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Conformational composition of propanol in gaseous state and in matrix isolation

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  • Taras Shevchenko National University of Kyiv
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Conformational analysis of the experimentally recorded IR absorption spectra of propanol in a gaseous state and in a low-temperature argon matrix at 20 K and at 35 K was carried out for different spectral ranges. It showed that the conformational composition of the samples in gas and in matrix isolation is different. In gaseous propanol, Gt conformers predominate, while in matrix isolation the most energetically favorable form is Tg conformer, which prevails in percentage at both considered temperatures of an Ar matrix.
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Conformational composition of propanol in gaseous state and in matrix
isolation
Iryna Doroshenko (1), Laziz Meyliev (2), Bahrom Kuyliev (2)
((1) Taras Shevchenko National University of Kyiv, Kyiv, Ukraine, (2) Karshi State University,
Karshi, Uzbekistan)
Conformational analysis of the experimentally recorded IR absorption spectra of propanol in
gaseous state and in a low-temperature argon matrix at 20 K and at 35 K was carried out for different
spectral ranges. It showed that the conformational composition of the samples in gas and in matrix
isolation is different. In gaseous propanol Gt conformers predominate, while in matrix isolation the
most energetically favorable form is Tg conformer, which prevails in percentage at both considered
temperatures of an Ar matrix.
Introduction
Propanol is one of the most useable alcohols for different applications, so it is
widely studied both by experimental and theoretical methods. It is known that
molecules of monohydric alcohols can form different cluster structures due to
hydrogen bonding [1-3]. Moreover, each molecule of alcohol with more than one atom
of carbon can be found in different conformations, which are formed as a result of
rotation of atoms around chemical bonds [4-6]. In the case of propanol (structural
formula CH2-CH3-CH3-OH), the molecule of which has two structural dihedrals, nine
stable configurations exist: one plane structure and four pairs of enantiomers (or
mirror-image pairs). Since enantiomers have similar energy and optical properties, the
difference between them is usually neglected. As a rule, propanol conformers are
designated by a symbolic title using a generally accepted scheme of dihedral angles
CCCO+CCOH via big+small letters for trans- (T, t), gauche- (G, g) and gauche'- (G',
g') conformers [6, 7].
Conformational properties of propanol were studied in supersonic jets [6, 8] and
cryogenic matrices [6, 9], in gas and liquid phases [7], in CCl4 solutions at low
concentrations [10]. The aim of the presented work is to analyze experimentally
registered IR absorption spectra of propanol in gaseous state and in matrix isolation,
and to compare conformational composition of the samples studied.
Experimental
Liquid propanol with purity > 99.9 from Fluka was used for experimental
investigations. Atmospheric oxygen and nitrogen were removed from the alcohol by
five freeze-pump-thaw cycles. Gas phase samples were obtained by the natural
evaporation in vacuum process from the liquid surface.
FTIR spectra of propanol in gaseous state were measured using conventional
single pass 10 cm gas cell equipped with KBr windows with the resolution 0.5 cm-1
using deuterated triglycine sulfate (DTGS) detector. The gas cell was filled with an
alcohol using standard vacuum technique. Pressure of the alcohol in the cell was set
close to its saturated pressure at 20o C: 2 mbar.
Conformational analysis of gaseous propanol
Spectral region of OH stretching vibrations
Fig. 1 presents the spectral band of the stretching vibrations of free hydroxyl
groups (3600 3750 cm-1) of gaseous propanol. A complicated shape of this band is
due to the vibrations of different conformers of propanol molecule as well as vibrations
of non-bonded hydroxyl groups of dimers and bigger clusters [11-13].
