J. Phys. Chem. 1983, 87, 536-537
538
Electron Spin Resonance Spectrum of Matrix Isolated Cu,' James A. Howard,' KeRh F. Preston, Roger Sutcllffe, National Research Council of Canada, Ottawa, Ontarlo, Canada K1A OR9
and Brynmor Mlle Department of Chemlstry and Bkchemistty, Liverpool Polytechnic, Liverpool, England, L3 3AF (Received: November 10, 1982)
The neutral copper cluster, 63Cu3,has been trapped in adamantane at 77 K and its electron spin resonance spectrum recorded. It consists of 16 sets of quartets with as,@) = 625.5 G, a6,(1) = 55.6 G, and g = 1.9925. These parameters are consistent with a slightly bent structure with a 2B2(C2J ground state. There is considerable interest in the geometry and The copper electron configuration of small metal ~lusters.~ clusters Cu, (n = 2-4) have been observed by UV and visible absorption spectroscopy3 and the atomization enthalpies studied' by high temperature mass spectrometry for n < 3. Clusters with n < 7 have been produced by ion bombardment of copper surfaces, the analysis being performed by classical trajectory method^.^ More recently Cu, (n = 1-13) have been generated in molecular beams by a variety of evaporation techniques and observed by photoionization followed by time-of-flight mass spectrometric analysis and by laser fluorescence spectroscopy.6 Magnetic circular dichroism spectroscopy has been applied to copper clusters but at present gives little information.' Resonance Raman spectroscopy has also been reported for Cup8 Our knowledge of the properties of these small clusters does, however, remain rather limited at the experimental level, since these spectroscopic techniques give relatively little information about the detailed structure of such clusters. There have been many theoretical studies of copper clusters by a variety of method^.^ There is, however, little (1) (a) Issued as NRCC No. 20945. (b) Cryochemical Studies. 5. For part 3, see J. H. Chenier, J. A. Howard, B. Mile, and R. Sutcliffe, J.Am. Chem. SOC.,in press. (2) See for example (a) "Diatomic Metala and Metallic Clusters", Symp. Faraday SOC.,No. 14 (1980); (b) J. L. Gole and W. C. Stwalley, Ed., 'Metal Bonding and Interactions in High Temperature Systems", ACS Symp. Ser., No. 179 (1982). (3) M. Moskovits and J. E. Hulee, J. Chem. Phys., 67, 4271 (1977). (4) K. Hilpert and K. A. Gingerich, Eer. Bumenges. Phys. Chem., 84, 739 (1980). (5) B. J. Garrison, W. Winograd, and D. E. Harrison, Jr., J. Chem. Phys., 69, 1440 (1978). (6) (a) D. R. Preuss, S.A. Pace, and J. L. Gole, J. Chem. Phys., 71, 3553 (1979); (b) D. E. Powers, S. G. Hansen, M. E. Geusic, A. C. Pulu, J. B. Hopkins, T. G.Dietz, M. A. Duncan, P. R. R. Langridge-Smith, and R. E. Smalley, J. Phys. Chem., 86, 2556 (1982); (c) J. L. Gole, J. H. English, and V. E. Bondybey, Zbid., 86, 2560 (1982); (d) S. J. Riley, E. K. Parks, C. R. Mao, L. G. Pobo, and S. Wexler, Ibid., 86,3911 (1982). (7! R. Grinter? S. Armstrong, U. A. Jayasooriya, J. McCombie, D. Norris, J. P. Sprmgall, ref 2a, p 94. (8) M. Moskovits and D. P. DiLella, ref 2b, p 153. (9) (a) R. C. Baetzold and R. E. Mack, J.Chem.Phys., 62,1613 (1976); (b) R. C. Baetzold, J. Phys. Chem., 82, 738 (1978); (c) R. P. Messmer, S. K. Knudson, K H. Johnson, J. B. Diamond, and C. Y. Yang,Phys. Reu. E , 13,1396 (1976); (d) A. B. Anderson, J. Chem. Phys., 68, 1744 (1978); (e) C. Bachmann, J. Demuynck, and A. Veillard, Gazz. Chim. Ztal., 108, 389 (1978); (0 C. Bachmann, J. Demuynck, and A. Veillard in 'Growth and Properties of Metal Clusters", J. Bourdon, Ed., Elsevier, Amsterdam, 1980, pp 269-78. See also ref 2a, p 170; (9) J. Demuynck, M.-M. Rohmer, A. Strich, and A. Veillard, J.Chem. Phys., 75,3443 (1981); (h) D. Post and E. J. Baerends, Chem. Phys. Lett., 86,176 (1982); (i) P. A. Cox, M. Bernard, and A. Veillard, ibid., 87,159 (1982); 6)R. P. Messmer, T. C. Caves, and C. M. Kao, ibid., 90,296(1982); (k) H. Tatewaki, E. Miyoshi, and T. Nakamura, J.Chem. Phys., 76,5073 (1982); (1) S. C. Richtsmeier, D. A. Dixon, and J. L. Gole, J.Phys. Chem., 86,3937 (1982); (m) S. C. Richtsmeier, J. L. Gole, and D. A. Dixon, Proc. Natl. Acad. Sci. U.S.A., 77,5611 (1980); (n) S. C. Richtsmeier, R. A. Eades, J. L. Gole, and D. A. Dixon, ref 2b, p 177.
