NMR IN MIXED LIQUID CRYSTALS

C.L. KhetrapalIndian Institute of Science, Bangalore-560 012, India

andNational Institutes of Health, Bethesda, Maryland-20892, USA

ABSTRACT

NMR studies in more than one andin mixed liquid crystals have led tonovel applications of NMR andenhanced the scope of NMR spectro-scopy of oriented systems inproviding spectral, structural andconformational information whichotherwise cannot be derivedconveniently. The results arec r i t i c a l l y evaluated for thefollowing types of experiments: (i)molecules dissolved in liquidcrystals with different orien-tat ional behavior, ( i i ) solutemolecules in mixed nematic-nematicmaterials of opposite diamagnetican i so t rop ie s , ( i i i ) dissolvedmolecules in mixed cholesteric-cholesteric and cholesteric-nematicliquid crystals and (iv) nematic-nematic liquid crystals themselves.Theoretical status of the experi-ments is described.

INTRODUCTION

NMR spec t r a of moleculesoriented in more than one and inmixed liquid crystals (1, 2) lead tonew applications of NMR. Thestudies have been undertaken inmixed nematic-nematic and nematic-c h o l e s t e r i c mixtures and themolecules dissolved therein with aview to enhance the scope of the NMRspectroscopy of oriented systems.For successfully employing thetechnique, i t is essential to obtainprecisely the spectral parameters

namely the direct dipolar couplings(D • • 's ) between the i n t e r -acting nuclei i and j , the indirectsp in - sp in couplings (J-^j's),the chemical shifts (j/.-1/-'sj , thequadrupole coupling constants(QCC's) and the anisotropies ofthe chemical shifts and the couplingtensors, from the spectra. Toderive the relative internucleardistances from the dipolar couplingswithout assumptions, the number of

D•U 's must not be

in NMR makes itthe informationpossible from aThe use of mixed

independentless than the sumJ of the geometricalparameters and the order parameters(S--'s) in the system. Theuse of more than one liquid crystal

possible to obtainwhich may not besingle experiment,liquid crystals of

opposite diamagnetic anisotropiesleads to more interesting results.Some such experiments are criticallyreviewed in th i s paper. Inaddition, studies of mixed nematic-nematic liquid crystals of oppositediamagnetic anisotropies themselveshave been undertaken in order tot h e o r e t i c a l l y understand theunder lying phenomena and todetermine the C-chemical shiftanisotropies of the individualcarbon atoms in liquid crystals.Results on mixed cholesteric-cholesteric and cholesteric-nematicliquid crystals are also presented(3, 4).

158 Bulletin of Magnetic Resonance

^k

MOLECULES DISSOLVED INLIQUID CRYSTALS WITH DIFFERENT

ORIENTATIONAL BEHAVIOR

In a system where the numberof derived dipolar couplings i sl e s s than the sum of the geo-met r ica l and order parameters tob e d e r i v e d , p e r f o r m i n gexperiments under differentorientation conditions such asusing more than one liquidcrystal is extremely useful. Ifthe experiments are performed intwo liquid crystals, the numberof dipolar parameters derivedbecomes double but the number ofgeometrical parameters to beobtained remains unchangedneglecting the solvent effects onmolecular geometry. The utilityof the experiments can bedemonstrated by the followingexamples:

(1) From NMR spectra of orientedAA'BB' and AA'BB'X systems wherethe d ipo la r couplings D,»(DA,g,) are much larger thanthe other coupling constants andwhere D^^i ^gg' i t ^ s notposs ib le to derive a l l thes p e c t r a l p a r a m e t e r s withacceptable precision. Such asituation is encountered in thes p e c t r a of p - d i s u b s t i t u t e db e n z e n e s in g e n e r a l andp-substituted f luorobenzenes inparticular. The spectra of theAA'BB'X t y p e s under suchconditions are insensitive to thechemical shift (VA~V]$) • anc* thecoupling parameters (DAX~̂ BX̂and ( D A A ' ~ D B B ' ) - Thus onecan only obtain DAg ( ^ )

