Infrared and theoretical calculations in 2-halocycloheptanones conformational analysis

Article abstract of DOI:10.1016/j.saa.2012.03.027

2-Halocycloheptanones (Halo = F, Cl, Br and I) were synthesized and their conformational analysis was performed through infrared spectroscopy data. The corresponding conformers geometries and energies were obtained by theoretical calculations at B3LYP/aug-cc-pVDZ level of theory in the isolated state and in solution. It was observed, by both approaches, that the conformational preferences were very sensitive to the solvent polarity, since its increase led to an increase in the population of the more polar conformer. An analysis of these conformational equilibria showed they suffer also the influence of stereoelectronic effects, like hyperconjugation and steric effects. These results were interpreted using natural bond orbital (NBO) analysis, which indicated that the electronic delocalization to the orbital π* _CO is directly involved in the stability increase of conformers I and II. The relative effect of the period of the halogen can also be noted, with changes in the conformational preferences and in the energies involved in the interactions of NBO.

Full text of DOI:10.1016/j.saa.2012.03.027

Spectrochimica Acta Part A 94 (2012) 277–287
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Infrared and theoretical calculations in 2-halocycloheptanones conformational
analysis
Thiago C. Rozadaa, Gisele F. Gauzea, Denize C. Favarob, Roberto Rittnerb, Ernani A. Bassoa,∗
a Departamento de Química, Universidade Estadual de Maringá, Av. Colombo, 5790, 87020-900 Maringá, PR, Brazil
b Instituto de Quimica, Universidade Estadual de Campinas, 13083-970 Campinas, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
2-Halocycloheptanones (Halo = F, Cl, Br and I) were synthesized and their conformational analysis was
performed through infrared spectroscopy data. The corresponding conformers geometries and energies
were obtained by theoretical calculations at B3LYP/aug-cc-pVDZ level of theory in the isolated state and
in solution. It was observed, by both approaches, that the conformational preferences were very sensitive
to the solvent polarity, since its increase led to an increase in the population of the more polar conformer.
An analysis of these conformational equilibria showed they suffer also the influence of stereoelectronic
effects, like hyperconjugation and steric effects. These results were interpreted using natural bond orbital
(NBO) analysis, which indicated that the electronic delocalization to the orbital ꢀ*C O is directly involved
in the stability increase of conformers I and II. The relative effect of the period of the halogen can also be
noted, with changes in the conformational preferences and in the energies involved in the interactions
of NBO.
Received 18 January 2012
Received in revised form 4 March 2012
Accepted 9 March 2012
Keywords:
2-Halocycloheptanones
Conformation analyses
Infrared
Theoretical calculations
NBO
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
also used density functional methods (DFT) calculations, but with-
out discuss the molecular geometry of the compounds [5].
The conformational study of cyclohexanones and other six-
membered rings is an important part of organic chemistry;
however, higher cycloalkanones have not received as much atten-
tion, partially due to the reasonable number of conformers present
in larger rings.
Conformations of cycloheptanone have been studied by molec-
ular mechanic calculations, which have indicated that the
chair/twist-chair conformations are the most stable [1]. Vibrational
spectral analyses of cycloheptanone have also been carried out [2].
These analyses have shown that cycloheptanone is a molecule that
presents non-rigid pseudorotation around chair/twist-chair struc-
tures.
IR analyses of the carbonyl stretching of 2-chloro- and 2-
bromine-cycloheptanone indicated that the C X and C O dipoles
of the most stable conformers are not coplanar [6]. Another study
with these compounds [7] using IR, UV, and dipole moment data
led to the early conclusion that in nonpolar solvents the axial
conformer is preferred, while in polar solvents the equatorial con-
former predominates. However, the other halogens and different
cycloheptanone conformations were not analyzed.
In the present work, IR spectroscopy and theoretical calcula-
tions were used to investigate the conformational preference of
2-halocycloheptanones and analyze the stereoelectronic effects
involved in the conformational preference of these little investi-
gated systems in organic physical chemistry.
Studies at B3LYP and CCSD(T) with 6−311+G(d,p) basis set have
shown that cycloheptanone adopts a more stable asymmetric con-
formation [3]. In a similar way, the analysis of the radical anion
of cycloheptanone through molecular mechanics calculations and
ESR spectra have also shown that the asymmetric conformation of
cycloheptanone is the most stable form [4].
