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13C NMR relaxation study on molecular motion of n-alkane chains in the rotator phase
221
Journal of Molecular Structure, 213 (1992) 227-232 Elsevier Science Publishers B.V., Amsterdam
13C NMR relaxation study on molecular motion of n-alkane chains in the rotator phase Shinji Ishikawa” and Isao Andob “Kao Research Laboratories, Ichikai-machi, Haga, Tochigi (Japan) ‘Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo (Japan) (Received 21 April 1992)
Abstract The 13C spin-lattice relaxation times (T,s) of n&H, were measured in the crystalline state, rotator phase and melting state, in order to clarify the molecular motion of the rotator phase. From these results it was found that in the rotator phase every methylene carbon has almost the same value of Z’, . This indicates that all-trans zigzag chains in the rotator phase rotate rapidly about their long axes.
INTRODUCTION
n-Alkanes with certain chain length on going from the crystalline to the liquid state have been clarified as taking the hexagonal crystallographic form in the narrow temperature range prior to the melting point. This was found by means of X-ray diffraction, which has served as a powerful tool for investigating crystallographic structures [l-3]. It has been proposed that n-alkane chains in the hexagonal phase become effectively cylindrical as a result of rotating about their long axes, where the hexagonal packing is closest; hence, the hexagonal form is named the “rotator phase”. In previous work, we have studied the relationship between the structure and W NMR chemical shift of some n-alkanes in the solid state, such as n-C,,H,, n-C,,H, and n-C&Hs6,which can take the rotator phase. This study revealed that the 13Cchemical shift value of the internal (int-) CH, carbons in n-C&H, moves from 34.2 to 33.3 ppm on going from the triclinic form to the rotator phase, and the 13Cchemical shift of the internal CH, carbons on n-C&H, and n-C!,,H,, moves from 32.8 (32.9) to 33.3ppm on going from the Correspondence to: Professor I. Ando, Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan.
0022-2860/92/$05.00 0 1992 Elsevier Science Publishers
B.V. All rights reserved.
228
S. Ishikawa and I. Ando/J. Mol. Struct., 273 (1992) 227-232
orthorhombic form to the rotator phase [4]. From these results, it can be said that 13Cchemical shift values of n-alkanes in the rotator phase are the same irrespective of chain length and, therefore, every n-alkane in the rotator phase has the same structural aspects. However, it is important to obtain detailed information about their molecular motions in order to discuss dynamical structures of the rotator phase. Thus, in this work we aim to measure the 13Cspin-lattice relaxation time ( Tl) of n-C&H, in the crystalline state, rotator phase and the melting state, and to clarify the dynamics of n-&H, in the rotator phase. EXPERIMENTAL
Material The sample of n-&H, used in this work was obtained from Tokyo Kasei Co. with a purity of > 98%; further purification was not performed. Measurement T,s of n-C&H, were determined by measuring the 13Ccross-polarization/ magic angle spinning (CP/MAS) NMR spectra at 60’ and 70°C using Torchia’s pulse sequence [5], and 13C pulse saturation transfer/magic angle spinning (PST/MAS) [6] at 80°C using the inversion-recovery pulse sequence as a function of recovery time (7). A JNM-GX 270 NMR spectrometer (67.8 MHz) with a variable-temperature (VT)/MAS accessory was used. In the PST technique, nOe enhancement is used to obtain the 13C signal. Therefore, this method effectively enhances peak intensity for mobile carbons undetected by the CP method. The sample of n-C,,H,, was contained in a cylindrical rotor made of zirconia, and the rotor was spun at 444.5 kHz. For all measurements, the spectral width was 2.7 kHz and data points were 8K. For Tl measurements using Torchia’s method, the contact time was 2ms, the repetition time was 5 s, and the number of accumulations was 200. For Tl measurement using the inversion-recovery method, the repetition time was 30 s and the number of accumulations was 20. The actual temperature of the sample considered here was corrected by a calibration curve of the temperature-dependence of the 13Cchemical shift of samarium acetate (obtained from Jeol Co.). RESULTS AND DISCUSSION
The n-C,,H, used here exists in the orthorhombic crystalline phase at 60°C, the rotator phase at 70°C, and the non-crystalline liquid-like phase at 80°C, according to an X-ray diffraction study. Taking these factors into account, the measurement temperature was chosen.
