5108
J. Phys. Chem. B 1997, 101, 5108-5111
Natural Abundance 2H NMR for Liquid Crystal Studies. Application to 4′-(Hexyloxy)-4-cyanobiphenyl Kazuteru Tabayashi DiVision of Material Science, Graduate School of Science and Technology, Kobe UniVersity, Kobe 657, Japan
Kazuyuki Akasaka* Department of Chemistry, Faculty of Science, and DiVision of Molecular Science, Graduate School of Science and Technology, Kobe UniVersity, Kobe 657, Japan ReceiVed: January 16, 1997; In Final Form: May 1, 1997X
Quadrupolar splitting in deuterium NMR (2H NMR) is a powerful means to evaluate microscopic orientational orders of liquid crystals. So far, because of the low sensitivity of 2H NMR at natural abundance, application of 2H NMR to liquid crystals has been considered possible only for samples with deuterium enrichment. However, in the present work, we show that by taking advantage of the high sensitivity of a recent high-field NMR spectrometer, detection of 2H NMR signals at the natural abundance of the 2H isotope is possible under proton decoupling. Assignments of the 2H NMR signals to specific chemical groups in the molecule may be made by comparing 2H chemical shift in the liquid crystalline phase with those of 1H chemical shift in the isotropic phase. Subsequent evaluation of quadrupolar splittings gives microscopic orientational orders of the molecule. With this technique, temperature dependence of microscopic orders of 4′-(hexyloxy)-4cyanobiphenyl is demonstrated.
Introduction Recently, a number of liquid crystalline materials have been newly synthesized for practical purposes, and there appears to be an increasing need for examining microscopic properties of these materials by a convenient, general technique. We have recently introduced a new NMR technique that enables measurements of local dipolar fields of individual protons allowing determination of microscopic orders of different chemical groups in a liquid crystalline molecule without performing any pretreatment of the sample, i.e., state-correlated two-dimensional 1H NMR spectroscopy (SC-2D).1-4 However, the application of this technique is currently limited to a sample in the nematic phase within several degrees below the clearing temperature Tc. 2H NMR is another well-established technique to study microscopic orders of liquid crystals. Under proton decoupling, the spin state of a deuterium atom is governed by the Zeemann interaction and the interaction of the quadrupole moment with an electric field gradient tensor, from which we obtain the microscopic orientational order of the C-2H bond (Dong, 1994).5 Unfortunately, since the sensitivity of 2H NMR at the natural abundance of the 2H isotope (0.015%) is quite low, enrichment of the 2H isotope must usually be made at specific positions of the molecule. However, specific deuteration of a liquid crystal is generally an enormously difficult task, and this has limited a wide application of 2H NMR to liquid crystals in spite of its high utility. Thus, at present, application of 2H NMR has been limited to several typical systems, including cyanobiphenyl, for example, 6OCB (4′-(hexyloxy)-4-cyanobiphenyl)d13 and -d15,6,7 6OCB-d13,8 and 6OCB-d21.9 Recently, natural abundance 2H NMR was performed on solid materials by a combination of magic angle spinning, high-power proton decoupling, and cross-polarization techniques10 or by * To whom correspondence should be addressed. Fax, +81-78-803-0839; Email,
[email protected]. X Abstract published in AdVance ACS Abstracts, June 15, 1997.