3600 3625 3650 3675 3700 3725
0,0
0,1
Gg
absorption, a.u.
wavenumber, cm-1
Gt
Gg'
D
D
Fig.1 IR absorption band of gaseous propanol corresponding to the stretching
vibrations of free OH groups
In [14] the following assignment of the absorption bands to different propanol
conformers was made: 3682 cm-1 Gt conformer, 3679 cm-1 Tt conformer, 3669
cm-1 Gg', 3660 cm-1 Tg and 3657 cm-1 Gg. As is seen from Fig.1, one can
distinguish an intense band at 3682 cm-1, which can be assigned to vibrations of Gt
conformer. In addition, there are bands at 3657 and 3669 cm-1, which can be assigned
to conformers Gg and Gg'. IR absorption bands of Tt and Tg conformers are not
observed in this part of the spectrum, or their intensity is too low. Therefore, we can
draw the conclusion that conformations with gauche- orientation of С С bond Gt,
Gg and Gg' dominate in the studied sample of gaseous propanol.
Moreover, spectral bands of molecules, which are proton acceptors in dimers,
with frequencies 3654, 3671 and 3675 cm-1 were found in [14]. All these three bands
were found in our sample, too. In fig. 1, D denotes the dimer bands. Besides these
identified bands there are other ones (for example, 3647 cm-1), which can belong to
vibrations of free OH groups in bigger clusters trimers and so on.
Spectral region 800 1300 cm-1
In the spectral region from 800 to 1300 cm-1 the stretching vibrations of a carbon
skeleton of the propanol molecule (i.e. С С and С О bonds) are located. Fig. 2
presents FTIR spectrum of gaseous propanol in the corresponding range.
800 850 900 950 1000 1050 1100 1150 1200 1250 1300
0
1
2
Gx
Tx
absorption, a.u.
wavenumber, cm-1
Tx
Gx
Gt
Tt
Fig.2 FTIR spectrum of gaseous propanol in the range 800 1300 cm-1
The stretching С С vibrations of conformers with trans-configuration in respect
to С С bond (they are denoted as Тх) have frequencies 881 and 885 cm-1 [14], and
the corresponding vibrations of conformers of Gx group are located in the range 850
860 cm-1. In the experimentally registered spectrum of gaseous propanol there are
vibrations at these frequencies, but they are of low intensity.
In the spectrum presented in Fig. 2 the most intense absorption is observed at
1065 cm-1. This band corresponds to a more high-frequency part of a doublet appearing
at interaction of the stretching С С and С О vibrations [14]. However, this band
gives few information about dominating conformations, since calculated frequencies
for all five conformers are located in the range 1050 1100 cm-1. The second, low-
frequency component of the doublet, is more informative: vibrations of Тх conformers
are observed at 1014 and 1025 cm-1, while vibrations of Gx conformers are located
between 965 and 972 cm-1 [14]. Therefore, the observed absorption maximum at
1020 cm-1 can be assigned to Тх vibrations, and the band at 972 cm-1 to Gx vibrations.
The comparison of intensities of these two bands allows concluding that Gx conformers
dominate in the investigated sample, which agree with the conclusions drawn from the
analysis of the spectral region of the stretching ОН vibrations.
In addition to skeleton vibrations, in this region there are also bending OH
vibrations. According to [14], bending OH vibrations in Gt conformer manifest at 1222
cm-1, and the corresponding vibrations of Tt conformer at 1235 cm-1. In the IR
spectrum in Fig. 2 one can notice both of these bands, and an intensity of Gt band is
higher than that of Tt band, indicating domination of Gt form over Tt one.
Spectral region of stretching С Н vibrations
Fig. 3 presents IR absorption spectrum of gaseous propanol in the spectral range
of the stretching С Н vibrations. According to [15], the absorption band near 2890
cm-1 corresponds to the symmetric stretching С-Н vibrations in СН3 group, and the
band at 2972 cm-1 to antisymmetric stretching С-Н vibrations in СН3 group. Between
them, the symmetric stretching С-Н vibrations of СβН2 group are located. According
to the assignment from [16 - 18], the band at 2942 cm-1 belongs to the conformer Tg,
the band at 2952 cm-1 to the conformer Gg. It means that the most intense vibrations
in this region correspond to conformers, which have ОН group in gauche- position.