DD AB 'and

A(D»y+DBX)

l

(DAA'+DBB'd f h>A A B By B X ) instead of theindividual coupling constants.Such a system needs two orderparameters for the specification,of molecular orientation andthree re la t ive internucleardistances for the fixation ofr e l a t i v e nuclear posit ions.C o n s e q u e n t l y , a t o t a l

of five geometrical and orderparameters are required to bedetermined, whereas the system ina single experiment provides onlyfour dipolar parameters. If,however, the studies are carriedout in two thermotropic solvents,the number of dipolar couplingparameters derived becomes eightand only two additional orderparameters are required to bedetermined, assuming that themolecular geometry does notchange with solvent. Suchexperiments thus permit thede te rmina t ion of moleculars t ruc ture . The u t i l i t y hasactually been demonstrated for2-(p-bromophenyl)-4,6-dichloro-pyrimidine (5). I t must- beemphasized that the method ismore instructive if the orderparameters in different experi-ments differ significantly.

(2) The proton NMR spectrum ofN,N-dimethyl uracil (6) provideseight HH-dipolar couplings in asingle experiment. In this case,three parameters are required forthe s p e c i f i c a t i o n of themolecular order and 8 geometricalparameters define the protonpositions for specific modes ofrotations of the methyl groups,under plausible assumptions (6).The 8 geometrical and the 3orientational parameters cannotbe derived from the 8 dipolarcouplings obtained from a singleexperiment. If, however, theexperiments are performed in twodifferent liquid crystals, thet o t a l of s ix or ienta t ionalparameters (3 for each experi-ment) and 8 geometrical para-meters can be derived from the 16dipolar couplings using a least-square-fit procedure. N, N-d imethy l u r a c i l has beeninvestigated in two differentliquid crystal solvents, namelyN-(p-methoxy benzylidene)-p'-n-butylaniline and p-n-butyl-p ' - methoxyazoxy-benzene and

Vol. 8, No. 3/4 159

the geome t r i ca l parametersderived (6). It has been foundthat the preferred conformationof the molecule is that where oneC-H bond of the methyl group(CH3 (3)) (Structure 1)

(Structure 1)

is in the ring plane pointingtowards the carbonyl group. Theresults were not sensitive to themode of rotation of the methylgroup (CH-}(4)) in line with thep r e d i c t i o n s of the bondpolarization hypothesis (7).

MOLECULES DISSOLVED INMIXED NEMATIC-NEMATIC

LIQUID CRYSTALS

In a mixture of nematicliquid crystals with oppositediamagnetic anisotropies, theD^ .• values change graduallywith relative concentrations ofthe two solvents (8, 9) until ata c r i t i c a l concentration andtemperature, they switch abruptlyto twice or half with opposites igns , depending, upon thedirection of approach of thec r i t i c a l p o i n t . A closeexamination of the results in thevicinity of the critical pointreveals the coexistence of twotypes of spectra at a particulart e m p e r a t u r e wi th d i p o l a rcouplings in one being twice withopposite sign to those in theother. They correspond to twotypes of orientations of theliquid crystal optic axis, onebeing preferentially parallel andthe other perpendicular to themagnetic field. The behavior is

theoret ical ly well understood(10-12) in terms of the Landautheory. I t is , however, notclear whether the two spectranear the cr i t ical point arisefrom cer ta in inhomogeneities(e.g. temperature and concen-tration) or whether there is a"true" coexistence of the twophases like in other first orderphase transitions. Though areport on the existence of the"Powder Pattern" is available(13), some further experimentsare e s s e n t i a l in order toconc lus ive ly e s t ab l i sh thep o i n t . Never the less , theexperiments lead to the followingnovel applications of NMR: (i)Determination of chemical shiftanisotropy without the use of areference compound or without thec h a n g e of e x p e r i m e n t a lc o n d i t i o n s , ( i i ) s epa ra tedetermination of the indirectspin-spin and direct dipol'arcouplings between heteronuclei,( i i i ) d e t e r m i n a t i o n ofdiamagnetic anisotropy of liquidc r y s t a l s , ( i v ) p r e c i s ed e t e r m i n a t i o n of s p e c t r a lparameters, (v) determination ofspect ra l parameters, such asthose in an oriented AB-System,which otherwise cannot beobtained from the line positions,(vi) determination of moleculars t r u c t u r e w i t h minimumdistortions and (vii) precisedetermination of quadrupolecoupling c o n s t a n t s . Sucha p p l i c a t i o n s have beenextensively explored (14-20).Besides determining the vibra-t i o n a l l y c o r r e c t e d r Q -structure, the chemical shiftanisotropy and the sign and themagnitude of the indirect HPcoupling in methyl dichloro-phosphine (21), the abnormalorientation of the molecule hasbeen interpreted in terms of theformation of a solvent-solutecomplex. Typical spectra areshown in Figure 1.

