Experimental and theoretical investigations on the tautomerism of 1-phenyl-2-thiobarbituric acid and its methylation reaction

Article abstract of DOI:10.1016/j.molstruc.2012.12.017

In the present study, 1-phenyl-2-thiobarbituric acid (1) was synthesized and the tautomerism of this compound was investigated by FT-IR spectroscopy, X-ray analysis and ¹H NMR study as well as quantum chemical calculations. It is found that com

Full text of DOI:10.1016/j.molstruc.2012.12.017

Journal of Molecular Structure 1036 (2013) 372–379
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Experimental and theoretical investigations on the tautomerism
of 1-phenyl-2-thiobarbituric acid and its methylation reaction
Gang Wang a, Jian Wang a, Han Nie a, Shen-Peng Tan a, Wei-Qun Shi b, Dong-Mei Zhao a,
Mao-Sheng Cheng a,
⇑
a Key Laboratory of Structure-Based Drugs Design & Discovery of Ministry of Education, School of Pharmaceutical Engineering, Shenyang Pharmaceutical University,
Shenyang 110016, PR China
b Key Laboratory of Nuclear Analytical Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, PR China
h i g h l i g h t s
"
"
"
Synthesis and structural identification of thiobarbituric derivatives are presented.
Tautomerism form was investigated by experimental methods and quantum calculations.
The methylation site was identified experimentally and theoretically.
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, 1-phenyl-2-thiobarbituric acid (1) was synthesized and the tautomerism of this
compound was investigated by FT-IR spectroscopy, X-ray analysis and 1H NMR study as well as quantum
chemical calculations. It is found that compound 1 exists in triketo form in the solid state and in CDCl3
solution while tautomerization was observed in DMSO-d6, DMF-d7 and CD3OD solution. The geometry
optimization of eight possible tautomers of 1-phenyl-2-thiobarbituric acid was performed in gas phase
and in different solvents using COSMO method. The calculated results are in good accordance with exper-
imental data. Additionally, the methylation reaction of 1-phenyl-2-thiobarbituric acid was investigated
for the first time by combination of experimental and theoretical methods. The methylation product
might be varied with three possible isomers because of the structural features of 1-phenyl-2-thiobarbi-
turic acid. We identified the obtained methylation product by X-ray diffraction and FT-IR spectroscopy,
and transition state search and energy calculation were conducted to elucidate experimental data, the
results revealed that the theoretical calculations are consistent with experimental data.
Received 28 September 2012
Received in revised form 10 December 2012
Accepted 10 December 2012
Available online 19 December 2012
Keywords:
1-Phenyl-2-thiobarbituric acid
Tautomerism
Methylation
Quantum chemical calculation
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
as the relative stability of tautomers of a compound may be crucial
for the ligand-receptor interaction [8–11]. In tautomerism re-
2-Thiobarbituric acid and its derivatives are a well known class
of compounds which have many applications in pharmaceutical
and chemical fields. They exhibit various biological activities such
as anticonvulsant [1,2], anesthetic [3], antitumor [4] and antibacte-
rial [5]. 2-Thiobarbituric acid is also applied in detecting aldehydes
and lipid oxides in food based on the reaction of 2-thiobarituric
acid with bifunctional aldehydes [6,7].
Additionally, 2-thiobarbituric acid has thioamide and b-dike-
tone moieties which could conduct thiol-thione and keto–enol tau-
tomerism simultaneously. Investigation on tautomerism of
pharmacologically active compounds is important to rationalize
their biological activity and to elucidate the mechanism of action
search, density functional theory (DFT) calculations are widely
used method in the determination of lowest energy structures of
tautomers, especially when the calculated results are confirmed
by experimental data [12–16]. Some theoretical and experimental
studies on 2-thiobarbituric acid and its derivatives have been
carried out due to their structure characteristic for tauomerism
research. Millefiori and Millefiori [17] estimated the relative
energies of thiobarbituric acid tautomers by means of AM1
method. Mendez et al. [18] investigated the molecular structure
of 2-thiobarbituric acid in solid, in polar solutions and on gold
nanoparticles finding that tautomerization is easy to occur in
methanol solution. Chierotti et al. [19] reported the tautomeric
polymorphs of 2-thiobarbituric acid demonstrating that 2-thiobar-
bituric acid family of crystal forms represents the richest collection
of examples of tautomeric polymorphism so far.