Absorption frequencies of conformers with hydroxyl group in trans- position are lower
by several tens of wavenumbers. However, in the registered spectrum absorption at
these frequencies is low.
2850 2900 2950 3000 3050
0,0
0,1
0,2
0,3
0,4
0,5
CH3- ss
absorption, a.u.
wavenumber, cm-1
CH3- as
Tg Gg
Fig. 3 IR spectrum of gaseous propanol in the region of the stretching С Н
vibrations
Conformational analysis of propanol in matrix isolation
Matrix isolation in low-temperature matrices of inert gases allows registering
vibrational spectra of individual molecules or small clusters without their interaction
with environment. Moreover, in contrast to the spectra of a substance in the gas phase,
the vibrational bands in the obtained spectra are not complicated by the rotational
structure. This fact allows making a detailed assignment of registered bands to different
conformers or clusters of different sizes. In this section, IR spectra of propanol in low-
temperature argon matrix are considered. For samples preparation, propanol vapor was
mixed with argon in ratio 1:1000 (one propanol molecule per 1000 argon atoms), then
the obtained gas mixture was deposited at a cooled to 20K window. As a result, one
can register IR spectra of propanol molecules or small clusters and analyze
conformational composition of propanol in matrix isolation.
Spectral region of stretching ОН vibrations
Fig. 4 presents IR spectrum of propanol trapped in an Ar matrix at 20 K in the
spectral region 3640 3680 cm-1. In this part of the spectrum, the stretching vibrations
of non-bonded hydroxyl group are manifested. These non-bonded OH groups can
belong to monomers and proton acceptors in dimers as well as bigger chain clusters.
3640 3650 3660 3670 3680
0,0
0,5
1,0
1,5
Gg'
Gg
Gt
D
D
absorption, a.u.
wavenumer, cm-1
Tg
Tt
D
Fig. 4 IR spectrum of propanol trapped in an Ar matrix at 20 K in the spectral region
of stretching OH vibrations
As is seen, the spectral band in fig. 4 has a complex contour with a number of
maxima. The maxima at 3657, 3659, 3660, 3663, and 3665 cm-1 are assigned,
respectively, to the vibrations of Gg, Tg, Gg', Tt and Gt conformers. This assignment
was made according to the results of IR absorption of propanol conformers in an Ar
matrix in this spectral region [19]. Analyzing relative intensities of the spectral bands
corresponding to the stretching OH vibrations of five different propanol conformers
one can say that conformers with gauche-configuration of ОН group (i.e. Gg, Tg and
Gg') dominate in the investigated sample. The intensity of the bands corresponding to
Tt and Gt conformers is essentially lower, indicating that the number of these
conformers (with OH group in trans- position) in the sample is much less. It is
interesting to note, that in the gas phase the domination of Gt conformer was observed
in this spectral region. Apparently, during the cooling of the gas mixture and the
deposition of the matrix, a transition from one form to another occurred, and the
conformational composition of the sample changed. This is confirmed by the results of
our quantum-chemical calculations [20], according to which the lowest energy barrier
(about 3.2 kJ/mol) is observed between Gt and Gg 'conformers, that is, the transition
from Gt conformation to Gg' conformation is the most probable.
In addition to the vibrations of propanol monomers, spectral bands of OH
stretching vibrations of free (i.e., not involved in the formation of hydrogen bonds)
hydroxyl groups of small propyl alcohol clusters are also observed in this frequency
range. Such free hydroxyl groups are inherent in molecules that act as proton acceptors
in the formation of larger dimers or open clusters. According to [14], IR absorption
bands of propanol dimers appear at frequencies of 3654, 3671, and 3675 cm-1. In the
spectrum in Fig. 4 all three of these bands are present (denoted by the letter D),
although they have a relatively low intensity. This fact indicates the presence of a small
(compared to monomers) amount of dimers in the sample under study. Note that dimers
were also present in the gas phase of propanol, as evidenced by the analysis of the
spectra of gaseous propanol in the same spectral range (see Fig.1).
