J. Phys. Chem. 1995, 99, 9523-9529
9523
Local Order, Conformation, and Interaction in Nematic 4-(n-Pentyloxy)-4’-cyanobiphenyl and Its One-to-one Mixture with 1-(4’-Cyanophenyl)-4-propylcyclohexane. A Study by State-Correlated lH Two-Dimensional NMR Spectroscopy Kazuyuki Akasaka* Department of Chemistry, Faculty of Science; and Division of Material Science, The Graduate School of Science and Technology; Kobe University, Kobe 657, Japan
Masaharu Kimura Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan
Akira Naito Department of Life Science, Faculty of Science, Himeji Institute of Technology, Hyogo 678-12, Japan
Hironori Kawahara and Mamoru Imanari JEOL LTD., Akishima, Tokyo 196, Japan Received: January 6, 1995; In Final Form: March 27, I995@
Microscopic ordering of the molecule of 4-(n-pentyloxy)-4’-cyanobiphenyl(50CB)and its one-to-one mixture with 1-(4’-cyanophenyl)-4-propylcyclohexane (PCH3) in their nematic phases has been obtained by measuring local dipolar couplings of individual protons in state-correlated two-dimensional (SC-2D) ‘H NMR experiments (Naito et al., J. Magn. Reson. 1990, 87, 429). Nematic-to-isotropic phase transitions (from within several degrees below T,) are realized within 5-25 ms by microwave irradiation within the NMR probe. In pure 50CB, the local order is found to decrease toward the end of the aliphatic chain. Secondary splittings of the ring protons give an estimate of the torsional angle between the two phenyl rings to be in the range 47-52’ at 67.1 ‘C. In a one-to-one mixture of 5 0 C B and PCH3, the local orders of both the rings and the aliphatic chain of 5 0 C B are significantly increased, while those of the rings of PCH3 are decreased. Interproton cross relaxation and spin diffusion are found to be quite sensitive to the dynamics and molecular interactions in pure and mixed liquid crystals.
Introduction Microscopic order of liquid-crystalline samples have been studied successfully using quadrupole couplings in the 2H NMR spectra of 2H-labeled samples.’ In principle, ‘H dipolar couplings can be also used for a similar purpose, but the application of ’H NMR spectroscopy to neat liquid-crystalline samples has been hampered by the complexity of the spectra with many overlapping transitions. We have recently been successful in developing a new technique in NMR spectroscopy that enables a rapid temperature jump of a dielectric material within an NMR probe by utilizing a pulsed m i c r ~ w a v e . ~When - ~ applied to a liquid crystal, this technique enables a phase transition from a nematic phase (close to the clearing temperature T,) into an isotropic phase within a period much shorter than T I . With this new technique, we proposed a new type of two-dimensional NMR experiment called “state-correlated two-dimensional NMR spectroscopy” (SC-2D)2 which correlates, for the first time, NMR signals of a molecule belonging to two different thermodynamic states, such as between the folded and unfolded conformers of a protein3or between the nematic phase and the isotropic phase.4 A IH SC-2D NMR experiment on ethoxybenzylidene-4-nbutylaniline (EBBA) revealed that the dipolar-coupled,complex pattem of the nematic phase spectrum (fl) can be divided into a set of much simpler subspectra of individual proton transitions, by correlating it to the dipolar-decoupled, chemical-shift dispersed signals in the isotropic phase (f2) where dipolar@
Abstract published in Advance ACS Abstracts, May 15, 1995.
0022-365419512099-9523$09.00/0
decoupling is realized by random molecular tumbling? However, since in our initial experiment a microwave pulse with a duration of some 200 ms had to be applied for making a phase transition, a poor signal-to-noise ratio, and spin diffusion-assisted overlap of signals prohibited a quantitative analysis of the s~bspectra.~ In the present work, by utilizing a high-power microwave source along with an improved coil arrangement for more efficient microwave irradiation, we shortened the microwave pulse to 5-25 ms and obtained SC-2D spectra with a much higher signal-to-noise ratio and resolution. This allowed the determination of order parameters of vectors connecting nearestneighbor protons in 4-(n-pentyloxy)-4’-cyanobiphenyl(50CB) 5 ’ 8 ’
2
3
H H
H H
a
p
y
8
0 CH&H&H&H$H3
NC H H 3
’
H H 2
’
6
5
in the nematic phase along with an estimate of the torsional angle of the biphenyl moiety. Second, changes in local orders of 50CB with l-(4’-cyanophenyl)-4-propylcyclohexane(PCH3) 5 ’ 8 ’ 2
3
’
2
’
3
6
5
were examined in a 1:l mixture of 50CB and PCH3.
