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Structural studies of n-alkanes by variable-temperature solid-state high-resolution 13C NMR spectroscopy
Journal of Molecular Structure, 248 (1991) 361-372 Elsevier Science Publishers B.V., Amsterdam
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Structural studies of n-alkanes by variable-temperature solid-state high-resolution 13CNMR spectroscopy Shinji Ishikawa”, Hiromichi Kurosub and Isao Andob “Kao Research Laboratories, Ichikai-machi, Haga, Tochigi (Japan) bDepartment of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-Ku, Tokyo (Japan) (Received 22 January
1991)
Abstract High-resolution r3C NMR spectra of some n-alkanes (~z-C,,H~~, n-Cz4Hs, n-C32H66 and n&H,) in the solid state have been measured by means of variable-temperature, solid-state, highresolution r3C NMR spectroscopy as a function of temperature, in order to clarify the structural change over a wide range of temperatures. From these results, it was found that the r3C chemical shift value of the internal CH, carbons in n-CH,,H5, moves from 34.2 to 33.3 ppm in going from the triclinic form to the rotator phase, and the i3C chemical shift of the internal CH, carbons of n-C19Hd0and n-CS2Hs6 moves from 32.8(32.9) to 33.3 ppm in going from the orthorhombic form to the rotator phase. This shows that the 13C chemical shift values of n-alkanes in the rotator phase are the same irrespective of their chain length. Further, it was found that the structure of the chain-end parts in the intermediate phase and the rotator phase of longer n-alkanes is similar to that in the noncrystalline phase in which fast transition between trans and gauche isomers occurs.
INTRODUCTION
n-Alkanes are known to have various crystallographic forms such as the orthorhombic, triclinic, monoclinic and hexagonal forms under certain conditions, in which the conformation is always the same all-trans zigzag, and to often have a solid-solid transition when the temperature is increased [ 11.So far, the structural analysis of n-alkanes has been studied mainly by X-ray diffraction which has served as a powerful tool. Recently, it has been demonstrated that solid-state, high-resolution NMR spectroscopy is a very powerful way to study structure and dynamics of molecules and polymers in the solid state [ 21. The structure and dynamics of nalkanes [ 3-51 and n-alkyl side chains protruding from a polymer chain [ 6-91 have been successfully studied by solid-state, high-resolution 13C NMR spectroscopy. The observation of the 13Cchemical shift could serve as a diagnostic means for distinguishing between the crystallographic forms. This was justi-
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fied from the results of the quantum-chemical calculation that the 13Cchemical shift displacement, which was observed on going from a specified crystallographic form to some other form, is caused by a local change of the electronic state through the difference in the intermolecular environment [ 10-111. Further, if we carefully observe solid-state, high-resolution 13CNMR spectra of nalkanes in the solid state over a wide range of temperatures and clarify the 13C chemical shift behaviour by theoretical calculations, it should be able to provide more detailed information on the structural and dynamic aspects, and another dimension which is not obtained by X-ray diffraction. The purpose of the present work is to measure carefully 13Cchemical shifts of the internal CH, groups and the terminal CH, and CH, groups in n-alkanes such as n-&H4,,, n-C24H50,n-C32H,, and n-C,,H,, in the solid state over a wide range of temperatures, and to discuss their structural and dynamic features from the 13C chemical shift values obtained and the results of the 13C chemical shift calculations as reported previously [lo], in addition to the results in the previous works on n-alkanes and n-alkyl side chains protruding from a polymer chain. EXPERIMENTAL
Materials All n-alkanes used in this work were purchased from Tokyo Kasei Co. The melting points were 32 ’ C, 51’ C, 70’ C and 86’ C for n-(&H,,, (purity > 99% ), (pun-CZ,H50 (purity > 99% ), n-C,,H,, (purity > 98% ) and n-C,,HS, rity > 96% ), respectively. NMR measurement 13CCP/MAS NMR spectra and 13CPST (pulse saturation transfer)/MAS NMR spectra were measured with a JNM-GX 270 NMR spectrometer (67.8 MHz) with a variable-temperature (VT) /MAS accessory. In the PST method [12], NOE enhancement is used to obtain the 13C signal. For this, the PST method effectively enhances peak intensity for mobile carbons [ 131. Samples were contained in a cylindrical rotor made of zirconia, melted at temperatures above the melting points, and slowly cooled to appropriate temperatures. The rotor was spun at 3-4 kHz. Contact time was 3 ms, repetition time was 5 s, the spectral width was 2.7 kHz, and data points were 8 k. The ‘H field strength was 1.6mT for the CP and the decoupling process. The number of accumulations was 100-1000. The 13Cchemical shifts were calibrated indirectly through external adamantane (29.5 ppm relative to tetramethylsilane (TMS) ). The real temperature of the sample considered here was corrected by the calibra-
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tion curve obtained from the melting points of palmitic acid, naphthalene, pnaphthol and cu-naphthol (obtained from JEOL Co.). RESULTS
According to an X-ray diffraction study on n-alkanes, the n4&H4,, used here exists as the crystalline phase in the orthorhombic form at temperatures below 15°C the rotator phase at 25°C and the noncrystalline phase at 35°C. According to such a situation, the measurement temperature was chosen. In Fig. 1 are shown 13CVT/MAS NMR spectra of n-C19H,, using the CP/MAS int-CH2
I 35 “C j-CH2
I
&Hz
y-CH2
CH3
noncrystalline phase
25 “C rotator phase
15°C orthorhombicform
0°C
orthorhombicform
-20 “C orthorhombic form
-40 “C orthorhombicform
/J
L A
I\
Fig. 1. VT 13C CP/MAS NMR spectra of ~z-C,~H,, at temperatures below 25°C and 13C PST/ MAS NMR spectrum at 35°C. Asterisks indicate ‘zero-hertz splitting’ which comes from quadrature detection.