⇑
Corresponding author. Tel.: +86 24 23986419; fax: +86 24 23995043.
E-mail address: mscheng@syphu.edu.cn (M.-S. Cheng).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.12.017

G. Wang et al. / Journal of Molecular Structure 1036 (2013) 372–379
373
Fig. 1. The eight possible tautomers of 1-phenyl-2-thiobarbituric acid.
1-Phenyl-2-thiobarbituric acid, a derivative of 2-thiobarbituric
2. Experimental
acid, is also an important pharmacophore which presented in some
compounds possessing different biological activities such as anti-
microbial and antitumor [20–22]. On the other hand, this com-
pound has an active methylene at 5-position which could react
with aldehydes by Knoevenagel condensation and has been ap-
plied in analytical chemistry [23] and chemical modification of
natural products [24].
All synthetic materials and reagents were commercially avail-
able and were used without further purification. Fourier’s transfor-
mation infrared (FT-IR) spectra were recorded on BRUKER IFS-55
FTIR spectrometer as KBr pellets with a resolution of 2 cmÀ1 in
the 4000–400 cmÀ1 range. 1H NMR spectra were recorded on
Bruker ARX-300 spectrometer at 300 MHz using tetramethylsilane
(TMS) as internal reference.
Recently, we become interested in the biological activity of
thiobarbituric acid derivatives. In the process of synthesizing
novel thiobarbituric acid derivatives, we synthesized 1-phenyl-
2-thiobarbituric acid and its methylated derivative. Similar to
2-thiobarbituric acid, tautomerism could also take place in
1-phenyl-2-thiobarbituric acid with 8 possible tautomeric forms
(Fig. 1). Whereas the N-phenyl substitution reduces a mobile H
atom compared with 2-thiobarbituric acid, so that the methylation
reaction with excess amount of methylating agent generate
dimethylated product at most in contrary to 2-thiobarbituric acid
which results in trimethylated product [25]. But because of the
structural features of 1-phenyl-2-thiobarbituric acid, the methyla-
tion site might be varied with three possible products (Fig. 2). In
the current study, 1-phenyl-2-thiobarbituric acid was synthesized
and its tautomerism equilibrium was examined experimentally in
solid state (IR, X-ray) and solution phase (NMR), supported by
quantum chemical calculations (DFT method). In addition, the
methylation reaction of 1-phenyl-2-thiobarbituric acid was firstly
investigated and the product was characterized by experimental
data (IR, X-ray) along with theoretical calculations (DFT).
2.1. Synthesis
The synthetic route of 1-phenyl-2-thiobarbituric acid 1 and its
methylated derivate 2 is shown in Fig. 3. For compound 1, we
adopted a similar reaction condition as the synthesis of 2-thiobar-
bituric acid [26]. Phenylthiourea (5.0 g, 32.8 mmol) and diethyl
malonate (5.2 g, 32.8 mmol) were refluxed in a freshly prepared
sodium ethylate solution (Na 0.75 g, 32.8 mmol, in 100 mL of eth-
anol) for 5 h. The reaction mixture was then cooled to room tem-
perature and the solvent was evaporated under reduced pressure.
The residue was dissolved in 30 mL of water, then pH was adjusted
to 7 by conc. HCl, the resulted precipitate was filtered and recrys-
tallized from ethanol to obtain slightly yellow crystals (5.2 g, 72%
yield).
