Ab initio
calculations were used to calculate normal mode frequencies and intensities of three stable conformations of npentane. The overall frequency region including IR and Raman spectra were analyzed to get the full spectra of npentane and to explore the relations between frequency and disorder in the overall frequency region. The bands in the IR spectra and in the Raman spectra were found to be associated with specific conformations, and therefore the characteristic bands of each conformation could be obtained. For the bands with the same vibrational modes, the trend with the change of conformation was also discussed.
INTRODUCTION
Vibrational spectroscopy has been used extensively to identify and characterize rotational isomeric structures of nalkanes, e.g., providing information on quantities such as configuration, conformation, chemical composition and orientation. A major contribution to the analysis of vibrational spectra, particularly in the conformational dependence of alkanes and polyethylene, was presented by Snyder and Schachtschneider with the empirically refined force fields.
1

3
But such force fields could not provide detailed insights into the conformation dependence of force constants and spectra, it was therefore important to have complete understanding of those properties that could be obtained by
ab initio
calculations.
4

7
With the current
ab initio
calculations, it would become possible to calculate vibrational frequencies and the corresponding intensities especially for the small molecules. Hence, small alkanes of trans (
t
) and gauche (
g
) conformations had been studied widely since their structure and bonding served as protypes for carbonbased chemical industry.
Ab initio
vibrational frequencies and optimized geometry research had been conducted on propane
8

10
at different levels of theory and the nbutane molecule also commanded obvious interest and had been studied extensively by theoretical methods.
11

18
However, for longer nalkanes, their stable conformers provided combinations of
t
and
g
rotational states that were available in nbutane, it was important to extend these studies.
Ab initio
methods also used to calculate pentane and hexane conformations with the basis sets of HF/421G,
17
,
18
HF/431G
4
and HF/631G,
14
,
19
which provided spectra studies of four stable conformers of npentane and 10 stable conformers of nhexane and the insights of the influence of conformation on these other molecular properties. In addition, Synder and Kim
20
had studied Raman spectra of liquid nalkanes C
_{4}
C
_{9}
and presented the overall Raman intensity in the low frequency range. Meanwhile, a wide range of experimental techniques, such as NMR,
21

23
IR
24

26
and Raman,
27
had been employed to reveal the molecular features of the nalkyl chains. However, the studies mentioned above only gave the amount of conformational disorder at a specific site in the
n
alkyl chains. Due to the limitation of experimental conditions, it is very difficult to get the overall frequency information of the conformational order of these systems by experiments. Hence
ab initio
calculations were of importance to solve this problem. At the same time, as a basic unit, it could provide some useful information of the conformation for long chain nalkanes by analyzing the results of
n
pentane. Therefore we could use
n
pentane as a model for long chain
n
alkanes, even though there were many more complex conformations. The
n
pentane undertook four conformations of the carbon backbone. With respect to the current study, the presence of three stable conformations of
n
pentane meant that the spectra could be interpreted as resulting from three spectroscopically different species, each having its own set of vibrational frequency and intensity parameters. According to the IR and Raman frequencies and intensities of each conformation, the detailed spectra information of the specific conformation and the corresponding correlation between frequency and vibrational mode could be obtained. Hence,
ab initio
calculations were useful tools to carry out the vibrational study of
n
alkanes.
CALCULATIONS
Ab initio
calculations were performed with the GAUSSIAN 98
28
programs. The HartreeFock (HF) method and the 631G basis set were used to obtain the optimized geometries and frequencies for each molecule in this work. For the nalkanes, the 631G basis set
19
,
29
was proved to be good enough. The internal and local symmetry coordinates for each molecule followed the definitions of the
tt
conformation of npentane (see
. 1
), both
gt
and
gg
conformations were just the same as the above description (see
. 2
and
. 3
). The optimized geometries of all the
n
pentane conformations at this level of theory were shown in
1
. In general, the HF
ab intio
calculated frequencies were higher than the corresponding experimental values due to neglect of contributions from electron correlation. In this work, the calculated and observed frequencies were not compared and no scaling of vibrational frequencies was applied. But the relations between the calculated frequencies and the conformations and the trend of frequency changes were discussed in detail.
