Conformational characteristics of aliphatic diesters derived from fluorinated diacids

Article abstract of DOI:10.1021/jp960629h

A comparative study of the polarity and conformational characteristics of diethyl hexafluoroglutarate (DEFG) and its hydrogenated diester counterpart, diethyl glutarate (DEG), is reported. The value at 30°C of the mean-square dipole moment 〈μ²〉 of the latter compound, 5.26 D², is nearly 2 times larger the value of this quantity for the fluorinated diester, which amounts to 12.1 D² at the same temperature. Values of the conformational energies associated with the rotational states about the skeletal bonds of the acid residue were calculated by combining molecular mechanics with the critical interpretation of the mean-square dipole moment of these compounds, finding that the rotational populations about the different skeletal bonds are similar in both molecules. The evaluation of 〈μ²〉 as a function of the conformational energies indicates that the experimental results are reproduced assuming that the conformation in which the C=O* group eclipses the C^α-C^β bond in the C^βC^αC*(O*)O residue is less stable than the alternative conformation in which the carbonyl group eclipses the C^α-F (or C^α- H) bond. The analysis suggests that the high polarity exhibited by DEFG, in comparison with that of DEG, arises from differences in the orientation and the modulus of the dipoles in both compounds rather than from differences in their conformational characteristics.

Full text of DOI:10.1021/jp960629h

13492
J. Phys. Chem. 1996, 100, 13492-13497
Conformational Characteristics of Aliphatic Diesters Derived from Fluorinated Diacids
Regina C. Reis-Nunes, Evaristo Riande,* Nelson C. Chavez, and Julio Guzma´n
Instituto de Ciencia y Tecnolog´ıa de Pol´ımeros, CSIC, 28006 Madrid, Spain
ReceiVed: February 29, 1996; In Final Form: April 24, 1996X
A comparative study of the polarity and conformational characteristics of diethyl hexafluoroglutarate (DEFG)
and its hydrogenated diester counterpart, diethyl glutarate (DEG), is reported. The value at 30 °C of the
mean-square dipole moment µ2 of the latter compound, 5.26 D2, is nearly 2 times larger the value of this
quantity for the fluorinated diester, which amounts to 12.1 D2 at the same temperature. Values of the
conformational energies associated with the rotational states about the skeletal bonds of the acid residue were
calculated by combining molecular mechanics with the critical interpretation of the mean-square dipole moment
of these compounds, finding that the rotational populations about the different skeletal bonds are similar in
both molecules. The evaluation of µ2 as a function of the conformational energies indicates that the
experimental results are reproduced assuming that the conformation in which the C*dO* group eclipses the
CR-Câ bond in the CâCRC*(O*)O residue is less stable than the alternative conformation in which the carbonyl
group eclipses the CRsF (or CRsH) bond. The analysis suggests that the high polarity exhibited by DEFG,
in comparison with that of DEG, arises from differences in the orientation and the modulus of the dipoles in
both compounds rather than from differences in their conformational characteristics.
Introduction
ficient of the unperturbed dimensions of polyesters in which n
g 4 shows a strong dependence on the conformational energies
associated with the rotational states about CR-C* bonds.8,9 In
fact, the only way that the theoretical results reproduce the
experimental ones is when gauche states about these bonds have
similar or lower energies than the alternative trans states.