agreement regarding the results of these studies. Thus, for example, it has been predicted that Cu3 is or bentk" and that CQ is linearsd or a trigonal bipyratnid.*J We report here the first ESR identification of the neutral triatomic copper cluster, Cu3, produced at 77 K in a rotating cryostat,1° which allows the geometry and electronic ground state of this cluster to be confirmed experimentally. In our studies of the reaction of copper atoms codeposited with acetylene in an adamantane matrix at 77 K we observed weak absorptions in addition to the spectra of Cu atoms and Cu mono- and bis(acety1ene)complexes." After annealing the sample to 113 K, these weak features sharpened appreciably (Figure 1) and became more amenable to analysis. (The use of isotopically pure 63Cu throughout these experiments further facilitated the interpretation of the ESR spectra.) A spectrum identical with that of Figure 1 was generated by irradiating Cu atoms cocondensed with adamantane with light (A > 320 nm) from a 250-W high-pressure mercury lamp. This provides us with evidence that acetylene is not playing an essential role in stabilizing the paramagnetic species. The spectrum of Figure 1 is due to a single unpaired electron which shows equal, large isotropic hyperfine interactions with two equivalent %u atoms and a further, small interaction with a third '%u atom. Of the 16 groups of lines expected for two equal, large 63Cu-hyperfine interactions showing second-order splitting, four were obscured by other unassigned absorptions in the g = 2 region. The field centers of the remaining 12 groups were used in conjunction with the Breit-Rabi equationlZto obtain an exact solution of the isotropic spin Hamiltonian: gieo= 1.9925, a,,@) = 625.5 G. The average of the measured splittings within the quartets led to am(1) = 55.6 G. Thus, the ESR spectral data lead us to conclude that we have observed a cluster of three copper atoms, entirely analogous to the Ag, species reported earlier.13 Assignment to the charged species C U ~may + be discounted because of the two different methods used to produce the triatomic cluster. If we use the appropriate one-electron parameter for 63Cu,the isotropic hyperfine interactions in Cu3 may be converted to 4s unpaired spin populations of 29% for each (IO) (a) J. E. Bennett and A. Thomas, R o c . R. SOC.London, Ser. A , 280,123 (1964); (b) J. E. Bennett, B. Mile, A. Thomas, and B. Ward, Adu. Phys. Org. Chem., 8 , l (1970); (c) A. J. Buck, B. Mile, and J. A. Howard, J.Am. Chem. Soc., in press. (11) (a) P. H. Kasai, D. McLeod, Jr., and T. Watanabe, J.Am. Chem. SOC.,102, 179 (1980); (b) J. H. B. Chenier, J. A. Howard, B. Mile, and R. Sutcliffe, ibid., in press. (12) A. R. Boate, J. R. Morton, and K. F. Preston, J. Magn. Reson., 24. 259 (19761. '(13) J. A. Howard, K. F. Preston, and B. Mile, J. Am. Chem. SOC.,103, 6226 (1981).
0022-365418312087-0536$01.50/0Published 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983 537
Letters 0 v=91244MHZ
,
,"
2000
Figure 1. ESR sped" of %u, In adamantane at 113 K. The stick diagram indicates the predicted line positions based on the ESR parameters.
of the terminal copper atoms and 2.6% for the central copper atom. The 4s spin population on the terminal two copper a t o m of Cu, is, therefore, appreciably lees than the corresponding 5s spin population in Ag,.13 Similarly, the 4s spin population on the unique Cu atom is about onehalf of the 5s spin population on the unique Ag atom in Ag,. The reason for the reduced s spin density on the terminal Cu atoms of Cu, is not immediately apparent because the ESR spectrum appears to be isotropic, suggesting little p and/or d orbital contribution to the semioccupied orbital. It should, however, be noted that copper atoms weakly bonded to a variety of ligands (C2H2,CzH4,CsHa, and HCN)lQ1lJ*have Cu hyperfine interactions -60% of the value in inert hydrocarbon matrices such as cyclohexane (14) J. A. Howard, B. Mile, and R. Sutcliffe,manuscript in preparation.
and adamantane. The s spin density at the central atom is probably negative and is due to spin polarization effects. The lower percentage relative to Ag3 is probably associated with the lower spin density on the terminal atoms. The isotropicg shift of -0.01is not as large as the g shift for Ag3 (Ag = -0.04)but it is still significant. It is in fact very similar to the value for K31Sand a little less than the value for NaS.16 The negative shift suggests that the unpaired electron acquires orbital angular momentum by coupling with a low-lying vacant molecular orbital having p and/or d character. The less negative value of Ag for Cu, relative to Ag, reflects the difference in the spin-orbit coupling constants for Cu and Ag. We can conclude that Cu, has the unpaired electron largely located in valence 4s atomic orbitals on the terminal atoms with no positive spin density on the central atom. By analogy with Ag, the structure is probably slightly bent with a 2Bzground electronic state in CZvsymmetry. Our sample of Cu, decayed rapidly at 143 K and between 77 and 143 K there was no evidence for geometrical isomeri~ation.'~~J~ Registry No. Cu8, 66771-03-7; adamantane, 281-23-2. (15) (a) G. A. Thompson, and D. M. Lindsay, J. Chem. Phys., 74,959 (1981); (b) G. A. Thompson, F. Tmhler, D. Garland, and D. M. Lindsay, Surf. Sci., 106, 408 (1981). (16) (a) D. M. Lindsay, D. R. Herechbach, and A. L. Kwiram, Mol. Phys., 32,1199 (1976); (b) ibid., 39,529 (1980); (c) D. M. Lindsay and G. A. Thompson, J. Chem. Phys., 77,1114 (1982).