364 TABLE 1 Observed 13Cchemical shift values of n-&H,, Temp. (“C)
-40 -20 0 15 25 35
in the solid state at various temperatures
13Cchemical shift (ppm)
Crystallographic form
P-CH,
int-CH,
(Y-CH,
CH,
34.3 34.3 34.3 34.4 34.6 32.6
32.9 32.9 32.9 32.9 33.4 30.4
25.0 25.0 24.9 24.9 24.1 23.3
15.2 15.1 15.1 15.0 14.7 14.4
orthorhombic orthorhombic orthorhombic orthorhombic rotator phase noncrystalline (melt)
method at temperatures below 25°C and the PST/MAS method at 35°C. The peak assignment of n-C,,H,, was straightforwardly performed by reference to data on liquid or dissolved n-alkanes and on solid n-alkanes as reported by VanderHart [3]. As shown in the 13CCP/MAS spectrum at 55”C, five peaks were assigned to the CH,, a-CH,, y-CH2, int-CH, and /3-CH, carbons, in the upfield direction. The int-CH, peak position is changed by temperature as shown by 13Cchemical shift values of 32.9,33.4 and 30.4 ppm at temperatures below 15°C at 25°C and at 35°C respectively, designated by dioi9, Sin,, and 6IA19,respectively. The 13Cchemical shift positions of the j.?-CH,peak are about 34.3, 34.6 and 32.6 ppm at temperatures below 15°C at 25°C and at 35°C respectively, and are designated by 6, 019, S, aI9 and 6, A19,respectively. The 13Cchemical shift positions of the cu-CH, peak are about 24.9, 24.7 and 23.3 ppm at temperatures below 15” C, at 25°C and at 35”C, respectively, and are designated by &0t0~~, &EW and Lw, respectively. The 13Cchemical shift positions of the CH3 peak are about 15.1, 14.7 and 14.4 ppm at temperatures below 15°C at 25°C and at 35°C respectively, and are designated by &o19, &, and &A~~,respectively. As seen from Fig. 1, in going from the crystalline phase to the rotator phase as the temperature is increased, the int-CH, and j?CH2 peaks move downfield, and the a-CH, and CH, peaks move upfield. All the 13Cchemical shift values determined are listed in Table 1.
~z-C&~H~~ used here exists as the crystalline phase, in the triclinic form, at temperatures below 45’ C, the rotator phase at 48 and 50” C and the noncrystalline phase at 55’ C. In Fig. 2 are shown 13C VT/MAS NMR spectra of nC,,H,, using the CP method at temperatures below 50’ C and the PST method at 55°C. The peak assignment of ~z-C!~~H~~ is performed as for n-CSH4,, as follows. The 13Cchemical shift positions of the int-CH, peak are 34.2,33.3 and
365 inl-CII2 55°C
/
40 “C _triclinic form
iJ
\
*
*
*
IL
A
33 “C triclinic .-
form
A_
I~I’II’~J’I~‘~“I”~l’J~‘~1’1’I~J~”11I’,
50
40
JO
20
IO
Fig. 2. VT 13CCP/MAS NMR spectra of n-C&H,, at temperatures below 50°C and 13CPST/ MAS NMR spectrum at 55°C. Asterisks indicate ‘zero-hertz splitting’ which comes from quadrature detection.
30.4 ppm at temperatures below 45”C, at 48 and 5O”C, and at %“C, respectively, and are designated by 6rrS0,SIR,, and &,20, respectively. The 13Cchemical shift positions of the P-CH, peak are 35.6,34.1 and 32.6 ppm at temperatures below 45 ‘C, at 48 and 50 oC, and at 55 “C, respectively, and are designated by 8, TZO, 6, R20 and 8, A209 respectively. The 13Cchemical shift positions of the cu-CH, peak are 25.8,24.3 and 23.2 ppm at temperatures below 45’ C, at 48 and 50 ’ C, and at 55 ’ C, respectively, and are designated by 8, rzO,6, a20and 8, A20, respectively. The 13Cchemical shift positions of the CH, peak are 15.9, z 14.5 and 14.4 ppm at temperatures below 45°C at 48 and 5O”C, and at 55”C, respectively, and are designated by &r2,,, &n2, and bA2,,, respectively. As seen from Fig. 2, in going from the crystalline phase to the rotator phase as the temperature is increased, all the peaks move upfield. When the alkane is melted,
366 TABLE 2 Observed r3C!chemical shift values of n-C&H,, in the solid state at various temperatures Temp. (“C)
33 40 45 48 50 55
Crystallographic form
r3C chemical shift (ppm)
P-C&
int-CHz
a-CH,
CH3
35.6 35.6 35.6 34.1 34.1 32.6
34.2 34.2 34.2 33.3 33.3 30.4
25.8 25.8 25.8 24.3 24.3 23.2
15.9 15.9 15.9 14.5 14.6 14.4
triclinic triclinic triclinic rotator phase rotator phase noncrystalline (melt)
I int_CHs
I
80°C noncrystalline
/3-‘332
phase
o-CHs
CH3
h :: :: I :I
-
70°C rotator phase
20°C orthorhombic
form
ortzhombic
form
h
~
&
Fig. 3. VT r3C CP/MAS NMR spectra of nJ&Hss at temperatures below 70°C and r3C PST/ MAS NMR spectrum at 80°C. Asterisks indicate ‘zero-hertz splitting’ which comes from quadrature detection.
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all the peaks shift further upfield. All the 13Cchemical shift values determined are listed in Table 2.
n-C&.HG6used here exists as the crystalline phase in the orthorhombic form at temperatures below 60’ C, the rotator phase at 70’ C, and the noncrystalline phase at 80°C. In Fig. 3 are shown 13C VT/MAS NMR spectra of ~z-C&~H~~ using the CP method at temperatures below 70°C and the PST method at 80’ C. The peak assignment of n-C32H66is performed as for n-C,,H,,. As seen from Fig. 3, on going from the crystalline phase to the rotator phase as the temperature is increased, int-CHz peak moves downfield and /?-CH,, a-CH, and CH, peaks shift upfield. The CH, carbon signal in the solid state appears as two overlapped peaks. All the 13Cchemical shift values determined are listed in Table 3.
n-C,,H,, used here exists as the crystalline phase in the orthorhombic form at temperatures below 80’ C, and the noncrystalline phase at 90’ C. At 70’ C a solid-solid transition occurs. In Fig. 4 are shown 13CVT/MAS NMR spectra of n&H90 using the CP method at temperatures below 80°C and the PST method at 90” C. The peak assignment of nJ&HgO is performed as for n-C1SH40. As seen from Fig. 4, at the solid-solid transition temperature (from the orthorhombic form to the intermediate phase as the temperature is increased), the int-CH, peak does not move, and the P-CH2, cr-CH, and CH, peaks shift upTABLE 3 Observed W chemical shift values of n-CS2H, in the solid state at various temperatures Temp. (“C) 0 20 40 60 70 80
r3C chemical shift (ppm)
Crystallographic form
P-CH,
int-CHx
a-CH,
CH3
34.1 34.2 34.3 34.3 _a 32.5
32.7 32.7 32.8 32.8 33.3 30.3
24.7 24.6 24.7 24.6 24.0 23.2
15.1,14.9b 15.1,14.9b 15.1,14.9b 15.1 14.4 14.4
“Overlapped with other carbon peak. bSee text.
orthorhombic orthorhombic orthorhombic orthorhombic rotator phase noncrystalline (melt)
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, int-CHs 90 “C
noncrystalline phase
a-CHs.