0 1995 American Chemical Society
":I::
9524 J. Phys. Chem., Vol. 99, No. 23, I995
Akasaka et al.
before temperature jump (67.1 "C)
I\
a
1
L
immediately after temperature jump (over 68.0 "C)
s
b
I
cooling 35 sec.
long range coupling
m
. 8
8
. a
e
* *
Figure 2. Proton DQF-COSY 2D NMR spectrum of 4-(n-pentyloxy)4'-cyanobiphenyl (50CB)in the isotropic liquid phase at 69.1 "C. Signals were assigned as indicated.
,c* 60
0
20
0
.2u
-IO
.GO
Figure 1. Proton NMR spectra of 4-(n-pentyloxy)-4'-cyanobiphenyl (50CB)upon rapid temperature jump up and slow cooling down. Temperature was jumped from 67.1 OC by applying a microwave pulse of 5 ms duration and cooling was made by continuous air flow.
Experimental Section Samples of 4-(n-pentyloxy)-4'-cyanobiphenyl (50CB, Tc = 68.0 "C, Merck) and 1-(4'-cyanophenyl)-4-propylcyclohexane (PCH3, Tc = 45.0 "C, obtained from Merck) were used without further purification. Standard 5 mm 0.d. glass sample tubes were used for the NMR measurements. The basic NMR system used for SC-2D measurements of liquid crystals was a JEOL GX-400 spectrometer, equipped with a temperature-jump proton probe which was a modification of a JEOL high-resolution probe for liquids. The microwave power was applied to the sample through an additional coil wound within the radiowave coil, the details of which will be published elsewhere. The microwave coil was tuned to a frequency of 2.46 GHz delivered from a high-power microwave generator (IDX Corp. IMG-2502-P) with a 1.3 kW magnetron (Toshiba 2M164) as a power source! For radio frequency (rf)excitation, a linear rf amplifier (Henry Radio) was used to deliver a reasonably high rf power at the sample position, so that the 90" rf pulse width was 16-18 ps at the frequency of 400 MHz. The pulse sequence used for the SC-2D experiment was the same as previously r e p ~ r t e d . ~ During -~ the transition period, microwave pulses of duration between 5 and 25 ms were applied, the duration of which depended on how far below T, (the clearing point or nematic-isotropic transition temperature) the temperature of the sample was in the evolution period. A JEOL variable-temperature accessory with air flow was used
to control the temperature in the evolution period as well as to cool the sample back to the original temperature after the temperature jump. The temperatures reported are corrected values against measured temperatures with a thermistor directly immersed into a liquid-crystal sample (Takara D641 type 1/100 temperature tracer with a SZL-64 type thermistor sensor, with the error warrant of less than 0.2 "C). For measurements of SC-2D spectra in the phase-sensitive mode, 32 data points were sampled in the tl domain (liquidcrystalline phase), each point being the result of four accumulations with phase cycling, giving a digital resolution of 1.56 kHz in the f l domain. A long repetition time of 3 min was employed to allow for sample cooling after each temperature-jump, giving a total measurement time of 3 (min) x 4 (times) x 32 (experiments) x 2 (real and imaginary) minutes or approximately 13 h.