6-Methoxy-2-(methylthio)-3-phenylpyrimidin-4(3H)-one 2 was
prepared by methylation of 1 following a classical procedure [27]
using dimethyl sulfate as methylating reagent. Potassium carbon-
ate (4.7 g, 34 mmol) was added to a solution of 1 (3.0 g, 13.6 mmol)
in 50 mL dimethylformamide, dimethyl sulfate (3.8 g, 29.9 mmol)
was added dropwise and the solution was stirred for 2 h at room
temperature. Then the reaction mixture was poured into cold
water and the solid resulted was filtered, and then recrystallized
from ethyl acetate to give white solid (2.5 g, 74% yield).
2.2. X-ray investigation
Single crystals of compound 1 and 2 suitable for X-ray diffrac-
tion analysis were grown by slow evaporation of ethanol solution
(for 1) and a mixed solvent of n-hexane: ethyl acetate (3:1) (for
2). The X-ray diffraction data were collected on a Bruker APEX-II
CCD diffractometer at 273 K using graphite-monochromated Mo
Ka radiation (k = 0.71073 Å). The crystal structure was solved by
Fig. 2. Three possible methylation products.

374
G. Wang et al. / Journal of Molecular Structure 1036 (2013) 372–379
Fig. 3. Synthesis of compound 1 and 2.
direct method and refined by the full-matrix leastsquares proce-
dure on F2 using SHELXL-97 program [28]. All non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were lo-
cated by geometrical calculation. A summary of basic crystallo-
graphic data and structure refinement details are given in Table 1.
CCDC 901812 and CCDC 901813 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB2 1EZ, UK; fax: +441223 336033).
was carried out at the same DFT level as used in the geometry
optimization.
3. Results and discussion
3.1. Description of crystal structure and optimized geometry
Single crystal X-ray analysis followed by quantum chemical
calculations for compound 1 and 2 were carried out in order to
identify the molecular structure and the tautomeric form in solid
state. The selected geometrical parameters for 1 and 2 are given
in Table 2. The displacement ellipsoid plot with numbering of the
atoms is shown in Fig. 4 and the packing of molecules in the unit
cell is shown in Fig. 5.
With regard to structural analysis of 1-phenyl-2-thiobarbituric
acid 1, it crystallizes in the monoclinic P21/n space group with
one crystallographically independent molecular in the asymmetric
unit. The C1AS1 bond length of 1.6342 (16) Å is typical for the CAS
double bond. The bond lengths of C2AO2 and C4AO1 are 1.209 (2)
Å and 1.2093 (19) Å which is typical for a carbonyl group [31].
These three features indicate that in the crystalline state com-
pound 1 exists in the triketone form. The dihedral angle between
the pyrimidone ring and phenyl ring is 87.165(49)° revealing that
the two rings adopt a approximately perpendicular conformation.
The bond lengths and angles in the pyrimidone ring are similar
to 2-thiobarbituric acid [18]. In the crystal, intermolecular
hydrogen bonds of N1AH1Á Á ÁO1 were found which stabilizing the
2.3. Computational methods
All computational calculations were performed by using
Amsterdam Density Functional (ADF) program [29]. The gas phase
geometry optimization for the eight possible tautomers N1AN8
and three possible methylation products were calculated by DFT
method using Becke’s three-parameters hybrid exchange–correla-
tion functional (B3LYP) [30] employing TZP basis set. The solution
phase geometry optimization of N1AN8 were performed by COS-
MO method for chloroform, ethanol and DMSO all of which have
different dielectric constants. Similarly the geometry optimization
of three possible methylation products in the reaction solvent N,N-
dimethylformamide (DMF) was implemented at the same level of
theory by COSMO method. The total energy and dipole moment
were obtained from the optimization output. The transition state
search for methylation reaction of 1-phenyl-2-thiobarbituric acid
Table 1
Crystal data and structure refinement for compound 1 and 2.
