3279
J . Phys. Chem. 1992,96, 3219-3282
Potential Energy Surfaces for Pt3
+ H and Pd3 4- H Interactions
Dingguo h i and K. Balasubramanian*qt Department of Chemistry, Arizona State University, Tempe, Arizona 85287- 1604 (Received: October 21, 1991)
We carry out complete active space multiconfiguration self-consistent-field (CAS-MCSCF) followed by multireference configuration interaction calculations (MRSDCI) which included up to 3.1 million configurations on Pt, + H and Pd, + H systems. Three low-lying electronic states were identified for Pd, + H while for Pt3+ H four electronic states were found. Both Pd3H and Pt,H were found to form triangular-pyramid structures in which the hydrogen atom is at the apex. The 2A2electronic state (C3")was found to be the lowest for both Pd3H and Pt3H. Spin-orbit effects were found to be significant for Pt3H. The energy to separate the hydrogen atom from the trimers was found to be comparable to the corresponding energy for the dimers. Pd3H was found to be fluxional in its ground state while in contrast Pt,H is rigid.
Introduction Reactivities of transition-metal clusters and metal-hydrogen bonding have been the topic of several theoretical and experimental s t u d i e ~ . ~ Reactivities -~~ of transition-metal clusters have been found to dramatically vary as a function of cluster size. Gas-phase studies of transition-metal ions with Hz have also revealed state specificities of metal-H, reactions.20 There has been considerable interest in recent years in the electronic properties of small transition-metal clusters and their reactivities with small molecules such as H2 and CO. Platinum is one of the most important materials employed in the heterogeneouscatalysis of hydrogenation reaction^.^-^ A comparative study of Pt with Pd and Ni materials has also been made. Multicentered metal-hydrogen bridge bonds appear to occur in both surfaces and inorganic complexes.21 In dissociative chemisorption processes one of the species appears to be formed in a trigonal metal-H-metal structure exhibiting bridge bonding. There is a fundamental question related to the dependence of reactivity on cluster size. In the present study we investigate the reaction of trimers of Pt and Pd with hydrogen. We consider several electronic states employing a complete active space multiconfiguration SCF (CAS-MCSCF) followed by multireference configuration interaction (MRCI) calculations which included up to 3.1 million configurations. In addition, for Pt3H systems we included spin-orbit coupling using the relativistic configuration interaction (RCI) method. Method of Calculation The complete active space multiconfiguration self-consistentfield (CAS-MCSCF) method followed by a higher-order multireference singles and doubles configuration interaction (MRCI) method were used in this study to investigate the low-lying electronic states of Pd3H and Pt3H. Both the CASSCF and MRCI calculations were made using the relativistic effective core potentials (RECPs) in refs 26 and 27, which retained the outermost 10 electrons of Pd and Pt explicitly in the valence shell. Ermler and c o - w o r k e r ~ ~have ~ , ~ optimized ~ (3s3p4d) valence Gaussian basis sets for the palladium and platinum atoms which were used in this study. For the hydrogen atom, we employed Van Duijneveldt's (Sslp/3slp) basis set.32 The basis sets and RECPs employed here were used before in theoretical studies of Pdgz5 and Pt2,24respectively. The structures of Pd3H and Pt3H were oriented such that Pd3 and Pt3were in the xy plane while the H atom was on the z axis. The H atom was above the plane of Pt3 and Pd,, but it was displaced vertically relative to the geometrical center of Pt3 and Pd,. The orbitals for higher-order CI calculations were generated using the CASSCF method. To keep the computations manageable, excitations from the lowest six d electrons of Pd or Pt were not included in the CAS-MCSCF but these orbitals were allowed to relax. The remaining 25 electrons of both Pd3H and PtpH were distributed in all possible ways among the active or-
'Camille and Henry Dreyfus Teacher-Scholar.
