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Journal of Molecular Structure 1036 (2013) 151–160
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structure, vibrational assignment, and NMR spectroscopy of 1,2-bis
(dichloroacetyl) cyclopentadiene
Sayyed Faramarz Tayyari a,⇑, Somayeh Laleh b, Mansoureh Zahedi-Tabrizi c, Mohammad Vakili b
a
Chemistry Department, Shahrood Branch, Islamic Azad University, Shahrood, Iran
Chemistry Department, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran
c
Chemistry Department, Alzahra University, Tehran, Iran
b
h i g h l i g h t s
" bis(dichloroacetyl)cyclopentadiene forms strong intramolecular hydrogen bond.
" The potential function for proton movement is a low barrier double minimum.
" The OH stretching vibration is observed at about 1700 cm
1
.
" The observed NMR chemical shifts are in agreement with the theoretical values.
a r t i c l e
i n f o
Article history:
Received 7 July 2012
Received in revised form 25 September 2012
Accepted 25 September 2012
Available online 2 October 2012
Keywords:
1,2-Bis (dichloroacetyl) cyclopentadiene
Intramolecular hydrogen bond
Very strong hydrogen bond
Density functional Theory
NMR
Vibrational assignment
a b s t r a c t
Molecular structure, intramolecular hydrogen bond (IHB), 1H and 13C chemical shifts, and vibrational
assignment of newly prepared 1,2-bis (dichloroacetyl) cyclopentadiene (DCACP), have been investigated
by means of density functional theory (DFT) calculations. In addition, the geometry of the most stable
conformer was also optimized at the MP2/6-31G level. By calculations at the B3LYP level, using 6311++G basis set, the IR band frequencies of the most stable conformer and its deuterated analogue
and the 13C and 1H chemical shifts were clearly assigned. In addition, the anharmonic vibrational wavenumbers in solution were also calculated at the B3LYP/6-31G level. All theoretical calculations and
experimental spectroscopy data are consistent with a very strong intramolecular hydrogen bond in this
f-diketone. According to the theoretical calculations, the enolated proton in DCACP slightly deviates from
half way between the two oxygen atoms, Cs symmetry, which suggests existence of a low barrier double
minimum potential for this system.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
It has been proposed that the short and strong hydrogen bonds
with low-barrier (67 kcal/mol) play important role in enzymatic
processes [1,2]. The information on intramolecular hydrogen bonding in particular is very useful to understand various molecular
properties. The intramolecular hydrogen bonding may be responsible for the molecular geometries as well as the stability of a certain
predominant conformation.
Among many intramolecularly hydrogen bonded systems, the
conjugated dicarbonyl compounds have been widely investigated
not only by theoretical approaches but also by experimental methods [3–13].
Conjugated dicarbonyl compounds, such as b-dicarbonyls and
1,2-diacyl-cyclopentadienes, are capable to form very strong intra⇑ Corresponding author. Tel.: +98 511 8780216.
E-mail address: [email protected] (S.F. Tayyari).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.09.069
molecular hydrogen bond. The strength of hydrogen bond in these
systems increases by p-electron conjugation with the chelated
rings [14].
In spite of extensive works on structure and properties of
hydrogen bonds in the enol form of b-dicarbonyl compounds, only
a few reports considering the hydrogen bonding in 1,2diacyl-cyclopentadienes are appeared in the literatures. These
compounds solely exist in the enol form, Fig. 1, characterized by
a seven-membered ring [15–19], and their proton chemical shifts
are consistent with a very strong hydrogen bond [17,20,21].
These compounds are classified by Gilli and Bertolasi [22] as fdiketone enols. The simplest member of this class of compounds is
6-hydroxy-1-formylfulvene (HFF), which has been the subject of a
number of practical [16–20] and theoretical [13] investigations.
The microwave spectrum of HFF [16] is consistent either a C2v or
a Cs with an O O distance near to 2.5 Å. An investigation on IR
spectrum of HFF [18], which only considers a few bands, supports
the presence of rapidly interconverting Cs forms, while the C2v nu-
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2.2. Instrumentation
Fig. 1. The enol form of DCACP and its atom numbering system.
clear symmetry is supported by the NMR data [21]. On the other
hand, Perrin and Ohta [23,24] by using the NMR method of isotopic
perturbation of equilibrium indicated existing of an asymmetric
structure for HFF. Solid state X-ray and neutron diffraction data
[17] indicate a slightly asymmetric and nearly linear hydrogen
bond with an O O distance of 2.513 and 2.550 Å, respectively,
which the latter is consistent with the calculated results
(2.556 Å) at the MP2/6-31G level by Millefiori and Alparone [13].
Deuteron quadrupole coupling constant studies suggests a low
barrier double minimum potential for HFF and its 1,6-diphenyl
derivative [25].
Except for HFF, the X-ray crystallography of two other members
of this class of compounds has been reported, which exhibit short,
strong, and nearly linear hydrogen bonds with a length at about
2.44 Å [15].
The aim of the present paper is to predict the structure, nature
of the intramolecular hydrogen bond of DCACP by means of density
functional theory (DFT). The calculated NMR chemical shifts and
FT-IR spectrum will be compared with the corresponding experimental values.
2. Experimental
2.1. Preparation
DCACP was prepared by modified procedures of Linn and Sharkey [26] as follows: phenyl-lithium was prepared from 21 ml
(0.2 mol) bromobenzene and 2.8 g (0.4 mol) lithium in dry ether
under an atmosphere of argon. To this solution, 13.2 g (0.2 mol)
freshly distilled cyclopentadiene in 30 ml dry diethyl ether was
added dropwise. To the resulting well-stirred suspension of the
lithium cyclopentadiene there was added, dropwise over a course
of 30 min at 0 °C, 19.2 g (0.2 mol) dichloroacetyl chloride in
30 ml dry diethyl ether. There was an immediate formation of a
bright yellow color, which gradually darkened. Stirring was continued for an additional 30 min then the reaction mixture was hydrolyzed with dilute aqueous acetic acid. The bright red ether layer
was separated and the aqueous layer extracted twice with 10 ml
ether. The combined organic solution was washed with water
and dried over anhydrous sodium sulfate. The solvent was removed by evaporation to leave a dark yellow solid. Recrystallization from cyclohexane gave 14.4 g (25%) yellow needles, m.p.
