1934
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
CORRESPONDENCE High-Resolution Nuclear Magnetic Resonance Spectrometry in Thermotropic Nematic Liquid Crystals with Negative Diamagnetic Anisotropy Sir: I t has hardly been possible to keep pace with the number of papers appearing on the subject of high-resolution NMR spectrometry in liquid crystals since the fundamental studies by Saupe and Englert (I, 2 ) . Investigations in this field have been carried out mainly in thermotropic nematic liquid crystals which had without exception a positive diamagnetic anisotropy A X = XII- X, XII and X, being the diamagnetic susceptibility parallel and perpendicular to the preferred direction of the long axis (director) of the liquid crystal molecules. Since every system aligns itself with the axis of its greatest susceptibility in the direction of the field, “diamagnetic positive” liquid crystals orient themselves with their director always parallel to a magnetic field (Figure la). In NMR measurements, it is a common technique to rapidly spin the sample tube oriented perpendicular to the magnetic field. in order to average out residual inhomogenities in the sample and thus to improve resolution. Since directiondependent properties of molecules are investigated by NMR in liquid crystals, a constant orientation of the dissolved molecules in the magnetic field is necessary. Therefore it has not been possible to use spinning techniques in nematic solvents, since spinning would destroy this orientation, unless the liquid crystal is oriented perpendicular to the magnetic field by an additional electric field, or the sample tube is aligned in the direction of the field as in superconducting magnets. N M R signals in a nematic solvent therefore generally manifest half-height width much larger than in isotropic solutions with sample spinning. Corresponding nonspinning spectra are consequently characterized by a comparatively low detection sensitivity (small signal/noise ratio) and resolution. T h e purely alicyclic trans,trans-4-alkyl-4’-cyanocyclohexylcyclohexanes (I)
AX>O
AXcO
(non splnning p r o b e )
l n o n spinning probe)
Y t
Figure 1. Orientation of the director in a nematic liquid crystal aligned by a magnetic field for molecules with positive diamagnetic anisotropy (a) and with negative diamagnetic anisotropy in a non-ordered (b) and ordered (c) state
Table I . Physical Data of trans, trans-4-Alkyl-(bicyclohexyl)-4’-carbonitriles Ie
-
R =
IC3H7-
0003-2700/78/0350-1934$01 0010
e
ClP
83
sl-sG
%-SH SH- N
32
S -N
AH
1
recently described by R. Eidenschink et al. (3) represent a new class of liquid crystals which, in contrast to previous concepts on conditions which influence liquid crystal properties, manifest stable and broad mesophases (see Table I). Maier and Saupe (4-6) who postulate clearing points to be proportional to molecular anisotropy of polarizability, molecules of type 1 should be expected to exhibit no enantiotropic nematic properties or, at the most, very low clearing points. According to their molecular structure, these liquid crystals have no UV absorption above 200 nm, and thus are interesting anisotropic solvents for UV and fluorescence spectroscopy (7). I n addition to a positive dielectric anisotropy A€ = ell - c, between +3 and +4, they possess a negative diamagnetic anisotropy (8)of approximately A X = -2 X lo-’ [cm3g-’1 with X, = -7.4 X [cm3g-l] (9). They are thus the first known class of thermotropic-nematic liquid crystals with this property, since no reference has hitherto been made to nematic substances with negative anisotropy of susceptibility except for a paper by E. Sackmann e t al. (10) on nematic systems
!LI-116;
MP
A C k-yIiN
I
l s p i n n i w probe]
-
ZLI-1167is a ternary eutectic of propyl-, pentyl- and heptylbicyclohexylcarbonitrileI. S , and N are abbreviations for smectic and nematic transitions. Monotropic transitions are in parentheses.
of compensated cholesteric liquid crystals. Because of their negative diamagnetic anisotropy, liquid crystalline bicyclohexane molecules should align themselves in a plane perpendicular to a magnetic field, initially without a preferred orientation of their longitudinal axis in between this plane (Figure lb). In a sample tube which is spun normal to the magnetic field, those molecules not oriented in the spinning axis must experience a torque, tending to align all of them homogeneously in the long axis of the tube, and thus forming a uniaxial nematic liquid crystal with the director field oriented in the rotation axis (Figure IC). Since dissolved molecules are aligned uniformly with the liquid crystal matrix, they no longer experience a change in orientation with respect to the magnetic field by sample spinning. Therefore liquid 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
1935
I IC!
$13
21!
!
..
4
C l Cl C l
I
I
-__I_
10
9
’
I
i_--__i
8
I
6
5
L
3
2
1
Opprn
Flgure 2. ’H NMR spectrum of a 0.5 M solution of 1,2,3,5-tetrachlorobenzenein the nematic solvent ZLI-1167 at room temperature measured with nonspinning sample tube (lower curve) and spinning sample tube (upper curve)
crystals of type I should permit the common spinning technique in NMR and hence enable an improved resolution comparable to that of measurements in isotropic solutions.
