UOLUME 51, NUMBER 13 PHYSICAL REVIEW LETTERS 26 SEPTEMBER 1983

Molecular Geometry of cis- and trans-Polyacetylene by Nutation NMR SpectroscopyC. S. Yannoni and T. C. Clarke

IBM Research Laboratory, San Jose, Califonzia 95193(Heceived 20 June 1983)

The carbon-carbon bond lengths in polyacetylene have been directly measured with useof nutation NMB spectroscopy. 1.36 and 1.44 A were found for the double and singlebonds, respectively, in trans-{CH), and 1.37 A was found for the double bond in thecps isomer. Over the temperature range 4.2-300 K, the dynamical single-double bondinterchange predicted for chains in trans-(CH)containigg a mobile soliton defect wasnot observed.PACS numbers: 72.15.Nj. 61.16.Hn, 61.65.+d, 76.60.-k

The bonding in linear polyenes has long been amatter of fundamental interest. ' The recent synth-esis of polyacetylene, ' the simplest long-chainpolyene, and the subsequent investigations of itstransport properties' have stimulated a numberof structural studies. ' Although the highly dis-ordered nature of as-grown polyacetylene filmshas prevented the direct determination of the bond-ing parameters in this material by the usual struc-tural characterization techniques, the existenceand magnitude of the bond alternation in the transmaterial have recently been inferred from x-rayscattering data obtained with use of streteh-aligned films. ' We wish to report here a directdetermination of the bond lengths in both cis- andtrans-(CH). ' These measurements, made onnonoriented samples, were carried out with useof nutation NMR spectroscopy, a method specifi-cally designed to measure interatomic distancesin amorphous solids or in materials which aredifficult to produce in single-crystal form. '

The main results of our study may be sum-marized as follows: (1) The double-bond lengthin cis-(CH) is 1.37 A; (2) spectra for the transisomer exhibit tzp0 bond lengths, 1.36 and 1.44Afor the double and single bonds, respectively, un-equivocally establishing bond alternation; (3) overthe temperature range 4.2-300 K, the trans spec-tra show no direct evidence of the dynamical inter-change of double and single bonds predicted tooccur as a result of rapid motion of neutral soli-ton defects in this material (see Pople and Walm-sley' and Su, Schrieffer, and Heeger').

In principle, the splitting due to the magneticdipole-dipole coupling in a "C NMR spectrumcontains the information necessary for the deter-mination of carbon-carbon bond lengths. ' Thissplitting varies inversely as the third power ofthe internuclear distance, and is manifested ina nonoriented sample as a Pake doublet with out-er shoulders. '" Nutation NMR spectroscopy can

be used to eliminate the effects of chemical-shiftanisotropy in the "C spectrum, which completelymask the Pake doublet. ' A description of the nu-tation experiment has been publ. ished previously, 'demonstrating that the method is capable of meas-uring bond lengths to l%%uo accuracy. Since themajor isotope of carbon ("C) is nonmagnetic, itis necessary to enrich with "C the two siteswhose internuclear distance is to be measured.To avoid broadening of the Pake doublet by long-range dipolar splittings, the "C spin pairs mustbe relatively isolated from other magnetic nuclei.We achieved this situation in polyacetylene bypolymerizing a mixture of 4'%%uo doubly-"C-enrichedacetylene in acetylene which has been doubly de-pleted in "C to provide a relatively spin-free hostmaterial. This produces a dilute, homogeneousdistribution of bonded "C pairs in the resulting(CH). Proton decoupling is used to remove di-polar broadening by the adjacent hydrogen nuclei.

Polyacetylene was prepared by the method ofIto, Shirakawa, and Ikeda. ' The isotopic label-ing required for these experiments was obtainedby mixing 4'%%uo of doubly-"C-enriched acetylene(~ 99 at. %%uo "CMSD Isotopes)withdoublydepletedacetylene (99.9'%%uo "C) before polymerization at195 K. The resulting cia films were packed intosample tubes under dry box conditions and thensealed under vacuum. cis samples were storedat 77 K until use to avoid isomerization to thetrans isomer. The A ass material was obtainedby heating in an evacuated sealed tube at 160 Cfor 1 h.