141.5–142.5 °C. Deuterated DCACP (D-DCACP) was prepared by
adding 3 ml D2O to a solution of 1 g DCACP in 5 ml CCl4. The mixture was left overnight, at room temperature in a dry box. Then the
organic layer was separated and dried over Na2SO4 and vacuumed
off to remove the solvent and water. This method was repeated
three times.
The NMR experiments were performed on dilute CDCl3 and
[2H6]-acetone solutions at 300 K.
The NMR spectra were obtained on a FT-NMR, Brucker DRX 500
AVANCE spectrometer equipped with a z-gradient accessory and
an inverse (or direct detection) 5 mm diameter probehead working
at 500.13 MHz for 1H and 125.76 MHz for 13C. The chemical shifts
were referenced to the signal of TMS.
The Mid-IR spectra of DCACP and D-DCACP were recorded as
KBr disk and CCl4 solution in the range of 4000–500 cm1 with resolution of 2 cm1 by averaging the results of 20 scans on a Bomem
MB-154 Fourier Transform Spectrophotometer.
The Far-IR spectra in the region 600–100 cm1 were obtained
using a Thermo Nicolet NEXUS 870 FT-IR spectrometer equipped
with a DTGS/polyethylene detector and a solid substrate beam
splitter. The spectrum was collected with a resolution of 2 cm1
by signal averaging the results of 64 scans.
3. Method of analysis
The Gaussian 09 [27] program was used for all quantum
mechanical computations. The full geometry optimizations for all
possible chelated and the corresponding open structures were performed at the B3LYP level [28,29] using 6-31G, 6-311G, and 6311++G basis sets. In addition, the geometry of the most stable
conformer was also optimized at the MP2/6-31G⁄⁄ level. For the
chelated conformers, the optimization and vibrational frequency
calculations were also performed at the B3LYP level, using 6311++G basis set. The frequencies for infrared fundamentals of
the vibrational modes of the most stable conformer and its deuterated analog were also calculated by anharmonic method at the
B3LYP/6-31G level, and we show below that these predictions
agree more satisfactorily with the measured frequencies.
The assignment of the calculated wavenumbers is aided by the
animation option of the GaussView 5 [30] graphical interface for
Gaussian programs, which gives a visual representation of the
shape of the vibrational modes.
To obtain the geometry of DCACP conformers, the ACHCl2
groups rotation minima were calculated separately, at the B3LYP/
6-31G level, for each of two groups, by assuming that the molecular geometry can relax during rotation (i.e. for each value of torsion angle the molecular geometry was fully optimized). The
potential scans for the internal rotation of both CHCl2 groups,
about C12AC6 and C11AC8 bonds (see Fig. 1), were obtained by
allowing the H17C12C6O7 and H16C11C8O9 dihedral angles to
vary from 0° to 180° by steps of 15°. By this method, four stable
chelated conformers were obtained which are shown in Fig. 2.
These stable conformers were further fully optimized in the gas
phase and in solution at the B3LYP level using 6-31G, 6311G, and 6-311++G basis sets.
To calculate the hydrogen bond strength of the chelated conformers, the OAH bond of the chelated conformers, A-I to A-IV,
were rotated about CAO bond by 180° and then the resulted structures were fully optimized to obtain the final open structures, B-I
to B-IV.
The absolute shieldings for all stable chelated conformers of
DCACP and tetramethylsilane (TMS) have been obtained using
the gauge-including atomic orbital (GIAO) method [31–35] at the
B3LYP/6-311++G level. The predicted 13C and 1H chemical shifts
are derived from equation d = ro r, where d is the chemical shift,
r is the absolute shielding, and r0 is the absolute shielding of TMS.
Acetone was selected for studying the relative stability of conformers in solution following the SCRF-PCM method [36], according to which the solute is embedded in the dielectric medium
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A-I (0.00)
A-II (0.90)
A-III (1.40)
A-IV (1.33)
B-I (22.79)
B-II (16.60)
B-III (24.32)
B-IV (18.38)
Fig. 2. The stable possible cis-enol: (A) conformers of DCACP and their corresponding open structures. (B) The relative energies are given in parentheses (in kcal/mol).
surrounded by a cavity shaped in the form of the solute [37,38].
The van der Waals radii suggested by Bondi [39] were adopted
for atoms. Specific solute–solvent effects are not taken into account
at this level of calculation and the obtained solvation energies correspond to the electrostatic contributions, which, however, play a
dominant role in tautomerization reactions [40].
4. Results and discussions
4.1. Molecular geometry
By rotation of CHCl2 groups around C6AC12 and C8AC11 bonds,
four stable chelated conformers were obtained, which are fully
optimized at the B3LYP level using 6-31G, 6-311G, and 6311++G basis sets. The relative energies in the gas phase and
in solutions of chloroform and acetone, dipole moments, and the
hydrogen bond strengths (EHB), the energy difference between
the chelated and corresponding open conformer (B structure in
Fig. 2) are collected in Table 1, whilst the geometrical parameters
for all stable conformers in the gas phase and in solution are compared in Table 2. In the last column of Table 2 the geometrical
parameters of B-IV are also listed.
The conformation of CHCl2 groups with respect to the plane of
molecule (Fig. 2) has great influence on the geometry of the
chelated ring. The O O distance varies from 2.447 Å in the conformation A-III to 2.493 Å in the conformation A-I (at the B3LYP/6311++G level). As the calculations at the MP2/6-31G and
B3LYP (using different basis sets) levels predict, A-I is the most stable conformation in the gas phase. However, the calculated relative
energies obtained at different level of theory for A-I to A-IV conformers are slightly different. The calculated relative energies for
other conformers obtained at various levels of theory are only
0.90–1.40 kcal/mol higher than that of A-I conformation. Of course,
such a little difference in energy is not enough to determine the
stability of a conformer. The optimization calculation in the solvent
media, using PCM method, without considering special solvent–
solute interactions, indicates that in solution A-IV is the most stable conformation. The stability increasing of A-IV, compared to that
of other conformers, is due to its considerably high value of dipole
moment. In the CH3CN solution, calculated at the B3LYP/6311++G level, the energies of A-I, A-II, and A-III conformers are
1.18–3.6 kcal/mol higher than that of A-IV conformer. By going
from the gas phase to acetone solution, the geometry of A-IV shows
the highest changes among all conformers. In solution, the O O
Table 1
Relative energies (kcal/mol) and dipole moments for DCACP conformers obtained at
the B3LYP level using various basis sets and different media.a.
