5 MARCH 1993
`VOL. 259 • PAGES 1369-1508
`
`$6.00
`
`IOOOI AN
`
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`£60I W £6/~2/60 SVaLLS9llO~ooooa
`IOOOI lISIO-S lllOOEIOllllllllElllElOOElllE SN:JJNX13a
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`Samsung Ex. 1030
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`ISSN 0036-8075
`5 MARCH 1993
`VOLUME 259
`NUMBER 5100
`
`SciENCE
`
`AMERICAN
`ASSOCIATION FOR THE
`ADV AN CEMENT OF
`SCIENCE
`
`NEWS & COMMENT ___
`
`1
`
`Bernadine Healy Bows Out
`Healy Highlights
`
`1388
`
`Biomedical Research: Animal Regulations 1 389
`Overturned
`
`Tropical Deforestation: Not Just a
`Problem in Amazonia
`
`Gene Therapy: A Speeding Ticket for
`NIH's Controversial Cancer Star
`
`1390
`
`1391
`
`Space Wars Begin to Take a Toll at UCSF 1392
`
`Gene Linked to Lou Gehrig's Disease
`
`1393
`
`What Might Cause Parasites to
`Become More Virulent
`
`Long Search for Sea Urchin Sperm
`Receptor Pays Off
`
`Mathematics: If You're Stumped,
`Try Something Harder
`
`~ 1402
`
`P"' 14 3
`
`14u4
`
`PERSPECTIVE
`
`Cells in Stress: Transcriptional Activation 140
`of Heat Shock Genes
`R. I. Morimoto
`
`ARTICLES
`
`1402 & 1442
`Parasitic virulence varies
`with host population
`structure
`
`SPECIAL REPORT
`
`Toxicology Goes Molecular
`A Breath Test for Cancer?
`Hot Field: Neurotoxicology
`
`Ken Olden Heals NIEHS's 'Split Brain'
`
`1398
`
`RESEARCH NEWS
`
`Did Venus Hiccup or Just Run Down?
`
`1400
`
`Radiative Climate Forcing by the Mount
`Pinatubo Eruption
`P. Minnis, E. F. Harrison, L. L. Stowe, G. G. Gibso
`F. M. Denn, D. R. Doelling, W. L. Smith, Jr.
`
`141 1
`
`1394
`
`Molecular Matchmakers
`A. Sancar and J. E. Hearst
`
`RESEARCH ARTICLE
`
`Sea Urchin Egg Receptor for Sperm:
`Sequence Similarity of Binding
`Domain and hsp70
`K. R. Foltz, J. S. Partin, W. J. Lennarz
`
`141 5
`
`-,
`
`P" 1421
`
`DEPARTMENTS
`
`1377
`
`THIS WEEK IN SCIENCE
`EDITORIAL
`Basic Research (III): Priorities
`LETTERS
`1381
`Pondering Greenhouse Policy: S. H. Schneider;
`H. Dowlatabadi and L. B. Lave; M. Oppenheimer;
`W. D. Nordhaus • Priority Envy: J. L. Sutton
`SCIENCESCOPE
`1387
`
`1379
`
`1405
`
`RANDOM SAMPLES
`BOOK REVIEWS
`1473
`The Origins of Agriculture, reviewed by J. McCorriston
`• The Savage Within, F. Spencer• Natural Selection,
`S. C. Stearns • The History and Development of
`Human Genetics, A.G. Motulsky • Vignettes: Highs
`and Lows • Books Received
`PRODUCTS & MATERIALS
`
`1479
`
`AAAS Board of Directors
`
`Board of Reviewing Editors
`
`F. Sherwood Rowland
`Retiring President,
`Chairman
`Eloise E. Clark
`President
`Francisco J. Ayala
`President-elect
`
`Robert A. Frosch
`Fl~rence P. Haseltine
`W1ll1am A. Lester, Jr.