1
2
Empirical formula
Formula weight
Wavelength (Å)
Crystal system
Space group
a (Å)
C
10H8N2O2S
C12H12N2O2S
248.31
0.71073
Monoclinic
P21/n
7.7784 (15)
16.972 (3)
9.6375 (19)
108.755 (2)
1204.7 (4)
4
220.25
0.71073
Monoclinic
P21/n
11.0748 (9)
7.3546 (5)
13.3517 (10)
113.603 (1)
996.53 (13)
4
b (Å)
c (Å)
b (°)
Volume (Å3)
Z
F(000)
456
1.468
0.30
520
1.369
0.26
Calculated density (Mg/m3)
l
(mmÀ1
)
Tmin, Tmax
h
Index ranges h, k, l
No. of measured reflections
No. of independent reflections
Crystal size (mm)
0.941, 0.955
3.1–26.1
À13/11, À5/9, À16/16
5065
1972 (Rint = 0.021)
0.2 Â 0.18 Â 0.15
0.949, 0.962
2.4–26.1
À6/9, À21/19, À11/11
6558
2373 (Rint = 0.026)
0.2 Â 0.18 Â 0.15
Refinement
Goodness-of-fit on F2
1.02
0.034
0.093
1.12
0.043
0.113
R [F2 > 2
r
(F2)]
wR(F2)
Data/restraints/parameters
5065/0/136
6558/0/156

G. Wang et al. / Journal of Molecular Structure 1036 (2013) 372–379
375
Table 2
Selected experimental and computed geometrical parameters for 1 and 2.
1
2
Parameter
Experimental
Calculated
Parameter
Experimental
Calculated
Bond length (Å)
S1AC1
1.6342 (16)
1.375 (2)
1.3878 (19)
1.370 (2)
1.386 (2)
1.451 (2)
1.209 (2)
1.2093 (19)
1.371 (2)
1.381 (2)
1.491 (2)
1.484 (2)
1.648
1.382
1.389
1.380
1.410
1.451
1.213
1.208
1.386
1.386
1.513
1.505
S1AC7
1.7563 (17)
1.8006 (17)
1.3463 (19)
1.432 (2)
1.224 (2)
1.300 (2)
1.366 (2)
1.374 (2)
1.420 (2)
1.4485 (19)
1.384 (2)
1.386 (2)
1.388 (2)
1.425 (2)
1.359 (2)
1.774
1.819
1.350
1.428
1.219
1.297
1.358
1.368
1.442
1.440
1.389
1.389
1.389
1.433
1.368
C1AN1
C1AN2
N1AC2
N2AC4
N2AC5
O2AC2
O1AC4
C5AC10
C5AC6
S1AC12
O1AC10
O1AC11
O2AC8
N1AC7
N1AC10
N2AC7
N2AC8
N2AC1
C1AC6
C1AC2
C2AC3
C8AC9
C9AC10
C4AC3
C2AC3
Bond angle (°)
N1AC1AN2
N1AC1AS1
N2AC1AS1
C2AN1AC1
C4AN2AC1
C4AN2AC5
C1AN2AC5
C10AC5AC6
C10AC5AN2
C6AC5AN2
O1AC4AN2
O1AC4AC3
N2AC4AC3
O2AC2AN1
O2AC2AC3
N1AC2AC3
C5AC10AC9
C2AC3AC4
116.33 (14)
120.69 (12)
122.98 (12)
127.34 (13)
124.35 (13)
116.51 (12)
118.71 (13)
122.02 (15)
119.06 (15)
118.87 (15)
120.54 (15)
122.13 (15)
117.30 (14)
121.49 (15)
122.46 (16)
116.05 (14)
118.69 (17)
117.86 (15)
115.92
119.89
124.19
129.03
124.58
116.50
118.91
121.21
119.47
119.31
121.13
121.63
117.22
121.33
123.77
114.89
119.27
117.49
C7AS1AC12
C10AO1AC11
C7AN1AC10
C7AN2AC8
C7AN2AC1
C8AN2AC1
C6AC1AC2
C6AC1AN2
C2AC1AN2
C1AC2AC3
C3AC4AC5
N1AC7AN2
N1AC7AS1
N2AC7AS1
O2AC8AN2
O2AC8AC9
N2AC8AC9
C10AC9AC8
O1AC10AC9
O1AC10AN1
C9AC10AN1
100.14 (8)
99.75
117.19 (14)
116.57 (14)
121.06 (13)
120.88 (13)
118.01 (13)
121.48 (15)
118.77 (14)
119.71 (14)
118.97 (16)
120.41 (16)
124.08 (14)
120.24 (12)
115.68 (11)
119.09 (15)
126.75 (15)
114.15 (14)
119.72 (15)
125.53 (15)
110.08 (14)
124.39 (15)
117.74
117.79
121.02
121.32
117.66
120.84
119.56
119.59
119.46
120.09
124.01
120.13
115.86
119.51
127.06
113.43
120.08
124.86
111.46
123.67
Torsion angle (°)
C4AN2AC5AC10
C1AN2AC5AC10
C4AN2AC5AC6
C1AN2AC5AC6
À89.94 (19)
97.35 (18)
87.65 (19)