Basis sets
6-31G e
6-311G e
6-311++Ge
MP2/631Ge
6-311G b
6-311G c
6-311++Gb
6-311++Gc
6-311++Gd
l d(Debye)
l e (Debye)
A-I
0.00(17.97)
0.00(16.48)
0.00(16.60)
0.00(15.09)
A-II
0.65(23.93)
1.01(20.88)
0.90(21.89)
0.85(20.39)
A-III
1.24(25.11)
1.37(20.97)
1.40(22.92)
1.31(19.93)
A-IV
0.97(18.56)
0.73(16.31)
1.33(17.05)
0.64(15.32)
C2v
1.46
1.47
1.63
1.93
0.54
1.16
0.42
1.05(15.56)
1.18(20.54)
4.4
3.0
2.53
3.58
2.30
3.44(19.94)
3.60(11.20)
1.4
1.2
1.70
2.13
1.58
2.14
2.21(15.80)
4.7
3.2
0.00
0.00
0.00
0.00(15.20)
0.00(13.88)
7.4
5.0
0.81
0.80
0.80
7.6
5.0
a
Figures in parentheses are corresponding EHB (in kcal/mol); l, dipole moment
(calculated with the 6-311++G** basis set).
b
Calculated in chloroform.
c
In acetone.
d
In CH3CN.
e
In the gas phase.
distance and CAO and C@O bond lengths increase by 0.004,
0.008, and 0.006 Å, respectively, while, the C1AC6 and C2AC8
bond lengths decrease by 0.005 and 0.007 Å, respectively.
The OAH bond lengths in all chelated conformers are considerably long, 1.027–1.057 Å, but in the open structures its value is
about 0.970 Å.
The calculated O O distance in the chelated frameworks of
DCACP is in good agreement with those observed for two other
members of this class of compounds, 2,3-diacetyl-5-nitrocyclopentadiene (NO2-DACP), 2.446(2) Å, and 2,3-dibenzoyl-5-nitrocyclopentadiene (NO2-DBCP) 2.433(2) Å [15]. On the other hand,
the calculated O O distance in the chelated DCACP is considerably shorter than those obtained for malonaldehyde (2.587 Å
[8]), hexafluoro-acetylacetone (2.592 Å [9]), and acetylacetone,
AA, (2.544 Å [11]), all calculated at the B3LYP/6-311++G level.
These results indicate formation of much stronger hydrogen bond
in this molecule compared with those in the enol form of b-diketones, which is consistent with the NMR results.
Comparing the geometrical parameters of chelated and open
conformations reveals that the cyclopentadiene ring participates
in the p-electron conjugation of the chelated ring. The C1AC2 bond
distance, 1.487 Å, and its very small change by going from open to
the chelated structure, 0.007 Å (see Table 2), indicates that this
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Table 2
Selected bond distances (Å) and bond angles (°) of calculated stable chelated DCACP at the B3LYP/6-311++G level.
A-I
Bond distances
C1AC2
C1AC5
C2AC3
C4AC5
C3AC4
C1AC6
C2AC8
C6AO7
C8AO9
C6AC12
C8AC11
OAH
H O
O O
C4AH14
C5AH15
C3AH13
C12AH17
C12ACl18
C11ACl20
C11AH16
Bond angles
OAH O
C2C1C5
C1C2C3
C2C3C4
C3C4C5
C2C1C6
C1C2C8
C1C6O7
C2C8O9
C6O7H10
C1C5H15
C5C4H14
C2C3H13
U1
U2
a
A-II
A-III
A-IV
B-IV
G
S
G
S
G
S
G
S
S
1.479
1.437
1.396
1.379
1.416
1.386
1.428
1.298
1.246
1.513
1.539
1.027
1.477
2.493
1.080
1.079
1.080
1.080
1.800
1.801
1.084
1.481
1.438
1.399
1.379
1.416
1.383
1.425
1.303
1.249
1.511
1.539
1.028
1.472
2.492
1.080
1.079
1.079
1.079
1.804
1.803
1.083
1.481
1.434
1.398
1.380
1.415
1.382
1.423
1.305
1.250
1.523
1.538
1.037
1.449
2.478
1.079
1.079
1.080
1.084
1.800
1.801
1.084
1.482
1.434
1.399
1.381
1.414
1.382
1.422
1.306
1.252
1.523
1.538
1.036
1.477
2.481
1.080
1.079
1.079
1.083
1.802
1.803
1.083
1.479
1.430
1.402
1.383
1.412
1.387
1.425
1.298
1.247
1.524
1.532
1.057
1.397
2.447
1.078
1.080
1.080
1.084
1.799
1.797
1.081
1.481
1.431
1.404
1.384
1.412
1.385
1.421
1.301
1.251
1.523
1.532
1.048
1.419
2.461
1.080
1.079
1.080
1.083
1.801
1.801
1.080
1.477
1.434
1.400
1.381
1.414
1.390
1.431
1.292
1.241
1.514
1.534
1.039
1.442
2.472
1.080
1.080
1.080
1.080
1.798
1.796
1.080
1.480
1.435
1.401
1.381
1.413
1.385
1.424
1.300
1.247
1.512
1.533
1.036
1.451
2.476
1.080
1.080
1.080
1.079
1.801
1.801
1.080
1.487
1.456
1.388
1.365
1.429
1.371
1.456
1.325
1.217
1.510
1.556
0.969
–
2.689
1.080
1.079
1.079
1.079
1.805
1.808
1.078
169.7
105.9
106.6
109.7
108.6
129.0
127.3
126.0
124.6
110.8
126.1
125.5
126.0
179.8
0.0
170.4
105.9
106.4
109.8
108.7
129.1
127.4
125.8
124.6
110.5
126.1
126
125.7
180.0
0.0
170.4
106.1
106.4
109.7
108.8
129.0
127.3
125.6
124.6
110.7
125.3
125.5
125.9
0.0
0.0
171
106
106.3
109.7
108.7
129.0
127.5
125.4
124.6
110.6
125.5
125.9
125.8
0.0
0.0
171.2
106.3
106.2
109.7
108.7
128.4
127.1
125.4
124.6
110.9
125.3
126.3
125.9
0.0
180.0
171.6
106.2
106.2
109.7
108.7
128.6
127.4
125.3
124.8
110.7
125.5
125.9
126.4
0.0
179.9
170.2
106.0
106.4
109.8
108.6
128.6
127.1
125.9
124.6
111.0
126.2
126.3
126.0
174.5
172.8
170.7
106.0
106.3
109.8
108.6
128.7
127.3
125.7
124.8
110.6
126.1
126.0
126.9
179.8
179.9
–
105.8
106.6
110.6
108.4
131.7
129.8
122.8
126.5
111.7
126.2
126.3
126.6
179.8
179.1
U1 and U2 are O7C6C12H17 and O9C8C11H16 dihedral angels, respectively; G and S stand for calculations in the gas phase and in acetone solution, respectively.
