The structure elucidation of mequindox and 1,4-bisdesoxymequindox: NMR analyses, FT-IR spectra, DFT calculations and thermochemical studies

Article abstract of DOI:10.1016/j.molstruc.2011.07.041

In the current work, we report a combined experimental and theoretical study on the molecular conformation, vibrational spectra, and nuclear magnetic resonance (NMR) spectra of mequindox (MEQ) and 1,4-bisdesoxymequindox (1,4-BDM). The geometric structure and vibrational frequencies of MEQ and 1,4-BDM have been calculated by density functional theory employing the B3LYP functional and 6-311++G(d,p) basis set. The ¹H and ¹³C NMR chemical shifts have been calculated by gauge-including atomic orbital method with B3LYP 6-311++G(2df,2pd) approach. The calculation results have been applied to simulate the infrared and NMR spectra of the compounds. The theoretical results agree well with the observed spectra. The bond dissociation enthalpy of MEQ and the heat of formation of MEQ and 1,4-BDM have also been computed.

Full text of DOI:10.1016/j.molstruc.2011.07.041

Journal of Molecular Structure 1004 (2011) 109–115
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
The structure elucidation of mequindox and 1,4-bisdesoxymequindox: NMR
analyses, FT-IR spectra, DFT calculations and thermochemical studies
a
a
a,b,
⇑
Jiaheng Zhang , Xin He , Haixiang Gao
a
Department of Applied Chemistry, China Agricultural University, Yuanmingyuan West Road 2#, Haidian District, Beijing 100194, PR China
Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, Hubei Normal University, Huangshi 435002, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In the current work, we report a combined experimental and theoretical study on the molecular
conformation, vibrational spectra, and nuclear magnetic resonance (NMR) spectra of mequindox (MEQ)
and 1,4-bisdesoxymequindox (1,4-BDM). The geometric structure and vibrational frequencies of MEQ
Received 6 July 2011
Accepted 22 July 2011
Available online 10 August 2011
and 1,4-BDM have been calculated by density functional theory employing the B3LYP functional and
13
6
-311++G(d,p) basis set. The 1H and C NMR chemical shifts have been calculated by gauge-including
Keywords:
Mequindox
atomic orbital method with B3LYP 6-311++G(2df,2pd) approach. The calculation results have been
applied to simulate the infrared and NMR spectra of the compounds. The theoretical results agree well
with the observed spectra. The bond dissociation enthalpy of MEQ and the heat of formation of MEQ
and 1,4-BDM have also been computed.
1
,4-Bisdesoxymequindox
DFT
Infrared spectra
NMR
Ó 2011 Elsevier B.V. All rights reserved.
BDE
1
. Introduction
Quinoxaline 1,4-dioxide derivatives (QdNOs) are an important
1- and 4-mono-N-oxides of MEQ were synthesized [5,6]. However,
only a few articles on 1,4-bisdesoxymequindox (1,4-BDM) have
been published, as the di-reduction products of the N-oxide moiety
of MEQ. Hence, obtaining detailed structure-related parameters and
vibrational frequencies, as well as nuclear magnetic resonance
(NMR) studies of these desoxy metabolites and their parent com-
pound are important.
class of benzoheterocycles that consist of one or two acyclic chain
moieties combined with a quinoxaline ring. Some QdNOs have
been regarded as interesting biologically active compounds and
have been utilized in the pharmaceutical, veterinary, and industrial
chemical fields since the 1970s [1]. With the development of the
domestic animal drug industry in the last decade, these com-
pounds have been widely used on cattle, swine, fish, and poultry
at subtherapeutic levels for improving feed efficiency [2]. These
drugs ameliorate the intestinal microflora, improve protein utiliza-
tion, and increase protein synthesis in vivo [3].
NMR is arguably one of the most sensitive and versatile analyt-
ical probes of molecular structure and dynamics of complex organ-
ic compounds. Recently, there has been increasing interest in the
use of density functional theory (DFT) to aid NMR analysis [7].
The studies of Chesnut [8] and Sauer et al. [9] in 1997 demon-
1
strated that the H chemical shifts of small molecules can be
Mequindox (2-methyl-3-acetylquinoxaline-1,4-dioxide; C11
H
10
obtained at accuracies of ca. ±0.1 ppm. When some larger basis sets
are employed, the accuracy of the calculation is still acceptable in
the case of complex targets.
N O ; MEQ) is a novel synthetic QdNO derivative used as an anti-
2 3
bacterial by the Lanzhou Institute of Animal Husbandry and Veter-
inary Drugs, Chinese Academy of Agriculture Sciences to treat pig
treponeme dysentery. Although this novel compound has pre-
sented good antibiotic activity and growth-enhancing activity,
the cytotoxicity, genotoxicity, and metabolomics of MEQ have
not been studied thoroughly [4].
ꢀ
ꢀ
In the research of Rablen et al., the GIAO/B3LYP/6-311++G
model was found to be a promising alternative. It showed the best
cost-to-performance ratio and provided predictions with a root-
mean-square deviation of <0.2 ppm for 80 medium-sized mole-
cules [10].
