 ml). The ether phase was dried
with anhydrous MgSO4, filtrated, and concentrated to afford crude
yellow liquid, which was further purified by chromatography using
light petroleum/ethyl acetate (2:1) as the eluant to provide color-
less needle crystals (3.03 g, 74.0%), mp. 60–62 °C. 1H NMR (CDCl3):
ꢀ131.9
ꢀ127.4
ꢀ113.3
3.8
3.5
1.2
ꢀ25.6
ꢀ29.8
0.8
a Yanagimoto MP-500 apparatus (uncorrected). The FT-IR spectra
(KBr pellets and in CCl4 solvent) were measured on a BIO-RAD
FTS 3000 infrared spectrometer in the 4000–400 cmꢀ1 range with
a 4 cmꢀ1 resolution. The FT-Raman spectra were measured using
Bruker RFS 100/S FT-Raman spectrometer with a 4 cmꢀ1 resolu-
tion. The 1H NMR spectra were recorded on a Bruker AV-400 spec-
trometer (400 MHz) at 298 K in CDCl3. The MS was performed on
the Thermo Finnigan Trace DSQ GC–MS spectrometer.
d
8.06 (d, J = 4.4 Hz, 1H), 7.50 (d, J = 8.4 Hz, 1H), 7.45 (dd,
J1 = 4.4 Hz, J2 = 8.4 Hz, 1H), 7.41–7.31 (m, 5H). MS: m/z 91 (100,
M+-NO2PyOꢂ), 124 (12.6).
2.2. Preparation of BNP
2.3. X-ray crystallography
2-Nitro-3-hydroxypyridine (2.80 g, 20 mmol) was added to a
solution of NaOH (0.80 g, 20 mmol) in 20 ml EtOH and stirred for
Single-crystal X-ray diffraction measurements were carried
out on a Bruker Smart 1000 CCD diffractometer using graphite

W. Sun et al. / Journal of Molecular Structure 1026 (2012) 133–139
135
Fig. 4. The optimized structures with selected structural parameters (bond lengths in angstroms and angles in degrees) for species involved in the BNP1 and BNP2. Selected
calculated structural parameters for the conformers are compared with X3LYP method (in parentheses).
monochromatized MoKa radiation (k = 0.71073 Å) with u and x
scan mode at 293(2) K. Semi-empirical absorption corrections
were applied using SABABS program. All structures were solved
by direct method and successive difference Fourier syntheses
(SHELXS-97), and refined by full-matrix least-squares procedure
on F2 with anisotropic thermal parameters for all non-hydrogen
atoms (SHELXL-97) [16]. Hydrogen atoms were generated geomet-
rically and refined with fixed thermal factors riding on the con-
cerned atoms. The two O atoms of the nitro group are disordered
over two sides. The ratio of site occupancies, from the refinement,
was 0.75:0.25. Crystallographic data and experimental details for
structural analyses are summarized in Table 1, and the selected
bond lengths and angles are presented in Table 2.
1750
1500
1250
1000
750
500
a
2.4. Theoretical method
b
c
Molecular geometries of the model complexes were optimized
without constraints via DFT calculations using the X3LYP and the
M06-2X functionals, as implemented in the Gaussian 09 suite of
programs [17]. It is known that these two functionals have good
performance in the calculation of compounds having weak interac-
tions. The 6-311Gꢃꢃ basis set [18] was used for all atoms. Frequency
calculations at the same level of theory have also been performed
-0.34
(-0.24)
O
-0.38
(-0.26)
O
N
-0.42
0.51
(0.21)
(-0.27)
N
d
-0.52
(-0.33)
O
1750
1500
1250
1000
750
500
Wavenumber (cm-1)
CH2
Fig. 6. The experimental IR spectra for BNP (1900–500 cmꢀ1) in (a) CCl4 solvent and
(b) KBr pellets and compared with the computational results via the methods of (c)
M06-2X and (d) X3LYP.