TABLE I: Dimensions of the Configuration Spaces in the CASSCF and MRSDCI Calculations m oIecuIe state CASSCF MRSDCI PdpH 2A2 364 3 080 791 2AI 364 2515 173
4E Pt,H
2A2 2E 4A2
4E
140 364 364 146 140
1182729 2941 106 2971 616 2342614 2 343 766
bitals. Although the point group of both Pd3H and Pt3H is a C3, group in general, for technical reasons all computations were made
( I ) Smalley, R. E. In Comparison of Ab Initio Quantum Chemistry with Experiment; Bartlett, R. J., Ed.; Reidel: New York, 1985; pp 53-65. (2) Wang, S. W.; Pitzer, K. S. J. Chem. Phys. 1983,79,3851. (3) Basch, H.; Topial, S. J. Chem. Phys. 1983,71,802. (4) Messmer, R. P.; Salahub, D. R.; Johnson, K. H.; Yang, C. Y. Chem. Phys. Lett. 1977,51, 84. (5) Demuth, J. E. Sur/. Sci. 1977,65,369. (6) Chrismann, K.; Ertl, G.;Pignet, T. Surf.Sci. 1976,54, 365. (7) Low, J. J.; Goddard, W. A. I11 Organometallics 1986,5, 609. (8) Poulain, E.; Garcia-Prieto, J.; Ruiz, M. E.; Novaro, 0.Inr. J . Quanfum Chem. 1986,24, 1 181. (9) Balasubramanian, K. J . Chem. Phys. 1987,87,2800. (10)Nakatsuji, H.; Matsuzaki, Y.; Yonezawa, J. J. Chem. Phys. 1988,88, 5759. (11) Balasubramanian, K.; Feng, P. Y. J. Chem. Phys. 1990,92, 541. (12) Balasubramanian, K.;Feng, P. Y.; Liao, M. Z . J . Chem. Phys. 1987, 87,3981. (13) Trevor, D.J.; Cox, D. M.; Kaldor, A. J. Am. Chem. SOC.1990,112, 3742. (14)Gustafsson, G.;Scullman, R. Mol. Phys. 1989,67,981. (15) Knight, L. B.; Cobranchi, S.T.; Herlong, J.; Kirk, T.; Balasubramanian, K.;Das, K. K. J . Chem. Phys. 1990, 92,2721. (16) Blomberg, M. R. A.; Bradenmark, V.; Pattersson, L.; Siegbahn, P. E. M. Inr. J . Quantum Chem. 1983,23, 855. (17) Nakatsuji, H.; Hada, M. Croat. Chim. Acta 1984,57, 1371. (18) Blomberg, M. R. A.; Siegbahn, P. E. M. J. Chem. Phys. 1983,78, 5682. (19) Noell, J. 0.;Hay, P. J. lnorg. Chem. 1982,21, 14. (20) Crabtree, B. Acc. Chem. Res. 1990, 23, 95. Armentrout, P. B.; Beauchamp, J. L. Acc. Chem. Res. 1989,22,315. Martinho, J. A.; Beauchamp, J. L. Chem. Reo. 1990,90,629. (21) Slocum, D. W., Moser, W. R., Eds. Catalytic transition metal hydrides; Ann. N.Y. Acad. Sci. 1983,415. (22) Balasubramanian, K. J. Chem. Phys. 1988,89,6310. (23) Ho, J.; Ervin, K. M.; Polak, M. L.; Gilles, M. K.;Lineberger, W. C. J . Chem. Phys. 1991, 95,4845. (24) Balasubramanian, K. J. Chem. Phys. 1987,87,6573. (25) Balasubramanian, K. J. Chem. Phys. 1989,91,307. (26) La John, L. A.; Christiansen, P. A.; Ross, R. B.; Atashroo, T.; Ermler, W. C. J. Chem. Phys. 1987,87,2812. (27) Ross, R. B.; Powers, J. M.; Atashroo, T.; Ermler, W. C.; La John, L. A.; Christiansen. P. A. J. Chem. Phys. 1990, 93, 6654. (28) Pitzer, R. M.; Winter, N. W. J . Phys. Chem. 1988,92,3061.
0022-365419212096-3279$03.00/0 0 1992 American Chemical Society
3280 The Journal of Physical Chemistry, Vol. 96, No. 8, 1992
TABLE 11:
Dai and Balasubramanian
Pro~ertiesof the Electronic States of P d d nitbout Spin-Orbit Effect
state
R,," A
R2tb A
2A2t 2A,