S. Zshikuwa and I. AndolJ. Mol. Strut.,
229
273 (1992) 227-232
L
10 5 3 2 1
0.5 0.3 --..--_---
--7-r
int-CHZ
0.2 set
PPln y77T--‘-,-‘--,““I
A”
10
20
10
Fig. 1. Partially relaxed *V! spectra of n-C,,H, in the crystalline state at 60°C, obtained by Torchia’s pulse sequence.
Figure 1 shows the partially relaxed spectra of n-C,,H, obtained by the Torchia’s pulse sequence at 60°C. The peak assignment of n-C&H, was performed in the same way as in our previous work. Four peaks were assigned to the CH,, c&H,, int-CH, and B-CH, carbons in the upfield direction. The P-CH, peak appears on the downfield foot of the int-CH, peak. As seen from Fig. 1, the int-CH, peak still remains at z = 40 s; this means that it has a long relaxation time. However, the CH, peak and the a-CH, peak gradually disappear as z is increased; this means that they have short relaxation times. Next, Fig. 2 shows the partially relaxed spectra of n-C,,H, obtained by Torchia’s pulse sequence at 70°C. The peak assignment is indicated in the figure. As seen from Fig. 2, as z is increased, every peak is decreased in the same manner; this indicates that every peak has almost the same relaxation time. Further, Fig. 3 shows the partially relaxed spectra of n-C&H, obtained by the inversion-recovery pulse sequence at 8OOC.As seen from Fig. 3, as z is decreased, the 13Cpeaks for more external CH, carbons rapidly decay. For example, in the spectrum at z = 2 s, although the int-CH, and fl-CH, peaks are still in the positive direction, the a-CH, peak disappears and the CH,
230
S. Ishikawa and I. Ando/J. Mol. Struct., 273 (1992) 227-232
12 11 10 9
8 7 6 5
3 set int-CHz
0.8
I
Y
V
1
v I
v
..J L
1.5
Y
2
L
Ai P-CH2
4
A YCH2
a-CH2
Ah_
CH3
10 set
A
A
int-CH2
-1_1
’ 40
30
8
’
’
I 20
’
’
’
’
I
10
Fig. 3. Partially relaxed 13Cspectra of n-C,,H, in the non-crystalline phase at 80°C, obtained by the inversion-recovery pulse sequence.
S. Ishikawa and I. Ando/J. Mol. Struct., 273 (1992) 227-232
231
TABLE 1 13Cspin-lattice relaxation time TI of n-C&H, at various temperatures Temp. (“C)
13CTI (5)
60
70 80
B-C%
int-CH,
y_CH,
c&H,
CH,
60 4.0 2.4
> 100 4.1 0.96
_ 3.9 1.4
11 3.8 3.2
1.6 4.8 5.5
peak reveals itself in the negative direction. Furthermore, in the spectrum at z = 0.8 s, the int-CH, peak is still in the positive direction, while the CH,, a-CH, and fl-CH, carbons are in the negative direction. Therefore, these results indicate that more external CH, carbons have longer relaxation times. The Tl values of n-C&H, obtained at various temperatures are summarized in Table 1. For convenience, these results are shown in Fig. 4. As seen from Table 1 and Fig 4, the order of magnitude of Tlvalues for the CH, carbons is a-CH, < /?-CH, < int-CH, in the crystalline phase, in contrast to a-CH, > /?-CH, > y-CH, > int-CH, in the melting state. According to Bloembergen-Purcell-Pound (BPP) theory [7], as the correlation time z, for molecular motion increases, Tlfirst decreases and then increases again passing through a minimum. Therefore, it can be said that the molecular motion of the CH, carbons of n-C,,H, in the crystalline state at 60°C is in
100
CH3
.-CHz
p2H~
Y-C%
intCHZ
Fig. 4. 13Cspin-lattice relaxation times NT, for n-C,,H, chain at various temperatures. N indicates the number of protons bonded to each carbon considered.