Similar behavior was found in the theoretical evaluation of the
temperature coefficients of the unperturbed dimensions of
polyesters derived from succinic acid.10
In spite of the efforts devoted to determine the conformational
energies associated with the rotational states about the skeletal
bonds of the acid residue of aliphatic diesters, a definite
conclusion regarding the values of these quantities has not been
reached. For example, from infrared and Raman spectroscopies,
Dirlikov et al.1 inferred that the conformation of the lowest
energy about CRsC* bonds in the CâCRC*(O*)O residue arises
when the CRsH bond eclipses the C*dO* group, that is, when
gauche states about these bonds are preferred over the alternative
trans state, in sharp contrast with the conclusions reached later
by Moravie and Corset2 from an analysis of the same type of
spectra, who reported the trans conformation is more stable than
the alternative gauche forms by the enthalpy difference of
∆H(CRsC*) ) 1.1 ( 0.3 kcal mol-1. Similar results to those
reported by Moravie et al. were obtained by Abe et al.3,4 from
the critical interpretation of both the NMR spectra and dipole
moments of aliphatic diesters. According to these studies,
gauche states about CR-C* bonds are more stable by ca. 1 kcal
mol-1 than the alternative trans form, whereas the trans form
about CR-Câ bonds was found to be more stable than the gauche
conformation in the aliphatic diesters, with the exception of the
dialkyl succinate,4 in which the gauche states seem to be
preferred over the corresponding trans states. In contrast to these
results, an intermediate value of ∆H(CR-C*) ) 0.08 kcal mol-1
also has been reported for methyl propionate from the analysis
of infrared and Raman spectra.5,6
If the spectroscopic studies regarding the stability of gauche
states about CR-C* bonds are not consistent among them, the
critical interpretation of the dipole moments of most aliphatic
diesters are not conclusive either. Actually, the correlation
between the dipoles associated with two ester groups in these
compounds decreases very rapidly as the number of methylene
groups, n, in the residue acid increases; for diesters with n g 4,
the dipolar correlation is relatively small.7 In other words, only
the dipole moments of dialkyl succinates and dialkyl propionates
show a significant sensitivity to the population of rotational
states about CR-C* bonds. However, the temperature coef-
The discrepancies we referred to above are investigated in a
comparative study of the polarity of both diethyl glutarate (DEG)
and diethyl hexafluoroglutarate (DEFG) carried out in this work.
On the other hand, another goal of this paper is to study how
the substitution of the hydrogen atoms in dialkyl esters for
fluorine atoms may affect the polarity of the resulting molecule.
It is expected that the presence of fluorine atoms may affect
not only the dipole moment associated with the ester groups
but also the conformational energies associated with the
rotational states about the skeletal bonds of the acid residue.
For that purpose, in parallel with the experimental determination
of the mean-square dipole moment of DEG, the value of this
quantity for DEFG is measured. The theoretical value of the
mean-square dipole moment of the latter compound is obtained
by calculating first the modulus and orientation of the dipole
moment associated with the ester group from the charge
distributions in the model compound CF3CF2COOCH3. Then,
the effect of the conformational energies on the mean-square
dipole moment of DEFG is analyzed by using the conventional
rotational isomeric state (RIS) model.11 These studies are of
interest to interpret and predict the physical properties of
materials containing fluorine atoms in their structure. It should
be pointed out in this regard that owing to the desirable
properties that fluorine atoms confer to polymers, among them
low-energy surface, good chemical resistance, and lower water
absorption, there is growing interest in the development of new
fluorinated materials.12-14
Experimental Section
Diethyl glutarate and diethyl hexafluoroglutarate were ob-
tained, respectively, by esterification with ethanol of glutaric
X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00629-6 CCC: $12.00 © 1996 American Chemical Society

Conformational Characteristics of Aliphatic Diesters
J. Phys. Chem., Vol. 100, No. 32, 1996 13493
see that the value of dꢀ/dw for DEFG is nearly 30% larger than
that of DEG, whereas dn/dw for the latter compound is ca. 2
times that of DEG. The results for µ2 , shown in the fourth
column of Table 1, reveal that the presence of fluorine atoms
in the acid residue produces a considerable enhancement of the
polarity of the resulting diester. The value of µ2 for DEG at
30 °C, 5.25 D2, compares very favorably with the results (5.24,
5.57, 5.71, and 5.34 D2) reported for this quantity in the
literature.4,17 As far as we know, the dipole moment of diethyl
hexafluoroglutarate has not been measured.