Internal coordinates of tt conformation of npentane.
Internal coordinates of gt conformation of npentane.
Internal coordinates of gg conformation of npentane.
Optimized geometries of conformers ofnpentane with 631G basis set
*Bond lengths in angstroms, bond angles in degrees.
RESULTS AND DISCUSSIONS
The calculated frequencies of the following molecules were used in the refinement:
tt
,
gt
and
gg
for
n
pentane. The
gg'
form was the least stable
19
because of the steric hindrance, and therefore the
gg'
conformation was not discussed in this paper. With respect to the optimized geometries of all pentane conformations, the CH bond lengths and the CCH bond angles were the least affected by conformation. On the other hand, the internal CC bond lengths and CCC angles were the most affected by conformational changes, there being a tendency for those to be larger than the terminal ones. The
tt
conformation had
C_{2v}
symmetry, while the
gg
conformation had
C_{2}
symmetry. And for
gt
conformation of
n
pentane, it had lower symmetry. Hence the vibrations associated with an assembly of conformational disordered chains were much more difficult to characterize. But they all had 45 fundamental frequencies including vibrational modes with IR and Raman active, those only with IR active or only with Raman active.
The low frequency region (0600 cm
^{−1}
) of the IR and Raman spectra was useful for analyzing polymethylene chain conformation. There remained some problems, however, concerning the interpretation of the spectra obtained by experiments. Hence the present analysis of calculated spectra of npentane was aimed at improving band assignments and exploring the relations between frequency and conformation in this region. With the changes of the conformation, the vibrational modes would become more complex. In order to characterize the vibrations of an assembly of disordered chains in detail, the coordinates are defined as follows: s is stretching; ss is symmetric stretching; as is antisymmetric stretching; b is bending; sb is symmetric bending; ab is antisymmetric bending; r is rocking; w is wagging; tw is twisting; t is torsion. The calculated IR spectra (0600 cm
^{−1}
) were shown in
. 4
and the assignments of main bands were listed in
2
. It was obvious that there were great differences between
tt
conformation and
gt
or
gg
conformation. For alltrans conformation there was only a single band, at 196.4 cm
^{−1}
, with appreciable intensity in the calculated IR spectra of npentane, whereas for
gt
conformation there were four bands with appreciable intensity, the band with the highest intensity was at 511.7 cm
^{1}
. The
gg
conformation had five bands with higher intensity. Different from
gt
conformation, the intensities of the bands of 414.9 cm
^{1}
and 517.9 cm
^{1}
were relatively higher. Especially interesting was that the vibrational modes mentioned above belonged to CCC bending mode, another kind of mode, at 305.6 cm
^{1}
(
gt
), 296.5 cm
^{1}
and 155.3 cm
^{1}
(
gg
), belonged to CC torsion mode or methyl torsion mode. The proposed assignments,
30
obtained by high resolution neutron scattering, to CCC bending mode were at 401 cm
^{1}
and 190 cm
^{1}
. And the earlier force fields
3
predicted that the methyl torsion modes were at 215 and 210 cm
^{1}
. Tomonaga
31
measured the far infrared spectra of
n
pentane and found that bands at 470 and 334 cm
^{1}
were assignable to the
gt
conformation and a band at 386 cm
^{1}
assignable to the
gg
conformation. The frequencies mentioned above were well in agreement with our calculated values, even though some of our assignments to conformations differed from those previously proposed.
