Experimental and theoretical investigations on the keto-enol tautomerism of 4-substituted 3-[1-methylpyrrol-2-yl)methyl]-4,5-dihydro-1H-1,2,4-triazol-5-one derivatives

Article abstract of DOI:10.1016/j.molstruc.2011.03.041

Keto-enol tautomerism is a key phenomenon which determines the pharmacological activity of compounds possessing the carbonyl group. In the present study we use X-ray analysis, IR spectroscopy as well as quantum chemical calculation to address the keto-enol tautomerism of 4-substituted 3-[1-methylpyrrol-2-yl)methyl]-4,5-dihydro-1H-1,2,4-triazol-5-one derivatives with antimicrobial activity. In the gas phase and in the crystalline state all the investigated molecules exist in the keto form while in the chloroform solution a small amount of the enol form in the equilibrium with the keto form is observed. The results of the calculations are in good agreement with experimental data obtained from X-ray analysis and IR spectroscopy in KBr but do not confirm the shift of the tautomeric equilibrium into the enol form in a chloroform solution.

Full text of DOI:10.1016/j.molstruc.2011.03.041

Journal of Molecular Structure 994 (2011) 313–320
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Experimental and theoretical investigations on the keto–enol tautomerism of
4-substituted 3-[1-methylpyrrol-2-yl)methyl]-4,5-dihydro-1H-1,2,4-triazol-5-one
derivatives
b
b
c
c
Monika Pitucha a, , Zbigniew Karczmarzyk , Waldemar Wysocki , Agnieszka A. Kaczor , Dariusz Matosiuk
⇑
a Department of Organic Chemistry, Faculty of Pharmacy, Medical University of Lublin, 6 Staszica St., 20081 Lublin, Poland
b Department of Chemistry, University of Podlasie, 3 Maja 54 St., 08110 Siedlce, Poland
c Department of Synthesis and Chemical Technology of Pharmaceutical Substances, Faculty of Pharmacy, Medical University of Lublin, 6 Staszica St., 20081 Lublin, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Keto–enol tautomerism is a key phenomenon which determines the pharmacological activity of com-
pounds possessing the carbonyl group. In the present study we use X-ray analysis, IR spectroscopy as well
as quantum chemical calculation to address the keto–enol tautomerism of 4-substituted 3-[1-methylpyr-
rol-2-yl)methyl]-4,5-dihydro-1H-1,2,4-triazol-5-one derivatives with antimicrobial activity. In the gas
phase and in the crystalline state all the investigated molecules exist in the keto form while in the chlo-
roform solution a small amount of the enol form in the equilibrium with the keto form is observed. The
results of the calculations are in good agreement with experimental data obtained from X-ray analysis
and IR spectroscopy in KBr but do not confirm the shift of the tautomeric equilibrium into the enol form
in a chloroform solution.
Received 15 December 2010
Received in revised form 16 March 2011
Accepted 16 March 2011
Available online 25 March 2011
Keywords:
Keto–enol tautomerism
X-ray structure determination
Quantum chemical calculation
1,2,4-Triazoles
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
the enol form of chlorthalidone binding to the active site of car-
bonic anhydrase.
Keto–enol tautomerism plays a crucial role in compounds pos-
sessing the carbonyl group. Investigations on the tautomeric equi-
librium in carbonyl compounds are very important to rationalize
their biological activity [1–7] and to understand biochemical pro-
cesses in which they take part. The best known examples of
keto–enol tautomerism in nature are nitrogenous bases in which
shifting the equilibrium to the keto form makes it possible to form
hydrogen bonds to maintain the double-strand structure of DNA.
Besides nucleic acids, many pharmacologically active compounds
have their activity determined by the relative greater stability of
one tautomer. Examples of such compounds are phosphorohydr-
azine derivatives of coumarin and chromone which have antibac-
terial and antitumor activity conditioned by the keto–enol
tautomerism [2]. Some compounds exhibiting keto–enol tautom-
erism which affects their opioid and serotonin receptor activity
have also been obtained by our research group [3–6]. Furthermore,
the greater stability of a keto or an enol form of a compound may
be crucial for the interaction with the target binding site. As it has
been recently shown by Temperini et al. [7], the binding site itself
may preferentially stabilize one of the tautomers as in the case of
Animportantclassofcompoundsexhibitingketo–enoltautomer-
ism are carbonyl derivatives of 1,2,4-triazole. The 1,2,4-triazole
system is present in a great number of compounds characterized by
different biological activities, such as: analgesic [8,9] antibacterial
[10–13], fungicidal [14], antinflammatory [15–17], antiviral [18,19]
and anti-cancer [20–22] properties. Based on these facts, many
1,2,4-triazol-5-one derivatives have been synthesized in our labora-
tory and their biological activities have been investigated [23–27].
