J. Phys. Chem. 1988, 92, 3079-3086
3079
Comparison of the Bindlng of Carbon, Nitrogen, and Oxygen Atoms to Single Nickel Atoms and to Nickel Surfaces Itai Panas, Josef Schiile, Ulf Brandemark, Per Siegbahn,* and Ulf Wahlgren Institute of Theoretical Physics, University of Stockholm, Vanadisvagen 9, S - 11346 Stockholm, Sweden (Received: June 29, 1987; In Final Form: October 30, 1987)
Multireference CCI calculations have been performed for the diatomic molecules NiC, NiN, and NiO, and for the cluster systems NiSC, NiSN,and Ni50. Of the diatomic molecules, NiN has by far the smallest dissociation energy, which we argue is primarily due to the small electron affinity of nitrogen. The bonding to nickel 3d, which is pronounced for the diatomic molecules, disappears almost entirely for the cluster systems. In the full geometry optimization of the NiSX systems, including cluster relaxation, the final geometries are rather similar for the three adsorbates, all being adsorbed below the surface. The energy gain in penetrating the surface is much larger for carbon and nitrogen than it is for oxygen, however. This is in line with the experimentallydetermined surface geometries, where carbon and nitrogen adsorb close to the surface and reconstruct the surface, whereas oxygen stays further out and does not reconstruct the surface. For oxygen the restoring forces from neighboring atoms in the surface are larger than the driving force to reconstruct the surface. The origin of the difference between the adsorbates is that the 2p, orbital, which points toward the surface, is almost doubly occupied for oxygen, whereas this orbital is only partially occupied for carbon and nitrogen.
I. Introduction There are interesting differences between carbon, nitrogen, and oxygen in their binding to nickel atoms and to nickel surfaces. For the diatomic molecules, NiC and NiO are similar with large dissociation energies and short bond distances',2 whereas NiN has a small dissociation energy and a longer bond d i ~ t a n c e . ~In the case of chemisorption on nickel surfaces, on the other hand, carbon and nitrogen behave similarly with a very short equilibrium height above the surface, 0.1-0.2 A,4,5whereas oxygen chemisorbs substantially further out at 0.8-0.9 A.5-6 There are often large similarities between results for small molecules and chemisorption results as, for example, pointed out in ref 7 and 8, but here is consequently a case where there are striking differences. There is no direct experimental determination of the dissociation energy of NiC, but in ref 1 an estimate was made on the basis of analogies with similar compounds. The estimated value was 80 kcal/mol, which was compared to a value of only 23 kcal/mol from a GVB calculation. It is well-known that calculations on transition-metal compounds are difficult, requiring an extensive treatment of electron- correlation for obtaining accurate results, but the large discrepancy between the GVB value and experiment for NiC is still remarkable. In the present work large multireference CCI calculations are performed to resolve this discrepancy between theory and experiments. There is no experimentally determined dissociation energy of NiN either, but a CCI calculation exists, and the result from that calculation is 40 kcal/mol with respect to 3DNiS3 For NiO there is a thermochemical determination of the dissociation energy with a value of 87 kcal/mol.2 Atomic oxygen chemisorption on Ni( 100) has been extensively studied. Previously there has been a slight controversy whether oxygen adsorbs precisely over the fourfold hollow position or dispkced from this position by 0.3 A in the (1 10) dire~tion.~This controversy is resolved by a later analysis and a later experiment5J0 (1) Kitaura, K.; Morokuma, K.; Csizmadia, I. G. Theochem 1982,88, 119. (2) Grimeley, R. T.; Burns, R. P.; Inghram, hk G. J . Chem. Phys. 1961, 35, 551. (3) Siegbahn, P. E. M.; Blomberg, M. R. A. Chem. Phys. 1984.87, 189. (4) Onuferko, H.; Woodruff, D. P.; Holland, B. W. Surf. Sci. 1979, 87, 357. (5) Wenzel, L.; Arvanitis, D.; Daum, W.; Rotmund, H. H.; Stour, J.; Baberschke, K.; Ibach, H., to be published. (6) Stahr, J.; Jeager, R.; Kendelewicz, T. Phys. Rev. Lett. 1982, 49, 142. (7) Muetterties, E. L. Bull. SOC.Chem. Belg. 1975, 84, 959. (8) Ertl, G. In Metal Clusters in Catalysis: Gates, B. C.; Guczi, L.; KnBzinger, H., Eds.; Elsevier: Amsterdam, 1986. (9) Demuth, J. E.; DiNardo, N. J.; Cargill, G. S.Phys. Reu. Lett. 1983, 50. 1373.
0022-3654/88/2092-3079$01.50/0
where the displacemenf is ruled out. The most accurate geometry for oxygen is 0.88 A directly over a fourfold hollow position determined by a SEXAFS mea~urement.~ Accurate determinations of the adsorption geometry for atomic carbon and nitrogen on Ni(100) are very recent. It turns out that both carbon and nitrogen, in contrast to oxygen, induce a substantial reconstruction of the surface leading to a p4g structure.4J1 The resulting geometries for carbon and nitrogen are 0.24 and 0.12 respectively, directly above the fourfold hollow site. The chemisorption energy-for atomic nitrogen on Ni( 100) has been determined by a desorption experiment to 135 kcal/molI2 assuming no barrier for desorption of NZ. Since N, is known to have an activation barrier for dissociation on nickel surfaces, the correct chemisorption energy for atomic nitrogen could be substantially below this value. There are two measurements of the chemisorption energy of atomic oxygen on Ni( 100). The first one is a calorimetric value of 115 kcal/mol,13 and the second one is an ESCA m e a ~ u r e m e n t 'combined ~ with the nitrogen chemisorption energy12 leading to 130 kcal/mol. The correct chemisorption energy for oxygen is probably somewhere in between these measurements. The analysis of the ESCA measurement made in ref 14 is very interesting from a theoretical point 6f view and will be discussed in detail in the present paper. For atomic carbon the only experimental value for the chemisorption energy is a measurement for a monolayer of graphite on Ni( 100).giving 170 k c a l / m ~ l . ' ~ This value is almost identical with the heat of sublimation of graphite, and it is therefore probably more characteristic of graphite for-mation than of chemisorption of atomic carbon on nickel. 170 kcal/mal could thus be a clear overestimation of the true chemisorption energy for carbon on nickel. The chemisorption of atomic carbon, nitrogen, and oxygen on Ni(100) is in the present paper studied by using the simplest possible model, a Ni5X cluster; see Figure 1. As a model of a real nickel surface this small cluster obviously has limitations. One such clear limitation is that when the cluster geometry is optimized, the reconstructions will be quite exaggeratedfor NiSX since the restoring forces from neighboring nickel atoms in the lattice are missing. However, for the present purpose this is not only a disadvantage since any tendency for reconstruction should be (10) Richter, H.; Gerhardt, U.Phys. Reu. Lett. 1983, 52, 1570. (11) Daum, W.; Cehwald, S.; Ibach, H. Surf. Sci. 1986, 178, 528. (12) Conrad, H.; Ertl, G.; Kuppers, J.; Latta, E. E. Surf. Sci. 1975, 50, 296. (13) Brennan, D.; Hayward, D. 0.;Trapnel, B. M. W. Proc. R. SOC. London, A 1960, 256, 81. (14) Egelhoff, W. F., Jr. Phys. Reu. E.: Condens. Mutter 1984, 29, 3861. (15) Isett, L. C.; Blakely, J. M. Surf. Sci. 1975, 47, 645.