Registered maxima of IR bands of propanol monomers and dimers in an Ar
matrix in the spectral region of the stretching vibrations of free hydroxyl groups as well
as their assignments are listed in Table 1.
Table 1 Spectral bands of stretching OH vibrations of propanol monomers and
dimers
Assignment
dimer, an acceptor molecule
Gg conformer
Tg conformer
Gg' conformer
Tt conformer
Gt conformer
dimer, an acceptor molecule
dimer, an acceptor molecule
Moreover, in the spectrum in Fig. 4 one can notice other low-intensity IR
absorption bands, for example, at frequencies of 3643, 3644, 3648 and 3650 cm-1,
which most likely belong to clusters consisting of three or more molecules. Note that
a band at 3647 cm-1 was also registered in gaseous propanol and was attributed to the
vibrations of trimers. Thus, it can be concluded that both in gaseous propanol and
during its isolation in a low-temperature argon matrix, not only individual propanol
molecules in different conformations exist there, but also hydrogen-bonded clusters
consisting of two, three and more molecules are formed.
It is interesting to consider how the conformational composition of the sample
under study changes with an increase in the temperature of the argon matrix. In fig. 5
the IR absorption spectrum of propanol in an argon matrix at 35 K in the spectral region
of stretching vibrations of the free hydroxyl group is presented.
3640 3650 3660 3670 3680
0,0
0,5
1,0
1,5
Gt
D
Gg'
absorption, a.u.
wavenumber, cm-1
Tt
DD
GgTg
Fig. 5 IR absorption spectrum of propanol isolated in an argon matrix at 35 K
in the region of OH stretching vibrations
As can be seen from Fig. 5, with an increase in the temperature of the argon
matrix to 35 K, the contributions of all propanol conformers remain in the absorption
band of the free OH group, but the intensity of the Tt conformer absorption band
noticeably increases compared to the other bands. This fact indicates a redistribution
of the number of different conformers in the sample during its heating: the percentage
of conformers with gauche orientation of the hydroxyl group decreases and the
proportion of Tt conformers increases. This redistribution is most likely due to
relaxation transitions from the Tg and Tg 'conformations to the Tt conformation, since
the calculated height of the energy barrier between the Tt, Tg, and Tg' conformers is
about 7 kJ/mol.
Spectral region of stretching С Н vibrations
Fig. 6 presents IR absorption spectrum of propanol in an Ar matrix at 20 K in
the spectral region of the stretching С Н vibrations. In this region, symmetric and
antisymmetric stretching vibrations in СН3 group at 2886 and 2973 cm-1 are the most
intense, similarly to the picture observed for the gas phase (see Fig. 3). Between these
two vibrations, there are symmetric stretching vibrations of CβH2 group. In contrast to
the corresponding absorption bands in the gas phase, for which a strong overlap is
observed (Fig. 3), the matrix isolation method allows the bands of different conformers
to be separated.
2850 2900 2950 3000 3050
0,0
0,5
1,0
1,5
СН3- ss
Gg'
Gg
Gt
absorption. a.u.
wavenumber, cm-1
Tg
Tt
СН3- as
Fig. 6 IR spectrum of propanol trapped in an Ar matrix at 20 K in the spectral
region of stretching С – Н vibrations
In Fig. 6 one can distinguish five bands of СβН2 vibrations corresponding to five
different propanol conformers [16, 18]: the band of Tt conformer at 2907 cm-1, the band
of Gt conformer at 2918 cm-1, the band of Gg' conformer at 2930 cm-1, the one of Tg
conformer at 2936 cm-1 and of Gg conformer at 2948 cm-1. From the distribution of the
relative intensities of these five bands, it can be concluded that conformers with gauche
orientation of the hydroxyl group, i.e. conformers of the Xg/g' type, predominate in the
sample under study. This conclusion completely coincides with the results of the
analysis of the spectral region of OH vibrations for this sample, which additionally
confirms the correctness of the conclusions made above.
