Pyrrole-2-carboxylic acid and its dimers: Molecular structures and vibrational spectrum

Article abstract of DOI:10.1021/jp0211786

The infrared and Raman spectroscopic study of pyrrole-2-carboxylic acid (PCA) confirms the formation of the cyclic acid dimer species in the solid state. The molecular structure, vibrational frequencies, and binding energies of cyclic dimers have been also examined using the density functional theory (DFT) at the B3LYP/ 6-311+G(d) level. In addition, a complete vibrational assignment is proposed for the both s-cis and s-trans PCA conformers. The vibrational assignments are supported by normal coordinate calculations utilizing force constants predicted using the DFT method. The atoms in molecules theory of Bader is also used to characterize hydrogen bonds.

Full text of DOI:10.1021/jp0211786

J. Phys. Chem. A 2002, 106, 10613-10621
10613
Pyrrole-2-carboxylic Acid and Its Dimers: Molecular Structures and Vibrational Spectrum
Alina T. Dubis, Sławomir J. Grabowski,* Dorota B. Romanowska,†
†
,†,‡
Tomasz Misiaszek,§ and Jerzy Leszczynski
,|
|
Institute of Chemistry, UniVersity of Białystok, Al. J. Piłsudskiego 11/4, 15-443 Białystok, Poland,
Department of Crystallography and Crystallochemistry, UniVersity of Ł o´ d z´ , ul. Pomorska 149/153,
9
0-236 Ł o´ d z´ , Poland, Institute of Physical and Theoretical Chemistry, Wrocław UniVersity of Technology,
Wybrze z˘ e Wyspia n´ skiego 27, 50-370 Wrocław, Poland, and Computational Center for Molecular Structure and
Interactions, Department of Chemistry, Jackson State UniVersity, Jackson, Mississippi 39217
ReceiVed: May 9, 2002; In Final Form: August 29, 2002
The infrared and Raman spectroscopic study of pyrrole-2-carboxylic acid (PCA) confirms the formation of
the cyclic acid dimer species in the solid state. The molecular structure, vibrational frequencies, and binding
energies of cyclic dimers have been also examined using the density functional theory (DFT) at the B3LYP/
6-311+G(d) level. In addition, a complete vibrational assignment is proposed for the both s-cis and s-trans
PCA conformers. The vibrational assignments are supported by normal coordinate calculations utilizing force
constants predicted using the DFT method. The “atoms in molecules” theory of Bader is also used to
characterize hydrogen bonds.
Introduction
CHART 1: The Cyclic Dimers of Carboxylic Acids
One of the aims of crystal engineering is to understand
intermolecular interactions and their role in crystal architec-
1
,2
tures. It is well-known that hydrogen bonds play a dominant
3
role in forming molecular networks in crystals. There are a lot
retical methods. The PCA molecule contains not only a carboxyl
group but also the N-H proton donating bond. Hence R2 (8)
motifs that may exist due to carboxyl groups may be replaced
by other motifs owing to N-H donors; O-H‚‚‚O bonds between
PCA molecules may be replaced by N-H‚‚‚O interactions. Such
a situation is not novel in the crystal structures of organic
compounds. For example, the existence of motifs containing
4-6
of reports and reviews on H-bond patterns in organic crystals.
Carboxyl groups are known as those which are often responsible
for the arrangement of molecules in crystals. Molecular packing
modes of carboxylic acids have been studied in depth. These
studies on crystals of carboxylic acids were based on X-ray and
neutron diffraction results. Carboxyl groups act as proton donors
OH bonds) and as proton acceptors (CdO bonds); hence they
can form hydrogen bonds between each other as cyclic dimers
2
7
8
,9
(
N-H‚‚‚O bonds was detected for the crystal structure of methyl
14
5
-methyl-3,4-diphenyl-1H-pyrrole-2-carboxylate. It is well-
8
or as open arrays forming, for example, catamer motifs.