In our previous studies, the toxicity of MEQ was determined to be
closely associated with the deoxidized metabolites, and the isomeric
Although much effort has gone into aiding these methods by
developing procedures and adjusting vibrational frequency scale
factors, only a few analyses of QdNO vibrational spectra have been
done using DFT methods. Yurdakul and Polat studied the Fourier
transform infrared (FT-IR) spectra of 2-hydroxyquinoxaline using
⇑
Corresponding author at: Department of Applied Chemistry, China Agricultural
University, Yuanmingyuan West Road 2#, Haidian District, Beijing 100194, PR
China. Tel.: +86 10 62734876; fax: +86 10 62733830.
ꢀ
ꢀ
the B3LYP/6-311++G DFT method, and they observed that the
infrared absorption of QdNOs computed by this method agree
E-mail address: hxgao@cau.edu.cn (H. Gao).
0
022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.07.041

1
10
J. Zhang et al. / Journal of Molecular Structure 1004 (2011) 109–115
reasonably well with the experimental data [11]. In the study of
form extracts were dried over anhydrous magnesium sulfate, fil-
tered and evaporated to dryness under vacuum. The residue was
purified by recrystallized from ether to yield a pale yellow acicular
crystal product of 1,4-bisdesoxymequindox (8.3 g, 0.0045 mol).
0
0
Miranda et al., a new heterocyclic dipyrido[3,2-f:2 ,3 -h]quinoxali-
no[2,3-b]quinoxaline was synthesized and characterized by IR
1
13
spectroscopy, H NMR and C NMR spectroscopy, and DFT calcu-
lation [12].
+
Yield: 82%; m.p. 85.2–86.1 °C; MS(ESI) m/z (%): 187.10 [M+H] .
+
With the development of investigations on the toxicity mecha-
nism of QdNOs, several studies involving experimental validation
or theoretical calculation have resulted in remarkable achieve-
ments [13–15]. Recently, attention has been paid to the detailed
thermodynamic analysis of the homolytic N–O cleavage and the
HRMS(ESI) calcd for C11
.2. Methods and calculations
Mequindox (98%) was provided by College of Veterinary Medi-
H
11
N
2
O [M+H] 187.2213, found 187.0954.
2
Å
formation of HO radicals, which leads to cytotoxicity by cleaving
cine, Huazhong Agricultural University (Wuhan, China). Sodium
hydrosulphite was purchased from Sigma–Aldrich. Other chemi-
cals used in synthesis were purchased from Guoyao chemical Co.
DNA strands. Thus, the N–O bond strength, which can be measured
by the N–O bond dissociation enthalpies (BDEs), should be investi-
gated [16–18]. Therefore, the calculation of N–O BDEs has been a
strategic goal in understanding the toxic properties of QdNOs.
In the current study, 1,4-BDM was synthesized from MEQ, and
(
ShangHai, China).
1H and 13C NMR spectra for solutions (in CDCl
3
using tetrameth-
ylsilane as internal standard) were recorded on a BrukerDSX-300
instrument. Mequindox forms yellow powder whereas 1,4-bis-
desoxymequindox forms pale yellow crystal at room temperature.
IR spectra of both the molecules were recorded in KBr disc. Perkin-
Elmer FT-IR Spectrum 100 spectrometer was used to record the IR
1
13
DFT calculations were done to obtain the theoretical H NMR,
C
NMR chemical shifts and vibrational frequencies. We also report
the experimental NMR and IR spectra of these QdNOs to compare
their calculated values and detect any relation between the values.
Moreover, heat of formation (HOF) of MEQ and 1,4-BDM was
calculated.
ꢂ1
spectra in the spectral range 450–4000 cm . The spectrum was
ꢂ1
ꢂ1
recorded with a scanning speed of 10 cm min and the spectral
ꢂ1
resolution of 4.0 cm
.
2
. Experimental
Full geometry optimizations of isolated molecules were accom-
plished by means of hybrid functional B3LYP [19,20] and the 6-
311++G(d,p) basis set with the Gaussian 09 code [21]. The opti-
mized structural parameters were used in the vibrational frequency
calculations at DFT B3LYP 6-311++G(d,p) level. The calculated
vibrational wave numbers were scaled with the scale factor, yield-
ing a good agreement between calculated assignments and experi-
mental data. The calculation of potential energy distribution (PED)
was done by GAR2PED software package [22] and assisted by
2.1. Synthesis
A solution of mequindox (12 g, 0.055 mol) in 95% EtoH (240 ml)
was heated under reflux for 0.5 h then sodium hydrosulphite (18 g,
.104 mol) was added in portions. The mixture was heated under
0
reflux for 2 h, dilute with cold water and the reaction mixture
was extracted with chloroform (3 ꢁ 50 ml). The combined chloro-
Table 1
Calculated geometry structure of MAQM.