Thesecompoundsweremainlysynthesizedbythecyclizationoftheir
semicarbazide derivatives in alkaline media. Searching for new bio-
logicallyactive1,2,4-triazolederivatives,weobtained,amongothers,
4-substituted-3-[(1-methylpyrrol-2-yl)methyl]-4,5-dihydro-1H-
1,2,4-triazol-5-one derivatives 1–6 (Fig. 1). These compounds were
screened for their antimicrobial [26] and central nervous system
activities [27]. In order to rationalize the biological activity of the
investigated compounds, we decided to study their keto–enol tau-
tomerism as it had been demonstrated that the relative stability of
the keto and the enol form may affect pharmacological activity in
similar systems [6]. The knowledge about relative stability of keto
and enol tautomers is an essential condition to perform structure–
activity studies (SAR) as well as molecular docking because the pres-
ence of a hydroxyl or a carbonyl group may dramatically change the
interactions of compounds 1–6 with bacterial enzymes. In general,
⇑
Corresponding author. Tel.: +48 815357373.
E-mail address: monika.pitucha@umlub.pl (M. Pitucha).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.03.041

314
M. Pitucha et al. / Journal of Molecular Structure 994 (2011) 313–320
Fig. 1. The synthesis and keto–enol tautomerism of the investigated compounds.
applicationofanycomputer-assisteddrugdesignmethodstoaseries
of compounds for which keto–enol tautomerism is possible, should
be preceded by determination which tautomer is energetically
privileged.
2.3. X-ray investigation of 1
Light brown prismatic crystals of 1 suitable for X-ray diffraction
analysis were grown by slow evaporation of an ethanol solution.
X-ray data were collected on the Bruker SMART APEX CCD diffrac-
tometer at room temperature; crystal sizes: 0.50 Â 0.25 Â 0.18
It is well established that quantum chemical calculations are
very useful in the determination of the most stable structure of
tautomers and the estimation of tautomeric equilibrium constants
[28,29], in particular when the results of the calculations are
confirmed by experimental data. Many such studies have been re-
ported for different classes of compounds, e.g. pyrazolones [30],
betaketonitriles [31], cyclohexanediones [32,33]. On the other
hand, 1,2,4-triazol-5-one system, bearing different substituents,
has been a subject of extensive experimental and computational
studies which were aimed to determine structural and spectral
properties of respective derivatives [34–40].
mm,
x scans. The structure was solved by direct methods using
SIR92 [41] and refined by full matrix least squares with SHELXL97
[42]. All hydrogen atoms were located from the map and their
D
q
coordinates were refined with isotropic displacement parameters
taken as 1.5 times those of the respective parent atoms. All crystal
and experimental data are listed in Table 1.
Molecular graphics were prepared using ORTEP3 for Windows
[45]. PARST [46] and PLATON [47] were used for geometrical
In the light of above, in this study, we report the analysis of the
tautomeric equilibrium between the keto and enol forms for the
1,2,4-triazole-5-one system in 1–6 using quantum chemical calcu-
lations in the gas phase and in solutions, supported by IR spectros-
copy and X-ray analysis in the crystalline state. We show that in
the gas phase and in the crystalline state all molecules exist in
the keto form while in the chloroform solution a small amount of
the enol form in the equilibrium with the keto form is observed.
Table 1
Crystal data and structure refinement for 1.
Empirical formula
Formula weight
Crystal system
Space group
Unit cell parameters
C16H18N4O2
298.34
monoclinic
P21
a = 7.6707(15) Å
b = 7.2466(14) Å
c = 14.307(3) Å
b = 103.37(3)°
773.7(3) Å3
2
Volume, V
2. Experimental
Molecular multiplicity, Z
Density (calculated)
Radiation
1.281 g/cm3
Cu Ka (k = 1.54178 Å)
2.1. Synthesis
Cell parameters from
h range for lattice parameters
61 Reflections
7.10–39.15o
4-substituted
3-[(1-methylpyrrol-2-yl)methyl]-4,5-dihydro-
Absorption coefficient,
Absorption correction
Tmin/Tmax
l
0.711 mmÀ1
Multi-scan [43]
0.8185/0.9323
3.17–70.04o
1H-1,2,4-triazol-5-one derivatives 1–6 were synthesized by the
intramolecular dehydrative cyclization of the respective 4-substi-
tuted-1-[(1-methylpyrrol-2-yl)acetyl]semicarbazides
from the reaction of 1-methylpyrrol-2-yl-acetic acid hydrazide
with the appropriate isocyanate) according to the procedure earlier
reported, Fig. 1 [26].
h range for data collection
Index ranges h, k, l
No. of measured reflections
No. of independent reflections
No. of observed reflection
(obtained
À9/9, À7/8, À17/17
8833
2579 (Rint = 0.0162)
2541 with I > 2r(I)
Refinement
Refinement method
Full-matrix least-squares on F2
0.0353, 0.1023
1.046
2579/1/255
0.0(2) [44]
2.2. IR spectra
Final R indices: R, wR(F2)
Goodness-of-fit on F2, S
Data/restraints/parameters
Absolute structure parameter
Extinction coefficient
Largest diff. peak and hole
IR spectra of 1–6 were measured in the solid state (KBr) and in
chloroform with
a
Magna IR-760 Nicolet spectrophotometer.
