RESEARCH
Liquid Crystals' Spectra Show Fine Structure PMR spectra of nematic solutions are used to determine orientation of molecules Proton and deuteron magnetic reso nance (PMR and DMR) spectra of oriented molecules in liquid crystals show detailed fine structure, accord ing to Dr. Alfred Saupe of Physikalisches Institut der Universitat Frei burg, West Germany; Dr. Gerhard Englert of Hoffmann-La Roche & Co., Ltd., Basel, Switzerland; and Dr. W. D. Phillips of Du Pont's central re search department, Wilmington, Del. (C&EN, March 9, page 3 9 ) . They get highly resolved PMR and DMR spectra of nematic-type ( molecules ordered in one dimension) crystals of compounds such as p-azoxyanisole and p-butoxybenzoic acid. But it's easier to analyze spectra of solutions in nematic liquid crystals such as benzene in ρ,ρ'-di-n-hexyloxyazoxybenzene, they told the Pitts burgh Conference on Analytical Chemistry and Applied Spectroscopy. Analyses of such spectra give accurate information about the anisotropic in teractions (which are lost in isotropic liquids or solutions), relative internuclear distances, and quadrupole interactions. These spectra can also be used to determine the average ori entation of solute molecules, Dr. Saupe says. Dr., Phillips and his co-workers at Du Pont, Dr. J. C. Rowell and Dr. L. R. Melby, find it easier to analyze dipole-dipole interactions of nematic phases when deuterium-substituted compounds are used. Additional splitting in the spectra of these deuterated nematogens and of dissolved
compounds such as CGDC gives deu terium quadrupole coupling constants. Anisotropy. The advantage of studying NMR spectra of liquid crys tals is that the systems retain the low viscosity of liquids, but the molecules are partially oriented. These liquid crystal phases are often as fluid as nor mal liquids, but they show anisotropic properties similar to those of crystals. The most obvious anisotropy of these liquids, Dr. Saupe says, is their very strong optical double refraction. Some organic substances exist in one or more of the liquid crystalline phases at temperatures between those where the solid crystalline phase and the normal isotropic liquid phase are found. The three main types of liq uid crystals are nematic, smectic, and cholesterinic. All liquid crystals (if they are homogeneously ordered) behave optically as uniaxial crystals. In the nematic phase, which is most suitable for NMR studies, the mole cules tend to orient themselves under the influence of a magnetic field (as in an NMR spectrometer) in one dimen sion, with the axis of least susceptibil ity of the molecular aggregates paral lel to the field. The diffusion of the molecules isn't hindered in the nematic phase. For NMR studies, this means that the intermolecular nuclear spin interactions are averaged out as they are in normal liquids. Thus the spectra show only the intramolecular interactions, Dr. Saupe says. Orientation. Fine structure appears
BENZENE. The PMR spectrum of benzene in a nematic solution is symmetric; all the protons have the same chemical 42
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MATRIX. Dr. Alfred Saupe (left) and Dr. Gerhard Engiert use symmetrical matrix to describe the average orienta tion of molecules in nematic phase
only in spectra of nematic liquid crys tals because only these systems are homogeneously ordered by the mag netic field of an NMR spectrometer. And only under such conditions is each molecule "seen" under the same average orientation by the magnetic field, Dr. Saupe notes. A field strength of 1000 to 2000 gauss is enough to orient the molecules so that the optical axis turns parallel to the field. But the molecules are not all paral lel in the nematic phase, Dr. Saupe points out. Rather, they fluctuate rapidly about the preferred orien tation. However, only the average orientation is important for intramo lecular interactions. The average ori entation is expressed as the degree of order, S, which is defined by the equa-
shift. Line widths range from 5 to 15 cycles per second with an over-all splitting of more than 2000 cycles per second
Nematic Liquid Crystal Systems Provide a Way to Obtain Deuterium Quadrupole Coupling Constants
When CH3 groups on p-azoxyanisole are replaced with CD3, the spectrum is simplified. The doublet splitting of 0.27 gauss in the spectrum of the deuterated compound may arise from H3—H3' and H5— H 5 ' interactions
2 tion S· = 1 - 3/2(sin 6>), where sin-0 is the the mean function of all angles, 0, between the long molecular and optical axes. S values range from 0.4 to 0.7 in nematic phases and depend strongly on the temperature of the system. The nematic ordering is caused by
ORDER. The degree of order of the tetrachlorobenzene molecules is 0.022 if the benzene ring is undistorted
The quadrupole splitting in 2,2',6,6'-tetra-p-azoxyphenetoleDi vanishes at 166° C. where the nematic phase breaks up to form the isotropic liquid. Disordering of the components of the nematic phase increase as temperature rises
dispersion forces. Therefore, the principal axis of S coincides ( or nearly coincides) with the principal axis of polarizibility. The axis of greatest polarizibility will tend mostly to orient parallel to the optical axis. This means that the S value that belongs to this axis will always be positive. Since the polarizibility often corresponds to the shape of the molecule, the same orientation will be preferred by closest packing of the molecules. The dipole-dipole forces due to permanent electrical dipole moments aren't important in most cases, Dr. Saupe believes. PMR spectra of pure compounds in the nematic phase show a detailed fine structure because the intermolecular direct dipole-dipole couplings are reduced, as in normal liquids, Dr. Eng-