`
`1374
`
`Alan Schriesheim
`Jean'ne M. Shreeve
`Chang-Lin Tien
`Warren M. Washington
`Nancy S. Wexler
`
`William T. Golden
`Treasurer
`Richard S. Nicholson
`Executive Officer
`
`John Abelson
`Frederick W. Alt
`Don L. Anderson
`Stephen J. Benkovic
`David E. Bloom
`Floyd E. Bloom
`Henry R. Bourne
`JamesJ. Bull
`Kathryn Calame
`C. Thomas Caskey
`Dennis W. Choi
`
`John M. Coffin
`Bruce F. Eldridge
`Paul T. Englund
`Richard G. Fairbanks
`Douglas T. Fearon
`Harry A. Fozzard
`Victor R. Fuchs
`Theodore H. Geballe
`Margaret J. Geller
`John C. Gerhart
`Roger I. M. Glass
`
`Stephen P. Goff
`Corey S. Goodman
`Stephen J. Gould
`Ira Herskowitz
`Eric F. Johnson
`Stephen M. Kosslyn
`Michael LaBarbera
`Charles S. Levings Ill
`Harvey F. Lodish
`Richard Losick
`Anthony R. Means
`
`Mortimer Mishkin
`Roger A. Nicoll
`William H. Orme-Johnson Ill
`Stuart L. P1mm
`Yeshayau Pocker
`Dennis A. Powers
`Ralph S. Ouatrano
`V. Ramanathan
`Douglas C. Rees
`Erkki Ruoslahti
`Ronald H. Schwartz
`
`Terrence J. Sejnowsk1
`Thomas A. Steitz
`Richard F. Thompson
`Robert T. N. Tjian
`Emil R. Unanue
`Geerat J. Vermeij
`Bert Vogelstein
`Harold Weintraub
`Zena Werb
`George M. Whitesides
`OwenN. Witte
`Keith Yamamoto
`
`SCIENCE • VOL. 259 • 5 MARCH 1993
`
`Samsung Ex. 1030
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`

`

`COVER
`horizontal scale is time from May to November 1991 .
`zonally averaged latitudinal spreading of the increase
`Volcanic aerosols reflect some of the sun's energy
`(yellow, orange, and red) in reflected shortwave radia(cid:173)
`back to space; as a result, Earth's climate is cooled.
`tion measured by the Earth Radiation Budget Experi(cid:173)
`See page 1411. [Image : P. Minnis et al., Atmospheric
`ment after the eruption of Mount Pinatubo in June 1991 .
`Sciences Division, NASA Langley Research Center]
`The vertical scale is latitude from 40° S to 40°N, and the
`
`REPORTS
`
`Crystal Structure and Optical Properties
`of Cd32S14(SC6H 5 ) 36•DMF4, a Cluster
`with a 15 Angstrom CdS Core
`N. Herron,]. C. Calabrese, W. E. Fameth, Y. Wang
`
`1426
`
`Stable Compounds of Helium and Neon:
`He@C60 and Ne@C60
`M. Saunders, H. A. Jimenez-Vazquez, R. J. Cross,
`R. J. Poreda
`
`1428
`
`On the Application of the Minimal
`Principle to Solve Unknown Structures
`R. Miller, G. T. DeTitta, R. Jones, D. A. Langs,
`C. M. Weeks, H. A. Hauptman
`
`1430
`
`A 2000-Year Tree Ring Record of Annual 1433
`Temperatures in the Sierra Nevada Mountains
`L. A. Scuderi
`
`Export of North American Ozone
`Pollution to the North Atlantic Ocean
`D. D. Parrish, J. S. Holloway, M. Trainer,
`P. C. Murphy, G. L. Forbes, F. C. Fehsenfeld
`
`1436
`
`Fossilization of Soft Tissue in the
`Laboratory
`D. E.G. Briggs and A. J. Kear
`
`Population Structure and the
`Evolution of Virulence in Nematode
`Parasites of Fig Wasps
`E. A. Herre
`
`1439
`
`~ 1442
`
`Structure-Based Discovery of Inhibitors
`of Thymidylate Synthase
`B. K. Shoichet, R. M. Stroud, D. V. Santi,
`I. D. Kuntz, K. M. Perry
`
`1445
`
`Deficiency in Rhabdomyosarcomas
`of a Factor Required for MyoD Activity
`and Myogenesis
`S. J. Tapscott, M. J. Thayer, H. Weintraub
`
`1450
`
`Antagonism of Catecholamine Receptor
`Signaling by Expression of Cytoplasmic
`Domains of the Receptors
`L. M. Luttrell, J. Ostrowski, S. Cotecchia,
`H. Kendall, R. J. Lefkowitz
`
`1453
`
`THI and TH2 Cell Antigen Receptors
`in Experimental Leishmaniasis
`S. L. Reiner, Z.-E. Wang, F. Hatam, P. Scott,
`R. M. Locksley
`
`1457
`
`1460
`
`1463
`
`1466
`
`Inhibition of Human Colon Cancer
`Growth by Antibody-Directed Human
`LAK Cells in SCIO Mice
`H. Takahashi, T. Nakada, I. Puisieux
`
`Optical Time-of-Flight and Absorbance
