Conformational analysis of meso- and (±)-2,3-dicyano-2,3-dicyclopropylbutane and 1,2-dicyanotetracyclopropylethane

Article abstract of DOI:10.1039/b103740b

The dipole moments of meso-2,3-dicyano-2,3-dicyclopropylbutane meso-1, (±)-2,3-dicyano-2,3-dicyclopropylbutane (±)-1 and 1,2-dicyanotetracyclopropylethane 2 in carbon tetrachloride and benzene have been measured over a range of temperatures. Analyses of the relative permittivity data show that at 25°C, meso-1 and (±)-1 favour the trans form. However, replacement of the methyl groups in 1 with cyclopropyl moieties completely reverses the trans ? gauche equilibrium, such that 2 exists in 74% gauche conformation. The experimentally derived values of the energy difference between the gauche and trans conformers and the gauche/trans population quotients were compared with values predicted by molecular orbital calculations. Theory predicts that the conformational preference of the substituted dicyanoethanes is strongly influenced by solvent polarity. The crystal and molecular structures of meso-1 and 2 were determined by single-crystal X-ray diffraction methods. Both compounds exist in the trans conformation in the solid state.

Full text of DOI:10.1039/b103740b

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Conformational analysis of meso- and (»)-2,3-dicyano-2,3-
dicyclopropylbutane and 1,2-dicyanotetracyclopropylethane¤
Yu-Lin Lam,* Ming Wah Wong, Hsing-Hua Huang and Eping Liang
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543.
E-mail: chmlamyl=nus.edu.sg
Received (in Montpellier, France) 25th April 2001, Accepted 22nd June 2001
First published as an Advance Article on the web 18th September 2001
The dipole moments of meso-2,3-dicyano-2,3-dicyclopropylbutane meso-1, (^)-2,3-dicyano-2,3-dicyclopropylbutane
(^)-1 and 1,2-dicyanotetracyclopropylethane 2 in carbon tetrachloride and benzene have been measured over a
range of temperatures. Analyses of the relative permittivity data show that at 25 ¡C, meso-1 and (^)-1 favour the
trans form. However, replacement of the methyl groups in 1 with cyclopropyl moieties completely reverses the
trans H gauche equilibrium, such that 2 exists in 74% gauche conformation. The experimentally derived values of
the energy di†erence between the gauche and trans conformers and the gauche/trans population quotients were
compared with values predicted by molecular orbital calculations. Theory predicts that the conformational
preference of the substituted dicyanoethanes is strongly inÑuenced by solvent polarity. The crystal and molecular
structures of meso-1 and 2 were determined by single-crystal X-ray di†raction methods. Both compounds exist in
the trans conformation in the solid state.
Preparation of compounds
Introduction
Our interest in polar and steric e†ects on rotational isomerism
2,3-Dicyano-2,3-dicyclopropylbutane.
2,3-Dicyano-2,3-di-
in symmetrically substituted ethanes has led us to investigate
the inÑuence of the cyano group in such molecules. Earlier
works on 1,2-dicyanoethane1 have shown that this compound
exists predominantly in the gauche form. However, studies on
dicyano compounds containing phenyl and alkyl
substituents2h5 have shown that these compounds exists
mainly in the trans conformation, with the exception of 9,9@-
dicyano-9,9@-biÑuorenyl, which favours the gauche conÐgu-
ration.6 Cyclopropyl groups are known to exhibit peculiar
chemical and physical properties that are signiÐcantly di†er-
ent from those of alkanes or cycloalkanes. They have been
shown to exhibit double bond character, qualifying them as
““functional carbon groupsÏÏ.7 We now report our Ðndings on
meso-2,3-dicyano-2,3-dicyclopropylbutane meso-1 (^)-2,3-
dicyano-2,3-dicyclopropylbutane (^)-1 and 1,2-dicyano-
tetracyclopropylethane 2 based on dipole moment determi-
nations, X-ray di†raction measurements and molecular orbital
calculations.