3
From the calculated results, we could find some bands that mainly occurred at the fold site of
gt
or
gg
conformation, thus these bands could be regarded as the characteristic bands. In this region, there was only one characteristic band: 511.7 cm
^{1}
for
gt
conformation, 517.9 cm
^{1}
for
gg
conformation respectively. But for
tt
conformation, the bands with higher intensity were considered as its characteristic bands, thus the band at 196.4 cm
^{1}
was the characteristic band of
tt
conformation. As far as the effect of changes in conformation was concerned, the change trend of the same vibrational modes was only discussed. For the vibrational mode of b(C
_{1,2,3}
,C
_{2,3,4}
,C
_{3,4,5}
) (stand for inphase CCC bending), with the change of conformation from
tt
to
gt
and
gg
, the frequency decreased from 428.6 cm
^{1}
, 358.1 cm
^{1}
to 287.8 cm
^{1}
. But the intensities of
gt
and
gg
conformations were stronger than that of
tt
conformation. For the vibrational mode of b(C
_{1,2,3}
, C
_{3,4,5}
)b(C
_{2,3,4,}
) (stand for outofphase CCC bending), the frequency increased from 511.7 to 517.9 cm
^{1}
with the conformation changed from
gt
to
gg
.
Calculated IR spectra of npentane in the region of 0600 cm^{1}.
Calculated IR frequency and description of 0600 cm1
Calculated IR frequency and description of 0600 cm^{1}
The calculated Raman spectra (0600 cm
^{1}
) were shown in
. 5
and the descriptions of main bands were listed in
3
. Compared with the calculated IR spectra, although the intensities of the main bands were higher than those of IR, the number of the bands with higher intensity was fewer than that of IR and the mode of vibration was only CCC bending. At the same time, there were significant differences between the bands of
tt
and
gt
,
gg
conformations. For alltrans conformation there was only a single band at 428.6 cm
^{1}
, with the highest intensity ,whereas for
gt
and
gg
conformation there were two main bands respectively, 511.7 and 358.1 cm
^{1}
with middle intensities for
gt
conformation, 517.9 and 287.8 cm
^{1}
with weak intensities for
gg
conformation. Snyder and Kim
20
presented the calculated Isotropic Raman spectra of npentane in the low frequency (0600 cm
^{1}
) and suggested that the band at 400 cm
^{1}
should belong to
tt
conformation, the bands at 340 cm
^{1}
and 470 cm
^{1}
should belong to
gt
conformation. Those were consistent with our calculated values. But the bands of
gg
conformation had not been mentioned. From our calculated results, although the intensities of bands of
gg
conformation were much weaker, the bands at 517.9 cm
^{1}
and 287.8 cm
^{1}
should belong to
gg
conformation. In addition, there were the same characteristic bands with those in the IR spectra, 511.7 cm
^{1}
for
gt
conformation, 517.9 cm
^{1}
for
gg
conformation. However the characteristic band of
tt
conformation was at 428.6 cm
^{1}
. Likewise, for the vibrational mode of b(C
_{1,2,3}
,C
_{2,3,4}
,C
_{3,4,5}
), the trend of frequency change with the conformation was the same as that in the IR spectra mentioned above.
Calculated Raman spectra of npentane in the region of 0600 cm^{−1}.