`Imaging of Biologic Media
`D. A. Benaron and D. K. Stevenson
`
`Requirement for a GTPase-Activating
`Protein in Vesicle Budding from the
`Endoplasmic Reticulum
`T. Yoshihisa, C. Barlowe, R. Schekman
`
`TECHNICAL COMMENTS
`
`The Energy Density of Water and Ice
`Nucleation
`L. Wilen; M. Lahav, M. Eisenstein,
`L. Leiserowitz
`
`1470
`HTLV-1 Provirus
`and Mycosis Fungoides
`S. J. Whittaker and
`L. Luzzatto; A. Bazarbachi,
`F. Saal, L. Laroche,
`B. Flageul, J. Peries,
`H. de The; W. W. Hall
`
`1426
`A chip of a
`semiconductor
`
`~ Indicates accompanying feature
`
`(cid:127) SCIENCE (ISSN 0036-8075) Is published weekly on Friday, except the last week In
`December, by the American Association for the Advancement of Science, 1333 H Street,
`NW, Washington, DC 20005. Second-class postage (publication No. 484460) paid at Washington,
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`ment of Science. The title SCIENCE is a registered trademark of the AAAS. Domestic individual
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`foster scientific freedom and responsibility, to Improve the effectiveness of science in the promotion
`of human welfare, to advance education in science, and to increase public understanding and appre(cid:173)
`ciation of the importance and promise of the methods of science in human progress.
`
`SCIENCE • VOL. 259 • 5 MARCH 1993
`
`1375
`
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`

`

`• REPORTS
`
`Crystal Structure and Optical Properties of
`Cd32S14(SC6 H5 ) 36·DMF 4 , a Cluster with
`a 15 Angstrom CdS Core
`
`N. Herron,* J. C. Calabrese, W. E. Farneth, Y. Wang
`Recrystallization of the solid Cd 10S4 (SC6 H5 ) 12 from a solution of pyridine and N,N-di(cid:173)
`methylformamide (DMF) results in the formation of the cluster Cd32S14(SC6 H5b 6 ·DMF 4 as
`pale yellow cubes. The structure consists of an 82-atom CdS core that is a roughly spherical
`piece of the cubic sphalerite lattice -12 angstroms in diameter. The four corners of the
`lattice are capped by hexagonal wurtzite-like CdS units, which results in an overall tet(cid:173)
`rahedral cluster -15 angstroms in diameter. This cluster dissolves intact in tetrahydrofuran
`where its absorption spectrum reveals a sharp peak at 358 nanometers at room temper(cid:173)
`ature and its emission spectra show a strong broad band at 500 nanometers.
`
`The use of very small molecular clusters as
`synthetic precursors to bulk, extended solids
`interests both the physics (I) and chemistry
`(2) communities. In the particular case of
`semiconductor materials, the small clusters
`themselves are of interest, given that the
`transport and optical behaviors vary as a
`function of crystallitic size. These phenom(cid:173)
`ena have led to a whole area of research into
`so-called nanoclusters or quantum dots (3).
`In this size regime (ten to a few hundred
`angstroms in particle diameter), quantum
`effects lead to severe perturbations in elec(cid:173)
`tronic properties. The clusters act as exam(cid:173)
`ples of the quantum mechanical particle in a
`box (4). Although research in this area has
`been intense, almost all efforts have suffered
`&om the same problem: attempts are made
`to relate properties to cluster size, but cluster
`size is crudely defined. In most cases, only a
`distribution of sizes is accessible because of
`the synthetic routes that are used to prepare
`such clusters (4). Although a few well(cid:173)
`defined molecular clusters of this type do
`exist (5, 6), these tend to be on the small
`end of the size spectrum where the properties
`are more like those of the molecules than
`like those of the bulk.