cyclopropylbutane 1, was prepared from methyl cyclopropyl
ketone by the method of Overberger and Lebovits.8 The reac-
tion mixture obtained was concentrated to yield a yellow oil
which upon recrystallisation with hexane gave meso-1 as a
white solid (1.62 g, 44% based on methyl cyclopropyl ketone);
mp 41 ¡C. Found: C, 76.66; H, 8.68; N, 14.90%. C
(mw 188) requires: C, 76.70; H, 8.51; N, 14.89%. d (500
H
N
12 16
2
H
MHz; CD Cl ; Me Si) 1.63 (s, 6H), 1.19È1.13 (m, 2H), 0.81È
2
2
4
0.74 (m, 4H), 0.69È0.62 (m, 2H), 0.51È0.45 (m, 2H). The
residual solution was puriÐed by Ñash chromatography over
silica gel (hexane) to yield (^)-1 as a colourless oil (0.85 g,
23% based on methyl cyclopropyl ketone). Found: C, 76.42;
H, 8.50; N, 14.76%. C
H
N (mw 188) requires: C, 76.70;
12 16
2
H, 8.51; N, 14.89%. d (500 MHz; CD Cl ; Me Si) 1.56 (s,
H
2
2
4
6H), 1.17È1.11 (m, 2H), 0.80È0.75 (m, 4H), 0.68È0.62 (m, 2H),
0.51È0.46 (m, 2H).
1,2-Dicyanotetracyclopropylethane. 1,2-Dicyanotetracyclo-
propylethane 2 has previously been prepared in two steps by
the reaction of bis(dicyclopropylmethylene)hydrazine 3 with
anhydrous hydrogen cyanide.9 Due to difficulties in obtaining
hydrogen cyanide, we decided to use sodium cyanide and
herein report the direct synthesis of 2 from 3.
Compound 3 was prepared from dicyclopropyl ketone by
the method of Bernlohr and co-workers.9 To a solution of
compound 3 (2.16 g, 10.0 mmol) in pentane (15 ml) was added
an ethanolic solution of sodium cyanide (4.41 g, 90.0 mmol)
and the mixture stirred vigorously. Concentrated HCl (7.46
ml, 90 mmol) was added dropwise10 and the mixture was
stirred for 8 days at 52 ¡C. The brown solution obtained was
Experimental
Reagents for the syntheses were purchased from Aldrich and
used without further puriÐcation. 1H NMR spectra were
recorded on a Bruker Advance DRX500 spectrometer. Ele-
mental analyses were determined with a Perkin-Elmer PE
2400 CHN elemental analyser and melting points were mea-
sured using a Thomas Hoover capillary melting point appar-
atus and are uncorrected.
¤ Electronic supplementary information (ESI) available: Table S1
concentrated, diluted with diethyl ether, washed with H O
Calculated geometries of meso-1, (^)-1,
2 and 4. see http://
2
www.rsc.org/suppdata/nj/b1/b103740b/
and puriÐed by silica gel chromatography (hexaneÈ
DOI: 10.1039/b103740b
New J. Chem., 2001, 25, 1325È1329
1325
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dichloromethane 3 : 1 and 1,2-dichloroethane) to give 2 as a
light yellow solid (0.86 g, 36%), mp 73 ¡C (lit.9 77È78 ¡C).
CCDC reference numbers 164827 and 164828. See http://
www.rsc.org/suppdata/nj/b1/b103740b/ for crystallographic
data in CIF or other electronic format.
Found: C, 79.87; H, 8.52; N, 11.46%. C
H
N (mw 240)
16 20
2
requires: C, 79.96; H, 8.39; N, 11.65%. d (500 MHz; CDCl ;
H
3
Molecular orbital calculations
Me Si) 1.32È1.27 (m, 4H), 0.82È0.77 (m, 4H), 0.71È0.60 (m,
4
12H).