2850 2900 2950 3000 3050
0.0
0.5
1.0
1.5
СН3- ss
СН3- as
Gt
Gg'
absorption, a.u.
wavenumber, cm-1
Tt
Gg
Tg
Fig. 7 IR spectrum of propanol trapped in an Ar matrix at 35 K in the spectral
region of stretching С – Н vibrations
As the temperature of the matrix rises to 35 K, no noticeable changes in the
spectrum in the spectral region of stretching CH vibrations occur. This is illustrated in
Fig. 7, which shows the corresponding part of the IR absorption spectrum of propanol
isolated in an argon matrix at a temperature of 35 K. The maxima of all the observed
absorption bands remain at the same positions as at a temperature of 20 K. Only a slight
narrowing of the bands of individual conformers can be noted, which leads to the
formation of sharper maxima.
Spectral region of skeletal vibrations
Fig. 8 shows the IR absorption spectrum of propanol isolated in an argon matrix
at a temperature of 20 K in the range of wavenumber from 800 to 1150 cm-1, where the
skeletal vibrations of the alkyl chain, i.e. stretching vibrations of C - C and C - O bonds,
are located. In this part of the spectrum, rather narrow bands are observed, making it
possible to assign them with high accuracy to various conformers of propanol.
850 900 950 1000 1050 1100
0,0
0,5
1,0
1,5
2,0 Gt, Tt, Tg
absorption, a.u.
wavenumber, cm-1
TgTt
Gx
Gg` Gg
Gt Tx
Fig. 8 IR absorption spectrum of propanol trapped in an Ar matrix at 20 K in
the spectral region 830 1120 cm-1
At 859 cm-1 a band of Gt conformer is observed, and at 883 cm-1 - of a mixture
of Tx conformers, similar to that in gaseous propanol (see Fig. 2).
One of the most intense absorption bands in this part of the spectrum at a
frequency of 968 cm-1 corresponds to the vibrations of the conformers of Gx group.
This assignment was made based on a comparison of the results of quantum-chemical
simulation of the IR absorption spectra of various propanol conformers performed by
the B3LYP/6-311+G(d), MP2/6-311+G(d) and MP2/aug-cc-pVTZ methods with the
experimentally recorded spectrum of propanol in an argon matrix [14]. Calculations
showed the presence of this band in the spectra of the conformers Gt, Gg, and Gg ', and
its absence in the spectra of the conformers Tt and Tg. Another band of approximately
the same intensity was recorded at a frequency of 1055 cm-1. According to the same
calculations, vibrations at this frequency are present in the spectra of the Tt, Tg, and
Gt conformers.
Using the results of quantum chemical calculations and experimental studies
from [14], we assign the bands at 1020 and 1024 cm-1 to the vibrations of Tg and Tt
conformers, respectively. At a frequency of 1066 cm-1, the absorption band of Gg'
conformer is observed, and at a frequency of 1102 cm-1 - of Gg conformer.
After analyzing the ratio of the intensities of the IR absorption bands of various
conformers in this part of the spectrum, it can be concluded that Gx-type conformers
prevail in number over Tx-type conformers. In addition, there are slightly more Tt
conformers than Tg conformers, and the number of Gg structures is slightly less than
Gg '.
When the sample was heated to 35 K, no significant changes in the spectral
pattern were observed. Fig. 9 shows the same region of the IR absorption spectrum of
propanol in an argon matrix, but at a temperature of 35 K. It can be seen that the only
difference from the spectrum at 20 K is the practically equal intensities of the bands
corresponding to the Tt and Tg conformers. In addition, an insignificant increase in
absorption on the right wing of the Gx band and a splitting of the Gg' band can be
observed, which may be associated with the appearance of additional bands in the
spectrum as a result of the formation of larger propanol clusters with an increase of the
sample temperature.