0 1988 American Chemical Society
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3080 The Journal of Physical Chemistry, Vol, 92, No. I 1 I988 I
TABLE I: CASSCF and CCI Results for the Diatomic Molecules NIX, with X Equal to Carbon, Nitrogen, and Oxygen Re, a0 De,kcal/mol 4(x)a we, cm-'
CASSCF CCI CASSCF CCI CCI CASSCF CCI NiC NiN NiO
3.44 3.26 3.15
3.21 3.27 3.20
73.5 12.1 44.7
75.0 -0.31 35.2 -0.46 70.2 -0.64
844
754
" q ( X ) is the charge on X.
Figure 1. The Ni,X system. Large circles denote Ni atoms, and the small circle is the adsorbate X, with X equal to carbon, nitrogen, or oxygen
clearly visible in the results. In cases where the surface does not reconstruct, the Ni5 cluster has been shown to be a quite adequate model for chemisorption. For both hydrogen and oxygen (in particular) chemisorption geometries and energies are qualitatively similar for a Ni5 cluster and a Ni41cluster, which is essentially converged in terms of cluster size.I6 The nickel atoms in the cluster are in the present study described as effective one-electron atoms by using a recently developed effective core potential (ECP)." Covalency effects from the 3d shells are investigated by replacing one of the ECP atoms by an all-electron atom. All-electron calculations have also been carried out on Ni5N, and comparisons are furthermore made to previous all-electron calculations on Ni5O.I7 The present calculations are not the first where the Ni5 cluster has been used as a model for chemisorption at the fourfold hollow site. Several other similar investigations exist in the literature: see, for example, ref 18-20, 11. Computational Details
The one-electron ECP's used for the nickel cluster atoms have been described in detail previously and contain a modeling of 3d relaxation effects.17 The basis set used for these atoms is a contracted 4s,lp set with a frozen 3s orbital. From this space a virtual 3s* orbital has been deleted. In some of the calculations the second-layer nickel atom in Ni5 has been replaced by an all-electron atom. This atom is described by the SDZC set (1) of Tatewaki and HuzinagaZ1 with one additional diffuse 3d function with exponent 0.1641 and two additional 4p functions with exponents 0.1 122 and 0.0355. This basis set is minimal basis contracted for the inner shells ls-3d, and the 1s-3p shells are frozen in their atomic shapes. In a few calculations all five nickel atoms were described with this basis set. The calculations on the diatomic molecules NiC. NiN, and NiO used the basis set described above for Ni except that the 3d basis was split into three functions. The carbon, nitrogen, and oxygen basis set, both for the diatomic and the cluster calculations, is a contracted 4s,3p setz2augmented with a diffuse, even-tempered, p function and a d function (exponent 1.0). The CASSCF methodz3has been used in all cases to generate orbitals. The active space used in each case will be denoted ( i j . k , n where i is the number of active orbitals in symmetry 1 etc. C, symmetry was used in all calculations. Dynamical correlation effects are described according to the multireference contracted C I (CCI) method.24 In general, all CASSCF configurations with a coefficient larger than 0.05 were (16) Panas, I.; Siegbahn, P.; Wahlgren. U., to be published. (17) Panas, I.: Siegbahn, P.: Wahlgren, U. Chem. Phys. 1987. 112, 325. (18) Walch. S. P.; Goddard, W . A., Ill Solid State Commun. 1977. 23, 907. (19) Bauschlicher, C. W., Jr.; Bagus, P. S. Phys. Reu. Lett. 1985,54, 349. (20) Bauschlicher, C. W., Jr. Inf.J . Quantum Chem. 1986, S20, 563. (21) Tatewaki, H.; Huzinaga. S.J . G e m . Phys. 1979, 7 1 . 4339. (22) Dunning, T. H . J . Chem. Phys. 1970, 53, 2823. (23) Siegbahn, P. E. M.; Almlof. J ; Heiberg. A.: Roos. B. 0. J . Chem. Phys. 1981, 74, 2384. (24) Siegbahn, P. E. M . Int. J . Quanrum Chem. 1983, 23, 1869.
selected as reference states. The multireference analogue of Davidson's correctionZSis included in all CCI energies reported here. The accuracy of the CCI method is discussed in ref 24, where it is shown that it is important to give all the configurations with large coefficients (greater than 0.05) full variational freedun. If this is done, the results are usually quite close to the corresponding uncontracted CI case. Comparisons between uncontracted C I and CCI calculations have recently been made for C2 and Si, with errors in De of 0.04-(0,= 5.77 eV) and 0.08 eV (De = 2.87 eV), respectively.26 The larger error for Siz is due to a less adequate reference space. For the A-X excitation energy in OH using a large reference space, the error in using CCI is only 8 cm-] ( T , = 34 01 3 cm-I), and the transition moment shows a correspondingly small error.27 The bond angle in NiH2 has also recently been used to judge the accuracy of the CCI method,** and a difference of 1 2 O was reported between the uncontracted CI result and the CCI results in ref 29. The potential curve for bending NiH, is, however, very flat, and the energy difference for a 12' change is only 0.3 kcal/mol. Furthermore, the method used in ref 29 to determine the bond angle was a two-dimensional fit and the one in ref 28 a one-dimensional fit, which is certainly also responsible for part of the difference. In the discussions in sections 111 and IV we make frequent use of the Mulliken population analysis. In spite of several other suggestions this is still the most widely used model for defining charges in a molecule. For obvious reasons the Mulliken charges should be used only as qualitative concepts. Even though artifacts with the population analysis are often pointed out, any convincing analysis showing that it is entirely useless in the present type of calculations is to our knowledge missing. This would also be contrary to our experience. An interesting demonstration of a case where the population analysis is misleading has been pointed out by Bagus and Bauschlicher for Ni(C0),.30 They showed that the large 4p population on nickel in this molecule was artificial due to the diffuse nature of these orbitals. We have for this reason not included any discussion of 4p charges in our analysis. The 4s charge i s for the same reason questionable, and we have avoided use of this charge as well. We consider the 3d population much more reliable, however, since these orbitals are more localized. Our experience is furthermore that the 2p charges on the carbon, nitrogen, and oxygen atoms are reasonably reliable with the limited basis sets we use. We find it meaningful to discuss changes on the order of 0.2 electron relating to these orbitals. We therefore trust changes of the total atomic charges of the same order. On the basis of the Mulliken population analysis we have also tried to define covalency and ionicity. The ionicity is simply related to the total charges, whereas the covalency is taken to be proportional to the overlap population. In a few cases we have further found it illustrative to use another definition of covalency. If in a T orbital with four electrons there is only one electron on one of the atoms, we have drawn the conclusion that there cannot be more than one covalent bond. A smaller covalency is obviously still possible. For two covalent bonds to be formed, two electrons on each atom would have been required. This definition of co(25) Davidson, E. R. In The World of Quanfum Chemistry; R.; Daudel, Pullman, B., Eds.; Reidel: Dordrecht, 1974. (26) Bauschlicher, C. W., Jr.; Langhoff, S.R. J . Chem. Phys. 1987, 87, 2919. (27) Bauschlicher, C. W., Jr.; Langhoff, S. R. J . Chem. Phys., in press. (28) Low, J. J.; Goddard. W. A., 111 Orgonometalk 1986, 5 , 609. (29) Blomberg, M. R . A.; Siegbahn, P. E. M. J . Chem. Phys. 1983, 78, 986. (30) Bauschlicher. C. W.. Jr.; Bagus, P. S. J Chem. Phys. 1984,81, 5889.