À85.07 (19)
À89.46
89.20
89.34
À89.46
C7AN2AC1AC2
C7AN2AC1AC6
C8AN2AC1AC2
C8AN2AC1AC6
À101.2(2)
80.8(2)
81.5(2)
À96.6(2)
91.93
À88.66
À88.14
91.27
molecular structure and the crystal packing. The hydrogen
bonding geometries are as follows: (N1AH1) = 0.859 Å,
(H1Á Á ÁO1) = 2.296 Å, d (N1Á Á ÁO1) = 3.063 Å.
calculated bond lengths and angles are in good accordance with
those observed in X-ray diffraction. The calculated energy mini-
mized molecules for 1 and 2 were aligned with the X-ray struc-
tures with root mean square deviation (RMSD) of 0.1684 Å for 1
and 0.1856 Å for 2 (Fig. 6).
d
d
The methylation product 2 also crystallizes in monoclinic P21/n
space group with one independent molecular in the asymmetric
unit. The crystal structure demonstrated that the methylation oc-
curred on sulfur atom and the oxygen atom which is far from phe-
nyl ring (isomer 2a). The C7AS1 bond length of 1.7563 (17) Å is in
accordance with S-Csp2 single bond [32], whereas the C12AS1
bond length of 1.8006 (17) Å is slightly longer than that of
C7AS1. The O1AC10 and O1AC11 bond lengths are 1.3463 (19) Å
and 1.432 (2) Å which is similar to a methoxyl group attached to
phenyl ring [33]. The bond lengths of C7AN1 and C9AC10 are
1.300 (2) Å and 1.359 (2) Å respectively, showing the double bond
character of these two bonds. The pyrimidine ring and phenyl ring
adopt similar conformation to that in compound 1 with the dihe-
dral angle be 81.162(56)°. The torsion angles of C11AO1AC10AC9
and C12AS1AC7AN1 are 9.807(249)° and À0.091(155)° demon-
strating the orientation of the two methyl groups.
3.2. IR spectra
IR spectroscopy provides recognizable adsorption bands for
functional groups so that it is a useful tool for molecular structure
identification [34]. The solid state IR spectra of the two compounds
is given in Supplemental material (Fig. S1). In the spectrum of com-
pound 1, a peak of medium intensity at 3258 cmÀ1 could be as-
signed to the NAH stretching frequency. And there are two
strong absorptions at 1730 cmÀ1 and 1703 cmÀ1 corresponding to
carbonyl stretching mode. These three absorption bands mainly
characterize the compound 1 to be the triketone form which is val-
idated by X-ray analysis. The absorptions at 1596 cmÀ1 and
1491 cmÀ1 are due to CAC stretching vibration of aromatic ring
[35]. The stretching and bending vibrations of the CH2 group at
ADF software was used to perform the geometric optimization
of compound 1 and 2 on the level of B3LYP with TZP basis set. Com-
parisons of the calculated geometric parameters with the experi-
mental values are shown in Table 2. The results indicate that the
pyrimidone ring were observed at 2920 cmÀ1 and 1472 cmÀ1
The absorption bands appear in 1100–1400 cmÀ1 should be
.

376
G. Wang et al. / Journal of Molecular Structure 1036 (2013) 372–379
Fig. 4. ORTEP plot of compound 1 and 2 (ellipsoids are shown at the 50% probability level).