`We report the single-crystal structure
`and optical properties of a large, well(cid:173)
`semiconductor molecular
`characterized
`cluster. It is a 15 A diameter crystalline
`fragment of bulk cubic CdS with an 82-
`atom core. This cluster contains all of the
`structural features of the bulk extended
`solid, yet it remains soluble in organic
`solvents and so retains many of the attrac(cid:173)
`tive properties (from a characterizational
`and processing viewpoint) of a discrete
`molecule.
`We reported previously (7) that when the
`discrete molecular species (NMe4)4Cd 10S4-
`(SC6H5) 16 (5) (Me, methyl) is heated un-
`
`Central Research and Development Department, The
`DuPont Company, Wilmington, DE 19880-0328.
`'To whom correspondence should be addressed.
`
`1426
`
`der carefully controlled conditions, the ma(cid:173)
`terial converts to bulk crystalline CdS. It does
`so in two distinct steps. First, at 250"C, it
`loses the tetramethylammonium cations and
`four of the thiophenolate "caps," which gen(cid:173)
`erates a solid, Cd 10S4(SC6H5) 12.-This step is
`followed by the loss, at 500°C, of the remain(cid:173)
`ing phenyl groups in the form of diphenyl
`sulfide which leaves behind crystalline CdS.
`intermediate composition, Cd 10S4-
`The
`(SC6H5) 12, is a pale-yellow solid with a broad
`x-ray diffraction pattern that indicates very
`small ( <25 A) sphalerite-phase (cubic) crys(cid:173)
`tallites of CdS. However, this solid is very
`soluble in pyridine, and such solutions can be
`recrystallized by the addition of DMF to the
`point of incipient precipitation. Large, clear(cid:173)
`yellow, cubic crystals formed (along with a
`small amount of yellow powder) during a
`period of several days at room temperature.
`The larger crystals appear t'o fracture when
`exposed to strong visible or ultraviolet (UV)
`light. The crystals are, therefore, routinely
`grown and handled under subdued lighting
`conditions. One such crystal was suitable for
`x-ray diffraction analysis.
`The solution of its structure (Fig. lA)
`reveals that recrystallization has led to the
`nucleation of a cluster with a stoichiometry
`ofCd32S 14 (SC6H5)36·DMF4 (8). This mol(cid:173)
`ecule is much larger than the original Cd 10
`cluster &om which it was prepared. It ap(cid:173)
`pears that this cluster assembled itself from
`CdS/SC6H5 species of various nuclearities
`that are in rapid exchange in the pyridine
`solution [ 113Cd nuclear magnetic resonance
`shows broad resonances in pyridine at all of
`the temperatures that we measured (7)].
`This cluster was apparently the least soluble
`species and therefore the one that preferen(cid:173)
`tially crystallized. The structural details re(cid:173)
`veal that the 82-atom core is constructed
`&om a large (-12 A diameter), roughly
`spherical chunk of the sphalerite form of
`bulk CdS whose requisite "dangling" sur(cid:173)
`face bonds have been terminated by wurtz(cid:173)
`ite-like (hexagonal) CdS units at four tetra-
`
`SCIENCE
`
`• VOL. 259
`
`• 5 MARCH 1993
`
`hedral comers. Thus, the cluster looks like
`a large tetrahedron whose points have been
`capped by DMF solvent molecules and is
`neutral in charge. The cluster core is -15
`A across (from the Cd atom at the vertex of
`the tetrahedron to the center of the oppo(cid:173)
`site tetrahedral face). All of the edges of the
`core are covered with phenyl rings of bridg(cid:173)
`ing and capping thiophenolate ligands. The
`structure is, in fact, a larger homolog of
`the large CdS cluster, Cd 17S4(SC6H5)2/-,
`which was prepared by Dance and co(cid:173)
`workers (6). It has all of the same structural
`features that are noted for that material
`(including the open clefts that run along
`each of the tetrahedral edges) except for (i)
`the capping of the Cd atoms that are at the
`vertices of the tetrahedron, which, in our
`case, is performed by DMF solvent but, in
`the structure of Dance and co-workers, is
`done by SC6H5 - units and (ii) the appear(cid:173)
`ance of four triply bridging sulfide ions in
`the centers of the tetrahedral faces. At the
`very center of the cluster is a ten-atom
`fragment with the same connectivity as bulk
`sphalerite-phase CdS with four tetracoordi(cid:173)
`nated cadmium and six tetracoordinated sul(cid:173)
`fide ions in an adamantyl arrangement.