PCH2
CH3
80 “C intermediate phase
orthorhombicform
32 “C!
Fig. 4. VT 13C CP/MAS
NMR
spectra of n-CIIHSo at temperatures below 80°C and 13C PST/
MAS NMR spectrum at 90°C. TABLE
4
Observed r3Cchemical shift values of n-C44HWin the solid state at various temperatures Temp. (“Cl 32 50 60 70 80 90
Crystallographic form
r3Cchemical shift (ppm) 8-CHz
int-CH,
cy-CH,
CH,
34.3 34.3 34.3 -a _a 32.6
32.8 32.8 32.9 33.0 32.9 30.3
24.8 24.8 24.8 24.2 24.0 23.2
15.2 15.2 15.2 14.5 14.4 14.4
“Overlapped with other carbonpeak.
orthorhombic orthorhombic orthorhombic intermediate phase intermediate phase noncrystalline (melt)
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field. When the alkane is melted, int-CHz, /3-CH, and a-CH, peaks shift upfield. All the 13C chemical shift values determined are listed in Table 4. DISCUSSION
Structural change of the int-CH,part of n-a&ones
by temperature elevation
Interchuin interaction Of the four samples used in this work, n-&H,, n-C32H66 and n-(&HgO take the orthorhombic crystalline structure where the trans zigzag planes are perpendicular to each other, and n-C,,H,, takes the triclinic crystalline structure where the trans zigzag planes are parallel to each other. The int-CH, peaks of the orthorhombic n-alkanes and the triclinic n-alkane appear at ~32.8 and 34.2 ppm, respectively, and their 13C chemical shift difference is w 1.4 ppm. It has been demonstrated that this difference comes from a local change of the electronic state through the difference in the intermolecular environment [lo]. Therefore, the 13C chemical shift behaviour provides useful information about the intermolecular interaction. Further, we will be concerned with the structural change in going from the crystalline phase to the noncrystalline phase by increasing the temperature. For this, we must consider two effects in order to understand the 13C chemical shift behaviour; one is the intermolecular interaction between n-alkane chains, and another is the conformation change. In going from the orthorhombic or triclinic crystalline phase to the rotator phase, the difference in 13C chemical shifts between the orthorhombic (&,,) or triclinic (6tri) form and the rotator phase (&,t) should be significant due to changes in intermolecular interactions between them. As seen from Table 2, on going from the triclinic form (S,, M 34.2 ppm) to the rotator phase (&,,z 33.3 ppm), an upfield shift of x 0.9 ppm ( =dstri_rot=6tri-6,,,) is observed. On the other hand, as seen from Tables 1 and 3, in going from the orthorhombic form (I&,,, z 32.8 ppm) to the rotator phase (a,,, w 33.3 ppm) a downfield shift of x 0.5 ppm ( =LI&,~~_~~~) is observed. (This does not conflict with the previous results reported by Miiller et al. [ 51.) It is noted that the 13C chemical shift values of the rotator phases in going from the orthorhombic and triclinic forms are the same (&,, z 33.3 ppm) irrespective of chain length, and are between the 13C chemical shift values of the orthorhombic and triclinic forms. These 13C chemical shift behaviours can be explained on the basis of the results obtained from the 13C chemical shift calculation by the tight-binding molecular orbital (TB-MO) method. Yamanobe et al. have carried out 13C chemical shift calculations for a set of three polyethylene chains as a function of interchain distance using the TBMO method within the CNDO/2 framework, taking into account the structural difference between the orthorhombic form and the triclinic form (the monoclinic form). It has been shown that the 13C shielding constants of the
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orthorhombic and triclinic polyethylenes are aO*= - 56.7 ppm and a,, = - 58.6 ppm, respectively, so the orthorhombic CH2 peak appears at about 1.4 ppm higher field than the triclinic CH, peak, where it is noted that the calculated value of o means shielding and the negative sign indicates deshielding. Using these calculated shielding constants, we estimate the 13Cchemical shift value of n-alkane in the rotator phase. As the all-trans zigzag n-alkane chains in the rotator phase are rotating rapidly about the direction of the chain axis, their chemical shift may be estimated as the averaged value over all the configurations under rotation. Therefore, the 13Cshielding constant in the rotator phase may be roughly estimated as the average of the shielding constants in the orthorhombic and triclinic forms, i.e. arot= - 57.6 ppm. This can explain why the 13C chemical shift of n-alkanes in the rotator phase is between those in the orthorhombic and triclinic forms. The detailed 13Cchemical shift calculations on the seven-chains model will appear using the TB-MO method in the near future. Conformational change n-Alkanes are known to adopt trans (7’) and two gauche (G and G’ ) conformations around the C-C bond as the preferred conformations. In the crystalline phase, an n-alkane chain takes the all-trans zigzag conformation as the most stable conformation. At any specified temperature where the n-alkane is in the rotator phase, the all-trans zigzag chains are rotating rapidly around the chain axis. At temperatures passing through the rotator phase or above the melting point, fast interconversion between trans and gauche conformations occurs, and so the CH, carbon resonance is shifted upfield due to the y-effect [ 