EPR
Spectra of y-Irradiated Ge-, Sn-, and Pb(0,CCH3),
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
853
Electron Paramagnetic Resonance Spectra of y-Irradiated Germanium, Tin, and Lead Tetraacetates‘ J. R. Morton,” K. F. Preston, and S. J. Strach2 Division of Chemistty, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR9 (Received November 7, 1978) Publication costs assisted by the National Research Council of Canada
EPR spectra observed in y-irradiated tetraacetates of Ge, Sn, and Pb at 100 K are characterized by large hyperfine interactions of the unpaired electron with the group 4 magnetic nuclei (73Ge,117,119Sn, and ‘07Pb). Hyperfine tensors for these interactions, derived from a careful analysis of the powder spectra, show that in the Ge and Sn centers the unpaired electron resides almost entirely in the valence s and p orbitals of the metal atom. In the case of the Pb radical the SOMO again possesses large Pb(6s) character, but negligible 6p character. In the linht of these findings. it is concluded that the centers are not simdv metal cations M3+.but are better descrged as tetraacetate radical anions M(Ac)i For Pb, an alternative strtcture consistent with the experimental facts is P ~ ( A C ) ~ .
Introduction EPR spectra attributed to Pb3+ions have been reported in a variety of matrices3 including the alkali halides, CaC03, CaFz, and llead tetraacetate. Although the unpaired electron is principally located in the Pb(6s) atomic orbital of these radical ions, significant unpaired spin density is located on neighboring nuclei. In the case of Pb3+ in KC1, for example, we have suggested4 that the spectrum could be better ascribed to a “molecular” complex PbCIB3-because of the similarity to the free radical PbF:-. With this comparison in mind, we decided to reexamine the spectrum of “Pb3+” in lead t e t r a a ~ e t a t e , ~ with a view to offering a more realistic description of the radical contained therein. For comparison purposes, we also examined the IEPR spectra of y-irradiated tin and germanium tetraacetates. Experimental Section Germanium tetraacetate was made from GeC1, and thallous acetate;6tin tetraacetate was prepared in a similar way, using Sn14as the starting material. Lead tetraacetate was obtained commercially. After recrystallization from acetic anhydride, the samples were sealed into 4-mm Suprasil tubes; and irradiated for 1-2 h at 77 K in a 3500-Ci 6oCoy cell. The EPR spectra of the irradiated samples were examined with a Varian E12 spectrometer which we have described e l ~ e w h e r e . ~ Results In all three cases, the EPR spectra of the irradiated samples showed, at N 100 K, exceedingly powerful features in the g = 2 region undoubtedly (because of the proton hyperfine structure) arising in part from organic free radicals. At other magnetic fields, however, we were able to detect signals from species in which there was no proton hyperfine interaction, but a large hyperfine interaction with the metal nucleus (germanium, tin, and lead all possess magnetic isotopes in detectable abundance). Only in the case of Ge(O2CK!H3),, however, was a “normal” EPR spectrum obtained (Figure 1). With tin and lead tetraacetates the metal hyperfine interactions were so large that “forbidden” transitions were observed and used to obtain the spectral parameters (g factors, hyperfine interactions) by standard methodsa8 The solution of the Hamiltonian was exact, except for the neglect of quadrupole interactions, and the implicit assumption that the principal axes of the g and hyperfine tensor were parallel.
TABLE I: EPR Spectrum of 7-Irradiated Ge(0,CCH,)4 at 96 Ka mI ti, G 1,G v , MHz g
+
4607.28
4501.8
9111.35
1798.4 4172.4
1895.3 4093.9
9108.85 9106.03
2003.2 3780.4
2069.0 3716.2
9108.86 9106.44
2214.8 3411.5
2269.1 3363.8
9108.88 9106.51
t
2450.7 3023.7
2497.8 3053.8
9109.12 9113.83
0.5 -
2724.5
2765.0
9109.25
4.5 t
3.5 t 2.5 t 1.5 -
1.9963 1.9963 1.9963 2.0012 1.9970 1.9970 2.0070 1.9944 1.9944 2.0169 1.9882 1.9882 2.0386 2.0001 2.0001
hfs, G -311.4 -288.8 -288.8 -309.4 -288.8 -288.8 -312.7 -288.9 -288.9 -320.0 -288.1 -288.1 -296.8 -286.3 -286.3
a Data for main site given. Two other (weaker) sites were also detected.