Fig. 5. The calculated NBO and Mulliken (in parentheses) charges at the X3LYP/6-
311Gꢃꢃ theoretical level.

136
W. Sun et al. / Journal of Molecular Structure 1026 (2012) 133–139
3500
3000
2500
2000
1500
1000
500
[23], the pyrimidine ring and the benzene ring are close to perpen-
dicular. The planar conformation of BNP can be explained by the
p-stacking effect between the layer packing of the BNP molecules
(Fig. 2). Such -stacking effect between pyridine and benzene rings
p
belongs to two BNP molecules prevent the rotation of benzene ring
around the C6AC7 single bond. However, we found that the ben-
zene ring is uncoplanar with the pyridine ring in the energetically
favored conformation of BNP resulting from the theoretical calcu-
lation, in which the BNP molecule was considered independently.
The dihedral angle of the benzene and pyridine ring planes is
ꢀ17.8(0)° from the X3LYP method. This can further demonstrate
a
that p-stacking effect is the main force to sustain BNP in a planar
conformation in the crystalline state.
In the crystal structure of BNP, it was found that the nitro group
was disordered and the site occupancy factors were refined to 0.75
(O2 or O3) and 0.25 (O20 or O30), respectively. The nitro group is
tilted out of the pyridine ring plane by a large angle of 66.4(4)°
for O2AN2AO3 and 76.4(4)° for O20AN20AO30, and the two compo-
nents vibrate by 37.1(4)° from each other around the C12AN2 sin-
gle bond. It seems that there exists a low rotational barrier
between the two components, which is so low that the two compo-
nents can vibrate from each other at room temperature. We calcu-
lated the corresponding transition states in the next section and
tried to find whether the nitro group vibrates through a small
barrier.
b
c
Due to the dipole–dipole interaction between the electron poor
pyridine ring and electron rich benzene ring, an interesting
p-stacking molecular pair of BNP was formed centrosymmetrically
(Figs. 2 and 3) with a separation distance of 3.568(5) Å, which indi-
cates the presence of C–T interactions. These centrosymmetric
pairs are further assembled into layers in the overall crystal pack-
ing, as what the normal planar organic molecules show (Fig. 2).
3500
3000
2500
2000
1500
1000
500
Wavenumber / cm-1
Fig. 7. The experimental Raman spectra of BNP and compared with the compu-
tational results via the methods of (b) X3LYP and (c) M06-2X.
3.2. Theoretical analysis
to identify all of the stationary points as minima (zero imaginary
frequencies) or transition states (one imaginary frequency), and
to provide free energies at 298.15 K which include entropic contri-
butions by taking into account the vibrational, rotational, and
translational motions of the species under consideration. To better
understand the nature of the intramolecular interaction, we per-
formed natural bond orbital (NBO) analysis for BNP with the
NBO 5.9 standalone package [19,20]. Furthermore, the Mulliken
charge distribution was calculated via the Gaussian 09 suite of pro-
grams [17]. The vibrational frequencies were scaled by 1.0 for
M06-2X/6-311Gꢃꢃ and 0.9703 for X3LYP/6-311Gꢃꢃ [21]. In conve-
nience, we define the torsion angle of C8AC12AN2AO2 as
The observation and analysis above prompt us to calculate the
rotational barriers of the nitro group, and these values may help
us to explain its disorder. Through our calculation, we indeed lo-
cate two conformational isomers (BNP1 and BNP2) equivalent by
rotation. The
U(NO2) for the two isomers calculated by X3LYP
method is 53.9° and 127.4°, respectively. Fig. 4 shows the opti-
mized structures with selected structural parameters for the con-
formational isomers BNP1 and BNP2. In Table 2, the calculated
structural parameters of the BNP1 are compared with their corre-
sponding experimental ones. The calculated geometries reproduce
well the important structural parameters. For example, calculated
geometric parameters for A1 by X3LYP method, O3AN2 = 1.214 Å,
N2AC12 = 1.493 Å, N1AC12 = 1.309 Å, agree well with experimen-
tally determined values for the BNP, O3AN2 = 1.220 Å, N2AC12 =
1.494 Å, N1AC12 = 1.315 Å, thus confirming that the basis sets
are adequate for present study. For a specific bond, we find that
M06-2X calculated bond length is slightly shorter than the bond
length calculated by X3LYP method. The deviation between the
theoretical and experimental value of bond length is less than
0.02 Å, which suggest that the two DFT functional methods are
reliable for the current study.
