J. Phys. Chem. 1983, 87, 2268-2271
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Cryochemical Studies. 7. Electron Spin Resonance Spectrum of Ag,' James A. Howard;
Roger Sutcllffe,
National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9
and Brynmor Mlle" Department of Chemistry and Biochemistry, Liverpool Polytechnic, Liverpool, England L3 3AF (Received: November 17, 1982; I n Final Form: April 20, 1983)
The neutral silver cluster lo7Ag5has been prepared in several inert matrices in a rotating cryostat at 77 K and its ESR spectrum recorded. The spectrum is anisotropic and can be analyzed in terms of AIl(2)= 594.5 MHz, A , (2) = 587.5 MHz, gll= 2.002, and g, = 2.085. The superhyperfiieinteractions associated with the perpendicular features of the spectrum give A1,(2) = 5.5 G and A1&) = 11G. This transition-metalpentamer has a distorted trigonal-bipyramidal structure with a 2B2 in CZuelectronic ground state. The SOMO has 5s and 4p and/or 4d orbital contributions of 0.6 and 0.2-0.3, respectively, from two equatorial Ag nuclei. The unique equatorial and axial Ag nuclei have a small percentage of negative unpaired 5s spin population.
Introduction There is increasing interest in the preparation and characterization of small homonuclear metal atom clusters because they appear to have unusual potential as catalysts, usage in chemical synthesis, and allow the study of metal-metal bonding and structure., Small silver clusters have received much attention both theoretically and experimentally because of the ease with which this metal aggregates. Several theoretical approaches have been used for these clusters ranging from semiempirical models such as extended Huckel, modified CNDO, diatomics-in-molecules (DIM), and X a theory to ab initio molecular orbital t h e ~ r y . ~ -Matrix '~ isolation techniques have allowed the silver clusters Ag, ( n < 10) to be studied by UV-visible ~ p e c t r o s c o p y . ~ ~Unfortunately, ~~~-~~ the results require
(1) (a) Issued as NRCC 21152. (b) Part 6. See A. J. Buck, B. Mile, and J. A. Howard, J. Am. Chem. Soc., 105, 3381 (1983). (2) See, for example, "Metal Bonding and Interactions in High Temperature Systems", ACS Symp. Ser., No. 179 (1982). (b) 'Diatomic Metals and Metallic Clusters", Faraday Symp. Chem. SOC.,No.14 (1980). (3) R. C. Baetzold, J. Chem. Phys., 55, 4355 (1971). (4) R. C. Baetzold, J. Chem. Phys., 65,4363 (1971). (5) R. C. Baetzold, J . Chem. Phys., 68, 555 (1978). (6) J. W. Mitchell, Photogr. Sci. Eng., 22, 1 (1978). (7) M. R. V. Sahyun, Photogr. Sci. Eng., 22, 317 (1978). (8) M. R. V. Sahyun in "Growth and Properties of Metal Clusters", J. Bourdon, Ed., Elsevier, Amsterdam, 1980, p 379. (9) S. C. Richtameier, J. L. Gole, and D. A. Dixon, Proc. Natl. Acad. Sci. U.S.A., 77, 5611 (1980). (10) S. C. Richtsmeier, R. A. Eades, J. L. Gole, and D. A. Dixon, ref 2a, p 177. (11) S. C. Richtsmeier, D. A. Dixon, and J. L. Gole, J . Phys. Chem., 86, 3937 (1982). (12) G. A. Ozin, H. Huber, D. McIntosh, S. Mitchell, J. G. Norman, Jr., and L. Noodleman, J. Am. Chem. SOC.,101, 3504 (1979). (13) H. Basch, J . Am. Chem. Soc., 103, 4657 (1981). (14) G. A. Ozin, ref 2b, p 1. (15) S. A. Mitchell and G. A. Ozin, J. Am. Chem. Soc., 100, 6776 (1978). (16) S. A. Mitchell, G. A. Kenney-Wallace, and G. A. Ozin, J. Am. Soc., 103, 6030 (1981). (17) G. A. Ozin and H. Huber, Znorg. Chem., 17, 155 (1978). (18) W. Schulze, H. U. Becker, and H. Abe, Ber. Bunsenges. Phys. Chem., 82, 138 (1978). (19) W. Schulze, H. U. Becker, and H. Abe, Chem. Phys., 35, 177 (1978). (20) H. Abe, W. Schulze, and D. M. Kolb, Chem. Phys. Lett., 60, 208 (1979).