160 Bulletin of Magnetic Resonance

DISSOLVED MOLECULES IN MIXEDCHOLESTERIC-CHOLESTERIC ANDCHOLESTERIC-NEMATIC LIQUID

CRYSTALS

The objective to study NMRspectra in cholesteric liquidcrystals is 3-fold:

(1) Such mate r ia l s possesssolubility properties which aredifferent from those in otheranisotropic media commonly usedin NMR experiments and hence suchinvestigations enhance the scopeof the method. Acetonitrile isthe only molecule studied so farwhere the r - s t ruc ture hasbeen derived in an opticallyc o m p e n s a t e d m i x t u r e ofc h o l e s t e r y l c h l o r i d e andcholesteryl nonanoate (3). Ther Q - v a l u e of the HCH bondangle determined is 109.2°which agrees well with themicrowave value of 109.3° (22).

(2) Distinct NMR spectra for dand 1_ enantiomers have beenobserved for optically activemolecules dissolved in compen-sated cholesteric liquid crystals(23, 24). This is due to thefact that the degree of order ofthe solute molecules differs forthe d and 1 geometry. The resultshave been demonstrated forracemic 3,3,3-trichloro propyleneoxide dissolved in a mixture ofcholesteryl chloride and choles-teryl myristate. The experimentsd e m o n s t r a t e t h e u s e ofcholesteric liquid crystals forthe study of optically activecompounds in order to obtainseparate information on the d andthe I forms. Such an informationcannot be obtained from nematicand lyotropic mesophases normallyused in NMR experiments. Thepotentials of such experimentshave to be explored since noother report is available to thebest of our knowledge.

(3) The c h o l e s t e r i c l iqu idcrystals derived from cholesterolderivatives have negative diamag-netic anisotropy. It is, there-fore, interesting to examine theorientational behavior of suchm a t e r i a l s when mixed withpositive diamagnetic anisotropynemat i c l i q u i d c r y s t a l s .Deuteron NMR spectra of moleculessuch as CgDg, CD-jCN andCDC1j d i s s o l v e d in suchmixtures have been investigated.The influence of the addition ofthe nematic liquid crystals suchas N-(p-methoxy benzylidene-p'-n-butylaniline (MBBA), trans-4-pentyl-4 (4- cyano-phenyl)-cyclohexane (S-1114) and aternary eu tec t ic mixture ofpropyl-, pentyl- and heptyl-b icyc lohexyl carboboni t r i le(ZLI-1167) on the deuteron NMRspectra of the molecules listedabove dissolved in the compen-sated mixtures of 1: 0.74 weightratio of cholesteryl chloride tocholesteryl nonanoate has beenexamined. Up to about 30 weightpercent of MBBA, only onequadrupole split doublet of thedissolved probe molecule isobserved. Around 30 weightpercent and above, two quadrupolespl i t doublets start appearing.Though the doubling of thequadrupole doublets is alsoobserved in mixtures with S-1114beyond a certain concentration,no such effect was observed forZLI-1167. However, addition ofZLI-1167 to the mixture con-taining compensated cholesterylmixture and MBBA sharpens boththe doublets . The r e su l t sindicate two domains in themixtures beyond about 30% of MBBA(Figure 2).

Vol. 8, No. 3/4 161

CH3PCI2

Ji 298K

J L l 1 JUL 299K

JUL 301K1350 0

Hz

1-1350

Figure 1: Proton NMR spectra of 7 weight percent solution of CH3PCI2oriented in a 1 : 1 weight percent of S-1114 and ZLI-1167

Spectrometer Frequency: 270 MHzTemperature: as indicated in the figureNumber of Scans: 1500

2500Hz

41.44 MHz deuteron NMR spectrum ofCQDQ in a mixture of cholesteryl chloridecholesteryl nonanoate : MBBA in theweight ratio of 1.0 : 0.73 : 0.73.

