Interaction in trans-2-halocyclohexanols - An infrared and theoretical study

Article abstract of DOI:10.1016/S0022-2860(01)00482-3

The O - H and C - O stretching frequencies of trans-2-halocyclohexanols in CCl₄ solutions have been measured and theoretical calculations have been performed to elucidate the main interactions, which are responsible for the conformational equil

Full text of DOI:10.1016/S0022-2860(01)00482-3

Journal of Molecular Structure 570 ?2001) 175±180
www.elsevier.com/locate/molstruc
Interaction in trans-2-halocyclohexanols Ð an infrared and
theoretical study
*
M.P. Freitas, C.F. Tormena, R. Rittner
Â
Ä
Instituto de Quõmica, Universidade Estadual de Campinas, CP 6154, 13083-970-Campinas, Sao Paulo, Brazil
Received 14 November 2000; revised 18 January 2001; accepted 18 January 2001
Abstract
The O±H and C±O stretching frequencies of trans-2-halocyclohexanols in CCl4 solutions have been measured and theore-
tical calculations have been performed to elucidate the main interactions, which are responsible for the conformational
equilibria in these systems. It can be concluded that hydrogen bonding is predominant for trans-2-¯uorocyclohexanol, leading
to a stabilization of the eq±eq conformation, while for the chlorine, bromine and iodine derivatives, besides hydrogen bonding,
gauche and steric interactions are also present. q 2001Elsevier Science B.V. All rights reserved.
Keywords: trans-2-Halocyclohexanols; Intramolecular interactions; Theoretical calculations; Infrared spectroscopy
1. Introduction
mation of this molecule. Similarly Bodot et al. [6]
suggest, from their IR and NMR studies, the impor-
tance of intramolecular hydrogen bonding in the
conformational equilibrium of trans-2-chlorocyclo-
hexanol.
However, for bromine, iodine and even for chlorine
substituents in trans-2-substituted cyclohexanols, the
relationship between the predominance of equatorial±
equatorial ?eq±eq) conformation and the occurrence
of hydrogen bonding is not well established, since
other interactions, such as gauche, steric and
dipole±dipole interactions can contribute substan-
tially to the conformational equilibrium in these
systems.
trans-1,2-Disubstituted cyclohexanes are useful
model compounds for the investigation of the factors
governing conformational equilibria [1,2]. Molecular
orbital theory has been applied to problems of organic
chemistry very effectively. Some theoreticians have
sought to correlate calculated properties like elec-
tronic densities, dipole moments, orbital energies,
etc. with corresponding experimental observations
[3], but the analysis of the interactions which are
important to determine the conformational prefer-
ences in 1,2-disubstituted cyclohexanes have not yet
been clearly established [4].
Abraham et al. [5] have demonstrated through
NMR and theoretical data the strong OH´´´F hydrogen
bonding in trans-2-¯uorocyclohexanol, which is
responsible for the predominance of an eq±eq confor-
2. Experimental
2.1. Syntheses
* Corresponding author. Tel.: 155-19-3788-3150; fax: 155-19-
3788-3023.
trans-2-Chloro-, bromo- and iodo-cyclohexanol
were prepared through the reaction of cyclohexene,
E-mail address: rittner@bitline.com.br ?R. Rittner).
0022-2860/01/$ - see front matter q 2001Elsevier Science B.V. All rights reserved.
PII: S0022-2860?01)00482-3