Calculated Raman frequency and description of 0600 cm1
Calculated Raman frequency and description of 0600 cm^{1}
Due to the intermolecular interactions, many vibrations had coupling in some degree. Thus the number and complexity of the bands in the region of 7001300 cm
^{−1}
increased, especially for the
gt
conformation with lower symmetry. Hence the approximate nature of the vibrational modes was only described in this paper. The calculated IR spectra were shown in
. 6
and the assignments of main bands were listed in
4
. Early study had reported
3
that there were two bands whose vibrational mode was methylene rocking (inphase) for
n
pentane, the band at 733 cm
^{−1}
with higher intensity belonged to
gt
conformation and the band at 728 cm
^{−1}
with lower intensity belonged to
tt
conformation. And this result was consistent with our calculated values, for
tt
conformation at 803.4 cm
^{−1}
, for
gt
conformation at 811.9 cm
^{−1}
and for
gg
conformation at 807.2 cm
^{−1}
. Based on the rule of characteristic bands mentioned above, the characteristic bands of each conformation were as follows: 803.4 cm
^{−1}
, 995.4 cm
^{−1}
and 1172.7 cm
^{−1}
for
tt
conformation; 844.2 cm
^{−1}
, 915.8 cm
^{−1}
and 1174.1 cm
^{−1}
for
gt
conformation;807.2 cm
^{−1}
and 922.6 cm
^{−1}
for
gg
conformation in this region. On the other hand, there were other two important vibrational modes: CC stretching mode and methyl rocking mode. And both of the bands often appeared at the same time. For the vibrational mode r(C
_{1}
H
_{1,2,3}
,C
_{5}
H
_{10,11,12}
)s(C
_{1,2}
,C
_{2,3}
)s (C
_{3,4}
,C
_{4,5}
), when the conformation changed from
tt
to
gt
and
gg
, the calculated frequency would shift about 4 cm
^{−1}
at most, 995.4 cm
^{−1}
for
tt
conformation, 991.8 cm
^{−1}
for
gt
conformation and 992.8 cm
^{−1}
for
gg
conformation. Another apparent character was at near 1300 cm
^{−1}
, the r(C
_{1}
H
_{1,2,3}
,C
_{5}
H
_{10,11,12}
)s(C
_{1,2}
,C
_{2,3}
,C
_{3,4}
,C
_{4,5}
) vibrational mode also varied, 1278.0 cm
^{−1}
for
tt
conformation, 1278.1 cm
^{−1}
for
gt
conformation and 1281.2 cm
^{−1}
for
gg
conformation. However, the intensity of
gg
conformation was so weak that it was not listed in the
4
.
Calculated IR spectra of npentane in the region of 7001300 cm^{1}.
Calculated IR frequency and description of 7001300 cm1
Calculated IR frequency and description of 7001300 cm^{1}
The calculated Raman spectra (7001300 cm
^{−1}
) were shown in
. 7
and the assignments of main bands were listed in
5
. Compared with the calculated IR spectra, there was a great difference between the spectra of
tt
conformation and the
gt
,
gg
conformation. For the vibrational mode of r(C
_{1}
H
_{1,2,3}
, C
_{5}
H
_{10,11,12}
),s(C
_{2,3}
,C
_{3,4}
), the band at 951.4 cm
^{−1}
was sharp for
tt
conformation, but the bands were broad for
gt
conformation at 915.8 cm
^{−1}
and for
gg
conformation at 922.6 cm
^{−1}
. In the meanwhile, the intensity decreased gradually. Similarly, the vibrational mode of s(C
_{1,2}
,C
_{2,3}
,C
_{3,4}
,C
_{4,5}
)r(C
_{1}
H
_{1,2,3}
,C
_{5}
H
_{10,11,12}
),
tt
at 1278.0 cm
^{−1}
,
gt
at 1278.1 cm
^{−1}
and
gg
at 1281.3 cm
^{−1}
, altered in the same way. With respect to the characteristic bands of each conformation, the bands for
tt
conformation were at 951.4 cm
^{−1}
, 1132.8 cm
^{−1}
and 1278.0 cm
^{−1}
; for
gt
conformation at 844.2 cm
^{−1}
, 915.8 cm
^{−1}
and 1174.1 cm
^{−1}
; for
gg
conformation at 922.6 cm
^{−1}
, 1083.8 cm
^{−1}
and 1126.2 cm
^{−1}
in this region.
Calculated Raman spectra of npentane in the region of 7001300 cm^{1}.