The proton-decoupled "C nutation spectrum ofa cps sample prepared in this manner is shown asthe solid curve in Fig. 1. The sharp peak in thecenter arises from isolated "C nuclei in the sam-ple. The symmetrically disposed satellites andshoulders are the features of a Pake doublet aris-ing from the "C-"C dipolar coupling of adjacent"C nuclei in the polyacetylene. With use of a

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VOLUME 51, NUMBER 13 PHYSICAL REVIEW LETTERS 26 SEPTEMBER 1983

1000 Hz

~~

~ ~ ~

~ ~

0

y ~ ~

~ ~

~~

~ ~

~~

~~

~ O~~

~

'f000 HzI=

FIG. 1. Solid line: proton-decoupled ' C nutationNMH spectrum at 77 K of doubly labeled cis-(CH).Dotted curve: simulation of nutation spectrum of cis-(CH)using a 1.37-A C-C bond length. The peak in themiddle is due to isolated '3C nuclei.

FIG. 2. Solid line: proton-decoupled '3C nutationNMR spectrum at 77 K of doubly labeled trans-(CH).Dotted curve: simulation resulting from superpositionof '3C nutation spectra using bond lengths of 1.36 and1.44 A. Again, the central peak is due to isolated "Cnuclei.

comprehensive simulation program, "this patterncan be fitted with the desired carbon-carbon bondlength as the only adjustable parameter. The bestfit to the observed spectrum corresponds to a dis-tribution of bond lengths with a mean value of1.37 A (see below); the simulation is shown asa dotted curve in Fig. 1. It is important to notethat a nutation spectrum attributable essentiallyto one carbon-carbon bond distance is obtainedfor the cis isomer Altho. ugh a Priori this resultcould be interpreted in terms of complete bondequalization in the cis-(CH), this explanation isunlikely for the cps isomer both on theoreticalgrounds" and in light of the failure to observebond equalization in the more favorable Sanscase (see below). Rather the observation of onlyone bond distance indicates that in the Ziegler-Natta polymerization, the carbons of a givenmonomer unit end up either singly or doubly bond-ed to one another, but not both. The observedbond length of 1.37 A strongly suggests that themechanism of the Ziegler-Natta reaction leavesthe original carbon pair doubly bonded in the re-sulting poly me r."

The situation becomes more complex on iso-merization of this sample to trans-(CH). Thenutation spectrum of trans-(CH)at 77 K is sig-nificantly different from that of its czs counter-part, as shown in Fig. 2. The doublet maximapeaks are now split and a step is observed in thewings of the Pake doublet. Both of these new fea-tures indicate two overlapping Pake doublets, andprovide unequivocal evidence for a system com-prised of not one but two carbon-carbon bonds of

different lengths. " The simulation (dotted curve)shown in Fig. 2 is a composite of two Pake pat-terns of comparable intensity corresponding tobond lengths of 1.44 and 1.36 A, which may beassigned to the single and double bonds, respec-tively, in trans-(CH). These bond lengths arein very close agreement with those predicted byGrant and Batra using the tight-binding approxi-mation to calculate the band structure of Aans-polyacetylene. "

The observation of both bond lengths clearlyconfirms the interpretation of previous x-raystudies in terms of a bond-alternated structure, 'and the bond-alternation parameter calculatedfrom our results is on the order of 0.03 A, inagreement with that derived by Fincher ef, al.However, it must be noted that if, as has beensuggested on the basis of Raman studies, "thereis a distribution of conjugation lengths in thismaterial, then one might also expect a distribu-tion of bond lengths to be observed. Indeed, wefind the features of the polyacetylene spectra tobe broadened considerably when compared to pre-vious nutation studies of aeetie acid" and to thenarrow linewidths obtained in polyacetylene bymagic-angle spinning techniques. " Thus, thebond lengths derived in the current experimentsdo correspond to the average values in a narrowdistribution of bond lengths.

The generation of approximately equal popula-tions of singly and doubly bonded labeled carbonpairs in trans-(CH)starting with only doubly

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VOLlJME 51, NUMBER 13 PHYSICAL REVIEW LETTERS 26 SEPTEMBER 1983

bonded pairs in the cis material is intriguing.Moreover, spectra of the ferns isomer are sub-stantially the same over the temperature rangefrom 4.2 to 300 K. Motion of neutral solitonsalong a polymer chain would lead to the inter-change of single and double bonds, but the obser-vation of two discrete Pake doublets requiresthat if the single and double bonds are dynamical-ly equilibrating, their rate of interconversionover this entire temperature range must be slowcompared to the difference in splitting of the twoPake doublets. Otherwise the nutation spectrumwould have shown a single Pake doublet charac-teristic of the average bond length. Since thesmallest observable difference in splitting is onthe order of 150 Hz, any dynamic interchangewould have to be occurring at a rate less than 1.5& 10' sec '. Given knowledge of the average con-jugation length in trans-(CH), the lack of co-alescence in the nutation spectrum could be usedto place an upper limit on the diffusion rate ofsoliton defects in this material. Unfortunately,reliable estimates of the conjugation length arenot yet available. Moreover, this analysis wouldbe complicated by the likelihood that many ormost of the conjugated segments in the trans iso-mer do not contain a mobile spin. "

We thank the U. S. Office of Naval Research forpartial support of this work. We are also grate-ful to R. D. Kendrick for his assistance in bothinstrumentation and implementation of the simu-lation program, and to H. -M. Vieth (Free Univer-sity, Berlin) for stimulating discussions and helpwith the nutation experiment.