0.0028(14)
Before IR measurements, chloroform (analytically pure) was dehy-
drated using CaCl2 and then distilled off with dephlegmator (with
no moisture).
+0.157/À0.139 eÅÀ3
0.068
(D/r)
max

M. Pitucha et al. / Journal of Molecular Structure 994 (2011) 313–320
315
calculations. All calculations were performed using WINGX ver.
1.64.05 package [48]. CCDC-618948 for 1 contains the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retriev-
ing.html (or from the Cambridge Crystallographic Data Centre
(CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)
1223 336 033; email: deposit@ccdc.cam.ac.uk).
2.4. Theoretical calculations
The molecular structure of 1 in the ground state was optimized
with the Hartree–Fock and B3LYP DFT (the variant of DFT method
using Becke’s three-parameter hybrid functional (B3) [49] with
correlation functional such as the one proposed by Lee, Yang, and
Parr (LYP) [50]) applying 6-31G(d,p) basis set as included in Gauss-
ian03 [51]. The choice of the computation level, in particular of
basis set, was dictated by a compromise between the accuracy of
expected results and the resources required. The X-ray structure
of 1 was used as the starting conformation for the calculation. In
order to identify low energy conformers, five selected degrees of
torsional freedom, N2AC3AC6AC7, C3AC6AC7AN8, C5AN4AC21
AC22, C23AC24AO27AC28 and C24AO27AC28AC29 were varied
from À180° to +180° in steps of 10°, and the respective molecular
energy profiles were determined at the molecular mechanics level
with the MOE Molecular Operating Environment [52]. To investi-
gate the keto–enole tautomerism of the compound, the population
of the tautomers was estimated using non-degenerate Boltzman
distribution on the level of 6-31G(d,p)/HF and 6-31G(d,p)/B3LYP.
The energy was calculated for isolated molecules (gas phase) and
the molecules in chloroform (e = 4.7113) and water (e = 78.3553)
solutions. Calculations were performed using the Polarizable Con-
tinuum Model (PCM) [53]. This method creates a solute cavity via a
set of overlapping spheres. Gaussian03 was also used to calculate
vibrational frequencies (with 6-31G(d,p)/HF and 6-31G(d,p)/
B3LYP methods). The vibrational band assignments were made
using the Gauss-View Molecular Visualization program [54,55].
Vibrational frequencies were scaled by 0.8992 [56] for 6-
31G(d,p)/HF and by 0.9608 for 6-31G(d,p)/B3LYP [57]. Addition-
ally, alternative scaling factors of IR frequencies were proposed
for this group of compounds by taking the average ratio of exper-
imental and computed frequencies, separately for each band and
each environment. The following software was also used for
visualizing results: Chimera [58], Mercury [59], VegaZZ [60], Ya-
sara Structure [61], PyMol [62] and MOE Molecular Operating
Environment [52].
3. Results and discussion
3.1. IR spectra
In order to determine the keto–enol tautomerism of the inves-
tigated compounds 1–6, first the IR spectra were recorded in solid
state (KBr) and in chloroform solution. Then, the IR frequencies
were calculated with Gaussian03 applying the Hartree–Fock and
B3LYP DFT methods and 6-31G(d,p) basis set. All experimental
spectra for 1–6 exhibit characteristic bands for the C@O and
NAH groups in KBr and additional weak and broad bands for the
OAH group in chloroform solution. The characteristic bands
(experimental and computed, both raw and scaled) involved in
keto–enol tautomerism of analyzed compounds are collected in
Table 2.
The calculated ‘‘raw’’ HF and DFT frequencies are usually
significantly overestimated in comparison to the experimental val-
ues because of the lack of electron correlation, an insufficient basis
set and anharmonicity [63]. Thus, we used the scaling factors

316
M. Pitucha et al. / Journal of Molecular Structure 994 (2011) 313–320
Table 3
The applied scaling factors.
available in literature to enhance the computation results. Vibra-
tional frequencies were scaled by 0.8992 [56] for 6-31G(d,p)/HF
and by 0.9608 [57] for 6-31G(d,p)/B3LYP. This led to the accor-
dance of experimental and computed frequencies for the spectra
recorded in chloroform (with the exception of frequencies for the
hydroxylic group) but not in the solid state. Therefore it was
decided to check what scaling factors would be suitable for this
group of compounds by calculating the average ratio of experimen-
tal and computed frequency for each method in each environment
separately. The obtained scaling factors were 0.8540 and 0.9179 for
the spectra in KBr, for HF and DFT method, respectively and varied
significantly from the scaling factors available in literature. The
same procedure applied to the spectra recorded in chloroform
resulted in the factors 0.8510 and 0.9177 for HF and DFT methods,
respectively. To improve further the calculated frequencies, the
scaling factors were calculated for each band separately which is
a common practice [64,65], with the scaling factors reported up
to now the range from about 0.78 to slightly above 1. The summary
on the applied scaling factors is given in Table 3. The final scaling
was performed with the separate scaling factors for each band.