Germanium Tetraacetate. The EPR spectrum of yirradiated Ge(02CCH3)4showed, at 96 K, signals characteristic of an anisotropic hyperfine interaction with a single 73Ge ( I = 9/2) nucleus (Figure 1). The ten transitions of the mr manifold were spread over 2800 G, and each showed additional structure characteristic of an axial hyperfine tensor (Table I). The data were analyzed in pairs (fmr) in order to generate the g factor and 73Ge hyperfine interaction by computerized diagonalization of the spin matrix. Bearing in mind that we are dealing with a powder spectrum the data are reasonably self-consistent. The following mean values were obtained for the g factor and the 73Gehyperfine interaction: 811 = 2.012 f 0.018 all = -310.1 f 8.6 G g, = 1.995 f 0.004
a, = -288.2 f 1.1 G
where the errors are one standard deviation. T i n Tetraacetate. A t 107 K the EPR spectrum of y-irradiated Sn(0zCCH3)4consisted (apart from spectra of organic free radicals) of a broad feature at g = 1.994 thought to be due to the nonmagnetic (even mass) isotopes of tin. In addition, near 5000 G, two groups of three lines due to the mr = 1/2 transitions of l17Sn and l19Sn were observed. A simple formula, which we have used elsewhere,’ indicates (a) that the l17Sn, 119Snhyperfine interactions are approximately -8000 G and (b) there should
0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83,
854
No. 7, 1979
J. R. Morton, K. F. Preston, and
S.J. Strach
TABLE IV: Spin Densities in Valence s and p Atomic Orbitals
___ 73Ge
Il9Sn
isotropic hfs, MHz 13 h e Y n rii *( 0 ns character anisotropic hfs, MHz
-827.5 -2361 0.35 -22.8
0 . 4 ~ ~ 7 ~3, ( r -
-48.0 0.47
-23595 -43738 0.54 -2661“ -157 -729.4 0.36 0.21
(
np character
*(”Pb
35917 81720 0.44
1‘ 1“ :::! la 2 0;i
652.1
“
Obtained by resolution of a nonaxial tensor into two mutually perpendicular tensors, each corresponding t o positive unpaired spin density in a p orbital. -1
-
1
Figure 1. EPR spectrum of y-irradiated Ge(02CCH,), powder at 96 K showing the parallel and perpendicular features of the Ge ( I = 9/2) species for the main site. Strong lines toward the center of the spectrum are due to H atoms, organic free radicals, and spin-free isotopes of the Ge species. Other lines are due to small amounts of the Ge ( I = 9/2) species in two other sites. Changes in relative intensities of the three sites with temperature permitted an unequivocal assignment of the lines of the main site.
TABLE 11: ml = 1/2Transitions in t h e EPR Spectrum of ?-Irradiated Sn(0,CCH3)4at 113 K
-
isotope ”Wn
‘I9Sn
a
Isotropic g =
magnetic field, G
microwave hfs freq, MHz interaction,“ G
4881.2 9007.21 5041.4 9007.01 5144.4 9006.99 4914.5 9007.16 5073.2 9007.01 5184.0 9006.98 1.994assumed.
-7940 -8112 -8222 -8303 -8472 -8589
TABLE 111: EPR Spectrum of y-Irradiated W(O,CCH,), at 107 K and Microwave Frequency 9108.4 MHz axis x y z a
ESR (-l/2) 5376.9 5382.3 5421.9
NMR (2)“ 9879.7 9843.3 9773.6
g
2.0081 2.0094 2.0048
a2019
G
12765 12765 12820
Formally forbidden transition; see ref 8.