U
(NO2). It should be noted that we use gas-phase calculation to
reproduce the molecular structures and vibrational frequencies of
solid phase compounds.
3. Results and discussions
3.1. Crystal structure
The bond lengths and angles in BNP are normal (Fig. 1). Due to
the p–
p
conjugation, the O1 atom adopts sp2 hybridization and
both of the C7 and O1 atoms are almost coplanar with the pyridine
ring plane. The deviation of C7 atom from the least-squares of pyr-
idine plane is ꢀ0.069(5) Å. In addition, the torsion angle of
C1AC6AC7AO1 is only 0.7(6)°. Therefore, the pyridine and the
benzene rings are almost coplanar with a small dihedral angle of
ꢀ2.5(9)°. However, this quasi-planar conformation does not exist
in some other benzyloxy heterocyclic compounds. For example,
in 2-amino-6-benzyloxy-4-(N-methylamino)-5-nitrosopyrimidine
[22] and 4-amino-6-benzyloxy-2-(methylsulfanyl) pyrimidine
The current DFT calculations allow us to measure the energy
barriers of conformational isomerization between isomers BNP1
and BNP2. Through nitro group rotating around the C12AN2 single
bond, we locate two transition states with very small barriers of
2.5 and 1.6 kcal molꢀ1 from the X3LYP method, respectively. The
barriers are even smaller than the CAH bond rotation in ethane
along the CAC bond, which needs a barrier of 3 kcal molꢀ1. From
the very low barrier height (1.6 kcal molꢀ1), we conclude that the
nitro group can free rotate in the room temperature. The optimized

W. Sun et al. / Journal of Molecular Structure 1026 (2012) 133–139
137
Table 3
The theoretical (M06-2X 6-311Gꢃꢃ and X3LYP 6-311Gꢃꢃ) and experimental (KBr pellets and in CCl4 solvent) frequencies (cmꢀ1) for BNP.
No.
Theoretical
X3LYP
Experimental
Assignmentsc
b
Ia
M06-2X
Ia
CCl4
KBr pelletsb
1
2
3
4
5
6
7
8
9
3121
3107
3103
3098
3088
3085
3077
3062
2950
2908
1
3
7
6
8
1
1
3
9
8
3234
3218
3222
3214
3197
3202
3181
3166
3080
3033
1748
1694
0
3
2
1
2
1
0
1
4
4
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
s(PyAH)
s(PhAH)
as(PyAH)
as(PhAH)
as(PhAH)
as(PyAH)
as(PhAH)
as(PhAH)
as(CH2)
3092 vw
3069 w
3092 w
3067 w
3036 w
2909 w
2884 w
3036 w
2918 w
2874 w
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
61
62
63
64
65
s(CH2)
as(NO2)
sk(Ph)
100
0
1607
1606
1586
1584
1561
1490
1466
1451
1444
1426
2
100
4
37
14
6
1599 m
1599 m
as(NO2),
sk(Ph)
m
sk(Py)
1670
1685
1650
1551
1528
1514
1502
1497
1493
0
5
9
1570 s
1549 vs
1499 m
1572 s
1541 vs
1497 m
sk(Py),
sk(Py)
sk(Ph)
m
as(NO2)
5
0
20
22
3
23
22
d(CH2)
d(CH2),
22
16
38
1466 m
1454 m
1433 s
1468 m
1450 s
1431 s
m
sk(Py)
m
m
m
m
sk(Ph),
sk(Py)
msk(Py)
s(NO2), d(CH2)