The temperature dependence of the dipole moment of DEG,
expressed in terms of 103d ln µ2 /dT, amounts to 1.9 K-1
,
somewhat higher than the value reported earlier for this
property.4 The low value of the temperature coefficient for
DEFG, 0.6 K-1, suggests that the dipole moment of this
compound is, in comparison, rather insensitive to changes in
temperature. It should be pointed out, however, that a few tenths
of error in the determination of the mean-square dipole moment
can produce significant changes in d ln µ2 /dT, and conse-
quently, µ2 rather than its temperature coefficient should be
used in the conformational analysis.
Theoretical Calculations
Figure 1. 1H NMR spectrum of diethyl glutarate (DEG).
The bond angles required in the calculations were defined as
C*CC ) CCC ) 112° for both DEG and DEFG. The
rotational states for the latter molecule, calculated by using
semiempirical potential functions, were taken to occur at 0°,
(120°. In the evaluation of the dipole moment of DEG, the
rotational angles used were those suggested earlier by Abe et
al.4: 0°, (123° for CR-C* and 0° and 0°, (113° for CR-Câ.
The dipole moment associated with the ester group of the DEG
compound amounts to4 1.75 D; it is located in the plane of this
group, forming an angle of 123° with the CR-C* bond.18,19 The
dipoles associated with the DEFG molecule were obtained from
the charge distributions calculated for a model compound of
the diester. Molecular charges were computed with the Sybyl
Molecular Modeling Package17 employing the MOPAC program
and the AM1 procedure.21,22 The usefulness of this approach
to determine the dipole moments of fluorinated compounds was
tested by calculating the dipole moments of trifluoroacetalde-
hyde and methyl fluoroformate and comparing them with the
experimental results. The calculated values amount to 1.90 and
2.54 D for the former and latter compound, respectively, which
are in reasonable agreement with the experimental dipole
moments reported for these compounds in the literature:23 1.65-
1.92 D for trifluoroacetaldehyde and 2.61 D for methyl
fluoroformate. In view of the good agreement obtained between
the calculated and experimental results, we proceeded to
determine the distribution of charges in the CF3CF2COOCH3,
model compound of the fluorinated diester. The distribution
acid and hexafluoroglutaric acid. The reaction proceeded in
nitrogen atmosphere, under reflux, for 16 h. The mixture
alcohol-water and the remaining alcohol were removed from
the reaction medium by distillation. Dry ethanol was added 2
more times and then removed until the esterification was
complete as indicated by the absence of acid groups in the IR
spectra. In the esterification of glutaric acid, about 1% (v/v)
concentrated chlorohydric acid was added to the reaction
medium. The diesters DEG and DEFG were distilled at 100
°C (10-2 mmHg) and 74 °C (10-2 mmHg), respectively, and
1
finally characterized by NMR spectroscopy. The H NMR
spectra for DEG and DEFG, together with the assignment of
the resonance peaks, are given in Figures 1 and 2, respectively.
In Figure 2, the 13C NMR spectrum for DEFG is also shown.
Values of the dielectric permittivity (ꢀ) of solutions of the
compounds in benzene were obtained with a capacitance bridge
(General Radio, Type 1640 A) coupled with a three-terminal
cell, at 10 kHz. Increments in the values of the dielectric
permittivity with respect to that of the solvent (∆ꢀ ) ꢀ - ꢀ1)
were measured at each temperature of interest for several
solutions in which the weight fraction of solute, w, was in the
interval 0.015-0.003. Increments in the index of refraction (n)
of the solutions of the compounds in benzene with respect to
that of the solvent (∆n ) n - n1) were measured with a Phoenix
refractometer. Values of the mean-square dipole moment µ2
at different temperatures were determined by means of the
method of Guggenheim and Smith:15,16
of residual charges for the CF2 group is F-0.118e - C+0.235e
-
F-0.118e. Hence, taking into account that the length of the C-F
bond is 1.36 D, the dipole moment associated with the C-F
bond has a value of 0.77 D, and consequently, each of the two
C-C skeletal bonds adjacent to a CF2 group can be considered
to have a dipole moment of 0.80 D. Moreover, from the
distribution of charges of the COOCH3 residue, one finds that
this group has a dipole moment (µE) of 1.965 D located in the
xy plane, its direction forming an angle of 15 ° with the C*-
(O*)-O bond of the ester group. It can be seen in Figure 3
that the dipole moments of the central C-C bonds of the DEFG
cancel out so that the dipoles contributing to the polarity of
this compound are restricted to a dipole of 0.80 D, ma and mb
located along the C-C* bonds, and the dipole µE indicated in
the figure.