Bond length (Å)
MEQ
1,4-BDM
Exp.
Bond angle (°)
MEQ
1,4-BDM
Exp.
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C1–C6
C13–C14
C14–C17
C13–C23
C17–C19
C3–N12
C4–N11
C13–N11
C14–N12
N11–O16
N12–O15
C17–O18
C1–H7
C2–H8
C5–H9
C6–H10
C19–H20
C19–H21
C19–H22
C23–H24
1.378
1.405
1.407
1.405
1.378
1.410
1.404
1.524
1.492
1.506
1.399
1.401
1.359
1.352
1.279
1.283
1.208
1.083
1.081
1.081
1.084
1.097
1.090
1.090
1.094
1.086
1.093
1.373
1.418
1.424
1.416
1.375
1.418
1.447
1.513
1.503
1.513
1.354
1.359
1.318
1.318
1.377
1.401
1.401
1.404
1.359
C1–C2–C3
C2–C3–C4
C3–C4–C5
C4–C5–C6
C5–C6–C1
C6–C1–C2
C13–C14–C17
C14–C17–C19
C14–C13–C23
H7–C1–C2
119.18
120.28
119.91
119.29
120.74
120.60
121.92
117.19
124.53
119.64
122.86
117.90
119.70
120.26
119.75
118.49
119.09
122.38
120.02
119.84
120.10
121.92
124.53
119.01
117.19
108.26
111.05
109.44
109.58
111.80
109.58
119.68
119.83
119.26
119.79
120.92
120.52
123.04
117.24
123.80
120.08
122.03
118.20
119.25
120.78
120.00
118.59
119.03
121.77
119.81
119.9
119.0
120.9
120.6
121.5
120.0
1.406
1.514
1.477
120.1
120.1
120.1
119.3
1.380
1.388
1.357
1.309
1.296
H8–C2–C1
H9–C5–C4
H10–C6–C1
C3–C4–N11
C4–C3–N12
C3–N12–C14
C4–N11–C13
N12–C14–C13
N11–C13–C14
C4–N11–O16
C3–N12–O15
C13–C14–C17
C14–C13–C23
C14–C17–O18
C14–C17–C19
C17–C19–H20
C17–C19–H21
C17–C19–H22
C13–C23–H24
C13–C23–H25
C13–C23–H26
121.6
116.8
120.1
125.2
117.5
120
1.216
1.084
1.084
1.083
1.084
1.089
1.092
1.092
1.092
1.092
1.092
0.950
0.950
0.950
0.950
123.04
123.80
121.22
117.24
108.45
110.65
110.65
111.37
111.37
108.27
119.0
127.3
0.980
0.980
0.980
C23–H25
C23–H26
Selected dihedral angles
C5–C4–N11–C13
C2–C3–N12–C14
C4–N11–C13–C23
C3–N12–C14–C17
C13–C14–C17–O18
C13–C14–C17–C19
ꢂ179.54
179.70
179.85
179.33
ꢂ61.07
114.91
ꢂ180.00
180.00
179.99
179.99
ꢂ0.01
ꢂ179.5
179.4
178.7
176.9
109.5
109.5
109.5
179.99

J. Zhang et al. / Journal of Molecular Structure 1004 (2011) 109–115
111
the C2–H8 and C5–H9 bonds are the shortest, and the rest fall in
the range of 1.081–1.097 Å. In addition, N11–O16 and N12–O15
in MEQ, and C17–O18 in MEQ and 1,4-BDM have lengths of
1
.279, 1.283, 1.208, and 1.206 Å, respectively.
In MEQ and 1,4-BDM, the calculated values of the C–C–C angles
for the benzene ring are around the typical hexagonal angle of
1
20°, in contrast to the quinoxaline ring, in which the substitution
leads to some changes in the bond angles.
C3–N12–C14 and N12–C14–C13 in MEQ and 1,4-BDM show an
obvious deviation from 120° and are consistent with the experi-
mental values. The benzene ring and quinoxaline moiety are essen-
tially planar, as evident in the dihedral angles of C5–C4–N11–C13
(
(
ꢂ179.54 for MEQ and ꢂ180.00 for 1,4-BDM) and C2–C3–N12–C14
179.70 for MEQ and 180.00 for 1,4-BDM). This finding agrees with
the characteristics of other substituted quinoxalines [26].
3.2. Vibrational frequency
Theoretical (unscaled and scaled) and experimental vibrational
ꢂ1
ꢂ1
frequencies (cm ), peak intensities (km mol ) and the assign-
ments of IR spectra of MEQ and 1,4-BDM.
MEQ consists of 26 atoms, so it has 72 normal vibrational
modes; it belongs to the point group C . 1,4-BDM, which has 24
1
atoms and 66 normal modes of fundamental vibrations resulting
from the di-reduction of the N-oxide moiety of MEQ, also possesses
1
C point group symmetry. The frequencies calculated at B3LYP lev-
els are overestimated compared with the experimental values. This
is due to neglect of the anharmonicity present in real systems.