be weak, “forbidden”, transitions above 20 kG (at 9000 MHz). After unsuccessful attempts to detect the latter, it was decided to assume a g factor of 1.994 and calculate the l17Sn,I19Sn hyperfine interactions from the mI = 1 / 2 transitions (Table 11). This method appears to be reasonably satisfactory, since the ratio of the isotropic hyperfine interactions a119/a117is 1.045, compared to their magnetic-moment ratio, 1.047. Lead Tetraacetate. Analysis of the EPR spectrum of 7-irradiated lead tetraacetate posed problems of a different nature. Two transitions of the 207Pb(I= 1/2) hyperfine manifold were observed near 5.4 and 9.8 kG (Table 111), indicating a z07Pbhyperfine interaction of approximately 12 kG. However, each transition was split into three components, corresponding to the directions of the three principal axes. There were, therefore, six possible ways of labeling these components and no easy way of selecting the correct one. It was not possible to obtain the principal values of g from spectral features arising from nonmagnetic isotopes of P b since such features were completely masked by the strong signal from organic free radicals. The spectrum was observed at 8.8 and 9.6 GHz in the hope that such a change in the microwave frequency would, after
generation of the g and hyperfine tensors, enable us to select the correct choice of axes. Unfortunately, the frequency change was insufficient to provide a definite answer probably because, in a powder spectrum, there is insufficient information to allow for a possible noncoincidence of the principal axes of the g and hyperfine tensors. The assignment of axes was finally chosen on the basis of (a) minimum g anisotropy, and (b) optimum fit of the data at 8.8 and 9.6 GHz.
Discussion The first step in the identification of a free radical is to obtain a description of its semioccupied orbital. This is usually done by estimating the spin density in the various contributing atomic orbitals by comparing the observed hyperfine interaction(s) with estimates of (8~/3)y,y,#~(O) and 0.4y,y,(r-3) for the valence s and p orbitals, respectively. We have recently computed a set of these parameters from Herman and Skillman’s HartreeFock-Slater wave f u n ~ t i o nand , ~ it does appear that, in contrast to earlier values, these parameters can be used sucessfully with the heavier elements. In Table IV we compare the isotropic and anisotropic components of the 73Ge, l19Sn, and 207Pbhyperfine interactions with the corresponding one-electron parameters, thus obtaining an estimate of the spin densities in the valence s and p atomic orbitals. We see immediately an anomaly between the result for lead tetraacetate and those for tin and germanium tetraacetates. In the case of the 73Ge and l19Sn hyperfine interactions the unpaired electron spin is largely accounted for, although the considerable spin density in the np orbitals suggests that identification of these species as Ge3+ or Sn3+ is improbable. For lead tetraacetate, however, only half of the spin density is accounted for, although it is almost all in the Pb(6s) atomic orbital. These considerations lead us to suspect, therefore, that we are not dealing with the spectra of Ge3+,Sn3+,and Pb3+ions, but instead with “molecular” free radicals whose central atom is germanium, tin, or lead. It would appear possible, moreover, that the lead-centered free radical is different in kind from the germanium- and tin-centered species. There is a paucity of reliable data on the hyperfine interaction of the metal in heavy-metal-containing free radicals, so that there are no well-documented prototypes with which to compare the present species. Jackel and Gordy’s datalo for SnH3 have been criticized by Bennett and Howard,ll whose data on Sn(CH3)3is no less suspect because of computed Sn(5p) spin densities well in excess of unity. Isotropic data for SnH,- are reasonably well established,12 being obtained from an isotropic spectrum; GeH,- and PbH4- are unknown. We are inclined to identify the species trapped in the yirradiated tin and germanium tetraacetates as the negative ions M(OAC)~-, rather than the positive ions M3+.
CNDO Study of a Faujasite Six-Ring
The large, isotropic central-atom hyperfine interactions are reminiscent of those of the well-known phosphoranyl radicals,13with which M(0Ac)c would be isoelectronic (at least as far as the valence electrons are concerned). The 31Pisotropic hyperfine interactions in PHI and PF4 are 1456 MHz14 and 3717 MH2,15 respectively, whereas the '19Sn hyperfine interaction in SnH, is -6245 MHz.12 With these data in mind, the I19Sn hyperfine interaction of -23.6 GHz for Sn(0Ac); is not unreasonable. The phosphoranyl radicals also possess CaUrather than Oh symmetry (i.e., the four ligands are equivalent in pairs, axial and equatorial). This low symmetry permits inclusion in the semioccupied orbital of M(np) atomic orbitals, generating considerable hyperfine anisotropy (Table IV). Describing these species as M3+ ions cannot adequately explain this anisotropy. In the case of lead tetraacetate, however, there is very little hyperfine anisotropy, and the spin density in Pb(6p) is only 0.05. Since only 50% of the spin is accounted for, we feel that the free radical is best described as P ~ ( O A C ) ~ rather than Pb3+. The 6s/6p spin-density ratio implies an interbond angle of 920,16and missing spin resides in various ligand orbitals. A possible alternative structure for the lead center is a lead tetraacetate anion having a tetrahedral geometry. Symmetry requirements would prevent a direct contribution of Pb(6p) orbitals to the al semioccupied molecular orbital of such a species, and thus account for the small 207Pb hyperfine anisotropy. The crystal
The Journal of Physical Chemistry, Vol. 83,
No. 7,
1979
855
structures of the tetraacetates are, unfortunately, unknown, so that there is no immediately apparent reason for the adoption of a different geometry by the lead radical.