Binding of C, N, and 0 Atoms to Nickel valency is related to the bond order concept. 111. The Diatomic Molecules NiC, NiN, and NiO The results for the diatomic molecules NiC, NiN, and NiO are given in Table I. NiN has a much smaller dissociation energy than NiO, which in turn is slightly smaller than that for NiC. The calculated values are 35, 70, and 75 kcal/mol, respectively. The major reason for the difference in the dissociation energies is the difference in the electron affinities. The experimental electron affinities are 1.25 eV for carbon,31around 0.0 eV for nitrogen, and 1.46 eV for oxygen.32 The larger dissociation energy for carbon than for oxygen is a result of a larger covalency for carbon due to triple bond contributions. We have chosen to correlate the bond strength in N i x with the electron affinity of X, which we feel is rather natural in molecules that are at least partly ionic. Another very related explanation for the weak bond in NiN is to emphasize the stability of the 2p3 configuration of the nitrogen atom according to Hund's rules. It is of course due to this stability that the electron affinity of nitrogen is so small. The stability of this configuration also leads to a larger ionization energy for nitrogen than for carbon and oxygen. This latter property seems, however, less relevant for NiN since nitrogen becomes negative. The CASSCF calculations used a (3,2,2,0) active space (see section I1 for definition) with 8 active electrons for NiC, 9 electrons for NiN, and 10 electrons for NiO. The inactive and frozen orbitals were (8,2,2,1) for all three molecules. The number of reference states in the CCI calculations was 15 for NiC, 10 for NiN, and 16 for NiO. The number of electrons correlated was 14 for NiC, 15 for NiN, and 16 for NiO. Since NiN has a very different dissociation energy from NiC and NiO, a large corresponding difference in the wave functions might be expected. The population analysis shows that NiN follows the trend of NiC and NiO very well, however. The total charge transfer between the atoms in the molecule is 0.31 for NiC, 0.46 for NiN, and 0.64 for NiO, which does not follow the trend in the electron affinities but rather the size of the nuclear charge. The 3d occupation on Ni is very close to 8.5 for all three diatomic molecules, and the 4s occupation is between 0.8 and 1.O. At the CASSCF level there are three active u orbitals with rather similar occupation numbers for the three molecules. Two of the u orbitals are strongly occupied with occupations between 1.85 and 1.95, while the third one is weakly occupied with an occupation between 0.15 and 0.25. These three orbitals can be characterized as a (4s 2p,) covalent bond, a (4s - 3dJ hybrid pointing away from the nuclear axis, and finally the third orbital, which is a mixture of a (4s 3du) hybrid and the antibonding (4s - 2p,) orbital. The 2p, occupation is 1.26 for carbon, 1.49 for nitrogen, and 1.56 for oxygen, again showing a stronger polarization for the larger nuclear charge. -In the K symmetry the bonds are formed between 3d, and 2p, atomic orbitals. The 2p, occupation is 1.11 for carbon, 2.01 for nitrogen, and 3.12 for oxygen. The 3d, population is quite similar in the three molecules: 2.81 in NiC, 2.94 in NiN, and 2.80 in NiO. Since a quantitative definition of covalency does not exist (see section 11), a division of the bonding in the molecules into ionic and covalent components is somewhat arbitrary. One way is to use the population analysis and determine the maximum number of possible covalent bonds. In the K symmetry for NiC it is thus clear that there can not be more than 1.1 1 covalent bonds, since the occupation of 2p, of carbon is 1.11. The remaining part of the double bond would then be assigned as ionic. For NiN and NiO not only the bonding but also the antibonding a bond is occupied. In NiO the total number of K bonds is thus only 1.5, of which at most 0.88 can be covalent since the 2p, occupation is 3.12. Another way of looking at the bonding in NiC is to view it as having a triple bond between Ni+ and C- with the I bonds strongly polarized over to Ni'. This type of triple bond is quite similar in origin to the one found in the C O molecule, which has the strongest bond of all first-row diatomic molecules. The reason
+
+
(31) Seman, M. L.; Branscomb, L. M. Phys. Rev. 1962, 125, 1602. (32) Hotop, H.; Linekrger, W. C. J . Phys. Chem. Rex Data 1975,4,539.
The Journal of Physical Chemistry, Vol. 92, No. 11, 1988 3081 NiC is more bound than NiO in spite of the smaller electron affinity for carbon is clearly that NiC has a stronger covalency as depicted by the overlap populations. For NiC the total overlap population is 0.86 compared to 0.62 for NiN and 0.54 for NiO. With the large similarity in the bonding between the three molecules the ground states are easily deduced once the ground state for one of them is determined. From what is described above it is clear that NiC with its polarized triple bond has a closed-shell 'E' ground state. Adding a 2p electron from nitrogen in NiN leads to a *l'Iground state for this system, and with the additional 2p electron present on oxygen the ground state for NiO is 32-. The equilibrium bond distance is extremely sensitive to the correlation treatment. At the CASSCF level NiC has a much longer bond distance than NiN and NiO, whereas at the CCI level NiN has the longest bond distance. We have also found that the bond distance depends critically on the choice of reference space. It seems clear that an accurate determination of the bond distance requires a much more careful analysis than we have made here. There has been only one previous calculation on NiC. This calculation used the GVB method, describing each of the three bonds with a pair of GVB orbitals.] The computed dissociation energy for NiC from that calculation was 23 kcal/mol as compared to the present result of 75 kcal/mol and the experimental estimate of 80 kcal/mol. One reason for the remarkable difference in the calculated results is that in ref 1 the dissociation energy was computed with respect to the 3F(d8s2)state of nickel whereas we considered dissociation to the 3D(d9s)state. Experimentally 3F and 3D are almost degenerate, but in our calculation the splitting is 9 kcal/mol and in the GVB calculation it should be around 30 kcal/mol. A much more favorable dissociation energy of around 50 kcal/mol can therefore be generated for the GVB calculation if dissociation to 3D is considered instead. The remaining discrepancy to our result is mainly due to additional near-degeneracy effects which are included in our CASSCF calculation. The most important difference between our CASSCF calculation and the GVB calculation is that we used three active u orbitals, none of which has an occupation number close to 2.0 or 0.0, whereas the GVB calculation used only two active u orbitals. A CASSCF calculation without the third active u orbital lowered the dissociation energy by 11 kcal/mol. The (3,2,2,0) CASSCF value for the dissociation energy is 73 kcal/mol, which is very close to the CCI value 75 kcal/mol. The similarity between the CASSCF result and the CCI result does not mean that dynamical corrglation effects are negligible for NiC, but it is rather a result of canceling effects. There is a loss of dynamical correlation in the nickel 3d shell upon bond formation since the total 3d population drops. On the other hand, the dynamical correlation energy on the ligand X in N i x increases due to the additional electronic charge. For NiN and NiO the dynamical correlation effects on the dissociation energy do not cancel as they do for NiC, and the CASSCF values are therefore quite different from the CCI values for these molecules. i t is of course somewhat arbitrary to choose to calculate the dissociation energies with respect to the d9s state of nickel rather than the d8s2state, since the d population in the molecule is around 8.5. It is, however, our experience that consistently better results are obtained for covalently bound nicker compounds if dissociation to the d9s state is considered, irrespective of the level of treatment. It is only with this asymptote that the CASSCF and CCI values are so close to each other for NiC. It should also be pointed out that at the CCI level De only drops by 9 kcal/mol if dissociation to d8s2is considered, which is a considerably smaller drop than the 30 kcal/mol in the GVB calculation described above. The results of the present calculation on NiN and our previous calculation' are qualitatively similar but show some quantitative differences. Our present dissociation energy is 35 kcal/mol compared to our previous value of 40 kcal/mol. The origin of this difference is a superposition error in our previous calculation mainly due to the omission of diffuse p functions on nitrogen. The present CCI bond distance is 3.27 a,, and the previous is 3.43 a,. This difference arises mainly from the use of a larger reference space with 10 states compared to only 7 reference states used in
3082
Panas et al.
