J. Phys. Chem. 1995, 99, 4655-4660
4655
DRIFTS, XPS, XAS, and ab Initio Study of Lanthanide Oxides Supported on y-A& M. J. Capitiin, M. A. Centeno, P. Malet,* I. Carrizosa, and J. A. Odriozola Departamento de Quimica Inorghnica e Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla-CSIC, Spain
A. Miirquez and J. Fernhndez Sanz Departamento de Quimica Fisica, Universidad de Sevilla, Spain Received: September 22, 1994; In Final Form: December 16, I994@
DRIFTS, XPS, and XAS experiments have been carried out on Ln2Ody-Al203 catalysts. The experimental results have also been modeled by using SCF MO ab initio calculations on clusters simulating the La2O3/ y-Al~O3catalysts. A strong interaction between the supported phase and the support is observed that results in the modification of the stretching frequency of A1-0 bonds, the binding energy of the lanthanide cations, and the white line intensity at the lanthanide Lm XAS spectrum. All these modifications can be explained in light of the a b initio calculations as a result of charge redistribution between aluminium and lanthanide cations through an oxygen bridge. This charge redistribution is a function of the number and nature of the cations of the second coordination sphere of the lanthanide element, which justify the differences in chemical reactivity of the catalysts as a function of the loading and calcination temperature.
Introduction The interaction of rare earth oxides with the surface of y-AlzO3 is a subject of interest in catalysis. The presence of small amounts of lanthanide oxides on the alumina surface hinders surface sinterizationl and modifies the acid-base properties of the surface.2 In a previous paper we have shown that the supported phase of Sm203/A1203 catalysts changes its crystalline structure as a function of the calcination temperat~re.~ This modification of the structure results in altering the activity and selectivity of C2 in the oxidative coupling of methane. The activity and selectivity in this reaction have been shown to correlate with the catalyst basic it^.^ Therefore, the data obtained for Sm203/ A1203catalysts as a function of the calcination temperature were interpreted as a consequence of the modification of the electronic structure of the Sm-0 active sites on modifying the crystalline structure of the active phases3 In the present work, the interaction of the supported lanthanide phase with the support is studied through spectroscopic measurements that provide information on the electronic structure of the surface species. These results are analyzed using MO SCF ab initio calculations of model clusters in an attempt to understand the lanthanide cation-support interaction resulting in a modification of the catalytic properties of Ln203/A1203 systems.
Experimental Section Ln203/~-A1203catalysts (Ln = La, Sm, Lu) were prepared according to a method previously d e s ~ r i b e d . ~ In . ~ brief, lanthanide nitrate solutions are poured over y-AlzO3 powders (ALUMINUMOXID C, Degussa, surface area 70 f.1 m2/g after calcining at 873 or 1173 K, 4 h) in adequate amounts for obtaining catalysts containing 10, 20, or 40% by weight of lanthanide oxides. The solution was evaporated to dryness and oven dried at 383 K overnight and the solid calcined in air at 873 K, 4 h. Since structural changes were detected in Sm2O3/ A1203 samples when calcined at higher temperature^,^ a second @
Abstract published in Advance ACS Abstracts, March 1, 1995.
set of samarium-supported samples calcined at 1173 K, 4 h, was also studied. DRIFT spectra were taken in a Nicolet 510P instrument in which a diffuse reflectance cell (Spectra-Tech) was fitted. The experimental setup has been described in detail elsewhere.6 Reasonable signal-to-noise ratios were obtained after coadding 200 interferograms. X-ray diffraction (XRD) patterns were taken in a Siemens Krystalloflex D-500 using Cu radiation and pyrolitic graphite as monochromator. XPS experiments were recorded in a Leybold-Heraeus (LHS10) instrument (Mg &, vacuum better than 1 x lo-*, 11 kV, 20 mA). All the spectra were referenced to the spurious carbon at 284.6 eV. X-ray absorption experiments were recorded at the LEIedge of lanthanum (5491 eV), samarium (6721 eV), and lutetium (9250 eV). Sm and La spectra were measured at station 8.1 at the S.R.S. (Daresbury Laboratory, U.K.), using a double silicon crystal working at the (111) reflection as monochromator, detuned 20% in intensity to minimize higher harmonics. Lu spectra were recorded at station EXAFS I11 at LURE DCI (France); in this case monochromatization was obtained by a double silicon crystal working at the (311) reflection and also detuned 20% in intensity. The measurements were carried out in transmission mode using optimized ion chambers as detectors. Samples were pressed into self-supporting pellets with an absorbance of ca. 2.5 ( A p x < l), using boron nitride when necessary and measured at room temperature.