Calculated Raman frequency and description of 7001300 cm1
Calculated Raman frequency and description of 7001300 cm^{1}
The calculated IR spectra in the range of 13001700 cm
^{−1}
were shown in
. 8
and the descriptions of main bands were listed in
6
. In general, the bands were much better defined than those in the region of 7001300 cm
^{−1}
, the vibrational modes mainly included the methylene wagging and bending, the methyl symmetric and antisymmetric bending. From
. 8
, we could find that the intensities of the bands in the lowerfrequency region were distinctly weaker than those in the higherfrequency region. Hence emphasis was put on the higherfrequency. The band at 1540.6 cm
^{−1}
of
tt
conformation was the coupling between methylene wagging mode and CC stretching mode, the same vibrational mode was found at 1523.8 cm
^{−1}
for
gt
conformation and at 1538.9 cm
^{−1}
for
gg
conformation respectively. In addition, the shoulders at about 1578 cm
^{−1}
belonged to inphase and outofphase methyl symmetric bending. According to the calculated results, the frequency of this mode was independent of conformation, e.g., for inphase methyl symmetric bending,
tt
at 1578.1 cm
^{−1}
,
gt
at 1577.3 cm
^{−1}
and
gg
at 1578.3 cm
^{−1}
. The mode of outofphase symmetric bending had the same character. In the higherfrequency region, the vibrational mode was the coupling between methyl antisymmetric bending and methylene bending. With the decrease of symmetry, the coupling would become more serious because of the great effect of conformation. For
tt
conformation, there were two sharp bands, at 1656.9 cm
^{−1}
and 1672.0 cm
^{−1}
respectively, but it was almost impossible for
gt
and
gg
conformation. At the same time, the same vibrational mode was very difficult to find, therefore this was not discussed in this paper. With respect to the characteristic bands of each conformation, the characteristic bands of
tt
conformation were at 1656.9 cm
^{−1}
and 1672.0 cm
^{−1}
; for
gt
conformation at 1523.8 cm
^{−1}
and 1660.5 cm
^{−1}
; for
gg
conformation at 1652.5 cm
^{−1}
, 1667.0 cm
^{−1}
and 1674.3 cm
^{−1}
in this region.
Calculated IR spectra of npentane in the region of 13001700 cm^{1}.
Calculated IR frequency and description of 13001700 cm1
Calculated IR frequency and description of 13001700 cm^{1}
From the calculated Raman spectra in the range of 13001700 cm
^{−1}
(shown in
. 9
) and the assignments of main bands (listed in
7
), we could see that the bands with higher intensity were mostly located at two sites: one at about 1461 cm
^{−1}
, the other at about 1660 cm
^{−1}
. For the former, the methylene twisting mode played an important role. For the latter, however, it comprised of three kinds of vibrational modes, viz., methyl symmetric bending, methyl antisymmetric bending and methylene bending. The methyl antisymmetric bending mode was independent of the conformational change, e.g.,
tt
at 1657.6 cm
^{−1}
,
gt
at 1657.2 cm
^{−1}
and
gg
at 1661.6 cm
^{−1}
. And the vibrational mode of inphase methylene twisting mode had the same character. Hence, it was impossible to find some characteristic bands for each conformation from the higher intensities mentioned above. According to our calculations, some bands with weaker intensity were of great interest in the lowerfrequency region. Among the three conformations, each conformation had its own bands, and thus the characteristic bands could be found. Hence, the bands at 1461.5 cm
^{−1}
and 1655.4 cm
^{−1}
belonged to
tt
conformation; 1431.2 cm
^{−1}
, 1653.0 cm
^{−1}
and 1660.5 cm
^{−1}
to
gt
conformation; 1459.1 cm
^{−1}
and 1674.3 cm
^{−1}
to
gg
conformation. Of course, these results would be confirmed by the experiments in the further work.
Calculated Raman spectra of npentane in the region of 13001700 cm^{−1}.