2. Experimental and theoretical methods
2.1. Synthesis of the compounds
1H and 13C NMR studies of non-substituted cycloalkanones pro-
vided experimental data of chemical shifts in several solvents, and
2.1.1. 2-Fluorocycloheptanone (1)
Synthesized by means of direct fluorination of the
corresponding
ketone
with
1-chloromethyl-4-fluoro-1,4-
diazoniabicyclo[2,2,2]octane bis-tetrafluoroborate, as described in
the literature [8]. 1H NMR ı (400.13 MHz, CDCl3): 5.05 (1H, ddd);
2.56 (2H, m); 1.99 (2H, m); 1.83 (2H, m); 1.63 (4H, m). 13C NMR
∗
Corresponding author. Tel.: +55 4430113662; fax: +55 4430114125.
E-mail address: eabasso@uem.br (E.A. Basso).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.03.027

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T.C. Rozada et al. / Spectrochimica Acta Part A 94 (2012) 277–287
ı (100.62 MHz, CDCl3): 208.9 (d); 95.5 (d); 40.0 (s); 31.4 (d); 28.4
(s); 25.3 (d); 23.2 (s).
at 250 ◦C, a duration of 32 min, and injections of 1.0 ␮L of solutions
containing 1.0 mg/5.0 mL of solvent. In this case the solvent used
was ethyl acetate, and the carrier gas was helium (99.999%) with a
2.1.2. 2-Chlorocycloheptanone (2)
flow of 1.0 mL min−1
.
Obtained by reacting the corresponding ketone with sulfuryl
chloride, as described in the literature for similar compounds [9].
1H NMR ı (300.06 MHz, CDCl3): 4.44 (1H, dd); 2.76 (1H, m); 2.54
(1H, ddd); 2.25 (1H, dddd); 1.93 (3H, m); 1.61 (4H, m). 13C NMR
ı (75.46 MHz, CDCl3): 206.5; 64.2; 40.4; 34.4; 29.1; 26.1; 24.2. IR
2.5. Theoretical calculations
Theoretical calculations were performed using the software
package Gaussian 03 [12]. Geometries were optimized with the
density functional B3LYP [13] and the aug-cc-pVDZ [14] basis set.
For the iodine atom, a mixed basis set function, LanL2DZ-ECP, was
used [15]. Besides the optimization in vacuum, the compounds
were also optimized in the same solvents used in the IR analy-
sis (CCl4, CHCl3, and CH3CN) with the same level of theory of the
vacuum calculations and the polarization continuum model (PCM)
[16] solvation model. The optimized geometries were submitted
to frequency calculations to characterize them as a minimum or
transition state and to obtain the thermodynamic properties and
simulate the frequencies of IR absorption.
ꢁC (cm−1): 1718.4. GC–MS: 10.441 min (145.943 m/z).
O
2.1.3. 2-Bromocycloheptanone (3)
Prepared from the corresponding ketone and N-
bromosuccinimide (NBS) in the presence of ammonium acetate
[10]. 1H NMR ı (300.06 MHz, CDCl3): 4.38 (1H, dd); 2.87 (1H,
ddd); 2.50 (1H, ddd); 2.37 (1H, dddd); 1.99 (3H, m); 1.76 (1H, m);
1.57 (2H, m); 1.38 (1H, m). 13C NMR ı (75.46 MHz, CDCl3): 206.5;
53.9; 39.6; 34.4; 29.8; 27.0; 25.2. IR ꢁC (cm−1): 1710.7. GC–MS:
O
11.745 min (191.871 m/z).
In addition to the DFT methods, energy calculations were
performed with the ab initio method MP2 [17] and the
6−311++G(3df,3dp) [18] basis set for hydrogen, carbon and oxy-
gen atoms. For iodine atom was used the LanL2DZ-ECP basis set.
These energy calculations were made both in the vacuum and in
the presence of solvents, in this case employing the PCM calcula-
tion using the integral equation formalism (IEF-PCM) [19] solvation
model and the description of the molecular cavity through bond’s
atomic radii.
2.1.4. 2-Iodocycloheptanone (4)
These compounds were prepared through the formation of the
lithium enolate of the corresponding ketones with LDA (lithium
diisopropylamine) and subsequent reaction with molecular iodine
[11]. 1H NMR ı (300.06 MHz, CDCl3): 4.57 (1H, dd); 2.95 (1H, ddd);
2.40 (2H, m); 2.05 (2H, m); 1.87 (2H, m); 1.51 (2H, m); 1.22 (1H, m).