bond slightly participates in the conjugated p-electron system.
However, elongation of C@C bonds and shortening of CAC bonds
of the cyclopentadiene ring by about 0.013–0.021 Å (compare the
last two columns of Table 2), upon formation of intramolecular
hydrogen bond, strongly supports participation of cyclopentadiene
ring in the p-electron delocalization of the chelated ring. Changing
from open conformers to the chelated structures causes a large
lengthening of C@O and C1@C6 (0.03 and 0.014 Å, respectively)
and shortening of CAO and C2AC8 bond distances (0.025 and
0.032 Å, respectively). These results suggest establishing of high
degree of p-electron delocalization in the chelated ring upon formation of intramolecular hydrogen bond, which in turn assists
the increasing of the hydrogen bond strength. These results interpret the formation of very strong hydrogen bond in 1,2-diacyl
cyclopentadiene systems and support the resonance-assisted
hydrogen-bonding hypothesis (RAHB) of Gilli et al. [14] for this
system.
As it is shown in Table 2, the OO distance of stable conformers
lays in the 2.49–2.45 Å range, and the hydrogen bond system is
nearly linear, with an O–HO angle of about 170–171°. These results suggest formation of a very strong hydrogen bond in DCACP
conformers, which is supported with an EHB in the 16.6–22.9 and
15.2–19.9 kcal/mol range in the gas phase and in acetone solution
(calculated at the B3LYP/6-311++G level), respectively. As it is
shown in Table 1, the corresponding values calculated at the
MP2/6-31G level is slightly different from those obtained at
the B3LYP/6-311++G level. The EHB’s of conformers A-II and AIII are considerably higher than that of conformers A-I and A-IV.
This difference caused by three factors: (1) difference in hydrogen
bond strength in the chelated conformers. (2) Hydrogen bonding
formation between OH and the two Cl atoms of the CHCl2 groups
in the open structures of A-I and A-IV, which reduces the energy
of the trans-enol forms. (3) Repulsion between enolated and CHCl2
group’s H atoms in the open structures of B-II and B-III, which
cause an increase in their energies. By considering these factors,
therefore, it may conclude that 16.6 and 15.2 kcal/mol is the lower
limit for the hydrogen bond strength in DCACP in the gas phase and
solution, respectively.
According to the calculated results (Table 2), the C6AC12 and
C8AC11 bond lengths in A-IV are 1.512 and 1.533 Å, respectively,
which are considerably longer than that calculated for CACH3 bond
lengths in AA, 1.497 and 1.513 Å [11]. The Electrostatic effects can
rationalize this lengthening of the CAC bonds qualitatively if we
assume that the small but negative net charge on the CH3 carbon
becomes positive upon chlorination. In this picture, the attractive
interaction (CdAC+d) in CACH3 becomes repulsive (C+dAC+d) if
chlorination occurs on terminal carbon atoms. This simple electrostatic mode has been proposed to explain the abnormally high value of CACF3 bond lengths, 1.521 and 1.543 Å, in HFAA [9].
The most important geometrical parameters for the A-IV conformer related to the intramolecular hydrogen bond, calculated
in the gas phase and in solution, are listed in Table 3. As it is shown
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Table 3
The most important geometrical parameters for the A-IV conformer.
A
Gas
B
C
Table 4
1
H chemical shifts (ppm) of some 1,2-diacylcyclopentadienes.
D
Sol.
Gas
Sol.
Gas
Sol.
Gas
Sol.
Bond distance (Å)
C1AC6
1.390
C2AC8
1.431
C6AO7
1.292
C8AO9
1.241
OAH
1.039
H O
1.442
O O
2.472
1.385
1.424
1.300
1.247
1.036
1.451
2.476
1.389
1.431
1.291
1.240
1.039
1.441
2.471
1.385
1.425
1.297
1.245
1.037
1.447
2.476
1.394
1.429
1.292
1.249
1.054
1.405
2.452
1.389
1.424
1.298
1.254
1.051
1.414
2.459
1.387
1.435
1.304
1.249
1.024
1.482
2.499
1.386
1.432
1.308
1.251
1.025
1.479
2.497
Bond angle (°)
OAH O 170.2
170.7
170.3
170.9
171.2
171.8
171.0
171.5
a
A, B, C stand for 6-311++G, 6-311G, and 6-31G basis sets, respectively; D,
calculated at the MP2/6-31G level; sol, acetone solution.
in Table 3, the calculated OO distance for the most stable conformer of DCACP varies from 2.452 to 2.503 Å. However, it has
been shown that calculation at the B3LYP/6-31G overestimates
the hydrogen bond strength and gives considerably shorter OO
distance compared to that obtained at the B3LYP/6-311++G level
[41–43], therefore it may be concluded that the hydrogen bond
distance in A-IV conformer is in the 2.472–2.503 Å range. All calculations, except calculations at the MP2/6-31G level, indicate that
the OO distance in solution is slightly longer than in the gas
phase. As it is also shown in Table 3, the length of polar bands,
CAO and C@O, in solution increases, whilst the CAC and C@C bond
lengths are slightly decreased compared to those in the gas phase.
