J . Phys. Chem. 1986, 90, 4239-4245
4239
Cu, Ag, and Au Atom-Molecular Oxygen Complexes: Matrix Isolation ESR Study Paul H. Kasai* and Paul M. Jones IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598 (Received: January 24, 1986)
ESR spectra of complexes generated between Cu, Ag, and Au atoms and oxygen molecules cocondensed in rare gas matrices were examined. Formations of mono(dioxygen) complexes, Cu(02), Ag(O,), and Au(Oz), and bis(dioxygen) complexes, C U ( O ~and ) ~ Ag(02),, were established, and their g tensors and the metal nuclear hyperfine coupling tensors were determined. Qualitative analyses of the spin Hamiltonian parameters indicate that the complexes are ionic pairs of the form M’(0,)and M+(02),-, but Cu(0,) and Ag(0,) have a bent end-on structure, whereas Au(0,) has a symmetric side-on structure.
Introduction Complexes formed between oxygen molecules and group 11 metal atoms (Cu, Ag, and Au) cocondensed in cryogenic matrices have been examined by many authors. Earlier, on the basis of IR spectra of various isotopic species, Ogden and his co-workers demonstrated the formation of a bis(dioxygen) complex, Cu(02),, in argon and krypton matrices.’ Later using a similar technique, Ozin and his co-workers showed that, while the Ag atoms form both the mono(dioxygen) and bis(dioxygen) complexes, Ag(02)1,2, the Au atoms form only mono(dioxygen) ~ o m p l e x . ~The - ~ formation of mono(dioxygen) Cu is certain, but its IR spectrum has not been as firmly established as for the other species. Recently Tevault et aL4 obtained IR spectral evidences showing nonequivalence of the two oxygen atoms in Ag(0,). Photoisomerization of C u ( 0 2 ) to a linear OCuO has also been reported by T e v a ~ l tand , ~ by Ozin et ale6 Here and throughout the text the notation (0,) is used to designate two oxygen atoms retaining essential integrity of the oxygen molecule within a complex. Cocondensation of alkali metal atoms and oxygen molecules in cryogenic matrices also leads to formation of mono- and bis(dioxygen) complexes of the respective atoms. Both the vibrationa17and ESR8-Io spectra of these species have been thoroughly examined. It has been demonstrated that both the mono- and bis(dioxygen) complexes of alkali metal atoms are ionic and are best described as M’(0,)- and Mf(04); and that the two oxygen atoms in M+(O,)- are equivalent. Recently Howard et al. reported on ESR spectra of mono(dioxygen) complexes of Cu, Ag, and Au atoms generated at 77 K in adamantane.” On the basis of the hyperfine structure due to 170nuclei, they concluded that the two oxygen atoms in Cu(0,) were not equivalent, while those in Ag(Oz) and Au(0,) were equivalent. They also concluded that the bis(dioxygen) complexes C U ( O ~and ) ~ Ag(02)2detected earlier in rare gas matrices were not formed in the adamantane matrix. The reported ESR spectrum of “ C u 0 2 in adamantane”, however, clearly indicates the presence of second species. The ESR spectra of nonlabeled Ag(Oz) and Au(0,) were not shown. Reported in this paper are the ESR spectra of C U ( O ~ ) , , ~ , Ag(02)l,z,and Au(02) complexes generated in rare gas matrices (neon and argon). The ESR spectra of CU(O,)~,,and Ag(02)1,2 (1) Darling, J. H.; Garton-Sprenger, M. B.;Ogden, J. S. Faraday Symp. Chem. SOC.1973, 8, 75. (2) McIntosh, D.; Ozin, G. A. Inorg. Chem. 1977, 16, 59. (3) McIntosh. D.: Ozin.G. A. Inorn. Chem. 1976, 15, 2869. (4) Tevault, D. E.; Smardzewski, R.-R.; Urban, M. W.; Nakamoto, K. J. Chem. Phys. 1982, 77, 577. (5) Tevault, D. E. J . Chem . Phys. 1982, 76, 2859. (6) Ozin, G. A,; Mitchell, S. A.; Garcia-F’rieto, J. J Am. Chem Soc. 1983, 105,6399. (7) See a review article by: Andrew, L. In Cryochemistry;Moskowits, M., Ozin, G. A., Eds.; Wiley: New York, 1976; pp 195-229, and referencescited therein. (8) Adrian, F. J.; Cochran, E. L.; Bowers, V. A. J . Chem. Phys. 1973.59, 56. (9) Lindsay, D. A.; Herschbach, D. R.;Kwirian, A. L. Chem. Phys. Lett. 1974, 25, 175. (10) Lindsay, D. A.; Herschbach, D. R.; Kwiriam, A. L. J . Phys. Chem. 1983,87, 2113. ( 1 1) Howard, J. A.; Sutcliffe, R.;Mile, B. J. Phys. Chem. 1984,88, 4351.
