Concentration and solvent effects on the conformational equilibrium of cis-3-ethoxycyclohexanol by 1H NMR and IR spectroscopy

Article abstract of DOI:10.1016/j.molstruc.2005.11.009

NMR data, in CCl₄, show that an increase in the concentration of cis-3-ethoxycyclohexanol (cis-3-ECH) shifts the conformational equilibrium from the ax-ax conformer (X_ax-ax=51% at 0.01 mol L^-1), stabilized by an intramolec

Full text of DOI:10.1016/j.molstruc.2005.11.009

Journal of Molecular Structure 788 (2006) 16–21
www.elsevier.com/locate/molstruc
Concentration and solvent effects on the conformational equilibrium
1
of cis-3-ethoxycyclohexanol by H NMR and IR spectroscopy
Paulo R. de Oliveira , Danilo S. Ortiz, Roberto Rittner
*
Physical Organic Chemistry Laboratory, Chemistry Institute, State University of Campinas, Caixa Postal 6154, 13084-971 Campinas, SP, Brazil
Received 21 September 2005; received in revised form 7 November 2005; accepted 8 November 2005
Available online 20 December 2005
Abstract
NMR data, in CCl
4
, show that an increase in the concentration of cis-3-ethoxycyclohexanol (cis-3-ECH) shifts the conformational equilibrium from the
K1
ax–ax conformer (Xax–axZ51% at 0.01 mol L ), stabilized by an intramolecular hydrogen bond (IAHB), to the eq–eq conformer (Xeq–eqZ67%
K1
at 0.40 mol L ), which forms intermolecular hydrogen bonds (IEHB), as confirmed by IR data. The Dn values, obtained by IR spectra, indicated that cis-
3
-ECH presents a stronger IAHB than the cis-3-methoxycyclohexanol, due to the increase in substituent steric and group electronegativity effects.
K1
The percentage of eq–eq conformer also increases with the solvent basicity, from 51% (DG
K1
–
eqeq axax
ZK0.03 kcal mol
in CCl4 to 97%
K1
(DGeqeq–axaxZK2.05 kcal mol ) in DMSO. Values of 4.58 and 5.39 kcal mol , for the IAHB strength in cis-3-ECH, were obtained from the
theoretical data at CBS-4M and B3LYP/6-311CG** levels, respectively.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Conformational analysis; Intramolecular hydrogen bond; NMR spectroscopy; IR spectroscopy; Theoretical calculations
1
. Introduction
different solvents. The IAHB energy in the gas phase was also
determined, using theoretical calculations.
Hydrogen bonding plays an important role in determining
the three-dimensional structures adopted by proteins and
nucleic acids, like in DNA and RNA structures where the
double helixes are formed due to the presence of hydrogen
bonds between the strands [1–4].
2
. Experimental
2.1. Spectra
Substituted six-membered rings are useful models for
studies of conformational analysis and hydrogen bonding.
Previous works have shown that if OH groups, of a dihydroxy
compound, are sufficiently close to each other, they may form
an intramolecular hydrogen bond [5]. Abraham et al. [6] have
The IR spectra were recorded on a BOMEM Model 100 FT-
K1
IR spectrometer, from 0.01, 0.03 and 0.10 mol L solutions
of the cis-3-ECH, in a NaCl cell with spacing of 0.5 mm. They
K1
were performed with 64 scans and resolution of 1 cm . The
1
H NMR spectra, for the study of solvent and concentration
effects, were recorded on a INOVA 500 spectrometer with
1
also demonstrated, through H NMR and theoretical data, that
1
probe temperature at 20 8C, operating at 499.88 ( H) and
the strong OH/F hydrogen bond in transK2-fluorocyclohex-
anol is responsible for the predominance of its eq–eq
conformer.
1
25.70 MHz ( C). Spectra were recorded at concentrations of
3
1
0
0
K1
.05 mol L
.40 mol L
for the study of solvent effects, and at 0.01–
in CCl4 (CCl /C D 9:1, the later for the
K1
The aim of the present work is to describe how inter- and
intramolecular hydrogen bonds (IEHB and IAHB, respect-
ively) affect the conformational equilibrium of cis-3-ethox-
ycyclohexanol (cisK3-ECH) at different concentrations and in
4
6 6
deuterium lock) for the study of concentration effects. In all
cases, SiMe4 (TMS) was used as internal reference. The
spectral window ensured a digital resolution of at least
0
.04 Hz/point, and zero-filling helped to further define line
shapes. Most FIDs were processed with Gaussian multipli-
cation, typically gfZ0.25 and gfZ0.35, for spectral resolution
improvement, without changes in the lb parameter. The typical
*
Corresponding author. Tel./fax: C55 19 3788 3150.