Calculated Raman frequency and description of 13001700 cm1
Calculated Raman frequency and description of 13001700 cm^{1}
The calculated IR spectra (31503270 cm
^{−1}
) were shown in
. 10
and the assignments of main bands were listed in
8
. In this region, the CH symmetric stretching mode and antisymmetric stretching mode were two main kinds of vibrational modes. Snyder
3
thought that the frequency and intensity for CH stretching tended to be independent of chain length. However the trend of frequency and intensity changes related to conformations was our major concern. Obviously, the bands of symmetric stretching lay in the lowerfrequency region, on the contrary, the bands of antisymmetric stretching lay in the higherfrequency region. Similar to other frequency regions, the number of bands with higher intensity of
tt
conformation was much less than that of
gt
and
gg
conformations. But the intensities in this region were much stronger than those in other regions. In the region of symmetric stretching, methyl symmetric stretching was an important vibrational mode, but a little wavenumber shift took place,
tt
at 3180.2 cm
^{−1}
,
gt
at 3180.9 cm
^{−1}
and
gg
at 3183.2 cm
^{−1}
. Besides, a band at 3175.6 cm
^{−1}
of
tt
conformation had so high intensity that it could be regarded as the characteristic band. Another characteristic band was at 3251.7 cm
^{−1}
. For
gt
and
gg
conformations, the characteristic bands of
gt
were at 3170.4 cm
^{−1}
and 3204.2 cm
^{−1}
, the characteristic bands of
gg
were at 3177.0 cm
^{−1}
and 3210.5 cm
^{−1}
in this region. In the higherfrequency region, the methyl antisymmetric stretching and methylene antisymmetric stretching were the main vibrational modes. For the outofphase methyl antisymmetric stretching, its frequency had a biggish shift,
tt
at 3250.1 cm
^{−1}
,
gt
at 3261.8 cm
^{−1}
and
gg
at 3256.3 cm
^{−1}
. In the spectra of
tt
conformation, the band with the strongest intensity was inphase CH antisymmetric stretching, at 3251.7 cm
^{−1}
, hence it could be considered as a characteristic band. In addition, one of the most intriguing frequencies was near 3200 cm
^{−1}
. It formed the division between the symmetric stretching and the antisymmetric stretching (dashed in
. 10
).
Calculated IR spectra of npentane in the region of 31503270 cm^{−1}.
Calculated IR frequency and description of 31503270 cm1
Calculated IR frequency and description of 31503270 cm^{1}
Calculated Raman spectra of npentane in the region of 31503270 cm^{−1}.
Calculated Raman frequency and description of 31503270 cm1
Calculated Raman frequency and description of 31503270 cm^{1}
The calculated Raman spectra (31503270 cm
^{−1}
) were shown in
. 11
and the assignments of main bands were listed in
9
. Different from the calculated IR spectra, the bands with higher intensities lay in the lowerfrequency region. But because of the coupling of each vibrational mode, it was very difficult to find the same mode. Although some bands with the approximate frequency, their vibrational modes were different, e.g., the band at 3160.4 cm
^{−1}
of
tt
conformation, the band at 3159.9 cm
^{−1}
of
gt
conformation and the band at 3160.2 cm
^{−1}
of
gg
conformation. On the other hand, the division between symmetric stretching and antisymmetric stretching shifted to near 3190 cm
^{−1}
(dashed in
. 11
), but there appeared an exception at about 3180 cm
^{−1}
, viz. the frequency of antisymmetric stretching was smaller than that of symmetric stretching. In the higherfrequency region, the methyl antisymmetric stretching mode was the main vibrational mode. The change trend of this mode was the same as that in IR spectra,
tt
at 3250.1 cm
^{−1}
,
gt
at 3261.8 cm
^{−1}
and
gg
at 3256.3 cm
^{−1}
. In the spectra of
gg
conformation, the characteristic bands were at 3170.4 cm
^{−1}
and 3204.2 cm
^{−1}
. For
gt
conformation, the characteristic bands were at 3177.0 cm
^{−1}
and 3210.5 cm
^{−1}
. For
tt
conformation, the characteristic bands were at 3182.2 cm
^{−1}
and 3217.3 cm
^{−1}
.