0022-3654/83/2087-2268$0 1.50/0
theoretical models or calculated electronic structures for their interpretation and assignment of absorptions. Similarly Raman spectroscopy has been applied to Ag, and AgPz1 Magnetic circular dichroism spectroscopic studies of Ag, and Ag, have been reported22and the atomization enthalpies measured by high-temperature mass spectrome t r ~ . , Several ~ ESR investigations of Ag atoms isolated in rare-gas matrices at low temperatures have been rep ~ r t e d . ~Recently, ~ - ~ ~ Ozin,,* using high concentrations of Ag in Ar and Kr and photoaggregation of matrices dilute in silver, obtained ESR spectra which were weak in Ag atoms and strong in anisotropic features centered at g 2. He concluded that the spectrum was a composite of two species, one consisting of sharp lines which were attributed to a range of Ag aggregates with molecular cluster properties and a broader ESR spectrum which was assigned to small silver microcrystallites. We recently reported the first positive assignment of the neutral silver trimer, Ag,, trapped in C6D6.29 It was suggested that this species is bent with a ,B2 (C2J ground state and not linear as was concluded from a Raman spectroscopic study.21 However, as Moskovits has pointed out the Raman evidence is not conclusive because many CZumolecules have very weak asymmetric stretches in the Raman. Moreover, the bend (vz) of Ag3 is expected to be a very low-frequency mode, perhaps lower than one can feasibly detect in a matrix Raman e ~ p e r i m e n t . ~Thus, ~ apart from the above deductions on Ag,, the only other experimental structural characterization for uncharged silver clusters, Ag, ( n > 21, is that of a "probable" uncharged Ag, cluster trapped in a Ag+-exchanged zeolite.31 I t should, however, be noted that Symons has identified Ag2+,Ag:+, and Ag43+by ESR spectroscopy in y-irradiated frozen s o l u t i ~ n s and ,~~~~~
-
(21) H. U. Becker, K. Manzel, R. Minkwitz, and W. Schulze, Chem. Phys. Lett., 55, 59 (1978). (22) R. Grinter, S. Armstrong, U. A. Jayasooriya, J. McCombie, D. Norris, and J. P. Springall, ref 2b, p 94. (23) K. Hilpert and K. A. Gingerich, Ber. Bunsenges. Phys. Chem., 84, 739 (1980). (24) P. H. Kasai and D. McLeod, Jr., J. Chem. Phys., 55,1566 (1971). (25) P. H. Kasai and D. McLeod, Jr., J . Am. Chem. Soc., 97, 6602 (1975). (26) P. H. Kasai and D. McLeod, Jr., J . Am. Chem. Soc., 100, 625 (1978). (27) P. H. Kasai, D. McLeod, Jr., and T. Watanabe, J. Am. Chem. Soc., 102, 179 (1980). (28) G. A. Ozin, J. Am. Chem. Soc., 102, 3301 (1980). (29) J. A. Howard, K. F. Preston, and B. Mile, J . Am. Chem. Soc., 103, 6226 (1981). (30) M. Moskovits and D. P. DiLella, ref 2a, p 153. (31) Y. Kim and K. Seff, J . Am. Chem. Soc., 99, 7055 (1977). Published 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 13, 1983
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C
V =9150.6 MHz. I
-0
""
'
Figure 2. Expanded scale detail of lo7Ag,absorptions in CBDlPshowing hyperfine splitting detail for (a) low-field, (b) center-field (at a lower receiver gain), and (c) high-field lines. 3250G
'J
Figure 1. Specbum of '07Ag5in deuteriocyclohexane at 77 K following careful annealing to remove the majority of other less stable species.