Fig. 5. The molecular packing of compound 1 and 2. Hydrogen bonds are shown as dashed lines.
attributed to aromatic CAH bending vibrations, NAH bending
vibrations and CAN stretching vibrations.
In the spectrum of compound 2, the NAH band and one of the
carbonyl absorptions in the spectra of compound 1 disappear re-
sult from methylation. The remaining C@O stretching vibration
shifts to lower frequency at 1674 cmÀ1 compared with compound
the conjugation effect between the pyrimidine ring and phenyl
ring. The CAH stretching and bending vibrations of methyl group
appear in 2939 cmÀ1, 1441 cmÀ1 and 1388 cmÀ1, respectively.
The strong absorption peak at 1241 cmÀ1 accounts for the CAO
stretching mode of methoxy group similar to a literature data [36].
1, this might because the formation of C10@C9AC8@O2
a,b-unsat-
3.3. 1H NMR study of 1-phenyl-2-thiobarbituric acid
urated ketone moiety causes conjugation effect which decrease the
force constant of C8@O2 carbonyl group. The absorptions at
1582 cmÀ1 and 1523 cmÀ1 could be assigned to aromatic ring
stretching mode, the absorption intensity of this two peaks is much
stronger than those in compound 1, this might due to the forma-
tion of C7@N1 and C9@C10 double bonds by methylation increases
The 1H NMR spectra of 1-phenyl-2-thiobarbituric acid were ta-
ken in deuterated chloroform (CDCl3), deuterated methanol (CD3-
OD), DMSO-d6 and DMF-d7 respectively in order to determine
the tautomerism tendency in different solvents. The spectra are
shown in Fig. S2. In CDCl3 (Fig. S2a), there is a singlet at

G. Wang et al. / Journal of Molecular Structure 1036 (2013) 372–379
377
which is shown in Fig. S3). In protic solvent CD3OD, we observed
that both the 5-CH2 methylene protons and the NAH proton
signals disappeared, only the aromatic hydrogen signals at
7.15–7.48 ppm remained. This could be interpreted by the
exchange process of the H’s on C-5 and NAH with D’s on deuter-
ated methanol. The exchange of the 5-CH2 protons could take
place through the OAH proton of one of the enol forms of
1-phenyl-2-thiobarbituric acid. Because of this, both the 5-CH2
proton and the OAH proton of enol form disappear in 1H NMR
spectrum taken in CD3OD. The possible tautomers and possible
H–D exchange process of compound 1 in CD3OD are shown in
Fig. S4. From the 1H NMR spectra in different solvents, it is con-
cluded that tautomerization of compound 1 occurred more easily
in polar solvent than in apolar solvent.
3.4. Theoretical calculations on 1-phenyl-2-thiobarbituric acid 1
The geometry optimization for the eight tautomers of 1-phenyl-
2-thiobarbituric acid 1 was performed in gas phase and different
solvent media at B3LYP/TZP level. We chose three types of solvent
with different dielectric constants (Chloroform = 4.9, Etha-
nol = 24.3, Dimethyl sulfoxide = 46.7). The total energy and dipole
moment of N1AN8 were obtained from the optimization output
and are listed in Table 3. From the result it can be seen that tauto-
mer N6 is the most energetically stable one both in gas phase and
solution phase. The energy gap between different tautomeric forms
and the most stable form N6 is listed in Table 4. The energy gap is
in the range of 8.16–33.07 kcal/mol for all investigated tautomers
in different phases. The second stable tautomer is N4 according
Fig. 6. Superimposition of X-ray structure (red) and gas phase optimized (blue)
structure of compound 1 and 2. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
3.88 ppm integrated to 2H which should be the 5-CH2 methylene
proton. And the signal at 9.42 ppm should be assigned to the
NAH proton of thioamide moiety. These two signals indicate that
compound 1 is presented in triketo form (2-C@S, 4,6-C@O) in
CDCl3 solution which is in accordance with the X-ray crystal struc-
ture. From the spectrum in DMSO-d6 (Fig. S2b), a polar but aprotic