s(NO2), (CH2)
(CH2), b(PhAH), b(PyAH)
b(PhAH)
1379
1368
1320
1296
1289
1281
1230
1220
1193
1171
1150
1131
1104
1080
1056
1024
1012
1003
989
983
962
959
903
898
857
839
836
803
791
784
731
715
695
684
619
608
587
543
523
480
448
403
392
54
13
3
47
5
76
1
29
0
1
0
2
22
2
1
1
18
10
1
0
0
0
1
1
14
1
2
1
18
1
17
4
11
8
1
4
1
0
0
2
1
0
1
1377 s
1381 s
x
1422
1361
1331
1332
1357
1287
1266
1247
1212
1179
1177
1160
1126
1102
1067
1089
1049
1020
1029
1013
1012
949
941
902
880
876
840
835
826
758
746
717
713
635
626
610
563
550
15
13
4
5
52
1
9
0
1
0
1
16
1
0
3
6
0
0
0
0
0
0
1
11
0
1
0
8
5
14
2
7
4
1
1
1
0
x
1312 m
1288 s
1306 s
m
m
sk(Py),
sk(Ph),
m
sk(Ph)
msk(Py)
m
1294 vs
1219 m
b(PyAH),
(CH2)
(PyAO)
s
1217 m
b(PyAH),
b(PhAH),
b(PhAH)
b(PhAH)
b(PyAH)
b(PyAH)
b(PhAH)
b(PyAH)
b(PhAH)
s
c
(CH2)
(CH2)
1117 s
1119 s
1022 m
m(PhCH2AO), c(PhAH)
c(PhAH),
m
(PhCH2AO)
b(Ph)
c(PhAH)
c(PyAH)
c(PhAH)
c(PhAH),
c(PyAH),
c
(PyAH)
910 w
862 m
899 w
866 s
c(PhAH)
d(NO2), b(Py)
c
c
m
c
c
c
c
c
(PhAH)
(PhAH)
841 w
795 m
733 s
689 s
(PhACH2), b(Py), b(Ph)
(PyAH)
(PyAH),
(PhAH)
(PyAH)
(PhAH)
x(NO2)
696 s
b(Py),
b(Ph)
c(PhAH)
613 w
590 w
563 vw
613 m
586 w
546 vw
b(Py), b(Ph)
sk(Py)
b(Ph), b(Py)
c
0
1
1
0
csk(Py)
csk(Py),
csk(Ph),
csk(Ph)
csk(Py)
494
463
414
411
c
sk(Ph)
457 w
463 w
415 w
csk(Py)
1
417 vw
a
b
c
Relative absorption intensities normalized with highest peak absorption equal to 100.
Intensity: vw, very weak; w, weak; m, medium; s, strong; vs very strong.
Vibrational modes:
m
, stretching; d, scissoring;
x, wagging;
q, rocking;
s
, twisting;
c, out-plane bending; b, in-plane bending; s, symmetric; as, asymmetric; sk, skeletal.
structures with selected structural parameters of the two transi-
tion states named TS1 and TS2 are also drawn into Fig. 4.
One may have the question that why the nitro group rotates
around the N2AC12 bond to adopt a tilted conformation. To
answer the question, we need to examine the structures shown
in Fig. 4.Check the structures of TS1 (U(NO2 = 0°)) that has the ni-
tro group parallel to the pyridine ring, we find that the calculated
bond length of N2AC12 bond (1.503 Å from X3LYP calculations) is

138
W. Sun et al. / Journal of Molecular Structure 1026 (2012) 133–139
longer than the bond length of N2AC12 bond in other structures. In
order to explain the bond length difference in these structures, we
also did a charge analysis for BNP. The NBO charge analysis in Fig. 5
shows that O atom of the 3-benzyloxy moiety and O atom in nitro
group all carry much negative formal charges. The approaching
two O atoms exists electron–electron repulsions of their lone pairs.