The Journal of Physical Chemistry, Vol. 92, No. 11, 1988
yy- - .. .
3 3296
33236
3 3296
4 7088
C
5.53
N
. .~
0
:2 :
--- "- -.
Figure 2. CASSCF results for geometries and CCI results for dissociation energies of Ni5X. Distances are in a. and energies in kcal/mol. The geometry optimization was made with a C,, symmetry constraint. The asymptotic energy is for a relaxed Ni, cluster.
the previous calculation. The sensitivity of the results is rather surprising and arises from strong balancing ionic and covalent effects in the bonding. A previous CI calculation on NiO has been reported by Bauschlicher et al.33 The results from that calculation are qualitatively the same as ours. The ground state is the same, the description of the bonding is the same, and the charge on oxygen is practically the same, about -0.7. There are some slight differences, however. Our bond distance, 3.20 ao, is longer than the value 3.16 a. in ref 33, the dipole moment is smaller in our calculation, 1.8 1 au compared to 2.36 au, and our we is 754 cm-I compared to 700 cm-I. These differences are due to differences in the reference spaces used. We used 16 reference states defined from a CASSCF calculation, whereas they used 4 reference states generated by an S C F calculation. Just as for NiN, we found that the bond distance is extremely sensitive to the reference space used. Part of the difference can also be due to the use of the CCI method in the present study as compared to the use of the uncontracted C I method in ref 33. For the accuracy of the CCI method, see section 11. As an overall picture, the results in ref 33 point at a more ionic NiO than our results do. This is reasonable since their results should be biased toward the SCF solution, which is known to be too ionic, whereas our results should be biased toward the CASSCF solution, which is in general too covalent. There is another reason why we expect that our solution could be a little bit too covalent, and that is that we have a basis set deficiency in the description of 0-. Our basis set (as that used in ref 33) has too few d functions and no f function on oxygen, and these basis functions are known to affect the electron affinity of oxygen substantially and should increase the ionicity. At the completion of this work an experimental determination of the bond distance and frequency of NiO appeared in the l i t e r a t ~ r e . The ~ ~ experimental result for the frequency is 828 cm-' compared to our value of 754 cm-I, and the bond distance is 3.08 a. compared to our value of 3.20 a,. We have already pointed out the choice of reference space as one possible source for these errors. Additional angular correlation functions for the nickel 3d shell and for oxygen should further shorten the bond distance and increase the frequency. IV. The Interaction between C, N, and 0 with Ni5 Two different types of geometry optimization, in which all nickel atoms are described by using one-electron E C P s , have been performed for the Ni5X system with X equal to carbon, nitrogen, and oxygen. In the first geometry optimization (see Figure 2), the only constraint was that the C4, symmetry was kept. This means that the Ni-Ni distance in the first layer was varied, the (33) Bauschlicher, C. W., Jr.; Nelin, C. J.; Bagus, P. S. J . Chem. Phys. 1985, 82, 3265. Bauschlicher, C. W., Jr. Chem. Phys. 1985, 93, 399. (34) Srdanov, V . I.; Harris, D. 0..submitted for publication in J . Chem.
Phys.
Figure 3. CASSCF results for geometries and CCI results for dissociation energies of NiSX,with the Ni5 geometry kept fixed. Distances are in a. and energies in kcal/mol. The asymptotic energy is for a Ni, geometry with bulk fixed distances.
TABLE II: Populations on X for N&X, with X Equal to Carbon, Nitroeen. and Orveena height. an X d X ) n(D,) n(p, ,,) o(Ni-Xa,) o(Ni-XF) 1.25 C -0.6 1.0 1.8 0.4 0.7 N -1.0 1.6 2.5 0.4 0.8 0 -0.9 1.9 3.1 0.2 0.5 -1.0 C -1.3 1.3 2.3 0.5 0.7 -0.7 N -1.7 1.5 3.3 0.3 0.6 -0.5 0 -1.1 1.8 3.4 0.2 0.3 " q is the
total charge, n is the occupation, and
o is the
overlap pop-
ulation. distance between the first and second layers in the cluster was varied, and the height above (or below) the surface of the adsorbate was varied. In the second type of geometry optimization, (see Figure 3), the geometry of the cluster was kepk fixed, using the bulk Ni-Ni distance of 4.7088 a,, and thus only the adsorbate height above the surface was varied. The geometry optimization was performed at the CASSCF level, but the energy at the optimized points was also computed at the CCI Davidson correction level. The most important populations are given in Table 11. The active space in the CASSCF calculations included the strongly occupied bonding orbitals with a large 2p contribution on the adsorbate and for each one of these orbitals a correlating orbital. This correlating orbital was an antibonding orbital for Ni5C but a nearly pure 3p orbital for Ni50. For Ni5N this orbital had mixed character. The number of reference'states ranged &tween four and seven, and all valence electrons were correlated. For Ni5C additional calculations were performed where the second-layer nickel atom was reptaced by an all-electron nickel atom. The reasons for doing this calculation are given below in subsection a. a . Comparisons of the Wave Functions for Ni&. In our calculations the ground state of the Ni, cluster is 4A2with the 'E state only 1 kcal/mol higher in energy. Whether the ground state of Ni5 is actually 4A2or 2E is beyond the scope of the present study and would require much more accurate calculations including also 3d correlation effects. For a more thorough discussion on this point see ref 35, where the ground state of Cu, is discussed. The orbital occupation of the 4A2 state is la:2aile2. The l a , orbital is bonding between all cluster atoms while the other orbitals are partly bonding and partly antibonding between the cluster atoms. For all three systems Ni5X there are bonds of E symmetry parallel with the surface between the surface atoms and the adsorbate 2p, and 2p, orbitals. These E bonds correspond to the K bonds for the diatomic molecules. The degree of polarization of the E bonds toward the adsorbate varies with the nuclear charge of the adsorbate, not with the electron affinity, just as in the diatomic case. At a point 1.25 a. above the surface, the 2p,,, population on carbon is 1.8, on nitrogen 2.5, and for oxygen 3.1 electrons. The corresponding numbers for the diatomic molecules are 1.1, 2.0, and 3.1, showing large differences compared to the
+
(35) Bagus, P S , Seel, M Phys Rec E Condenr Lfatter 1981,23, 2065
Binding of C, N, and 0 Atoms to Nickel cluster results for carbon and nitrogen but no difference for oxygen. A population near 3.0 as for oxygen indicates the formation of at most a single covalent bond of E symmetry, whereas a population near 2.0 as for carbon indicates the possible formation of a covalent double bond in this symmetry (see section I1 for the definition of covalency). In the A, symmetry for Ni5X the 2p, population on carbon is 1.O, on nitrogen 1.6, and on oxygen 1.9. The corresponding populations for the diatomic molecules are 1.3, 1.5, and 1.6. The main differences are consequently that the possible covalency (see section I1 for definition) m this symmetry becomes larger for carbon and drops to a very small value for oxygen in Ni,X. The 2s population for Ni5X is 1.8 for carbon and close to 1.9 for nitrogen and oxygen, showing only a small degree of sp hybridization. The gross atomic charges on X in NiSX are -0.6 for carbon, -1 .O for nitrogen, and -0.9 for oxygen at the same point (1.25 a. above the surface). The charge on nitrogen is consequently largest even though this is the adsorbate that has by far the smallest electron affinity. This surprising effect has two origins. The first