Thus, restricting them to the analyzed functional groups, the pro-
posed scaling factors may lead to an enhancement of the calculated
IR frequencies for the similar compounds [65]. As a result of this
part of the study it may be concluded that the keto form is the
one existing in the solid state. In chloroform solution besides
the keto form a small amount of an enol form is observed and equi-
librium between both keto and enol forms is reached. Additionally,
1H NMR spectra in chloroform were recorded for the investigated
compounds but they did not exhibit the signal of the enol
from.
Scaling
KBr
Chloroform
HF/6-
31G(d,p)
B3LYP/6-
31G(d,p)
HF/6-
31G(d,p)
B3LYP/6-
31G(d,p)
Refs. [45,46]
Average for all the 0.8540av
bands
0.8992
0.9608
0.9179
0.8992
0.8510
0.9608
0.9177
Separate for each band
NAH
C@O
OAH
0.8519
0.8560
NA
0.9110
0.9248
NA
0.8892
0.8801
0.7836
0.9501
0.9447
0.8584
NA = not available.
Table 4
Experimental and computed geometrical parameters of 1.
Parameter
Experimental
Computed, 6-31G(d,p)
HF DFT
Bond
O5AC5
N1AC5
N1AN2
N2AC3
N4AC3
N4AC5
N4AC21
N8AC7
N8AC9
N8AC12
C3AC6
C6AC7
C7AC11
C9AC10
C10AC11
1.2323(17)
1.346(2)
1.3875(17)
1.292(2)
1.3842(18)
1.3845(19)
1.432(2)
1.372(3)
1.381(3)
1.455(3)
1.4986(19)
1.492(2)
1.378(3)
1.346(5)
1.394(4)
1.197
1.352
1.370
1.270
1.381
1.387
1.426
1.370
1.363
1.449
1.502
1.506
1.362
1.358
1.421
1.218
1.375
1.379
1.303
1.386
1.416
1.427
1.384
1.376
1.455
1.505
1.506
1.383
1.377
1.421
3.2. X-ray and computed structure of 1
In order to identify the molecular structure and the tautomeric
form in the solid state of the investigated compound, X-ray struc-
ture analysis followed by quantum chemical calculations for 1
were performed. The selected geometrical parameters for the crys-
tal structure of 1 are given in Table 4. A view of the molecule with
numbering of the atoms is shown in Fig. 2 and the packing of mol-
ecules in the crystal network is presented in Fig. 3.
Bond angle
C5AN1AN2
C3AN2AN1
C3AN4AC5
C3AN4AC21
C5AN4AC21
C9AN8AC7
C9AN8AC12
C7AN8AC12
N2AC3AN4
N2AC3AC6
N4AC3AC6
O5AC5AN1
O5AC5AN4
N1AC5AN4
C7AC6AC3
C11AC7AN8
C11AC7AC6
N8AC7AC6
C10AC9AN8
C9AC10AC11
C7AC11AC10
112.37(13)
104.90(13)
107.44(12)
128.37(12)
123.90(11)
106.9(2)
113.26
105.10
107.62
128.59
123.67
108.56
124.18
126.78
111.59
123.69
124.71
129.38
128.20
102.41
114.53
108.20
128.79
123.01
109.14
106.64
107.46
114.21
104.47
108.07
128.63
123.28
108.83
124.56
126.27
111.74
122.97
125.20
129.99
128.51
101.51
114.11
107.74
129.49
122.76
108.69
107.01
107.73
126.5(2)
The difference electron density map for molecule 1 revealed the
position of the H atom in the vicinity of the N1 atom. Additionally,
the C5AO5 bond length of 1.2323(17) Å is typical for the bond
length in a carbonyl group [66]. These two facts explicitly indicate
that the keto tautomeric form of 1 is the only one existing in the
crystalline state. Bond lengths and angles in the triazolone and pyr-
role ring systems do not differ significantly from those reported for
other related structures [67,68]. The 1,2,4-triazole (A), pyrrole (B)
and phenyl (C) rings are planar to within 0.006(1), 0.007(3) and
0.024(1) Å and the dihedral angles between the planes of these
rings are A/B = 63.14(7)°, A/C = 55.55(5)°, B/C = 62.36(7)°. The 3-
[(1-methylpyrrole-2-yl)methyl] substituent adopts gauche–
gauche conformation with torsion angles N2AC3AC6AC7 and
C3AC6AC7AN8 of À98.5(2)° and 85.2(2)°, respectively. The ethoxy
group is almost co-planar with the phenyl ring and it adopts trans–
trans conformation with the torsion angles C23AC24AO27AC28
= 174.35(16)° and C24AO27AC28AC29 = À176.90(19)°. The con-
formation of the molecule observed in the crystal of 1 is forced
by steric repulsion between two large substituents in the adjacent
3 and 4 positions of the triazole ring. In the crystal structure the
molecules related by the 21 symmetry axis are joined in molecular
chains parallel to the Y crystallographic axis by N1AH11Á Á ÁO5 (1
126.36(18)
111.29(12)
123.19(14)
125.52(15)
128.49(15)
127.52(15)
103.99(11)
115.28(13)
108.48(18)
129.0(2)
122.40(17)
109.7(3)
107.5(2)
107.4(3)
Torsion angles
N2AC3AC6AC7
C3AC6AC7AN8
C5AN4AC21AC22
C23AC24AO27AC28
C24AO27AC28AC29
À98.5(2)
À101.43
76.05
84.01
À103.53
75.54
61.14
85.2(2)
52.12(19)
174.35(16)
À176.90(13)
À178.13
À179.19
178.93
179.87
= 2.87(4) Å and C28AH282Á Á ÁCg1 = 125(2)°; Cg1 is the centroid of
the pyrrole (x, 1 + y, z) ring.