27kBTM
4πFNA(ꢀ1 + 2)2
0
0 - 2n1
(1)
dꢀ
dw
dn
dw
µ2
)
[
(
)
(
)
]
where kB and NA are, respectively, the Boltzmann constant and
Avogadro’s number, F is the density of the solvent, and M is
the molecular weight of the solute. The error involved in the
determination of µ2 was estimated to be (2%.
Experimental Results
Values of dꢀ/dw and dn/dw at 30, 40, 50, and 60 °C for both
DEG and DEFG are given respectively, in the second and third
columns of Table 1. At each temperature of interest, one can

13494 J. Phys. Chem., Vol. 100, No. 32, 1996
Reis-Nunes et al.
Figure 2. 1H and 13C NMR spectra of diethyl hexafluoroglutarate (DEFG).
TABLE 1: Dielectric Results for Diethyl Glutarate (DEG)
and Diethyl Hexafluoroglutarate (DEFG)
compd
DEG
T, °C
2n1(∂n/∂w2)0
(∂ꢀ/∂w2)0
µ2 , D2
30
40
50
60
30
40
50
60
0.17
0.15
0.13
0.11
0.32
0.31
0.31
0.31
3.11
2.98
2.89
2.76
4.64
4.41
4.23
4.06
5.25
5.30
5.48
5.54
DEFG
12.10
12.10
12.20
12.33
The critical interpretation of the 1H NMR spectra and dipole
moments of aliphatic diesters with general formula H3CO(*O)-
*C(CH2)nC*(O*)OCH3 led Abe and co-workers3,4 to conclude
that the energy of gauche states about CR-C* bonds, EσR, is 1
kcal mol-1 above that of the alternative trans states. According
to these authors, however, gauche states about CR-Câ bonds
have an energy Eσâ which seems to be dependent on the number
of methylene groups in the residue acid. Thus, values of Eσâ
) -0.6, 0.3, and 0.8 kcal mol-1 have been reported, respec-
tively, for the methyl esters of succinic, glutaric, and adipic
acids.4 Rotations of different sign about two consecutive bonds
were considered to have energies Eω about 2 kcal mol-1 higher
than the alternative tt states.4
Figure 3. Schematic representation of diethyl glutarate and diethyl
hexafluoroglutarate (DEFG) in the all-trans conformation showing the
dipoles associated with the molecules.
0.8 kcal mol-1 above that of the corresponding trans states.
Rotations of different sign about two consecutive bonds were
considered to be forbidden. It should be pointed out that
preliminary calculations showed that the mean-square dipole
moments of both DEG and DEFG are nearly insensitivity to
the values of the second-order energies, Eω and Eω′, for values
Molecular mechanics calculations carried out in this work
suggest that gauche states about CR-C* bonds of DEFG have
an energy EσR ca. 0.1 kcal mol-1 below that of the alternative
trans states, whereas the energy Eσâ about Câ-CR bonds is ca.
of this quantities higher than 1.4 kcal mol-1
.

Conformational Characteristics of Aliphatic Diesters
J. Phys. Chem., Vol. 100, No. 32, 1996 13495
Figure 4. Dependence of the mean-square dipole moment of diethyl
glutarate (DEG) and diethyl hexafluoroglutarate (DEFG) on the energy
associated with gauche states about C*-CR bonds.
Figure 5. Variation of the mean-square dipole moment of diethyl
glutarate (DEG) and diethyl hexafluoroglutarate (DEFG) with the
energy associated with gauche states about Câ-CR bonds.