solvent, it is observed that the signal at 3.86 ppm become a broad
singlet, and a new broad singlet appeared at 5.14 ppm compared
with the spectrum in CDCl3. The signal at 3.86 ppm should be as-
signed to 5-CH2 methylene proton. The new appearing singlet at
5.14 ppm should belong to the vinylic hydrogen at C-5 of the enol
form of 1-phenyl-2-thiobarbituric acid [37], and the very broad
singlet between 11.11 ppm and 11.62 ppm probably belong to
OAH proton signal of enol form. Additionally the signals at 7.15–
7.46 ppm and 12.68 ppm should be assigned to aromatic hydrogen
and NAH proton respectively. The coexistence of 5-CH2 methylene
and 5-CH vinylic proton signals indicates that the tautomerization
of compound 1 occurred in DMSO-d6. From the integral values of
5-CH2 and 5-CH vinylic proton it is concluded that the ratio of
keto:enol tautomers is 38%:62%. The spectrum in another polar
and aprotic solvent DMF-d7 is shown in Fig. S2c, the singlet at
3.99 ppm should be assigned to 5-CH2 methylene protons and
the singlet at 5.25 ppm should belong to the 5-CH vinylic hydro-
gen. Besides the aromatic hydrogen signal at 7.28–7.49 ppm, there
are two active hydrogen signals at 12.55 and 12.89 ppm which
should be assigned to OAH proton of enol form and NAH proton
respectively. Therefore, we could conclude that tautomerism of
compound 1 also occurred in DMF-d7. The ratio of keto:enol form
is 44%:56% according to the integral values. But only according to
1H NMR spectra, we could not identify which enol form exists in
DMSO-d6 and DMF-d7 solution (It could be tautomer N3 or N4
to calculated energy with the
DE to be 8.16–10.29 kcal/mol in
gas and different solvents. The calculated energy support the
experimental data obtained from IR spectra in KBr pellets and X-
ray single crystal diffraction for the solid state and from 1H NMR
spectrum for the CDCl3 solution because it has shown that in this
two phases 1-pheny-2-thiobarbituric acid exists in the most ener-
getically stable tautomer N6. It is found that with the increasing of
the solvent dielectric constants, the calculated dipole moment va-
lue also increase for most of the 1-phenyl-2-thiobabituric acid tau-
tomers. Depending on the increase of solvent polarity, the
molecules with higher dipole moment are more easily stabilized
by solvent. This may be one of the reasons of why the tautomeriza-
tion occurs easier in DMSO-d6 and CD3OD solution.
3.5. Theoretical calculations on the methylation of 1-phenyl-2-
thiobarbituric acid
To the best of our knowledge, the methylation reaction of 1-
phenyl-2-thiobarbituric acid 1 has not been investigated till now.
We carried out the methylation reaction of 1-phenyl-2-thiobarbi-
turic acid using 2.2 eq of dimethyl sulfate as methylating reagent.
The molecular weight of the obtained product is 248 detected by
ESI-MS, concluding that its a dimethylated product. This result is
Table 3
The calculated energy and dipole moments of N1AN8.
Tautomers
Gas
CHCl3
EtOH
DMSO
Total energy (a.u)
l
(D)
Total energy (a.u)
l
(D)
Total energy (a.u)
l
(D)
Total energy (a.u)
l (D)
N1
N2
N3
N4
N5
N6
N7
N8
À1043.8478
À1043.8327
À1043.8530
À1043.8596
À1043.8510
À1043.8726
À1043.8487
À1043.8417
2.2135
7.9322
3.5041
5.8736
5.4896
1.7823
1.9663
6.2191
À1043.8499
À1043.8166
À1043.8476
À1043.8556
À1043.8459
À1043.8693
À1043.8443
À1043.8395
5.7530
15.5452
5.6359
8.8855
7.3248
1.6748
3.6574
7.8788
À1043.8379
À1043.8200
À1043.8427
À1043.8508
À1043.8425
À1043.8666
À1043.8384
À1043.8336
3.1495
11.8284
6.4249
8.5776
7.9617
1.5705
4.3997
8.8953
À1043.8459
À1043.8182
À1043.8424
À1043.8503
À1043.8422
À1043.8667
À1043.8375
À1043.8340
7.0865
14.9991
6.5193
8.6705
8.0471
1.5796
4.4808
9.0293

378
G. Wang et al. / Journal of Molecular Structure 1036 (2013) 372–379
Table 4
compound 1 at DFT level agree well with the experimental data.