Results and Discussion Spectroscopic Measurements. Figure 1A shows the DRIFT spectra of pure y-AlzO3 and LazO3/A1203-supported systems as a function of the lanthanum loading. The most distinctive feature is the band peaking at 1050-1000 cm-', which does not appear for bulk La203. Therefore, this band should be ascribed to vibrational modes involving A13+ cations that shift to lower wavenumbers on increasing the lanthanum loading. As shown in Figures 1B-D, this behavior is common for all the LnzO3/
0022-3654/95/2099-4655$09.00/0 0 1995 American Chemical Society
Capitin et al.
4656 J. Phys. Chem., Vol. 99, No. 13, 1995
A
I
Lu
{-
v cm - 1 Sm
1050
1000 1
I
I
La
1200
1100
1000
u cm
900
20
0
-t
40
% Ln,O,
Figure 1. Diffuse reflectance infrared spectra of La203/A1203 catalysts calcined at 873 K as a function of the rare earth loading (A) and plot of the variation of the peak position for this vibrational mode as a function of the lanthanide loading of Lu20dA1203 (B), Sm203/Al203 (C), and Laz031A1203 (D). Open symbols stand for catalysts calcined at 873 K and solid symbols for catalysts calcined at 1173 K.
1
A1203 systems, although the observed frequencies depend on the nature of the lanthanide cation. The assignment of this vibration mode is far from clear; however, the presence of bands at 1160 and 1080 cm-' in y-A100H has been ascribed to the in-plane bending (&H) of A1,OH (m = 2, 3) groups7 For a-A100H the frequency associated with these modes has been found at 1060, 1020,969, and 914 cm-l, and bands in the 1100-900 cm-' region are found for all the oxides, hydroxides, and oxohydroxides of al~minium.~ The shift to lower wavenumbers on going from y-A100H to a-A100H might be associated with the structural change since the a-phase can be described by NO6 octahedra sharing corners, while the y-phase may be considered as close to y-Al2O3, where AP+ cations randomly occupy tetrahedral and octahedral holes of a cubic lattice.8 Therefore, a tentative explanation of the shift would be to ascribe it to the change in the coordination number of the A13+ cations. Following this reasoning, the observed shift for Lnz03/A1203 catalysts might be associated with a modification of the coordination number of surface aluminum cations from tetrahedral to octahedral coordination due to the presence of the lanthanide cations. Even if this possibility is considered, other effects have to be expected for explaining the modification of the vibrational frequency as a function of the nature of the rare earth cation. Thus, in Figure 2 the frequency of this mode is plotted against the ionic radius of the Ln3+ cations for catalysts having a rare earth loading of 20% Ln2O3 w/w. The value for pure A1203 vs the radius of the A13+ cation was also included. It can be observed that the vibrational frequency decreases as the ionic ratio increases. This behavior was observedgfor C03*anions in various carbonates, the frequency of each vibration mode decreasing on increasing the radius of the cation, and has to be ascribed to a second-neighbor effect, thus suggesting in our case that the formation of AI-0-M bonds, where M stands for the lanthanide cation, is responsible for the observed shifts.
1050
1000
'
0
C 0.6
0.8
1 .o
I 1.2
Figure 2. Frequency of the AI-0 mode of the different catalysts studied plotted against the ionic radius of the second neighbor of the
aluminum cations of the support for catalysts having a rare earth loading of 20% Ln203 w/w. Structural data discard the modification of the coordination polyhedra of aluminum cations as the main reason for the observed frequency shift on increasing the lanthanide loading. XRD patterns show that supported phases are well dispersed for Ln2O3 loadings up to 20% w/w (10 f 1 Ln3+/nm2),since diffraction lines different from those corresponding to y-Al203 do not appear. This result is in agreement with previous datal0 that calculate the monolayer capacity for the La203/A1203 system as 10 La3+/nm2. For the highest loading tried (40% Ln2O3 w/w) XRD indicates that crystalline phases appear in all the samples under study, although their nature depends on the
J. Phys. Chem., Vol. 99, No. 13, 1995 4657
Lanthanide Oxides Supported on y-A1203 % Sm203
0
20
40
80
60
100
B A
%
134
v
x
0
L
a, C 0
132
c .U
.-C
m
130
n
3
3 W
x .cn C
AI(2s)
0, -4-
C -
3
110
133.5
120
130
140
150
Binding Energy (eV) XP spectra of pure SmzO3 and 20% Sm203/Al~03calcined at 873 K (A). Modification of the binding energy of Sm(4d) levels as a function of the lanthanide loading for Sm203/A1203 catalysts (B).
