Infrared spectrum, molecular structure and theoretical calculation of 2-pyridone-6-carboxylic acid

Article abstract of DOI:10.1016/j.molstruc.2006.10.045

The X-ray and infrared spectroscopic analysis of 2-pyridone-6-carboxylic acid are reported. The crystals of investigated molecule belong to P2₁/c of the monoclinic system, a = 11.714 ?, b = 3.7088?, c = 18.223? and β = 123.71°. The molecule is found in the ketonic form. Comprehensive studies of the molecular structures and vibrational frequencies and infrared intensities of the molecule have been performed by using Hartree-Fock, density functional B3LYP and second-order Moller-Plesset MP2 methods with the 6-31G+(d, p) basis set. The calculated geometrical parameters of investigated molecule in gas phase were compared with the experimental X-ray data.

Full text of DOI:10.1016/j.molstruc.2006.10.045

Journal of Molecular Structure 839 (2007) 76–83
www.elsevier.com/locate/molstruc
Infrared spectrum, molecular structure and theoretical calculation
of 2-pyridone-6-carboxylic acid
a
b
b
c
d,
*
H. Arslan , A. Sßeng u¨ l , S. Ayg u¨ n , N. Karadayı , S. Haman Bayarı
a
Department of Physics, Zonguldak Karaelmas University, 67100 Zonguldak, Turkey
Department of Chemistry, Zonguldak Karaelmas University, 67100 Zonguldak, Turkey
b
c
Department of Physics, Ondokuz Mayıs University, 55159 Samsun, Turkey
Department of Physics Education, Hacettepe University, Beytepe, 06800 Ankara, Turkey
d
Received 27 July 2006; received in revised form 19 October 2006; accepted 24 October 2006
Available online 26 December 2006
Abstract
The X-ray and infrared spectroscopic analysis of 2-pyridone-6-carboxylic acid are reported. The crystals of investigated molecule
˚
˚
˚
belong to P2 /c of the monoclinic system, a = 11.714 A, b = 3.7088A, c = 18.223A and b = 123.71ꢀ. The molecule is found in the ketonic
1
form. Comprehensive studies of the molecular structures and vibrational frequencies and infrared intensities of the molecule have been
performed by using Hartree–Fock, density functional B3LYP and second-order Moller–Plesset MP2 methods with the 6-31G+(d,p)
basis set. The calculated geometrical parameters of investigated molecule in gas phase were compared with the experimental X-ray data.
ꢁ
2006 Elsevier B.V. All rights reserved.
Keywords: Molecular structure; X-ray; Infrared; Theoretical calculation; 6-Hydroxypicoline; 2-Pyridone-6-carboxylic acid
1
. Introduction
keto–enol tautomerisation has long been of much interest
because such interactions may catalyze certain biochemical
transformations [6]. Adjacent donor and acceptor groups
within the molecules are known to form rigid dimers in
the solid state. The PCA possesses complementary hydro-
gen-bonding sites that are adjacent to one another; the
N–H and the acidic O–H groups as donors and the carbon-
yl C@O and the ketonic C–O groups as acceptors. A sim-
ilar tautomerism occurs for 2-hydroxynicotinic acid and
2-mercaptonicotinic acid [7] and for 6-methyl-2-pyridone
[5]. The tautomerisation in favor of pyridine form is greatly
enhanced by the electron realizing effect of the OH group
due to the basicity of the hetero aromatic nitrogen atom
[5,8,9]. Supramolecular synton as an important concept
in crystal engineering introduced by Desiraju, is typically
applied to non-covalent interactions, such as hydrogen
bonds [10]. Supramolecular synthons are the recognition
motifs between self-complementary constituent functional
groups (or building blocks) that can be used to propagate
networks or supramolecular assemblies [11]. Hydrogen
bonding is a directional interaction essential for a great
Picolinic acid (PA) is a very interesting model, as it is a
biologically important ligand incorporated into some
enzymes and its molecule is an active agent in some drugs
[
1–3].
The ligands possessing oxygen atom or nitrogen and
oxygen atoms are of great interest in the field of supramo-
lecular chemistry [4]. 6-Hydroxypicolinic acid is character-
ized by enol-ketonic tautomerism [4] (ketonic form can be
namely as 2-pyridone-6-carboxylic acid: PCA) as shown in
Scheme 1.