known that unlike the O-H‚‚‚O hydrogen bonds, there are no
Sinha et al.10 suggested that the combination of infrared and
Raman spectroscopy is an ideal method to establish whether
carboxylic acid molecules are hydrogen bonded as cyclic dimers
or whether they form other patterns. For example, the infrared
spectra of benzoic acid crystals have been demonstrated to show
3
examples of strong N-H‚‚‚O bonds. The experimental X-ray
diffraction or neutron diffraction data support this statement
because the N-H bond can be only slightly stretched by H-bond
formation from 1.01 to 1.06 Å, in contrast to O-H‚‚‚O
15
systems where for short H‚‚‚O contacts of 1.2-1.3 Å the O-H
1
1
the coexistence of two kinds of cyclic configurations. Ac-
cording to the Etter terminology the cyclic dimers of carboxylic
acids form the eight-membered ring (Chart 1) denoted as R2 (8).
This means that eight atoms (CdO‚‚‚HsOsCdO‚‚‚HsO)
form two H-bonds usually related through the center of
inversion. The superscript designates the number of acceptor
centers, and the subscript, the number of donors within the motif.
The R2 (8) motif often exists within crystal structures because
it may be formed not only by OsH‚‚‚O H-bonds but also by
other bonds as well. For example, for the crystal structure of
bond length may be 1.15-1.2 Å. The elongation of the donating
12
bond within H-bridges is often treated as a measure of H-bond
2
16
strength.
The spectroscopic investigations of pyrrole-2-carboxylic acid
17
have been performed earlier. On the basis of vibrational spectra
8
the possible arrangements of the molecules in crystals have been
predicted. As mentioned above, the studies of PCA conformers
and of the possible PCA dimers have been performed here using
FT-IR and Raman spectroscopic methods, ab initio calculations,
2
18
and the Bader atoms-in-molecules (AIM) theory.
2
the phenazine-3,5-dinitrobenzoic acid complex the R2 (8) motifs
exist for phenazine-acid dimers connected by CsH‚‚‚O and
Experimental Section
OsH‚‚‚N bonds.13
Pyrrole-2-carboxylic acid was prepared by hydrolysis19 of
methyl pyrrole-2-carboxylate, MPC (prepared by Bailey’s
method ). A sample of methyl pyrrole-2-carboxylate (0.8 mmol)
was dissolved in 10 mL of methanol. The mixture was treated
with a solution of 40 mmol of KOH in 2 mL of water and stirred
overnight at 50 °C. The solution was acidified and extracted
with ether. The extract was concentrated on a rotary evaporator,
The aim of this study is to investigate the conformations of
pyrrole-2-carboxylic acid (PCA) using spectroscopic and theo-
20
*
Corresponding author. E-mail address: slagra@krysia.uni.lodz.pl.
University of Białystok.
†
‡
§
|
University of Ł o´ d z´ .
Wrocław University of Technology.
Jackson State University.
1
0.1021/jp0211786 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/10/2002

1
0614 J. Phys. Chem. A, Vol. 106, No. 44, 2002
Dubis et al.
CHART 3: Structures of the Possible Cyclic Dimers of
CHART 2: Structures of Pyrrole-2-carboxylic Acid
Monomers: a) s-cis (syn) Conformer b) s-trans (anti)
Conformer
the Pyrrole-2-carboxylic Acid
and the obtained white solid was crystallized from methanol.
1
Mp: 204-208 °C. The structure was proved by H NMR and
FT-IR spectra.
N-Methylpyrrole-2-carboxylic acid was prepared by hydroly-
sis of methyl N-methylpyrrole-2-carboxylate according to the
procedure described for PCA.
The FT-IR spectra were recorded using a Nicolet Magna IR
550 Series II spectrometer. The sample was measured in CCl4
solution (0.001M) in a KBr cell of 0.2 mm path length, as a
KBr pellet and Nujol mull on KBr plates. A spectral resolution
-
1
of 4 cm was used.
The Raman spectrum was taken using a Jobin Yvone T64000
spectrograph equipped with a CCD camera cooled by liquid
+
nitrogen as a detector. The 514.5 mn excited line of an Ar
laser was used. A powder sample was measured.