Figure
supported lanthanide. Thus, a mixed oxide with a perovskite structure, La.4103, appears when calcining 40% La203/Al203 at 873 K. The formation of the perovskite structure implies the modification of the coordination polyhedra of tetrahedral aluminum cations on the surface of the sample to octahedral coordination. However, evidence for this change of coordination is not found in the 40% Lu203/A1203 sample, since C-Lu203 is the only phase detected by XRD. As previously r e p ~ r t e d , ~ 40% Sm20dA1203 shows an intermediate behavior, and after calcining at 1173 K the formation of a perovskite structure can be observed by XRD, while for the same loading and a lower calcination temperature (873 K) the formation of an oxide-like structure is evident. As shown in Figure IC, the formation of the perovskite structure, which implies the modification of the coordination polyhedra of aluminum cations at the surface of the sample, does not alter significantly the vibration frequency of the intense IR band at about 1000 cm-’, which is independent of the calcination temperature for Sm203/A1203 catalysts. Therefore, the observed shift should be better ascribed to the presence of lanthanide cations in the second coordination sphere of the aluminum ones. While the vibration frequency of the A1-0-M bonds is modified as a function of the lanthanide coverage, the electronic
TABLE 1: Binding Energies (BE) of Lanthanide Levels of the Studied Catalysts catalyst level BE (eV) 3dvz 833.5 La203 10%La203/A1203 3dm 835.0 3d5iz 1083.5 Smz03 10% Sm203/A1203 Wiz 1084.5 4dsiz 196.1 LUzo3 10%L~203/A1203 4dwz 196.9 structure of the lanthanide cation is also modified. Figure 3 shows the XPS spectra of the Sm(4d) level for pure Sm2O3 and that supported on y-ALO3 (20%Sm2O3 w/w). The Sm(4dwz) peak in the pure oxide is at 131.0 eV, which is in accordance to literature results for this oxide.” After supporting samarium oxide on Al2O3, the peak shifts to 133.5 eV, while the Al(2s) peak remains in the position that corresponds to y-Al203 (120 eV). The displacement of the Sm(4dsl2) binding energy is a function of the samarium loading but is independent of the calcination temperature. A similar behavior is observed for all the supported rare earth oxides studied, either at the Ln(3d) or Ln(4d) level (Table 1).
Capitin et al.
4658 J. Phys. Chem., Vol. 99, No. 13, 1995
A 4
3.5
Lu
. - - - -La203 10%La20,/A1,0,
A.
A
3.0
\ i
ul
c +-Q)
-8
A A-
2.5
B x
x ,T
3.5
2
3.0 ;ti” v)
C 0)
c
c -
2.5
0 5450
5500
5550 0
20
40
60
80
100
% Ln203
Energy (eV)
Figure 4. Modification of the white line intensity at the La LUIedge on supporting La203 on y-Al203 (A). Effect of the rare earth loading on the white line intensity for Lu203/A1203 (B) and Sm203/A1203(C) catalysts. Open symbols stand for catalysts calcined at 873 K and solid symbols for catalysts calcined at 1173 K. Values for crystalline Lu203. SmzO3, and SmA103 (67% SmzO3) are included for comparison. Figure 3b shows the dependence of the binding energy of the Sm(4d) level with the samarium loading. It shows that, on increasing the loading, the binding energy approaches that found in the pure oxide. This behavior has been previously described for La203/A1203 catalysts12 and has been associated with the presence of lanthanum ions in low coordination following a previous suggestion for dispersed lutetium ions in y - A l ~ 0 3 . ~ ~ However, Haack et a1.I2 found a value of 833.2 eV for the binding energy of the La(3d) level in AlLa03 and of 833.2 eV for the same level in LazO3. According to their hypothesis on increasing the coordination number of the lanthanum cation, their binding energy should shift to lower values; that is, the binding energy for AlLa03, where La3+ cations have a coordination number 12, should be lower than the value for these ions in La203, where they have a coordination number 7. Moreover, analysis of the EXAFS spectra of La20dA1203 and Sm203/A1203 at the lanthanide Lm edge3,5shows that the number of oxygen atoms in the first shell of samarium cations is higher for a 10% Sm203/A1203 sample than for the bulk oxide. However, the reverse is true for La203 samples, the oxygen coordination number of lanthanum cations being slightly higher in the bulk oxide than in a 10% La203/A1203 sample. These data discard the changes in the oxygen coordination number of the lanthanide cation as the main reason for the observed shifts in their XPS spectra, which are always displaced to higher binding energies when supporting the oxides in y-AlzOs (Table l), in spite of the different behavior observed for oxygen coordination numbers. The X-ray absorption spectra of the studied catalysts show the same general characteristics as the DRIFTS and XPS measurements: the spectrum is a function of the lanthanide loading and is independent of the calcination temperature, and all the rare earth cations behave similarly. Figure 4A shows the XANES spectra of the La LIEedge for bulk La203 and a La203/A1203 sample. The most distinctive feature is the presence of a fairly intense white line with a higher intensity for the supported sample than for the bulk oxide. A similar behavior has been previously described in glasses of