Standard ab initio and density functional calculations were
carried out using the GAUSSIAN 98 series of programs.15
Geometry optimizations were performed at the B3-LYP16
level using the split-valence polarized 6-31G* basis set.17
Higher-level relative energies were computed at the B3-LYP/
6-311&G** level, using the B3-LYP/6-31G* optimized geome-
tries and including the zero-point energy (B3-LYP/6-31G*
value, scaled by a factor of 0.9804).18 For 1,2-dicyanoethane,
the gauche/trans equilibrium was also investigated at the
G3(MP2) theory.19 In the present study, we have employed
the OnsagerÏs self-consistent reaction Ðeld (SCRF) theory20 at
the dipole level to examine the soluteÈsolvent interaction. The
cavity radii were derived from the calculated molecular
volumes, computed using a Monte Carlo method at the HF/6-
31G* level.21 The calculated gauche/trans energy di†erences
Dipole moment determination
Relative permittivities were determined with a heterodyne-
beat meter11 and densities and refractive indices by standard
procedures.12 All solvents were carefully distilled and dried
before use. The physical constants required for the relative
permittivity have been given previously.4,13
Crystal structure determination and reÐnement
Single crystals of meso-1 were obtained from hexane and of 2
from benzeneÈhexane.
Crystal data for meso-1: C
H
N , M \ 188.27, mono-
12 16
2
clinic, colourless prisms, a \ 7.3831(7), b \ 8.2193(8),
(*H ) for various compounds are summarized in Table 3. The
c \ 9.7902(9) A, b \ 100.292(2)¡, V \ 584.6(1) AŽ 3, space group
0
free energy di†erences (*G) were computed from the equation
P2 /n, Z \ 2, D \ 1.070
g cm~3, crystal dimensions:
1
x
*G \ *H [ T *S, where *S is the entropy change and
0.40 ] 0.30 ] 0.30 mm, k(Mo-Ka) \ 0.064 mm~1, 2748 reÑec-
T
T
T
0
*H \ *H ] (H [ H ).
tions measured, 1023 unique (R \ 0.0292) which were used
in all calculations. The Ðnal R and R were 5.99 and 18.17%,
T
0
int
w
respectively, [for I [ 2p(I)].
Results and discussion
Crystal data for 2: C
H
N , M \ 240.34, monoclinic,
16 20
2
colourless prisms, a \ 7.0406(3), b \ 14.7756(4), c \ 6.9714(3)
Dipole moment measurements
A, b \ 104.185(1)¡, V \ 703.12(5) A
Ž
3, space group P2 /c,
1
The results of the dipole moment measurements of meso-1,
(^)-1 and 2 are presented in Table 1 with standard notation.
Three concentration dependencies, namely those of the rela-
tive permittivities, densities and refractive indices (ae , bd and
Z \ 2,
D \ 1.135
g
cm~3.
Crystal
dimensions:
x
0.50 ] 0.46 ] 0.40 mm, k(Mo-Ka) \ 0.067 mm~1, 4330 reÑec-
tions measured, 1728 unique (R \ 0.0227) which were used
in all calculations. The Ðnal R and R were 4.38 and 11.79%,
int
1
1
w
cd ) were determined for each solvent at the three tem-
respectively, [for I [ 2p(I)].
12
peratures. Using the least squares method, the experimental
Data collections were performed at room temperature using
a Siemens R3m/V200 di†ractometer with Mo-Ka radiation
(j \ 0.710 60 A). Lorentz and polarization corrections, struc-
ture solution by direct methods, full-matrix least-squares
reÐnements and preparation of Ðgures were all performed by
programs in the SHELXTL-Plus PC program package.14 All
non-hydrogen atoms were reÐned anisotropically, whereas
hydrogen atoms were placed at calculated positions and
assigned isotropic displacement coefficients 1.6 times that of
the atom to which they are attached.
values of the slopes ae , bd and cn 2 [given by eqn. (1)] at
1
1
1
inÐnite dilutions of the compounds (w denoting the solute
2
weight fraction) and the respective molar polarisation, refrac-
tions and dipole moments were calculated.
