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IR study of the cholesteryl n-alkanoates conformation mobility
Journal
ofMokcuhr
141
Structure, 218 (1990) 141-146
Elsevier Science PublishersB.V., Amsterdam-Printedin TheNetherlands
IR STUDY OF THE CHOLESTERYL n-ALKANOATES CONFORMATION MOBILITY
G.A. PLJCHKOVSKAYA'and A.A.
YAKUBOV2
'Institute of Physics of UkrSSR Academy of Sciences, 252650, Kiev-28 (U.S.S.R.) 2Samarkand State University, 703000, Samarkand (U.S.S.R.)
ABSTRACT IR-spectroscopy methods have been used to study conformational mobility of alkyl chains of cholesteryl alkanoates in different phases. It is shown that cholestergl alkanoates may be divided into three groups with similar conformation and packing of alkyl chains in solid state and mesomorphism types within each of them.
INTRODUCTION The idea of Bernal and Crowfoot (ref. 1) upon connection between crystal structure and molecular organization in mesophase was further developed in numerous papers (see, e.g., ref. 2). Nevertheless in the most of both experimental and theoretical papers the problems of change of the shape of individual molecules at solid crystal (SC) - liquid crystal (LC) - isotropic liquid (II,)transitions have not been discussed. In different models of mesophases one deals with packings of rigid cylindrical molecules. It is, however, known that rotation about single bonds in molecules, i.e. existence of rotated conformers, is possible. In the present work conformational mobility in homologic series of cholesteryl n-alkanoates (C&A) CH3(CH2)n_2COOC27H45 (thereafter referred to as C,), where carbon atoms number in starting alkanoic acid n is equal to 1 to 16 and 18, is investigated with the aid of IR spectroscopy methods. IR absorption spectra of ChA have been recorded using UR-20 spectrophotometer at a resolution I...2 cm-1 in the 400 to 4000 cm-1 spectral region in the temperature range of 100 to 400K. Sample temperature was maintained constant using automatic thermoregulator with error of + IK. Thermophysical (ref. 3) and X-ray structural (ref. 4) investigations have shown that physical properties of CM do not exhibit W22-2860/90/$03.50
0
1990Elsevier SciencePublishers
B.V.
142
such features characteristic to most of homologic series as evenodd alternation, but they do exhibit different types of solid and liquid crystal structure and of polymorphism that are however similar within few consequent groups of ChA homologues. Investigation of ChA IR absorption spectra (ref. 5) whioh carry information on conformational changes and intermolecular interactions are of great interest. However, the indications of conformational mobility of ChA molecules have not been investigated to a sufficient extent. Structural and spectral data have not been compared. There is no information on ChA IR spectral studies
at low temperatures
(below
293K).
RtiSULTS&m DISCUSSION It has been established
that
stitution
by other
of
one homologue
phase
transitions
do not
as well
strongly
as sub-
influence
parameters of vibration bands of steroid nucleus and isooctyl chain and the most sensitive to these changes are parameters of CH3(CH2)n_2-alkyl chain vibration bands, namely, rocking q(CH2) q (CH2) (1180...1350 cm”) bands (720...900 cm-l) and wagging (Fig.
1).
mologue to
Investigation
number n,
of
these
temperature
make conclusions
about
parameters
and phase
conformation
dependence
state
upon ho-
makes it
and alignment
possible
of
alkyl
chains. In the tinguish
CH2 groups asing
spectra 736 cm-’ within
temperature
of
first
two homologues
band assigned
to
rocking
isooctyl
radical.
(down to
IOOK) leads
In the to
(Fig.