176
M.P. Freitas et al. / Journal of Molecular Structure 570 62001) 175±180
Fig. 1. Possible conformers for the trans-2-halocyclohexanols ?halo ˆ F, Cl, Br, I).
water and the corresponding N-halosuccinimide,
according to a procedure described in literature [7],
it being necessary to heat under re¯ux to obtain the
chlorohalohydrin.
trans-2-Fluorocyclohexanol was obtained by
re¯uxing cyclohexene oxide ?5.0 ml, 24.7 mmol),
KHF2 ?5.0 g, 64.1mmol) and diethyleneglycol
?20 ml) during 2 h at 1808C, to give a yield of 2.0 g
?16.9 mmol, 69%).
tion, through the BOMEM program, which utilizes
Lorentzian and Gaussian functions.
Further dilution ?,0.01mol l 21) of the samples
yielded to no changes in the O±H and C±O stretching
frequencies observed values. An additional band
corresponding to any possible intermolecular inter-
action, as observed by Shiratori et al. [9] for the halo-
acetones, was not detected, indicating that at
0.03 mol l21 intramolecular interactions were negli-
gible. However, the intensities of the bands were
very low at that concentration, leading to nonreprodu-
cible values for the measurements of I.
2.2. Theoretical calculations
The energies and the more stable geometries for the
conformers of the trans-2-halocyclohexanols were
obtained from potential energy surface ?PES) through
DFT calculations with the Becke3LYP method with
6-31gpp ?for ¯uorine, chlorine and bromine substitu-
ents) and 3-21g ?for the iodine substituent) basis sets
of the Gaussian 98 program [8]. HOMO and LUMO
orbitals for the more stable eq±eq molecules were
visualized performing AM1calculations over the
optimized geometries at B3LYP/6-31gpp and 3-21g
levels, respectively.
3. Results and discussion
Theoretical calculations at B3LYP/6-31gpp ?for the
F, Cl and Br derivatives) and 3-21g ?for the iodine
derivative) levels have shown that the more stable
conformers for the trans-2-halocyclohexanols are
the ones shown in Fig. 1. It must be borne in mind
that the geometries from DFT are quite correct [10],
although the corresponding energies may present
deviations from the values obtained through more
accurate methods ?e.g. MP2). However, their relative
values can be used to compare the stabilities of the
conformers within a limited set.
For all the halogen derivatives, the eq±eq g2
conformation, which brings the OH hydrogen close
to the halogen atom, allowing the establishment of a
hydrogen bond, is far more stable than the others in
the vapor phase ?Table 1) and these results can be
2.3. Infrared measurements
The FTIR spectra were recorded on a BOMEM
MB-100 spectrometer from CCl4 solutions of
0.03 mol l21, in a NaCl cell with spacer of 0.5 mm.
They were performed with 20 scans and resolution of
1cm 21. The intensities of the O±H stretching vibra-
tions for axial±axial and equatorial±equatorial
conformations were measured after band deconvolu-

Table 1
Dihedral angles ?u,8), conformer energies ?E, kJ mol21) and dipole moments ?m, D) for trans-2-halocyclohexanols
Conformer
Fa
Cla
Bra
Ib
u?H±C±O±H)
Erel
m
u?H±C±O±H)
Erel
m
u?H±C±O±H)
Erel
m
u?H±C±O±H)
Erel
m
eq±eq
ax±ax
g1
257.9
67.9
11.97
0
2.98
1.81
3.04
1.22
1.71
1.98
252.4
62.3
12.51
0
3.55
2.18
3.58
1.59
2.33
2.58
253.8
64.9
15.06
0
3.48
2.22
3.48
1.55
2.21
2.48
252.4
60.0
10.50
0
3.70
2.15
3.68
1.78
2.45
2.68
g2
anti
g1
g2
178.9
253.9
64.6
12.47
9.58
8.28
12.05
178.0
250.9
68.2
12.09
12.38
12.38
14.60
176.1
251.4
67.5
14.90
11.09
11.97
13.76
178.3
254.0
66.6
8.54
7.70
8.87
13.18
anti
167.1
161.0
162.2
156.9
a
Values obtained using the B3LYP/6-31gpp level.
Values obtained using the B3LYP/3-21g level.
b