Keto–enol tautomerism has been the subject of continu-
ous interest in chemistry. The electronic effects and the
position of the substituents, pH and the solvent polarity
are the major factors that influence this tautomerism [5].
In particular, the proton transfer (PT) associated with
*
Corresponding author. Tel.: +90 312 297 86 06; fax: +90 312 297 86
00.
E-mail address: bayari@hacettepe.edu.tr (S.H. Bayarı).
0
022-2860/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.10.045

H. Arslan et al. / Journal of Molecular Structure 839 (2007) 76–83
77
a donor-rich molecule, prevents the pyridine–carboxylic
acid interactions and serves as template in assembling the
molecules around it with different packing motif [20]. On
the other hand, in the absence of water molecule, the title
compound exists in the tautomeric equilibrium with the
oxo tautomer, the latter being preferred in the solid state.
Two molecules are connected by two N–HÁ Á ÁO hydrogen
bonds around an inversion centre to form cyclic dimer,
and the dimers are linked in a planar fashion by O–
HÁ Á ÁO hydrogen bonds to form a sheet [21]. The carbonyl
O atoms do not take part in hydrogen bonding. Engineer-
ing the carboxylic acid dimer clearly requires the absence of
successful competitors (i.e. pyridine N atom or water mol-
ecule). A very recent study on carboxyl donors shows that
recognition of COOH with pyridine is favored 10 times
more through an O–HÁ Á ÁN hydrogen bond compared to
dimer O–HÁ Á ÁO [18]. For example, in the crystal structure
of 5-amino-2-benzoic acid, two identical acid molecules
form A–A type homodimer in the absence of any compet-
itor [22].
Due to its small size and excellent H-bond capability,
water plays a critical role in many of the structures in which
it is found [23]. These strong H-bonds play important role
in chemical and enzymatic interaction and are, therefore,
subject of great current interest to shed lights on the influ-
ence of the hydration on the stability of the tautomers in
different media [23].
To perform this study, we have used both an experimen-
tal and theoretical approach. The single crystal X-ray
structure of the 2-pyridone-6-carboxylic acid (PCA) has
been determined.
OH
OH
HO
N
O
N
H
O
O
enol form
ketonic form
Scheme 1.
number of supramolecular constructions. In particular, the
conventional hydrogen bonds of the type O–HÁ Á ÁN,
À1
N–HÁ Á ÁO (energies 20–40 kJ mol ) usually termed as the
strong ones, dictate the molecular assembling in the studied
class, and also prevail in drug and biological molecules
[
12–14]. The PT forces molecules to produce hydrated
structures involving intermolecular H-bonds between the
water molecules and the –COOH groups of the adjacent
molecules to assemble supramolecular networks via hydro-
gen-bonded dimer synthon [15]. Hydrogen bond pattern of
PCA in the solid state is given in Scheme 2. The coordinat-
ed water molecules and the amido groups of PCA can act
as hydrogen bond donors, while the carboxylate groups
and the carbonyl oxygen atoms of PCA can act as hydro-
gen bond acceptors.
In general, water prefers to accept hydrogen bonds from
OH and N–H donors in macromolecular structures [16].
–
The O–H group of the carboxylic acid is a very strong
donor but a very poor acceptor, whereas the water mole-
cule is a good acceptor but only a moderate donor [17].
The large difference in electronegativity between the H
atom and O atoms makes the O–H bonds of water mole-
cule inherently polar, with partial atomic charges of
around +0.4 on each H atom and À0.8 on each O atom
The FT-IR spectrum of sample (crystal form) was
recorded. The theoretical calculations were performed for
isolated molecule using the HF, MP2 and DFT methods.
A general assignment of the fundamental vibrational
modes was proposed on the basis of the calculated results
of ketonic form. DFT calculation of the PCA dimer also
performed in this paper and is compared with the mono-
mer PCA.
[
18]. So, the inclusion of water molecules opens the possi-
bility for a strong O–HÁ Á ÁOw motif due to formation of
hydrogen bonding in a hierarchical fashion: best donor to
best acceptor; second-best donor to second-best acceptor,
etc [19]. The present supramolecular motif accords well
with this criterion, in which the water molecule donates
hydrogen bonds to the dimer through O –HÁ Á ÁO
and
oxo
w
O –HÁ Á ÁO
and accepts hydrogen bond from the
carbonyl
w
2. Experimental
adjacent dimer via O –HÁ Á ÁO . The inclusion of water,
acid
w
2.1. Preparation of crystal
O
6
-Hydroxypicolinic acid was purchased from Aldrich. It
N
H
O
was dissolved in an ammonia solution and set to reflux for
1 h. After evaporation of the solution to dryness under
vacuum, the brown residue was crystallized in an etha-
nol–water (1:1 v/v) solvent mixture to form needle shape
colorless crystals of suitable for X-ray analysis [24].