2)
one can dis-
vibrations spectrum splitting
of
three
of
C2 decre-
of
this
(A$\/, -15 cm-‘) band into four narrow components indicating the reordering of isooctyl radicals whereas at 300K the hindered rotation of these radicals is possible. In the spectra of C3 and higher homologues alkyl chain rocking vibration bands whose frequencies decrease with increasing homologue number n are present in this region. Frequency of the most intensive band, i.e. of the first member P, of the rooking vibration series, becomes practically constant and equal to 720 cm-' beginning from C7 or C8. Broad bands of these vibrations at 300K indicate the presence of non-regular gauche conformations of alkyl and isooctyl chains in C, to CS. When decreasing temperature down to 100K homologues C, to CS with even n show intensity increase and splitting of rocking bands suggesting the phase transition in solid state accompanied with reordering of the alkyl broad
143
1-I
;ig,, 1. IR spectra of ChA (n=15): 1 - SC (lOOK), 2 - SC (295K), - smectic LC (342K), 4 - aholesteric LC (349K), 5 - IL (355K). and isooctyl n< 8 form
radicals. cholesterio
It
should
mesophase
be pointed
out
that
ChA with
only.
-1 of 720 cm In the spectra of Cg to C,2 at IOOK in the region the single intensive band has been observed that is typical for triclinic packing of fully extended alkyl chains, as in the case of even n-paraffins, where planes of all alkyl chains are parallel, and elementary cell contains only one chain. At 300K intensity of this band, proportional to the number of CH2 groups in all-trans section, decreases and its breadth increases which indicates the possibility of hindered rotation of alkyl chains and forming gauche conformers. For Cg to C,2 monotropic transition to smectic phase is typical. In the spectra of C,3 to C,6 and C,a Davydov splitting of rocking bands has been observed. Its magnitude for the P, at IOOK is the same (71...12 cm") as those of the fully extended chains of n-paraffins which contain two molecules per cell. Thus, we come to the conclusion that in C,3...C,a alkyl chains are fully extended (all-trans conformation), are closely packed and form orthorhombic subcell. One should note, however, that spectra of are somewhat different from those of C,3 to C,5: the ‘16 and ‘18
144
145
magnitude
of
the Davydov
splitting
and additional
maxima in the
(Fig.
fact
the to
2).
This
spectrum
of
more loose
teric
phases,
smectic
The peculiarities responding quency of
bound of
bands
increasing
first
IL
real
cases
(ref.
of
free
i.e.
and fixed
number of of
moment of
the
of
of
of
the
end group
and the
(14
CH2 groups)
1OOK and absorption
ing
of
at
calculated
(dimensionless
theory
pole
end group
moment of
shown in Fig. pancy to
its
3.
number is
is
experiment that
is
the
are not
Change of dramatic
of
change
of
of
that
of
frequency
of
shift:
calculation frequency center,
with
dipole
interaction
of
the
palmitate of
chain
described
in
shift of
consist+ (ref.
0.53;
6) di-
1.2)
are
band intensity
experiment.
upon its
was
limiting
coefficient
neglected
Discre-
number with
interaction
respect
between
CH2
neighbourhood.
at melting
interaction
this
zone
dependence
agreement
immediate
ChA spectra
decrease
the
frequency
band position
result in
For
of
of
IR spectra
between
spectrum
are:
we
long-
7) which
necessary:
interaction
evident
in a good
in dependence
groups
slight
It
parameters
of
on the method
(ref.
approach
from
The advantage
to calculate
from
of
band at
peculiarities
based
chain.
bands
spectra
transition
CH2 group. solid cholesteryl
using 20.;
is
of
the
spectra
coefficient
neighbouring spectrum of
upon homo-
broad
these
Zbinden
is
end group with the The experimental 14 groups
at
functions.
parameters
end group
in of
intermediate
ends of the
those
optical
6) that
Green’s
cases
varied
vibration
to
the possibility for
independent
band series
(ref.
to
in ChA spect-
correspondence
To interprete
to those
is
CH2 groups
appearance
theory
retarded 5)
chains,
a small
the
as compared
in
of 1).
compounds
dependent
theory
applied
(Fig.
transition
with those of corband 1180 cm-1 at the low-fre-
CH2 groups;
to apply
aliphatic
two time
upon
of
in number and position
and disappearance
that
and choles-
as compared
intensive
(n-l>
note
smectic
phase.
is
with
present and in
can be assigned
monotropic
2);
the
shift
only
from C4 (Fig.