E-mail addresses: pro@iqm.unicamp.br (P.R. de Oliveira), rittner@iqm.
unicamp.br (R. Rittner).
1
conditions for H spectra were: 128 transients, 32 k data points,
pulse width 378, sweep width ca. 3000 Hz and acquisition time
0
022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.11.009
1
(AT) ca. 2.7 s; and for C NMR spectra: 1024 transients, 32 k
3

P.R. de Oliveira et al. / Journal of Molecular Structure 788 (2006) 16–21
17
data points, pulse width 458, sweep width ca. 10,000 Hz and
(62%; 2.3 g) and 2-cyclohexen-1-ol. cisK3-ECH was further
purified through column chromatography, using hexane–ethyl
acetate (10:1) as eluent and silica gel 230–400 mesh. The main
fractions were analyzed by GC-MS, with helium as the carrier
1
AT 1 s. Assignment of the signals in H and C NMR spectra
13
K1
of cisK3-ECH, at a concentration of 0.30 mol L , were
performed through gCOSY and HSQC experiments. The
quantum chemical calculations were made with the Gaussian
gas. The GC-MS column was DB1 SUPELCO. Similar
1
98 package [7]. Optimized geometries were computed at the
B3LYP level of theory [8–11], using the 6–311CG** basis
fractions were combined and the solvent evaporated.
NMR (500 MHz, CDCl
CH ), 3.45 (tt, 7.61, 3.74, H3), 2.03 (m, H2e), 1.84 (m, H5e),
1.73 (m, H6e), 1.72 (m, H4e), 1.56 (m, H2a), 1.45 (m, H4a),
H
), d3.73 (tt, 7.87, 3.84, H1), 3.50 (m,
3
sets [12]. The IAHB strength was computed at Hartree-Fock
(
2
HF), MP2 and B3LYP levels of theory, using the 6–311CG**
1
3
basis sets, and also with the CBS-4M method [12–15]. The
potential energy surfaces (PES) were obtained at HF and
B3LYP levels, with 6–311CG** as basis sets, by changing the
C –C –O–H dihedral angle for the ax–ax1 rotamer by 108 until
1.42 (m, H6a), 1.29 (m, H5a), 1.20 (t, 7.00, CH
(125 MHz, CDCl ), d75.7 (C3), 68.3 (C1), 63.7 (C7), 39.3
(C2), 34.7 (C6), 30.4 (C4), 18.2 (C5), 15.6 (C8);
3
). C NMR
3
2
1
completing 3608. For each 108, the structure obtained was
optimized.
3. Results and discussion
3.1. Concentration effects
2
.2. Compound
1
H NMR spectra of 9 samples in CCl (with 10% C D ),
4
6 6
cis-3-Ethoxycyclohexanol (cisK3-ECH): 8.3 g (60 mmol)
of aluminium chloride was placed in a round-bottomed flask
fitted with magnetic stirrer and external ice bath. 3.0 g
with concentrations of 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30,
K1
.35 and 0.40 mol L , were recorded and the results are
0
displayed in Table 1.
3
(
30 mmol) of 2-cyclohexen-1-one were added gradually and
Table 1 shows that the coupling constant ( J) values for the
H-1 and H-3 hydrogen of cis-3-ECH (Fig. 1) increase
significantly on increasing the concentration, which is clearly
due to a change in the conformer populations. For dilute
solutions, the ax–ax conformer is favored, but the increase in
concentration makes eq–eq the predominant conformer.