S1089-5647(97)00225-3 CCC: $14.00
rotor-synchronized magic angle spinning.11 However, neither method allows us to obtain information on anisotropic interactions and chemical shifts simultaneously. In the present paper, we will demonstrate that, by taking advantage of the substantial increase in sensitivity of high-field NMR spectrometers in recent years, it is now possible to obtain useful 2H NMR spectra of liquid crystals at natural abundance of the 2H isotope under conditions of 1H dipolar decoupling with reasonable signal-to-noise ratios, from which one may deduce microscopic orientations of many individual chemical groups in the molecule. The experiments can be performed on a high-resolution NMR spectrometer primarily designed for liquids. We will also demonstrate that assignments of quadrupolar couplings to specific chemical sites may be made by correlating the chemical shifts of the 2H NMR signals in the liquid crystalline phase to those of the 1H NMR signals in the isotropic phase that are independently assignable. In this report, we will demonstrate the first example of such a study performed on 6OCB in the nematic phase. Experimental Section 6OCB was a gift from Merck Japan. This material was used without further purification. 2H NMR spectra were recorded on a JEOL high-resolution NMR spectrometer at 61.25 MHz (400 MHz for 1H), equipped with a JEOL multinuclear probe designed primarily for liquid samples in a 5 mm outer diameter tube, and partly on a Bruker DMX-750 at 115.15 MHz (750 MHz for 1H). The temperature of the sample was controlled with a gas-flow temperature controller to within a fluctuation of (0.1 °C. For most measurements carried out at 61.25 MHz, a simple single-pulse sequence with a π/4 pulse of 6 µs and 16K data points was used with a total repetition time of 0.4 s. Nonselective proton decoupling was performed with the WAUGH sequence with a spectral width of 100 kHz. The field-frequency lock was not used, but the field stability was good enough to allow for a long time accumulation without significant broadening of the lines. For a measurement of one spectrum with a © 1997 American Chemical Society
Natural Abundance 2H NMR Studies
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5109
Figure 1. Natural abundance 2H NMR spectra of 6OCB (4′-(hexyloxy)4-cyanobiphenyl) in the nematic phase obtained with a single radio frequency pulse under proton decoupling: (a) Fourier transformed without applying a window function and (b) Fourier transformed after applying a trapezoidal window function. The measurement was made at 60 °C with 180 000 scans.
sufficient signal-to-noise ratio, we needed to accumulate typically 180 000 scans for 20 h at 61.25 MHz, but only 20 000 scans for 2 h at 115.15 MHz. Results and Discussion 1. Detection of Natural Abundance 2H NMR Signals in the Nematic Phase of 6OCB. Free induction decay (FID) signals at natural abundance of the 2H isotope were obtained under nonselective proton decoupling that eliminated all the 2H1H dipolar couplings. Even so, the deuteron signals were superimposed on the acoustic ringing that, after Fourier transformation, gave rise to a distorted base line (see Figure 1a). 2H NMR measurements in ordered systems are usually performed with a quadrupole echo sequence which eliminates acoustic ringing.5 In the present experiments perfomed on NMR instruments designed primarily for liquid samples, however, the use of a quadrupole echo sequence was not feasible primarily due to a too long π/2 pulse (12 µs). To eliminate the effect of ringing, another multiple-pulse sequence has been proposed that takes advantage of differences in longitudinal and transverse relaxation rates between the signal and the ringing.12 The sequence also requires a series of π/2 and π pulses but was applied successfully to the case where quadrupolar splittings are relatively small, e.g., p-azoxyanisole (PAA) (data not shown). Another method for eliminating the effect of ringing is simply to acquire FID signals after excitation with a single radio frequency pulse and then apply a suitable window function to the FID before Fourier transformation so that the rapidly decaying ringing component may be filtered away. With a trapezoidal function applied on the FID, a spectrum was obtained with a reasonably flat base line (see Figure 1b). The advantage of this method is that it can be used with an arbitrarily short radio frequency pulse so that a sufficiently wide spectral range may be covered even with a spectrometer primarily designed for liquids. The result in Figure 1b shows that, by eliminating the effect of the acoustic ringing, natural abundance 2H NMR spectra of liquid crystals could be obtained with sufficient signal-to-noise
Figure 2. Temperature dependence of 1H-decoupled natural abundance 2H NMR spectra of 6OCB in the nematic phase.