232
S. Zshikawa and 1. AndolJ. Mol. Struct., 273 (1992) 227-232
the slow motion region, i.e. 07, > 1, and that the molecular motion of the CH, carbons of n-C&H, in the non-crystalline state at 80°C is in the extreme narrowing region, i.e. 07, -K 1. However, for Tlvalues in the rotator phase at 70°C, c&H, x P-CH, x +2H, x int-CH,. This means that every CH, carbon in the rotator phase is undergoing molecular motion with the same mode. Therefore, it can be said that all-trans zigzag chains in the rotator phase rotate about their long axes. As seen from Table 1,the Tlvalues of a-CH,, /?-CH,, y-CH, and int-CH, carbons in n-C,,H, decrease as the temperature is increased. This means that the molecular motion in the rotator phase is in the slow motion region. In contrast, the Tlvalue of the CH, carbons in n-C,,H, increases as the temperature is increased. This means that the molecular motion of the CH, carbons is in the extreme narrowing region; this is due to rapid rotation about the methyl C,, axis. REFERENCES 1 2 3 4 5 6
A. Miiller, Proc. R. Sot., London, Ser. A, 138 (1932) 514. M.G. Broadhurst, J. Res. Natl. Bur. Stand., Sect. A, 66 (1962) 241. G. Ungar, J. Phys. Chem., 87 (1983) 689. S. Ishikawa, H. Kurosu and I. Ando, J. Mol. Struct., 248 (1991) 361. D.A. Torchia, J. Magn. Reson., 30 (1978) 613. T. Fujito, K. Deguchi, M. Ohuchi, M. Imanari and M.J. Albright, 20th Meeting on NMR, Tokyo, November, 1981, p. 68. 7 N. Bloembergen, E.M. Purcell and R.V. Pound, Phys. Rev., 73 (1948) 679.
Journal of Molecular Structure, 213 (1992) 227-232 Elsevier Science Publishers B.V., Amsterdam
13C NMR relaxation study on molecular motion of n-alkane chains in the rotator phase Shinji Ishikawa” and Isao Andob “Kao Research Laboratories, Ichikai-machi, Haga, Tochigi (Japan) ‘Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo (Japan) (Received 21 April 1992)
Abstract The 13C spin-lattice relaxation times (T,s) of n&H, were measured in the crystalline state, rotator phase and melting state, in order to clarify the molecular motion of the rotator phase. From these results it was found that in the rotator phase every methylene carbon has almost the same value of Z’, . This indicates that all-trans zigzag chains in the rotator phase rotate rapidly about their long axes.
INTRODUCTION
n-Alkanes with certain chain length on going from the crystalline to the liquid state have been clarified as taking the hexagonal crystallographic form in the narrow temperature range prior to the melting point. This was found by means of X-ray diffraction, which has served as a powerful tool for investigating crystallographic structures [l-3]. It has been proposed that n-alkane chains in the hexagonal phase become effectively cylindrical as a result of rotating about their long axes, where the hexagonal packing is closest; hence, the hexagonal form is named the “rotator phase”. In previous work, we have studied the relationship between the structure and W NMR chemical shift of some n-alkanes in the solid state, such as n-C,,H,, n-C,,H, and n-C&Hs6,which can take the rotator phase. This study revealed that the 13Cchemical shift value of the internal (int-) CH, carbons in n-C&H, moves from 34.2 to 33.3 ppm on going from the triclinic form to the rotator phase, and the 13Cchemical shift of the internal CH, carbons on n-C&H, and n-C!,,H,, moves from 32.8 (32.9) to 33.3ppm on going from the Correspondence to: Professor I. Ando, Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan.
0022-2860/92/$05.00 0 1992 Elsevier Science Publishers
B.V. All rights reserved.