0022-3654/86/2090-4239%01 SO10
observed here are quite similar to those observed from the alkali metal counterparts. However, the deviation of the g tensor from the spin only value is significantly smaller, and the magnitude of the metal nuclear hfc (hyperfine coupling) interaction is larger than the corresponding quantities of the alkali metal atom species. The difference can be accounted for by a bent, end-on structure I, as opposed to the symmetric, side-on structure I1 of the alkali metal atom species.
,o=o M
o=o M
I
I1
Also it was found that the metal nuclear hfc constants of Cu(Oz) and Ag(Oz) generated in neon matrices are almost twice as large as those of the same species generated in argon matrices. A high sensitivity of the bent structure on the environment is evident. The ESR spectrum of Au(0,) was unusually complicated by the nuclear quadrupole interaction of the 19’Au nucleus. Analysis of the quadrupole interaction suggests that Au(02) may have the side-on structure 11.
Experimental Section A liquid helium cryostat that would enable trapping of vaporized metal atoms in an inert gas matrix and examination of the resulting matrix by ESR has already been described.12 In the present series of experiments, Cu, Ag, and Au atoms were generated from a resistively heated tantalum cell (1400, 1300, and 1500 “ C for Cu, Ag, and Au, respectively) and were trapped in rare gas matrices containing controlled amounts of oxygen molecules (0.1-3%). In the Au experiment, in order to prevent alloying with the Ta cell, gold pellets were placed in an alumina tube, capped with a molybdenum plug, and then placed in the Ta cell. From the line widths of the ESR signals of the metal atoms trapped in oxygen-free matrices under similar conditions, the metal atom concentration was estimated to be - O . I % . l 3 The cryostat is constructed in such a way that the matrix gas mixture can be routed through a tube passing through the liquid nitrogen reservoir before deposition onto the cold finger, or introduced directly from the room temperature inlet. For certain metal/oxygen/rare-gas combinations the two methods of deposition led to the difference of presence or absence of the orientation effect (i.e,, the orientation of the trapped species within the matrix relative to the flat surface of the cold finger).14*15Thus when relevant, the matrix will be labeled as either “precooled” or “nonprecooled” depending upon which deposition method was employed. The ESR spectrometer used was an IBM Model ER200D. A low-frequency field modulation (375 Hz) was used for the signal detection. All the spectra were observed while the matrix was maintained at -4 K. The spectrometer frequency locked to the (12) Kasai, P. H. Acc. Chem. Res. 1971, 4, 329. (13) Kittle, C.; Abraham, E. Phys. Res. 1953, 90, 238. (14) Kasai, P. H.; Weltner, Jr., W.; whipple, E. B.J . Chem. Phys. 1965, 42, 1120. (15) Kasai, P. H.; Whipple, E. B.;Weltner, Jr., W. J . Chem. Phys. 1966, 44. 2581.