I
d\
v=9145.2MHz
1 1
n
I 1 I
I
lOOG +
several silver clusters, containing Ag,' and Ag,' have been chara~terized.~"~~ We now report the preparation of the neutral silver cluster Ag, in a rotating ~ r y o s t a t and ~ ' ~ the ~ deduction of its geometry and probable electronic ground state by ESR spectroscopy.
Experimental Section The experimental technique used to deposit metal atoms from a small high-temperature furnace onto a continually II renewed frozen matrix deposited from a previous jet onto Flgure 3. Spectrum of Io7Ag5In adamantane at 77 K after annealing. the surface of a rotating drum at 77 K has been described p r e v i o ~ s l y . ~For ~ the present experiments light from a by using a computer program made available by Drs. J. 250-W extrahigh-pressure mercury lamp (A > 320 nm, R. Morton and K. F. Preston (NRCC). using a Corning 0-52 filter) was focussed on the surface layer between the silver and matrix material (adamantane, Results and Discussion benzene, or cyclohexane) jets. Thus the silver atoms were ESR spectra obtained at 77 K from Io7Ag,photolyzed irradiated continuously on the new surface of the deposit immediately after deposition, in any of the matrix supports before being covered by the next layer of inert matrix. In are complex and contain transitions from several silver this respect the present in situ photolysis is significantly species. The best spectral resolution is obtained in C6DI2 different from those where samples are irradiated after and the spectrum that is assigned to Ag, is present at this deposition has been completed and with the metal atom temperature. I t is possible, with careful annealing, to trapped in the bulk matrix rather than on the s u r f a ~ e . ~ ? * ~remove the transitions from Ag atoms and Ag3 irreversibly Isotopically pure Ag (98.22% lo7Ag)was purchased from from the overall spectrum leaving the spectrum shown in Oak Ridge National Laboratory (Oak Ridge, TN) and used Figure 1. This spectrum consists principally of a triplet in these experiments because of the anticipated complexity of multiplets suggesting two equivalent Ag nuclei (I= 'I2). of the spectrum from natural silver. PerdeuteriocycloAn exact solution of an isotropic spin Hamiltonian using hexane and perdeuteriobenzene were obtained from Merck M I = f l components of the triplet gives the following Sharpe and Dohme, Canada Ltd. and adamantane, cyparameters: A1&) = 204.6 G and g = 2.082 (7). The clohexane, and benzene from Aldrich. expected line intensities for the triplet (1:2:1) is not obDeposits were removed from the drum at 77 K and tained because the lowest field line is asymmetric while transferred to a tube suitable for ESR investigation. ESR the central line is broader than the other two lines. Closer spectra were recorded on a Varian E-4 spectrometer. The examination of the central feature reveals that it is an microwave frequency was measured with a Systron-Donner overlapping doublet of quintets with a separation of the Model 6057 frequency counter and the magnetic field with quintets of -13 G which is in good agreement with the a Varian E-500 NMR gaussmeter. ESR parameters were second-order splitting43expected from two nuclei with Z calculated from exact solutions of the spin H a m i l t ~ n i a n ~ ~ = ' I 2and equivalent hyperfine interactions of 204.6 G. This confirms the assignment of the triplet to two Ag nuclei with equivalent and large hyperfine interactions. (32)C.E.Forbes and M. C. R. Symons, Mol. Phys., 27,467 (1974). The line shape of the MI = 1 component indicates a (33)D. R. Brown, T. J. V. Findlay, and M. C. R. Symons, Trans. randomly oriented species with anisotropic g and/or hyFaraday SOC.,72,1792 (1976). (34)R. Hesse and L. Nileon, Acta Chem. Scand., 23,825 (1969). perfine coupling tensors. The parameters given above are, (35)P. J. Birker and G. C. Verschoor, J. Chem. SOC.,Chem. Commun., therefore, almost certainly the perpendicular components 322 (1981). of an axially symmetric spin Hamiltonian. Unfortunately, (36)H. Dietrich, W. Storck, and G. Manecke, J. Chem. SOC.,Chem. Common., 1036 (1982). the parallel features cannot be assigned unambiguously (37)J. E.Bennett and A. Thomas, R o c . R. SOC.London, Ser. A, 280, in C6DI2. 123 (1964). ~~~-~ The superhyperfine interactions associated with each (38)J.E.Bennett, B. Mile, A. Thomas, and B. Ward, Adu. Phys. Org. Chem., 8, 1 (1970). line of the triplet are shown in expanded scale in Figure ~~~
(39)A. J. Buck, B. Mile, and J. A. Howard, J. Am. Chem. SOC.,in press. (40)G.A. Ozin, S. A. Mitchell, and J. Garcia-Prieto, Angew. Chem., Int. Ed. En& 21,381 (1982). (41)P. H. Kasai, J. Am. Chem. SOC.,104,1165 (1982).