series
smaller,
are
One should to
cholesteric
wagging
are
bands
end it
transitions
whose position
1250 cm”
for
n,
chains.
series
SC to LC and to
this
greater
number beginning
this
chain
of
bands
of wagging
alkyl
to
intensity
rocking
at low temperatures
and Cl8 have
n-paraffins;
n-paraffins
were
long
enanthiotropic
and C,6
bands
region evident
with
of
and enanthiotropic
ra are:
logue
most
homologue
packing
to C,5 have
cl3
is
of
(Fig.
coefficient the
broad
I)
can be described accompanied band at
with
1250 cm”
as
146
Fig. 3. Experimental (a) and calculated according to (ref. 6) (b) cl6 spectra in the region of wagging vibrations. corresponds to end CH2 group (neighbouring to CO-group); bands of collective vibrations of residual chain are weak because the great dipole moment of end group is not more included in these vibrations; band at 1170 cm" belongs to C-O vibration. The strong decrease of interaction coefficient can naturally be aasigned to rotated conformers formation. ACKNOWLEDGEMENTS The authors express their thanks to I.V. Sekirin for computing theoretical ChA spectra. REFERENCES 1 J.D. Bernal, D. Crowfoot, Trans. Farad. Sot., 29 (1933) 10321049. 2 R.F. Brajan, Zhurn. struct. chimia, 23 (1982) 154-174. 3 G.J. Davis, R.S. Porter, B.M. Barrel, Mel, Cryat. Liquid Cryst., 11 (1970) 319-330. 4 N.G. Guerine, B.M. Craven, J.C.S. Perkin II, No.10 (1979) 5
6 7
1414-1419.
L.S. Gorbatenko, Theses Cand. Soi., Rostov-na-Donu, 1975. T.A. Gavrilko, G.A. Puchkovskaya, I.V. Sekirin, N.M. Chepilko, Preprint of Institute of Physics of UkrSSR Academy of Scienoes, No. 83/24, Kiev, 1983. R. Zbinden. Infrared spectroscopy of polymer molecules, Mir, Moscow, 1966.
ofMokcuhr
141
Structure, 218 (1990) 141-146
Elsevier Science PublishersB.V., Amsterdam-Printedin TheNetherlands
IR STUDY OF THE CHOLESTERYL n-ALKANOATES CONFORMATION MOBILITY
G.A. PLJCHKOVSKAYA'and A.A.
YAKUBOV2
'Institute of Physics of UkrSSR Academy of Sciences, 252650, Kiev-28 (U.S.S.R.) 2Samarkand State University, 703000, Samarkand (U.S.S.R.)
ABSTRACT IR-spectroscopy methods have been used to study conformational mobility of alkyl chains of cholesteryl alkanoates in different phases. It is shown that cholestergl alkanoates may be divided into three groups with similar conformation and packing of alkyl chains in solid state and mesomorphism types within each of them.
INTRODUCTION The idea of Bernal and Crowfoot (ref. 1) upon connection between crystal structure and molecular organization in mesophase was further developed in numerous papers (see, e.g., ref. 2). Nevertheless in the most of both experimental and theoretical papers the problems of change of the shape of individual molecules at solid crystal (SC) - liquid crystal (LC) - isotropic liquid (II,)transitions have not been discussed. In different models of mesophases one deals with packings of rigid cylindrical molecules. It is, however, known that rotation about single bonds in molecules, i.e. existence of rotated conformers, is possible. In the present work conformational mobility in homologic series of cholesteryl n-alkanoates (C&A) CH3(CH2)n_2COOC27H45 (thereafter referred to as C,), where carbon atoms number in starting alkanoic acid n is equal to 1 to 16 and 18, is investigated with the aid of IR spectroscopy methods. IR absorption spectra of ChA have been recorded using UR-20 spectrophotometer at a resolution I...2 cm-1 in the 400 to 4000 cm-1 spectral region in the temperature range of 100 to 400K. Sample temperature was maintained constant using automatic thermoregulator with error of + IK. Thermophysical (ref. 3) and X-ray structural (ref. 4) investigations have shown that physical properties of CM do not exhibit W22-2860/90/$03.50
0
1990Elsevier SciencePublishers
B.V.