`The Cd-S bond length in this core is
`2.503(4) A (where the number in paren(cid:173)
`thesis is the error in the last digit), which is
`significantly less than that found for bulk
`CdS (2.519 A). The individual clusters of
`the crystal structure interact only by means
`of nonbonded contact of the phenyl groups
`of the capping thiophenolate ligands, as
`depicted in the packing diagram (Fig. lB).
`One fascinating aspect of the packing struc(cid:173)
`ture is the presence of very large intercluster
`voids, which is similar to what is found in
`zeolites. Channels -8 A in diameter pro(cid:173)
`vide access to large spherical cavities -16
`A in diameter between the individual clus(cid:173)
`ters. These spaces appear to be essentially
`free of adsorbed solvent molecules.
`With such a well-defined molecular clus(cid:173)
`ter, we examined the intrinsic optical prop(cid:173)
`erties of a single-sized, 15 A quantum dot
`without ambiguities from size dispersion or
`poorly defined surfaces. On photoexcita(cid:173)
`tion, the crystals emit green light at -520
`nm (Fig. 2A). The excitation spectrum
`shows absorption bands at 325 and 384 nm
`with a weak shoulder at 435 nm (Fig. 2A).
`In pyridine solution, the optical spectra
`and emission properties are quite different
`from those of the solid and revert to the
`precursor material,
`the
`behavior of
`Cd 10S4(SC6H5) 12, in pyridine (7); this re(cid:173)
`sult is consistent with the view that pyri(cid:173)
`dine causes the rapid fracture of the cluster
`core into a dynamic mixture of species with
`lower nuclearities. The instability of clus(cid:173)
`ters of this type in coordinating solvents
`such as pyridine and DMF has been well
`documented (5). We propose that this is a
`
`Samsung Ex. 1030
`
`

`

`Fig. 1. (A) Crystal structure of Cd32S14(SC6H5) 36·DMF 4 core.
`All phenyl groups have been omitted for clarity, but their
`orientations with respect to the cluster are implied by the
`nonterminated stick bonds that protrude from the thiopheno(cid:173)
`late S atoms. The spheres represent Cd (green), sulfide S
`(yellow). thiophenolate S (red), and N (blue) atoms. Selected
`bond lengths (in angstroms with error in the last digit in
`parentheses) are Cd-S2- = 2.468(4) (triply bridging S in
`center of cluster's tetrahedral face), 2.503(4) (central ada(cid:173)
`mantyl core), 2.538(8), 2.532(4). and 2.537(5) A (second shell out from central core); Cd-SC6H5 = 2.495(5), 2.569(5), 2.546(5), 2.503(5), 2.554(5),
`and 2.560(5) A; Cd-N = 2.33(4) A. (B) Crystal packing diagram of Cd32S14(SC6H5 ) 36·DMF4 that shows four molecules at the corners of a face of the
`unit cell. The phenyl rings have been reduced to red hexagons, and the cores of the clusters are represented by green tetrahedra that are centered
`on the Cd atoms and defined by the attached S (yellow) and N (blue) atoms.
`
`result of the ability of the coordinating
`solvent to both compete with the surface(cid:173)
`capping thiophenolate ligands for the metal
`sites and to immediately passivate dangling
`bonds, which arise as individual cluster
`bonds break and fragments detach from the
`cluster surface.