Computational Details
All DFT calculations were carried out using the Gaussian 98
program.2 The geometry of conformers of pyrrole-2-carboxylic
acid (PCA) considered in this paper (s-cis and s-trans conform-
ers, Chart 2) has been optimized at the B3LYP/6-311+G(d)
and B3LYP/6-311++G(d,p) levels of theory. The geometry
optimizations have been performed for three PCA dimers (A,
B, and C; Chart 3) at the B3LYP/6-311+G(d) level. However,
in the case of dimers the optimizations were carried out with
symmetry restrictions. The equivalence of monomers forming
complexes was assumed. In other words, for each kind of dimer
two associated monomers are related by the center of inversion.
For the A and B dimers (Chart 3) the centers of inversion lie in
the middle of the eight-membered rings and hence the cen-
trosymmetric dimers are considered as is usual for dimers of
1
vibrational potential energy distribution (PED) among the
normal coordinates.
In addition, the atoms-in-molecules theory (AIM) of Bader18
was applied for the localization of bond critical points (BCPs),
which were analyzed in terms of electron densities and their
2
6
Laplacians.
Results and Discussion
Vibrational Spectra. The molecules of carboxylic acids
usually exist as centrosymmetric dimers in crystals with the
center of inversion within the eight-membered ring formed by
8
carboxylic acids in crystals. For the C dimer the center of
inversion lies within a ten-membered ring.
3,8
two carboxyl groups (Chart 1). The PCA molecule contains
H-bond energies were computed as the difference in energy
between the complex, on one hand, and the sum of isolated
monomers on the other hand. Basis set superposition error
OsH, CdO, and NsH groups and hence may generate the
formation of three types of hydrogen-bonded dimers (Chart 3).
For the A and B cases the complexes are linked through two
CdO‚‚‚HsO bonds and for the C dimers there are two
intermolecular interactions of NsH‚‚‚OdC type. The last type
of interaction is the so-called heterogenic association where the
carboxyl group is hydrogen bonded to another functional group.
(
BSSE) was corrected by the counterpoise procedure of Boys
22
and Bernardi. The IR spectra of monomers and dimers
considered in this study were also calculated by applying the
same codes of Gaussian 98.21 The DFT (B3LYP) approach was
used to predict the vibrational wavenumbers and intensities and
the corresponding optimized geometries of the molecules. The
predicted vibrational wavenumbers were scaled down by a single
factor of 0.98. The aforementioned method overestimates the
calculated harmonic frequencies, and for this reason, the scaling
factors have been proposed in the literature to correct for
2
2
For the A and B dimers there are R (8) motifs containing two
12
equivalent OsH‚‚‚O hydrogen bonds that are replaced by the
2
2
R (16) motif in the C dimer. It seems that the R (16) motif is
2
2
2
not as frequent as the R (8) motif, and it is probably less stable.
2
The C dimer could be also less stable than the A and B
complexes because NsH‚‚‚O bonds are not as strong as Os
2
3,24
15
anharmonicity.
H‚‚‚O ones. This is in agreement with the spectroscopic results
The transformation of the force field from Cartesian to
internal coordinates was carried out. The internal coordinates
following the UPAC recommendation25 are given in Table 1.
Such determined force constants were used to calculate the
based on the shifts of the wavenumbers of the H-bonded OsH
2
7
and NsH stretching absorption bands.
The dimers considered here are of resonant type (resonance-
1
5
assisted hydrogen bonds, RAHBs). For dimers A and B Os

Pyrrole-2-carboxylic Acid and Its Dimers
J. Phys. Chem. A, Vol. 106, No. 44, 2002 10615
bending vibrational modes30,31 is straightforward and therefore
will not be discussed here.