13C NMR ı (75.46 MHz, CDCl3): 208.0; 38.4; 35.8; 32.1; 30.3; 29.0;
25.6. IR ꢁC (cm−1): 1701.0. GC–MS: 12.371 min (237.907 m/z).
O
Calculations involving the theory of natural bonding orbitals
(NBO) were made using the structures optimized in vacuum and
the B3LYP/aug-cc-pVDZ level through module NBO 5.0 [20] of the
Gaussian 03 program.
2.2. Infrared experiments
Infrared analyses were performed in a BOMEN FT-IR MB-100
spectrometer using 0.03 mol L−1 solutions and a NaCl cell with
an optical path of 0.5 mm to observe the carbonyl band in the
fundamental stretching region (1800–1600 cm−1) and a quartz
cell with 1.0 cm of optical path for the first overtone region
(3500–3300 cm−1). The solutions were prepared with solvents with
different polarities (CCl4, CHCl3 and CH3CN) so that the effect of the
medium on the conformational preference could be assessed. The
spectra were acquired with 64 scans and resolution of 1 cm−1. The
percentage of conformers was determined after the deconvolution
of the bands in the fundamental stretching regions of the carbonyl
group and in its first overtone using the program GRAMS, available
in the BOMEM equipment, which utilizes Lorentzian and Gaussian
functions.
3. Results and discussion
According to molecular mechanics studies by Langley et al. [21],
there are basically two conformation families for cycloheptanone:
The cycloheptane ring in the chair/twist-chair conformation (C/TC)
or in the boat/twist-boat conformation (B/TB) (Fig. 1).
The relative conformational energies (Table 1) for the cyclohep-
tanone indicated the structure C/TC-2 with the carbonyl group in
position two in relation to the symmetry axis C2 of cycloheptane as
the structure of lowest energy, but with an energy difference for the
C/TC-1 structure, which has a C2 symmetry of only 0.70 kcal mol−1
.
However, the frequency calculations characterized the latter struc-
ture as a transition state and further optimization with the halogen
substituted in the ␣ position led to the more stable structure C/TC-
2. The optimized and frequency calculations, in vacuum and in the
solvent presence, were performed for all the ring conformations
containing the halogen in position 2. These calculations showed
that the conformation C/TC-2 is the most stable one. The others
conformations were either less stable than or converged to C/TC-2.
Considering the energy difference observed, the structure C/TC-2 of
2.3. NMR experiments
The spectra were obtained in a Varian spectrometer, model Mer-
cury Plus BB operating at 300.06 MHz for the hydrogen nucleus and
75.46 MHz for the carbon nucleus. A Bruker Avance III spectrome-
ter was also used operating at 400.13 MHz and 100.62 MHz for 1
H
and 13C, respectively. Samples were prepared as solutions of 20 mg
of solute in 0.7 mL of CDCl3 and probe temperature around 25 ◦C
were used. Me4Si was used as an internal reference under typical
operating conditions for 1H (spectrum window around 4000 Hz and
number of data points equal to 32k, zero filled with 128k, to give a
final digital resolution of 0.03 Hz).
Table 1
Relative conformational energies of cycloheptanone (kcal mol−1).
a
b
Conformation
Erel
Erel
C/TC-1c
C/TC-2
C/TC-3
C/TC-4
B/TB
0.70
0.00
2.35
2.45
2.62
0.80
0.00
1.69
2.40
2.07
2.4. GC–MS experiments
The chromatograms were taken in a Focus DSQ II appara-
tus from Thermo-Finnigan with a DB5 column (5% phenyl, 95%
polydimethylsiloxane). The analyses were performed with a tem-
perature gradient with initial temperature of 40 ◦C and maximum
temperature of 250 ◦C, with the injector and the ionization source
a
B3LYP/aug-cc-pVDZ.
MP2/aug-cc-pVDZ.
b
c
Transition state.

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279
Fig. 1. Conformations of cycloheptanone.
cycloheptanone was taken as the initial point in the optimization
of 2-halocycloheptanones.
the data obtained from the deconvolution for compounds 1–4 are
given in Table 3.
From the structure C/TC-2 of cycloheptanone, we found four
non-equivalent possibilities of substitution in the ␣ position, two
equatorial conformations (I and III), and two axial conformations
(II and IV), as shown in Fig. 2. Some parameters obtained with the
optimization and frequency calculations in vacuum, as well as the
relative energy for compounds 1–4, are given in Table 2.