ratios under proton decoupling. A number of very sharp signals were observed in pairs, approximately symmetrically spaced with respect to the center of the spectrum. Since 2H dipoledipole couplings are absent in the spectrum due to the extremely low abundance of the 2H isotope and since 1H-2H dipolar couplings have been eliminated by the nonselective 1H decoupling pulse, the observed doublet splittings should represent quadrupolar couplings of individual C-2H bonds. By referring to 2H NMR signals of a series of specifically deuterated compounds in previous literatures,6,7 it is clear that the signals from all the C-2H bonds of 6OCB6,7 were detected in Figure 1b. Figure 2 shows the temperature dependence of the 1Hdecoupled natural abundance 2H NMR spectra in the entire temperature range of the nematic phase of 6OCB. It is clear that all the quadrupolar couplings decrease with temperature. 2. Signal Assignments in the Natural Abundance 2H NMR Spectrum of 6OCB in the Nematic Phase. Apart from the signal-to-noise ratio, another requirement for the successful application of natural abundance 2H NMR to liquid crystal studies is the assignment of signals to specific C-2H bonds without performing specific deuteration. We will show below that signal assignments are possible, at least to a certain extent, without performing specific deuteration, and compare the result with the assignments by Emsley et al. using specific deuteration.7 In this regard, we recognize that the signals are not totally symmetrically arranged with respect to the center of the spectrum (see Figure 1b). The slight assymmetry of the signals must arise from slight differences in chemical shifts (δ) for different deuterons in the liquid crystal, which have become noticeable due to the outstanding sharpness of the resonance lines in the 1H-decoupled natural abundance 2H NMR spectrum, thanks to the absence of both 2H-1H and 2H-2H dipolar couplings. Within the first-order analysis of the spectrum, chemical shifts for individual deuterons in the liquid crystalline
5110 J. Phys. Chem. B, Vol. 101, No. 26, 1997
Tabayashi and Akasaka
TABLE 1: Comparison of 2H Chemical Shifts of 6OCB in the Nematic Phase with Assigned 1H Chemical Shifts of 6OCB in the Isotropic Phase, Both with Methyl Signals as Reference (δ ) 0)a 2H 2
H signala δ (ppm)
1
H typeb δ (ppm)
7 4.3 R1 5.9
8 5.5 R2 6.2
9 4.6
Chemical Shifts in the Nematic Phase 10 1 2 5.7 2.5 0.8
3 0.5
Assigned 1H Chemical Shifts in the Isotropic Phase R3 R4 C1 C2 C3 6.4 6.5 2.9 1.8 1.1
4 0.7 C4 0.9
5 0.6 C5 0.9
6 0 C6 0
a The values in the nematic phase are given as averages of chemical shifts at seven different temperatures from 58 to 72 °C, each determined from the center of the quadrupolar doublets with estimated errors of (0.5 ppm. b From Figure 1. c From Figure 3.
Figure 3. Comparison of (a) 1H NMR spectrum (400 MHz) and (b) natural abundance 2H NMR spectrum (61.25 MHz) of 6OCB in the isotropic phase. Measurements were made at 80 °C with a single scan for 1H NMR and 10 000 scans for 2H NMR. Chemical shifts are referred to the methyl signals. The 1H signals were assigned as shown by measurement of DQF-COSY.
phase can be given rather accurately from the centers of the quadrupolar splittings in the 2H NMR spectrum. The secondorder contributions, estimated to be less than 0.1 ppm, are within the experimental error of the chemical shift measurement ((0.5 ppm) and may be totally neglected. In this way, we determined chemical shifts for individual quadrupole-split lines in the entire range of the nematic phase of 6OCB from 58 to 72 °C using the data in Figure 2, with the methyl deuteron signal (signal 6 in Figure 1; easily recognized from its intensity) as reference. As the assigned shifts did not show any systematic variation with temperature, only averaged values over the temperature range from 58 to 72 °C are listed in Table 1. We observe that the chemical shifts determined in this way show a considerable variation, presumably reflecting chemical environments. In an effort to make assignments of these signals to specific deuterons, we first try to correlate these 2H chemical shift values in the nematic phase with those in the isotropic phase. The 2H NMR spectrum of 6OCB in the isotropic phase shows several peaks with a chemical shift variation (Figure 3b). However, the signals are too broad to make assignments for individual deuterons. Figure 3a shows a corresponding 1H spectrum of 6OCB in the isotropic phase, which should, in principle, have identical chemical shifts with Figure 3b. Therefore, the signal assignments were made for the 1H spectrum by using a standard two-dimensional NMR technique, i.e., DQF-COSY, and are shown in Figure 3a. The chemical shift values of the assigned 1H signals are listed in Table 1, again with the chemical shift of the methyl group as reference. In Table 1, we compare the 2H chemical shifts determined in the nematic phase with the assigned 1H chemical shifts in the isotropic phase. We note that chemical shifts in the two phases do not exactly coincide, making immediate assignments of the 2H signals not feasible. However, we note that the 2H chemical shifts consist of two distinct groups with larger δ