Gaussian03 was used to optimize the geometry of the investi-
gated compound on the level of Hartree–Fock and B3LYP DFT with
6-31G(d,p) basis set. Some selected computed geometrical param-
eters along with the X-ray analysis parameters are listed in Table 4.
À x, À1/2 + y, Àz) hydrogen bond: N1AH11
H11Á Á ÁO5 = 1.88(3) Å, N1Á Á ÁO5=2.810(2) Å, NAH11Á Á ÁO5=166(2)°.
Additionally, the intermolecular CAHÁ Á Á contact involving the
pyrrole ring is observed with C28Á Á ÁCg1=3.575(3)Å, H282Á Á ÁCg1
=
0.95(3) Å,
p

M. Pitucha et al. / Journal of Molecular Structure 994 (2011) 313–320
317
Fig. 2. A view of compound 1 showing the atom-numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level.
The computed structures were aligned with the X-ray structure
with root mean square deviation (RMSD, not taking into consider-
ation hydrogen atoms) of 0.4242 Å and 0.2592 Å for HF and B3LYP
DFT, respectively (Fig. 4). The good accordance between experi-
mental and computed data is also indicated by correlation coeffi-
cients, R2. These are 0.9896 and 0.9824 for bond lengths and
0.9910 and 0.9865 for bond angles for HF and B3LYP DFT, respec-
tively. The discrepancies between the experimental and computed
results may be explained by the fact that the theoretical calcula-
tions refer to the gaseous phase whereas the X-ray results refer
to the solid state in which the existence of the crystal field along
with the intermolecular interactions which have connected the
molecules together changes the bond lengths and angles [63].
Although in general DFT-based geometry should be a better
approximation of the experimental geometry due to inclusion of
electron correlation, in the case of the investigated compound this
trend is reversed as indicated by the correlation coefficients given
above. The largest difference between experimental and computed
bond length was 0.035 Å for the HF method and 0.031 Å for B3LYP
DFT whereas the values for bond angles were 2.32° and 2.48°,
respectively. Much greater discrepancies are observed in the values
of torsion angles which are in the range of 3–30°. Thus, in order to
define the preferred position of the methylpyrrol group to the
1,2,4-triazole system, and the ethoxy substituents to the phenyl
ring, conformational analysis was performed on the five degrees
of conformational freedom: N2AC3AC6AC7, C3AC6AC7AN8,
C5AN4AC21AC22, C23AC24AO27AC28 and C24AO27AC28AC29
at the molecular mechanics level with the MOE Molecular Operat-
ing Environment [52]. Molecular energy profiles referring to the
rotations about the selected torsion angles are depicted in Fig. 5.
According to the results the low energy domains for
N2AC3AC6AC7 are located at À110° and 30°, while they are
located at À180° to À140° and at 75° for the torsion angle C3AC6
AC7AN8. Next, the low energy domains for C5AN4AC21AC22 are
located at À130° and 45°, while they are located at À180° and 180°
for the torsion angles C23AC24AO27AC28 and C24AO27AC28A
C29. The results of conformational analysis are in good accordance
with the results of the X-ray analysis and the quantum chemical
calculations for 1.
To characterize the structure of 1 in more detail, the Mulliken
atomic charges and natural population analysis (NPA) atomic
charges for the non-hydrogen atoms of the title compound calcu-
lated at HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels were calcu-
lated and collected in Table S1 in Supplementary Information.
Negative charge is accumulated on heteroatoms (nitrogen and oxy-
gen atoms) whereas the largest positive charge is accumulated on
C3 and C5 carbon atoms.
3.3. Theoretical calculations of keto–enol tautomerism
The energy for both the keto and enol tautomers of 1–6 was cal-
culated for isolated molecules (gas phase) and the molecules in
chloroform and water solutions. The results of the calculations
are presented in Table 5.