Besides the theoretical calculations, the IR spectra were
obtained to observe the stretching behavior of the carbonyl with
the variation of the solvent, and through the analytical deconvolu-
tion of the bands, to estimate the number and molar fraction of the
conformers present in the solution using the intensity (absorbance)
and the relative area of each band. The carbonyl band is appro-
priate for this type of analysis as it has a strong absorption in a
relatively clear region of the IR spectrum. The region of the first
overtone of the carbonyl was also analyzed to prevent the inter-
ference of Fermi’s resonance on the fundamental stretching [22].
Although each conformer may has a distinct molar absorptivity for
3.1. 2-Fluorocycloheptanone (1)
The relative energy data in Table 2 indicate a preference for
conformer II when the molecule in vacuum was considered. This
conformer has the lowest dipole moment (2.28 D), and thus it is not
favored in solvents with greater polarity. Conformer I has an inter-
mediate dipole moment (3.47 D) and energy relative to conformer
II of 1.69 kcal mol−1. Conformer III has the lowest relative energy in
relation to conformer II in vacuum (1.27 kcal mol−1) and the highest
dipole moment (4.80 D), which must favor this conformer in sol-
vents with greater dielectric constant. Despite its greater relative
energy in vacuum, conformer IV has a high dipole moment (4.52
D), which may favor it in polar condensed phases.
Based on the wavenumber of the calculated carbonyl stretch-
ing, it was possible to attribute the bands observed in IR spectra
(Fig. 3) to the possible conformers and to estimate their respec-
tive populations in solvents with different polarities (CCl4, CHCl3,
and CH3CN). In CCl4, four distinct bands were observed in the fun-
damental stretching but only three on the carbonyl first overtone.
On the first overtone, the band with the highest wavenumber was
assigned to conformer IV, the intermediate band to conformer III
and the band with the greatest intensity and the lowest wavenum-
ber to conformer II. In CHCl3 and CH3CN these three bands were
also observed both on fundamental and first overtone region.
The deconvolution data shown in Table 3 indicate that con-
former II is preferred in CCl4, corresponding to 50% of the
equilibrium. As the medium polarity increases, the population of
conformer II decreases to 21% and 5% in CHCl3 and CH3CN, respec-
tively, while the population of III increases to 35%, 60%, and 75%
in CCl4, CHCl3, and CH3CN, respectively. The percentage of IV also
increases with the increase in the medium polarity.
C
O absorption [23] the use of the same extinction coefficients for
different conformations is very often carried out to estimate the
percentages of conformers by IR at a single temperature, and this
procedure is usually well accepted [24–26]. The discussion of the
theoretical calculations and the IR analysis are given below, and
3.2. 2-Chlorocycloheptanone (2)
According to the analysis of the dipole moment data and the
relative energy of the conformers of 2, conformer II is expected to
occur in a significant amount in apolar solvents, but to decrease as
the polarity of the medium increases, as it has the lowest dipole
moment (2.68 D). Conformer I has an intermediate dipole moment
(3.78 D) and the lowest energy in vacuum and probably has a sig-
nificant population in the different solvents. Despite the highest
Fig. 2. Conformations of 2-halocycloheptanones (X = F, Cl, Br, and I).

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T.C. Rozada et al. / Spectrochimica Acta Part A 94 (2012) 277–287
Table 2
Parameters calculated at the B3LYP/aug-cc-pVDZ level (LanL2DZ-ECP for iodine) for 1–4.
Compound
Conformer
Parameters
a
b
c
e
f
g
ꢂX
ωX
rC
ꢃ d (D)
ꢄC
(cm−1
)
IꢄC
(km mol−1
)
Erel (kcal mol−1
)
˚
(A)
C
C
O
C2 C1
X
O
O
1
I
II
III
IV
110.31
−150.05
−17.04
−12.62
106.56
110.05
108.86
108.80
1.419
1.416
1.397
1.399
3.47
2.28
4.80
4.52
1777.46
1767.76
1784.93
1789.85
153.7
165.3
179.7
169.9
1.69
0.00
1.27
1.84
2
3
4
I
II
III
IV
99.03
−133.08
−29.25
−16.00
106.50
111.07
110.37
110.38
1.839
1.840
1.818
1.820
3.78
2.68
4.70
4.54
1735.88
1721.04
1749.14
1749.86
161.2
175.3
140.1
140.7
0.00
0.11
1.46
1.96
I
II
III
IV
97.55
−128.10
−33.27
−21.13
106.19
110.82
110.44
110.32
2.004
2.006
1.981
1.987
3.80
2.83
4.62
4.50
1729.73
1715.56
1747.06
1744.43
160.9
177.5
129.1
132.9
0.00
0.55
2.13
2.64
I
II
III
95.54
−117.92
−38.04
106.38
109.77
110.65
2.217
2.222
2.192
3.66
3.06
4.30
1757.56
1742.60
1779.17
166.9
184.0
125.8
0.00
0.99
2.87
a
ꢂX
ωX
dihedral angles between the halogen and carbonyl oxygen.
angles between the halogen and carbon 1.