Results Figure 1 shows the changes in the one-dimensional 'H NMR spectrum of 50CB upon a temperature jump and cooling. Thanks to the improved experimental setup, the nematic-toisotropic phase transition from T = 67.1 "C, i.e., 0.9 "C below the clearing temperature Tc (Tc = 68.0 "C), occurred within 5-10 ms of microwave irradiation. Longer microwave pulses were necessary for the transitions from lower temperatures (cf. Figure 3). The 'H NMR spectrum of 50CB below Tc is dominated by interproton dipolar couplings, a potentially useful source of information but, as such, is quite complex for any first-hand analysis. The slight distortion of the baseline is due to the finite pulse width (16- 18 ps) insufficient to cover the entire frequency range of k 2 5 kHz. In contrast, the spectrum above T, is that of the isotropic liquid state, without dipolar coupling but with isotropic chemical shifts (and J couplings), and is therefore easily analyzable by a conventional 2D NMR technique. Figure 2 shows a DQF-COSY spectrum of 50CB at a fixed temperature (69.1 "C) slightly above T, in the isotropic phase, which allowed explicit assignments of the 1D signals to
State-Correlated 'H Two-Dimensional NMR Spectroscopy specific protons as indicated in the figure except for the y and
6 proton signals. Figure 3a-c shows state-correlated two-dimensional (SC2D) spectra of 50CB between the nematic phase and the isotropic phase for three different temperatures of the nematic phase, Le., 67.1,65.1, and 63.1 "C. Compared to the relatively short microwave pulse (5 ms) which attained the phase transition at 67.1 "C, longer microwave pulses of duration 20 and 25 ms were required to attain the transition from the lower temperatures. The complex asymmetric one-dimensional 'H NMR spectrum below Tc (in the nematic phase) for the whole proton system is now correlated with the individual proton transitions above T, (in the isotropic phase), which were already assigned in Figure 2. The patterns of the correlated signals are best displayed as cross sections along the f l direction for those values of f2 corresponding to frequencies of transition of individual protons in the isotropic phase. We note that each cross section shows a nearly symmetric pattem with respect to its center of resonance and has its own characteristic shape which indicates that the cross section is practically free from mixing with others, while its center of resonance is characteristically displaced owing to its own chemical shift value in the nematic phase. Figure 4 shows an SC-2D spectrum of PCH3 (Tc = 45.0 "C, the temperature before the transition was 43.1 "C) obtained with a microwave pulse duration of 5 ms, the signal assignments in the isotropic phase being made by an independent experiment (DQF-COSY). We notice that the cross sections of the ring protons and those of the aliphatic protons (both cyclohexyl and propyl) differ considerably, but cross sections are quite similar among the ring protons themselves and among the aliphatic protons themselves, indicating that nearly complete mixing of the subspectra took place among each group of protons. Figure 5 shows an SC-2D spectrum of a 1:l mixture (by weight) of 50CB and PCH3 at 50.1 "C (T, = 51.0 "C) measured with a microwave pulse duration of 7 ms. Although the separation of the signals is not complete even in the isotropic phase, a number of lines were assignable to individual proton transitions of 50CB and PCH3 by performing a DQF-COSY experiment on the mixture (data not shown), as indicated in the figure as 5(- - -) and P(- - -), respectively. Analysis of the Results We consider the nature of the cross sections obtained in the SC-2D experiment performed on a liquid crystal. Let us consider how the kth cross section is obtained along the f l axis. In the evolution period, all the protons evolve under the strong coupling Hamiltonian in the liquid-crystalline phase, but in the t2 domain the signal from the kth proton is separately detected. This means that the multiproton spectrum in the liquidcrystalline phase are separated into subspectra of individual protons characterized by their local dipolar couplings. The situation is analogous to the case of the proton dipolar couplingchemical shift correlation experiment performed on a single crystal by Schuff and Haeberlen, in which case a homonuclear decoupling pulse was used to eliminate dipolar couplings in the t2 d ~ m a i n . The ~ major splitting of the signal of the kth proton caused by interaction with lth proton can then be used to evaluate microscopic order parameter (&) (with respect to the external magnetic field) of a vector connecting proton k to proton I, under the assumption of axial symmetry of the ordering matrix, through the relation6