141, according to which a carbon resonance appears 5.3 ppm upfield if its carbon three bonds away is in the gauche rather than trans conformation. Such a situation means that the observed chemical shift can be expressed by [ 151 &IX=&I - Wf, where &,,,*and 8, are the observed 13Cchemical shift and the 13Cchemical shift for the trans conformation, respectively, fp is the fraction of gauche conformations, and y is attributed to the chemical shift difference between trans and gauche conformations (Tonelli [ 141 proposed y= +5.3 ppm). In n-alkanes, the energy difference between the trans and gauche conformations is about 600 cal mol-’ (corresponding to f,=O.357) [161,and so the 13Cchemical shift in the liquid or dissolved state is smaller by about 3 ppm than that in the solid state. In fact, the noncrystalline int-CH, peak of n-alkanes considered here appears 3 ppm upfield of the int-CH2 peak in the rotator phase. Structural change of the chain termini by temperature elevation n-C,,H, and n-&HgO take the same orthorhombic form in the nG&b crystalline state as determined by the 13C chemical shift values of their int-
371
CH2 carbons. The 13C chemical shift behaviour of the carbons in the chain ends, especially cu-CH, and CHBcarbons, is varied even in the crystalline state. As seen from Fig. 3, the CH, carbons signal of n-C&Hs6 consists of two peaks at temperatures below 40” C. This means that the CH, groups are in two kinds of structures in which the magnetic environments are different. At this stage, the details of the exact structures of the terminal CH, carbons are not clear. As seen from Tables l-4, the difference in 13Cchemical shifts of the &CH, peaks between the orthorhombic form and triclinic form (dSo,,,,i) is z 1.4 ppm. This difference is equal to the corresponding difference in the int-CH, carbons. In the a-CH, and CH, carbons, the differences in 13Cchemical shifts between the two crystalline forms are 1.0 and 0.8 ppm, respectively. From these results, it is shown that the difference (dS,,,_t,i) decreases gradually towards the chain end. This means that the difference in interchain interactions between the orthorhombic and triclinic forms becomes small for the terminal carbons. Next, we are concerned with the intermediate phase which appeared in nC,,H,,. In this n-alkane the transition to the rotator phase by temperature elevation does not occur. In the terminal carbons the structural change occurs, but it is different from the case of short n-alkanes. This is the so-called “intermediate phase”. The peaks of the terminal carbons such as cu-CH, and CH,, in going from the crystalline phase to the intermediate phase, is shifted upfield by = 0.6 ppm. The chemical shift value of the terminal CH, carbons at 80’ C is 14.4 ppm, and is the same as that in the noncrystalline phase. From this, it can be said that the terminal CH, carbons are in the noncrystalline phase, in which fast transition between the trans and gauche conformers around the second C-C bond from the chain terminus occurs. Nevertheless, it is seen from Table 4 that the int-CH2 carbons are in the orthorhombic form. Namely, the intermediate phase can be defined as the state in which int-CH, carbons are in the crystalline state but terminal carbons are in the noncrystalline state. Finally, we are concerned with the terminal CH3 and CH, carbons of nW-Lo, n-CZ4H5,,and n-C,,H,, in the rotator phase. In the rotator phase, the 13Cchemical shift values of the int-CH, carbons of n-(&H,,, n-CZ4H5,,and nC,,H, agree with each other, but those of the terminal CH, and CH, carbons depend on the chain length of the n-alkanes. This means that even if two nalkanes are in the same rotator phase, the spatial environments in the terminal parts are slightly different from each other. If the terminal carbons in the rotator phase take the all-trans zigzag conformation, it can be expected that the /?-CH, peak may appear at about 34.7 ppm. This comes from the experimental result that the int-CH, carbons in the rotator phase appear at lower field by 0.5 ppm compared with the int-CH, carbons in the orthorhombic form and also at higher field by 0.9 ppm compared with those in the triclinic form. Nevertheless, the 13Cchemical shift values of the j?-CH, carbons of n-C1SH,0 and nC,,H,,, in the rotator phase are actually S,,,, = 34.6 ppm and S,,,, = 34.1 ppm,
372
respectively (8, nz2for n-C&H66 cannot be obtained by the overlapping of the P-CH, and int-CH, peaks). The experimental values are at somewhat higher field than the expected ones. It can be said that such an upfield shift comes from fast transition between trans and gauche conformers in the chain termini. The terminal CH, and CH, carbons of n-CS2Hs6 appear at somewhat higher field than those of n-C,,H,, or n-C,,H,,. From this, it can be said that the terminal CH, and CH, carbons of longer n-alkanes in the rotator phase are undergoing fast transition between trans and gauche conformers, and the gauche fraction may be larger than in the case of shorter n-alkanes. From the above results, it is shown that 6, RI9> 6, nz4> 8, aS2, 6, ni9 > 6, uz4> 6, nS2and 6ER19 > dERZ.4) &R,,* As the 13C chemical shift value of the terminal CH3 carbons of n-C32H66 in the rotator phase is 30.4 ppm, it can be said that these carbons remain in the noncrystalline phase in spite of the rotator phase.