Regardless of the level of calculations, the calculated harmonic fre-
quencies are usually scaled down to improve agreement with the
experimental values; in the current study, the scaling factor
Fig. 1. Optimized molecular structures and atomic numbering of meq and 1,4-BDM.
MOLEKEL program which offers visualized presentation of vibra-
tion modes [23,24]. Nuclear shielding were computed at IEFPCM/
B3LYP 6-311++G(2df,2pd) basis set by the gauge-including atomic
orbital (GIAO) method. The SCRF theory via integral equation for-
malism version of the polarizable continuum model (IEFPCM)
including the effect of environment was applied to correct the rel-
ative ground state energies obtained for isolated molecules in gas
0
.9668 was used [27].
Assignments of several vibrational frequencies in the experi-
mental FT-IR spectra of MEQ and 1,4-BDM are shown in Table 2.
The theoretical wavenumbers (unscaled and scaled) for the various
fundamental vibrational modes predicted by DFT methods using
the 6-311++G(d,p) basis set are presented in Table 2. IR spectra
from experiments and those predicted by the DFT B3LYP method
are shown in Figs. 2 and 3.
1
13
phase. The H and C chemical shifts is obtained as d =
r
ref
ꢂ
r,
1
13
where
r
ref is the shielding constant of H and C in tetramethylsil-
ane (TMS) and
r
is the corresponding reference parameter in the
molecule of interest. Subsequently, the B3LYP 6-311++G(2df,2pd)
approach was used to further optimize the geometry and to obtain
the natural population analysis of these two compounds. The abso-
lute enthalpies of all compounds were used to estimate the stan-
dard molar enthalpies of formation and enthalpies of N–O bond
dissociation are B3LYP 6-311++G(2df,2pd) values.
For all aromatic compounds, the carbon–hydrogen stretching
ꢂ1
vibrations commonly appear in the region of 3000–3100 cm
[
28]. In the experimental spectrum, the C–H stretching frequencies
ꢂ1
ꢂ1
ꢂ1
are slightly over 3100 cm (3104 cm for MEQ and 3103 cm
for 1,4-BDM). Scaled theoretical C–H stretching modes were found
ꢂ1
to be 3058, 3072, 3101, and 3102 cm for MEQ, and 3065, 3076,
ꢂ1
3
088, and 3093 cm for 1,4-BDM. The vibrations assigned to aro-
3
. Results and discussion
matic C–H stretch predicted theoretically at B3LYP/6-311++G(d,p)
are in good agreement with the experimental assignment. As indi-
cated by the TED, these modes involve approximately 100% contri-
3.1. Geometrical structure
bution suggesting that they are pure stretching modes. CH
3
ꢂ1
The optimized structural parameters of MEQ and 1,4-BDM cal-
stretching modes were found in the region 2922–3064 cm for
ꢂ1
culated by the DFT-B3LYP level with 6-311++G(d,p) as the basis
sets are listed in Table 1, in accordance with the atom numbering
scheme of the molecule shown in Fig. 1. Since the exact crystal
structures of these compounds are not available at present, the
optimized structures can only be compared with 3-methylquinox-
aline-2-carboxylic acid 4-oxide, which has a highly similar crystal
structure (determined by X-ray studies) [25]. As shown in Table 1,
most of the computed bond lengths and bond angles are slightly
larger than the corresponding experimental data. This can be ex-
plained by the fact that the experimental results were based on
the solid phase, whereas theoretical calculations were based on
the gaseous phase, wherein intermolecular Coulombic interactions
among neighboring molecules were ignored.
MEQ and 2947–3042 cm for 1,4-BDM. The C–H in-plane bending
ꢂ1
frequencies are within 1000–1300 cm and the three out-of-plane
ꢂ1
bending vibrations are in the range of 750–1000 cm
.
The C–H in-plane bending vibrations of MEQ and 1,4-BDM are
ꢂ1
all within 1097–1328 cm ; these correlate well with the frequen-
ꢂ1
cies calculated by the DFT method (1083–1321 cm for MEQ and
ꢂ1
1109–1323 cm
for 1,4-BDM). The C–H out-of-plane vibration
ꢂ1
was observed at 776 cm for both molecules, with theoretical
wavenumbers of 761 cm for MEQ and 753 cm for 1,4-BDM.
Two peaks at 1446 cm for MEQ and 1416 cm for 1,4-BDM were
also observed, and were assigned to the scissoring of CH3, which
has predicted frequencies at 1439 cm and 1426 cm
The strong experimental IR band observed at 1515 cm in MEQ
was assigned to C–C stretching; a similar peak in the 1,4-BDM
spectrum (at 1514 cm ) has medium intensity.
ꢂ1
ꢂ1
ꢂ1
ꢂ1
ꢂ1
ꢂ1
.
ꢂ1
The DFT data show that the C–C bond length is 1.373–1.524 Å.