In view of the discrepancies concerning the energies of the
rotational populations about CR-C* bonds in aliphatic diesters,
values of µ2 were obtained by using the conventional RIS
scheme for -2 < EσR < 2 kcal mol-1, where EσR is the energy
of gauche states about these bonds with respect to the corre-
sponding trans states. In the evaluation of the mean-square
dipole moment of DEG, it was assumed that the gauche
population about CR-Câ bonds has an energy, Eσâ, 0.7 kcal
mol-1 above that of the alternative trans states. The curve
depicting the variation of µ2 with EσR for DEG, shown in
Figure 4, indicates that in changing this energy from 1.0 to -2
kcal mol-1, µ2 decreases from 7.11 to 4.66 D2. This behavior
is a consequence of the fact that an increase of the gauche
population about CR-C* bonds will decrease the fraction of
all-trans conformation in which the dipoles associated with the
ester groups are in a parallel direction. On the contrary, owing
to the fact that in the all-trans conformation the dipoles in DEFG
are almost in an antiparallel direction, an increase of the gauche
population about CR-C* bonds will increase the polarity of the
compound, as is reflected in Figure 4, where the dependence
of the mean-square dipole moment on EσR for this compound
is shown. It can be seen in this curve that in decreasing EσR
from 1.2 to -2 kcal mol-1, the value of µ2 for DEFG will
increase from 5.12 to 14.1 D2. In other words, although the
variation of µ2 with EσR follows opposite trends in DEG and
in DEFG, in order to reproduce the experimental results, it is
necessary to postulate that EσR < 0 in both cases.
Figure 6. Variation of the temperature dependence of the mean-square
dipole moment of diethyl glutarate (DEG) with the energy associated
with gauche states about CR-C* bonds.
result, 5.25 D2. In the same way, the experimental mean-square
dipole moment of DEFG is reproduced for EσR ) -0.5 kcal
mol-1 and Eσâ ) 0.7 kcal mol-1; for this set of values, µ2
)
The curves showing the dependence of µ2 on Eσâ for DEG
and DEFG, represented in Figure 5, were calculated assuming
EσR ) -1 kcal mol-1 and Eω ) Eω′ ) 1.4 kcal mol-1. For the
reasons outlined above, the variation of the polarity of the
compounds with Eσâ follows opposite trends in both com-
pounds. Thus, µ2 for DEG diminishes from 8.52 to 4.53 D2
when Eσâ increases from -1.2 to 1 kcal mol-1. On the contrary,
the polarity of DEFG increases as Eσâ goes up. As can be seen
in Figure 5, in increasing Eσâ from -1.2 to 1 kcal mol-1, the
value of µ2 changes from 6.21 to 11.31 D2 for this latter
compound. Very good agreement between theoretical and
experimental values of DEG is achieved using the set of
conformational energies EσR ) -0.40, Eσâ ) 0.7, and Eω )
Eω′ ) 1.4 kcal mol-1 for which the value of µ2 at 30 °C is
5.31 D2, which compares very favorably with the experimental
11.7 D2 at 30 °C, also in good agreement with the experimental
result, 12.1 D2, at the same temperature.
It is worth investigating how the conformational energies
affect the temperature coefficient of the dipole moments of DEG
and DEFG. The dependence of this parameter on the first-order
energies for DEG, represented in Figure 6, shows that keeping
EσR ) -1.0 kcal mol-1, d ln µ2 /dT > 0 for Eσâ > 0 kcal
mol-1. The temperature coefficient of the dipole moment of
DEG is also positive for values of EσR e 0 kcal mol-1 if Eσâ
) 0.7 kcal mol-1. As for the temperature dependence of the
dipole moment of DEFG, the calculations represented in Figure
7 reveal that d ln µ2 /dT > 0 in the whole range of conforma-
tional energies, except in the interval in which EσR > 600 cal
mol-1 (Eσâ ) 0.7 kcal mol-1) or Eσâ < -500 cal mol-1 (EσR
) -500 cal mol-1) for which d ln µ2 /dT > 0.