It can be seen from Table 5, that the keto form is the one exist-
ing in the crystalline state and this form is the most energetically
stable in the analyzed solutions. The population of the enol form
estimated using non-degenerate Boltzman distribution is less than
0.01% in all considered states. The energy differences between the
keto and enol forms change in the relatively narrow range of 8.19–
22.30 kcal/mol for all investigated molecules 1–6 and do not
depend on the type of substituent in 4 position of 1,2,4-triazole
ring and polarity of solvent. The results of the calculations are in
good agreement with the experimental data obtained from X-ray

318
M. Pitucha et al. / Journal of Molecular Structure 994 (2011) 313–320
Fig. 3. The molecular packing of 1, viewed down the a axis. Dashed lines indicate intermolecular hydrogen bonds (symmetry code: (i) =1 À x, À1/2 + y, Àz).
Fig. 4. The alignment of the X-ray and computed (A–HF, B– 3LYP DFT) structures of the investigated compound.
analysis and IR spectroscopy in KBr but do not confirm the shift of
the tautomeric equilibrium towards the enol form in a chloroform
solution.
The obtained results enrich the knowledge about structural
chemistry of 1,2,4-triazol-5-ones and in this manner are important
for basic research. It is worth stressing that only few studies have

M. Pitucha et al. / Journal of Molecular Structure 994 (2011) 313–320
319
Fig. 5. Molecular energy profiles of the investigated compound against the selected degrees of torsional freedom: AAN2AC3AC6AC7; BAC3AC6AC7AN8;
CAC5AN4AC21AC22; DAC23AC24AO27AC28 and EAC24AO27AC28AC29 The red line denotes the final conformer obtained with the B3LYP DFT method.
been reported for similar systems [29,40]. However, as it has been
already mentioned, the significance of the obtained results on the
keto–enol tautomerism of compounds 1–6 is mainly connected
with their pharmacological activity. Determination that the keto
form of 1–6 is a privileged one was a prerequisite for future SAR
and molecular docking studies.
Table 5
The stabilization energy
DE (kcal/mol) for the tautomeric forms of 1–6.
Compound Form
R
Gas phase
Chloroform
Water
RHF/
SCF
DFT
RHF/
SCF
DFT
RHF/
SCF
DFT
D
E
D
E
D
E
D
E
D
E
D
E
1
2
3
4
5
6
Ketone
Enol
Ketone
Enol
Ketone
Enol
Ketone
Enol
Ketone
Enol
Ketone
Enol
0
0
0
0
0
0
4. Conclusions
21.05
0
16.16
0
19.68
0
20.51
0
18.29 19.34
0
17.03 15.41
0
19.38 17.86
0
20.16 18.54
0
16.90 18.58
0
15.63 15.03
0
17.36 17.02
0
18.08 17.53
0
16.14
0
8.19
0
16.47
0
17.04
0
0
0
The results of experimental (X-ray, IR spectra) and computa-
tional studies indicate that, in the gas phase and in the crystalline
state, all the investigated molecules exist in the keto form while in
the chloroform solution a small amount of the enol form in the
equilibrium with the keto form is observed. The results of the
above studies are of crucial for chemistry of 1,2,4-triazol-5-ones
as well as for future SAR and docking studies for this series of
compounds.
0
0
0
0
0
0
22.30
0
15.53
22.21 21.58
0
15.54 14.86
21.37 21.41
0
14.77 14.52
20.96
0
17.98
0
0

320
M. Pitucha et al. / Journal of Molecular Structure 994 (2011) 313–320
[28] T.I. Vakulskaya, L.I. Larina, N.I. Protsuk, V.A. Lopyrev, Mag. Res. Chem. 47
Acknowledgments
(2009) 716.
[29] M.D. Davari, H. Bahrami, Z.Z. Haghighi, M. Zahedi, J. Mol. Model. 16 (2010) 841.
[30] N.R. Jadeja, R.N. Shirsat, E. Suresh, Struct. Chem. 16 (2005) 515.
[31] D. Ruiz, J. Giussi, A. Albesa, M. Schiavoni, J. Furlong, P. Allegretti, Spectrochim.
Acta A Mol. Biomol. Spectrosc. 77 (2010) 485.
[32] V. Lacerda Jr., M.G. Constantino, G.V.J. da Silva, Á. Cunha Neto, C.F. Tormena, J.
Mol. Struct. 828 (2007) 54.
[33] A.K. Samanta, P. Pandey, B. Bandyopadhyay, T. Chakraborty, J. Mol. Struct. 963
(2010) 234.
[34] H. Yüksek, O. Gürsoy, I. Cakmak, M. Alkan, Mag. Res. Chem. 43 (2005) 585.
[35] L. Xu, G. Fang, X. Li, J. Yuan, X. Hu, W. Zhu, H. Xiao, G. Ji, J. Mol. Graph. Model.
26 (2007) 415.