0 1986 American Chemical Societv
4240 The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
Kasai and Jones
TABLE I: g Tensors, Metal Nuclear Hyperfiie Coupling Tensors of Dioxygen Complexes of Cu, Ag, and Au Atoms, and the Au Nuclear Quadrupole Term of the Au Complex“*b
complex/matrix Cu(O,)/Ne Cu(OJ/Ar Ag(Oz)/Ne Ag(Oz)/Ar Au(O,)/Ar Na(Oz)/ArC
gz
2.0854 2.0854 2.0551 2.0750 2.104 (2) 2.1112
gx
(11)
C U ( O ~ ) ~ / N ~ , A 2.0 ~ 156 Ag(02)2/Ne,Ar 2.010 (2) Na(02)2/Wd
2.0440
2.0074 2.007 1 2.0064 2.0070 2.004 (2) ( I ) 2.0063 2.0075 2.000 (2) 2.0092
gY 2.0020 2.0017 2.0030 2.0025
A, 88.9 56.9 63.2 27.1 33 (2) 3.4
2.0029 2.005 1 1.997 (2) 2.0027
(11)
27.3 110 3.3
Ax 84.0 54.0 63.0 26.4 33 (2) (1) 1.8
AY 83.8 52.1 64.0 27.8
29.5
26.8
2.9
2.3
PI1
41 (2) 3.5
“The hfc constants are for the 63Cu,Io7Ag, and 197Aunuclei, and A’s and pi,are given in G. *Experimentaluncertainties are f0.0006 for the g tensors, and 0.5 G for the hfc terms except for Au(0,) and Ag(02)2, as noted. ‘From ref 9. dFrom ref 10. sample cavity was typically 9.428 GHz, and the microwave power employed was -20 pW. For photoirradiation of the matrix a high-pressure xenon-arc lamp (Oriel, 1 kW) was used with a set of sharp cut-off filters. Spectral Results and Analyses Cu Atom-Dioxygen Complexes. The ESR spectrum of Cu atoms (3dI04s1)isolated in rare-gas matrices has already been analyzed.I6 There are two.naturally abundant Cu isotopes, 63Cu (natural abundance = 69%, I = 3/2, p = 2.2206&), and 65Cu (natural abundance = 31%, I = 3/2, p = 2.37908,). Owing to the extremely large hfc interactions with these nuclei, the ESR spectrum of Cu atoms observed a t an X-band frequency shows only two resonance transitions; one corresponds to the N M R -3/2), and occurs at 1.5 and transition (Ms = ‘I2,M I = -1/2 2.0 kG for 63Cuand 65Cu,respectively, and the other corresponds to the ESR transition (Ms = -‘Iz ‘I2,MI = -3/2), and occurs at 5.8 and 6.0 kG. Figure l a shows the ESR spectrum observed from the Cu/ oxygen(0.2%)/neon system in the 3.1-3.5 kG range. The ESR signals of the isolated C u atoms discussed above are thus outside the range; the signals seen here were observed only when the Cu atoms and oxygen molecules were cocondensed. As indicated in the figure the signals were recognized as two overlapping quartets A and B. The quartet structure must surely be due to the hfc interaction with the Cu nucleus; the average spacing of quartet A (- 85 G) is several times larger than that of quartet B, however. The line shape of individual component is roughly that expected from an axially symmetric spin Hamiltonian. Thus, as indicated in the figure, the highest field parallel component of quartet A is masked by quartet B,while the inner pair perpendicular components of quartet A and the outer pair of quartet B overlap completely. When the experiment was repeated by using a higher concentration of oxygen (Oh%), the intensity of quartet A decreased, while that of B increased. When the oxygen amcentration was increased to 3%, only the signals due to quartet B were observed with increased line width reflecting the paramagnetism of oxygen molecules. Thus on the basis of these observations and in cognizance of the results of earlier optical we assigned ~, No quartets A and B to C u ( 0 2 ) and C U ( O ~ )respectively. orientation effect was observed for the Cu dioxygen complexes generated in neon matrices. Since a sufficient number of components are seen free from obstruction, analysis of quartet A was straightforward. The spectrum was analyzed in terms of an orthohombic spin Hamiltonian (1). The z axis is to be identified with the direction of
-
= g,P(ffS,)
+ g,B(ffJ,) + g Y P w p J + A,I& + AXIS, + A g p y (1)
the parallel components, and Ais represent the hfc tensor of the Cu nucleus. The g tensor and the 63Cuhfc tensor thus determined for Cu(02) generated in a neon matrix are shown in Table I. Figure 2a shows, in an expanded scale, the quartet B section of the spectrum observed from the Cu/oxygen(0.6%)/neon system. (16) Kasai, P. H.; McLecd, Jr., D. J . Chem. Phys. 1971, 55, 1566.