values of 4.3-5.7 and smaller δ values of 0-2.5. It is fairly obvious that the shifts of the former group correspond to those of the aromatic protons (5.9-6.5 ppm) in the isotropic phase, whereas the shifts of the latter group correspond to those of the aliphatic protons (0-2.9 ppm) in the isotropic phase. Thus signals 7-10 from 4.3 to 5.7 ppm must belong to the aromatic C-2H groups, R1 through R4. Signals 1-5 from 2.5 to 0.6 ppm must belong to the aliphatic C-2H groups, C1 through C5. Furthermore, within the aliphatic groups, signal 1 at 2.5 ppm in the nematic phase may safely be assigned to the C1-2H group which resonates at 2.9 ppm in the isotropic phase. Identification of signals 2-5 to each of the groups from C2-2H through C52H is more difficult, because of the very similar chemical shift values in the nematic phase. However, it may be generally assumed that the magnitude of quadrupolar splitting has a decreasing tendency from C1 to C6 with increasing disorder along the aliphatic chain, modified with an alternating oddeven effect.5 On this assumption, signals 2, 3, 4, and 5 may be tentatively assigned to C2-, C3-, C4-, and C5-2H, respectively. However, assignments of signals 3 and 4 were actually reversed by Emsley et al.,7 reflecting the alternating odd-even effect,5 on the basis of measurements of 2H longitudinal relaxation rates. In the rest of the paper, we follow their assignments of signals 3 and 4. After the assignments are complete as above, the upfield shifts of all the aromatic deuterons by about 1 ppm from 5.9-6.5 ppm in the isotropic phase to 4.3-5.7 ppm in the nematic phase are noted, provided that the methyl group resonates at the same position in the two phases (Table 1). These upfield shifts should reflect the chemical shift anisotropy of the phenyl deuterons such that the deshielding effects from the ring currents diminish in the nematic phase in which the aromatic rings are oriented fairly parallel to the applied magnetic field. Quantitative analysis of such chemical shift differences is premature at present but can be a source of information of liquid crystals in future studies. On the other hand, within the aromatic core, differentiation among the ring deuteron signals is extremely difficult, without performing specific deuteration in the ring. In the fully deuterated rings, only two peaks were observed for the C-2H groups of the rings due to line broadening by 2H dipole-dipole couplings.6 In the natural abundance 2H NMR signals of Figure 2b, the sharpness of the signals allowed detection of four separate peaks from the two aromatic rings of 6OCB, indicating nonequivalent microscopic orderings among the four pairs of the C-2H bonds. Even so, the quadrupolar splittings obtained from natural abundance 2H NMR spectra coincided well with those obtained by Emsley et al. using specifically deuterated 6OCB-d13.6 For example, at 65 °C, the methyl splitting was 13.5 kHz in our natural abundance 2H NMR data and 13.2 kHz in 6OCB-d13,6 while the R-methylene splitting was 48.4 kHz in our data and 49.0 kHz in 6OCB-d13.6 3. Temperature Dependence of Microscopic Orientational Orders in 6OCB in the Nematic Phase. Having established assignments of some of the individual C-2H quadrupolar
Natural Abundance 2H NMR Studies
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5111 0.33. Therefore, the natural abundance 2H NMR as presented here may not be suited for estimating absolute values of the orientational orders of the para-axis of the ring between different substances. Nonetheless, it provides a general and convenient means to evaluate microscopic orientational orders of a liquid crystalline material under different conditions, such as in the temperature of measurement and in the molecular composition of the liquid crystalline material. Figure 4 shows the dependence of the orientational orders of the rings (Szz) and the orders of the C-2H bonds (SCD) of 6OCB on temperature determined from the data in Figure 3. Microscopic orders of the molecule, for example, in the core and in the side chain, could easily be estimated from natural abundance 2H NMR without specific deuteration. Concluding Remarks
Figure 4. Plots of the order parameters of the C-2H bonds (SCD, open symbols) and the para-axes of the rings (Szz, filled symbols) of 6OCB against temperature, obtained from the experimental data using eqs 1 and 2, respectively. The angle θ between the C-2H bond and the paraaxis of the ring is assumed to be 60° in eq 2. Labels 1, 2, 3, ... refer to corresponding signals in Figure 1.