lert points out. However, the over-all splitting of the spectra means the intramolecular interactions are still of comparable magnitude to those values in solids. Although the angles between the long axes of the individual elongated molecules and the direction of the magnetic field differ from each other at any instant, the rapid fluctuation of the molecules causes the intramolecular couplings to average out to sharp values. The individual molecules behave like isolated oriented spin systems, which accounts for the sharp PMR signals. Matrix. Because of the large number of interacting protons in nematic systems, the spectra are extremely complex, Dr. Englert says. So it's better to study the spectra of molecules (having only a few protons) disMAR. 16, 1964 C&EN
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solved in a liquid crystalline matrix. In such systems (using normal highresolution techniques), Dr. Englert obtains highly resolved, simple spec tra, superimposed on a much broader background signal caused by the nematic compound. PMR spectra of anisotropic molecules dissolved in a liquid crystalline matrix (usually 5 to 30 mole%) can give information about the type and degree of molec ular orientation. This information can also be used to measure the anisotropy of chemical shifts. In some cases, interproton distances and the absolute sign of the indirect spinspin coupling constant can be deter mined. Dr. Englert studied the spectrum of a two-spin system, consisting of 1,2,4,5-tetrachlorobenzene in nematic p,p'-di-n-hexyloxyazoxybenzene at 93° C. The spectrum of the simplest case of an isolated system of two in teracting magnetic nuclei shows a doublet of sharp lines with equal in tensities. Dr. Englert finds only the sharp lines for the dissolved two-spin system. The much broader signal of the nematic compound isn't detect able. Because of symmetry, the two pro tons of the solute have the same chem ical shift. If the indirect J-coupling of ordinary liquid systems is neglected, the splitting of the doublet is propor tional to S/r 3 , where S is the degree of ordering of the axis connecting the two protons and r is the distance be tween them. Dr. Englert assumes an
undistorted benzene ring with a pro ton-proton distance of 4.96 Α., and calculates an absolute value of 0.022 for S. He obtains similar S values from the spectra of other aromatic two-spin systems. These values com pare to S values of 0.4 to 0.7 for the long molecular axis in pure nematic crystals. This indicates that the ar rangement of the aromatic solute mol ecules is more random than the elon gated molecules of a pure nematic phase, Dr. Englert says. He and Dr. Saupe have also obtained high-reso lution spectra of nematic solutions of three, four, five, and six interacting protons. Complex. Although the PMR spec tra of nematic phases such as p-azoxyanisole (PAA) and p-azoxyphenetole (PAP) are highly resolved, they are extremely complex, Dr. Phillips says. Even worse, some of the chemical shifts are comparable to some of the smaller dipole-dipole spacings in the NMR spectra of these nematic phases. This factor is probably responsible for the asymmetries in the spectra, he notes. But when CH 3 groups are replaced by CD 3 in PAA, for example, the spec trum is greatly simplified, Dr. Phillips says. The spectrum of the CD 8 deriv ative shows a major doublet of spacing of 3.09 gauss. Each of these doublet components is further split into dou blets of spacing of 0.27 gauss. The smaller doublet splitting of 0.27 gauss in the spectrum of PAA-CD 8 may arise from Ho—H/ and H-—H-/ in-
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teractions, Dr. Phillips believes. Nematic systems such as PAA and PAP (where the molecules are in a relatively mobile phase and are oriented in a magnetic field) provide an ideal opportunity to obtain deuterium quadrupole coupling constants, Dr. Phillips says. Effects of nuclear quadrupole coupling constants can, in principle at least, be observed for component atoms of the molecules of the nematic substance, and for molecules which can be embedded in the nematic phase without destroying the structure of the regions of closely aligned molecules or "swarms," he adds. In the latter case, the molecules of the nematic phase serve as a matrix for orienting the solute molecules under the influence of an external magnetic field. Deuterium quadrupole constants help in clarifying electronic structures of molecules, but few have been measured so far, Dr. Phillips says. Quadrupole effects on nuclear resonances of noncrystalline solids are used in determining molecular structure and solid-state effects. The DMR spectrum of 2,2',6,6 r tetra-p-azoxyanisole-d 4 ( PAA-D 4 ) shows two triplets of internal spacing of 1.5 gauss separated by 14.0 gauss. This triplet structure arises from H—D dipole-dipole interactions, and is not of interest in the quadrupole couplings. But the major doublet spacing of 14.0 gauss arises from quadrupolar perturbation of the nuclear spin levels, and manifests the deuterium quadrupole coupling constant, the Du Pont scientist says. Temperature. Dipole-dipole splittings in PAA, PAP, and their deuterium derivatives are temperature dependent, since disordering and mobility of the components of the nematic phase increase as temperature rises. Quadrupole splitting in the DMR spectrum of PAP-D 4 vanishes at 166° C , where the nematic phase breaks up to form the isotropic liquid. The DMR spectrum of C 6 D G dissolved in p,p'-di-n-hexyloxyazoxybenzene in the nematic phase shows a simple doublet of spacing of 25.4 gauss (at 73° C. with 15% CGDG by weight) with fine structure that can't be resolved further. The existence of the quadrupole doublet in the DMR spectrum shows that the benzene is at least partially ordered, and that the p,p'-di-n-hexyloxyazoxybenzene is acting as an ordering matrix, Dr. Phillips adds.
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