As a result, the nitro group in BNP adopts a tilted conformation to
mental spectrum, the band at 1022 cmꢀ1 (in solid state) in FT-IR
and 1022 cmꢀ1 in FT-Raman is assigned to the stretching
m(OACH2Ph) mode coupled with out-plane bending c(PhAH)
mode. The corresponding theoretical bands are located at
1012 cmꢀ1 from the X3LYP method and 1089 cmꢀ1 from the
M06-2X method.
The experimental band at 862 cmꢀ1 (CCl4 solvent) in FT-IR and
867 cmꢀ1 in FT-Raman spectrum are assigned to d(NO2) model
[25]. The wavenumbers of d(NO2) model calculated by the X3LYP
method (857 cmꢀ1) are in very good agreement with the corre-
sponding experimental data.
avoid the repulsion. When
U(NO2) was restricted to 0°, the calcu-
lated distances between O1 and O2 atoms are only 2.59 and 2.56 Å
from the X3LYP and M06-2X methods, respectively. However, in
the tilted conformation of the X-ray result, O1AO2 distance is en-
larged to 2.884(5) Å to minimize the repulsion.
The experimental frequencies at 841 cmꢀ1 and 795 cmꢀ1 in so-
lid state with weak to medium intensity are assigned to the out-
plane bending of PhAH and PyAH. But in CCl4 solvent, these fre-
3.3. Vibrational frequency analysis
quencies are overlapped by m(CACl) vibration of the solvent. The
The vibrational frequencies of BNP were calculated and com-
pared with the experimental IR spectra as well as the FT-Raman
spectrum. Figs. 6 and 7 show the IR spectra and the FT-Raman
spectrum, respectively. Table 3 presents the detailed description
of the IR absorption spectra.
experimental frequencies at 733 cmꢀ1 and 689 cmꢀ1 in solid state
with strong intensity assigned to the out-plane bending of PhAH
for mono-substituted benzene contain compounds [26].
3.3.4. The prediction of Raman spectra by density functional theory
Fig. 7 show the experimental FT-Raman spectrum compared
with theoretical methods predicted Raman spectrum. In the
FT-Raman spectrum, the experimental determined frequencies of
3091 cmꢀ1, 3079 cmꢀ1, 3068 cmꢀ1, and 3054 cmꢀ1 are assigned
3.3.1. Frequencies above 2800 cmꢀ1
The experimental spectrum shows very weak band at
3092 cmꢀ1 (in CCl4 solvent) in FT-IR and a strong band at
3091 cmꢀ1 in FT-Raman spectrum, which correspond to the
to stretching m(PyAH) and m(PhAH) vibrations. The X3LYP func-
tional predicted values are 3103 cmꢀ1, 3088 cmꢀ1, 3077 cmꢀ1
,
stretching
stretching
m(PyAH) vibrations [24,25]. The prediction of the
(PyAH) vibration in 3103 cmꢀ1 from the X3LYP method
and 3062 cmꢀ1, respectively. The deviations between experimental
values and X3LYP functional predict values for the specific vibra-
tions are smaller than 20 cmꢀ1. The experimental observed sym-
m
and 3222 cmꢀ1 from M06-2X method agree well with the experi-
mental band in FT-IR. The weak bands at 3069 cmꢀ1 and
3036 cmꢀ1 (in CCl4 solvent) in FT-IR are assignable to the stretch-
metric stretching
The X3LYP functional predicts a very similar values of 1379 cmꢀ1
for
(N@O). The experimental observed band 1066 cmꢀ1 is as-
m .