`When the crystals are dissolved in tet(cid:173)
`rahydrofuran (THF), however ( they are
`sparingly soluble in this, as well as DMF and
`acetonitrile solvents), the solution emits at
`-500 nm (Fig. 2B). The excitation spec(cid:173)
`trum shows sharp absorption bands at 313
`and 366 nm, the same as those observed in
`the solid crystal except that they are shifted
`to higher energy by -18 nm. The similarity
`between the THF-solution and the solid(cid:173)
`state spectra indicates that the cluster dis(cid:173)
`solves intact into this solvent and, further(cid:173)
`more, in the crystal there exist only weak
`cluster-cluster interactions, which is consis(cid:173)
`tent with the cluster packing that is revealed
`in Fig. lB. The Cd32S14(SC6H5)36·DMF4
`solid should therefore be regarded as a molec(cid:173)
`ular crystal, similar to what has been found in
`the case of C60 fullerene crystals.
`The lowest absorption band of Cd32S14-
`(SC6H5)36·DMF4 dissolved in THF is locat(cid:173)
`ed at 358 nm (Fig. 2C). This transition has
`a large absorption cross section (the extinc(cid:173)
`tion coefficient is -84 ,500 M- 1 cm - 1).
`The position of the peak is insensitive to
`
`the effect of solvent polarity (from THF to
`acetonitrile), which
`indicates
`that
`the
`ground state or the corresponding excited
`state has a vanishingly small dipole mo(cid:173)
`ment. Both the large absorption cross sec(cid:173)
`tion and the vanishing dipole moment are
`signatures of an ideal quantum-confined
`"exciton state" (4), to use a bulk semicon(cid:173)
`ductor term. The possibility that this band
`is a result of charge transfer from cadmium
`to thiophenolate, which would yield a large
`excited-state dipole moment, can be elim(cid:173)
`inated on the basis of these data.
`In spite of the large oscillator strength of
`the 358-nm state, the luminescence spectrum
`is dominated by a low-lying excited state that
`emits in the green (Fig. 2B). There is a very
`efficient relaxation process that leads to the
`formation of this second, lower lying excited
`state. Previous research on small CdS parti(cid:173)
`cles (9) has often revealed such an emission
`band that is at a much lower energy than the
`absorption edge. This band has usually been
`loosely associated with defects largely because
`the samples have not been sufficiently defined
`to permit a more definitive description. This
`defect concept is no longer valid for our
`well-defined cluster, and such an emission
`band must be attributable to an intrinsic
`excited state of the cluster. This excited
`state has a very low oscillator strength (be(cid:173)
`cause it cannot be observed in the absorp-
`
`SCIENCE • VOL. 259
`
`• 5 MARCH 1993
`
`A
`
`384nm
`
`2x 107
`~ .;
`C .,
`£ 1 X 107
`
`2 X 107
`~ .;
`C :g 1 X 107
`
`BulkCdS
`
`350
`450
`550
`Wavelength (nm)
`
`650
`
`Fig. 2. (A and B) Excitation (solid line) and lumi(cid:173)
`nescence (dotted
`line) spectra of Cd32S, 4-
`(SC6H5)36·DMF 4 cluster (A) as polycrystalline sol(cid:173)
`ids at 6.5 K and (B) in THF i:11 77 K. The spectra
`were taken with a reflection geometry. The exci(cid:173)
`tation wavelength for the emission spectra was
`320 nm, and the monitoring wavelength for the
`excitation spectra was 520 nm. (C) Absorption
`spectrum of Cd32S14 (SC6Hs)36·DMF4
`in THF at
`room temperature compared with the bulk CdS
`absorption spectrum. All
`three graphs were
`scaled arbitrarily.
`
`1427
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`

`tion spectrum), yet it dominates the excited(cid:173)
`state properties. It has a broad emission band
`without any detectable vibronic structure
`even at 6 K. All of these properties are
`characteristic of a charge-transfer state.
`Questions about the nature of this excited
`state and how it evolves with increasing
`cluster size remain to be addressed.
`Finally, the solubility of this new crystal(cid:173)
`line cluster may be used in the production of
`polymer films, such as polyvinylcarbazole,
`doped with the CdS cluster simply by spin(cid:173)
`coating from a pyridine solution. Such films
`have been found to exhibit good photocon(cid:173)
`ductive properties (10), which points to a real
`utility for such soluble semiconductor clusters
`in the preparation of materials that go into
`electronic devices made for this and other
`thin-film applications.