'J. E. Lennard-Jones, Proc. Roy. Soc. London, Ser.A 158, 280 (1937); C. A. Coulson, Proc. Roy. Soc.London, Ser. A 169, 413 (1939); H. C. Longuet-Higginsand L. Salem, Proc. Roy. Soc. London, Ser. A 251,172 (1959); J. A. Pople and S. H. Walmsley, Mol. Phys.5. 15 (1962)

T. Ito, H. Shirakawa, and S. Ikeda, J. Polym. Sci.,Polym. Chem. Ed. 12, 11 (1974); H. Shirakawa andS. Ikeda, Polymer J. 2, 231 (1971).

3For reviews, see W. D. Gill, T. C. Clarke, andG. B. Street, in "Electrical Properties of Polymers, "edited by A. M. Herman (Dekker, New York, to be pub-

lished); S. Etemad, A. J. Heeger, and A. G. MacDiar-mid, Annu. Rev. Phys. Chem. 33, 443 (1982).

4R. H. Baughman, S. L. Hsu, L. R. Anderson, G. P.Pez, and A. J. Signorelli, Molecuhzr Metals, editedby W. E. Hatfield (Plenum, New York, 1979), p. 187;R. H. Baughman, S. L. Hsu, G. P. Pez, and A. J.Signorelli, J. Chem. Phys. 68, 5405 (1978).

'C. R. Fincher, Jr., C.-E. Chen, A. J. Heeger,A. G. MacDiarmid, and J. B. Hastings, Phys. Rev. Lett.48, 100 (1982).

6Preliminary accounts of these results have beenpresented in Proceedings of the Sixth InternationalConference on Chemistry of the Organic Solid State,Freiburg, 4-8 October 1982 (to be published), and inProceedings of the International Conference on theChemistry and Physics of Conducting Polymers, LesArcs 11-15December 1982 (to be published).

'C. S. Yannoni and R. D. Kendrick, J. Chem. Phys.74, 747 (1981).

W. P. Su, J. R. Schrieffer, and A. J. Heeger, Phys.Rev. Lett. 42, 1698 (1979), and Phys. Rev. B 22, 2099(1980).

~G. E. Pake, J. Chem. Phys. 16, 327 (1948).' D. E. Horne, R. D. Kendrick, and C. S. Yannoni, J.

Magn. Reson. 52, 299 (1983)."The implications of these results for the mechanism

of Ziegler-Natta polymerization will be presentedseparately; T. C. Clarke, C. S. Yannoni, and T. J.Katz, to be published.

' We have also considered the possibility that the twoPake doublets arise from the anisotropy of the elec-tron-mediated exchange coupling between the bondedcarbons (Jcc). We have shown, however, that theanisotropy in Jcc is negligible in our previous nutationstudies on acetic acid (Ref. 10). Moreover, simula-tions using reasonable estimates for Jcc [K. Zilm andD. Grant, J. Am. Chem. Soc. 103, 2913 (1981)I do notresult in any splitting. Finally, the fact that the cisspectrum is not split also suggests that the J anisotropyis not responsible for the splitting observed in the transma, terial.

'3P. M. Grant and I. P. Batra, Solid State Commun.29, 225 (1979), and Synth. Met. 1, 193 (1980).

'4H. Kuzmany, Phys. Status Solidi (b) 97, 521 (1980);D. B. Fitchen, Mol. Cryst. Liq. Cryst. 83, 95 (1982).

'5M. M. Maricq, J. S. Waugh, A. G. MacDiarmid,H. Shirakawa, and A. J. Heeger, J. Am. Chem. Soc.100, 7729 (1978); T. C. Clarke, J. C. Scott, and G. B.Street, to be published; P. Bernier, F. Schue, J. Sledz,M. Rolland, and L. Giral, Chem. Scripta 17, 151 (1981).

'6J. C. Scott and T. C. Clarke, to be published;M. Mehring, in Proceedings of the International Con-ference on the Chemistry and Physics of ConductingPolymers, Les Arcs, 1115 December 1982 (to bepublished) .

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