O
H
O H
Ow
O
H
H
N
H
O
O
H
H
N
O
O
H
O
2.2. X-ray analysis
O
H
H
Ow
H
O
O
N
X-ray diffraction data were collected on a STOE IPDS
O
II diffractometer with graphite monochromatized MoKa
˚
radiation (k = 0.71073 A) at 296 K. A summary of crystal-
Scheme 2.
lographic data, experimental details and refinement results

78
H. Arslan et al. / Journal of Molecular Structure 839 (2007) 76–83
Table 1
Crystal data and structure refinement for 2-pyridone-6-carboxylic acid
Empirical formula
Formula weight
Crystal system
Space group
C
6
H
7
NO
157.13
Monoclinic
P 2 /c
4
1
˚
Unit cell dimensions (A, ꢀ)
a = 11.714(2)
b = 3.7088(7)
c = 18.223(4)
a = 90.00
b = 123.711(13)
c = 90.00
658.6(2)
˚
3
Cell volume, (A )
Z (molecule/cell)
Density calculated (mg m
4
À3
)
1.585
F
000
328
0.135
À1
Absorption coefficient (mm
Crystal size (mm )
Temperature (K)
Index ranges
h range (ꢀ)
Absorption correction
Reflections collected and observed (>2sigma)
Refinement method
)
3
0.37 · 0.17 · 0.06
296(2)
À146h 613, À46k64,À176l621
1.75 to 228.79
Integration
3957 and 2460
2
Full-matrix least-squares on F
Data/restrains/parameters
1194 / 3 / 106
0.0484, 0.1181
0.849
0.193 and À0.206
R,R
Goodness-of-fit on F
Largest diff. Peak and hole (e/ A
w
[I > 2sigma(I)]
2
˚
À3
)
for 2-pyridone-6-carboxylic acid is given in Table 1. For
compound, the H atoms were refined using a riding model,
expression (iii) second-order Moller–Plesset MP2. The cal-
culation was carried out with one functional base: 6–
31G+(d,p). All theoretical calculations were performed
using the Gaussian 03 W package of programs [28] running
on a PC. Minimum-energy structure of the PCA dimer was
also calculated with the DFT using the B3LYP functional
and the large 6-311G(2d,2p) basis set.
˚
with fixed C–H distances of 0.93 A [U (H) = 1.2U ] and
iso
eq
˚
N–H distance of 0.82 A [U (H) = 1.2U ] and O–H dis-
iso
eq
˚
tance of 0.82 A [U (H) = 1.5U ]. The H atoms of water
was refined freely [O–H = 0.842(18)–0.845(18) A and
iso
eq
˚
U (H) = 1.5U ].
iso
eq
Data collection: X-AREA [19]; cell refinement: X-AR-
EA; data reduction: X-RED [25] program(s) used to solve
structure: SHELXS97 program(s) used to refine structure:
SHELXS97 [26]; molecular graphics: ORTEP-3 [27]. The
crystallographic information file is deposited in the
Cambridge Crystallographic Data Center as CCDC
3. Results and discussion
3.1. Description of the structure
The molecular structure of PCA with the atom number-
ing is shown in Fig. 1. The compound crystallizes in the
6
14189.
space group P2 /c with four molecules in the unit cell.
1
2
.3. Spectroscopic measurements
Two strong N–HÁ Á ÁO bonds link identical keto-amino acid
molecules to form a dimer homosynthon (Scheme 2). The
inclusion of water molecules in the packing pattern fulfill
the hydrogen bonding requirements in a hierarchical way
to build up a supramolecular herringbone structure in the
solid state (Fig. 2). The geometry parameters of these
hydrogen bonds are listed in Table 2.