origin is an increased electron attraction from a larger nuclear charge which explains why carbon becomes less negative than nitrogen and oxygen. The size of the nuclear charge does not explain why nitrogen obtains more charge than oxygen, however. The reason for this is that the charge flow between the cluster and the adsorbate takes place in the bonding orbitals, where the overlap is largest. Nitrogen with its 2p3 occupation can form three bonds with the cluster, but, oxygen with its 2p4 occupation can form only two bonds, and there will thus be a larger charge flow over to nitrogen than over to oxygen. (The argument does not change if the negative ions are considered instead, since N- still can form one more bond than 0-.) The total overlap populations in NiSX are quite interesting and show some of the more important differences between the adsorbates. The total overlap populations (at 1.25 a, above the surface) between the adsorbate and the cluster is for carbon 1.1, for nitrogen 1.2, and for oxygen 0.7. There is consequently a significantly larger covalency in the binding for carbon and nitrogen than for oxygen. The difference between nitrogen and oxygen clearly has to do with the larger number of possible bonds for nitrogen, as discussed above. The similarity between the total overlap populations for carbon and nitrogen is not difficult to understand either. In the E symmetry the binding is very similar for carbon and nitrogen, both having a polarized double bond, and this symmetry dominates the overlap population. The total overlap population in the E symmetry is 0.7 for carbon and 0.8 for nitrogen. In the A, symmetry carbon has at the onset of the bond formation a zero occupation of the 2p, orbital, but this orbital mixes strongly with the binding cluster orbital, which is initially doubly occupied. After mixing, the carbon 2p, occupation is around 1.O and the total overlap population in the A, symmetry is 0.4. The same overlap population is found for nitrogen, which forms a bond between the singly occupied 2p, orbital and the singly occupied cluster orbital. The contribution to the total overlap population from the adsorbate 2s orbital is not negligible: 0.15 for nitrogen and 0.10 for carbon. The differential overlap populations between the adsorbates and the second-layer nickel atoms also show important differences. For carbon this overlap population is 0.24 and for nitrogen 0.21, but for oxygen it is only 0.12 (this is also at the point 1.25 a. above the surface). This shows that already at this height above the surface there is a larger covalent driving force for carbon and nitrogen than for oxygen to penetrate into the surface and cause reconstruction (see further below). The reason for these differences in the overlap populations are already mentioned above. Nitrogen has a singly occupied 2p, orbital which forms a bond with a singly occupied cluster orbital with a large contribution from the second-layer nickel atom. Carbon binds to the second-layer nickel atom through a mixing between the initially empty 2p, orbital and the doubly occupied cluster orbital. Oxygen, on the other hand, has a 2p, orbital, which is almost doubly occupied and therefore does not form bonds to the cluster. All the above results refer to a geometry with the adsorbate 1.25 a, above the surface. The charge on the adsorbate continually
The Journal of Physical Chemistry, Vol. 92, No. 11, 1988 3083 increases as the adsorbate moves down toward and through the surface. A maximal charge is obtained close to the absolute energy minimum (see Figure 2). For nitrogen, which has the highest charge of the adsorbates as discussed above, the charge increases from -1 .O at 1.25 a, above the surface to a value of -1.7 at the optimal geometry in Figure 2. A similar large increase in the charge is obtained for carbon from -0.6 to a value of -1.3 at the geometry in Figure 2. The charge increase on oxygen is much smaller than for nitrogen, from -0.9 at 1.25 a. to -1.1 at the geometry in Figure 2. This means that also the ionic driving force to penetrate the surface is smaller for oxygen than for carbon and nitrogen. The reason for this is again that the 2p, orbital on oxygen already at the height 1.25 a, becomes essentially doubly occupied and therefore can not accommodate more charge as oxygen moves down toward the surface. The overlap population between the adsorbate and the second-layer nickel atom increases as the corresponding distance decreases as expected, and this overlap population stays much larger for carbon and nitrogen than for oxygen. For the overlap population between the adsorbate and the surface atoms there are two compensating effects when the distance between the adsorbate and the surface is varied. In the E symmetry there is an increase while in the A, symmetry there is a decrease in the overlap population as the adsorbate approaches the surface. The overall effect leads to a maximum in the overlap population at around 2.0 a, above the surface, which is in the region of the energy minimum when the geometry of the cluster is kept fixed (see Figure 3). An open question when one-electron ECP’s are used to model metal atoms is to what extent 3d involvement would change the picture obtained. 3d orbital relaxation is always present to some extent, but this effect is modeled by our ECP. In the case of carbon, nitrogen, and oxygen chemisorption there is, however, a clear possibility of direct covalent bonding between the metal 3d orbitals and the adsorbate. For oxygen chemisorption we have already shown, by a comparison to all-electron calculations, that the involvement of 3d in the covalent bonding is truly The effect on the chemisorption energy can for oxygen be estimated to be only a few percent. To test the 3d covalency effect for carbon, calculations were done on Ni5C, where the second-layer nickel atom was described by an all-electron atom. The geometry of NisC was chosen to be such that the distance between the second-layer nickel atom and carbon was taken to be nearly optimal for NiC bonding, 3.2 a* The distance between the surface atoms was 5.5 ao, and the carbon height above the surface was 0.1 a,. A start wave function was then generated for the CASSCF calculation with a triple bond between carbon and the second-layer atom just like in the diatomic NiC case (see section 111). These bonding orbitals are purely 3d bonding in the E symmetry and have a large 3d contribution in the A, symmetry. After converging the CASSCF wave function, the 3d contribution had almost entirely disappeared in the E symmetry and the contribution was small in the A, symmetry, and the energy was significantly lowered. Carbon therefore clearly prefers to bind to the sp band of the cluster, just like oxygen. A similar test calculation for NisN gave the same result. The energetic effect of the slight remaining 3d contribution to the bonding was small for all systems Ni5X. The chemisorption energy difference between the all-ECP calculation and the calculation with one all-electron nickel atom was only a few percent. The reason direct covalent bonding to 3d becomes unimportant for adsorbates in the hollow positions is clearly related to the large distances between the adsorbate and the nearest metal atoms. This leads to a very small overlap between the adsorbate 2p orbitals and the 3d orbitals. This overlap is small already for the diatomic molecules and leads to rather weak bonds involving the 3d orbitals. The strength of the covalent 3d bonds can be estimated in the case of NiO by comparison to CuO, where there is only a very small 3d covalency. The binding energy for NiO is 87 kcal/mol and for CuO 65 kcal/m01,~~ giving a 22 kcal/mol effect from 3d (36) Smoes, S.; Mandy, F.; Vander Auwera-Mahieu, A,; Drowart, J. Bull. SOC.Chim. Belg. 1972, 81, 45.