splittings (∆ν) in the liquid crystalline phase, we can estimate SCD, the microscopic orders of the C-2H bonds with respect to the static magnetic field from eq 1.
(1)
We may further evaluate the order parameter of the ring, Szz, from eq 2 by assuming an axial symmetry for the orientational order of the ring (Sxx ) Syy) and the angle θ that the C-2H bond makes with the para-axis of the rings to be 60°.
3 ar 3 cos2 θ - 1 Szz ∆ν ) QCD 2 2 ar QCD ) 185 kHz
Acknowledgment. We thank Merck Japan for providing a sample of 6OCB. We also thank Mr. K. Takahashi of JEOL for his enduring help with the instrument. References and Notes
3 al ∆ν ) QCD SCD 2 al QCD ) 168 kHz
Information on the microscopic orders of the C-2H bonds can readily be obtained from 2H NMR measurements at natural abundance of a liquid crystalline sample in the nematic phase on a conventional NMR spectrometer primarily designed for liquids that operates at a relatively high field (9.4 T or more). Although at 9.4 T signal accumulation for as long as 20 h is needed, this experimental time can be shortened to 2 h at 17.6 T. Extension of this technique to phases other than the nematic appears to be promising with a spectrometer having a higher decoupling power. Natural abundance 2H NMR will extend the utility of NMR spectroscopy to a range of liquid crystalline samples far wider than that hitherto been explored by 2H NMR spectroscopy.
(2)
The choice of 60° for θ is rather arbitrary. In 5CB-d8(4′pentyl-4-cyanobiphenyl), Emsley et al. obtained 60° for the outer and 61° for the inner angles of biphenyl C-2H bonds from the 2H-2H dipole couplings.13 If the angle θ deviates by 1°, e.g., from 60 to 61°, then Szz changes rather sensitively from 0.39 to
(1) Naito, A.; Nakatani, H.; Imanari, M.; Akasaka, K. J. Magn. Reson. 1990, 87, 492-432. (2) Naito, A.; Imanari, M.; Akasaka, K. J. Magn. Reson. 1991, 92, 85-93. (3) Akasaka, K.; Kimura, M.; Naito, A.; Kawahara, H.; Imanari, M. J. Phys. Chem. 1995, 99, 9523-9529. (4) Naito, A.; Imanari, M.; Akasaka, K. J. Chem. Phys. 1996, 105, 4504-4510. (5) Dong, R. Y. Nuclear Magnetic Resonance of Liquid Crystals; Spring-Verlag: New York, 1994. (6) Emsley, J. W.; Luckhurst, G. R.; Parason, P. J.; Timimi, B. A. Mol. Phys. 1985, 56, 767-774. (7) Emsley, J. W.; Foord, E. K.; Gandy, P. J. F.; Turner, D. L. Liq. Cryst. 1994, 17, 303-309. (8) Counsell, C. J. R.; Emsley, J. W.; Luckhurst, G. R.; Sachdev, H. S. Mol. Phys. 1988, 63, 33-47. (9) Dong, R. Y.; Ravindranath, G. Liq. Cryst. 1994, 17, 47-63. (10) Aliev, A. E.; Harris, K. D. M.; Apperley, D. C. Chem. Phys. Lett. 1994, 226, 193-198. (11) Reichert, D.; Olender, Z.; Poupko, R.; Zimmermann, H.; Luz, Z. J. Chem. Phys. 1993, 98, 7699-7710. (12) Patt, S. L. J. Magn. Reson. 1982, 49, 161-163. (13) Emsley, J. W.; Luckhurst, G. R.; Stockley, C. P. Mol. Phys. 1981, 44, 565-580.