178
M.P. Freitas et al. / Journal of Molecular Structure 570 62001) 175±180
Table 2
C±O and O±H stretching wavenumbers ?cm21) and intensity ratios
bonds. From the density matrix of the theoretical
calculations, the bond order between F and O
?which represents the gauche effect) and the bond
order between the hydrogen bonded to the oxygen
and F ?which represents the hydrogen bonding) are
obtained. The former presented an almost null value,
actually negative ?20.0036), which means that a
repulsive interaction between O and F lone pairs
predominates slightly over the attractive interaction,
involving nO and nF orbitals via the sCpC orbital [3].
However, the latter value is positive ?0.0157), which
suggests that, in fact, hydrogen bonding must be
occurring in the eq±eq g2 conformation of trans-2-
¯uorocyclohexanol.
Although, each line of Fig. 2 correspond to just four
points and bear no statistical signi®cance, it is very
impressive that Fig. 2c ?nOH?ax±ax) versus x, for ax±ax
conformers) is strictly linear ?correlation coef®cient
0.998), and it agrees with the expectation that for
these conformers no interaction between the halogen
and OH group can occur.
For the remaining halogens ?Cl, Br and I), the
observed linearity ?or close to linearity) in Fig. 2
does not mean that no intramolecular interaction is
occurring in the eq±eq g2 conformation, which is
far more stable than all other conformations in these
trans-2-halocyclohexanols. Thus, while there is a
signi®cant stabilization of the eq±eq g2 conformers,
the same does not occur for the ax±ax conformers,
although their gauche ?ax±ax g2 and g1) conforma-
tions are more stable than the anti ones, for the whole
series.
for the eq±eq and ax±ax conformers of trans-2-halocyclohexanols
Substituent
nC±O
nO±H
Ieq±eq/Iax±ax
eq±eq
eq±eq
ax±ax
3633
F
1082
3614
41.4
Cl
Br
I
08135912
2.366211
1078
1073
3581
3563
3620
3616
11.6
5.5
extended also to nonpolar solutions, such as CCl4, in
which the infrared spectra were performed.
The prevalence of the eq±eq g2 conformation, in
comparison to the remaining ones, allows us to
consider just this conformer for the analysis of the
interactions involving trans-2-halocyclohexanols.
This predominance is represented by the ratio
between the O±H stretching vibration intensities of
the eq±eq and ax±ax conformers, Ieq±eq/Iax±ax
?Table 2). However, these ratios are estimation,
since the true values depend on the molar absorptivity
of the individual O±H bond vibrations, which are
different for each conformer [6]. It should be noted
that the O±H and C±O stretching vibrations were
chosen for performing the study of the trans-2-halo-
cyclohexanols here described, since they are easily
identi®ed and do not suffer noticeable interference
from other vibrations, while the C±halogen stretching
vibration, which have been used for conformational
studies of halocyclohexanes [11] could not be unequi-
vocally assigned in the present case.
In the absence of any kind of intramolecular inter-
actions, a parallelism between nOH?eq±eq) or nC±O?eq±eq)
and the halogen electronegativity ?x) should be
observed, since the bond order depends on the substi-
tuent electronegativity and correlates also with the C±
O and O±H bond force constants. The observed
trends, approximately represented by straight lines
in Fig. 2, indicate that the ¯uorine derivative deviate
signi®cantly in Fig. 2a ?nOH?eq±eq)) and to a minor
extent in Fig. 2b ?nC±O?eq±eq)). These deviations, due
to nOH?eq±eq) and nC±O?eq±eq) values, which are smaller
than expected from the ¯uorine electronegativity, are
the result of the weakening of the O±H and C±O
bonds. Both the known gauche effects [3,12] as
hydrogen bonding F´´´HO [5] will favor the eq±eq
g2 conformation and weaken the O±H and C±O
Therefore, for the trans-2-X-cyclohexanols
?X ˆ Cl, Br and I) intramolecular interactions must
also be present, as suggested by the results of theore-
tical calculations. Bond orders between Cl, Br, I and
the OH hydrogen are positive ?attractive interaction),
an indication of hydrogen bonding, but signi®cantly
weaker than those of the ¯uorine derivative. The steric
effect ?an ax±ax conformation presents 1,3-diaxial
repulsions) and the gauche interaction ?Cl . Br . I)
should also be important. Although the calculations
demonstrate negative values of bond order between
the halogen ?X) and the oxygen atoms ?20.0049,
20.0089 and 20.0034 for Cl, Br and I, respectively),
which would represent a predominance of nX and nO
steric repulsion over nX and nO attraction via the sCpC
orbital [3], the graphic representation of the HOMO

M.P. Freitas et al. / Journal of Molecular Structure 570 62001) 175±180
179
Fig. 2. a) vC±Oꢀeq±eq†, b) vO±Hꢀeq±eq† and c) vO±Hꢀax±ax† vs. electronegativity parameter x of the halogen.

180
M.P. Freitas et al. / Journal of Molecular Structure 570 62001) 175±180
reveals a bonding interaction between nX and nO,
except for iodine. However, it should be borne in
mind that the I±O bond order value was calculated
using a different basis set in relation to F, Cl and Br.
Obviously, the increase in the halogen atom size
results in an increase of nX and nO repulsion, leading
to the data of Table 2, where Ieq±eq/Iax±ax decreases
from 41.4 when X ˆ F to 5.5 when X ˆ I. This means
that the ax±ax conformation population increases
aproximately from 2% when X ˆ F to 15% when
X ˆ I.Thelast®gureswereobtainedtakingintoaccount
that Ieq±eq/Iax±ax < peq±eq/pax±ax ?p ˆ percentage) and
peq±eq 1 pax±ax ˆ 100%: Again, these values are quali-
tative, since the molar absorptivity values between the
conformers must be different.
Thus, it can be concluded that the main factor for
the preference of the eq±eq over ax±ax conformation
in trans-2-¯uorocyclohexanol is hydrogen bonding,
while for the chloro-, bromo- and iodo-derivatives,
the gauche effect and hydrogen bonding, as well as
the steric effect, are important. The increase of the
halogen atom size leads to a signi®cant increase in
the ax±ax conformation population.
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