I
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The Journal of Physical Chemistry, Vol. 92, No. 11, 1988
covalency. This is a very small energy, and it is with this perspective not surprising that oxygen can form more efficient bonds directly with the valence orbitals of the cluster, for which the overlap is much larger. For binding in on-top positions, where the overlap with valence orbitals centered on neighboring atoms is also small, the situation is again different. In this position one therefore expects a substantially larger 3d covalency contribution to the bonding. The better overlap with the valence orbitals on many different centers in the hollow position than in the on-top position is a major reason why the adsorbate prefers the former adsorption site. b. Optimized Geometries and Energiesfor Ni& The results from the two different types of geometry optimizations, described in the beginning of this section, are given in Figures 2 and 3. We should emphasize once more that none of these optimized geometries should be directly compared with results from surface experiments, since the type and amount of reconstruction of a real surface is naturally very different. The only exception may be the oxygen result in Figure 3 where the cluster geometry was kept fixed, since oxygen is known not to reconstruct the surface. A more detailed comparison with oxygen chemisorption is given in ref 16 where larger, more realistic clusters are also used. The first striking fact from the results in Figures 2 and 3 is the similarity between the three adsorbates. In the case of the free geometry optimization in Figure 2 all of the adsorbates penetrate through the surface to a point 0.8 f 0.3 below the surface, and in the case of the frozen cluster geometry in Figure 3 the adsorbates stay at a very similar height above the surface, around 2.1 f 0.2 a,. An important conclusion from these results is that there is a driving force to penetrate and reconstruct the surface for all three adsorbates. The energy gains in relaxing the cluster are, however, quite different. For carbon the energy gain is 33 kcal/mol, for nitrogen it is 26 kcal/mol, whereas for oxygen the energy gain is much smaller, only 6 kcal/mol. This is clearly in line with the experimental fact that carbon and nitrogen adsorb close to the surface and reconstruct the surface, whereas oxygen does not reconstruct the ~ u r f a c e . ~A- ~conclusion that can be drawn from the populations discussed above in subsection a is that there is both an ionic and a covalent component in the driving force to penetrate through the surface. The covalent driving force, as indicated by the overlap population between the adsorbate and the second layer nickel atom, is smallest for oxygen. This is due to an essentially doubly occupied pz orbital on oxygen. The ionic driving force is related to the number of bonds formed between the adsorbate and the cluster, which is also smallest for oxygen due to the doubly occupied 2p, orbital. Another important conclusion from the results in Figures 2 and 3 is that the driving force to reconstruct the surface is not proportional to the chemisorption energy, since nitrogen in the calculations has a smaller chemisorption energy than oxygen and yet gains more in energy by the reconstruction. A geometry optimization was also performed for the bare Ni, cluster. This optimization resulted in a surface Ni-Ni distance of 4.8 ao, which is very close to the bulk Ni-Ni distance, and a first to second layer distance of 4.2 ao, which is longer than the corresponding bulk distance of 3.3 ao. The energy difference between the optimized geometry and the bulk geometry is 5 kcal/mol. Describing the second-layer nickel atoms with all its electrons shortens the distance between the layers to 3.5 a. but lengthens the surface Ni-Ni distance to 5.0 ao. The energy difference to the bulk geometry is now 2 kcal/mol. In order to check the cluster size dependence of the present results, a geometry optimization was also performed for a Nil& cluster, varying the same parameters as in Figure 2. This cluster has four atoms in the first layer, five in the second layer, and four in the third layer. The optimization resulted in a very similar geometry and a chemisorption energy which is 9 kcal/mol larger than that for Ni,C. This energy difference between the different clusters is in line with the variations found previously for Ni,O c1usters.l6 Cluster molecules with carbon, nitrogen, and oxygen ligands threefold coordinated to iron atoms have recently been synthes-
Panas et al. i ~ e d . ~The ' differences in the geometry for the three ligands are rather small, which is in line with the similarity of the optimized structures in Figures 2 and 3. This is another clear example that the difference in the geometry found on surfaces for the three adsorbates is a result of balancing forces, where the driving force to penetrate the surface is also present for oxygen but it is weaker than for carbon and nitrogen. A comparison of the dissociation energies for the diatomic molecules and the chemisorption energies for Ni,X shows that the binding to the cluster is significantly more efficient. The main differences between the binding in N i x and NisX are that X is much more ionic in Ni5X and that the 3d contribution to the bonds have almost disappeared in Ni,X. The trend in the dissociation energies are the same in the two types of systems and have the same origin. Nitrogen has the smallest dissociation energy since it has the smallest electron affinity, and carbon has a larger dissociation energy than oxygen since it has a larger covalency contribution to the bonding. In previous studiesI6Js we have made comparisons of oxygen chemisorption energies on nickel and copper clusters containing from 5 to 25 atoms. The experience from those studies is that in particular the difference in chemisorption energies between Ni,O and Cu,O was very stable for all clusters and quite well reproduced already by a five-atom cluster. If this information is transferred to the present case, we would expect that our calculated chemisorption energy differences between the three adsorbates should be particularly significant. With this asumption we would predict that carbon should have the largest chemisorption energy, 15-25 kcal/mol larger than that for oxygen. Given the e~perimental'~*'~ and theoretically predicted16chemisorption energy for oxygen of about 125 kcal/mol, the carbon chemisorption energy would therefore be about 145 kcal/mol. The difference between the oxygen and nitrogen chemisorption energies is expected to be smaller with a slightly larger energy for oxygen; see discussion below in subsection c. The calculations on Ni5C with an all-electron nickel atom in the second layer gave only minor energetic differences to the all-ECP results at a point 1.O a. above the surface. Correlating all electrons, including the 3d electrons, gave a chemisorption energy of 135 kcal/mol. Without correlating the 3d electrons the result increased to 140 kcal/mol. These results can be compared to the all-ECP result in the same point, which was 135 kcal/mol. A CASSCF calculation was also performed for Ni5N where the Ni atoms were treated with all their electrons. The active space was very small with only the A, bond correlated. The geometry was chosen with nitrogen 0.2 a. above the surface and a surface Ni-Ni distance of 5.0 ao. At this point the CASSCF interaction energy is only weakly attractive by 6 kcal/mol. The all-ECP result at the same CASSCF level is for this geometry 14 kcal/mol, which is a rather satisfactory result considering the short distances involved. The accuracy for N i 5 0 at this geometry is probably slightly higher than that for N&N, since the ECP was calibrated against all-electron calculations for Ni,O at geometries close to that in Figure 3.17 We have already mentioned that the distortions of the Ni-Ni distances in our cluster are larger than is seen on a real surface due to missing neighboring atoms, but the distortions obtained in the calculations are expected to be exaggerated also for other reasons. In particular, correlation effects from the 3d shells are missing. For Cuz, excitations from 3d4s (of core-valence (CV) type) are known to account for as much as 40% of the dissociation energy.39 CV correlation effects are also known to substantially reduce the size of the 4s orbital, which should also simplify penetration through the surface for an adsorbate. It is therefore expected that the inclusion of CV correlation effects should decrease the distortions in the cluster substantially, and calculations (37) Schauer, C. K.; Shriver, D. F. Angew. Chem., Int. Ed. Engl. 1987, 26, 255. ( 3 8 ) Mattsson, A.; Panas, I.; Siegbahn, P.; Wahlgren, U.; Akeby, H. Phys. Rev. B: Condens. Mafter, in press. (39) Bauschlicher, C. W., Jr. Chem. Phys. L e f t . 1983, 97, 204.