In the case of C–H bonds, C19–H20 of MEQ is the longest, whereas
ꢂ1

1
12
J. Zhang et al. / Journal of Molecular Structure 1004 (2011) 109–115
Table 2
Theoretical (unscaled and scaled) and experimental vibrational frequencies (cm ), peak intensities (km mol ) and the assignments of IR spectra of MEQ and 1,4-BDM.
ꢂ1
ꢂ1
Mode
MEQ
Exp.
Assignment
1,4-BDM
Exp.
Assignment
Unscaled
freq.
Scaled
freq.
Intensity
Unscaled
freq.
Scaled
freq.
Intensity
1
2
3
4
5
6
7
8
9
55
68
91
101
136
162
163
193
234
277
306
352
362
404
423
452
459
503
515
561
568
615
636
668
683
711
714
787
845
871
886
970
996
1010
1019
1034
1043
1052
1074
1120
1140
1167
1215
1225
1237
1298
1362
1366
1384
1402
1425
1448
1453
1462
1468
1484
1488
1534
1545
1638
1644
1782
3022
3044
3088
3105
3149
3169
3181
3195
53
66
88
98
131
157
158
187
226
268
296
340
350
391
409
437
444
486
498
542
549
595
615
646
660
687
690
761
817
842
857
938
963
3.2
2.9
3.1
6.2
0.6
2.0
0.1
3.8
3.2
2.0
4.1
0.6
0.1
22.7
0.5
2.0
3.0
5.0
2.0
4.6
4.2
52.4
9.7
5.9
6.1
0.4
1.7
63.9
13.9
0.6
0.0
22.2
2.7
0.0
22.9
3.8
11.1
1.5
55.4
61.1
1.3
s
s
s
s
s
s
s
NCCO(100)
NCCO(100)
OCCH(100)
OCCH(100)
NCCH(64),
26
71
116
123
175
200
242
296
307
335
379
430
472
500
511
524
593
597
630
657
704
721
779
805
828
882
917
944
981
1003
1027
1034
1036
1053
1076
1147
1162
1217
1259
1286
1332
1345
1368
1390
1411
1423
1459
1462
1464
1475
1498
1516
1592
1592
1651
1755
3048
3048
3103
3107
3136
3146
3170
3182
3194
3199
25
69
112
119
169
193
234
286
297
324
366
416
456
483
494
507
573
577
609
635
681
697
753
778
801
853
887
913
948
970
993
1000
1002
1018
1040
1109
1123
1177
1217
1243
1288
1300
1323
1344
1364
1376
1411
1413
1415
1426
1448
1466
1539
1539
1596
1697
2947
2947
3000
3004
3032
3042
3065
3076
3088
3093
4.1
0.0
2.4
0.0
2.7
0.5
1.4
0.0
4.1
2.2
4.1
8.5
0.2
0.0
0.1
8.2
0.0
14.6
1.3
26.3
0.8
1.7
80.3
0.0
13.0
0.1
1.7
59.8
1.2
1.0
8.8
1.3
1.4
2.1
20.2
22.3
8.3
61.0
13.3
26.5
34.9
4.0
s
NCCC(59),
bC–C(59), bRing(19)
Ring(48), CH3(38)
CCCH(98)
dNCC(80)
CCCH(95)
Ring(83)
Ring(79)
CH3(76)
sNCCO(32)
c
s
c
s
NCCO(35)
NCCH(51), bN–O(25)
NCCH(50), bN–O(26)
s
c
c
c
c
N–O(57),
Ring(82), bCH3(17)
Ring(31)
Ring(56), bN–O(20), bC–C(17)
bCH3(68), bN–O(25)
CH3(93)
bN–O(86)
Ring(84), bN–O(14)
dNCC(97)
sNCCH(20)
c
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
7
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
bCH3(51),
c
dCCC(72), dCNC(13)
dCCC(50), dCNC(40)
bRing(59), bCH3(32)
dNCC(58), dCCC(21)
c
c
dNCC(71), bC@O(25)
cRing(91)
dCCC(66), bC@O(21)
dCCC(47), dCNC(23)
dCCC(68), bC–C(17)
bRing(67), bCH3(10)
c
c
Ring(55),
Ring(96)
cC–H(41)
c
457vw
c
c
Ring(92)
Ring(63),
552vw
573vw
615 m
637w
657w
683vw
776 m
c
c
CH3(14)
C@O(30), dCCC(24)
C@O(33)
C@O(45)
515w
552w
573w
615s
dCNC(39),
dCCC(63),
c
dCCC(40), dCCN(2),
bN–O(79)bC–H(17)
c
Ring(52),
c
t
C–C(27)
dCNC(57), tC–C(17)