On the other hand, we explored the methylation reaction of 1-phe-
nyl-2-thiobarbituric acid for the first time. There are three possible
dimethylation products for compound 1 and we just observed one
product in the reaction. The methylated site was identified exper-
imentally (X-ray, FT-IR) and theoretically (transition state search,
energy calculation), the results revealed that theoretical calcula-
tions are in good agreement with experimental data. In summary,
this work demonstrates the combination of experimental and the-
oretical methods to improve knowledge of tautomerism equilib-
rium and reactivity of 1-phenyl-2-thiobarbituric acid and may
help in synthesizing novel thiobarbituric acid derivatives.
The energy gap between N6 and other tautomers (kcal/mol).
Tautomers
Gas
CHCl3
EtOH
DMSO
N1
N2
N3
N4
N5
N6
N7
N8
15.56
25.04
12.30
8.16
13.55
0
12.17
33.07
13.62
8.60
14.68
0
18.01
29.24
15.00
9.91
15.12
0
13.05
30.43
15.25
10.29
15.37
0
15.00
19.39
15.69
18.70
17.69
20.71
18.32
20.52
Acknowledgements
Table 5
The energy of possible methylated products in gas phase and DMF solution.
The authors would like to thank Scientific Computing & Model-
ling (SCM) for allowing us to utilize their software for this study,
and also thank Beijing Computing Center for their computational
resource. We also gratefully acknowledge financial support from
the National Natural Science Foundation of China (Grants
81230077 and 81102379) and the Education Department of
Liaoning Province (Grant L2011174).
Gas (a.u)
DMF (a.u)
2a
2b
2c
À1122.4382
À1122.3992
À1122.4217
À1122.4219
À1122.3796
À1122.4092
as expected because the N-phenyl substitution reduces a mobile H
atom consequently reduces a methylation site. But 1-phenyl-2-
thiobarbituric acid has three potential methylation site (sulfur
atom and two oxygen atoms). And as observed in the 1H NMR spec-
trum in DMF-d7, we found that tautomerization of compound 1 oc-
curs but the exact tautomer (N3 or N4) could not be identified.
Accordingly we conclude that there might be three possible meth-
ylated products (Fig. 2) of 1-phenyl-2-thiobarbituric acid using
DMF as reaction solvent. But we only observed one product after
the reaction so that a problem arise that which two sites had been
methylated. In order to determine the methylation site, theoretical
calculations were carried out to assist experimental data. First we
searched the transition state of the methylation reaction using ADF
software at DFT level finding that there is no transition state in the
reaction process, the energy kept decreasing during the whole pro-
cess. Consequently the methylation reaction will generate the most
energetically stable product. Therefore, we calculated the energy
for the three possible methylated products 2a–2c in gas phase
and in its reaction solvent (DMF). The calculated energy is pre-
sented in Table 5. It can be seen from Table 5, that 2a is the most
energetically stable one among the three isomers both in gas phase
and in DMF solution. The energy sequence of the three isomers is
2a < 2c < 2b. So that 2a should be the theoretical product for the
methylation reaction of 1-phenyl-2-thiobarbituric acid. As de-
scribed in Section 3.1, the molecular structure of methylation reac-
tion product was experimentally identified by X-ray diffraction to
be 2a which verified the calculated results. And no other isomers
were observed. In summary, the methylation reaction of 1-phe-
nyl-2-thiobarbituric acid was investigated theoretically (DFT) and
experimentally (X-ray, IR) and we found that the calculated results
are in good accordance with experimental data.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.12.
017.
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