The molar fraction (X) and free energy difference
(DGeqeq–axax) for eq–eq and ax–ax conformers (Fig. 1) of
cisK3-ECH were determined through Eqs. (1)–(3), taking
the hydrogen coupling constant values of the eq–eq and
the mixture was homogenized with stirring. 50 mL of ethanol
were added and the reaction mixture was stirred at room
temperature for 2 h. Water was added to destroy the excess of
AlCl . The reaction mixture was extracted with diethyl ether,
3
dried over MgSO , filtered and the solvent was evaporated. The
4
product obtained (3-ethoxycyclohexanone) was gradually
added to a 3-necked 250 mL round bottomed flask containing
a suspension of lithium aluminum hydride (0.6 g, 16 mmol) in
tetrahydrofuran (60 ml), under stirring at K10 8C and a
nitrogen atmosphere. The mixture was allowed to warm to
room temperature and stirred for another 1.5 h. Water was
carefully added to destroy the excess of lithium aluminum
hydride. The reaction mixture was extracted with diethyl ether,
3
3
ax–ax conformers individually ( J
3
Z JH1a/H6aZ11.23,
H1a/H2a
3
3
3
JH1e/H2eZ J
Z2.73, J
Z JH3a/H4aZ11.23 and
H3a/H2a
H1e/H6e
3
3
JH3e/H2eZ JH3e/H4eZ3.42), obtained from optimized struc-
tures utilizing the MM3 method through the PCMODEL
[16] program, with Haasnoot–Altona equations [17], since
3
the experimental data for vicinal coupling constants ( Jobs)
are averaged values (Table 1).
dried over MgSO , filtered and the solvent was evaporated. The
4
product could not be distilled, due to decomposition, but was
analyzed using GC-MS, indicating a mixture of cis-3-ECH
Table 1
3
Hydrogen H-1 and H-3 coupling constants ( J), equatorial–equatorial molar fractions (Xeq–eq) and energy differences (DGeqeq–axax) for cis-3-ECH at different
4
concentrations, utilizing CCl as solvent
3
3
3
3
3
J
3
3
J
3
a,b
X
a,b
DG
Conc.
J
H1/H2a and JH1/H6a
J
H1/H2e and JH1/H6e
H3/H2a and JH3/H4a
H3/H2e and JH3/H4e
0.01
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
6.90
7.08
7.23
7.47
7.66
7.86
8.07
8.26
8.44
3.60
3.62
3.64
3.69
3.78
3.85
3.91
3.98
4.04
–
–
–
–
0.49
0.51
0.53
0.56
0.58
0.60
0.63
0.65
0.67
0.02
K0.03
K0.07
K0.14
K0.19
K0.25
K0.31
K0.37
K0.43
7.13
7.45
7.64
7.85
8.05
8.23
8.41
3.53
3.64
3.72
3.78
3.85
3.93
3.99
K1
In Hz. Concentration in mol L . For a mixture of CCl
4
6 6
/C D (9:1).
a
3
3
Molar fraction and DGeqeq–axax obtained from experimental coupling constants ( JH1/H2a and
J
H1/H6a) and calculated by the PCMODEL program for H-1
3
3
3
3
hydrogen ( JH1a/H2aZ JH1a/H6aZ11.23 and JH1e/H2eZ JH1e/H6eZ2.73, obtained from Eq. (1)).
b
K1
In kcal mol
.

18
P.R. de Oliveira et al. / Journal of Molecular Structure 788 (2006) 16–21
OH
OCH CH
(a)
2
3
OCH CH
99
2
3
H3
H1
HO
ax-ax
eq-eq
9
8
Fig. 1. Conformational equilibrium for cisK3-ECH.
3
3
3
3
97
XeqKeq Z ð J K J
obs
Þ=ð J
K JH1e=H2eÞ
(1)
(2)
H1e=H2e
H1a=H2a
3
3
3
3
Xeq–eq Z ð J K J
Þ=ð J
K JH3e=H2eÞ
obs
H3e=H2e
H3e=H2e
9
6
Since Xeq–eqCXax–axZ1, Eqs. (1) or (2) was used according
to the observable J values (Eq. (1) is for H1 and Eq. (2) for H3),
3700
3600
3500
Frequency (cm )
3400
3300
-1
0
the free energy difference (DG ) can be readily obtained from
K1 K1
(b)
Eq. (3), where RZ0.00199 kcal mol
K
, TZ298 K and
9
9
K ZXeq–eq/Xax–ax for cis-3-ECH isomer.
1
0
DG ZKRT ln K
(3)
96
1
The molar fraction of the diequatorial conformer (eq–eq) and
9
3
the free energy (DGeqeq–axax) between the eq–eq4ax–ax
K1
conformers, for a concentration range of 0.01–0.40 mol L
in CCl are also given in Table 1. Positive DGeqeq–axax values
4
90
show that the ax–ax conformer predominates for low
concentrations. Its stability can be attributed to an IAHB,
while an increase in concentration makes DGeqeq–axax negative,
favoring the eq–eq conformer due to the prevalence of an IEHB
over the IAHB.