EXPERIMENTAL Measurements were performed with a Varian NMR spectrometer EM-360 at room temperature. Commercial sample tubes (Wilmad Glass Co.) were used with 5.0-mm o.d., 4.2-mm i.d., and 190-mm length. The nematic solvent was ZLI-1167 (product of E. Merck, Darmstadt, and EM Laboratories, Elmsford N.Y.). This solvent is a ternary eutectic of the three “diamagnetic negative” propyl-, pentyl-, and heptylbicyclohexylcarhonitriles,which is liquid crystalline between +8 and +83 O C . The commercially available 1,2,3,5-tetrachlorobenzene (Schuchard, Munich) was purified by recrystallization and dissolved in ZLI-1167 at 80 O C , Le., above the clearing point of the nematic solution, in order to achieve a homogeneous sample. RESULTS AND DISCUSSION Figure 2 illustrates the ‘H NMR spectrum of a 0.5 M nematic solution of 1,2,3,5-tetrachlorobenzeneusing a nonspinning sample tube (lower curve). I t is characterized by a half-height width of 6-8 Hz, which concurs with that published by G. Englert and A. Saupe in 1964 (2). When the sample tube is spun a t a speed greater than 20 Hz (all other instrumental parameters remained unchanged), a dramatic change takes place (see upper curve). Resolution and signal-to-noise ratio are increased by a factor of 3, whereas splitting remains unchanged. Despite the high concentration of dissolved substance, a spectrum is obtained with a half-height width of the signals of 2 Hz, Le., a resolution common in isotropic solvents. Simultaneously, the probe, cloudy before rotating, has become transparent without any light scattering over the total sample volume, indicating a high-ordered liquid crystal with Sllnear +1 and S , near -0.5, SIIand S , being the order parameter in the rotation axis and in magnetic field direction, respectively. When spinning is stopped, the original low resolutions and small signal-to-noise ratio again is to be observed, whereas the optical appearance of the probe remains unchanged even for hours and also outside the magnet gap of the spectrometer. By changing from “dynamic” to “static” drive mode, improvement and deterioration of resolution can be reproduced. Thus the observed
improvement cannot be related to molecular ordering phenomena but has to be explained as an effect of averaging out magnetic field inhomogeneities in a probe, in which magnetic field and spinning procedure have transformed the nematic solution into a uniaxial liquid crystal, the director of which oriented orthogonal to the field and in the axis of rotation. Only in such a nematic system, spinning does not affect the orientation of molecules under investigation and makes possible a reduction of inhomogeneities by sample spinning. Measurements on benzene have shown that, with appropriate experimental skill, a resolution even better than 1.0 Hz can be achieved in this new type of nematic solvents (11). Although the nematic solvent ZLI-1167 has a relative small negative diamagnetic anisotropy, nonetheless the specific behavior, theoretically expected from such a type of liquid crystals, can be observed experimentally and utilized already in a magnetic field of 14 000 Gauss as demonstrated. Thus, bicyclohexanes of type I represent a new class of anisotropic solvents, which open up in conventional magnets a high resolution NMR spectrometry in nematic liquid crystals previously possible only in isotropic solvents.
LITERATURE CITED (1) A. Saupe and G. Englert, Phys. Rev. Len., 11. 462 (1963) (2) G. Englerl and A . Saupe, Z.Naturforsch A, 19, 172 (1964). (3) R. Eldenschink,D. Erdmann, J. Krause, and L. Pohl, Angew. Chem.. 90
(2),133 (1978). (4) W. Maier and A. Saupe. Z. Naturforsch. A, 13, 564 (1958). (5) W. Maier and A . Saupe, 2. Naturforsch. A , 14,882 (1959). (6) W. Maier and A . Saupe, 2 . Naturforsch. A, 15,287 (1960). (7) H. Wedel and W. Haase, Chem. Phys. Left, 55 (l),96 (1978). (8) L. Pohl, R. Eidenschink,J. Krause, and G. Weber, Phys. Left. A, 65 (2), 169 (1978). (9) H. Schad, G.Baur, and G. Meier, presented at the Seventh International Liquid Crystal Conference, Bordeaux. France, July 1978. (10) E. Sackmann, E. Meiboom, and L. C. Snyder, J. Am. Chem. SOC.,69, 5981 (1967). (11) P. Diehl, Universitat Basel, Switzerland, private communication, 1978.
Ludwig Pohl* Rudolf Eidenschink E. Merck Darmstadt, Germany RECEIVED for review January 16. 1978. Accepted August 16, 1978.