TABLE 1: Local Symmetry Coordinates of PCA (Atom
Numbering as in Chart 2)
symmetry coordinatea
descriptionb
Figure 1 presents the FT-IR spectra of pyrrole-2-carboxylic
acid taken as a KBr pellet (curve a), Nujol mull (curve b), and
diluted CCl4 solution (curve c), displayed together to facilitate
their comparison. The solubility of the PCA in nonpolar solvents
is very low; however, the IR spectrum was successfully
obtained.
S1 ) r(C1-N5)
S2 ) r(C1-C2)
S3 ) r(C2-C3)
S4 ) r(C3-C4)
S5 ) r(C4-N5)
S6 ) r(C4-C7)
S7 ) r(C7-O8)
S8 ) r(C7-O9)
S9 ) r(N5-H6)
S10 ) r(O9-H13)
S11 ) r(C1-H10)
S12 ) r(C2-H11)
S13 ) r(C3-H12)
νCN
νCC
νCC
νCC
νCN
νCC
νCO
νCO
νNH
νOH
νCH
νCH
νCH
δNH
δCH
δCH
δCH
δCC
δCO
δCO
δOH
δring
PCA shows a free νNH band at 3465 cm- in CCl4 solution.
The position of this band is exactly the same as for the s-cis
conformer of methyl pyrrole-2-carboxylate (MPC). The experi-
mental and theoretical IR studies of MPC were presented in
1
32
our previous paper. The theoretically estimated values of the
S14 ) â(C1-N5-H6) - â(C4-N5-H6)
S15 ) â(N5-C1-H10) - â(C2-C1-H10)
S16 ) â(C1-C2-H11) - â(C3-C2-H11)
S17 ) â(C2-C3-H12) - â(C4-C3-H12)
S18 ) â(C3-C4-C7) - â(N5-C4-C7)
S19 ) â(O8-C7-C4) - â(O8-C7-O9)
S20 ) â(O9-C7-C4) - â(O8-C7-O8)
S21 ) â(H13-O9-C7)
-1
free νNH modes are 3576 and 3593 cm for the s-cis and s-trans
PCA conformers, respectively (Table 3). The condensed phase
spectrum of PCA (KBr pellet and Nujol mull) revealed strong
-
1
absorptions at 3358 and 3356 cm , respectively, attributed to
the vibration of the hydrogen-bonded N-H group. The N-H
out-of-plane bending mode additionally confirms involvement
of the N-H group in intermolecular interactions. Solid-state
S22 ) â(C3-C4-N5) - 0.809â(C2-C3-C4) -
0
0
.809â(C1-N5-C4) + 0.309â(C3-C2-C1) +
.309â(C2-C1-N5)
-1
spectra show an absorption band at 602 cm , whereas dilute
S23 ) -1.118â(C2-C3-C4) + 1.118â(C1-N5-C4) -
δring
1
.809â(C3-C2-C1) + 1.809â(C2-C1-N5)
CCl4 solution revealed the absence of a δN-H band of a bonded
S24 ) γ(H6-N5-C4-C1)
γCH
γCH
γCH
γCH
γCC
γCO
γCO
γOH
τring
32
N-H group.
S25 ) γ(H10-C1-C2-N5)
S26 ) γ(H11-C2-C1-C3)
Formation of associates causes typical changes in the
vibrational frequency of the free O-H group. Even in quite
dilute CCl4 solution the PCA, as a large majority of carboxylic
acids, exists essentially as a dimeric species. As a consequence,
the spectrum in the condensed phase (KBr disk and Nujol mull)
exhibits absorption due to the presence of the dimers. The bands
S27 ) γ(H12-C3-C2-C4)
S28 ) γ(C7-C4-C3-N5)
S29 ) γ(O8-C7-C4-C3)
S30 ) γ(O9-C7-C4-C3)
S31 ) γ(H13-C9-C7-C4)
S32 ) τ(C3-C2-C1-N5) + 0.309τ(N5-C4-C3-C2) +
0
0
.309τ(C1-N5-C4-C3) - 0.809τ(C4-C3-C2-C1) -
.809τ(C2-C1-N5-C4)
-
1
at 2917, 2906, 2746, 2701, 2637, 2627, 2574, and 2515 cm
S33 ) -0.118τ(C2-C1-N5-C4) +
τring
were observed. In very dilute solution only a small portion of
the monomer is present, which was confirmed by a weak band
0
1
1
.118τ(C4-C3-C2-C1) +
.809τ(C1-N5-C4-C3) -
.809τ(N5-C4-C3-C2)
-
1
of the stretching vibration of the free OH group at 3550 cm .