8
7
3
700
3600
3500
Frequency (cm )
3400
3300
-1
The IR spectra of cisK3-ECH at 0.01, 0.03 and
K1
(c) 95
90
0
Fig. 2. IR spectra of cis-3-ECH at low concentrations (0.01 and
.10 mol L , in CCl , were recorded and are shown in
4
K1
.03 mol L ) show two OH bands attributed to the two
0
conformers, the high frequency vibration corresponding to the
free OH (eq–eq conformer) and the other to OH bonded to the
OCH CH substituent (ax–ax conformer), which predominates
8
8
7
5
0
5
2
3
in relation to the eq–eq conformer. A third band, at lower
frequency, appears in the spectrum of the more concentrated
K1
solution (0.10 mol L ), which is due to an intermolecularly
bonded OH, as shown in Fig. 2(c). The OH stretching
frequency and the frequency shifts (Dn), between free and
bonded OH, are given in Table 2.
70
3
700
3600
3500
Frequency (cm )
3400
3300
It has been reported by Kuhn [18] that the conformational
equilibria of trans-cyclohexane-1,2-diol and cis-cyclohexane-
-1
1
,3-diol are shifted towards the diequatorial and diaxial
Fig. 2. Infrared spectra of cis-3-ECH in CCl
K1
4
solutions: 0.01 (a), 0.03 (b)
conformers, respectively, which have the OH groups close to
each other, allowing the formation of hydrogen bonds. Kuhn
and 0.10 mol L
(c).
[
transK1,2-diol (32 cm ) to the cis-1,3-diol (75 cm ), while
18] noted that the frequency shifts (Dn) increase from the
K1
K1
Table 2
OH Infrared frequencies for cisK3-ECH
Badger [19] showed that the strength of the hydrogen bond
increases with the increase in Dn values. Thus, it can be
a
b
c
Conc.
Free OH
Bonded OH
Dn
K1
concluded that the Dn average value 99G1 cm
(Table 2),
0
0
.01
.03
3622
3621
3621
3523
3523
3522
99
98
99
obtained for the cisK3-ECH, indicates that its IAHB energy is
much larger than for the reported diols, probably because the
OCH CH oxygen is a better hydrogen acceptor than the OH
0.10
2
3
K1
K1
In cm , with 1 cm digital resolution.
group [20]. Moreover, the Dn values obtained indicate also that
cis-3-ECH presents an IAHB stronger than that for cisK3-
a
b
c
K1
Concentration in mol L
The OH intermolecularly bonded frequency is w3450 cm
Frequency difference between free and bonded OH bands in cm
.
K1
.
K1
methoxycyclohexanol (92G3 cm ) previously reported [21].
K1
.

P.R. de Oliveira et al. / Journal of Molecular Structure 788 (2006) 16–21
19
Table 3
3
Hydrogen H-1 and H-3 coupling constants ( J), equatorial–equatorial molar fractions (Xeq–eq) and energy differences (DGeqeq–axax) for cis-3-ECH, in solvents of
different dielectric constants (3) and basicities (SB)
3
3
3
3
3
3
3
3
a,c
X
a,b,c
Solvent
SB
3
J
J
H1/H2a and
H1/H6a
J
J
H1/H2e and
H1/H6e
J
H3/H2a and
H3/H4a
J
J
H3/H2e and
H3/H4e
DG
J
c
CCl
CDCl
CD CN
Acetone-d
Pyridine-d
DMSO-d
4
0.04
0.07
0.29
0.48
0.58
0.65
2.24
4.81
7.08
7.37
3.62
3.62
–
–
0.51
0.54
0.87
0.89
0.91
0.97
K0.03
K0.09
K1.12
K1.25
K1.34
K2.05
3
7.63
3.72
3.94
3.99
4.19
4.10
d
d
3
37.50
20.70
12.40
46.70
–
–
10.21
10.38
10.49
10.99
d
d
6
–
–
5
10.78
d
–
4.16
d
–
6
K1
In Hz. Concentration: 0.05 mol L . Ref. [23].
a
3
Molar fraction and DGeqeq–axax obtained from experimental coupling constants ( JH3/H2a and H4a) and calculated by the PCMODEL program for H-1 hydrogen
3
3
3
3
(
see Table 1) and H-3 hydrogen ( JH3a/H2aZ JH3a/H4aZ11.23 and
J
H3e/H2eZ JH3e/H4eZ3.42).
b
c
d
K1
In kcal mol
.
3
3
Values obtained from experimental coupling constants ( JH1/H2a and JH1/H6a).
2 2 3
Overlapped by CH hydrogens of OCH CH group.