By increasing the OO distance the OAH bond length decreases.
The relative energies of C2v structure of A-IV (putting the H
atom in the midway between two O atoms) are also listed in Table 1. As it is shown in Table 1, the relative energy of the C2v structure is less than 2 kcal/mol in the gas phase and less than 1 kcal/
mol in solution, which supports existence of a very strong hydrogen bond in DCACP with low barrier potential to proton tunneling
in the system.
Solvent
dOH
dH
(13,15)
dH
(14)
dH (R)
Ref.
HFF
DACP
DACP
DBCP
CDCl3
CCl4
CDCl3
CCl4
16.1
18.0
18.15
18.45
7.17
7.2
7.32
7.15
6.37
6.25
6.39
6.35
[20]
[21]
[20]
[21]
DBCP
CDCl3
18.43
7.24
6.46
D-t-BuCP
DTCP
Bis(pNO2)BCP
NO2DACP
DCACP
DCACP
DCACP
CCl4
CDCl3
CDCl3
19.2
18.29
18.23
7.48
7.83
7.25
6.25
6.55
6.6
8.57
2.5
2.53
7.9–7.25
(m)
7.76–
7.81(m)
1.47
7.2,7.66,7.68
7.96, 8.39
CDCl3
CDCl3
CD3CN
d6Acetone
19.35
17.34
17.40
17.41
7.86
7.57
7.83
8.02
–
6.57
6.72
6.68
2.6
6.78
7.33
7.59
[20]
T.W
T.W
T.W
Gas
Gas
Gas
Gas
Acetone
Acetone
Acetone
Acetone
17.03
16.91
18.75
19.05
16.74
16.87
18.11
17.78
8.57
7.87
7.91
7.58
8.13
8.37
8.18
8.01
6.70
6.71
6.70
6.67
6.94
6.91
6.95
6.92
6.29
6.58
6.64
6.99
6.78
6.44
7.22
7.26
Calculatedb
A-I
A-II
A-III
A-IV
A-I
A-II
A-III
A-IV
[20]
[21]
[20]
[20]
a
HFF, hydroxyl formylfulvene; DACP, 1,2-diacetylcyclopentadiene; DBCP, dibenzoylcyclopentadiene; D-t-BuCP, bis(t-butanoyl)cyclopentadiene; T.W, this work;
DTCP, 1,2-dithenoylcyclopentadiene.
b
Calculated at the B3LYP/6-311++G** level.
Table 5
Calculated and observed
HFFb
DCACP
DCACP
Calculatedc
A-I
A-II
A-III
A-IV
A-I
A-II
A-III
A-IV
4.2. Analysis of NMR spectra
The observed and calculated 1H and 13C NMR chemical shifts for
DCACP are listed in Tables 4 and 5, respectively. In Table 4, the 1H
chemical shifts of DCACP are compared with those of several 1,2diacyl-cyclopentadiene compounds. According to Tables 4 and 5,
good agreements exist between the calculated and observed
results.
The full 1H spectrum could be assigned on the basis of signal
integrations and splitting patterns. The full 13C spectrum of DCACP
was assigned by comparing with the corresponding spectrum of
HFF [23] and calculated results.
The data in Table 4 indicates that the hydrogen bond in DCACP
is stronger than that in HFF but slightly is weaker than that in 1,2diacetyl-cyclopentadiene, DACP. The weakening of the hydrogen
bond upon substitution of H atoms of the methyl groups by chlorine atoms could be attributed to the electron withdrawing character of Cl atoms. Para-substitution of phenyl groups in DBCP by NO2,
bis (p-NO2B) CP, shifts the 1H signal of the hydrogen bonded proton
from 18.43 ppm to 18.23 ppm (see Table 4), which supports weakening of the hydrogen bond by an electron-withdrawing group in R
position. On the other hand, substitution of the 4-position of cyclopentadiene ring of DACP by NO2 group (NO2DACP) considerably
shifts the 1H signal downfield (ca. 1.2 ppm), i.e. increasing the
strength of the bond. These effects could be explained as follows:
In the p-NO2 substituted DBCP, the nitro group acts as a withdrawing group, which reduces the electron density on the carbonyl
Compound
a
b
c
13
C chemical shifts for some 1,2-diacylcyclopentadienes.a
Solvent
C6,8
C1,2
C3,5
C4
R
CDCl3
CDCl3
Acetone-d6
176.0
176.88
177.33
126.5
120.92
121.44
141.2
140.27
142.62
125.2
125.82
126.72
–
66.27
67.09
Gas
Gas
Gas
Gas
Acetone
Acetone
Acetone
Acetone
184.45
183.11
184.74
183.02
185.96
188.16
187.76
185.19
123.73
123.97
125.07
125.57
127.71
127.43
128.31
129.02
146.80
144.58
143.90
142.53
151.40
151.33
150.78
151.7
128.08
128.12
127.58
126.77
133.55
132.87
133.30
133.37
88.80
86.59
85.68
83.24
90.97
92.47
90.12
88.36
Rf
70.28
68.54
67.79
65.80
78.77
79.68
78.10
76.71
R, CHCl2-carbon; Rf, corrected according to Ref. [47].
Data from Ref. [23].
Calculated at the B3LYP/6-311++G** level.
group of the chelated ring, therefore, decreases the hydrogen bond
strength. The similar behaviors have been observed for the enol
form of b-diketones when the terminal group substituted by electron withdrawing groups [9,41]. On the other hand, in the case of
substitution of NO2 in the 4-position of the CP ring in DACP, the nitro group enhances the p-electron delocalization by resonating
with the CP and chelated rings. This behavior is very similar when
the hydrogen atom of 3-position of AA is replaced by NO2 group
(the proton chemical shifts of enolated proton of AA and NO2-AA
are 15.4 [44] and 16.86 ppm [45], respectively).