850 900 950 1000 1050 1100
0,0
0,5
1,0
1,5
2,0 Gx
Tx
Gt
Gg'
absorption, a.u.
wavenumber, cm-1
Tt
Gg
Tg
Fig. 9 IR absorption spectrum of propanol trapped in an Ar matrix at 35 K in
the spectral region 830 1120 cm-1
Spectral region of bending ОН vibrations
Let us consider separately the spectral region 1100 - 1300 cm-1, where bending
OH vibrations of propanol are manifested. The corresponding part of the recorded IR
absorption spectra of propanol isolated in an argon matrix at temperatures of 20 K and
35 K is shown in Fig. 10 and 11, respectively. As can be seen, vibrations of all five
propanol conformers are recorded in this region, the assignment of which was carried
out on the basis of our quantum-chemical calculations, as well as the results of
modeling presented in [14].
1150 1200 1250
0,0
0,5
absorption, a.u.
wavenumber, cm-1
TgTt
Gg`
Gg
Gt
Fig. 10 IR absorption spectrum of propanol in an Ar matrix at 20 K in the
spectral region of bending OH vibrations
1150 1200 1250
0,0
0,5
Gg
Gt
Gg'
absorption, a.u.
wavenumber, cm-1
Tt
Tg
Fig. 11 IR absorption spectrum of propanol in an Ar matrix at 35 K in the
spectral region of bending OH vibrations
Quantum-chemical simulation of IR absorption spectra of various propanol
conformers shows that bending OH vibrations of Tt conformer appear at a frequency
of 1232 cm-1, and ones of Tg conformer at a frequency of 1228 cm-1. The
corresponding absorption band of the Gt conformer is located at a frequency of
1222 cm-1, and the vibrations of the Gg conformer manifest as a shoulder of this band
at 2019 cm-1. For Gg' conformer the calculated frequencies of bending OH vibrations
are somewhat lower - about 1130 cm-1.
All the bands listed above are observed at both temperatures, but with slightly
different intensity distributions. In addition, the spectrum contains bands at frequencies
of 1240, 1246, and 1252 cm-1. Since the intensity of these bands increases with
increasing temperature, it can be assumed that they belong to propanol clusters.
Conclusions
Conformational analysis of the experimentally recorded IR absorption spectra of
propanol in gaseous state and in a low-temperature argon matrix, carried out for
different spectral ranges, showed that the conformational composition of the samples
in these two cases is different.
Analyzing the region of OH stretching vibrations in gaseous propanol, the
predominance of Gt structures, as well as the presence of the Gg and Gg' structures,
was observed. This composition of the sample in the gas phase is also confirmed by
the results of the analysis of the spectral region of bending OH vibrations, which
indicate the predominance of the Gt form over Tt, and the region of skeletal vibrations,
according to which conformers of the Gx type prevail over those of Tx.
When propanol is isolated in an argon matrix, a redistribution occurs between
the ratios of conformers in the sample. Analysis of the range of OH stretching
vibrations shows the predominance of the Tg conformer, but the number of Gg and Gg'
conformers is not much less. The slight predominance of structures of the Tx type over
Gx is also evidenced by the distribution of the intensities of the recorded absorption
bands in the spectral intervals of bending OH vibrations and skeletal vibrations of the
alkyl chain. The spectral region of stretching СН vibrations turned out to be the least
informative both in the case of a gaseous sample and in the case of matrix isolation.
Thus, the results of our studies show that gaseous propanol contains the largest
number of Gt conformers, which are the most stable of the five possible propanol
conformers according to a number of quantum mechanical calculations [8, 14, 21]. The
propanol molecules isolated in a low-temperature argon matrix are influenced by the
environment; therefore, in this case the most energetically favorable form is Tg
conformer, which prevails in percentage.
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