Another band characteristic of the dimeric acid species arises
from the O-H out-of-plane deformation vibration, which
a
Normalization constants are not given here. Definitions are in terms
of standard valence coordinates: rij is the bond length between atoms
i and j; âijk is the valence angle between angle atoms i, j, and k with
the j central atom, âijkl is the out-of plane angle between the i-j bond
and the plane defined by the j, k, and l atoms; τijkl is the torsion
-
1
appears as a broad band of intermediate intensity at 884 cm .
In the diluted solution this band is less intense.
In the spectrum of the KBr pellet νCO bands at 1685 and 1665
(
dihedral) angle between planes defined by the i, j, k and j, k, l atoms.
Vibrational modes: ν, stretching; δ, bending in-plane; γ, bending out-
-
1
b
cm are broad and overlapped (Figure 1a). The spectrum of
the mull form also shows a broad and strong doublet at 1680
of-plane; τ, torsion.
-
1
and 1661 cm (Figure 1b) associated with the stretching
vibration of a hydrogen-bonded carbonyl group. The spectrum
of a diluted carbon tetrachloride solution (Figure 1c) revealed
H‚‚‚O hydrogen bonds are assisted by H13sO9sC7dO8
conjugated systems (see Chart 3) and can be called as R1 rings
-
1
2
28
strong νCO absorption at 1671 cm . The experimentally
observed absorption of the carbonyl group can be confirmed
by using DFT calculated vibrational frequencies. The lowest
νCO frequency was theoretically estimated for the B dimer (Table
[
R2 (8)]. NsH‚‚‚O hydrogen bonds of dimer C are assisted
2
by two conjugated systems: the R1 ring [R2 (8)] H6sN5sC4d
C7sO8 and a more extended H6sN5sC1dC2sC3dC4s
C7dO8 R5 ring [R2 (16)], giving origin to a conjugated structure
that can be called an (R1+R5) ring [R2 (8) R2 (16)]. The
similar intermolecular NsH‚‚‚O hydrogen bonds assisted by
resonance were investigated previously
2
-
1
2
2
28
4). Its value of 1671 cm is the closest to the experimental
-
1
νCO frequency of 1670 cm
recorded in CCl4 solution.
Additionally, the IR spectrum of N-methylpyrrole-2-carboxylic
28
for the crystal
acid, run as a CCl4 solution, shows absorption of the carbonyl
structures. However, the PCA or its simple derivatives were
not investigated from the crystallographic point of view. The
Cambridge Structural Database (CSD)29 was searched in this
study for the PCA structure and its derivative structures (all
possible substituents instead of H-atoms within the pyrrole ring;
only the NsH bond was kept); no entries were found.
-1
group at the same wavenumber, e.g., 1670 cm , as PCA (Figure
3
3
2
). It has been shown previously that N-alkylpyrroles substi-
tuted in the R-position exist primarily in the more stable syn
conformation (s-cis). Thus, in nonpolar solvents the N-H group
takes no part in the hydrogen bond formation and, as a result,
PCA forms a hydrogen bond via the carboxylic group. These
observations indicate that in the condensed phase, the carbonyl
oxygen of PCM is involved in two types of interactions B and
C, whereas in tetrachloride solution the B dimer is dominant.