here Av is the dipolar splitting, y is the magnetogyric ratio of
J. Phys. Chem., Vol. 99, No. 23, 1995 9525 proton, h is the Planck constant divided by 2n, r is the distance between dipolar-coupled protons, and 6 is the angle between the interproton vector and the external magnetic field. It is fairly straightforward to evaluate the largest splittings for individual protons directly from the cross sectional patterns (subspectra) of 50CB, as shown by arrows in Figure 3a. The major doublet splittings of about 10 kHz for the ring protons could be attributed to the dipolar couplings between the nearestneighbor protons only 2.45 8, apart, parallel to the rings, Le., pairs of 2-3, 5-6, 2'-3', and 5'-6' protons. Likewise, the major doublet splittings for the methylene protons and the unresolved triplet splitting for the methyl protons can be attributed to the mutual dipolar couplings (geminal couplings) within the same methylene protons and within the same methyl protons, respectively. The major doublets in the experimental cross sections of the ring protons such as those in Figure 3a show splittings into further doublets with much smaller coupling constants of 2-3 kHz. Indeed, each proton of 50CB should be coupled to several other protons, and therefore in principle each of the cross sectional spectra should show a multiple pattem whose complete analysis requires computer simulation with all these coupling constants taken into account. Unfortunately, the resolution of the experimental cross sectional spectra of Figure 3 are not high enough to render analysis of these couplings by spectral simulation, Therefore, we limit our discussion only to the secondary splittings of 2-3 kHz which can be estimated directly on the ring proton spectra of Figure 3a. Under the assumption that the biphenyl ring flips rapidly about its para axis, calculation using the C-C inter-ring distances of 1.493 and 1.52 8, for biphenyl and 4,4'-difluorobiphenyl, respectively, standard C-C distances of 1.400 8, and C-H distances of 1.082 8, and standard angles of 120" for the biphenyl part7 predicts splittings due to couplings between the proton pairs across the rings (2-6, 3-5, 2'-6', 3 ' 3 ' ) to be only 1.0 kHz and much less for other pairs (such as 2-5 and 3-6). On the other hand, similar calculation for the nearest inter-ring proton pairs, Le., 2-6' and 2'-6, predicts the experimentally observed values of 2-3 mz, for a single conformation with the rotation angle of 47-52" between the two rings of the biphenyl group. Thus the secondary splittings in the subspectra of the aromatic protons can be assigned to the nearest inter-ring proton interactions. Indeed, our experience on the SC-2D study of liquid crystals so far tells that the double-doublet nature of the cross section of the ring protons is observed only in liquid crystals with a biphenyl moiety but not with phenyl rings separated by heteroatoms (e.g., EBBA and APAPA). The fact that the secondary splittings were observed not only in (2,6) and (2',6') protons but also in (33) and (3'5') protons having no direct inter-ring interactions (Figure 3) is probably due to strong cross relaxation within the ring protons. The determined torsional angle 47-52" is larger than those (31.8-35.4") reported for pure biphenyl or meta- or parahalogenated biphenyls at 30 "C but is smaller than those reported for ortho-halogenated biphenyls (68.1-77.4").* Discussion The local order parameters &) obtained from the experimental major splittings Av (Figure 3) and eq 1 are plotted in Figure 6 for the nematic phase of 50CB at three different temperatures. At all the temperatures studied, evaluated local orders are, as expected, the same within the accuracy of measurement for any proton in the two rings. However, it becomes smaller and smaller as the position of the methylene protons becomes more distant from the ring along the aliphatic chain. The latter
Akasaka et al.
9526 J. Phys. Chem., Vol. 99, No. 23, 1995 50CB
za
50CB
o
30
10
20
(65.1 "C)
20
50CB
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(67.1"C)
10
0
-10
-20
(63.1"C)
- =
P
-
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a
0
10
20
Figure 3. Nematic phase-isotropic phase correlated two-dimensional proton NMR spectra of 4-(n-pentyloxy)-4'-cyanobiphenyl(50CB), together with cross sections at frequencies of chemical shifts of individual protons in the isotropic phase. Temperatures were jumped from (a, top) 67.1, (b, middle) 65.1, and (c, bottom) 63.1 O C with microwave pulses of duration 5 , 20, and 25 ms, respectively.