3.1
3.2
3.3
3A
3.5
KG
F i e 1. (a) ESR spectrum observed from the Cu/oxygen(O.Z%)/neon system. (b) ESR spectrum observed from the Cu/oxygen(O.S%)/argon system. Two overlapping quartets A and B of axial symmetry are seen
in each spectrum as indicated. Here the effect of quartet A is minimal. The spin Hamiltonian parameters of C U ( O ~generated )~ in a neon matrix thus assessed are also shown in Table I. Figure 2b is a computer-simulated spectrum of C U ( O ~based ) ~ on these parameters. In Figure 3 the spectrum observed from the Cu/oxygen(0.2%)/neon system (Figure la) is compared with that simulated assuming the presence of 1 part Cu(Oz) and 0.8 part Cu(02),. In simulating the spectrum the signals due the 63Cuand 65Cuisotopic species were separately considered and superimposed. Figure l b shows the ESR spectrum observed from the Cu/ oxygen(OS%)/argon (precooled) system in the 3.1-3.5-kG range. This spectrum was also recognized as two overlapping quartets, A and B. When the experiment was repeated with less oxygen (O.l%), the intensity of quartet A remained strong, but quartet B appeared only as “shoulders” of the other. The quartets A and
Cu, Ag, and Au Atom-Molecular Oxygen Complexes
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4241
+
Figure 2. (a) Quartet B section of the spectrum observed from the Cu/oxygen(0.6%)/neon system. The solid arrow indicates the position of g, (=2.00). (b) Computer-simulated spectrum of C U ( O ~based ) ~ on parameters given in Table I.
+
Figure 4. (a) ESR spectrum observed from the Cu/oxygen(O.S%)/argon system. (b) Computer-simulated spectrum for a mixture of 1 part Cu(0,)and 0.2 part C U ( O ~assuming )~ the parameters given in Table I. The arrow indicates the g, position. Y
z
Y
X I 6’C” I
t t l t Xl”C“1
Figure 5. ESR spectrum of the Cu/oxygen(O.l%)/argon (nonprecooled) system observed when the external field was parallel to the plane of the trapping surface (upper trace) and when the field was perpendicular to the plane (lower trace). Figure 3. (a) ESR spectrum observed from the Cu/oxygen(0.2%)/neon system. (b) Computer-simulated spectrum for a mixture of 1 part Cu(0,)and 0.8 part Cu(02), assuming the parameters given in Table I.
The arrow indicates the g, position.
B were thus respectively assigned to Cu(02) and Cu(02)2 generated in an argon matrix. The g tensors and the 63Cuhfc tensors of these species are also shown in Table I. It is noteworthy that in an argon matrix (- 54 the average hyperfine spacing of Cu(02) G) is considerably smaller than that of Cu(02) in a neon matrix )~ in (-85 G). In contrast the parameters of C U ( O ~generated argon and neon matrices are, within the experimental accuracy, identical. Figure 4 compares the ESR spectrum observed from the Cu/oxygen(O.S%)/argon (precooled) system (Figure 1b) and that simulated on the basis of parameters given in Table I and assuming . the presence of 1 part C u ( O 3 and 0.2 part C U ( O ~ ) ~Understandably, the formation of the bis(dioxygen) complex is much more difficult in an argon matrix. For the Cu dioxygen complexes generated in a “precooled” argon matrix, no orientation effect was observed; however, when generated in a “nonprecooled” argon matrix, a strong orientation
effect was observed. Figure 5 shows the ESR spectra of the Cu/oxygen(O.l%)/argon (nonprecooled) system. The upper trace was obtained when the magnetic field was parallel to the flat surface of the cold finger. The lower trace was obtained when the field was perpendicular to the plane of the rod. The orientation effect is most conspicuously manifested in the Cu(02) spectrum (quartet A). Suffice it to note that the signals in the z and y directions (seeTable I) are dominant in the former field direction, and the signals along the x axis appear like a “single-crystal case” in the latter field direction. Photoirradiation of argon matrices containing both Cu(02) and C U ( O ~with ) ~ “red light” (A > 580 nm) resulted in bleaching of the signals due to the latter species only. Irradiation of the matrix with UV light (A > 370 nm) eliminated both species. No new ESR signals appeared as a result of the irradiation. A g Atom-Dioxygen Complexes. The ESR spectrum atoms (4d1°5s’) isolated in rare-gas matrices is well-knc! There are two naturally abundant Ag isotopes, lo7Ag (n:. I ..,.a1 abundance = 51%, Z = p = -0.1 130/3,), and Io9Ag(narural abundance = 49%, I = = -0.12998,). The ESR spectrum of Ag atoms thus appear as two sets of sharp doublets with the respective spacings of -650 and -750 G. t5:.
%
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The Journal of Physical Chemistry, Vol. 90,No. 18, I986
Kasai and Jones
c
A
n
3:2
313
3.4
KC
Figure 6. (a) ESR spectrum observed from the Ag/oxygen(0.3%)/neon system. (b) Computer-simulatedspectrum of Ag(0,) based on parameters given in Table I. The hyperfine splittings due to the Ag nuclei are shown; the solid arrow indicates the g, position.