CONCLUSIONS
In this work, we investigated the overall IR and Raman spectra of
n
pentane by
ab initio
calculations. The HartreeFock (HF) method and the 631G basis set provided a good description of normal mode frequencies and intensities of three stable conformations of
n
pentane. Analyzing the IR and Raman spectra respectively, the frequency region was divided into four parts: 0600 cm
^{−1}
, 7001300 cm
^{−1}
, 13001700 cm
^{−1}
and 31503270 cm
^{−1}
. Each region was discussed in detail and some characteristic bands with corresponding vibrational modes were reassigned. For different conformations, there were some different characteristic bands. Some of them could not be obtained by experiments, especially for some broad bands. But in the present work, this question could be avoided. Hence, it was possible to find the overall spectra information including IR and Raman of each conformation. The bands in the IR spectra were found to be associated with specific conformations: 196.4 cm
^{−1}
, 803.4 cm
^{−1}
, 995.4 cm
^{−1}
, 1172.7 cm
^{−1}
, 1656.9 cm
^{−1}
, 1672.0 cm
^{−1}
, 3175.6 cm
^{−1}
and 3251.7 cm
^{−1}
for
tt
conformation; 511.7 cm
^{−1}
, 844.2 cm
^{−1}
, 915.8 cm
^{−1}
, 1174.1 cm
^{−1}
, 1523.8 cm
^{−1}
, 1660.5 cm
^{−1}
, 3170.4 cm
^{−1}
and 3204.2 cm
^{−1}
for
gt
conformation; 517.9 cm
^{−1}
, 807.2 cm
^{−1}
, 922.6 cm
^{−1}
, 1652.5 cm
^{−1}
, 1667.0 cm
^{−1}
, 1674.3 cm
^{−1}
, 3177.0 cm
^{−1}
and 3210.5 cm
^{−1}
for
gg
conformation. In the Raman spectra, the bands were as follows: 428.6 cm
^{−1}
, 951.4 cm
^{−1}
, 1132.8 cm
^{−1}
, 1278.0 cm
^{−1}
, 1461.5 cm
^{−1}
, 1655.4 cm
^{−1}
, 3182.2 cm
^{−1}
and 3217.3 cm
^{−1}
for
tt
conformation; 511.7 cm
^{−1}
, 844.2 cm
^{−1}
, 915.8 cm
^{−1}
, 1431.2 cm
^{−1}
, 1653.0 cm
^{−1}
, 1660.5 cm
^{−1}
, 3170.4 cm
^{−1}
and 3204.2 cm
^{−1}
for
gt
conformation; 517.9 cm
^{−1}
, 922.6 cm
^{−1}
, 1083.8 cm
^{−1}
, 1126.2 cm
^{−1}
, 1459.1 cm
^{−1}
, 1674.3 cm
^{−1}
, 3177.0 cm
^{−1}
and 3210.5 cm
^{−1}
for
gg
conformation. Likewise, the trend of frequency changes with the same vibrational mode was discussed. According to the trend discussed above, we could predict the change of conformation for experiments. At the same time, we also wanted to demonstrate the applicability and usefulness of
ab initio
calculated spectra for elucidating the spectra of long chain
n
alkanes and their conformations in the future work.
Acknowledgements
This work is supported by the NSFC (20073004, 20473012). Fundamental Foundation of BIT and the TransCentury Training Program Foundation for the Talents by the Ministry of Education of China are also gratefully acknowledged.
Aljibury A. L.
,
Snyder R. G.
,
Strauss H. L.
,
Raghavachari K.
1986
J. Chem. Phys.
84
6872 
DOI : 10.1063/1.450691
DeFrees D. J.
,
Levi B. A.
,
Pollack S. K.
,
Hehre W. J.
,
Binkley J. S.
,
Pople J. A.
1979
J. Am. Chem. Soc.
101
4085 
DOI : 10.1021/ja00509a013
Shashikala S.
,
Jurgen W.
,
Klaus A.
2002
J. Phys. Chem. B.
106
878 
Frisch M. J.
,
Trucks G. W.
,
Schlegel H. B.
,
Scuseria G. E.
,
Robb M. A.
,
Cheeseman J. R.
1998
GAUSSIAN 98, Revision A. 9.
Gaussian, Inc.
Pittsburgh, PA