The analysis of the observed bands in the IR and Raman
spectra was supported by theoretical calculations. The frequen-
cies and IR intensities of the s-cis and s-trans conformers of
pyrrole-2-carboxylic acid were calculated; and the proposed
description of bands is presented in Tables 2 and 3. Furthermore,
the DFT PCA dimer frequencies are given in Table 4. The
assignment of the majority of the pyrrole ring stretching and
On the basis of the difference in wavenumbers of the acid
νCO measured in the gas and crystal form, Lauti e´ et al.17 assumed
that PCA is likely to associate in cyclic dimers. The obtained
difference, around 4.5%, is close to the analogous values for

10616 J. Phys. Chem. A, Vol. 106, No. 44, 2002
Dubis et al.
TABLE 2: Theoretical (B3LYP/6-311++G** Level of Theory) Wavenumbers and IR Intensities for PCA
a
Scaled by factor 0.98. b PED’s lower than 10% are not included.
benzoic and trifluoroacetic acids, which are known to associate
in cyclic forms. The theoretically calculated differences between
νCO of the monomers and of the cyclic associates are 2.95%,
Additional support for this conclusion is provided by the
comparison of the calculated at B3LYP/6-311+G(d) level dipole
moments of both conformers. The value for the s-cis form is
1.42 D, whereas for the s-trans form it amounts to 3.34 D. The
dipole moments of selected pyrroles have been also calculated
2
.7%, and 1.7% for the A, B, and C dimers, respectively. This
provides additional support for the experimental IR evidence.
Furthermore, the interactions of solvents of different polarities
with pyrrole-2-carboxylic acid and N-methylpyrrole-2-carboxylic
acid were also studied. It was observed that the shift of the νCO
band of these two acids is of the same type in diethyl ether
3
4
previously on the basis of a group’s moments and compared
with their measured values. It was concluded that 2-acylpyrroles
exist in solution almost exclusively in the syn conformation.
3
3
Furthermore, Chadwick pointed out that the greater stability
of the syn- over the anti-rotamer of pyrrole-2-carbaldehyde and
-2-ketones in a low polarity solvent is due to the lower dipole
moment of this form. A similar conclusion may be drawn in
the case of pyrrole-2-carboxylic acid. In this case the syn-
rotamer has a much lower dipole moment than the anti rota-
mer.
-
1
-1
(
1
1711 cm ), dioxane (1706, 1707 cm ), and acetonitrile (1713,
-
1
710 cm ). The carbonyl band is symmetric and has a regular
shape. This is another piece of evidence suggesting that the
pyrrole-2-carboxylic acid, as well as its N-methyl derivative,
exist in only one conformation in solution, most likely in the
s-cis form.

Pyrrole-2-carboxylic Acid and Its Dimers
J. Phys. Chem. A, Vol. 106, No. 44, 2002 10617
TABLE 3: Theoretical (B3LYP/6-311+G* Level of Theory) IR Data for Pyrrole-2-carboxylic Acid Monomers
a
Scaled by 0.98. b Vibrational modes: ν, stretching; δ, deformational (all kinds). Superscripts: s, symmetrical; as, asymmetrical; ip, in-plane;
oop, out of plane.
In the study of the vibrational behavior of carboxylic acids,
the combination of infrared and Raman spectroscopy seems to
be a good method to decide whether a carboxylic acid is
H-bonded through a cyclic dimer pattern. For instance, the νCd
O band of the formic acid cyclic dimer was observed at 1754
intensities are based on the HF calculations). The out of phase
stretching vibration of the carbonyl group gives intensive bands
at 1695, 1670, and 1703 cm in the IR spectrum of dimer A,
B, and C, respectively.
-
1
Figure 3 shows the Raman spectrum of the crystal PCA. The
-
1
-1
35-37
-1
cm in the infrared and at 1670 cm in the Raman.
The calculated spectra of dimers A, B, and C revealed two
broad and medium intensity band at 1622 cm is assigned to
ip
CO
the stretching vibration ν of the H-bonded CdO group. The
ip
CO
carbonyl absorptions, which are assigned to the in phase (ν
)
theoretically estimated νCO frequencies for three cyclic dimers
op
and out of phase (ν ) stretching modes for intermolecular
are higher than Raman experimental frequencies by 73, 48, and
CO
-
1
hydrogen-bonded CO groups. Their intensities differ signifi-
cantly from each other in the Raman and IR spectra. The in
phase, symmetrical stretching vibration of the CO group is active
81 cm for dimers A, B, and C, respectively. Thus, the
experimental value of the νCO is closest to the theoretically
predicted value for dimer B. The significant difference of 63
-1
in the Raman. The ab initio calculated frequencies are 1672,
cm between the in phase and out of phase stretching vibration
of the CO groups may be treated as direct evidence of the
existence of the cyclic dimers.