the system K 2 0 Si02 La203,14 although no explanations have been given for this fact. A smooth decrease in the white line intensity on increasing the samarium or lutetium content of the solids can also be observed in Figure 4B,C. For Sm203/A1203 catalysts neither the calcination temperature nor the crystalline structure of the compound shows a significant influence on the white line intensity. Thus, values for bulk SmA103 and Sm2O3 have been included in Figure 1C and follow the general trend vs Sm2O3 content observed for the supported samples, in spite of the different structure of both compounds and the coordination of samarium cations by oxygen atoms, which is 7 in Sm2O3 and 12 in the SmA103 structure. Therefore, white line intensities have to be related only to the Ln/Al atomic ratio and to the nature of the rare earth cation. Taking into account the selection rules for this type of electronic transition, the observed white line has to be assigned to a 2p 5d tran~iti0n.l~ So, the population of the 5d level of the lanthanide cations decreases on decreasing the loading. As the reduction of the lanthanide cations cannot be considered feasible,16a modification of the electronic structure of the rare earth cation has to be invoked. The reported spectroscopic measurements point to a strong interaction between the supported, phase and the support that results in the modification of the electronic structure of the active sites present in the catalyst surface. Theoretical Calculations. The interactions between the support and the supported phase have been modeled through a cluster approximation, lanthanum being chosen for having a closed shell. Three clusters have been selected as representative of the y-AlzO3 and La203 interaction: (OH)3AlOAl(OH)3, (OH)3AlOLa(OH)3,and (OH)3LaOLa(OH)3, Chart 1. The bond angles, distances, and energies after optimizing the geometry” are shown in Table 2. The most relevant characteristic among the optimized parameters is the relative independence of them on the nature of the second neighbor of tht metallic center; for example, the La-Ot distance is 2.325 A when the second neighbor is La and 2.320 8, when it is Al. In general, both distances and angles fall within 5% on changing La to A1 as
-
J. Phys. Chem., Vol. 99,No. 13, 1995 4659
Lanthanide Oxides Supported on y-A1203
CHART 1: Optimized geometries of MM’O(OH)6 clusters: AlzO(OH)a (A); La20(0H)6 and LaA1O(OH)6 (B). Bond lengths and angles are given in Table 2
TABLE 4: Net Charges for the Different Clusters Studied qM
Al2O(OH)6 La20(0H)6 AlLaO(OH)6
A1 La La A1
1.669 1.671 1.712 1.659
gob -1.285 -1.144
-1.207
90t
qH
-1.004 -0.996 -0.997 -0.995
0.327 0.296 0.294 0.311
TABLE 5: Protonic Affinities for the Studied Clusters PA (kcal mol-’)
A120(0H)6
La20(0H)6
ALaO(OH)6
180.2
205.7
207.0
properties of the normal coordinate. The normal coordinate Q can be represented as a linear combination of the individual coordinates having the same symmetry:
TABLE 2: Parameters for the Optimized Geometries of MM’O(OH)( Clusters AlLaO(0Hk
&-Ob
dM-0, do+
(A) (A) (A)
0-M-0 M-0-H
M-0-M’ SPG
ET
ALO(OH)~
La20(0H)6
1.718 1.782 0.949 116.9 123.7 178.8 C2Y -117.5842
2.259 2.320 0.950 110.9 168.6 180.0
AI
La
2.241 1.716 2.325 1.798 0.950 0.949 110.9 112.8 160.1 128.3 180.0 c1
D3d
-175.4225
- 146.4902
TABLE 3: Intrinsic Frequencies (0’)for the Internal Coordinates of the Studied Clusters A~~O(OH)~ La20(0H)6 ALaO(OH)6
M
W’M-Ob
W’M-Oi
A1 La La A1
890 505 541
688 489 470 690
848