A
d*eB Ad*dB Ad*n2
B
ae \
bd \
cn 2 \
2 w2?0
(1)
1
1
1
dw
dw
dw
2 w2?0
2 w2?0
By measuring the dielectric e†ects in extremely dilute solu-
tions of the compounds in non-polar solvents like carbon
Table 1 Molar polarization, refractions and dipole moments at inÐnite dilution of meso-2,3-dicyano-2,3-dicyclopropylbutane meso-1, (^)-2,3-
dicyano-2,3-dicyclopropylbutane (^)-1 and 1,2-dicyanotetracyclopropylethane 2
Conc. range
T /¡C
Solvent
(105 w )
ae
b
c
P /cm3
R /cm3
ka/10~30 C m
2
1
2
D
meso-1 (R \ 53.305 cal)
7
25
45
7
25
45
CCl
230È485
260È560
240È565
298È602
266È590
240È571
4.04
4.09
3.93
3.93
3.84
3.63
[0.618
[0.591
[0.729
0.068
0.140
0.142
133.4
136.4
141.7
193.7
191.2
198.3
6.41 ^ 0.03
6.74 ^ 0.02
7.14 ^ 0.04
8.37 ^ 0.04
8.55 ^ 0.03
8.75 ^ 0.04
D
4
CCl
4
0.022
53.08
54.14
CCl
4
Benzene
Benzene
Benzene
[0.003
(^)-1 (R \ 53.305 cal)
7
25
45
7
25
45
CCl
240È500
540È750
230È640
410È900
330È725
310È775
5.27
5.54
5.21
4.83
4.57
4.50
[0.557
[0.530
[0.607
0.176
0.142
0.158
155.1
163.2
164.1
217.2
216.5
220.5
7.22 ^ 0.02
7.65 ^ 0.02
8.01 ^ 0.03
9.09 ^ 0.04
9.43 ^ 0.02
9.76 ^ 0.03
D
4
CCl
4
0.028
53.37
53.19
CCl
4
Benzene
Benzene
Benzene
[0.004
2 (R \ 67.784 cal)
7
25
45
7
25
45
CCl
150È400
200È430
220È420
400È900
330È800
380È700
8.32
7.57
6.34
5.94
5.18
4.16
[0.741
[0.635
[0.763
0.228
0.180
0.096
281.4
264.8
246.5
321.5
301.3
270.7
10.37 ^ 0.02
10.26 ^ 0.02
10.07 ^ 0.03
11.31 ^ 0.04
11.19 ^ 0.02
10.74 ^ 0.03
D
4
CCl
4
0.055
0.047
71.53
71.55
CCl
4
Benzene
Benzene
Benzene
a P \ 1.05R
D
D
1326
New J. Chem., 2001, 25, 1325È1329

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Fig. 1 Thermal ellipsoid and Newman projection diagrams of meso-1.
tetrachloride and benzene, we can interpret our present
system as consisting of isolated molecules immersed in a well-
deÐned environment. The dipole moments were determined
using the method of LeFevre and Vine.12,22
contrast with those of meso-2,3-dicyano-2,3-dicyclopentyl-
butane and meso-2,3-dicyano-2,3-dicyclohexylbutane. In the
latter two compounds, the dipole moment decreases with
increasing temperatures, indicating that the gauche conformer
is more stable than the trans form in this solvent. This obser-
vation further conÐrms the stabilization e†ect of benzene on
the gauche conformer.
meso-2,3-Dicyano-2,3-dicyclopropylbutane meso-1. Table 1
shows that the dipole moment of this compound increases
with increasing temperature in both carbon tetrachloride and
benzene solutions, indicating that the trans conformer is more
stable than the gauche, and is higher in population in these
solvents. Application of the Lennard-JonesÈPike method of
analysis23 to our dipole moment data in carbon tetrachloride
yields a *E( \ E [ E ) value of 5.21 kJ mol~1 and a gauche
(»)-2,3-Dicyano-2,3-dicyclopropylbutane (»)-1. The dipole
moments of (^)-1 in carbon tetrachloride and benzene are
higher than those of meso-1 in the corresponding solvents.