Substitution of t-butyl for the CH3 groups in DACP shifts the
signal of OH proton 1.2 ppm downfield, i.e. considerably increases the hydrogen bond strength. It is interesting that the
similar behavior is observed for b-diketone system. The chemical
shifts of AA [44] and 1,3-di (t-butyl) propane-1, 3-dione [46] are
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S.F. Tayyari et al. / Journal of Molecular Structure 1036 (2013) 151–160
observed at 15.4 and 16.3 ppm, respectively, which causes a
downfield shift of 0.9 ppm upon substitution of both CH3 groups
by t-butyl. This effect could be attributed to the pushing of
both oxygen atoms together by the bulky group, which results
in a stronger hydrogen bond. As it is indicated in Table 4, the
results of experimental proton chemical shifts are in good agreement with the corresponding calculated results for A-IV
conformer.
Table 5 reveals that the predicted 13C chemical shifts obtained
at the B3LYP level, except for CHCl2 groups’ carbon, are in agreement with the experimental results. A large deviation is observed
for the chemical shift of the chlorinated carbons, which is
21.27 ppm for the most stable conformer in solution, A-IV. A
similar behavior for double chlorinated carbon atoms has been
reported by Koivisto et al. [47]. These authors found a deviation
of 14.2–18.7 ppm for CHCl2 groups in polychlorinated dihydro-
Table 6
Theoretical and experimental vibrational frequencies of DCACP(A-IV).a
Theoretical
Experimental
F1
IIR
F2
AR
Fan
F3
3120
3111
3102
3094
3090
2306
1611
1579
1526
1499
1433
1394
1380
1294
1282
1278
1258
1225
1222
1201
1122
1082
1070
998
938
942
885
839
794
781
775
757
751
709
698
676
629
585
561
534
496
377
367
318
311
234
232
218
187
177
166
145
101
77
34
29
11
2
2
4
1
1
1712
968
806
61
83
266
194
208
73
40
239
290
36
30
0
33
173
106
24
19
1
2
5
37
7
220
14
5
69
506
0
343
2
11
4
7
3
10
1
1
0
2
7
0
1
0
0
1
0
0
0
0
3116
3105
3095
3085
3074
2318
1628
1601
1529
1499
1438
1397
1379
1311
1284
1278
1257
1227
1224
1198
1121
1110
1072
998
941
927
867
842
796
782
775
760
748
720
711
692
646
590
568
534
495
376
369
318
311
233
231
217
184
180
166
144
101
77
35
27
11
242
53
58
23
19
84
76
107
20
7
58
22
82
235
27
15
21
4
6
1
94
1
4
62
13
0
5
28
28
7
10
10
7
41
30
13
2
1
0
22
1
5
17
8
7
0
1
0
0
1
3
4
2
4
4
2
2
3134
3117
3109
3138
3081
1538
1617
1583
1511
1385
1440
1405
1334
1220
1256
1312
1257
1229
1259
1197
1113
2022
1066
998
939
927
880
837
788
774
771
748
757
710
687
675
622
588
572
522
507
384
342
321
306
235
229
212
186
185
169
148
93
70
17
6
NC
3116
3105
3097
3084
3079
2142
1604
1588
1518
1488
1427
1389
1370
1288
1273
1260
1244
1201
1198
1188
1110
1073
1061
987
927
922
867
824
785
766
761
747
742
700
687
667
619
578
554
528
490
374
369
314
308
232
229
215
187
176
165
143
100
75
33
27
16
IRCCl4
3092(1)
3039(1)
2925 (2)
2852(1)
1718(14)
1626(32)
1603(100)
1492(13)
1461(12)
1440(21)
1426(36)
1408(33)
1315(3)
1261(44)
1261
1253(26)
1228(5)
1203(8)
1171(1)
1099(2)
1074(11)
1049(14)
1028(6)
948(2)
918(1)
841(3)
725(3)
700(2)
657(20)
657
564(4)
512(7)
Assignments
IRSolid
3124(5)
3102(13)
3037(20)
2925 (2)
2853(1)
1696(15)
1613(100)
1568(43)
1497(43)
1484(18)
1449(28)
1419(30)
1401(24)
1312(5)
1265(80)
1265
1261(83)
1246(9)
1202(35)
1167(3)
1106(6)
1088(20)
1043(23)
1003(5)
944(3)
928(2)
884(1)
840(17)
795(97)
788(25)
761(22)
751(29)
731(39)
708(21)
657(97)
657
590(6)
561(17)
537(6)
500(10)
237(10)
1
2
3
mCH(R)
mCH(R)
mOH
dOH,mC@C
dOH,mC@O
4, msCAC@O, mCAO
5, dOH
6, mCAC, mCAO
7,dCAH(R), mCAC,mCAO
8,msC@C
9, mCAC, dCH(R)
10, dCH(R)
11, dCH(R)
11, dCH(R)
cCH(R)
cCH(R)
dCH(R), 12
13
cOH
15
15.mCAR
16
17
18
19
dR-C@O, nOHO, dR-C@O, 21
20
22
dR-C@O, dR-C@O
23
msCCl2
24, msCCl2
25
maCCl2, cOH
26 maCCl2
27
dCACAR, dC@CAO, 21
dC5C1C6, dC1C2C3
dC1C2C7, dC2C1C6
mOHO
dC@O, CCl2sci
mOHO, CCl2 sci.
28, cC@O, cCAO
29
xCCl2
cCAO, cC@O, 28
dCACAR
qR
27, cC@O, cCAO
qR
pCAR
sR
pCP
sCP
a
F1 and F2, stand for scaled (by 0.9609 and 0.9859 for higher and lower than 1700 cm1) wavenumbers calculated at the B3LYP/6-311++G** in acetone solution and gas
phase, respectively; Fan, and F3, stand for anharmonic and scaled (by 0.9539 and 0.9705 for higher and lower than 1700 cm1) wavenumbers, calculated at the B3LYP/631G** level in acetone solution, respectively; NC, not converged; R, CHCl2 group; m, stretching, d, in-plane bending; q, in-plane rocking; p, out-of-plane bending mode; x,
wagging; sci, scissoring; CP, cyclopentadiene frame; m,O-H. . .O, hydrogen bond stretching.