c
c
t
c
C–H(92)
Ring(44),
C–C(61),
C–H(94)
636 m
656w
683w
694w
776s
dCCC(75), dCNC(10)
C–C(69), CH3(24)
c
C–H(40)
c
c
829 m
965w
tC–N(12)
dCCC(89)
dCCC(66),
c
dCCC(49),
dCCC(54),
c
t
c
c
dCCC(48),
bC–H(49),
bC–H(94)
dCH3(98)
c
CH3(24)
dCCC(39), dCNC(28)
C–H(90)
t
c
c
c
c
c
c
c
C–C(63), bC@O(25)
C–H89)
C–H(96)
830s
c
t
CH3(36)
C–N(18),
t
C–C(13)
861w
C–H(100)
C–C(56), dCH3(28)
C–H(100)
C–H(61),
cCH3(22)
CH3(74), bC@O(12)
CH3(42),
CH3(78)
CH3(73),
c
t
C–H(34)
C–C(20)
976
985
C–H(100)
1000w
1050 m
1097s
966 m
t
c
C–C(44)
CH3(36)
1000
1008
1017
1038
1083
1102
1128
1175
1184
1196
1255
1317
1321
1338
1355
1378
1400
1405
1413
1419
1435
1439
1483
1494
1584
1589
1723
2922
2943
2985
3002
3044
3064
3075
3089
bC–H(91)
bC–H(76),
bC–H(42),
t
t
t
t
t
dCH3(96)
dCH3(72), tC–C(12)
t
dCH3(88)
dCH3(58), bC–H(19)
dCH3(99)
1000 m
1016 m
1050s
t
t
C–N(11)
C–C(29), tC–N(16)
1149 m
1216 m
c
CH3(40),
t
t
t
t
t
t
C–C(26),
C–N(29)
Ring(41)
N–O(29),
N–O(20)
C–C(22)
c
C@O(11)
Ring(71), bC–H(17)
Ring(62), bC–H(17)
Ring(77)
1097s
bC–H(66),
bC–H(46),
bC–H(55),
bC–H(69),
bC–H(67),
t
t
t
t
1274 m
4.3
1.8
t
C–N(12)
C–N(59),
tC–C(35)
1149 m
1186 m
1216 m
1.2
C–C(39), dCH3(29), bC–H(19)
34.8
14.7
54.2
311.7
126.6
29.0
4.7
42.3
25.4
22.0
13.6
23.9
10.9
9.3
1.1
10.0
2.3
69.1
3.5
244.4
1.1
7.2
5.6
1328vs
1384vs
Ring(41),
C–C(30),
t
N–O(32),
t
C–C(22)
t
C–N(28), bC–H(20)
Ring(49), bC–H(32)
1275s
1328vs
N–O(61), bC–H(22)
N–O(61), Ring(28)
t
dCH3(90)
C–C(91)
t
1416 m
1514 m
dCH3(87)
0.2
dCH3(81)
dCH3(85),
dCH3(85), bC–H(11)
dCH3(81), bC–H(12)
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
C–C(92)
C–C(60), bC–H(20)
Ring(91)
Ring(99)
Ring(287)
C@O(96)
sCH3(100)
sCH3(100)
asCH3(99)
73.2
16.0
10.8
7.3
10.0
2.9
49.6
3.4
3.4
4.3
209.6
2.7
2.8
3.3
1383 m
1417s
t
C–C(11)
t
Ring(54), bC–H(39)
1602w
1700vs
dCH3(78)
dCH3(77), bC–H(16)
1446 m
1515s
t
t
t
t
t
t
t
t
t
t
t
t
t
Ring(87)
Ring(74), dCH3(13)
Ring(83)
C–C(88)
C@O(99)
5.0
asCH3(99)
1602 m
1701vs
12.8
15.0
1.8
4.7
8.6
asCH3(100)
asCH3(100)
asCH(Ring)(99)
sCH(Ring)(100)
asCH(Ring)(100)
sCH(Ring)(100)
sCH3(99)
sCH3(100)
2970w
asCH3(100)
asCH3(99)
asCH3(100)
asCH3(100)
asC–H(Ring)(99)
asC–H(Ring)(100)
3.3
3.0
4.2
5.6
11.9
3104w
7.5

J. Zhang et al. / Journal of Molecular Structure 1004 (2011) 109–115
113
Table 2 (continued)
Mode
MEQ
Exp.
Assignment
1,4-BDM
Exp.
Assignment
Unscaled
freq.
Scaled
freq.
Intensity
Unscaled
freq.
Scaled
freq.
Intensity
7
7
1
2
3226
3227
3119
3120
7.6
3.4
t
t
asC–H(Ring)(100)
sC–H(Ring)(100)
Scaled factors: B3LYP/6-311++G(d,p): 0.9668.
– stretching (s – symmetric; as – asymmetric), b – in-plane,
t
c – out-of-plane bending, x – wagging, d – scissoring, s – torsion. s – strong, m – medium, w – weak, v – very.
Fig. 3. Experimental and calculated FT-IR spectrums of 1,4-BDM.
Fig. 2. Experimental and calculated FT-IR spectrums of MEQ.