The internal geometry of the PCA molecule in the crys-
tal conform the predominant form of the keto tautomer in
which the phenolic OH group has become deprotonated
and the ring N atom has become protonated as shown in
Fig. 1. The hydrogen bonding and an increase in resonance
energy as a result of the H-bonded dimers should facilitate
the PT process. The role of resonance assisted bonding
(RAHB) in strengthening intermolecular N–HÁ Á ÁO=C
bonds in a number of small molecule crystal structures with
The FT-IR spectrum of crystal form of the title com-
À1
pound in the region 400–4000 cm
2
were recorded at
À1
cm resolution on a Mattson 1000 spectrometer using
KBr pellets.
.4. Theoretical methods
The geometric parameters of monomeric PCA in the cal-
2
culations were employed from crystal structure data. To
calculate optimized geometric structure, the harmonic fre-
quencies and infrared intensities, three quantum mechani-
cal methods were used: (i) Hartree–Fock (HF), (ii) the
density functional theory (DFT) hybrid method B3LYP
with non-local correlation provided by the Lee–Yang–Parr

H. Arslan et al. / Journal of Molecular Structure 839 (2007) 76–83
79
Table 2
Hydrogen bonds distances (A) and angles (ꢀ) in PCA
˚
D–H. . .A
d(D-H)
d(H. . .A)
d(D. . .A)
<(DHA)
#
1
O2–H2A. . .O4
0.820
0.860
0.842(18)
0.845(18)
1.74
2.04
2.09(2)
1.94(2)
2.554(3)
2.882(3)
2.911(4)
2.783(3)
171.8
164.5
165(4)
173(3)
#
2
N1–H1. . .O1
O4–H4A. . .O3
#
3
O4–H4B. . .O1
Symmetry transformations used to generate equivalent atoms. #1,Àx,
y+1/2, Àz+1/2; #2, Àx+1, Ày+2, Àz+1; #3, Àx+1, Ày+1, Àz+1.
Here the water molecule is a double proton donor and a
single proton acceptor. So, it should play an important role
for the stabilization of the dimer via O–HÁ Á ÁO hydrogen
bonding to the oxo group of the one tautomer and to the
carboxylic carbonyl group of the symmetry related comple-
mentary tautomer (head-to-head stacking dimer). It does
not only stabilize the dimers, but also links the adjacent
chains of the H-bonded dimers via accepting H-bond from
the O–H group of the neighboring PCA molecule. These
interactions link the dimers and the adjacent puckered
chains into the herringbone motif in the crystalline state
(Fig. 2).
The selected bond lengths in the PCA (with water) and
in the anhydrous form of PCA [21] are given in Table 3
for comparison. Both structures have similar bonding pat-
tern, but those in the hydrated form are slightly longer
(Table 3). This can be attributable to the stronger hydrogen
Fig. 1. ORTEP 3 drawing of 2-pyridone-6-carboxylic acid in with 40 %
probability displacement ellipsoids and the numbering system.
conjugated rings has been comprehensively discussed by
Bertolasi et al. [29]. The PCA molecules form dimers via
two symmetry-related N–HÁ Á ÁO hydrogen bonds (Table
2
). Neighboring dimers are then connected by a water mol-
ecule through short O–HÁ Á ÁO hydrogen bonds to yield a
puckered tape structure (Fig. 2).
Fig. 2. A packing diagram for 2-pyridone-6-carboxylic acid. Hydrogen bonds are indicated by dashed lines.