J. Phys. Chem., Vol. 99, No. 23, 1995 9527
State-Correlated 'H Two-Dimensional NMR Spectroscopy
PCH3
I
20
AV
10
0
I
-10
.20
Figure 4. Nematic phase-isotropic phase correlated two-dimensional proton NMR spectrum of l-(4'-cyanophenyl)-4-propylcyclohexane (PCH3), together with cross sections at frequencies of individual proton transitions in the isotropic phase. Temperature was jumped from 43.1 "C with a microwave pulse of duration 5 ms.
A
,
.i-
**I
20
10
0
-10
-20
Figure 5. Nematic phase-isotropic phase correlated two-dimensional proton NMR spectrum of a 1:1 mixture (by weight) of 4-(n-pentyloxy)-4'cyanobiphenyl (50CB) and 1-(4'-cyanophenyl)-4-propylcyclohexane (PCH3). Temperature was jumped from 50.1 "C with a microwave pulse of duration 7 ms.
observation is in qualitative agreement with what had been disclosed previously from *H NMR spectra of *H-labeled n-octylcyanobiphenyls.' Furthermore, the local orders of the
two rings clearly depended on the temperature of observation. At 67.1 "C, only 0.9 "C below T,, the local order was the lowest, but it increased considerably at lower temperatures, as also
Akasaka et al.
9528 J. Phys. Chem., Vol. 99, No. 23, 1995
Figure 6. Local orders of the vectors connecting the nearest proton pairs (k, I ) in 4-(n-pentyloxy)-4'-cyanobiphenyl(50CB) with respect to the external magnetic field. Evaluation is made for each observed proton ( k ) and is plotted against each proton pair (k, r) as deduced from the dipolar coupling obtained from Figure 3 using eq 1. The error bars were estimated from line widths of the cross-sectional spectra in Figure 3.
shown previously from 2H NMR.' Furthermore, if we compare the patterns of the cross sections among parts a-c in Figure 3, we note that more spectral mixing occurs among the cross sections as the microwave pulse duration, Le., the transition period, becomes longer (compare a, b, y , 6, E among Figure 3a-c). This spectral mixing occurs because of exchange of longitudinal magnetizations during the transition period in which the sample partly spends in the liquid-crystallinephase. In principle, exchange should occur by two mechanisms in the liquid crystalline phase; namely, cross polarization or dipolar oscillation between the coupled protons due to the static part of the dipolar interaction and cross relaxation between the coupled protons due to the zero-frequency component of the fluctuating part of the dipolar interaction. In the present case, the dipolar oscillation may not be apparent, since it will rapidly damp out in less than milliseconds because of dipolar couplings with other protons and relaxation processes, leaving mainly the effect of cross relaxation. Since the molecules under study constitute dipolar-coupled multiproton systems, consecutive cross relaxations may occur leading to spin difision. If we examine the patterns of Figure 3 in more detail, we note that the mixing is most complete within the ring protons and extends much less along the chain. This indicates that cross relaxation and spin diffusion is a sensitive parameter reflecting the dynamics of dipolar couplings among protons in the liquid-crystalline phase. For PCH3, the subspectra of the ring protons are essentially a doublet with a splitting of 12.7 kHz, giving 0.56 as the order of the ring (Table l), whereas those of the aliphatic protons have another doublet component with a larger splitting in addition to the 12.7 kHz doublet which apparently comes from the ring protons by spin diffusion. Major dipolar splittings of the ring protons 3, 5, and the 6 methylene protons of 50CB are compared between pure 50CB and its 1:l mixture with PCH3 (Figure 7). We note that the local orders of the ring and the 6 methylene protons of 50CB in the mixture at 50.1 "C are much smaller than that of pure 50CB expected at the same temperature. In fact, the local orders of 50CB in the mixture at 50.1 "C are comparable to those of pure 50CB at about 66 "C. Table 1 compares the
TABLE 1: Comparison of Dipolar Splittings and Order Parameters of the Ring Protons in 50CB, PCH3, and Their 1:l Mixture at a Reduced Temperaturd dipolar wlittine (kHz) ~
50CB 50CB (mixture) PCH3 PCH3 (mixture)
order Darameter
reduced temperature F
0.45 0.55 0.56 0.42
0.99 0.99 0.99 0.99
~~~
10.1 12.4 12.7 9.4
Reduced temperature F = T,,(K)/T,(K).