This is similar to the behaviour of most meso/(^) diastereo-
isomers, including that of 2,3-dicyano-2,3-dicyclopentylbutane
and 2,3-dicyano-2,3-dicyclohexylbutane.2,3
Analysis of the data in Table 1 shows that the dipole
moments obtained in both solvents increase with increasing
temperature, indicating that the trans conformer is more
stable than the gauche and is higher in population in these
solvents. Application of the Lennard-JonesÈPike method of
analysis23 to our dipole moment data yields *E values of 4.48
and 3.83 kJ mol~1 in carbon tetrachloride and benzene,
respectively, and the percentage gauche population at 25 ¡C
was found to be 25% in carbon tetrachloride and 30% in
benzene. These results contrast with (^)-2,3-dicyano-2,3-di-
cyclopentylbutane and (^)-2,3-dicyano-2,3-dicyclohexylbu-
tane which exist predominantly as the trans conformer in
carbon tetrachloride but prefer the gauche conÐguration in
benzene solution.2,3
g
t
conformer dipole moment (k ) of 15.26 ] 10~30 C m.
g
Assuming that k is independent of temperature, an esti-
g
mate of the gauche conformer population (x%) in solution can
be made from eqn. (2), which on substituting the observed
moment and k values yields a population of 19% gauche and
g
81% trans at 25 ¡C.
100k
2
obs
k 2
x \
(2)
g
Lennard-JonesÈPike analysis of the dipole moment data in
benzene gives gauche conformer dipole moment of
a
13.74 ] 10~30 C m and *E value of 2.86 kJ mol~1. This cor-
responds to a population of 38% gauche and 62% trans con-
formers at 25 ¡C. These results are comparable with 2,3-
dicyano-2,3-dimethylbutane, which exists in carbon tetra-
chloride and benzene as 18 and 43% gauche conformation,
respectively.4 Since the dielectric constants of carbon tetra-
chloride and benzene are similar in value, the larger gauche
population indicated in benzene is consistent with earlier
observations that polar solutes and aromatic solvents interact
to form weak complexes and when rotational isomers are
involved, there is a tendency for speciÐc solvent interactions to
be established with the more polar isomer.24 The preferential
stabilization of the gauche conformer is explicable in terms of
the gauche conÐguration having two cyano groups closer
together. The p-electron cloud of benzene would interact with
these polar groups in the region away from the bulkier cyclo-
propyl and methyl moieties. Compared with the gauche con-
former, the trans form would have the cyano groups
““sandwichedÏÏ between the cyclopropyl and methyl moieties
and thus cause hindrances during p-complex formation.
1,2-Dicyanotetracyclopropylethane 2. The large dipole
moments in carbon tetrachloride and benzene means that the
polar gauche conformer must be present in high proportion in
these solvents. From Table 1, it can also be seen that the
dipole moment of the compound decreases with increasing
temperature, indicating that the gauche conformer is more
stable than the trans, and is higher in population.
Application of Lennard-JonesÈPike analysis23 to our dipole
moment data in carbon tetrachloride yields a *E value of 4.33
kJ mol~1 and a gauche conformer dipole moment (k ) of
g
11.90 ] 10~30 C m. From eqn. (2), the percentage gauche
population at 25 ¡C was found to be 74%. These results con-
trast with 1, which favours the trans isomer when it is in
carbon tetrachloride and benzene solutions. This di†erence in
conformation behaviour can be understood on the basis of the
trans H gauche equilibrium; the balance between steric, elec-
trostatic and stereoelectronic factors. The trans form possesses
two alkylÉ É Éalkyl interactions, whereas the gauche form con-
tains three such interactions. Notwithstanding these steric
repulsions, the gauche conformer becomes the preferred
isomer when the methyl moieties are replaced by the planar
cyclopropyl groups. Since 2 exists in the trans conformation in
It is interesting to compare the results of meso-1 with those
found in other members of the meso-2,3-dicyano-2,3-dicyclo-
alkylbutane series (n \ 5, 6).2,3 In carbon tetrachloride,
where no speciÐc soluteÈsolvent interactions are expected, the
gauche population is 26% for meso-2,3-dicyano-2,3-dicyclo-
pentylbutane and decreases to 16% in meso-2,3-dicyano-2,3-
dicyclohexylbutane. However, in benzene, the results of meso-1
New J. Chem., 2001, 25, 1325È1329
1327

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the solid state (as shown by our X-ray data), the preference for
the gauche conformer in solution may plausibly be attributed
to the greater solvent stabilization of the polar rotamer in 2 so
as to reverse the trans H gauche equilibrium, even in nonpolar
solvents like carbon tetrachloride and benzene.