80
H. Arslan et al. / Journal of Molecular Structure 839 (2007) 76–83
Table 3
˚
The selected bond lengths (A), bond and torsion angles (ꢀ) for PCA
a
X-ray PCA H
2
O
X-ray PCA
HF 6-31+G(d,p)
B3LYP 6-31+G(d,p)
MP2 6-31+G(d,p)
˚
Bond lengths (A)
C1–C2
C1–C6
C5–O1
C6–O3
C1–N1
C4–C5
C5–N1
C6–O2
1.349(4)
1.495(4)
1.253(4)
1.203(4)
1.366(4)
1.428(4)
1.374(4)
1.306(4)
1.362(4)
1.489(4)
1.266(4)
1.210(4)
1.376(4)
1.426(4)
1.354(4)
1.312(4)
1.342
1.493
1.225
1.197
1.368
1.452
1.359
1.306
1.371
1.491
1.249
1.223
1.369
1.448
1.388
1.333
1.377
1.486
1.258
1.229
1.368
1.443
1.386
1.339
Bond Angles (ꢀ)
C2–C1–N1
O1–C5–N1
N1–C5–C4
O3–C6– C1
C1– N1–C5
C2–C1–C6
C1–C2–C3
C3–C4–C5
O1–C5–C4
O3–C6–O2
O2–C6–C1
120.5(3)
120.1(3)
115.0(3)
122.1(3)
123.8(3)
124.6(3)
118.4(4)
121.6(3)
124.8(3)
125.8(3)
112.1(3)
119.9(3)
120.0(3)
116.7(3)
121.1(3)
123.5(3)
120.7(3)
118.8(3)
120.3(3)
123.3(3)
125.4(3)
113.6(3)
120.9
120.7
115.1
122.0
124.4
124.1
117.8
120.8
124.2
124.5
113.5
120.3
120.2
114.5
122.6
124.8
124.6
118.2
121.2
125.3
124.3
113.1
120.2
120.3
114.3
122.7
125.0
124.7
118.1
121.4
125.4
124.7
112.5
Torsion angles (ꢀ)
N1–C1–C2–C3
C2–C1–C6–O2
N1–C1–C6–O2
N1–C1–C6–O3
C2–C1–N1–C5
O1–C5–N1–C1
À1.9(4)
À3.6(4)
177.6(3)
À2.3(4)
1.8(4)
—
—
—
—
—
—
0.0
0.1
0.0
À0.4
0.1
2.8
À179.9
179.7
À0.3
À177.7
0.1
2.3
0.0
À0.1
À0.1
À179.7(3)
À179.9
À179.9
À179.9
a
Taken from Ref. [21].
˚
bonds between the water molecule and PCA molecules
C5–O1 bond length of 1.253 (4) A is considerably shorter
than the C–O(H) bond distances noted in 6-chloro-2-
(
vide supra).
˚
˚
The C–O bond lengths in the carboxylic acid group
˚
and
hydroxypyridine (1.35 A) and 3-hydroxypyridine (1.32 A),
but very close to those in 6-methyl-2-pyridone (1.247 (6)–
1.270 (5) A) in which the carbonyl O atoms also involve
[
C6(@O3)–O2–H]
of
C6–O2 = 1.306
(4) A
˚
˚
C6@O3 = 1.203 (4) A (Table 3) are comparable with those
found for dipicolinic acid [30] in which the corresponding
bond lengths are 1.319 (2) and 1.217 (2) A, respectively,
H-bonding toward formation of catemer motif [5]. There
are differences in the C–C bond lengths within the ring
(Table 3), indicating that the aromaticity of the pyridine
ring is disturbed [5].
˚
and also conform to the average values tabulated in Inter-
national Tables for Crystallography [31] for an aromatic
˚
carboxylic acid in which C@O is 1.226 (20) A and C–O is
The geometry of PCA optimized at the HF/6-31G level
is given in Fig. 3. The optimized geometrical parameters of
the PCA molecule are reported in Table 3, together with
the experimental values obtained from the X-ray data. It
should be noted that the bond lengths of the isolated mol-
ecule calculated at the B3LYP/6-31G+ (d,p) level are in
very good agreement with the X-ray data. (Correlation
coefficients: CC: 0.9822, 0.9806 and 0.9775 for DFT/
B3LYP, HF and MP2 methods, respectively). The calculat-
ed bond angles of molecule are also in good agreement with
the experiment.
Fig. 4 presents the geometry of anhydrous PCA dimer
optimized at the B3LYP/6-311G (2d,2p) level (geometric
parameters are not given). It is compared with the experi-
mental data. A detailed comparison with the experiment
is not justified, because calculations correspond to an ideal
hydrogen-bonded dimer and experimental data were
˚
1
.305 (20) A. The corresponding bond lengths in the 5-ami-
˚
˚
no-2-nitrobenzoic acid are 1.283 (2) A and 1.225 (2) A [22].
The carboxylic acid groups pointing opposite sides in the
dimer participate in two strong H-bonds: while the C@O
accepts a H-bond from water molecule through O4–
˚
H4AÁ Á ÁO3 R = 2.911 (4) A and OHO angle 165 (4)ꢀ,
OO
the O–H group donates a H-bond to another water mole-
i
˚
cule through O2–H2AÁ Á ÁO4 ROO = 2.554 (3) A and
OHO angle 171.8 (3)ꢀ. The ring C@O groups of the each
tautomer are also H-bonded to water molecule through
iii
˚
O4–H4BÁ Á ÁO1 ROO = 2.783 (3) A and OHO angle 173
(
3)ꢀ.