`
`REFERENCES AND NOTES
`
`1. S. Datta, J. K. Furdyna, A. L. Gunshor, Superlat(cid:173)
`tices Microstruct. 1, 317 (1985); L. E. Brus, J.
`Chem. Phys. 80, 4403 (1984) .
`2. C. N. Rao and J. Gopalakrishnan, New Directions
`in Solid State Chemistry (Cambridge Univ. Press,
`Cambridge, 1986), chap. 3; L. Spanhel and M.
`Anderson, J. Am. Chem. Soc. 113, 2826 (1990) .
`3. C. R. Berry, Phys. Rev. 161 , 848 (1967); M. L.
`Steigerwald and L. E. Brus, Acc. Chem. Res. 23,
`183 (1990); A Henglein, Top. Curr. Chem. 143, 113
`(1988); Y. Wang, N. Herron, W. Mahler, A Suna, J.
`Opt. Soc. Am. B. 6, 808 (1989).
`4. Y. Wang and N. Herron, J. Phys. Chem. 95, 525
`(1991).
`
`5. I. G. Dance, A Choy, M. L. Scudder, J. Am.
`Chem. Soc. 106, 6285 (1984); I. G. Dance, Aust.
`J. Chem. 38, 1745 (1985).
`6. G. S. H. Lee, D. C. Craig, I. Ma, M. L. Scudder, T.
`D. Bailey, I. G. Dance J. Am. Chem. Soc. 110,
`4863 (1988) .
`7. W. E. Farneth, N. Herron, Y. Wang, Chem. Mater.
`4, 916 (1992) .
`8. A light-yellow, clear cubic block, 0.39 mm on a
`side, was mounted on an Enraf-Nonius (Delft,
`Holland) CAD4 diffractometer (graphite mono(cid:173)
`chromatized MoKa radiation, wavelength of
`o. 71069 A) and cooled to - 70°C. The cubic unit
`cell parameter was determined from the Bragg
`angles of 25 computer-centered reflections as a
`= 21 .856(2) A. The space group was assigned as
`P23 (No. 195), and the structure later confirmed
`this assignment. We collected 12,924 data points,
`of which 1,672 were unique with I ;;,, 3.0o(/) ,
`where I is intensity. The structure was solved by
`direct methods and was refined by full-matrix
`least-squares techniques with scattering factors
`from International Tables for X-ray Crystallogra(cid:173)
`phy (Mathematical Tables Series, Kluwer Aca(cid:173)
`demic, Norwell, MA, 1974), vol. IV, including the
`anomalous terms for Cd and S. The final refine(cid:173)
`ment of 239 parameters (data to parameter ratio
`= 6.91 ), in which Cd, S, N, and C were anisotropic
`and H was fixed, converged at R = 0.042 and ~
`= 0.039. The single molecule for each unit cell
`resides on a site of T symmetry.
`9. N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc.
`112, 1322 (1990).
`10. Y. Wang and N. Herron, Chem. Phys. Lett., in press.
`11. DuPont Contribution 6304. Supplementary data
`(atomic coordinates, bond lengths, and angles)
`are available on request. Atomic coordinates will
`also be deposited with the Cambridge crystallo(cid:173)
`graphic data files . We thank J. B. Jensen and S. J.
`Harvey for technical assistance and W. J. Mar(cid:173)
`shall for x-ray manipulations.
`
`31 August 1992; accepted 4 December 1992
`
`Stable Compounds of Helium and Neon:
`He@C60 and Ne@C60
`Martin Saunders, Hugo A. Jimenez-Vazquez, R. James Cross,
`Robert J. Poreda
`It is demonstrated that fullerenes, prepared via the standard method (an arc between graphite
`electrodes in a partial pressure of helium), on heating to high temperatures release 4He and
`3He. The amount corresponds to one 4He for every 880,000 fullerene molecules. The 3 He/4He
`isotopic ratio is that of tank helium rather than that of atmospheric helium. These results
`convincingly show that the helium is inside and that there is no exchange with the atmosphere.