13496 J. Phys. Chem., Vol. 100, No. 32, 1996
Reis-Nunes et al.
out that for the two compounds for which correlation between
the ester dipoles is more important, dimethyl succinate and DEG,
good agreement between theoretical and experimental results
is also found for EσR e 0.4 This conclusion is corroborated by
the analysis of the temperature coefficient for the unperturbed
dimensions of aliphatic polyesters, such as poly(neopentyl glycol
succinate)10 and poly(neopentyl glycol adipate),8 whose dimen-
sions are extremely sensitive to the gauche population about
CR-C* bonds. From thermoelastic experiments carried out on
poly(neopentyl glycol succinate), one finds that 103 ln r2 0/dT
) 0.52 ( 0.24 K-1. Studies performed on the evolution of
this magnitude with EσR, keeping Eσâ ) 0.7 kcal mol-1, show
that d ln r2 /dT is negative and nearly independent of EσR for
values of this energy lying in the range 1-0.5 kcal mol-1
;
however, the temperature coefficient undergoes a sharp increase
as EσR diminishes in the interval 0.5-0 kcal mol-1, the
theoretical results becoming close to the experimental ones for
values of EσR e 0.10 Theoretical calculations10 also performed
on the polarity of this polymer show that its experimental dipolar
correlation coefficient is reproduced better by assuming that EσR
e 0. Similar results are found in the analysis of the temperature
coefficients of the unperturbed dimensions of poly(neopentyl
glycol adipate)8 and other polyesters prepared by condensation
of both adipic and sebacic acids with 2,2-bis[4-(2-hydroxy-
ethoxy)phenyl]propane.9
Figure 7. Influence of the conformational energies of gauche states
about CR-Câ bonds on the temperature dependence of the mean-square
dipole moment of hexafluoroglutarate (DEFG).
Discussion
Summing up, the evidence at hand arising from the theoretical
analysis of both the dipole moments of aliphatic diesters and
the temperature coefficient of the unperturbed dimensions of
aliphatic polyesters seems to suggest that EσR < 0 and Eσâ >
0 for the DEG and DEFG compounds. The difference in
polarity observed in both compounds arises from the differences
in orientation of the dipoles associated with the diesters in the
fluorinated compound caused by inductive effects of the fluorine
atoms. These conclusions are important to predict the confor-
mational properties of polyesters containing fluorine atoms in
the acid residue of the repeating unit.
The calculations described above suggest that the gauche
population about C-C* bonds in diethyl glutarate is preferred
by ca. 500-1000 cal mol-1 over the alternative trans population.
To assume EσR ) 1.0 kcal mol-1, as has been suggested by
some authors,2-4 would give µ2 ) 7.11 D2 for Eσâ ) 0.7 kcal
mol-1; this quantity would reduce to 6.25 D2 for Eσâ ) 0.3
kcal mol-1. Moreover, the calculations indicate that the dipole
moment of DEG undergoes a significant decrease with increas-
ing temperature, in contrast with the experimental results which
suggest that the temperature coefficient is clearly positive.
Therefore, in order to each agreement between the theoretical
and experimental results of both µ2 and d ln µ2 /dT, it is
necessary to postulate that EσR e 0.
As first noted, DEFG exhibits a mean-square dipole moment
whose value at 30 °C is almost 2 times larger than that of its
hydrogenated homologous compound, DEG. In the study of
the evolution of the mean-square dipole moment with the
conformational energies, it was observed that DEG in the all-
trans conformation exhibits the maximum polarity due to the
fact that the dipoles of the ester groups are in nearly parallel
direction, whereas the opposite occurs in DEFG as a conse-
quence of the almost antiparallel direction of the dipole in this
conformation. Therefore, the changes in polarity with the
gauche population about the skeletal bonds of the acid residue
follow opposite trends in both polymers. Thus, the dipole
moment diminishes as the gauche population increases in DEG,
whereas the polarity of DEFG increases as such a population
increases. However, the fact that agreement between theoretical
and experimental results of µ2 is found for the same values of
the conformational energies seems to suggest that differences
in the modulus and orientation of the dipoles in both compounds,
rather than differences in conformational characteristics, are
responsible for the much higher polarity of DEFG. The
differences observed in the orientations of the dipole moments
also explain the much lower temperature coefficient of the dipole
moment of DEFG in comparison with that of DEG.