Figure 6a shows the ESR spectrum observed from the Ag/ oxygen(0.3%)/neon system in the 3200-3450-G range. The signals due to isolated Ag atoms are thus outside the range. The signals seen here were observed only when the Ag atoms and oxygen molecules were cocondensed. The solid arrow indicates the position corresponding to g, (=2.00). Other than the weak signals near this position, the entire spectrum can be accounted for by a system with a g tensor of uniaxial nature (g, >> g, > g,,) and an essentially isotropic hfc interaction with one Ag nucleus, as indicated. The spectrum was assigned to Ag(02) generated in a neon matrix. Its g tensor and the lo7Ag hfc tensor are shown in Table I. Figure 6b is a computer-simulated spectrum based on these parameters. Figure 7a shows the ESR spectrum observed from the Ag/ oxygen( l%)/argon (precooled) system. The spectrum can be readily recognized as that of a system having an uniaxially symmetric g tensor (g,,>> g, z g,) and a hfc interaction with one Ag nucleus. The spectrum was thus assigned to Ag(02) in an argon matrix. Its g tensor and the Ag hfc tensor are also given in Table I. The hyperfine splittings here are too small to resolve individual isotopic components. Figure 7b is a computer-simulated spectrum based on these parameters. It is intriguing that the Ag hfc constant of Ag(02) in a neon matrix is twice as large as that of the same species in an argon matrix. Figure 8 shows the ESR spectra of the Ag dioxygen complex region observed from the Ag/oxygen( l%)/neon and Ag/ oxygen( 3%)/argon (precooled) systems. A higher oxygen concentration thus leads to increased line width of the Ag(0,) signals and appearance of a broad, slightly assymetric singlet at g = 2.0 in both the neon and argon matrices. The new singlet is assigned to Ag(02),. It is formed much less in an argon matrix, and there the signal overlaps with the higher field perpendicular component of Ag(02). The g tensor and the upper limit of the Ag hfc tensor of Ag(02)2assessed from the line width are presented in Table I. No orientation effect was observed for the Ag dioxygen complexes generated in neon matrices, nor for the Ag dioxygen
I
I
I
3.2
3.3
3.4
KG
Figure 7. (a) ESR spectrum observed from the Ag/oxygen(l%)/argon system. (b) Computer-simulatedspectrum of Ag(02) based an parameters given in Table I. The solid arrow indicates the g, position.
ii
A
3.2
3.3
ll
3.4
KG
Figure 8. (a) ESR spectrum observed from the Ag/oxygen(l%)/neon system. (b) ESR spectrum observed from the Ag/oxygen(3%)/argon system. The arrow indicates the g, position.
complexes formed in precooled argon matrices. A strong orientation effect was noted, however, for the complexes generated in a nonprecooled argon matrix. Again the signals of the z and y directions dominated the spectrum when the field was parallel
Cu,Ag, and Au Atom-Molecular Oxygen Complexes
Figure 9. (a) ESR spectrum observed from the Au/oxygen(3%)/argon system. The spectrum is complicated by the I9’Au quadrupole term; the positions of the parallel and the “normal”perpendicular transitions are indicated. (b) Computer-simulatedspectrum based on parameters given in Table I. The stick diagram at the bottom indicates the positions and relative intensities of the normal and forbidden transitions (Am,= h2) at the perpendicular orientation.
to the plane of the rod, and the signals in the x direction dominated the spectrum when the field was perpendicular to the plane of the
rod. Photoirradiation of an argon matrix containing Ag(02) and perhaps some Ag(02)2with UV light (A > 370 nm) resulted in total disappearance of their ESR signals. No new signals appeared as a result of irradiation. One experiment was performed using a krypton matrix. The ESR spectrum observed from Ag/oxygen(2%)/krypton was identical with that observed from an argon matrix (Figure 7a). Au Atom-Dioxygen Complex. The ESR spectrum of Au atoms (5d1°6s1) isolated in rare-gas matrices is well-known.I6 The spectrum comprises a sharp, widely spaced quartet (spacings between successive lines of 1000 G) due to the hfc interaction with the Ig7Au nucleus (natural abundance = loo%, I = 3/2, 1 = 0.14398,). Figure 9a shows the ESR spectrum (3.1-3.5 kG) observed from the Au/oxygen(3%)/argon system. The sharp, isotropic signal a t 3200 G is the second highest field hyperfine component ( m , = of the isolated Au atoms. The rest of the signals were observed only when the Au atoms and oxygen molecules were codeposited. We assigned these signals to Au(02). The signals flanking the gold atom hyperfine component at 3200 G were recognized as some of the parallel components of Au(02), as indicated. Many of the remaining signals have shapes typical of perpendicular components. We thence postulated that the most, if not all, of the remaining signals were the perpendicular components of Au(02), and their positions and intensities were strongly altered by the quadruple interaction of the Au nucleus. Attempts to generate similar Au atom-dioxygen complex in a neon matrix were not successful. Inability to trap a sufficient amount of Au atoms in a neon matrix is probably responsible for the failure. A spin Hamiltonian for a system with an axially symmetric g tensor, a hfc tensor with a nucleus, and a quadrupole interaction with the same nucleus may be written as follows.