second neighbor. The only difference worth mentioning is a contraction of the La-Ob distance when La is substituted for aluminum in the second coordination shell. Among the calculated frequencies for these models several bands appear at frequencies ranging from 1200 to 900 cm-’, in good accordance with the experimental result (Figure 1). A band at w = 1107 cm-’ is calculated for the Al2O(OH)6 cluster that shifts to 986 cm-’ for the LaAlO(OH)6one. Although these bands are mainly associated with the stretching motion of the Al-Ob bonds, this normal mode has stretching and bending components of A1-OH and Al-0-A1 bonds, and it is difficult to extract clear conclusions about the nature of the observed experimental shift in the DRIFTS spectrum. For solving this problem, we have calculated the intrinsic frequencies (0’) according to the method proposed by Boatz and Gordon.22The intrinsic frequencies associated with the internal coordinates that can be considered relevant in the assignment of the experimental spectrum of y-AlzO3 are shown in Table 3. Thus, while the calculated frequency (a)shifts more than 120 cm-’ on substituting A1 for La, it can be observed that the intrinsic frequency (w’) that is directly related to the bond force constant shows a shift of only 50 cm-’. So, at least, part of the experimental shift has to be ascribed to the symmetry
When the component is incorporated, and since La is heavier than Al, the observed frequency should decrease, even assuming that the La-0 bond force constant is of the same order. The net charges associated with every atomz3in the different clusters obtained from Mulliken population analysis are reported in Table 4. The analysis of the net charge associated with lanthanum shows a slight dependence on the nature of the element in the second coordination sphere, being lower for a second neighbor La than a second neighbor Al. The reverse effect can be observed for the case of aluminum, indicating that the formation of A1-0-La bonds involves some charge transfer from lanthanum to aluminum across the oxygen bridge. This result allows us to discard the effect of the coordination number as the main factor responsible for the XPS shift, as previously proposed.12 In a previous report we have shown by EXAFS that the number of A1 and Sm neighbors in the second coordination sphere of the supported samarium cations is a function of the 10ading.~ In this way, on increasing the average number of samarium cations in the second coordination sphere (Le. raising the loading), the net charge on the lanthanide cation should decrease. If we assume that the observed binding energy is only a function of the effective nuclear charge felt by the electron in the probed level, that is, there is an absence of final state effects, the binding energy should shift to lower values on increasing the average coordination number of lanthanum in the second coordination sphere, but the coordination number of oxygen in the first coordination sphere does not necessarily have to be modified. For obtaining a description of the oxide ions donating power, the gas phase protonic affinity of the clusters has been obtained, eq 1:
The obtained values, Table 5 , show a trend that fits fairly well the experimental sequence for the zero point charge of the surface of Laz03/A1203.z This indicates that in the absence of other effects the donating power of the oxide ions bound to aluminum and lanthanum ranges within those for pure alumina and lanthana. Figure 5 shows the electron density in d orbitals of lanthanum as a function of the second neighbor. It can be observed that the d level population depends on the nature of the atoms present in the second coordination sphere, being higher when the second neighbor is La than when it is Al. This result provides a
Capitiin et al.
4660 J. Phys. Chem., Vol. 99, No. 13, 1995 1.10
1.08
1.06
1.04
1.02
1 .oo
0.98
X=
I
I
H
AI(OH),
I
La(OH)3
(OH),-La- 0-X Figure 5. Calculated d orbital electron density as a function of the nature of the second neighbor of La in (OH)3LaOX clusters.
qualitative explanation for the observed lowering in white line intensity as a function of the lanthanide cation loading.
Conclusions Spectroscopic measurements and ab initio calculations have shown that, on supporting rare earths on alumina, a modification of the electronic structure of the lanthanide cation occurs that should affect the catalytic properties of rare earth oxides. In general, the lanthanide cation presents a strong interaction with the support which results in a redistribution of the electronic density over AlOLn ensembles. This charge redistribution over AlOLn ensembles implies a modification of the acid-base properties of the active site resulting in a tuning of the strength of the acid-base pairs, the acidity of the lanthanide cations when isolated on the support surface being higher while the basicity of the oxide cations is higher than that corresponding to y-AlpO3 but smaller than in pure rare earth oxides.