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M.G. Broadhurst, J. Res. Natl. Bur. Stand., A66 (1962) 241. See (a) J. Schaefer and E.O. Stejskal, in G.C. Levy (Ed.), Topics in Carbon-13 NMR Spectroscopy, Vol. 3, Wiley-Interscience, New York, 1979, p. 284. (b) CA. Fyfe, Solid State NMR for Chemists, C.F.C. Press, Canada, 1983. (c) R.A. Komoroski (Ed.), High Resolution NMR Spectroscopy of Synthetic Polymer in Bulk, VCH Publishers, Deerfield Beach, FL, 1986. (d) H. Saito and I. Ando, in G.A. Webb (Ed.), Annu. Rep. on NMR Spectroscopy, Vol. 21, Academic Press, London, 1989, p. 210. (e) I. Ando, T. Yamanobe and T. Asakura, in J.W. Emsley, J. Feeney and L.H. Sutcliffe (Eds. ), Progress in Nuclear Magnetic Resonance Spectroscopy, Vol. 22, Pergamon Press, Oxford, 1990, p. 349. D.L. VanderHart, J. Magn. Reson., 44 (1981) 117. T. Sorita, T. Yamanobe, T. Komoto, I. Ando, H. Sato, K. Deguchi and M. Imanari, Makromol. Chem., Rapid Commun., 5 (1984) 657. M. Mijller, H.-J. Cantow, H. Drotloff, D. Emeis, K.-S. Lee and G. Wagner, Makromol. Chem., 187 (1986) 1237. M. Tsukahara, T. Yamanobe, T. Komoto, J. Watanabe, I. Ando and I. Uematsu, J. Mol. Struct., 159 (1987) 345. T. Yamanobe, M. Tsukaraha, T. Komoto, J. Watanabe, I. Ando, I. Uematsu, K. Deguchi, T. Fujito and M. Imanari, Macromolecules, 21 (1988) 48. M. Okabe, T. Yamanobe, T. Komoto, J. Watanabe, I. Ando and I. Uematsu, J. Mol. Struct., 213 (1989) 213. L.J. Mathias, Polym. Commun., 29 (1988) 352. T. Yamanobe, T. Sorita, T. Komoto, I. Ando and H. Sato, J. Mol. Struct., 131 (1985) 267. H. Kurosu, I. Ando and T. Yamanobe, J. Mol. Struct. (Theochem), 201 (1989) 239. T. Fujito, K. Deguchi, M. Ohuchi, M. Imanari and M.J. Albright, The 20th Meeting of NMR, Tokyo, 1981, p. 68. K. Kubo, I. Ando, T. Shiibashi, T. Yamanobe and T. Komoto, J. Polym. Sci., B29 (1990) 57. A.E. Tonelli and F.C. Schilling, Act. Chem. I&s., 14 (1981) 223. W.L. Earl and D.L. VanderHart, Macromolecules, 12 (1979) 762. N.J. Sheppard and G.J. Szasz, J. Chem. Phys., 17 (1962) 86.
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Structural studies of n-alkanes by variable-temperature solid-state high-resolution 13CNMR spectroscopy Shinji Ishikawa”, Hiromichi Kurosub and Isao Andob “Kao Research Laboratories, Ichikai-machi, Haga, Tochigi (Japan) bDepartment of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-Ku, Tokyo (Japan) (Received 22 January
1991)
Abstract High-resolution r3C NMR spectra of some n-alkanes (~z-C,,H~~, n-Cz4Hs, n-C32H66 and n&H,) in the solid state have been measured by means of variable-temperature, solid-state, highresolution r3C NMR spectroscopy as a function of temperature, in order to clarify the structural change over a wide range of temperatures. From these results, it was found that the r3C chemical shift value of the internal CH, carbons in n-CH,,H5, moves from 34.2 to 33.3 ppm in going from the triclinic form to the rotator phase, and the i3C chemical shift of the internal CH, carbons of n-C19Hd0and n-CS2Hs6 moves from 32.8(32.9) to 33.3 ppm in going from the orthorhombic form to the rotator phase. This shows that the 13C chemical shift values of n-alkanes in the rotator phase are the same irrespective of their chain length. Further, it was found that the structure of the chain-end parts in the intermediate phase and the rotator phase of longer n-alkanes is similar to that in the noncrystalline phase in which fast transition between trans and gauche isomers occurs.
INTRODUCTION
n-Alkanes are known to have various crystallographic forms such as the orthorhombic, triclinic, monoclinic and hexagonal forms under certain conditions, in which the conformation is always the same all-trans zigzag, and to often have a solid-solid transition when the temperature is increased [ 11.So far, the structural analysis of n-alkanes has been studied mainly by X-ray diffraction which has served as a powerful tool. Recently, it has been demonstrated that solid-state, high-resolution NMR spectroscopy is a very powerful way to study structure and dynamics of molecules and polymers in the solid state [ 21. The structure and dynamics of nalkanes [ 3-51 and n-alkyl side chains protruding from a polymer chain [ 6-91 have been successfully studied by solid-state, high-resolution 13C NMR spectroscopy. The observation of the 13Cchemical shift could serve as a diagnostic means for distinguishing between the crystallographic forms. This was justi-
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B.V.
362
fied from the results of the quantum-chemical calculation that the 13Cchemical shift displacement, which was observed on going from a specified crystallographic form to some other form, is caused by a local change of the electronic state through the difference in the intermolecular environment [ 10-111. Further, if we carefully observe solid-state, high-resolution 13CNMR spectra of nalkanes in the solid state over a wide range of temperatures and clarify the 13C chemical shift behaviour by theoretical calculations, it should be able to provide more detailed information on the structural and dynamic aspects, and another dimension which is not obtained by X-ray diffraction. The purpose of the present work is to measure carefully 13Cchemical shifts of the internal CH, groups and the terminal CH, and CH, groups in n-alkanes such as n-&H4,,, n-C24H50,n-C32H,, and n-C,,H,, in the solid state over a wide range of temperatures, and to discuss their structural and dynamic features from the 13C chemical shift values obtained and the results of the 13C chemical shift calculations as reported previously [lo], in addition to the results in the previous works on n-alkanes and n-alkyl side chains protruding from a polymer chain. EXPERIMENTAL
Materials All n-alkanes used in this work were purchased from Tokyo Kasei Co. The melting points were 32 ’ C, 51’ C, 70’ C and 86’ C for n-(&H,,, (purity > 99% ), (pun-CZ,H50 (purity > 99% ), n-C,,H,, (purity > 98% ) and n-C,,HS, rity > 96% ), respectively. NMR measurement 13CCP/MAS NMR spectra and 13CPST (pulse saturation transfer)/MAS NMR spectra were measured with a JNM-GX 270 NMR spectrometer (67.8 MHz) with a variable-temperature (VT) /MAS accessory. In the PST method [12], NOE enhancement is used to obtain the 13C signal. For this, the PST method effectively enhances peak intensity for mobile carbons [ 131. Samples were contained in a cylindrical rotor made of zirconia, melted at temperatures above the melting points, and slowly cooled to appropriate temperatures. The rotor was spun at 3-4 kHz. Contact time was 3 ms, repetition time was 5 s, the spectral width was 2.7 kHz, and data points were 8 k. The ‘H field strength was 1.6mT for the CP and the decoupling process. The number of accumulations was 100-1000. The 13Cchemical shifts were calibrated indirectly through external adamantane (29.5 ppm relative to tetramethylsilane (TMS) ). The real temperature of the sample considered here was corrected by the calibra-
363
tion curve obtained from the melting points of palmitic acid, naphthalene, pnaphthol and cu-naphthol (obtained from JEOL Co.). RESULTS
According to an X-ray diffraction study on n-alkanes, the n4&H4,, used here exists as the crystalline phase in the orthorhombic form at temperatures below 15°C the rotator phase at 25°C and the noncrystalline phase at 35°C. According to such a situation, the measurement temperature was chosen. In Fig. 1 are shown 13CVT/MAS NMR spectra of n-C19H,, using the CP/MAS int-CH2
I 35 “C j-CH2
I
&Hz
y-CH2
CH3
noncrystalline phase
25 “C rotator phase
15°C orthorhombicform
0°C
orthorhombicform
-20 “C orthorhombic form
-40 “C orthorhombicform
/J
L A
I\
Fig. 1. VT 13C CP/MAS NMR spectra of ~z-C,~H,, at temperatures below 25°C and 13C PST/ MAS NMR spectrum at 35°C. Asterisks indicate ‘zero-hertz splitting’ which comes from quadrature detection.