The average C–N–C angle in pyridine molecules in the
CSD [24] is 117.3 (2)ꢀ, while the average for pyridinium
ion is 122.0 (2)ꢀ. The C1–N1–C5 angle of 123.8 (3)ꢀ
corresponds more closely to the protonated state. The ring

H. Arslan et al. / Journal of Molecular Structure 839 (2007) 76–83
81
atoms in the carboxylic group. Most of the deviations
observed are a consequence of interactions within the crys-
tal. The shorter bond is regarded as the carbonyl group,
whereas the longer one as the hydroxyl group. Then we
examined and identified all the plausible hydrogen-bond
donors and/or acceptors within the hydrogen-bonding dis-
tance of a carboxylic group. Next, we checked the distance,
bond angle and dihedral angles between the potential
hydrogen-bonding partners. A hydrogen bond is expected
˚
to meet all three criteria: <3.2 A for the hydrogen-bond
length, 109 ± 15ꢀ for the C–OÁ Á ÁX angle.
3.2. The observed and calculated fundamental modes of
vibrations
Fig. 3. The DFT/B3LYP/6-31G(d,p) optimized structure of 2-pyridone-
-carboxylic acid.
6
To our best knowledge, no complete experimental vibra-
tional data have been reported for hydroxypicolinic acids.
To our knowledge, there have also been no quantum
mechanical calculations on this molecule to examine the
proposed vibrational assignments. These calculations are
valuable for providing insight into the vibrational
spectrum.
We report in the present paper, the detailed vibrational
analysis of ketonic form of this molecule as isolated unit.
We also discuss the calculated infrared data of COOH
and NH groups for the dimer structure. In Table 4, the
wave numbers, experimental intensities and assignments
of the bands occurring in the infrared spectrum of the
monomer PCA are given. Table 4 also collects the calculat-
ed harmonic vibrational wave numbers of molecule by
means of the HF, the DFT (B3LYP) and MP2 approaches
with the 6-31G+(d,p) basis set, together with the experi-
mental data. The calculated (DFT/B3LYP/6-31G+(d,p))
and experimental infrared spectra of the molecule are
shown in Fig. 5.
It should be remembered that the comparison is between
an isolated molecule calculation and and experimental data
were obtained for the crystal. The predicted spectra are cal-
culated using the harmonic approximation; anharmonicity
and Fermi resonance are not included.
Comparison of the calculated and experimental results
indicates the density functional BLYP/6-31G+(d,p) method
is more accurate in predicting fundamental vibrational fre-
quencies than the Hartree–Fock and MP2 methods.
The detailed comparison at first does not seem to indi-
cate good agreement between the calculated and experi-
mental frequencies for the O–H, N–H and C@O
stretching modes. As discussed in Section 3.1, the differenc-
es are caused by the hydrogen bonding in the experimental
system. The wave numbers of the C–H stretching vibra-
tions are fairly stable, whereas the N–H stretching vibra-
tion is significantly environment dependent. There is a
great difference between calculated frequencies of the N–
H stretching mode in DFT/B3LYP (3378), HF (3574)
and MP2 (3389). The frequency is remarkably decreased
for dimer structure where the N–H bond is involved in
the Nl–H–O1 hydrogen bond. A weak band observed in
Fig. 4. Geometry of the 2-pyridone-6-carboxylic acid dimer, optimized at
the B3LYP/6-311G (2d, 2p) level.
obtained for the crystal. The calculated values reproduce
the experimental bond lengths and angles reasonably well.
The largest differences are for the bonds which involve
hydrogen atoms. The H-bond equilibrium distance for
˚
the B3LYP/6-311G (2d,2p) calculations is (NÁ Á ÁO) 2.79 A
in excellent agreement with the experimental value NÁ Á ÁO
˚
2
.873 A [21].
The agreement between the calculated and the experi-
mental angles is worse than for the bond lengths. The larg-
est discrepancies are for the angles which involve oxygen

82
H. Arslan et al. / Journal of Molecular Structure 839 (2007) 76–83
Table 4
À1
À1
Comparison of the experimental FT-IR wavenumbers (cm ) and the theoretical wavenumbers (cm ) of the 2-pyridone-6-carboxylic acid monomer (vs:
very strong, s: strong, m: medium, w: weak, vw: very weak, br: broad)
Assignment
Exp.