dicyclopentylbutane2 and meso-2,3-dicyano-2,3-dicyclohexyl-
butane,3 which also adopt the trans conformation in the solid
state. The bond lengths and angles of 2 compare well with the
corresponding ones of meso-1. The cyclopropyl rings in both
molecules adopt a propeller-like arrangement with the C(6)È
C(4)ÈC(1)ÈC(1A) angle of meso-1 being 88.6(1)¡, and the C(4)È
C(2)ÈC(1)ÈC(1A) and C(7)ÈC(5)ÈC(1)ÈC(1A) angles of 2 being
85.3(2) and 87.8(2)¡, respectively.
X-Ray structure determination
Bond lengths, bond angles and torsion angles based on the
Ðnal atomic positions are shown in Table 2. Fig. 1 and 2
depict the structures and deÐne the atomic numbering of
meso-1 and 2, respectively.
Molecules of meso-1 and 2 exist in the trans conformation
in the crystalline state. This is similar to meso-2,3-dicyano-2,3-
Molecular orbital calculations
We shall Ðrst examine the rotational equilibrium of the parent
analogue, 1,2-dicyanoethane 4. Experimental studies have
shown that 4 exists as mixture of trans and gauche forms in
condensed phases, with the latter being the lower energy
form.1,25,26 In particular, Fitzgerald and Janz have demon-
strated, based on infrared spectroscopy, that pure 1,2-dicya-
noethane exists entirely in the gauche conÐguration below
[43.7 ¡C.25 On the other hand, the trans rotamer was pre-
dicted to be the preferred structure in the gas phase.1 We have
calculated the gauche/trans equilibrium of 1,2-dicyanoethane
in the gas phase (e \ 1), in a nonpolar (e \ 2) and a polar
(e \ 40) medium. The trans rotamer is predicted to be the
more stable conÐguration, by 4.8 kJ mol~1, in an isolated
state. The calculated gauche/trans free energy di†erence at 298
Table 2 Bond lengths, bond angles and torsion angles of meso-1 and
2a
meso-1
2
N(1)ÈC(2)
C(1)ÈC(1A)
C(4)ÈC(5)
C(4)ÈC(6)
C(1)ÈC(4)
C(1)ÈC(2)
C(1)ÈC(3)
C(5)ÈC(6)
C(2)ÈC(1)ÈC(4)
C(4)ÈC(1)ÈC(3)
C(2)ÈC(1)ÈC(1A)
C(3)ÈC(1)ÈC(1A)
C(2)ÈC(1)ÈC(3)
C(5)ÈC(4)ÈC(1)
C(4)ÈC(1)ÈC(1A)
C(6)ÈC(4)ÈC(1)
C(1A)ÈC(1)ÈC(4)ÈC(6)
C(3)ÈC(1)ÈC(4)ÈC(6)
C(2)ÈC(1)ÈC(4)ÈC(6)
C(2)ÈC(1)ÈC(4)ÈC(5)
1.138(3)
1.576(4)
1.488(3)
1.498(3)
1.525(3)
1.482(3)
1.552(3)
1.471(4)
108.8(2)
110.3(2)
106.7(2)
110.5(2)
108.8(2)
121.0(2)
111.7(2)
124.8(2)
[88.6(3)
148.1(2)
28.9(3)
C(1)ÈC(8)
C(2)ÈC(4)
C(2)ÈC(3)
C(5)ÈC(7)
C(1)ÈC(1A)
C(1)ÈC(5)
C(1)ÈC(2)
C(6)ÈC(7)
1.480(2)
1.501(4)
1.508(2)
1.492(2)
1.595(2)
1.534(2)
1.533(2)
1.485(3)
1.499(2)
1.496(2)
1.139(2)
109.3(1)
107.1(1)
111.5(1)
123.6(1)
121.9(1)
108.8(1)
108.9(1)
111.2(1)
124.2(1)
122.3(1)
32.8(2)
110.3(2)
[113.7(2)
[85.3(2)
79.2(2)
[149.1(1)
160.7(1)
151.8(1)
42.7(2)
151.8(1)
[39.9(2)
[30.3(2)
[76.2(2)
87.8(2)
K (*G ) is 4.7 kJ mol~1 (B3-LYP/6-311&G** ] ZPE level).