C
C O
b
c
d
e
f
C2 C1
rC bond length of the carbon–halogen.
ꢃ dipole moment.
ꢄC carbonyl frequency.
IꢄC carbonyl frequency intensity.
X
O
O
g
Relative energy of the conformers.
energy of conformer III, its dipole moment is high enough (4.70
D) to give significant percentages in solvents with moderate and
high polarities. Due to the greater energy of IV and its high dipole
moment (4.54 D), it should have a considerable percentage only in
more polar solvents.
but was the second in CH3CN. In the last solvent, the population
of conformer IV was similar to that of II. The analysis in the first
overtone of carbonyl revealed the same behavior as the analysis of
the fundamental stretching.
The IR analysis of 2 through the deconvolution of the car-
bonyl stretching bands in the fundamental and the first overtones
(Fig. 4) indicated three bands in CCl4 and CHCl3 (a and b, respec-
tively). The band with the lowest wavenumber was assigned to
conformer II, the most intense band, of intermediate frequency,
to the more stable conformer I, and the band with the highest
wavenumber, to conformer III. In CH3CN, a fourth band with the
highest wavenumber was assigned to conformer IV. The popula-
tion of each conformer was estimated based on the intensities and
the relative areas of each band in the IR spectra (Table 3).
3.3. 2-Bromocycloheptanone (3)
The theoretical calculations of 3 revealed a greater preference
for conformer I than that observed for 2. Again, conformer II had
the lowest dipole moment (2.83 D) and conformers III and IV, the
highest values (4.62 and 4.50 D, respectively). However, the rela-
tive energy of conformer I for the latter two increased, mainly for
IV (from 1.96 to 2.64 kcal mol−1 in 2 and 3, respectively), which
probably does not have a significant population in equilibrium in
the condensed phases.
In relation to the effect of the solvent, there was a decrease in the
percentage of II in equilibrium as the medium polarity increased,
while the percentage of III increased until it became the main con-
former in CH3CN. Conformer I predominated in CCl4, and CHCl3
The deconvolution (Fig. 5) gave two bands in CCl4; the one with
the highest wavenumber was attributed to conformer I, and that
with the lowest intensity and wavenumber to conformer II. In
CHCl3 and CH3CN, a third band with a wavenumber higher than
Table 3
Percentage of conformers for 1–4 obtained from the average between the relative intensities and areas of the fundamental and first overtone carbonyl stretching in the IR
spectra in selected solvents.
Compound
Conformer
ꢄC fundamental
ꢄC overtone
O
O
CCl4
CHCl3
CH3CN
CCl4
CHCl3
CH3CN
1
I
II
III
IV
5
50
35
10
21
60
19
5
75
20
61
27
12
22
61
17
2
I
II
III
IV
55
32
13
51
19
30
36
10
41
13
54
32
14
51
21
28
36
11
42
11
I
II
III
86
14
72
18
10
70
12
18
79
21
71
18
11
68
19
19
3
4
I
II
III
81
16
3
79
21
78
15
7
75
18
7
76
14
10

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281
Fig. 3. Carbonyl absorption band in the IR spectrum of 2-fluorcycloheptanone (1) in (a) CCl4, (aꢀ) CCl4 (first overtone), (b) CHCl3, (bꢀ) CHCl3 (first overtone), and (c) CH3CN.
that of conformer I was observed and attributed to conformer III.
This band was relatively less intense than that of conformer II in
CHCl3 but stronger than in CH3CN, as expected, due to the greater
dipole moment of III. Conformer I predominated in all the analyzed
solvents with estimated populations of 86%, 72%, and 70% in CCl4,
CHCl3, and CH3CN, respectively. Again, the results obtained in the
analysis of the fundamental stretching and of the first overtone of
carbonyl agreed.
which was also greater. However, the high dipole moment of III
(4.30 D) suggests that in more polar solvents, a small fraction of
this conformer must be favored.