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S.F. Tayyari et al. / Journal of Molecular Structure 1036 (2013) 151–160
camphenes calculated at the B3LYP/6-311G level. Koivisto et al.
[47] suggested the following equation for correction of calculated
13
C chemical shift of double chlorinated carbons:
dobs ¼ 0:791dcalc þ 6:817
with a standard deviation of 2.314 ppm. This correction for the 13C
chemical shift of CHCl2 groups of DCACP is given in Table 5, which
indicates good agreement with the experimental value.
4.3. Analysis of the IR spectra
The calculated scaled harmonic and anharmonic vibrational frequencies and their assignments for the most stable conformer of
DCACP and its corresponding deuterated analogous in solution
along with the observed infrared frequencies are listed in
Tables 6 and 7, respectively. As it is shown in these tables, calculated wavenumbers obtained for solution and gas phase, except
for a few cases, are almost the same. According to Tables 6 and
7, the calculated anharmonic and scaled harmonic wave numbers,
except for OH stretching and out-of-plane bending modes, are very
Table 7
Theoretical and experimental vibrational frequencies of D-DCACP(A-IV).a
Theoretical
a
Experimental
F1
F2
IIR
AR
Fan
F3
IRSolid
3120
3111
3102
3094
3090
1752
1592
1526
1507
1431
1407
1387
1292
1283
1279
1260
1225
1221
1205
1162
1109
1069
972
941
937
885
839
791
791
780
773
754
749
708
697
677
625
584
561
523
494
376
359
317
308
234
232
217
186
178
166
145
100
77
33
28
10
3116
3105
3095
3085
3074
1770
1619
1533
1512
1439
1410
1384
1308
1285
1279
1258
1227
1225
1202
1162
1106
1071
972
940
926
866
842
810
793
781
775
758
748
719
709
692
641
590
567
523
494
375
361
318
307
232
230
216
183
180
165
143
100
77
35
27
11
2
3
2
2
3
713
726
36
65
86
7
173
39
46
44
80
25
18
9
61
4
53
25
7
0
2
3
57
22
58
127
18
1
72
217
1
132
0
4
1
3
3
5
0
0
0
1
4
0
0
0
0
0
0
0
0
0
242
53
58
23
19
8
69
62
44
79
48
52
249
22
31
20
4
6
1
38
44
4
61
15
0
5
27
1
28
7
10
13
7
42
27
13
2
1
0
19
2
4
16
10
6
0
1
0
0
1
3
4
2
4
4
2
2
3134
3117
3109
3139
3082
1322
1588
1516
1221
1437
1413
1284
1245
1147
1307
1257
1229
1261
1194
1178
1104
1061
977
938
927
879
836
824
781
774
773
744
754
707
687
674
621
587
572
513
507
384
339
321
301
235
230
211
186
186
168
146
95
70
19
4
NC
3116
3104
3097
3084
3079
1685
1592
1519
1472
1427
1403
1368
1281
1266
1264
1246
1201
1198
1190
1157
1099
1060
962
927
922
867
823
784
783
765
760
744
740
699
686
667
615
577
554
517
489
372
361
313
305
232
228
214
185
175
164
143
99
75
33
27
16
3124(5)
3102(13)
3037(20)
2965
2925
?
1604(100)
1513(22)
1475(26)
1440(40)
1418(28)
1393(80)
1315(26)
1268(74)
1257(47)
1248(28)
1201(36)
1230(11)
1201
1157(29)
See footnotes of Table 6; m, O-D. . .O, hydrogen bond stretching.
1070(30)
972(11)
948(3)
928(4)
838(33)
756(50)
794(82)
785(50)
785
742(56)
729 (60)
719 (97)
645(89)
559(31)
499(10)
236 (12)
Assignments
1
2
3
mCH(R)
mCH(R)
mOD, mC@CAO, mC@O
mC@O, mC@C, mOD
4, mC@C, mC@O, mOD
5, mC@O, mC@C, mOD
6, mCAC, mCAO
7,dCAH(R), mCAC,mCAO
8,msC@C
9, mCAC, dCH(R)
10, dCH(R)
11, dCH(R)
11, dCH(R)
cCH(R)
cCH(R)
12, dCH(R), dOD
dOD, 13
13, dOD
14
dOD, 15, mCAR
17
16
18
19
cOD
dRAC@O, 1
20
22
dRAC@O, dRAC@O
23
msCCl2
24, msCCl2
25
maCCl2, cOD
26,.maCCl2
27
21, dCACAR, nOD O
dC5C1C6, dC1C2C3
dC1C2C7, dC2C1C6
mOD O
dC@O,CCl2sci
mOD O, CCl2 sci.
28, cC@O, cCAO
29
xCCl2
cCAO, cC@O, 28
dCACAR
qR
28, cC@O, cCAO
qR
pCAR
sR
pCP
sCP
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S.F. Tayyari et al. / Journal of Molecular Structure 1036 (2013) 151–160
close to each other. The deconvoluted IR spectrum of DCACP in the
1900–1000 cm1 region is given in the Fig. 3. Comparison between
IR spectra of DCACP and its deuterated analogue is given in Fig. 4.
The assignments of the experimental frequencies are based on
the observed band frequencies and intensity changes in the infrared spectra of the deuterated species and confirmed by establishing one to one correlation between observed and theoretically
calculated frequencies.
The calculated frequencies are slightly higher than the experimental values. Two factors may be effective for the discrepancies
between the observed and calculated spectra of these compounds.
The first is caused by the environment. DFT calculations have been
done at the gas phase and the experimental frequencies are obtained from solid state. The second reason for this discrepancy is
the fact that the experimental values are anharmonic frequencies
while the computed data are harmonic frequencies. The overestimation of the computed wavenumbers is quite systematic and
can be corrected by applying appropriate scaling factors or scaling
equations [48,12,49]. In Tables 6 and 7 the calculated harmonic frequencies are scaled by different scaling factors for the regions
above and below 1700 cm1, recommended by Tayyari et al. [48].