364 TABLE 1 Observed 13Cchemical shift values of n-&H,, Temp. (“C)
-40 -20 0 15 25 35
in the solid state at various temperatures
13Cchemical shift (ppm)
Crystallographic form
P-CH,
int-CH,
(Y-CH,
CH,
34.3 34.3 34.3 34.4 34.6 32.6
32.9 32.9 32.9 32.9 33.4 30.4
25.0 25.0 24.9 24.9 24.1 23.3
15.2 15.1 15.1 15.0 14.7 14.4
orthorhombic orthorhombic orthorhombic orthorhombic rotator phase noncrystalline (melt)
method at temperatures below 25°C and the PST/MAS method at 35°C. The peak assignment of n-C,,H,, was straightforwardly performed by reference to data on liquid or dissolved n-alkanes and on solid n-alkanes as reported by VanderHart [3]. As shown in the 13CCP/MAS spectrum at 55”C, five peaks were assigned to the CH,, a-CH,, y-CH2, int-CH, and /3-CH, carbons, in the upfield direction. The int-CH, peak position is changed by temperature as shown by 13Cchemical shift values of 32.9,33.4 and 30.4 ppm at temperatures below 15°C at 25°C and at 35°C respectively, designated by dioi9, Sin,, and 6IA19,respectively. The 13Cchemical shift positions of the j.?-CH,peak are about 34.3, 34.6 and 32.6 ppm at temperatures below 15°C at 25°C and at 35°C respectively, and are designated by 6, 019, S, aI9 and 6, A19,respectively. The 13Cchemical shift positions of the cu-CH, peak are about 24.9, 24.7 and 23.3 ppm at temperatures below 15” C, at 25°C and at 35”C, respectively, and are designated by &0t0~~, &EW and Lw, respectively. The 13Cchemical shift positions of the CH3 peak are about 15.1, 14.7 and 14.4 ppm at temperatures below 15°C at 25°C and at 35°C respectively, and are designated by &o19, &, and &A~~,respectively. As seen from Fig. 1, in going from the crystalline phase to the rotator phase as the temperature is increased, the int-CH, and j?CH2 peaks move downfield, and the a-CH, and CH, peaks move upfield. All the 13Cchemical shift values determined are listed in Table 1.
~z-C&~H~~ used here exists as the crystalline phase, in the triclinic form, at temperatures below 45’ C, the rotator phase at 48 and 50” C and the noncrystalline phase at 55’ C. In Fig. 2 are shown 13C VT/MAS NMR spectra of nC,,H,, using the CP method at temperatures below 50’ C and the PST method at 55°C. The peak assignment of ~z-C!~~H~~ is performed as for n-CSH4,, as follows. The 13Cchemical shift positions of the int-CH, peak are 34.2,33.3 and
365 inl-CII2 55°C
/
40 “C _triclinic form
iJ
\
*
*
*
IL
A
33 “C triclinic .-
form
A_
I~I’II’~J’I~‘~“I”~l’J~‘~1’1’I~J~”11I’,
50
40
JO
20
IO
Fig. 2. VT 13CCP/MAS NMR spectra of n-C&H,, at temperatures below 50°C and 13CPST/ MAS NMR spectrum at 55°C. Asterisks indicate ‘zero-hertz splitting’ which comes from quadrature detection.
30.4 ppm at temperatures below 45”C, at 48 and 5O”C, and at %“C, respectively, and are designated by 6rrS0,SIR,, and &,20, respectively. The 13Cchemical shift positions of the P-CH, peak are 35.6,34.1 and 32.6 ppm at temperatures below 45 ‘C, at 48 and 50 oC, and at 55 “C, respectively, and are designated by 8, TZO, 6, R20 and 8, A209 respectively. The 13Cchemical shift positions of the cu-CH, peak are 25.8,24.3 and 23.2 ppm at temperatures below 45’ C, at 48 and 50 ’ C, and at 55 ’ C, respectively, and are designated by 8, rzO,6, a20and 8, A20, respectively. The 13Cchemical shift positions of the CH, peak are 15.9, z 14.5 and 14.4 ppm at temperatures below 45°C at 48 and 5O”C, and at 55”C, respectively, and are designated by &r2,,, &n2, and bA2,,, respectively. As seen from Fig. 2, in going from the crystalline phase to the rotator phase as the temperature is increased, all the peaks move upfield. When the alkane is melted,
366 TABLE 2 Observed r3C!chemical shift values of n-C&H,, in the solid state at various temperatures Temp. (“C)
33 40 45 48 50 55
Crystallographic form
r3C chemical shift (ppm)
P-C&
int-CHz
a-CH,
CH3
35.6 35.6 35.6 34.1 34.1 32.6
34.2 34.2 34.2 33.3 33.3 30.4
25.8 25.8 25.8 24.3 24.3 23.2
15.9 15.9 15.9 14.5 14.6 14.4
triclinic triclinic triclinic rotator phase rotator phase noncrystalline (melt)
I int_CHs
I
80°C noncrystalline
/3-‘332
phase
o-CHs
CH3
h :: :: I :I
-
70°C rotator phase
20°C orthorhombic
form
ortzhombic
form
h
~
&
Fig. 3. VT r3C CP/MAS NMR spectra of nJ&Hss at temperatures below 70°C and r3C PST/ MAS NMR spectrum at 80°C. Asterisks indicate ‘zero-hertz splitting’ which comes from quadrature detection.