Binding of C, N , and 0 Atoms to Nickel including these effects would thus be of significant interest in the present context. Models to include CV effects in cluster calculations are in progress,@ based on the method described in ref 41. In a recent paper Muller, Wuttig, and Ibach (MWI)42have done model calculations for carbon and oxygen chemisorption using a Ni9 cluster and the X, method. They obtained excellent agreement with the experimentally known frequencies and equilibrium heights above the surface, much better than we obtain. Particularly striking is the perfect agreement for the oxygen frequency, even though the phonon coupling of 80 cm-' (about 20% of the vibrational frequency)43does not seem to have been accounted for. For the experimentally much more uncertain chemisorption energies of carbon and oxygen, MWI obtained 263 and 182 kcal/mol, respectively, which can be compared with our estimates of 145 and 125 kcal/mol, respectively. As an explanation of the differences between carbon and oxygen chemisorption, MWI have pointed at the large contributions from covalency to 3d for carbon as compared to oxygen. This explanation is in complete contradiction to the present results, which show only minor contributions from 3d to the covalent bonding for both carbon and oxygen. This difference between the results of MWI and our results is probably the origin of the large difference in the obtained chemisorption energies. c. The Nitrogen-Oxygen Chemisorption Energy Difference. Egelhoff has by the use of ESCA experiments determined the chemisorption energy difference between nitrogen and oxygen A(N-0) on the Ni( 100) surface.I4 There are two actual measurements: first the l s binding energy of chemisorbed atomic nitrogen of 397.60 eV and second the excitation energy from 1s to l a , of the gas-phase nitrogen molecule of 400.64 eV.44 The latter measurement is used to derive an excitation energy from 1s to 2p for the nitrogen atom in the gas phase, and this is accomplished by the use of a Born-Haber cycle and the equivalent core approximation (ECA). A value of 397.38 eV for this excitation energy is found by using the dissociation energies, 9.76 eV for N 2 and 6.5 eV for NO, together with the 1s to l a , excitation energy in N2. The difference between the 1s binding energy of 397.60 eV and the derived excitation energy of 397.38 eV should, again by using a Born-Haber cycle and the ECA, be equal to A(N-0). Nitrogen should thus have a larger chemisorption energy than oxygen by 5 kcal/mol. Several calculations were set up to analyze the approximations made in ref 14. In the first calculation the 1s to 2p excitation energy on the nitrogen atom was calculated. The result is 397.6 eV, which is in good agreement with the value of 397.38 eV derived in ref 14. This agreement is partly fortuitous since the value in ref 14 relies on the validity of the Born-Haber cycle and the ECA and since there are two compensating effects in the calculations: the 1s orbital relaxation is not accounted for in the excited state and the present basis set is not quite adequate for describing the Is2 correlation energy of the ground state. In the second set of calculations a geometry of Ni5N was used in reasonable correspondence with the experimentally determined chemisorption geometry of nitrogen on Ni( The nitrogen to surface height was taken to be 0.2 ao, the Ni-Ni surface distance was 5.2 ao, and the first to second layer distance was 3.5 ao. The binding energy of the nitrogen 1s was then calculated as the excitation energy from 1s to the Fermi level. The Fermi level is in the present case the lowest unoccupied valence level. The result from this calculation is 397.6 eV, which happens to be in perfect agreement with the measured value. The agreement between our calculated value and experiments is another, rather sensitive, indication that our model of the chemisorption of nitrogen is very reasonable. There are two key assumptions in the analysis that (40) Pettersson, L. G. M.; Siegbahn, P.; Akeby, H.; Wahlgren, U., to be published. (41) Miiller, W.; Flesch, J.; Meyer, W. J. Chem. Phys. 1984, 80,3297. (42) Miiller, J. E.; Wuttig, M.; Ibach, H. Phys. Reu. Lett. 1986, 56, 1583. (43) Andersson, S.;Karlsson, P.-A.; Persson, M. Phys. Reu. Let?. 1983, 51, 2378. (44) Hitchcock, A. P.; Brion, C. E. J. Electron. Spectrosc. Relal. Phenom. 1980, 18, 1.
The Journal of Physical Chemistry, Vol. 92, No. 11, 1988 3085 lead to the A(N-0) value in ref 14. First it is assumed that nitrogen and oxygen have the same chemisorption geometry and second the validity of the ECA is assumed. The first assumption has later shown by experiments not to be correct. Nitrogen adsorbs at a height 0.2 a. above the surface and oxygen at a height 1.7 a. above the surface. The error in the ECA approximation is equal to the chemisorption energy difference between ECA oxygen (Nwith a 1s hole) and real oxygen. At the point 0.2 a. above the surface, our calculations indicate that the ECA error is 14 kcal/mol. The chemisorption energy for nitrogen at this point is 100 kcal/mol, for oxygen 114 kcal/mol, and for ECA oxygen 100 kcal/mol. These energies are with respect to a long-distance calculation for a cluster with bulk geometry. Since our chemisorption energy for oxygen at the experimentally determined geometry is 106 kcal/mol, our value for A(N-0) is -6 kcal/mol, based on experimental geometries for both nitrogen and oxygen chemisorption. The A(N-0) value of + 5 kcal/mol in ref 14 is used to derive a value for the chemisorption energy of oxygen based on the measured chemisorption energy for nitrogen of 135 kcal/mol.'2 As mentioned in the Introduction, this latter value is overestimated since the bamer for dissociation of N2is neglected. The overestimation of the chemisorption energy of nitrogen probably more than compensates for the error in A(N-0), and the final value for the chemisorption energy of oxygen of 130 kcal/mol is therefore slightly overestimated. On the basis of our calculations, here and in ref 16, we suggest a chemisorption energy for oxygen of about 125 kcal/mol and for nitrogen of about 120 kcal/mol. The variation of the excitation energy from the nitrogen 1s to the Fermi level, E( 1s-F), and of the nitrogen 1s ionization energy, IP( Is), was finally studied by doing calculations on Ni5N with two different geometries, with nitrogen 0.2 and 2.0 a. above the surface. The difference in E( 1s-F) between the two points is 1.O eV with the higher value at 0.2 ao. The difference in IP(1s) between the two points is 0.2 eV with the higher value at 2.0 a,, instead. The nitrogen charge at 0.2 a. is -1.6, and at 2.0 a. it is -1.0. We draw two conclusions from these results. First, the variation of E ( 1s-F) is only related to differences in the Fermi level at the two points and not to differences of IP( 1s). Second, neither the change in the E(1s-F) nor in the IP(1s) corresponds well to the large charge change on nitrogen (as the corresponding ESCA shift for a molecule normally would do), in particular not the E( 1s-F), since E( 1s-F) even increases as the charge increases. We should finally point out that even with the above-mentioned problems with Engelhoffs approach it is still in our opinion a very useful experimental technique for obtaining atomic chemisorption energies. The model assumptions are clear and are not difficult to correct for, as we have indicated above. The combination of the experimental shifts and accurate calculations therefore seems an extremely interesting route to follow in the future for obtaining in particular the larger atomic chemisorption energies, such as those for carbon and oxygen, which have been found nearly impossible to obtain by using other techniques such as desorption measurements.