These results can be attributed to differences in the ethoxy
group properties in comparison with hydroxy and methoxy
groups, as discussed in Section 3.4 (hydrogen bonding).
3.3. Theoretical calculations
The geometries for the stable conformers of cisK3-ECH
were obtained through theoretical calculations using Gaussian
98 [7], with the 6-311CG** basis set [11] from B3LYP [8–10]
level of theory. The relative energies and dipole moments of
3
.2. Solvent effects
K1
conformers with DE!3.0 kcal mol
are given in Table 4,
K1
Table 3 presents the H-1 and H-3 coupling constants for cis-
K1
-ECH at a low concentration (0.05 mol L ), in different
while the ones with DEO3.0 kcal mol were not considered,
since they represent a very small proportion in the equilibrium.
Table 4 shows 2 and 6 stable rotamers for ax–ax and eq–eq
conformers, respectively, and the corresponding geometries
are presented in Fig. 3. The rotamers ax–ax1 and eq–eq1, for
ax–ax and eq–eq conformers, respectively, are the most stable
3
solvents, to avoid interference of IEHB. H-1 and H-3 coupling
constants increase with the increase of solvent polarity,
showing that ax–ax is more stable than the eq–eq conformer
in less polar solvents, such as CCl and CDCl , due to the
4
3
favorable formation of an IAHB (Fig. 1), in the absence of
solvation effects.
ones, and ax–ax1 is more stable than eq–eq1 by
K1
.12 kcal mol , reinforcing the important role of an IAHB
1
in the conformational equilibrium of cis-3-ECH.
The experimental coupling constant (7.08 Hz) in CCl4,
associated with the calculated values obtained from the
PCMODEL program, were introduced into Eqs. (1) and (3),
leading to 51% for the eq–eq conformer in the confor-
mational equilibrium at room temperature. Therefore, the
steric effect is less important or smaller than the effect of
stabilization provided by an IAHB, for less polar solvents,
while the opposite is observed in more polar solvents, when
the eq–eq conformer becomes more stable. This behavior
can be explained by taking into account that a polar solvent
Moreover, it can be noted that ax–ax2 is also more stable than
K1
eq–eq1, sinceit can also form an IAHB, but it is 0.09 kcal mol
less stable than ax–ax1, because in the ax–ax2 rotamer one
OCH CH oxygen lone pair is pointing inside the cyclohexane
2
3
ring, increasing the steric effect, a rather different geometry in
comparison to ax–ax1. Along with ax–ax1 and ax–ax2, there are
Table 4
has a larger affinity for the OH and OCH CH groups [22].
2
3
Conformer relative energies (DE) and dipole moments (m) for cisK3-ECH
(ax–ax and eq–eq) at the B3LYP/6-311CG** level
Then, the solvent molecules can easily approach these groups
when they are equatorial, and the eq–eq conformer can be
more efficiently solvated. The increasing stabilization of
a
Conformer
DE
m
ax–ax1
ax–ax2
0.00
0.09
–
3.39
2.82
–
eq–eq conformer by the solvation effect is confirmed by the
3
data in DMSO, a very polar solvent. Thus, J
b
is
0.99 Hz in DMSO, leading to eq–eq conformer population
H3/H2a and H4a
ax–ax3
1
of 97%.
eq–eq1
eq–eq2
eq–eq3
eq–eq4
eq–eq5
eq–eq6
1.12
1.24
1.31
1.30
1.21
1.18
2.30
2.33
2.96
2.14
0.89
2.10
However, data from Table 3 show that the eq–eq proportion
is also large when the solvent is pyridine (91%), which is less
polar (3Z12.40; Table 3) than acetone and acetonitrile.
Therefore, these results show that the solvent basicity [23]
K1
In kcal mol . In debye.
(
SB, Table 3) is also an important effect in this conformational
a
Fig. 3.
b
K1
equilibrium.
DEO3.0 kcal mol
.