Monomer
HF 6-31G+(d,p)
DFT /B3LYP 6-31G+(d,p)
MP2 6-31G+(d,p)
m(OH)
m(NH)
m(CH)
m(CH)
m(CH)
m(C@O)
m(C@O)
3302
3278
3193
3161
3136
1735
1678
1634
1586
1476
1435
1381
1357
1275
1246
1178
1147
1080
1045
1011
941
3611
3574
3304
3278
3243
1835
1756
1719
1647
1536
1474
1417
1385
1317
1287
1206
1178
1106
1078
1061
1005
935
3306
3289
3245
3217
3194
1748
1697
1635
1598
1515
1455
1391
1377
1319
1188
1182
1148
1100
1067
1014
951
3209 w
3120 w
3095 m
3006 w
1715 vs
1636 vs
1596 vs
1536 vs
1433 s
1396 m
1331w
1295 s
1262 vs
1231sh
1164 m
1135 w
1067 w
1031 mw
999 ms
935 w
908 s
c
r
d(NH)+m (CC)
m(CC) +
m(CC)+d(NH)+d(CH)
d(CH))+d(NH)
m(C–O)+d(NH)+d(OH)
d(NH)+d(CH)
m(CN)+d(CH)
m(CC)+d(CH)
d(CH)+d(OH)
d(CH)+m(C-OH)
d(CH)+m(CC)
c(CH)
Ring breathing
c(CH)+c(NH)
c(CH)
913
922
c(NH)+c(C@O)
r
889 s
882
912
886
m(NC)+c(CH)+m(C–C–O)
822 s
824
852
827
c(CH)+c(COOH)
771 s
768
798
766
c
c
ring + c(CO)
ring + c(NH)+c(CH)
c
755 s
748
727
673
589
765
715
686
623
749
736
696
612
717 m
651 m
597 s
c(NH) +c(OH)
c(C@O)
c
+ cring
ring + c(OH) + c(CH)
c
558 s
576
611
589
s
ring
538 s
541
565
542
c (COOH)
476 w
455 w
423 w
502
458
415
315
529
469
430
392
487
436
386
344
c
s
s
s
ring + c(C@O)
ring + s(COOH)
r
ring
ring
190
208
190
À1
the IR spectrum of molecule at 3209 cm is assigned to the
NH stretching band of the ring.
groups. They showed a strong correlation between the
C@O stretching frequency and the hydrogen bond number
À1
À1
The assignment of in-plane d (NH) vibration is very dif-
ficult because of the presence of aromatic ring vibrations in
this region. We observed band at 1596 cm which can be
of a COOH group: 1759–1776 cm
for zero, 1733–
À1
1749 cm for one and 1703–1710 cm for two hydrogen
À1
À1
bonds (HO–C@O). We observed a band at 1715 cm in
largely assigned to the d(N–H) bending vibration mode of
ring. The predicted spectra for this mode showed strong
coupling between coordinates, N–H bending (mainly con-
tributed) and the ring stretching coordinates.
the infrared spectrum of sample. Therefore, we can say a
low C@O stretching frequency is an indication of two
hydrogen-bond.
À1
The band at 1636 cm is assigned to stretching vibra-
In the IR spectrum of the molecule, the band corre-
sponding to the OH group vibrations is absent. The pres-
tions m(C@O) of the ring. This observation also shows that
the molecule is keto form in agreement with X-ray results.
r
À1
ence of the stretching vibrations m(C@O) at 1715 cm
The wave number value of calculated (C@O) higher than
r
À1
and the in-plane d(NH) band at ca. 1596 cm in the IR
the experimental m(C@O) that would suggest that calcula-
r
spectrum of the molecule (Table 4) suggest that the keto
form predominates in the solid state.
Nea et al. [32] reported that the C@O stretching fre-
quency is a sensitive infrared structural sensor for detecting
and monitoring the number of hydrogen bonds of COOH
tions underestimate the strength of intermolecular interac-
tions in the structure. Such difference may be due to the
band anharmonicity.
The c OHÁ Á ÁO vibration exhibits a moderate band in the
À1
region 905 ± 65 cm . In the unbound state the c(OH)

H. Arslan et al. / Journal of Molecular Structure 839 (2007) 76–83
83
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We gratefully acknowledge support of this work by the
Turkish Scientific and Technical Research Council [Grant
No: TBAG-2450 (104T060)] and also by the Zonguldak
Karaelmas University Research Fund [Grant No: 2004-
(
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