The analysis of the deconvolution (Fig. 6) of the fundamental
stretching of carbonyl for 4 indicated a clear preference for one
of the conformers. CCl4 showed three bands. The band with the
lowest wavenumber was assigned to conformer II, the strongest
band and of intermediate wavenumber, to conformer I, and the
band of lowest intensity and highest wavenumber to conformer
III. In CHCl3, the band relative to III was observed only in the first
overtone. The data in Table 3 show that the population of I remains
stable, while that of III slightly increases as that of II decreases.
The preference for conformer I was even more pronounced in
compound 4 than in 2 and 3. In contrast, for conformer II, it had a
smaller decrease in relation to observed in 2 and 3. This is related
to the greater dipole moment of II in compound 4 in relation to the
other compounds.
3.4. 2-Iodocycloheptanone (4)
For this compound, the optimization calculations resulted in
only three conformers, with conformer I being the most stable.
In this case, the dipole moment of conformer II (3.06 D) is close
to that of I (3.66 D), but the difference in energy between them
(0.99 kcal mol−1) is greater than in the case of compounds 2 and
3, like the difference in energy for conformer III (2.87 kcal mol−1),

282
T.C. Rozada et al. / Spectrochimica Acta Part A 94 (2012) 277–287
Fig. 4. Carbonyl absorption band in the IR spectrum of 2-chlorocycloheptanone (2) in (a) CCl4, (aꢀ) CCl4 (first overtone), (b) CHCl3, (bꢀ) CHCl3 (first overtone), (c) CH3CN and
(cꢀ) CH3CN (first overtone).
3.5. Solvation calculations
conformer II increased with the increase in the medium polarity,
which can be explained by the smaller dipole moment of II in rela-
tion to the other conformers. The difference in energy of conformers
III and IV regarding conformer I decreased with the increase in the
polarity of the medium, which also agreed with the difference in
dipole moment between them.
The parameters of comparison with the data obtained in IR anal-
ysis were obtained through solvation calculations for the three
solvents used. Energy calculation at a higher level also was per-
formed and the results are given in Table 4.
The solvation calculations presented the same tendency
observed in the IR analysis. For 1, the most stable conformer in vac-
uum and in CCl4 was II, while in CHCl3 and CH3CN, conformer III
was more stable, with an increase in the relative energy of II with
the increase in the medium polarity. The relative energy of con-
former I was the highest in the solvation calculations of 1, which
indicates that this conformer does not contribute effectively to the
equilibrium of this compound.
3.6. Conformational preference
It has been suggested that the orbital interaction
ÁX → ꢀ*C
is important in the conformational preference of
2-halocyclohexanones [27–30] and that the overlapping of these
orbitals increases as the volume of halogen and the proximity in
O
energy of the orbitals ÁX and ꢀ*C
increase. For the cyclohex-
O
For 2–4, conformer I was the most stable in all solvents, with the
exception of 2 in CH3CN, in which conformer III was calculated with
only 0.05 kcal mol−1 more stable than I. The difference in energy for
anones, this interaction is greater for the axial conformer than
for the equatorial one and increases in the order F < Cl < Br < I,
according to the difference in axial-equatorial energy of these

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283
Fig. 5. Carbonyl absorption band in the IR spectrum of 2-bromocycloheptanone (3) in (a) CCl4, (aꢀ) CCl4 (first overtone), (b) CHCl3, (bꢀ) CHCl3 (first overtone), and (c) CH3CN
and (cꢀ) CH3CN (first overtone).
compounds, which in the vapor phase is 0.45, 1.05, 1.50, and
1.90 kcal mol−1, respectively [31].
2-halocyclohexanones. The interaction ÁX → ꢀ*C
was observed
O
only in conformers I and II in 2-halocycloheptanones, and the
interaction
X → ꢀ*C was far more pronounced in these same
To investigate these orbital interactions, second-order pertur-
bation theory analyses were performed with software NBO 5.0
of Gaussian 03. This type of calculation describes the chemical
structure of a molecule based on the bonds between two atoms
and lone pairs in a way very similar to the Lewis structures and
allows analyzing charge transfers of bonding orbitals to antibond-
ing orbitals, as well as the associated energetic implications. The
main orbital interactions observed for 2-halocycloheptanones are
shown in Table 5. These orbital interactions have a stabilizing
character and the greater the interaction energy, the greater the
resulting stabilizing effect.
C
O
conformers, which contributed to the lower energy of conformer
I and II observed in the calculations with the molecules in the
vacuum. With the increase in halogen, these interactions in
conformers III and IV were also observed, although with lower
intensity than those observed for conformers I and II. For com-
pound 1, the effect of the volume of the substituent was still
important, as the calculations show that conformer II is the most
stable in vacuum.