`The amount found corresponds with a prediction from a simple model based on the expected
`volume of the cavity. In addition, the temperature dependence for the release of helium implies
`a barrier about 80 kilocalories per mole. This is much lower than the barrier expected from
`theory for helium passing through one of the rings in the intact structure. A mechanism involving
`reversibly breaking one or more bonds to temporarily open a "window" in the cage is proposed.
`A predicted consequence of this mechanism is the incorporation of other gases while the
`"window" is open. This was demonstrated through the incorporation of 3He and neon by heat(cid:173)
`ing fullerene in their presence.
`
`An intriging
`feature of buckminster(cid:173)
`fullerene is the hollow interior, which is
`large enough to enclose atoms. In contrast
`
`M. Saunders, H. A Jimenez-Vazquez, R. J. Cross,
`Department of Chemistry, Yale University, New Haven,
`CT06511 .
`R. J. Poreda, Department of Geology, University of
`Rochester, Rochester, NY 14627.
`
`1428
`
`with other molecules that bind ligands
`through noncovalent forces, buckminster(cid:173)
`fullerene and its higher homologs are like
`closed bottles. Atoms or small molecules
`can be contained and held without need for
`binding interactions. Smalley et al. have
`reported
`the preparation of metallo(cid:173)
`fullerenes (I). Although there was early
`
`SCIENCE • VOL. 259
`
`• 5 MARCH 1993
`
`discussion about whether the metals might
`be covalently attached to the outside (2),
`the present evidence indicates that there
`are metal atoms inside the carbon cages
`(3-6). Cram et al. (7) have suggested the
`name carcerands for closed-surface com(cid:173)
`pounds that trap atoms and molecules in(cid:173)
`side. Schwarz et al. have reported that
`collisions of C60 cations with 4He and 3He
`in a mass spectrometer produce ions which
`have the additional mass of helium (8, 9).
`In order to provide evidence that the heli(cid:173)
`um is inside, the ions were neutralized and
`then reionized with a four-sector tandem
`mass spectrometer. Helium retention was
`interpreted as indicating that the helium
`was inside (IO). It would be extremely
`difficult to recover macroscopic quantities
`of fullerenes containing helium or other
`rare gas atoms after formation of such ions
`in a mass spectrometer.
`How then could one prepare quantities
`of buckminsterfullerene containing helium?
`We considered the possibility that this had
`already been done. The common method of
`preparing bulk quantities of the fullerenes
`involves operating an arc between graphite
`electrodes in helium at about 150 torr. As
`each molecule closes, there is a chance that
`a helium atom will end up inside. How
`could one detect this helium? If enough
`were present, one might see it directly in
`the mass spectrum of buckminsterfullerene.
`However, the ion with four 13C atoms (and
`no helium) will generally be present. This
`ion has a mass so similar to the ion with no
`13C but with 4He that only extremely high
`resolution would resolve them. If only a
`small quantity were present, it would not be
`seen.
`Helium itself can be detected with ex(cid:173)
`tremely high sensitivity in a mass spectrom(cid:173)
`eter. One of us (R. J.P.) has been using an
`instrument designed
`to measure helium
`content in geological samples ( 11). A sam(cid:173)
`ple, after evacuation is heated to high
`temperature in an all-metal furnace and the
`released gases are analyzed. Helium and
`neon were measured with a VG 5400 noble
`gas mass
`spectrometer
`fitted with a
`Johnston electron multiplier with pulse(cid:173)
`counting electronics on the axial collector
`!see (I I) for details]. A resolution of 550
`(iim/m) achieved complete baseline separa(cid:173)
`tion of 3He+ from HD+. A "high side"
`Faraday cup with a resolution of 200 was
`used for the 4He measurement. Absolute
`abundances of 3He, 4He, and 22Ne were
`calculated by peak height comparison to an
`air standard of known size (0.101 cm3
`standard temperature and pressure) and are
`accurate to ±3%. A split of the gas was also
`measured for 40 Ar abundance on a Oycor
`quadrupole mass spectrometer by peak
`height comparison with the air standard.
`The small amount of 22Ne in all tempera-
`
`Samsung Ex. 1030
`
`