142
such features characteristic to most of homologic series as evenodd alternation, but they do exhibit different types of solid and liquid crystal structure and of polymorphism that are however similar within few consequent groups of ChA homologues. Investigation of ChA IR absorption spectra (ref. 5) whioh carry information on conformational changes and intermolecular interactions are of great interest. However, the indications of conformational mobility of ChA molecules have not been investigated to a sufficient extent. Structural and spectral data have not been compared. There is no information on ChA IR spectral studies
at low temperatures
(below
293K).
RtiSULTS&m DISCUSSION It has been established
that
stitution
by other
of
one homologue
phase
transitions
do not
as well
strongly
as sub-
influence
parameters of vibration bands of steroid nucleus and isooctyl chain and the most sensitive to these changes are parameters of CH3(CH2)n_2-alkyl chain vibration bands, namely, rocking q(CH2) q (CH2) (1180...1350 cm”) bands (720...900 cm-l) and wagging (Fig.
1).
mologue to
Investigation
number n,
of
these
temperature
make conclusions
about
parameters
and phase
conformation
dependence
state
upon ho-
makes it
and alignment
possible
of
alkyl
chains. In the tinguish
CH2 groups asing
spectra 736 cm-’ within
temperature
of
first
two homologues
band assigned
to
rocking
isooctyl
radical.
(down to
IOOK) leads
In the to
(Fig.
2)
one can dis-
vibrations spectrum splitting
of
three
of
C2 decre-
of
this
(A$\/, -15 cm-‘) band into four narrow components indicating the reordering of isooctyl radicals whereas at 300K the hindered rotation of these radicals is possible. In the spectra of C3 and higher homologues alkyl chain rocking vibration bands whose frequencies decrease with increasing homologue number n are present in this region. Frequency of the most intensive band, i.e. of the first member P, of the rooking vibration series, becomes practically constant and equal to 720 cm-' beginning from C7 or C8. Broad bands of these vibrations at 300K indicate the presence of non-regular gauche conformations of alkyl and isooctyl chains in C, to CS. When decreasing temperature down to 100K homologues C, to CS with even n show intensity increase and splitting of rocking bands suggesting the phase transition in solid state accompanied with reordering of the alkyl broad
143
1-I
;ig,, 1. IR spectra of ChA (n=15): 1 - SC (lOOK), 2 - SC (295K), - smectic LC (342K), 4 - aholesteric LC (349K), 5 - IL (355K). and isooctyl n< 8 form
radicals. cholesterio
It
should
mesophase
be pointed
out
that
ChA with
only.
-1 of 720 cm In the spectra of Cg to C,2 at IOOK in the region the single intensive band has been observed that is typical for triclinic packing of fully extended alkyl chains, as in the case of even n-paraffins, where planes of all alkyl chains are parallel, and elementary cell contains only one chain. At 300K intensity of this band, proportional to the number of CH2 groups in all-trans section, decreases and its breadth increases which indicates the possibility of hindered rotation of alkyl chains and forming gauche conformers. For Cg to C,2 monotropic transition to smectic phase is typical. In the spectra of C,3 to C,6 and C,a Davydov splitting of rocking bands has been observed. Its magnitude for the P, at IOOK is the same (71...12 cm") as those of the fully extended chains of n-paraffins which contain two molecules per cell. Thus, we come to the conclusion that in C,3...C,a alkyl chains are fully extended (all-trans conformation), are closely packed and form orthorhombic subcell. One should note, however, that spectra of are somewhat different from those of C,3 to C,5: the ‘16 and ‘18
144
145
magnitude
of
the Davydov
splitting
and additional
maxima in the
(Fig.
fact
the to
2).