The greater energy of interaction observed for conformers I and
II can be understood based on the geometry of each conformer,
in which the position of the substituent is important for an
appropriate overlapping of orbitals to occur. The closer to 180◦
or 0◦ the angle between the orbitals involved is, the better the
The main interactions observed were ÁX → ꢀ*C
and
These interactions increase with the increase
in the halogen volume, in a way similar to that reported for
O
X → ꢀ*C
.
O
C

284
T.C. Rozada et al. / Spectrochimica Acta Part A 94 (2012) 277–287
Fig. 6. Carbonyl absorption band in the IR spectrum of 2-iodocycloheptanone (4) in (a) CCl4, (aꢀ) CCl4 (first overtone), (b) CHCl3, (bꢀ) CHCl3 (first overtone), and (c) CH3CN.
Table 4
Percentage of conformers for 1–4a calculated in different media.
Compound
Conformer
B3LYP/aug-cc-pVDZ
MP2/6−311++G(3df,3dp)
Vacuum
CCl4
CHCl3
CH3CN
Vacuum
CCl4
CHCl3
CH3CN
1
I
II
III
IV
4.7
82.0
9.6
4.4
55.5
30.7
9.4
3.0
25.3
55.7
16.0
0.9
5.6
78.3
15.2
2.1
95.0
2.2
2.2
74.3
9.6
1.6
44.3
24.1
30.0
0.6
15.1
36.9
47.4
3.7
0.7
13.9
2
3
I
II
III
IV
51.2
42.5
4.4
51.4
32.0
12.3
4.3
47.0
17.7
27.4
7.9
36.7
8.5
40.0
14.8
51.7
43.7
2.0
55.9
32.6
5.1
51.8
23.9
10.4
13.9
36.7
12.9
20.4
30.0
1.9
2.6
6.4
I
II
III
IV
69.7
27.6
1.9
74.4
19.6
4.3
71.7
16.5
8.7
64.1
11.9
17.8
6.2
82.1
15.4
1.3
83.9
10.9
2.8
82.1
7.9
5.2
73.9
5.4
9.9
0.8
1.7
3.1
1.2
2.4
4.8
10.8
4
I
II
III
83.6
15.7
0.7
83.2
15.7
1.1
76.8
21.7
1.5
87.9
9.0
3.1
91.4
7.7
0.9
92.9
5.9
1.2
94.1
4.6
1.3
94.3
3.6
2.1
a
LanL2DZ-ECP for iodine.

T.C. Rozada et al. / Spectrochimica Acta Part A 94 (2012) 277–287
285
Table 5
Main NBO orbital interactions energies (kcal mol−1) and ꢀ*C occupancy calculated at the B3LYP/aug-cc-pVDZ level (LanL2DZ-ECP for iodine) for 1–4.
O
Compound
Conformer
Orbital interaction
Occupancy ꢀ*C
O
ÁX → ꢀ*C
X → ꢀ*C
→
*
C
→
*
C
ꢀC
→
*
C
ÁO
→
*
C X
˙
O
C
O
C
X
O
C
O
X
O
X
1
I
II
III
IV
2.02
0.66
–
1.92
0.62
–
0.75
1.59
–
–
0.82
–
3.87
1.62
–
–
1.71
–
8.56
7.02
0
0.0683
0.0775
0.0809
0.0783
–
–
–
–
–
–
0
2
3
4
I
II
III
IV
2.55
1.30
–
4.61
2.37
0.52
–
0.54
1.65
–
–
0.77
–
3.71
3.03
–
–
1.34
–
11.41
10.46
0.52
0
0.0839
0.0799
0.0782
0.0772
–
–
–
–
–
I
II
III
IV
2.41
1.37
–
6.20
3.56
0.86
0.66
–
1.67
–
–
0.75
–
3.79
3.47
0.51
–
–
1.30
–
12.40
12.12
1.37
0.0924
0.0846
0.0788
0.0785
–
–
–
–
0.66
I
II
III
2.08
1.51
–
9.30
6.71
1.70
–
1.44
–
–
0.60
–
3.28
3.61
0.56
–
1.05
–
14.66
14.92
2.23
0.1043
0.0960
0.0814
Table 6
overlapping. For I and II, the dihedral angle (Table 2) between the
halogen and the oxygen of the carbonyl is favorable to the orbital
overlapping, while for III and IV, this interaction is not favored
Relative energy (kcal mol−1) before and after deletion of the ꢀ*C O orbital calculated
at the B3LYP/aug-cc-pVDZ level (LanL2DZ-ECP for iodine) for 1–4.