1
I
(3, 2) (5, 6) I (mixture)
14-
-
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10-
m
-2
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3
6-
2 0
a 4-
0
6 (mixture)
2-
I
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I
65
50
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40
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Figure 7. Microscopic orders of the rings of 4-(n-pentyloxy)-4'cyanobiphenyl(50CB) with respect to the static magnetic field at three different temperatures below its T, and in the 1:l mixture with 1-(4'cyanophenyl)-4-propylcyclohexane (PCW).
dipolar splittings and the order parameters of the ring protons in the mixture with the corresponding values in pure 50CB and PCH3 at a reduced temperature of Te,(K)/Tc(K) = 0.99. It is obvious that by the mixing the order of the ring of 50CB increased while the order of the ring of PCH3 decreased. The strong mixing of subspectra in pure PCH3 indicates a distinctly high efficiency of spin diffusion in PCH3, as compared to that in 50CB and in other nematic-phase liquid crystals so
State-Correlated 'H Two-Dimensional NMR Spectroscopy
J. Phys. Chem., Vol. 99, No. 23, 1995 9529
far examined by SC-2D by the authors' group. This result seems to suggest that at 43.1 "C PCH3 does not exist in the nematic phase but probably in the smectic phase in which intermolecular interaction is stronger and mobility is more severely restricted. We also note that the cross sections of PCH3 in the mixture (Figure 5 ) are mutually far more distinct from each other than those of pure PCH3 (Figure 4),indicating that the efficiency of spin diffusion in PCH3 is considerably reduced by the presence of 50CB. These observations indicate that spin diffusion is quite sensitive to intermolecular interactions between liquid-crystalline molecules and suggests that it be used to detect direct intermolecular interactions in liquid crystals.
Acknowledgment. This work was supported by the Asahi Glass Foundation and, in part, by a Grant-in-Aid for Developmental Research from the Ministry of Education, Science and Culture of Japan. The samples of 50CB and PCH3 were kindly donated by from Merck, Japan.
Concluding Remarks
References and Notes
SC-2D spectroscopy allows elucidation of local-order parameters without the need for deuteration and therefore is readily applicable to a wide range of liquid crystalline samples. Second, specific 2H labeling is not necessary for assignments of the cross sectional spectra, since SC-2D NMR allows their automatic assignments through cross peaks to the signals in the isotropic phase which are readily assignable by the conventional 2D method. Third, information on local conformation, as exemplified above by the torsional angle of the two phenyl rings, could be obtained through the analysis of fine structures of the cross sectional spectra. Finally, cross relaxation and spin diffusion
can be a unique means to elucidate dynamic intramolecular as well as intermolecular interactions in liquid-crystalline molecules. Although the temperature range of measurement for SC-2D NMR spectroscopy of liquid crystals is limited to those close to T,,the method has those unique advantages over the conventional *H NMR spectroscopy.
(1) Counsell, C. J. R.; Emsley, J. W.; Luckhurst, G . R.; Sachdev, H. S. Mol. Phys. 1988, 63, 33; J. W. Emsley, G . R. Luckhurst, E. J. Parsona and B. A. Timimi, Mol. Phys. 1985, 56, 767. (2) Naito, A.; Nakatani, H.; Imanari, M.; Akasaka, K. J. Magn. Reson.
1990, 87, 429. (3) Akasaka, K.; Naito, A.; Imanari, M. J. Am. Chem. SOC. 1991,113, 4688. (4) Naito, A,; Imanari, M.; Akasaka, K. J. Magn. Reson. 1991, 92, 85. (5) Schuff, N.; Haeberlen, U. J. Magn. Reson. 1983, 52, 267. (6) Prasad, J. S. J. Chem. Phys. 1976, 65, 941. (7) Niedelberger, W.; Diel, P.; Lunazzi, L. Mol. Phys. 1973, 26, 571. (8) Field, L. D.; Stemhell, S. J. Am. Chem. SOC. 1981, 99, 5249.
Jp9500807