(42)A. R. Boate. J. R. Morton, and K. F. Preston. J . M a m . Reson.. 24,259 (1976). (43)R.W. Fessenden, J. Chem. Phys., 37,747 (1962).
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2. They do not depend on the nature of the matrix (C&2 or C6H12) and must be associated with further silver nuclei. The best fit between observed and simulated spectra is obtained for three additional Ag atoms, two being equivalent with AlO7(2)= 5.5 G and a unique one having approximately twice the hyperfine interaction (hfi) of the other two, i.e., AlO7(1)= 11 G. We therefore assign the spectrum shown in Figure 1 to the neutral silver cluster Ag,. A simpler spectrum of Ag, is obtained in adamantane after annealing and recooling (Figure 3). Thus the superhyperfine interactions are not resolved and the parallel components of the anisotropic spectrum appear to be resolved. Analysis of this spectrum gives A,, = 594.5 MHz (212 G), A, = 587.5 MHz (201 G), g,, = 2.002, and gL = 2.085. The superhyperfine structure of Ag5 in CJI1, is gradually lost on warming (to as high at 240 K) and the signal intensity diminishes. The signal returns on cooling the sample and very little of the loss of intensity on warming is the result of irreversible decay. The reversible decrease and increase of the signal intensity on raising and lowering the temperature suggests some dynamic motion or matrix orientation effect and is under further study. It is worth noting that there is no evidence for a spectrum that can be assigned to Ag5 with five equivalent Ag atoms analogous to the dynamic Jahn-Teller molecule observed for K3.44 The fact that the ESR spectrum of Ag, can be seen in samples that have not been photolyzed, albeit in lower yields, as well as samples that have been photolyzed leads us to conclude that we have observed the neutral Ag, and not the charged cluster Ag:'. The charge-transfer complex (Ag2)-(Ag3)+,a structure suggested by a referee, seems unlikely in view of the expected instability of (Ag,)- in a nonsolvating matrix such as C6DI2.Furthermore the singly occupied molecular orbital (SOMO) for such a complex would be largely that of an antibonding orbital constituted from the two 5s orbitals of the Ag, and would thus have much more than the 60% s character found experimently (see below). We are as yet unsure of the mechanism by which Ag, is formed; photolysis may serve to photoexcite Ag atoms and small Ag clusters present and produce further aggregation. On the other hand, it may cause matrix softening by a relaxation process45which causes high localized temperatures in the matrix allowing diffusion and reaction. Five structures are possible for Ag,: (I) linear (D-h), (11) pentagon ( D 5 h ) , (111) body-centered square (D4J; (IV) square pyramid (C4J, and (V) trigonal-bipyramid (DSh). Structures I11 and IV can be discounted since they would contain four equivalent Ag atoms. The flat symmetric pentagon structure would be expected to undergo JahnTeller distortion.G The expected unpaired spin populations would, however, be quite different from those observed. By analogy with calculations that have been performed on Cu, the central Ag atom in the linear (or zigzagged) structure is predicted to have the highest unpaired spin population followed closely by the two terminal Ag atoms with the other internal Ag atoms having much smaller values.47 Our observation of two equivalent Ag atoms with large hfi and three with small hfi is clearly at variance with the linear structure. We are, therefore, left (44) G . A. Thompson, F. Tischler, D. Garland, and D. M. Lindsay, Surf. Sci., 106, 408 (1981). (45) D. M. Kolb and D. Leutloff, Chem. Phys. Lett., 55 264 (1978). (46) H. A. Jahn and E. Teller, Proc. R. SOC.London,Ser. A , 161, 220 (1937). (47) C. Bachmann, J. Demuynck, and A. Veillard, ref Zb, p 170.