367
all the peaks shift further upfield. All the 13Cchemical shift values determined are listed in Table 2.
n-C&.HG6used here exists as the crystalline phase in the orthorhombic form at temperatures below 60’ C, the rotator phase at 70’ C, and the noncrystalline phase at 80°C. In Fig. 3 are shown 13C VT/MAS NMR spectra of ~z-C&~H~~ using the CP method at temperatures below 70°C and the PST method at 80’ C. The peak assignment of n-C32H66is performed as for n-C,,H,,. As seen from Fig. 3, on going from the crystalline phase to the rotator phase as the temperature is increased, int-CHz peak moves downfield and /?-CH,, a-CH, and CH, peaks shift upfield. The CH, carbon signal in the solid state appears as two overlapped peaks. All the 13Cchemical shift values determined are listed in Table 3.
n-C,,H,, used here exists as the crystalline phase in the orthorhombic form at temperatures below 80’ C, and the noncrystalline phase at 90’ C. At 70’ C a solid-solid transition occurs. In Fig. 4 are shown 13CVT/MAS NMR spectra of n&H90 using the CP method at temperatures below 80°C and the PST method at 90” C. The peak assignment of nJ&HgO is performed as for n-C1SH40. As seen from Fig. 4, at the solid-solid transition temperature (from the orthorhombic form to the intermediate phase as the temperature is increased), the int-CH, peak does not move, and the P-CH2, cr-CH, and CH, peaks shift upTABLE 3 Observed W chemical shift values of n-CS2H, in the solid state at various temperatures Temp. (“C) 0 20 40 60 70 80
r3C chemical shift (ppm)
Crystallographic form
P-CH,
int-CHx
a-CH,
CH3
34.1 34.2 34.3 34.3 _a 32.5
32.7 32.7 32.8 32.8 33.3 30.3
24.7 24.6 24.7 24.6 24.0 23.2
15.1,14.9b 15.1,14.9b 15.1,14.9b 15.1 14.4 14.4
“Overlapped with other carbon peak. bSee text.
orthorhombic orthorhombic orthorhombic orthorhombic rotator phase noncrystalline (melt)
368
, int-CHs 90 “C
noncrystalline phase
a-CHs.
PCH2
CH3
80 “C intermediate phase
orthorhombicform
32 “C!
Fig. 4. VT 13C CP/MAS
NMR
spectra of n-CIIHSo at temperatures below 80°C and 13C PST/
MAS NMR spectrum at 90°C. TABLE
4
Observed r3Cchemical shift values of n-C44HWin the solid state at various temperatures Temp. (“Cl 32 50 60 70 80 90
Crystallographic form
r3Cchemical shift (ppm) 8-CHz
int-CH,
cy-CH,
CH,
34.3 34.3 34.3 -a _a 32.6
32.8 32.8 32.9 33.0 32.9 30.3
24.8 24.8 24.8 24.2 24.0 23.2
15.2 15.2 15.2 14.5 14.4 14.4
“Overlapped with other carbonpeak.
orthorhombic orthorhombic orthorhombic intermediate phase intermediate phase noncrystalline (melt)
369
field. When the alkane is melted, int-CHz, /3-CH, and a-CH, peaks shift upfield. All the 13C chemical shift values determined are listed in Table 4. DISCUSSION
Structural change of the int-CH,part of n-a&ones
by temperature elevation
Interchuin interaction Of the four samples used in this work, n-&H,, n-C32H66 and n-(&HgO take the orthorhombic crystalline structure where the trans zigzag planes are perpendicular to each other, and n-C,,H,, takes the triclinic crystalline structure where the trans zigzag planes are parallel to each other. The int-CH, peaks of the orthorhombic n-alkanes and the triclinic n-alkane appear at ~32.8 and 34.2 ppm, respectively, and their 13C chemical shift difference is w 1.4 ppm. It has been demonstrated that this difference comes from a local change of the electronic state through the difference in the intermolecular environment [lo]. Therefore, the 13C chemical shift behaviour provides useful information about the intermolecular interaction. Further, we will be concerned with the structural change in going from the crystalline phase to the noncrystalline phase by increasing the temperature. For this, we must consider two effects in order to understand the 13C chemical shift behaviour; one is the intermolecular interaction between n-alkane chains, and another is the conformation change. In going from the orthorhombic or triclinic crystalline phase to the rotator phase, the difference in 13C chemical shifts between the orthorhombic (&,,) or triclinic (6tri) form and the rotator phase (&,t) should be significant due to changes in intermolecular interactions between them. As seen from Table 2, on going from the triclinic form (S,, M 34.2 ppm) to the rotator phase (&,,z 33.3 ppm), an upfield shift of x 0.9 ppm ( =dstri_rot=6tri-6,,,) is observed. On the other hand, as seen from Tables 1 and 3, in going from the orthorhombic form (I&,,, z 32.8 ppm) to the rotator phase (a,,, w 33.3 ppm) a downfield shift of x 0.5 ppm ( =LI&,~~_~~~) is observed. (This does not conflict with the previous results reported by Miiller et al. [ 51.) It is noted that the 13C chemical shift values of the rotator phases in going from the orthorhombic and triclinic forms are the same (&,, z 33.3 ppm) irrespective of chain length, and are between the 13C chemical shift values of the orthorhombic and triclinic forms. These 13C chemical shift behaviours can be explained on the basis of the results obtained from the 13C chemical shift calculation by the tight-binding molecular orbital (TB-MO) method. Yamanobe et al. have carried out 13C chemical shift calculations for a set of three polyethylene chains as a function of interchain distance using the TBMO method within the CNDO/2 framework, taking into account the structural difference between the orthorhombic form and the triclinic form (the monoclinic form). It has been shown that the 13C shielding constants of the
370