References and Notes (1) NRCC No. 17185. (2) NRCC Research Associate 1977-1979. (3) J. R. Morton and K. F. Preston, "Magnetic Properties of Free Radicals", Landolt-Bornstein, Vol. 9a, H. Fischer and K.-H. Heliwege, Ed., Springer-Verlag, Berlin, 1977, p 263. (4) A. R. Boate, J. R. Morton, and K. F. Preston, J. Magn. Reson., 29, 243 (1978). (5) J. I. Isoya, H. Ishizuka, A. Yamasaki, and S. Fujimara, Chem. Left., 397 (1972). (6) P. W. Schenk, "Handbook of Preparative Inorganic Chemistry", Vol. 1, G. Brauer, Ed., Academic Press, New York, 1963, p 726. (7) A. R. Boate, J. R. Morton, and K. F. Preston, J . Phys. Chem., 80, 2954 (1976). (8) A. R. Boate, J. R. Morton, and K. F. Preston, J . Magn. Reson., 24, 259 (1976). (9) J. R. Morton and K. F. Preston, J . Magn. Reson., 30, 577 (1978). (10) G. S. Jackel and W. Gordy, Phys. Rev., 176, 443 (1968). (11) J. E. Bennett and J. A. Howard, Chem. Phys. Lett., 15, 322 (1972). Our more recent compilation' of $*(O) and ( r 3 )however, , improves the spin count in Sn(CH,),. (12) J. R. Morton and K. F. Preston, Mol. Phys., 30, 1213 (1975). (13) P. J. Krusic, W. Mahler, and J. K. Kochi, J . Am. Chem. SOC.,94, 6033 (1972). (14) A. J. Colussi, J. R. Morton, and K. F. Preston, J . Chem. Phys., 62, 2004 (1975). (15) R. W. Fessenden and R. H. Schuler, J. Chem. phys., 45, 1845 (1966). (16) P. W. Atkins and M. C. R. Symons, "The Structure of Inorganic Radicals", Elsevier, Amsterdam, 1967, p 257.
A CNDO Study of the Electronic Structure of Faujasite Type Six-Rings as Influenced by the Placement of Magnesium and by the Isomorphous Substitution of Aluminum for Silicon W. J . Mortier," Katholieke Universitelt Leuven, Centrum voor Oppervlaktescheikunde en Colloble Scheikunde, De Croylaan 42, 8-3030 Heverlee, Belgium
P. Gieerllngs, C. Van Alsenoy, and H. P. Figeys Free University of Brussels (U.L.B. and V.U.B.), Department of Organic Chemisiy, F.D. Rooseveltlaan 50, B- 1050 Brussel, Belgium (Received June 28, 1978; Revised Manuscript Received December 1 I, 1978)
A CNDO study has been made of the electronic structure of the main cation-exchange site in faujasite-type zeolitic structures, the framework six-ring. The following isolated molecules and ions were considered as models: Si6018H12,(Sis018H12Mg)2t,(Si3A13018H12)3-, and (Si3Al3Ol8HI2Mg)-.Some general statements can be made concerning the properties of the aluminosilicate framework and the six-ring site. First, isomorphous substitution of A1 for Si, or the presence of an exchangeable cation, results in only small variations in framework oxygen charges and a considerable delocalization of these charges is observed. The charge on Si is more highly variable than that on AI. This explains the difference in sensitivities of the SiK, and AlK, emission energies upon isomlorphous substitution. Second, protons attached to aluminate tetrahedra are more acidic than those attached to silicate tetrahedra. Finally, the molecular electrostatic potential (MEP) seems to be very sensitive to residual charges. This influence, as indicated by the features of the MEP pattern, accounts for the fundamentally different adsorption properties of X and Y zeolites toward CO and COz. Further, the increase in the CO stretching frequency observed when passing from X to Y zeolites, both containing divalent cations, is also explained by the MEP.