with the trigonal-bipyramidalstructure (V). Consequently, Ag, is isostructural with Cu5&which has a trigonal-bipyramidal structure with a,,@) = 608 G, a6,(2) = 15 G, a,(l) = 30 G, and g = 2.055. The values of All and A, for the two large Ag hfi can be converted to Aisoand Adipby using the formulae All= A&,,+ 2Adipand A, = Ak0- Adip.49If we assume that A,, and Adipare both positive Ai, = 592 MHz and Adip= 2.3 MHz and the Ag Mi is virtually isotropic. Using the atomic parameter for unit spin population in a 5s orbital of lWAgl5O we can calculate a 5s contribution (p,,) to the SOMO of -0.3 for each of the two Ag nuclei. The remaining three Ag nuclei have small and negative spin populations of -0.017 for the unique Ag nucleus and -0.008 for the other two Ag nuclei. These values of pb are very similar to values of pas for C U , . ~The ~ values of the atomic parameters for unit spin population in 5p and 4d orbitals on lo7Ag are 2P/5 = 21.6 MHz and 2P/7 = 15.4 M H Z . ~Clearly ~ a substantial p and/or d contribution to the SOMO will only produce a small value for Adiv Our value of Adipsuggests p5p = 0.1 or p4d = 0.15 and total spin counts close to 1. The average g factor of 2.057 for Ag, is similar to the value than we have reported for Cupa It is, however, quite possible that we only observed the perpendicular features of the anisotropic spectrum of Cu5 even though 2P/5 and 2P/7 are larger for Cu than Ag39and the spectrum appeared more isotropic than the spectrum from Ag,. If this is the case g,, for Ag, would be greater than g, for Cu5 which is consistent with the relative spin-orbit coupling constants of these two elements.49 The large and positive Ag for Ag, is a consequence of spin-orbit interaction between the ground state and a neighboring filled level which introduces p and/or d orbital character into the SOM0.49 The fact that g, > g,,does, however, favor a d rather than p orbital contribution. We have concluded,48 with the aid of semiempirical IND0/2 calculations, that the electronic ground state for Cu, is probably 2Bzin CZu.That is the equatorial atoms of a D3,, array of five copper atoms has undergone a Jahn-Teller d i ~ t o r t i o nby ~ ~opening up the equatorial triangle and that most of the 4s spin population is on the two equatorial atoms that have been forced apart, i.e. 5
'4 2
c.--lr" b5
2
8 L,
We were unable to perform semiempirical INDO/2 calculations on Ag,. We do, however, conclude, because of the similarities between the unpaired spin populations and g factors for Cu5 and Ag,, that they have the same structure and electronic ground state. It should, however, be noted that we cannot completely discount a 2B1ground state with most of the unpaired 5s spin population equally divided between the apical atoms 4 and 5. In conclusion, despite the somewhat tentative assignment in the detailed nature of the SOMO the ESR results allow the first definitive experimental delineation in favor (48) J. A. Howard, R. Sutcliffe, J. S. Tse, and B. Mile, Chem. Phys. Lett., 94, 561 (1983). (49) M. C. R. Symons, "Chemical and Biochemical Aspects of Electron Spin Resonance Spectroscopy", Wiley, New York, 1978. (50) J. R. Morton and K. F. Preston, J. Magn. Reson., 30,577 (1978).
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of a three-dimensional structure for a Ag, cluster and its characterization as a trigonal-bipyramid with Jahn-Teller distortion. Acknowledgment. We thank Drs. J. R. Morton and K. Preston for many helpful discussions and for making their computer programs available to us. We also thank
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Dr. J. S.Tse for helpful discussions regarding the theoretical aspects of this problem and Mr. J. H. B. Chenier for much technical assistance. We are r>articularlv indebted to a referee for several helpful comments espeiially those regarding the anisotropic nature of the spectrum. Registry No. Ag,, 64475-46-3.