This article was partially prepared during the postdoctoral
fellowship of Agnieszka A. Kaczor, funded by the Foundation for
Polish Science (FNP, Kulomb outgoing fellowship). Calculations
with Gaussian03 were performed under a computational grant
by Interdisciplinary Center for Mathematical and Computational
Modeling (ICM), Warsaw, Poland, Grant No. G30-18. We thank
Grzegorz Bakalarski, MSc, from the ICM for his help with
Gaussian03 software. We also thank Prof. dr Hab. Zofia Rza˛czyn´ ska
from the Department of General and Coordination Chemistry of
Maria Curie-Skłodowska University, Lublin, Poland for access to
the GaussView software.
[36] H. Yüksek, M. Alkan, I. Cakmak, Z. Ocak, S. Bahçeci, M. Calapog˘lu, M. Elmastas,
A. Kolomuç, H. Aksu, Int. J. Mol. Sci. 9 (2008) 9, 12.
[37] H. Yüksek, M. Alkan, S. Bahçeci, I. Cakmak, Z. Ocak, H. Baykara, O. Aktas, E.
Agyel, J. Mol. Struct. 873 (2008) 142.
[38] H. Ma, H. Xiao, J. Song, X. Ju, W. Zhu, K. Yu, Chem. Phys. 344 (2008) 79.
´
[39] M. Pitucha, P. Borowski, Z. Karczmarzyk, A. Fruzinski, J. Mol. Struct. 919 (2009)
170.
Appendix A. Supplementary material
[40] N. Süleymanoglu, R. Ustabas, Y. Bingöl Alpaslan, Y. Ünver, M. Turan, K. Sancak,
J. Mol. Struct. 989 (2011) 101.
[41] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J. Appl. Crystallogr. 26
(1993) 343.
[42] G.M. Sheldrick, SHELX97 (Includes SHELXS97, SHELXL97, CIFTAB), Programs
for Crystal Structure Analysis (Release 97–2), University of Göttingen,
Germany, 1997.
[43] G.M. Sheldrick, SADABS (Ver. 2.06), University of Göttingen, Germany, 2002.
[44] H.D. Flack, Acta Crystallogr. A39 (1983) 876.
[45] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[46] M. Nardelli, Comput. Chem. 7 (1993) 95.
[47] A.L. Spek, J. Appl. Crystallogr. 36 (2003) 7.
[48] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2011.03.041.
References
[1] K. Zborowski, A. Korenova, M. Uher, M.L. Proniewicz, J. Mol. Struct. (Theochem)
683 (2004) 15.
[2] J. Nawrot-Modranka, E. Nawrot, J. Graczyk, Eur. J. Med. Chem. 41 (2006) 1301.
[3] D. Matosiuk, S. Fidecka, L. Antkiewicz-Michaluk, I. Dybała, A.E. Kozioł, Eur. J.
Med. Chem. 36 (2001) 783.
[4] D. Matosiuk, S. Fidecka, L. Antkiewicz-Michaluk, J. Lipkowski, I. Dybala, A.E.
Koziol, Eur. J. Med. Chem. 37 (2002) 761.
[5] D. Matosiuk, S. Fidecka, L. Antkiewicz-Michaluk, I. Dybala, A.E. Koziol, Eur. J.
Med. Chem. 37 (2002) 845.
[6] K. Sztanke, S. Fidecka, E. Kedzierska, Z. Karczmarzyk, K. Pihlaja, D. Matosiuk,
Eur. J. Med. Chem. 40 (2005) 127.
[7] C. Temperini, A. Cecchi, A. Scozzafava, C.T. Supuran, J. Med. Chem. 52 (2009)
322.
[8] H. Kumar, S.A. Javed, S.A. Khan, M. Amir, Eur. J. Med. Chem. 43 (2008) 2688.
[9] M. Moise, V. Sunel, L. Profire, M. Popa, J. Desbrieres, C. Peptu, Molecules 14
(2009) 2621.
[10] H. Bayrak, A. Demirbas, N. Demirbas, S.A. Karaoglu, Eur. J. Med. Chem. 44
(2009) 4362.
[11] M.A. Al-Omar, Molecules 15 (2010) 502.
[12] W.A. El-Sayed, O.M. Ali, S.R. El-Dakkony, A.A. Abdel-Rahman, Z. Naturforsch.
65c (2010) 15.
[13] R.C. Sanjeeva, R.L. Sanjeeva, K.G. Rajesh, A. Nagaraj, Chem. Pharm. Bull. 58
(2010) 1328.
[14] R.K. Singla, B.G. Varadaraj, J. Enzyme. Inhib. Med. Chem. 25 (2010) 696.
[15] E. Dogdas, B. Tozkoparan, F.B. Kaynak, L. Eriksson, E. Kuepeli, E.E. Yesilada, M.
Ertan, Arzneim-Forsch. 57 (2007) 196.