V. Conclusions CCI calculations have been done on the diatomic molecules NiC, NiN, and NiO and for the clusters Ni5C, Ni5N, and Ni50. For the diatomic molecules the a bonds are bonds between the ligand 2p and nickel 3d. Also, the u bond has a large 3d contribution. For the cluster systems the 3d contribution to the bonding almost entirely disappears and the chemisorption bonds are instead formed with the cluster sp band. The ionic contribution to the bonding is large already for the diatomic molecules but becomes even larger for the cluster systems. Perhaps surprisingly the charge on the ligand does not depend on the electron affinity. For the diatomic molecules the charge is instead set by the size of the nuclear charge and therefore increases monotonically from carbon to oxygen. For the cluster systems the charge is actually largest for nitrogen even though nitrogen has by far the smallest electron affinity. The reason for this is that the excess charge on the adsorate is set by a polarization of the bonds to the cluster,
J . Phys. Chem. 1988, 92, 3086-3091
3086
so that the charge depends both on the nuclear charge and the number of bonds. Carbon and nitrogen form one more bond than oxygen. Since the ionicity is large, the electron affinity of the ligand is important for the size of the dissociation energy, however, and for the diatomic molecules, NiN is much more weakly bound than NiC and NiO. For the same reason NiSN is more weakly bound than the other clusters, but here the larger ionicity and covalency nearly compensate for the poor electron affinity. Carbon forms the strongest bonds both in the diatomic molecule and in the clusters since both the covalency and the electron affinity are large. Experimentally carbon and nitrogen are found to adsorb close to the surface of Ni(100) whereas oxygen adsorbs further out. The reasons for this different behavior are twofold. First, the covalency to the second-layer nickel atom is much smaller for oxygen, which in turn is due to an almost doubly occupied 2p, orbital (pointing down toward the surface) for oxygen, which cannot form covalent bonds to the second-layer nickel atom. For both carbon and nitrogen the 2p, orbital is far from doubly occupied. This difference between carbon and nitrogen on the one
hand and oxygen on the other can be seen in the overlap populations already rather far above the surface. The second reason for the larger force to reconstruct the surface for carbon and nitrogen compared to oxygen is that the ionicity increases much more for the former atoms as the atom penetrates the surface. This again is connected with the occupation of the 2p, orbital for oxygen which already at a large height above the surface becomes almost doubly occupied and therefore cannot accommodate more charge as the atom moves down toward the surface. The final geometries for Ni5X are very similar for all three adsorbates, both after a full geometry optimization as well as after an optimization with the cluster geometry fixed. Oxygen consequently also penetrates the surface for these clusters but gains much less energy by doing so than carbon and nitrogen. On a real surface the restoring forces from neighboring atoms are strong enough to keep oxygen outside but cannot resist the stronger force from carbon and nitrogen. Registry No. NiC, 12167-08-7; N i N , 54485-02-8; NiO, 1313-99-1;
Ni5C, 112138-30-4; Ni5N, 112138-31-5; Ni50, 108599-79-7; C,
7440-
44-0; N , 17778-88-0; 0, 17778-80-2; Ni, 7440-02-0.
Propensity Rules for Vlbratlon-Rotation- Induced Electron Detachment of Diatomic NH 4- eAnions: Application to NH--+
Grzegorz Chalasinski: Rick A. Kendall,*Hugh Taylor, and Jack Simons* Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12 (Received: August 14, 1987)
The vibration-rotation-induced electron detachment (VRIED) of diatomic anions has recently become accessible to detailed experimental analysis. Continuing our efforts in this area of nonadiabatic couplings, we attempt to establish more specific angular momentum propensity rules for such processes. In particular, we show how characteristics of the final electronic state may be predicted depending on the symmetry of the'initial state and the symmetry of the vibrational or rotational motion that causes the ejection of the electron, and we show how the rates of electron ejection should vary with rotational quantum number. The rotational structure of the VRIED event recently reported by Neumark et al. for NH- ( J . Chem. Phys. 1985, 83, 4364) is rationalized without our model.
I. Introduction The vibration-rotation spectroscopy of negative ions has become a rapidly developing field due to the development of new ion sources and the application of modern techniques of tunable laser spectroscopy,' fast ion beams, and other ion modulation techniques.'** One of the first anions whose vibrational and rotational structure was studied by these techniques was NH-.3 The infrared vibration-rotation spectrum of NH- was obtained by autodetachment spectroscopy in a coaxial laser-ion beam spectrometer. Rotational transitions within the u = 0 to u = 1 vibrational absorption of NH- were sampled, and subsequent autodetachment was observed. The non-instrument-limited line widths of these autodetachment resonance peaks revealed some of the dynamics of the autodetachment process. In particular, the observed increase of the autodetachment rate with rotational energy was much faster than expected if vibrational-to-electronic energy transfer were the primary detachment mechanism (which might be anticipated because most of the energy is in the V = 1 vibrational degree of freedom). Moreover, the experimental data suggest that transitions in which the rotational quantum number decreases by a single unit dominate the ejection process. The nature of the pr orbital of NH- from which detachment occurs and the fact that this orbital is not strongly affected by vibrational motion suggest 'Permanent address: Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland. *University of Utah Graduate Research Fellow.
0022-3654/88/2092-3086$01.50/0
that rotational rather than vibrational motion could provide a more efficient avenue for energy transfer to the electronic degrees of freedom. Part of the purpose of this paper is to examine the relative roles of vibrational and rotational motion in this process. The conceptual framework for vibration-rotation-assisted autoionization in Rydberg states of neutral molecules was formulated by Berry over 20 years ago.4 For negative ions this treatment has been extended by Simons et al.5-8 The present paper represents a continuation of these earlier studies, and its purpose is to derive qualitative rotational propensity rules for vibrationrotation-induced electron detachment (VRIED) in the particular (1) Jones, P. L.; Mead, R. D.; Kohler, B. E.; Rosner, S . D.; Lineberger, W. C. J . Chem. Phys. 1980, 73, 4419. Hefter, U.; Mead, R. D.; Schulz, P. A.; Lineberger, W. C. Phys. Rev. A 1983, 28, 1479. (2) Bae, Y. K.; Cosby, P. C.; Peterson, J. R. Chem. Phys. Lert. 1986, 126, 266. Tack, L. M.; Rosenbaum, N. H.; Owrutsky, J.; Saykally, R. J. J . Chem. Phys. 1986,85,4222. Gruebele, M.; Polak, M.; Saykally, R. J. J . Chem. Phys. 1987,86, 1698. 1987,86,6631. Rosenbaum, N. H.; Owrutsky, J. C.; Tack, L. M.; Saykally, R. J. J . Chem. Phys. 1986, 84, 5308. (3) Neumark, D. M.; Lykke, K. R.; Andersen, T.; Lineberger, W. C. J . Chem. Phys. 1985, 83, 4364. (4) Berry, R. S. J . Chem. Phys. 1966, 45, 1228. (5) Simons, J. J . A m . Chem. SOC.1981, 103, 3971. (6) Acharya, P. K.; Kendall, R. A.; Simons, J. J . A m . Chem. SOC.1984,
106, 3402. (7) Acharya, P. K.; Kendall, R. A.; Simons, J. Contrib.-Symp. A t . Surf. Phys. 1984, 84, 1. ( 8 ) Acharya, P. K.; Kendall, R. A.; Simons, J. J . Chem. Phys. 1985,83, 3888.
0 1988 American Chemical Society