Since the mixing of several bands is possible in the region, the
assignment of C–N vibration is a challenging task. Moreover, most
using the B3LYP 6-311++G(2df,2pd) basis set. In the current study,
1
13
the calculated H and C isotropic chemical shielding for trimeth-
ylsilane ref at the B3LYP/6-311++g(2df,2pd) are 31.62 and
83.67 ppm, respectively. The theoretical and experimental chem-
of the C–C and C–N vibrations are associated with the whole ring:
r
ꢂ1
vibrations occurring in the region 1417–1602 cm
and 1000–
1
ꢂ1
1
186 cm are assigned to
t
C–C (ring) and bC–H (ring). These spe-
ical shifts, isotropic shielding tensors, and the peak assignments for
MEQ and 1,4-BDM are presented in Table 3.
cific bands have been also found in the IR spectrum of other quin-
oxaline molecules [29].
The linear correlation between the calculated and experimental
values was determined, the result of which are presented in Fig. 4.
Mode No. 35 of MEQ and Mode No. 30 of 1,4-BDM at the calcu-
ꢂ1
A good correlation between predicted and observed 13C and 1
H
lated frequencies of 985 and 970 cm , respectively, are assigned
to the C–C and C–N stretching and C–N bending vibrations; corre-
chemical shifts was found. To remove systematic errors and de-
crease deviation, the calculated values were corrected according
ꢂ1
sponding bands at ꢃ965 cm were observed in the experimental
d
calcꢂintercept
FT-IR spectra. The strong bands of MEQ and 1,4-BDM observed at
to the equation, d ¼
[30]. The slope and intercept were
slope
ꢂ1
ꢃ615 cm are assigned to C–C and C–N bending and C–N stretch-
obtained by plotting the calculated data against the experimental
shifts to be assigned. In the present study, the intercept and slope
are 0.2063 and 1.0595, respectively. As demonstrated in Table 3,
the mean absolute error between the calculated chemical shifts
and the experimental values is 4.19 ppm, whereas the corrected
mean absolute error is 0.91 ppm.
ing; similar bands are present in the predicted spectra, at calcu-
ꢂ1
lated frequencies of 595and 609 cm
.
As seen in Figs. 2 and 3, the characteristic wavenumber of the
t
(C@O) mode has one of the strongest bands, and the intensities
ꢂ1
ꢂ1
for this mode (1701 cm for MEQ and 1700 cm for 1,4-BDM)
ꢂ1
are in accordance with the calculated values (1723 and 1697 cm ).
Typically, aromatic carbons give signals in overlapped areas of
the spectrum, with chemical shift values from 100 to 150 ppm.
The highest chemical shift of carbon is that of C17 for both MEQ
and 1,4-BDM because it is linked to an electron-withdrawing ke-
tone oxygen atom. Due to the influence of electronegative nitrogen
atoms, the chemical shifts of C3, C4, C13, and C14 are significantly
different in the range of 136–153 ppm. Shifts of other carbons
3
.3. NMR spectra
After initial optimization of the molecular structures with the
1
3
6
-311++G(d,p) basis set, gauge-including atomic orbital C and
1
H chemical shift calculations of MEQ and 1,4-BDM were done

1
14
J. Zhang et al. / Journal of Molecular Structure 1004 (2011) 109–115
Table 3
The experimental, calculated and corrected 1H and 13C isotropic chemical shifts (ppm) with respect to TMS of MEQ and 1,4-BDM.
Assignment
MEQ
Exp.
1,4-BDM
Exp.
Assignment
MEQ
Exp.
1,4-BDM
Exp.
Cal.
Corrected
Cal.
Corrected
Cal.
Corrected
Cal.
Corrected
H7
H8
H9
H10
H20
H21
H22
H24
H25
H26
7.89
8.58
8.62
7.89
2.74
2.74
2.74
2.53
2.53
2.53
8.16
8.94
8.99
8.21
2.31
3.60
2.51
2.57
2.19
2.60
7.51
8.24
8.29
7.55
1.99
3.21
2.18
2.23
1.87
2.26
7.82
8.06
8.06
7.84
2.98
2.98
2.98
2.85
2.85
2.85
8.14
8.53
8.41
8.24
3.45
3.45
2.12
3.29
3.29
2.82
7.49
7.85
7.74
7.58
3.06
3.06
1.80
2.91
2.91
2.47
C1
C2
C3
C4
C5
C6
C13
C14
C17
C19
C23
131.49
120.23
136.79
137.86
120.00
132.54
139.69
138.89
194.21
29.90
137.90
127.06
145.95
146.31
126.79
138.95
149.83
148.33
210.01
32.32
129.96
119.73
137.56
137.90
119.47
130.95
141.22
139.81
198.02
30.31
128.35
129.75
139.69
147.08
129.46
131.87
153.00
142.57
201.20
27.69
136.58
137.70
147.77
150.71
136.25
140.08
164.47
149.28
213.30
31.46
128.72
129.77
139.27
142.05
128.40
132.02
155.03
140.70
201.12
29.50
13.84
17.34
16.17
24.34
30.41
28.51
MAE
CMAE
4.19
0.91
In ppm based on the TMS reference, computed at the B3LYP/6-311++G(2df,2pd) level with IEFPCM model in chloroform.