[16] G. Aktay, B. Tozkoparan, M. Ertan, J. Enzyme Inhib. Med. Chem. 24 (2009) 898.
[17] B. Jiang, Y. Zeng, M.J. Li, J.Y. Xu, Y.N. Zhang, Q.J. Wang, N.Y. Sun, X.M. Wu, Arch.
Pharm. 343 (2010) 500.
[18] V. Vivet-Boudou, J. Paillart, A. Burger, R. Marquet, Nucleosides, Nucleotides
Nucleic Acids 26 (2007) 743.
[19] A. El-Essawy, W.A. El-Sayed, S.A. El-Kafrawy, A.S. Morshedy, A.H. Abdel-
Rahman, Z. Naturforsch. 63c (2008) 667.
[20] R. Lesyk, O. Vladzimirska, S. Holota, L. Zaprutko, A. Gzella, Eur. J. Med. Chem. 42
(2007) 641.
[49] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[50] C. Lee, W. Yang, R.G. Parr, Phys. Rev., B 37 (1988) 785.
[51] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery, T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,
J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O.
Yazyev, A.J. Austin; R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian 03 (Revision C.02) Gaussian, Inc., Wallingford CT, 2004.
[52] MOE Molecular Operating Environment 2009.10, Chemical Computing Group.
[53] S. Miertuš, J. Tomasi, Chem. Phys. 65 (1982) 239.
[54] R.I.I. Dennington, T. Keith, J. Millam, GaussView (Version 4.1.2.), Semichem Inc,
Shawnee Mission, KS, 2007.
[55] A. Frisch, R.I.I. Dennington, T. Keith, J. Millam, A.B. Nielsen, A.J. Holder, J.
Hiscocks, GaussView Reference (Version 4.0.) Gaussian Inc, Pittsburgh, 2007.
[56] A.P. Scott, L. Random, J. Phys. Chem. 100 (1996) 16502.
[57] Vibrational frequency scaling factors. <http://cccbdb.nist.gov/vsf.asp>.
[58] E.F. Pettersen, T.D. Goddard, C.C. Huang, G.S. Couch, D.M. Greenblatt, E.C.
Meng, T.E. Ferrin, J. Comput. Chem. 25 (2004) 1605.
[59] C.F. Macrae, I.J. Bruno, J.A. Chisholm, P.R. Edgington, P. McCabe, E. Pidcock, L.
Rodriguez-Monge, R. Taylor, J. van de Streek, P.A. Wood, J. Appl. Crystallogr. 41
(2008) 466.
[60] A. Pedretti, L. Villa, G. Vistoli, J. Comput. Aided Mol. Des. 18 (2004) 167.
[61] E. Krieger, G. Vriend, Bioinformatics 18 (2002) 315.
[62] The PyMOL Molecular Graphics System, Version 0.99, Schrödinger, LLC.
[63] N. Özdemir, M. Dinçer, A. Çukurovalı, O. Büyükgüngör, J. Mol. Model. 15 (2009)
1435.
[21] K. Sztanke, T. Tuzimski, J. Rzymowska, K. Pasternak, M. Kandefer-Szerszen´ , Eur.
J. Med. Chem. 43 (2008) 404.
[22] A. Pachuta-Stec, J. Rzymowska, L. Mazur, E. Mendyk, M. Pitucha, Z. Rza˛czyn´ ska,
Eur. J. Med. Chem. 44 (2009) 3788.
[23] M. Dobosz, M. Pitucha, I. Dybała, A.E. Kozioł, Collect. Czech. Chem. Commun.
68 (2003) 792.
[64] G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093.
[65] A.A. El-Azhary, A.A. Al-Kahtani, J. Phys. Chem. A 109 (2005) 4505.
[66] F.H. Allen, O. Kennard, D. G. Watson, L. Brammer, A.G. Orpen, R. Taylor, Typical
interatomic distances: organic compounds, in: International Tables for
Crystallography, Vol. C, Kluwer Academic Publishers, Dordrecht/Boston/
London, 1995, pp. 685.
[67] C.H. Stam, H.C. van der Plas, Acta Crystallogr. B32 (1976) 1288.
[68] U. Corouh, B. Kahveci, S. Sasmaz, E. Agar, Y. Kim, A. Erdönmez, Acta Crystallogr.
C59 (2003) o47.
[24] M. Dobosz, A. Maliszewska-Guz, M. Struga, Z. Kleinrok, E. Wielosz-
Tokarzewska, E. Jagiełło-Wójtowicz, Acta Pol. Pharm. 58 (2001) 35.
[25] M. Wujec, M. Pitucha, M. Dobosz, Heterocycles 68 (2006) 779.
´
[26] M. Pitucha, B. Polak, R. Swieboda, U. Kosikowska, A. Malm, Z. Naturforsch. 64b
(2009) 570.
[27] M. Pitucha, A. Chodkowska, R. Duda, E. Jagiełło-Wójtowicz, Z. Naturforsch. 64c
(2009) 615.