-
Here the term invovling PIIrepresents the quadrupole interaction. The quadrupole term mixes the nuclear spin states, induces forbidden transitions, and alters the positions and intensities of the normal transition^.'^ When the PIIand A terms are of equal magnitude, accurate assessment of these effects requires exact diagonalization of the Hamiltonian matrix given by (2). An elegant algorithm for such diagonalization has been shown by Lund et a1.,18 and a powder pattern simulation program based on its results has been described re~ent1y.l~ For a situation where PIlN A , an analysis of the exact solution of the Hamiltonian above reveals that (1) the positions of the parallel components are not affected by the quadrupole term, and (2) the low-field pair of “normal” perpendicular components ( m , = +3/z and +ll2) are shifted downfield by 2P,,,while the high-field pair of “normal” perpendicular components (m, = - l l 2 and -3/2) are shifted upfield by 2 4 and the spacing within each pair remains exactly at A*. The four signals indicated in the figure were thus identified as the normal perpendicular components. The axially symmetric g tensor, the Ig7Auhfc tensor, and the quadrupole term PI, of Au(02) thus determined are shown in Table I. Figure 9b is a computer-simulated spectrum based on these parameters. The stick diagram at the bottom indicates the positions and the relative intensities of the normal (Am, = 0) and forbidden (Am, = k2) transitions expected at exactly the perpendicular orientation. The agreement between the observed and computed spectra is considered reasonable; the variance is ascribed to the adoption of an axially symmetric spin Hamiltonian (rigorously speaking, an orthorhombic spin Hamiltonian is required in either the end-on or the side-on structure) and/or the presence of some singlet signal (of carbonaceous origin) at the g = 2.00 region.
Discussion The g tensors and the metal nuclear hfc tensors of C U ( O ~ ) ~ , ~ , Ag(02)1,2and Au(02) determined in the present study are compiled in Table I together with those reported earlier for Na(02)1,z.9J0For all the dioxygen complexes of Cu, Ag, and Au listed here we note that both the isotropic and dipolar components of the metal hfc tensors are extremely small compared with the values expected from a unit spin density in the ns and np valence orbitals of the respective metal atoms.20 Hence, as in the case of alkali metal dioxygen complexes, the ESR spectra seen here and must essentially be manifests of the superoxide ion, (02)-, As stated earlier the stoichiometry bis(dioxygen) anion, (02)z-. of each complex was decided on the basis of oxygen concentration dependency of its ESR spectrum and in comparison with results of the I R studies.’-3 Figure 10 depicts the valence molecular orbitals of an superoxide ion, (Oz)-, the degeneracy between the A* orbitals of which is removed by the presence of a cation in either the end-on structure I or the side-on structure 11. If A > 6 >> X, where X is the spin-orbit coupling constant of the oxygen atom, the principal g tensor of the resulting superoxide ion is given as follows.21q22
Here the z axis is along the internuclear direction of the 02-ion, and the cation is in the the x-z plane. If follows that g, > g, > gy = g,, and that Ag,(=g, - g,) is a measure of the separation between the A,* and ry*orbitals. However, one should note that the unpaired electron resides in the ry*orbital, and since the (17) See, for example: Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions;Oxford Univesity: London, 1970; pp 178-186. (18) Lund, A.; Thuomas, K.; Maruani, J. J. Magn. Reson. 1978,30, 505. (19) Kasai, P. H.; Jones, P. M. J . Am. Chem. SOC.1985, 107, 813. (20) For these atomic values see ref 16 and: Lindsay, D. M.; Kasai, P. H. J . Magn. Reson. 1985, 64, 221. (21) Kanzig, W.; Cohen, M. H. Phys. Rev. Lett. 1959, 3, 509. (22) Kasai, P. H. J . Chem. Phys. 1965, 43, 3322.