However, to show the effect of solvent and anharmonicity, the
vibrational frequencies were also obtained in solution (acetone)
at the B3LYP/6-311++G level, and calculating the anharmonic
wavenumbers at the B3LYP/6-31G level in solution.
The notations for cyclopentadienyl group vibrations are according to the illustrations in Fig. 5.
4.3.1. CH stretching modes
We assigned three bands at above 3000 cm1 to the CH stretching
vibrations of CP ring. These bands occur at 3124, 3102, and 3037 cm1
2.0
1.5
1.0
0.5
0.0
1800
1600
1400
1200
1000
Fig. 3. The deconvoluted IR spectrum of DCACP in the solid state.
2.5
2.0
1.5
1.0
0.5
0.0
1600
1200
800
Fig. 4. Comparing the IR spectra of DCACP and D-DCACP in the 1800–500 cm1
region.
in the solid phase. Two relatively weak bands at 2925 and 2853 cm1
are assigned to the CH stretching modes of the CHCl2 groups.
4.3.2. OH stretching mode
The most interesting fundamental in IR spectrum of the titled
compound is the OH stretching vibration, mOH. According to the
harmonic approximation calculations, obtained at the B3LYP/6311++G level in the solution and in the gas phase, we expect to
observe this band as a very strong band at about 2300 cm1, for
the most stable conformer (A-IV). By using the 6-31G basis set
the corresponding wavenumber obtained at about 2140 cm1.
However, the observed IR spectrum does not exhibit any broad
band in this region. As it is shown in Table 6, by considering the
anharmonicity, calculations at the B3LYP/6-31G level predict
the OH stretching to be occurred at about 1500 cm1. This result
shows that this vibration should be highly anharmonic. With such
a huge anharmonicity calculating the exact wavenumbers is almost impossible, because the shape of the potential function is
very sensitive to the level of calculation and used basis set.
However, it is well known that strong intramolecular hydrogen
bonding leads to large anharmonic effects, resulting in a shift of
mOH towards lower wavenumbers and possibly enhanced coupling
with other modes. The recent theoretical analysis by Szczepaniak
et al. [50] demonstrates how anharmonic effects associated with
strong intramolecular hydrogen bonding may lead to a drastic shift
of mOH towards lower wavenumbers and redistribution of the
associated IR intensity over several other modes. As it is shown
in Fig. 4, a weak and broad band is observed at about 1700 cm1,
which disappears in the deuterated analogous. This band is attributed to the OH stretching mode. Deconvolution of the IR spectrum
of DCACP (in the solid state) in the 1800–1000 cm1 range, Fig. 3,
shows a weak and broad band at about 1700 cm1.
4.3.3. 1700–1300 cm1 region
The deconvoluted IR spectrum of DCACP in the solid phase
shows two bands at 1613 and 1568 cm1 which involve C@C and
C@O stretching motions, which the latter is strongly coupled to
the OH in-plane bending vibration. In the IR spectrum in solution
these bands show a blue shift and occur at 1626 cm1 and
1603 cm1, respectively. The phase sensitivity of these two vibrational modes is well predicted by the calculated results (see Table 6). Upon deuteration the former was observed at 1604 cm1
and assigned to the C1@C6, which in the light and deuterated compounds it is coupled to the OH in-plane bending and OD stretching
vibrations, respectively. The 1568 cm1 band is disappeared in the
deuterated compound and a new band appears at 1157 cm1. The
corresponding band in deuterated AA is observed at 1079 cm1
[51], which supports much stronger hydrogen bond in DCACP than
that in AA.
The strong IR band at 1419 cm1 is assigned to the symmetric
C2AC8 and C6AO7 stretching mode, which is strongly coupled to
the cyclopentadiene (CP) ring’s CAH in-plane bending movements.
The weak band at about 1312 cm1 is attributed to the CAC
stretching of the terminal groups, which is coupled to the CAH
bending of the CP ring.
4.3.4. Below 1300 cm1
In the solid phase, the strong IR band observed at 1261 cm1 is
assigned to the coupled CH in-plane-bending vibrations of the CP
ring and terminal groups.
The two IR bands at 1228 and 1203 cm1 in solution are assigned to the CH in-plane and out-of-plane bending modes of the
CHCl2 groups, respectively.
The relatively broad band observed at 1088 cm1 in the solid
state IR spectrum is assigned to the OH out-of-plane bending vibration. The corresponding band in AA was observed at 940 cm1 [51],
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159
Fig. 5. The notations for cyclopentadienyl group vibrations.
which also indicates much stronger hydrogen bond in DCACP than
in AA. The corresponding band in the IR spectrum of b-diketones
appears in the 850–970 cm1 range [8,9,11], which is also in agreement with much stronger intramolecular hydrogen bond in DCACP
than that in the enol forms of b-diketones. The corresponding band
in the deuterated analogue was observed as a relatively broad band
at 756 cm1.
The strong bands at 708, 657, and 590 cm1 are caused by CACl
stretching vibrations and the 237 cm1 band is related to the CCl2
wagging mode.
5. Conclusions
A conformation analysis at DFT-B3LYP level using 6-31G, 6311G, and 6-311++G basis sets was preformed for DCACP. Four
stable conformers with little energy difference were obtained,
which their relative stabilities seem to be solvent dependent. From
comparison of observed and calculated NMR chemical shifts by
GIAO method, it is concluded that the H atoms of CHCl2 are engaged in an intermolecular hydrogen bond with the solvent
molecules.
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The optimized geometrical parameters of all stable chelated
conformers indicate presence of a very strong intramolecular
hydrogen bond in all chelated stable conformers with a hydrogen
bond strength more than 16.6 kcal/mol. The strength of the bond
is confirmed with a large proton chemical shift of about
17.4 ppm, which is almost solvent independent.
By comparing of the relative energies of the most stable conformer and the C2v structure of DCACP a low barrier double minimum potential energy is concluded for the intramolecular
hydrogen bond system in DCACP.
The OAH stretching, in-plane and out-of-plane bending and
OO stretching modes also indicate that the hydrogen bond
strength in DCACP is much stronger than that in the enol form of
b-diketones.
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