228
S. Ishikawa and I. Ando/J. Mol. Struct., 273 (1992) 227-232
orthorhombic form to the rotator phase [4]. From these results, it can be said that 13Cchemical shift values of n-alkanes in the rotator phase are the same irrespective of chain length and, therefore, every n-alkane in the rotator phase has the same structural aspects. However, it is important to obtain detailed information about their molecular motions in order to discuss dynamical structures of the rotator phase. Thus, in this work we aim to measure the 13Cspin-lattice relaxation time ( Tl) of n-C&H, in the crystalline state, rotator phase and the melting state, and to clarify the dynamics of n-&H, in the rotator phase. EXPERIMENTAL
Material The sample of n-&H, used in this work was obtained from Tokyo Kasei Co. with a purity of > 98%; further purification was not performed. Measurement T,s of n-C&H, were determined by measuring the 13Ccross-polarization/ magic angle spinning (CP/MAS) NMR spectra at 60’ and 70°C using Torchia’s pulse sequence [5], and 13C pulse saturation transfer/magic angle spinning (PST/MAS) [6] at 80°C using the inversion-recovery pulse sequence as a function of recovery time (7). A JNM-GX 270 NMR spectrometer (67.8 MHz) with a variable-temperature (VT)/MAS accessory was used. In the PST technique, nOe enhancement is used to obtain the 13C signal. Therefore, this method effectively enhances peak intensity for mobile carbons undetected by the CP method. The sample of n-C,,H,, was contained in a cylindrical rotor made of zirconia, and the rotor was spun at 444.5 kHz. For all measurements, the spectral width was 2.7 kHz and data points were 8K. For Tl measurements using Torchia’s method, the contact time was 2ms, the repetition time was 5 s, and the number of accumulations was 200. For Tl measurement using the inversion-recovery method, the repetition time was 30 s and the number of accumulations was 20. The actual temperature of the sample considered here was corrected by a calibration curve of the temperature-dependence of the 13Cchemical shift of samarium acetate (obtained from Jeol Co.). RESULTS AND DISCUSSION
The n-C,,H, used here exists in the orthorhombic crystalline phase at 60°C, the rotator phase at 70°C, and the non-crystalline liquid-like phase at 80°C, according to an X-ray diffraction study. Taking these factors into account, the measurement temperature was chosen.
S. Zshikuwa and I. AndolJ. Mol. Strut.,
229
273 (1992) 227-232
L
10 5 3 2 1
0.5 0.3 --..--_---
--7-r
int-CHZ
0.2 set
PPln y77T--‘-,-‘--,““I
A”
10
20
10
Fig. 1. Partially relaxed *V! spectra of n-C,,H, in the crystalline state at 60°C, obtained by Torchia’s pulse sequence.
Figure 1 shows the partially relaxed spectra of n-C,,H, obtained by the Torchia’s pulse sequence at 60°C. The peak assignment of n-C&H, was performed in the same way as in our previous work. Four peaks were assigned to the CH,, c&H,, int-CH, and B-CH, carbons in the upfield direction. The P-CH, peak appears on the downfield foot of the int-CH, peak. As seen from Fig. 1, the int-CH, peak still remains at z = 40 s; this means that it has a long relaxation time. However, the CH, peak and the a-CH, peak gradually disappear as z is increased; this means that they have short relaxation times. Next, Fig. 2 shows the partially relaxed spectra of n-C,,H, obtained by Torchia’s pulse sequence at 70°C. The peak assignment is indicated in the figure. As seen from Fig. 2, as z is increased, every peak is decreased in the same manner; this indicates that every peak has almost the same relaxation time. Further, Fig. 3 shows the partially relaxed spectra of n-C&H, obtained by the inversion-recovery pulse sequence at 8OOC.As seen from Fig. 3, as z is decreased, the 13Cpeaks for more external CH, carbons rapidly decay. For example, in the spectrum at z = 2 s, although the int-CH, and fl-CH, peaks are still in the positive direction, the a-CH, peak disappears and the CH,
230
S. Ishikawa and I. Ando/J. Mol. Struct., 273 (1992) 227-232
12 11 10 9
8 7 6 5
3 set int-CHz
0.8
I
Y
V
1
v I
v
..J L
1.5
Y
2
L
Ai P-CH2
4
A YCH2
a-CH2
Ah_
CH3
10 set
A
A
int-CH2
-1_1
’ 40
30
8
’
’
I 20
’
’
’
’
I
10
Fig. 3. Partially relaxed 13Cspectra of n-C,,H, in the non-crystalline phase at 80°C, obtained by the inversion-recovery pulse sequence.