20
P.R. de Oliveira et al. / Journal of Molecular Structure 788 (2006) 16–21
R
H
H
O
O
R
R
H
R
O
O
O
O
ax-ax3
ax-ax1
eq-eq1
ax-ax2
eq-eq2
R
R
H
H
O
O
O
O
O
O
H
eq-eq3
R
R
R
H
H
O
O
O
O
O
O
eq-eq4
eq-eq5
H
eq-eq6
Fig. 3. Stable rotamers for cisK3-ECH obtained, at the B3LYP/6-311Cg** level.
other possible rotamers for the diaxial conformer (e.g. ax–ax3,
K1
IAHB for conformer ax–ax, with (DE ) optimization of
hb
DEO3.0 kcal mol ), which are less stablebecause they cannot
the reference structure, through HF, B3LYP and MP2 levels
with 6–311CG** basis sets, and also by the CBS-4M method
[12–15], are presented in Table 5. All methods show that IAHB
be stabilized by an IAHB. Consequently, these rotamers are
also much less stable than eq–eq rotamers (Table 4), due to the
K1
1
,3-diaxial interactions, which are large and decisive in
energy is larger than 4.58 kcal mol and show that it is also
larger than in cisK3-methoxycyclohexanol [21], when both
the conformational equilibrium of the cis isomer. It can also
be observed that most of the other possible rotamers (eq–eq2,
eq–eq3, eq–eq4, eq–eq5 and eq–eq6) of the eq–eq conformer
present stabilities near to eq–eq1, which indicates that the
rotation of the C–O bond does not introduce large changes in
their energies.
(a)
6
5
4
3
2
1
0
ax-ax3
Data from Table 4 also show that the ax–ax1 rotamer
presents a larger dipole moment than the eq–eq1 rotamer.
Therefore, an increase in solvent polarity should shift
the equilibrium towards the ax–ax conformer. However, an
opposite behavior was observed, eq–eq being favored in polar
solvents (Table 3). This means that, for cisK3-ECH, eq–eq is
more stable in more polar solvent because solvation is more
important than the dipole moment effect.
ax-ax1
100
Dihedral Angle (C -C -O-H)
0
50
150
200
250
300
350
3
.4. Hydrogen bonding
2
1
Calculations of the PES (Potential Energy Surface) through
(b)
6
HF and B3LYP levels with the 6–311CG** basis set, for the
ax–ax conformer of cis-3-ECH, were performed to
observe changes in the position of the OH hydrogen, near or
far from OCH CH , which is followed by how changes in the
5
4
3
2
1
0
ax-ax3
2
3
C –C –O–H dihedral angle from the ax–ax1 geometry affect the
2
1
rotamer energy and the formation of a hydrogen bond (IAHB).
Fig. 4 shows two minima, one corresponding to the most stable
rotamer (ax–ax1), which forms an IAHB, and another one for a
less stable (ax–ax3) rotamer (Fig. 3) with the hydrogen far from
the OCH CH group. Both theory levels (Fig. 4(a) and (b)) lead
ax-ax1
50 100
2
3
to similar results.
Moreover, hydrogen bond energies are usually calculated as
the difference between the energies of a non-bonded and a
0
150
200
250
300
350
Dihedral Angle (C -C -O-H)
2
1
bonded species (DEZE KEbonded). The calculated energy of
ref
Fig. 4. PES for the ax–ax1 rotamer: (a) B3LYP/6-311Cg**, (b) HF/6-311Cg**.

P.R. de Oliveira et al. / Journal of Molecular Structure 788 (2006) 16–21
21
Table 5
Studies of the solvent effects showed that the solvent
basicity is also an important effect, since an increase in the
solvent basicity shifts the conformational equilibrium from the
ax–ax conformer towards the eq–eq conformer.
Intramolecular hydrogen bond energy (DEhb) of rotamer ax–ax1 in comparison
to ax–ax3, for the cis-3-ECH ax–ax conformer and for the corresponding
methoxy-derivative, at several theory levels
a
Theory level
DEhb
DEhb
HF/6-311Cg**
B3LYP/6-311Cg**
MP2/6-311Cg**
CBS-4M
5.09
5.39
5.77
4.58
5.13
4.78
–
Acknowledgements
4.41
The authors thank FAPESP for financial support of this
research and for a fellowship (to P.R.O.), CNPq for a
fellowship (to R.R.) and CENAPAD-SP for computer facilities
K1
.
In kcal mol
a
cisK3-Methoxycyclohexanol, Ref. [21].
(Gaussian 98). Professor C. H. Collins’ assistance in revising
this manuscript is also gratefully acknowledged.
B3LYP/6-311Cg** as CBS-4M methods are used. However, at
HF/6-311Cg** level, the results indicate that IAHB strength is
similar for both compounds, which may be due to the fact that
the Hartree Fock method does not include the effects of electron
correlation [11].
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2
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4
. Conclusion
(
[
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2
783.
[
[
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K1
DGeqeq–axaxZ1.12 kcal mol ) the ax–ax is much more stable
than the eq–eq conformer.
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