In chloroform using the IEFPCM model (
e
= 4.9) at B3LYP/6-311++G(2df,2pd) level.
P
1
N
i
MAE: mean absolute error is calculated as MAE ¼
jdcalc ꢂ dexpj.
N
P
1
N
N
i
CMEA: corrected mean absolute error is calculated as CMAE ¼
jdscale ꢂ dexpj.
2
2
1
1
50
00
50
00
y = 1.0595x + 0.2063
R² = 0.9995
5
0
0
0
50
100
150
200
250
Experimental Chemical Shift (ppm)
Fig. 4. The linear regression between the experimental and theoretical 1H and 13
C
NMR Chemical shifts of MEQ and 1,4-BDM.
belonging to the quinoxaline moiety fall within 120–132 ppm. The
observed chemical shift of protons of the quinoxaline moiety was
evidently larger, and protons in 2-methyl showed the lowest val-
ues in the results for MEQ and 1,4-BDM.
3.4. Thermochemical studies
As mentioned, the importance of the N–O bonds for selective
activity raises an increasing interest in the thermochemical study
of the energetic properties of quinoxaline species. The present
work reports theoretical results of the DFT calculations of the heat
of formation (HOF) for MEQ and 1,4-BDM, and BDE of the N–O
bonds. The EOF of MEQ and 1,4-BDM were estimated from the
isodesmic reaction approach and a working reaction involving
structurally similar reactants and products for which accurate
experimental EOF values are known.
Fig. 5. Working reactions to calculate HOF of MEQ and 1,4-BDM in the isodesmic
reaction approach.
The equation shown in Fig. 5 was used in the isodesmic reaction
approach, and the literature values of the EOF [D H°(g)] were the
f
following: 2,3-dimethyl-quinoxaline-1,4-dioxide,
D
f
H°(g) = 149.4 ±
H°(g) = 172.9 ±
ꢂ1
4
3
.5 kJ mol
[31]; 2,3-dimethylquinoxaline,
D
f
The N–O BDE is defined as the enthalpy change of the dissocia-
tion reaction in the gas phase at 298.15 K and 1 atm. MEQ has two
different N–O bonds, one closer to the methyl group, and the other
closer to the acetyl group. Due to the different chemical neighbor-
hoods, these bonds are expected to have different strengths and
dissociation energies. Consequently, the N–O BDE may be
described in terms of the first, second, total, and mean N–O BDE
values. The first N–O BDE is the energy required to break the weak-
est bonds in the di-N-oxide compound to yield the corresponding
N-oxide. The second N–O BDE is the energy required to break the
ꢂ1
ꢂ1
.0 kJ mol [31]; toluene,
H°(g) = ꢂ86.7 ± 1.7 kJ mol [33]. Similar approaches
for estimating the EOF of heteroaromatic nitrogen derivatives
including substituted quinolines) based on well-established experi-
mental values for anchor compounds, have been reported [34].
Using combination of experimental and B3LYP 6-311+
G(2df,2pd) values of anchor compounds based on the isodesmic reac-
f
DH°(g) = 50.1 ± 1.1 kJ mol [32]; and ace-
ꢂ1
tophenone,
D
f
(
a
+
tion approach, the theoretical EOF values of MEQ and 1,4-BDM were
found to be 44.9 and 51.1 kJ mol , respectively.
ꢂ1

J. Zhang et al. / Journal of Molecular Structure 1004 (2011) 109–115
115
supported by NSFC and Chinese Universities Scientific Fund (Project
No. 2011JS035) and the financial support (KY2010G20) from Hubei
Key Laboratory of Pollutant Analysis & Reuse Technology.
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2
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69.9 kJ mol , and the total and mean N–O BDEs of MEQ are
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14.0and 257.0 kJ mol , respectively.
4
. Conclusion
In the present investigation, we have examined the experimen-
tal and theoretical molecular conformation and performed the
vibrational and NMR analyses of MEQ and 1,4-BDM. The molecular
geometry and vibrational frequencies of MEQ and 1,4-BDM have
been examined through DFT calculations using the B3LYP func-
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1
13
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1
[
Optimized geometric structures have been found to be consis-
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theoretical values of the first, second, total, and mean N-OBDEs
for MEQ have been determined using the DFT method. Predicted
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[
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33] J.D. Cox, G. Pilcher, Thermochemistry of Organic and Organometallic
Compounds, Academic Press, New York, 1970. pp. 1–636.
values based on B3LYP 6-311++G(2df,2pd) are 244.1, 269.9,
ꢂ1
5
14.0, and 257.0 kJ mol , respectively.
[34] M.A.V. Ribeiro da Silva, M.A.R. Matos, Pure Appl. Chem. 69 (1997) 2295.
Acknowledgements
We are grateful to the973 Fund, the Ministryof Scienceand Tech-
nology, PR China (Grant No. 2009CB118801), Project 20977112