S. Ishikawa and I. Ando/J. Mol. Struct., 273 (1992) 227-232
231
TABLE 1 13Cspin-lattice relaxation time TI of n-C&H, at various temperatures Temp. (“C)
13CTI (5)
60
70 80
B-C%
int-CH,
y_CH,
c&H,
CH,
60 4.0 2.4
> 100 4.1 0.96
_ 3.9 1.4
11 3.8 3.2
1.6 4.8 5.5
peak reveals itself in the negative direction. Furthermore, in the spectrum at z = 0.8 s, the int-CH, peak is still in the positive direction, while the CH,, a-CH, and fl-CH, carbons are in the negative direction. Therefore, these results indicate that more external CH, carbons have longer relaxation times. The Tl values of n-C&H, obtained at various temperatures are summarized in Table 1. For convenience, these results are shown in Fig. 4. As seen from Table 1 and Fig 4, the order of magnitude of Tlvalues for the CH, carbons is a-CH, < /?-CH, < int-CH, in the crystalline phase, in contrast to a-CH, > /?-CH, > y-CH, > int-CH, in the melting state. According to Bloembergen-Purcell-Pound (BPP) theory [7], as the correlation time z, for molecular motion increases, Tlfirst decreases and then increases again passing through a minimum. Therefore, it can be said that the molecular motion of the CH, carbons of n-C,,H, in the crystalline state at 60°C is in
100
CH3
.-CHz
p2H~
Y-C%
intCHZ
Fig. 4. 13Cspin-lattice relaxation times NT, for n-C,,H, chain at various temperatures. N indicates the number of protons bonded to each carbon considered.
232
S. Zshikawa and 1. AndolJ. Mol. Struct., 273 (1992) 227-232
the slow motion region, i.e. 07, > 1, and that the molecular motion of the CH, carbons of n-C&H, in the non-crystalline state at 80°C is in the extreme narrowing region, i.e. 07, -K 1. However, for Tlvalues in the rotator phase at 70°C, c&H, x P-CH, x +2H, x int-CH,. This means that every CH, carbon in the rotator phase is undergoing molecular motion with the same mode. Therefore, it can be said that all-trans zigzag chains in the rotator phase rotate about their long axes. As seen from Table 1,the Tlvalues of a-CH,, /?-CH,, y-CH, and int-CH, carbons in n-C,,H, decrease as the temperature is increased. This means that the molecular motion in the rotator phase is in the slow motion region. In contrast, the Tlvalue of the CH, carbons in n-C,,H, increases as the temperature is increased. This means that the molecular motion of the CH, carbons is in the extreme narrowing region; this is due to rapid rotation about the methyl C,, axis. REFERENCES 1 2 3 4 5 6
A. Miiller, Proc. R. Sot., London, Ser. A, 138 (1932) 514. M.G. Broadhurst, J. Res. Natl. Bur. Stand., Sect. A, 66 (1962) 241. G. Ungar, J. Phys. Chem., 87 (1983) 689. S. Ishikawa, H. Kurosu and I. Ando, J. Mol. Struct., 248 (1991) 361. D.A. Torchia, J. Magn. Reson., 30 (1978) 613. T. Fujito, K. Deguchi, M. Ohuchi, M. Imanari and M.J. Albright, 20th Meeting on NMR, Tokyo, November, 1981, p. 68. 7 N. Bloembergen, E.M. Purcell and R.V. Pound, Phys. Rev., 73 (1948) 679.