1
1
651, and 1692 cm- for dimers A, B, and C, respectively, and
their intensities are of medium strength (data concerning

10618 J. Phys. Chem. A, Vol. 106, No. 44, 2002
Dubis et al.
TABLE 4: Theoretical (B3LYP/6-311+G* Level of Theory) and Experimental IR and Raman Data for Pyrrole-2-carboxylic
Acid Dimers
a
Scaled by 0.98. b Vibrational modes: ν, stretching; δ, deformational (all kinds). Superscripts: s, symmetrical; as, asymmetrical; ip, in-plane;
oop, out of plane. Abbreviation used: s, strong; m, medium; w, weak; v, very. d Measured as CCl
c
solution. e Measured as KBr pellet. Measured
f
4
as Nujol mull.
at 1328 cm- in IR is high, whereas in the Raman spectrum a
weak band was observed. Additionally, the broad and medium
intense γOH band is visible only in the IR spectrum of solid
PCA.
1
Other bands that may assist in the identification of acid dimers
are the coupled vibrations involving the CdO stretching and
-1
OH in plane deformation vibrations. The δOH band at 1391 cm
is rather weak in both spectra. The intensity of the νCsO band

Pyrrole-2-carboxylic Acid and Its Dimers
J. Phys. Chem. A, Vol. 106, No. 44, 2002 10619
4
Figure 1. IR spectra of pyrrole-2-carboxylic acid: (a) KBr pellet; (b) Nujol mull; (c) 0.001 M CCl solution.
4
Figure 2. IR spectra of pyrrole-2-carboxylic acid (PCA) and N-methylpyrrole-2-carboxylic acid (0.025 M; CCl ).
Geometries and Energetics. The results of the B3LYP/6-
11+G(d) level calculations indicate that the s-cis monomer
NsH bond (0.010-0.011 Å). A similar situation is observed
for the CdO bond; its elongation is smaller for the C dimer
than for the A and B complexes. It was pointed out earlier38 on
the basis of neutron diffraction results taken from the Cambridge
Structural Database that in the case of CdO‚‚‚HsO interac-
tions there is a correlation between the H‚‚‚O distance and Cd
O bond length: the shorter the H‚‚‚O distance, the longer the
CdO bond.
3
conformer is more stable than the s-trans conformer. The
difference in energies of these conformers amounts to 0.97 kcal/
mol. It was pointed out in the previous section that the B dimer
is dominant in solution. Table 5 presents some of the geometrical
parameters of complexes considered in this study. The results
show that the dimers formed through OsH‚‚‚O bonds are more
stable than the complex formed through NsH‚‚‚O bonds. The
elongation due to the H-bond formation of the OsH proton
donors (0.027-0.028 Å) is greater than the elongation of the
2
9
Interestingly, the O-H‚‚‚O angles are greater than N-H‚‚‚
O angles (Table 5) and it is well-known that linear H-bonds
and those that approach linearity are usually stronger than the

10620 J. Phys. Chem. A, Vol. 106, No. 44, 2002
Dubis et al.
Figure 4. Contour maps of the electron density for the pyrrole-2-
carboxylic acid dimers A, B, and C: (2)nuclear attractor NA; (b) ring
critical point RCP; (•) bond critical point BCP.