Acknowledgment. Financial support for this work has been obtained from Comision Interministerial de Ciencia y Tecnologia (PB88-0257, PB92-0665, and PB92-0662). References and Notes (1) Bettman, M.; Chase, R. E.; Otto, K.; Weber, W. H. J . Catal. 1989, 117, 447.
(2) Subramanian, S.; Chatta, M. S.; Peters, C. R. J. Mol. Catal. 1991, 69, 235. (3) Capitan, M. J.; Malet, P.; Centeno, M. A,; MuAoz-Paez, A,; Carrizosa, I.; Odriozola, J. A. J. Phys. Chem. 1993, 97, 9233. (4) (a) Maitra, A. M. Appl. Catal. A 1993, 104, 11. (b) Centeno, M. A.; Capitin, M. J.; Malet, P.; Carrizosa, I.; Odriozola, J. A. J . Catal. 1994, 148, 399. (5) Malet, P.; Benitez, J. J.; Capitan, M. J.; Centeno, M. A,; Carrizosa, I.; Odriozola, J. A. Catal. Letr. 1993, 18, 81. (6) Benitez, J. J.; Carrizosa, I.; Odriozola, J. A. Appl. Spectrosc. 1993, 47, 1760. (7) Ryskin, Ya. I. In The Infrared Spectra of Minerals; Farmer, V. C., Ed.; The Mineralogical Society: London, 1974. (8) Wells, A. F. In Srructural Inorganic Chemistry; Oxford University Press: London, 1965. (9) van der Marel, H. W.; Bentelspacher, H. In Atlas of Infrared Spectroscopy of Clay Minerals and their Admixtures; Elsevier: Amsterdam, 1976. (10) Xie, Y.; Quian, M.; Tang, Y. Sci. Sin. Ser. B 1984, 6, 549. (11) Rao, C. N. R.; Sarma, D. D. In Science and Technology of Rare Earth Materials; Academic Press: New York, 1980; p 291. (12) Haack, L. P.; de Vries, J. E.; Otto, K.; Chatta, M. S. Appl. Catal. A 1992, 82, 199. (13) Alvero, R.; Bemal, A.; Canizosa, I.; Odriozola, J. A. Znorg. Chim. Acta 1987, 140, 45. (14) Larson, E. M.; Wong, J.; Ellison, A. J. G.; Navrotsky, A,; Lytle, F. W.; Greegor, R. B. X-Ray Absorption Fine Structure; Hasnair, S., Ed.; Ellis Honvood: New York, 1991; p 328. (15) (a) Lytle, F. W.; Wei, P. S. P.; Greegor, R. B.; Via, G. H.; Sinfelt, J. H. J . Chem. Phys. 1979, 70, 4849. (b) Mansour, A. N.; Cook, J. W.; Sayers, D. E. J. Phys. Chem. 1984, 88, 2330. (16) Johnson, D. A. J. Chem. SOC.,Dalton Trans. 1974, 1671. (17) Ab initio Hartree-Fock calculations were undertaken using the pseudopotential approximation to describe core electrons. For La the HayW a d P ECPs were used, while for A1 and 0, the Duraud-Ba~thelat'~ pseudopotentials were chosen. The valence basis set was of double-c type: (10,5,3) contracted to 4,2,2] for LaI8 and (4,4) contracted to [2,2] for AI and 0.19 For H, the double-g basis of Dunningz0was taken. Calculations were carried out using the HONDO-8,4 program.z1 (18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (19) Duraud, Ph.; Barthelat, J. C. Theor. Chim. Acta 1975, 38, 283. (20) Dunning, T. H.; Hay, P. J. In Modem Theoretical Chemistry; Plenum Press: New York, 1976; Vol. 2. (21) Dupuis, M.; Chin, S.; Marquez, A. Modem Tools for Including Electron Correlation in Electronic Structure Studies: HONDO and CHEMStation; in Relativistic and Electron Correlation Effects in Molecules and Solids; Malli, G. L., Ed.; NATO AS1 Series; Plenum Press: New York, 1994. (22) Boatz, J. A,; Gordon, M. S. J . Phys. Chem. 1989, 93, 1819. (23) Although the use of the absolute values of atomic population obtained through Mulliken's method has been extensively criticized, its application to closely related structures allows one to obtain representative ~orrelations.2~ (24) See for instance: (a) Ammeter, J. H.; Biirgi, H. B.; Thibeault, J. C.; Hoffmann, R. J . Am. Chem. Soc. 1978, 100, 3686. (b) Whangbo, M.H.; Hoffmann, R. J . Chem. Phys. 1978, 68, 5498.
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