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The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
TABLE II: Isotropic Metal Hyperfii Coupling Constants and the Spin Densities in the Metal s Orbitals of M(02) of the Groups 1 and 11 Atoms
Cu(O,)/Ne Cu(O,)/Ar Ag(Oz)/Ne Ag(O,)/Ar Au(Oz)/Ar Na(Oz)/Ar,K Cs(02)/Ar,Kr
85.6 54.3 63.4 27.1 "3 1.9 5.0
2196 2196 646 646 1121 316 821
3.9 2.5 9.8 4.2 2.9 0.9 0.6
'The isotropic coupling constants determined for free atoms isolated 0.
'7Tx
0.10
in argon matrices (ref 16, and Jen, C. K.; Bowers, V. A.; Cochran, E. L.; Foner, S. N. Phys. Rev. 1962, 126, 1749).
+ AgCAr) 0
Ag ( N e ) 0
of electrons in the bonding axorbital into the vacant s orbital of the cation. If the ionic or the dative mode of interaction prevails, the complex should assume a symmetric, side-on structure. In the covalent mode a partial transfer of the extra charge in the ax* orbital into the metal s orbital is to be effected; it hence requires an asymmetric, end-on structure. It has been shown that the ionic mode prevails in the alkali metal atom series of M(02).' A correlation is hence expected between their Ag,'s and the ionic radii of the cations. The Na, K, and R b complexes in Figure 11 may indeed be showing such a ~ o r r e l a t i o n . However, ~~ the variation of Ag, within the alkali metal atom series is remarkably small in comparison with that observed among the Cu, Ag, and Au complexes. We suggest that the smaller Ag,'s, hence the larger separation between the a* orbitals, observed for Cu(02) and Ag(0,) reflect a contribution, if not dominance, of the covalent mode in these complexes. The inequivalence of the two oxygen atoms in these complexes (generated in argon matrices) is clearly shown by the IR spectra observed by Tevault et al.495 According to the ESR study of the "0-enriched complexes generated in adamantane by Howard et al.," however, the two oxygen atoms are not equivalent in Cu(02), but are equivalent in Ag(0,) and Au(02). A high sensitivity of the structure of Ag(02) on its environment is also demonstrated by the ESR spectra of this study (Le., neon vs. argon matrices). The valence s orbital of the Au atom (as given by its ionization potential, -9.2 eV) is significantly more stable than those of Cu (-7.7 eV) and Ag (-7.6 eV). We suggest that Au(02) has the side-on structure I1 dictated by the dative mode. Since the dative mode does not involve the ax*orbital, the separation 6 between the a* orbitals would be small; a larger Ag, observed for Au(0,) is thus accounted for. The side-on structure of Au(0,) is also supported by its 1 9 7 Aquadrupole ~ tensor. The spin Hamiltonian (2) assumes that the symmetry axis of the g tensor coincides with that of the quadrupole tensor. The. quadrupole coupling constant PI, is related to the nuclear quadrupole moment Q and the electric field gradient q as shown below.
* 1 .o
1.5
RC i )
Figure 11. Ag,'s (=gz - g,) of M(0,) of the alkali metal atoms (ref 9) and those of Cu, Ag, and Au atoms (Table I) plotted against the ionic radii of the respective atoms.