Solute Concentration: 2.7 weight percentTemperature : 17°C

41.44 MHz deuteron NMR spectrum ofCDCI3 in a mixture of ZLI-1167,cholesteryl chloride, cholesterylnonanoate and MBBA in weight ratio of1.0 : 0.075: 0.055 : 0.43.

Solute Concentration : 4.6 weight percentTemperature : 20°C.

Figure 2

162 Bulletin of Magnetic Resonance!

SPECTRA OF NEMATIC-NEMATICLIQUID CRYSTALS THEMSELVES

The ob j ec t i ve of such s t u d i e sis 2-fold:

(1) The Landau theory developed(10-12) for the behavior of mixedl iquid crystals of opposite-diamagnetic anisotropies is basedon the study of the orientationalbehavior of the liquid crystalsand was tested by experimentalresu l t s from studies of thedissolved molecules assuming thatthe solute orientation followsthe solvent orientation. Inorder to test the assumptions,2H-NMR study of MBBA-d2 andZLI-1167 mixtures without adissolved molecule was undertaken(25). The results show twoquadrupole-split doublets at the"critical" point where both theor ienta t ions "coexist". Theexperiments were performed on aJEOL FX-100 s p e c t r o m e t e roperat ing at 15.35 MHz fordeuterons and using a conven-tional electromagnet. A spinningof the sample around the axisorthogonal to the magnetic fieldresulted in the disappearance ofthe doublet corresponding to theor ienta t ion where the liquidcrystal director is preferen-t i a l l y a l i g n e d along thedirection of the magnetic field(Figure 3). The assumption thatthe solute molecules follow theorientation of the liquid crystalis , therefore, justified asestablished by this experiment.

(2) The 13C-NMR spectra of theliquid crystals MBBA, S-1114 andZLI-1167 were studied in thenatural abundance of C withhigh power proton decoupling and.cross polarization, on a BrukerMSL-300 spectrometer (26). Thespectra near the "critical" pointin the mixtures of l iqu idcrystals of opposite-diamagnetic

anisotropies were also inves-tigated. The appearance of thetwo spectra (Figure A) near thec r i t i c a l point permits theevaluation of the C-Chemicals h i f t a n i s o t r o p y of theindividual carbons in liquidcrystals without the use of areference compound or the changeof experimental conditions as forthe solute molecules. Though theexperiments have been completed,assignments of the lines to theindividual carbons has s t i l l tobe made. Though the earlier work(27, 28) on the assignments ofthe C-carbon lines may behelpful for the assignments whenthe l iquid crys ta l directoraligns preferentially along themagnetic field direction, theassignments for the alignment ofthe director orthogonal to themagnetic field has s t i l l to bemade. The C-resonance forthe cyanide carbon has beenunambiguously assigned and a

ilue of 350 ppm for theJC-chemical shift anisotropy

der ived. More experimentsleading to the assignments are inprogress.

CONCLUSIONS

The results reported in thispaper clearly demonstrate thepotentials of using more than oneliquid crystals or the mixedliquid c rys ta l s of oppositediamagnetic anisotropies leadingto novel applications of NMR.

ACKNOWLEDGEMENT

I am grateful to Dr. E.D.Becker for critical discussions.

If}

Vol. 8, No. 3/4 163

23° C

20 KHzProton decoupled deuteron NMRspectra of 0.3 : 1 weight ratio of MBBA-d2 : ZLI-1167 at 23, 24 and 25°C.

Figure 3

5000 -5000

20 KHzProton decoupled deuteron NMRspectra of 0.3 : 1 weight ratio of MBBA-d2 : ZLI-1167 at 24°C in a conventionalelectromagnet. Upper trace is withsample spinning and the lower one iswithout spinning of the sample.

o

n)co

bo

oc

303

Figure 4:13C-NMR spectrum of a mixture ofS-1114 and ZLI-1167 with broad bandproton decoupling at 289.9K where thespectra due to two orientations of theliquid crystal "coexist."

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