Acknowledgment. This work was supported by the DGI-
CYT through Grant PB92-0773. R. C. Reis-Nunes and N. A.
Chavez are indebted, respectively, to CAPES (Brazil) and the
Consejo Nacional de Investigaciones de Venezuela for their
support through grants.
References and Notes
(1) Dirlikov, S.; Stokr, J.; Schneider, B. Collect. Czech. Commun. 1971,
36, 3028.
(2) Moravie, R. M.; Corset, J. Chem. Phys. Lett. 1974, 26, 210; J. Mol.
Struct. 1975, 24, 91.
(3) Abe, A. J. Am. Chem. Soc. 1984, 106, 14.
(4) Abe, A.; Miura, I.; Furuya, H. J. Phys. Chem. 1987, 91, 6496.
(5) Bowles, A. J.; George, W. O.; Cunliffe-Jones, D. B. Chem.
Commun. 1970, 103.
(6) George, W. O.; Hassid, D. V.; Maddams, W. F. J. Chem. Soc.,
Perkin Trans. 2 1972, 1029.
(7) Riande, E.; Saiz, E. Dipole Moments and Birefringence of Polymers;
Prentice Hall: Englewood Cliffs, NJ, 1992.
(8) Riande, E.; Guzma´n, J.; Addabo, H. Macromolecules 1986, 19,
2567.
(9) Guzma´n, J.; Riande, E.; Salvador, R.; de Abajo, J. Macromolecules
1991, 24, 5357.
(10) Riande, E.; Guzma´n, J. J. Chem. Soc., Perkin Trans. 2 1988, 299.
(11) Flory, P. J. Macromolecules 1974, 7, 381.
(12) Lynn, M.; Worm, A. T. Fluorocarbon Elastomers. In Encyclopedia
Polymer Science and Engineering; Bikales, N. M., Overberger, C. G.,
Menges, G., Eds.; Wiley-Interscience: New York, 1987; Vol. 7, p 256.
(13) Antonucci, J. M. New Monomers for Use in Dentistry. In
Biomedical and Dentistry Applications of Polymers; Gebelein, G. G.,
Koblitz, F. F., Eds.; Plenum: New York, 1981.
Neither this study nor others based on the critical interpreta-
tion of the dipole moments of dialkyl esters3,4 permits us to
conclude that gauche states about C-C* bonds have higher
energies than the alternative trans states. It should be pointed
(14) Am. Chem. Soc., Polym. DiV. Polymer. Prep. 1990, 31 (1), (see
for example) 307, 318, 314, 320.
(15) Guggenheim, E. A. Trans. Faraday Soc. 1949, 45, 714.

Conformational Characteristics of Aliphatic Diesters
J. Phys. Chem., Vol. 100, No. 32, 1996 13497
(16) Smith, J. W. Trans. Faraday Soc. 1950, 46, 394.
(17) Ketelaar, J. A. A.; Van Meurs, N. Recl. TraV. Chim. Pays-Bas 1957,
76, 437.
(18) Saiz, E.; Hummel, J. P.; Flory, P. J.; Plavsic, M. J. Phys. Chem.
1981, 85, 3211.
(19) Mattice, W. L.; Saiz, E. J. Am. Chem. Soc. 1978, 100, 6308.
(20) Sybyl Molecular Modeling Package, ver. 6.0, Tripos Associates,
Inc.; St. Louis, MO 63144.
(21) QCPE, Department of Chemistry, Indiana University, Bloomington,
IN 47405.
(22) The MOPAC program (version 5.0) containing the AM1 procedure
is included in the Sybyl package by agreement between Tripos and QCPE.
(23) McClellan, A. L. Tables of Experimental Dipole Moments; Rahara
Enterprises: El Cerrito, CA, 1974.
JP960629H