Compound
Conformer
Erel (kcal mol−1
)
(Fig. 7). The occupancy data of orbital ꢀ*C
were also analyzed
O
(Table 5) and demonstrated the same tendency observed in the
Before Deletion
After Deletion
orbital interaction energies. The largest the occupancy of ꢀ*C
the bigger the electronic delocalization (hyperconjugation) to this
orbital.
To evaluate the importance of the interactions involving the
orbital ꢀ*C O, NBO calculations were performed without this orbital
so that the energy obtained for the molecule did not include the
electronic delocalization for this orbital. The differences in energy
between conformers are given in Table 6. These calculations indi-
O
1
I
II
III
IV
1.69
0.00
1.27
1.84
4.26
0.00
1.57
2.12
2
3
4
I
II
III
IV
0.00
0.11
1.46
1.96
2.86
0.00
0.81
1.46
I
II
III
IV
0.00
0.55
2.13
2.64
2.50
0.00
0.40
0.91
cate that the interactions involving the ꢀ*C orbital are actually
O
important for the stabilization of conformers I and II, mainly for
compounds 2–4. In these compounds, with the removal of only the
ꢀ*C orbital, conformer II became more stable, but had reduced
O
I
II
III
0.00
0.99
2.87
0.14
0.00
2.82
energy in relation to conformers III and IV. Without the ꢀ*C O, con-
formers III and IV became more stable than conformer I. However,
with the increase in the size of the halogen, the energy difference
between I and II becomes smaller. As a result, the calculations
of the deletion of the ꢀ*C
orbital indicated that this orbital is
O
directly involved in the greater stability observed for conformer I
in 2–4.
For 1, conformer IV has the lowest exchange energy, while for
2 and 3, it is conformer II, and for 4, it is conformer III. Despite the
greater exchange energy observed for conformer I, the stabilizing
Another factor that was evaluated was the exchange energy
[32,33] of each conformer, whose energy has a destabilizing charac-
ter. The energy of these interactions is given in Table 7. The analysis
of these data indicates that conformer I has the greatest exchange
energy, which indicates a greater destabilizing effect.
effect afforded by the interactions involving the ꢀ*C
orbital
O
compensates the destabilizing effect of the exchange energy,
which justifies the lower stability of this conformer after the
deletion of the ꢀ*C orbital. The preference for conformer II after
O
Fig. 7.
and ꢀ*C orbital representation for 2-halocycloheptanones.
O
C
X

286
T.C. Rozada et al. / Spectrochimica Acta Part A 94 (2012) 277–287
Table 7
Main exchange orbital interactions energies (kcal mol−1) calculated at the B3LYP/aug-cc-pVDZ level (LanL2DZ-ECP for iodine) for 1–4.
Compound
Conformer
Orbital interaction
X → ꢀC
→
ÁX → ÁO
ÁX → ꢀC
ÁX
→
ÁO
→
˙
C
O
C
X
C
O
O
C
O
C X
1
I
II
III
IV
3.29
1.04
–
–
–
–
0.52
–
0.72
–
–
–
–
–
–
0.96
1.01
4.01
2.37
2.07
1.64
1.33
0.59
0.63
–
–
–
–
2
3
4
I
II
III
IV
5.01
2.68
1.01
–
–
–
–
0.70
1.49
1.15
–
0.59
–
–
–
–
–
–
1.23
1.49
6.16
3.99
4.13
4.40
1.31
0.60
0.84
0.58
I
II
III
IV
5.77
3.37
1.36
0.63
–
–
–
0.58
1.36
1.06
–
0.68
–
–
–
–
0.61
–
–
1.39
1.75
6.83
4.34
4.80
5.46
0.97
0.79
1.11
I
II
III
6.26
4.57
1.76
–
0.76
0.83
–
–
–
0.95
–
0.75
–
–
–
–
–
1.18
7.21
5.33
4.52
the deletions can also be attributed to the small exchange energy
observed for this conformer.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2012.03.027.
4. Conclusion
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medium starts to contribute less to the conformational preference,
as the more polar conformers start to have higher relative energy
in relation to the more stable conformer with the increase in the
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Acknowledgments
The authors would like to thank the Brazilian Council for Sci-
entific and Technological Development (CNPq) for the fellowship
for E.A.B. and R.R. Thanks also go to Fundac¸ ão Araucária (Grant
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support and to the Coordination for the Improvement of Higher
Education Personnel (CAPES) for the scholarship for T. C. Rozada.
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