This
spectrum
of
more loose
teric
phases,
smectic
The peculiarities responding quency of
bound of
bands
increasing
first
IL
real
cases
(ref.
of
free
i.e.
and fixed
number of of
moment of
the
of
of
of
the
end group
and the
(14
CH2 groups)
1OOK and absorption
ing
of
at
calculated
(dimensionless
theory
pole
end group
moment of
shown in Fig. pancy to
its
3.
number is
is
experiment that
is
the
are not
Change of dramatic
of
change
of
of
that
of
frequency
of
shift:
calculation frequency center,
with
dipole
interaction
of
the
palmitate of
chain
described
in
shift of
consist+ (ref.
0.53;
6) di-
1.2)
are
band intensity
experiment.
upon its
was
limiting
coefficient
neglected
Discre-
number with
interaction
respect
between
CH2
neighbourhood.
at melting
interaction
this
zone
dependence
agreement
immediate
ChA spectra
decrease
the
frequency
band position
result in
For
of
of
IR spectra
between
spectrum
are:
we
long-
7) which
necessary:
interaction
evident
in a good
in dependence
groups
slight
It
parameters
of
on the method
(ref.
approach
from
The advantage
to calculate
from
of
band at
peculiarities
based
chain.
bands
spectra
transition
CH2 group. solid cholesteryl
using 20.;
is
of
the
spectra
coefficient
neighbouring spectrum of
upon homo-
broad
these
Zbinden
is
end group with the The experimental 14 groups
at
functions.
parameters
end group
in of
intermediate
ends of the
those
optical
6) that
Green’s
cases
varied
vibration
to
the possibility for
independent
band series
(ref.
to
in ChA spect-
correspondence
To interprete
to those
is
CH2 groups
appearance
theory
retarded 5)
chains,
a small
the
as compared
in
of 1).
compounds
dependent
theory
applied
(Fig.
transition
with those of corband 1180 cm-1 at the low-fre-
CH2 groups;
to apply
aliphatic
two time
upon
of
in number and position
and disappearance
that
and choles-
as compared
intensive
(n-l>
note
smectic
phase.
is
with
present and in
can be assigned
monotropic
2);
the
shift
only
from C4 (Fig.
series
smaller,
are
One should to
cholesteric
wagging
are
bands
end it
transitions
whose position
1250 cm”
for
n,
chains.
series
SC to LC and to
this
greater
number beginning
this
chain
of
bands
of wagging
alkyl
to
intensity
rocking
at low temperatures
and Cl8 have
n-paraffins;
n-paraffins
were
long
enanthiotropic
and C,6
bands
region evident
with
of
and enanthiotropic
ra are:
logue
most
homologue
packing
to C,5 have
cl3
is
of
(Fig.
coefficient the
broad
I)
can be described accompanied band at
with
1250 cm”
as
146
Fig. 3. Experimental (a) and calculated according to (ref. 6) (b) cl6 spectra in the region of wagging vibrations. corresponds to end CH2 group (neighbouring to CO-group); bands of collective vibrations of residual chain are weak because the great dipole moment of end group is not more included in these vibrations; band at 1170 cm" belongs to C-O vibration. The strong decrease of interaction coefficient can naturally be aasigned to rotated conformers formation. ACKNOWLEDGEMENTS The authors express their thanks to I.V. Sekirin for computing theoretical ChA spectra. REFERENCES 1 J.D. Bernal, D. Crowfoot, Trans. Farad. Sot., 29 (1933) 10321049. 2 R.F. Brajan, Zhurn. struct. chimia, 23 (1982) 154-174. 3 G.J. Davis, R.S. Porter, B.M. Barrel, Mel, Cryat. Liquid Cryst., 11 (1970) 319-330. 4 N.G. Guerine, B.M. Craven, J.C.S. Perkin II, No.10 (1979) 5
6 7
1414-1419.
L.S. Gorbatenko, Theses Cand. Soi., Rostov-na-Donu, 1975. T.A. Gavrilko, G.A. Puchkovskaya, I.V. Sekirin, N.M. Chepilko, Preprint of Institute of Physics of UkrSSR Academy of Scienoes, No. 83/24, Kiev, 1983. R. Zbinden. Infrared spectroscopy of polymer molecules, Mir, Moscow, 1966.