298
This is readily conÐrmed by higher-level calculations at the
C(3)ÈC(4)
C(5)ÈC(6)
N(1)ÈC(8)
G3(MP2) level (3.6 kJ mol~1). These calculated energy di†er-
ences agree well with the experimental estimate of D5 kJ
mol~1.1 The calculated dipole moments of the trans and
gauche forms are 0.0 and 18.19 ] 10~30 C m, respectively.
Thus, the polar gauche rotamer is expected to be stabilized
preferentially in a polarizable dielectric medium. Indeed, the
di†erential solvent stabilization is sufficiently strong that the
C(8)ÈC(1)ÈC(5)
C(8)ÈC(1)ÈC(1A)
C(2)ÈC(1)ÈC(1A)
C(4)ÈC(2)ÈC(1)
C(3)ÈC(2)ÈC(1)
C(2)ÈC(1)ÈC(5)
C(8)ÈC(1)ÈC(2)
C(5)ÈC(1)ÈC(1A)
C(7)ÈC(5)ÈC(1)
C(6)ÈC(5)ÈC(1)
C(8)ÈC(1)ÈC(2)ÈC(4)
C(1)ÈC(2)ÈC(4)ÈC(3)
C(1)ÈC(5)ÈC(6)ÈC(7)
C(1A)ÈC(1)ÈC(2)ÈC(4)
C(5)ÈC(1)ÈC(2)ÈC(3)
C(2)ÈC(1)ÈC(5)ÈC(7)
C(1A)ÈC(1)ÈC(5)ÈC(6)
C(5)ÈC(9)ÈC(10)ÈC(4)
C(8)ÈC(1)ÈC(5)ÈC(6)
C(5)ÈC(1)ÈC(2)ÈC(4)
C(8)ÈC(1)ÈC(2)ÈC(3)
C(8)ÈC(1)ÈC(5)ÈC(7)
C(2)ÈC(1)ÈC(5)ÈC(6)
C(1A)ÈC(1)ÈC(5)ÈC(7)
rotational equilibrium is reversed (*G \ [2.7 kJ mol~1)
[42.8(3)
298
even in a non-polar medium (e \ 2). In a polar medium of
e \ 40, the calculated *G
decreases further to [11.3 kJ
298
mol~1. Hence, our calculations conÐrm the observed strong
preference for the gauche form in the liquid phase.21 As with
1,2-dichloroethane,20a this change of conformational prefer-
ence is attributed to solvation e†ects. The calculated torsional
angle between the cyano groups of the gauche conformer is
67.1¡. Introduction of a dielectric medium reduces this tor-
sional angle signiÐcantly, by 9.2¡ on going from a vacuum to a
dielectric medium of e \ 40. The calculated change in the tor-
sional angle is in close accord with what might be expected
from a solvation e†ect. A decrease in the CCCC torsional
angle may lead to an increase in the total dipole moment of
the molecule, which will provide further stabilization. It is
1
1
1
1
1
a Symmetry transformations: A (meso-1): [ x, ] y, [ z; A (2): [x, ] y, [ z.
2
2
2
2
2
Fig. 2 Thermal ellipsoid and Newman projection diagrams of 2.
1328
New J. Chem., 2001, 25, 1325È1329