valence orbitals of the metal cation overlap much more with the ax*orbital, Ag, is a measure of the interaction between the metal cation and the doubly occupied a,* orbital of the superoxide ion. Figure 11 shows Ag,'s of M(02) reported earlier for the alkali metal atom series? and those of Cu, Ag, and Au atoms determined in the present study plotted against the ionic radiiz3of the respective atoms. Clearly a simple correlation does not exist between such quantities. That the dioxygen moiety in all of these complexes is also supported by the fact is essentially a superoxide ion, 02-, that the 0-0 stretching frequencies of these complexes are all within the range 1100 f 20 Three modes of interaction are plausible between the superoxide ion and the metal cations currently being considered: (1) the ionic mode based only on the Coulombic interaction between the ions, (2) the covalent mode where a a-bond is formed between the metal and one of the oxygen atoms based on the paired electrons from the ax* orbital, and ( 3 ) the dative mode resulting from shifting
3eqQ 'I
= 4I(2Z - 1)
--
3eQ a2v 4Z(2Z - 1)z
That the observed spectrum can be adequately accounted for by this Hamiltonian implies that the electric field gradient at the Au nucleus is symmetric about the z axis, and hence some electron density in the Au pz orbital but none in the other p orbitals. Such an electron density distribution can be realized only in the side-on structure where a back-donation of the extra charge from the antibonding ax*orbital occurs exclusively into the metal p, orbital. In either the end-on structure I or the side-on structure I1 the semifilled orbital of M+(Oz)- is the out-of-plane a,,* orbital and the cation is located at its nodal plane. The isotropic component of the metal hfc tensor must then arise from the exchange polarization scheme.z5 Recently Adrianz6has shown that, in such a situation, the contribution of direct polarization of the metal s orbitals by the unpaired electron in the a,,* orbital to the isotropic ~~~
(23) Cotton, F. A.; Wilkinson, G . Advanced Inorganic Chemistry, 4th ed.; Wiley: New York, 1980; p 14
~
~~
(24) See ref 9 for the anomalous trend shown by Cs(02). (25) See, for example, ref 17, Chapter 17. (26) Adrian, F J.; Jette, A. N. J . Chem. Phys. 1984, 81, 2415.
Cu, Ag, and Au Atom-Molecular Oxygen Complexes component is negligible, and the isotropic component results mainly followed from polarization of filled orbitals of the anion, (02)-, by partial transfer of the resulting spin density to the cation orbitals via overlap. The theory thus predicts the smallest spin density in the ns orbital of the cation for M+(02)- of the ionic mode, and increasingly larger spin densities for those involving the dative mode, and the covalent mode, respectively. The isotropic component generated by this scheme should be negative. Table I1 lists, for all the M(02) observed presently and those alkali metal atom species for which the metal hfc tensors have been firmly determined, (1) the isotropic component of the metal hfc tensor (calculated assuming that all the observed principal components are negative), (2) the hfc constant of the free metal atoms isolated in an argon matrix, and (3) the spin density in the metal ns orbital of M ( 0 2 ) determined therefrom. The overall trend seen in the last column is clearly in accord with the theory. The orientation effect observed for C u ( 0 2 ) and Ag(02) generated in nonprecooled argon matrices indicates that these molecules are preferentially oriented with their dioxygen group lying parallel to the surface of the cold finger and the metal atom positioned directly above (or below) it. Recall that the z axis is identified with the 0-0internuclear direction, and the metal atom is in the z-x plane. The orientation depicted above thus leads to observation of the z and y components of the spectrum when the magnetic field is applied parallel to the surface of the rod, and observation of only the x components when the field is perpendicular to the surface of the rod. It has been our experience and other’s that the orientation effect in rare-gas matrices is most often encountered with linear molecules (e.g., CuF2, BO, et^.)'^,^^ and molecules with a well-defined The molecular plane (e.g., NO2, NF2, C U ( N O ~ )et^.).'^*^^-^* ~, preferred orientation is usually that in which the molecular axis or the plane lies “flat”, parallel to the surface of the cold finger. Noted exceptions are diatomic molecules with a large dipole moment such as VO*9and ZnF.30 These molecules were found to be oriented with the molecular axis perpendicular to the surface of the rod. One can reconcile with the orientation observed for Cu(02) and Ag(02) by postulating that the dioxygen section of these complexes orients itself in the “normal” way and the metal (27) Knight, L. B.; Wise, M. B.; Davidson, E. R.; McMurchie, L. E. J . Chem. Phys. 1982, 76, 126. (28) Kasai, P. H.; Whipple, E. B. Mol. Phys. 1965, 9, 497. (29) Kasai, P. H. J. Chem. Phys. 1968, 49, 4979. (30) Knight, L. B.; Mouchet, A.;Beaudry, W. T.; Duncan, M. J . Magn. Reson. 1978, 32, 383.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4245 atom position is affected by the dipole moment of the complexes. It attests the strength of the ionic nature in these complexes, though the contribution of the covalent mode is clearly indicated by the g tensors and the metal hfc tensors. The structure of M(02), of the group 1 and group 11 metal atoms is known with less certainty. It has been shown that (1) the two dioxygen groups are equivalent, (2) there exists a weak “intermolecular” bonding between the two dioxygen groups, and (3) the complexes are ionic pairs of the form M+(02)