TABLE 6: Topological Parameters (Obtained from B3LYP/
6
-311+G* Wave Functions) for Complexes of the PCA (All
Values in au)
Figure 3. Raman spectrum of solid pyrrole-2-carboxylic acid (PCA).
cyclic dimer
F
XH
32FXH
F
H
‚‚‚O 32F
H
‚‚‚O
F
CdO
32FCdO
TABLE 5: Geometrical (Ångstroms and Degrees) and
Energy (kcal/mol) Parameters of the PCA Dimers
Considered in This Study (Results Obtained at B3LYP/
A
B
C
0.3147 -2.0832 0.0462 0.1449 0.3961 -0.3413
0.3163 -2.0977 0.0454 0.1435 0.3902 -0.3667
0.3243 -1.7270 0.0264 0.1094 0.4000 -0.3098
6
-311+G* Level of Theory, H-Bond Energies Corrected for
BSSE)
ring critical points. It was shown recently43 that for intermo-
lecular H-bonds, greater FRCP (electron density at the ring critical
point) values are observed for stronger H-bonds. For such
systems, the considered ring critical points are associated with
the ring created due to intermolecular H-bond formation. The
value of FRCP for the eight-membered ring of PCA dimers (A
and B) amounts to 0.008 au and for the ten-membered ring of
the C dimer it is only 0.003 au
cyclic dimer X-H* H‚‚‚O CdO X-H‚‚‚O H-bond energy
A
B
C
0.997 1.688 1.230
0.996 1.697 1.237
1.019 1.892 1.226
178.6
178.9
163.9
-8.3
-7.7
-5.7
a
NH bond length for monomeric PCA amounts to 1.008 Å (s-E)
and 1.009 Å (s-Z); OH bond length amounts to 0.969 Å for both
conformers.
bonds where the X-H‚‚‚Y angle is far from 180°. Additionally,
H‚‚‚O distances for the A and B complexes are shorter than
this distance for the C complex.
Conclusions
On the basis of spectroscopic investigations using both IR
and Raman techniques, we have revealed that the pyrrole-2-
carboxylic acid (PCA) forms cyclic acid dimers in the solid
state. The theoretically estimated IR frequencies for the B and
C dimers are very close to the experimentally observed
absorptions. Pyrrole-2-carboxylic acid exists in solution and in
the solid state in only one conformation, most likely in the s-cis
form.
The B3LYP/6-311+G(d) H-bond energies corrected for BSSE
are also given in Table 5. The single O-H‚‚‚O and N-H‚‚‚O
binding energies are provided. One can see that H-bonds are of
the medium strength, as mentioned in the previous section, and
they are assisted by conjugated systems. For the all cases
considered here two H-bonds stabilize each of the complexes.
One can conclude that the energetic results confirm the
geometrical data; the A and B complexes are more stable.
Additionally, the O-H‚‚‚O bond within the complex formed
by the s-trans conformers is stronger than such an O-H‚‚‚O
bond within the s-cis conformers dimer.
Figure 4 presents the contour maps of electron density for
the considered dimers. The positions of attractors, bond critical
points and ring critical points are also displayed. Table 6 presents
the topological parameters of the bonds involved in H-bond
formation (CdO, OsH, and NsH bonds): electron densities
at BCPs and their Laplacians. The same topological parameters
of H‚‚‚O contacts are also given.
Acknowledgment. This work was supported by State
Committee of Scientific Research of Poland (KBN), grants 3
T09A 061 19 and 4 TO9A 163 22. The authors wish to
acknowledge the Interdisciplinary Centre for Mathematical and
Computational Modeling (Warsaw University) and Wrocław
Supercomputing and Networking Center for computational
facilities. This work was facilitated in part by the NSF grant
No. 9805465 & 9706268, Wrocław University of Technology
Grant, and the Army High Performance Computing Research
Center under the auspices of the Department of the Army, Army
Research Laboratory cooperative agreement number DAAH04-
It is well-known that the electron density at H‚‚‚Y (Y is the
accepting center within the X-H‚‚‚Y bridge) well correlates
with the H-bond energy; thus the stronger the H-bond, the
9
5-2-0003/contract number DAAH04-95-C-0008.
3
9-41
greater the FH‚‚‚Y value.
Such a relationship is mainly
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