(N@O) vibration in FT-Raman is 1380 cmꢀ1
ing m(PhAH) vibrations of phenyl groups [26]. The experimental
observed bands at 2918 cmꢀ1 and 2874 cmꢀ1 (in solid state) in
FT-IR and 2919 cmꢀ1 and 2875 cmꢀ1 in FT-Raman are assigned to
m
signed to the in-plane bending b(PyAH), the X3LYP functional pre-
dict value for this vibration is 1056 cmꢀ1 and M06-2X functional
predict value is 1102 cmꢀ1. From the analysis, we can see that
the band positions and intensities of the Raman-active vibrations
predicated by X3LYP functional are well-reproduced the experi-
mental results.
the asymmetric stretching
mas(CH2) and symmetric stretching
ms(CH2) vibrations [26], respectively.
3.3.2. Frequencies from 2800 to 1370 cmꢀ1
In the experimental FT-IR spectrum, the asymmetric stretching
m
as(N@O) vibrations with medium to strong intensity are located at
1599 cmꢀ1 and 1570 cmꢀ1 in CCl4 solvent and at 1599 cmꢀ1 and
1572 cmꢀ1 in solid state, respectively [25]. The X3LYP functional
4. Conclusions
predicts asymmetric stretching
1606 cmꢀ1 and 1584 cmꢀ1. From the calculated eigenvectors, it is
clearly seen that these bands are connected with as(N@O) vibra-
tions coupled with pyridine ring vibrations sk(Py). Two bands
for the asymmetric stretching as(N@O) vibrations are also ob-
mas(N@O) vibrations are located in
In conclusion, the molecular structure of BNP was investigated
both experimentally and theoretically. A disordered nitro group
with a large dihedral angle of 66.4(4)° between it and its connected
m
m
pyridyl group as well as a centrosymmetric
p-stacking molecular
m
pair has been found in the crystalline state. The following theoret-
ical analysis reveals that the tilt of the nitro group can enlarge the
distance between the O atoms of the 3-benzyloxy and the nitro
groups from 2.59 to 2.88 Å and thus diminishes the repulsion be-
tween them. An energetically favored dihedral angle of
served in recently studies on the spectrum of aromatic nitro com-
pounds [25,27]. The deviations between experimental value and
X3LYP functional predict value of
while M06-2X functional overestimates the asymmetric stretching
as(N@O) vibrations. The 1608 cmꢀ1 in FT-Raman spectrum is as-
signed to sk(Ph) and the asymmetric stretching as(N@O) vibra-
tions are overlapped in this area. The experimental observed the
symmetric stretching (N@O) vibration coupled with wagging
(CH2) vibrations is 1377 cmꢀ1 in CCl4 solvent and 1381 cmꢀ1 in
m ,
as(N@O) are within 15 cmꢀ1
m
U
(NO2) = 53.9° from the X3LYP method coincides well with the
m
m
experimental data of (NO2) = 66.4(4)°. The existing two confor-
U
mational isomers of 3-benzyloxy-2-nitropyridine with equal en-
ergy explain well the disorder of the nitro group at room
temperature. The calculated rotational barrier height of the nitro
group is only 1.6 kcal molꢀ1. In addition, the vibrational frequency
analyses indicate that the X3LYP method can predict the vibra-
tional bands more closely than the M06-2X method.
m
x
solid state [25]. The X3LYP functional predicted value for this
vibration is 1379 cmꢀ1, which agrees well with the experimental
data.
3.3.3. Frequencies below 1370 cmꢀ1
The experimental frequency at 1288 cmꢀ1 (CCl4 solvent), with
Acknowledgements
strong intensity, in FT-IR and 1295 cmꢀ1 in FT-Raman is connected
with the stretching
m
(PyAO) vibration coupled with the in-plane
This work was supported by Grants from the NSFC (Nos.
20834002 and 20872109) and the Innovation Foundation of Tianjin
University. Parts of this work are also supported by High Perfor-
mance Computing Center of Tianjin University, China. The authors
bending b(PyAH) [24]. The calculated corresponding frequencies
are positioned at 1281 cmꢀ1 from the X3LYP method and
1357 cmꢀ1 from the M06-2X method, respectively. In the experi-

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