orthorhombic and triclinic polyethylenes are aO*= - 56.7 ppm and a,, = - 58.6 ppm, respectively, so the orthorhombic CH2 peak appears at about 1.4 ppm higher field than the triclinic CH, peak, where it is noted that the calculated value of o means shielding and the negative sign indicates deshielding. Using these calculated shielding constants, we estimate the 13Cchemical shift value of n-alkane in the rotator phase. As the all-trans zigzag n-alkane chains in the rotator phase are rotating rapidly about the direction of the chain axis, their chemical shift may be estimated as the averaged value over all the configurations under rotation. Therefore, the 13Cshielding constant in the rotator phase may be roughly estimated as the average of the shielding constants in the orthorhombic and triclinic forms, i.e. arot= - 57.6 ppm. This can explain why the 13C chemical shift of n-alkanes in the rotator phase is between those in the orthorhombic and triclinic forms. The detailed 13Cchemical shift calculations on the seven-chains model will appear using the TB-MO method in the near future. Conformational change n-Alkanes are known to adopt trans (7’) and two gauche (G and G’ ) conformations around the C-C bond as the preferred conformations. In the crystalline phase, an n-alkane chain takes the all-trans zigzag conformation as the most stable conformation. At any specified temperature where the n-alkane is in the rotator phase, the all-trans zigzag chains are rotating rapidly around the chain axis. At temperatures passing through the rotator phase or above the melting point, fast interconversion between trans and gauche conformations occurs, and so the CH, carbon resonance is shifted upfield due to the y-effect [ 141, according to which a carbon resonance appears 5.3 ppm upfield if its carbon three bonds away is in the gauche rather than trans conformation. Such a situation means that the observed chemical shift can be expressed by [ 151 &IX=&I - Wf, where &,,,*and 8, are the observed 13Cchemical shift and the 13Cchemical shift for the trans conformation, respectively, fp is the fraction of gauche conformations, and y is attributed to the chemical shift difference between trans and gauche conformations (Tonelli [ 141 proposed y= +5.3 ppm). In n-alkanes, the energy difference between the trans and gauche conformations is about 600 cal mol-’ (corresponding to f,=O.357) [161,and so the 13Cchemical shift in the liquid or dissolved state is smaller by about 3 ppm than that in the solid state. In fact, the noncrystalline int-CH, peak of n-alkanes considered here appears 3 ppm upfield of the int-CH2 peak in the rotator phase. Structural change of the chain termini by temperature elevation n-C,,H, and n-&HgO take the same orthorhombic form in the nG&b crystalline state as determined by the 13C chemical shift values of their int-
371
CH2 carbons. The 13C chemical shift behaviour of the carbons in the chain ends, especially cu-CH, and CHBcarbons, is varied even in the crystalline state. As seen from Fig. 3, the CH, carbons signal of n-C&Hs6 consists of two peaks at temperatures below 40” C. This means that the CH, groups are in two kinds of structures in which the magnetic environments are different. At this stage, the details of the exact structures of the terminal CH, carbons are not clear. As seen from Tables l-4, the difference in 13Cchemical shifts of the &CH, peaks between the orthorhombic form and triclinic form (dSo,,,,i) is z 1.4 ppm. This difference is equal to the corresponding difference in the int-CH, carbons. In the a-CH, and CH, carbons, the differences in 13Cchemical shifts between the two crystalline forms are 1.0 and 0.8 ppm, respectively. From these results, it is shown that the difference (dS,,,_t,i) decreases gradually towards the chain end. This means that the difference in interchain interactions between the orthorhombic and triclinic forms becomes small for the terminal carbons. Next, we are concerned with the intermediate phase which appeared in nC,,H,,. In this n-alkane the transition to the rotator phase by temperature elevation does not occur. In the terminal carbons the structural change occurs, but it is different from the case of short n-alkanes. This is the so-called “intermediate phase”. The peaks of the terminal carbons such as cu-CH, and CH,, in going from the crystalline phase to the intermediate phase, is shifted upfield by = 0.6 ppm. The chemical shift value of the terminal CH, carbons at 80’ C is 14.4 ppm, and is the same as that in the noncrystalline phase. From this, it can be said that the terminal CH, carbons are in the noncrystalline phase, in which fast transition between the trans and gauche conformers around the second C-C bond from the chain terminus occurs. Nevertheless, it is seen from Table 4 that the int-CH2 carbons are in the orthorhombic form. Namely, the intermediate phase can be defined as the state in which int-CH, carbons are in the crystalline state but terminal carbons are in the noncrystalline state. Finally, we are concerned with the terminal CH3 and CH, carbons of nW-Lo, n-CZ4H5,,and n-C,,H,, in the rotator phase. In the rotator phase, the 13Cchemical shift values of the int-CH, carbons of n-(&H,,, n-CZ4H5,,and nC,,H, agree with each other, but those of the terminal CH, and CH, carbons depend on the chain length of the n-alkanes. This means that even if two nalkanes are in the same rotator phase, the spatial environments in the terminal parts are slightly different from each other. If the terminal carbons in the rotator phase take the all-trans zigzag conformation, it can be expected that the /?-CH, peak may appear at about 34.7 ppm. This comes from the experimental result that the int-CH, carbons in the rotator phase appear at lower field by 0.5 ppm compared with the int-CH, carbons in the orthorhombic form and also at higher field by 0.9 ppm compared with those in the triclinic form. Nevertheless, the 13Cchemical shift values of the j?-CH, carbons of n-C1SH,0 and nC,,H,,, in the rotator phase are actually S,,,, = 34.6 ppm and S,,,, = 34.1 ppm,
372
respectively (8, nz2for n-C&H66 cannot be obtained by the overlapping of the P-CH, and int-CH, peaks). The experimental values are at somewhat higher field than the expected ones. It can be said that such an upfield shift comes from fast transition between trans and gauche conformers in the chain termini. The terminal CH, and CH, carbons of n-CS2Hs6 appear at somewhat higher field than those of n-C,,H,, or n-C,,H,,. From this, it can be said that the terminal CH, and CH, carbons of longer n-alkanes in the rotator phase are undergoing fast transition between trans and gauche conformers, and the gauche fraction may be larger than in the case of shorter n-alkanes. From the above results, it is shown that 6, RI9> 6, nz4> 8, aS2, 6, ni9 > 6, uz4> 6, nS2and 6ER19 > dERZ.4) &R,,* As the 13C chemical shift value of the terminal CH3 carbons of n-C32H66 in the rotator phase is 30.4 ppm, it can be said that these carbons remain in the noncrystalline phase in spite of the rotator phase.
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