The Challenge of d and f Electrons - ACS Publications - American

Chapter 8

Activation of Small Molecules by Transition Metal Atoms Theoretical Interpretation of Low-Temperature Experiments with Cu, Pd, and Pt Atoms

Downloaded by COLUMBIA UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: June 8, 1989 | doi: 10.1021/bk-1989-0394.ch008

O. A. Novaro Instituto de Física, Universidad Nacional Autónoma de México, 01000 México D.F., Mexico Theoretical-experimental results on transition metal atom-small mole cules systems are reported. The theoretical studies employ the PSHONDO-CIPSI sequence of programs that allow for variational and perturbational configuration-interaction calculations including up to 10 configurations for each ground and excited-states of the system. These theoretical results are contrasted with data from low-temperature ma trix isolation experiments on these same systems supported by infrared, visible-ultraviolet, epr and other spectroscopic techniques. Interesting correlations between theory and experiment are found, including the fol lowing: the photoactivation of H and methane by Cu atoms at low tem peratures are rationalized from a theoretical standpoint; the theoretical prediction of H activation by ground-state Palladium is verified exper imentally and the preference of insertion over abstraction reactions and the formation of Cu(N ) complexes serve to explain some extraordinary isotopic effects found in experiment. 6

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Matrix isolation experimental techniques [1-10] stand out among many other mod ern chemical research methods with regard to their ability to provide direct comparisons with quantum mechanical calculations. The use of photoexcitation methods to induce reactions [7-9] as well as the applications of multiple spectroscopic techniques to study such photochemical reactions allows for close control of the reaction parameters. Most of the high temperature and entropy effects, otherwise very large in thermochemical re actions, are therefore not present here and thus some of the limitations associated with applications of precise quantum mechanical calculations to kinetic processes disappear. Specifically the low temperature studies which concern elementary interactions of small molecules and transition metal clusters or atoms isolated over "inert" solid matri ces [5-10] are of high interest, especially now that the Schrodinger equation representing such interactions can be solved to relatively high precision using ab-initio configurationinteraction methods. Among such methods we could mention the CASSCF and CCI, GVB-CI, Monstergauss, PSHONDO and CIPSI [11-15] among other methods and pro grams, many of them mentioned and described in this book. The fact is that theoretical physicists and chemists in the recent past have developed very accurate methods for the study of d- and /-electron systems. Therefore, while several low temperature exper iments concerning transition metal atoms or clusters and their interactions with small molecules have appeared in the literature, simultaneously many quantum mechanical calculations are appearing on the same type of systems. Rarely in the history of quan0097-6156/89/0394-O106$06.00/0 c 1989 American Chemical Society

In The Challenge of d and f Electrons; Salahub, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


8. NOVARO

Activation of Small Molecules by Transition Metal Atoms 107

turn chemistry has a situation so tempting been found where theory can be compared with experiments concerning systems of great practical chemical interest and yet small enough and with such strict control of variables as to make direct comparisons feasible. However, actual theoretical experimental collaborations are not at all common. Some joint papers exist [16-18] and sometimes theoretical and a experimental papers on the same system are published back to back [19-20]. An example is that of Weltner and coworkers, who performed esr and endor studies of MnH supported over solid Argon at 4K [21] in order to compare with previous calculations on the same MnH molecules [22], among other similar efforts. The alternative situation of theoreticians directing their calculations of previous or current matrix isolation experiments is of course common also [22-31] but considering the potential benefits of following the experiments closely, we feel that in reality much more work should be done in this direction. In particular we believe that in order to be really relevant to the understanding of the kinetics, calcula tions should not only aim at obtaining the structure of a molecule but very importantly at determining reaction coordinates and activation barriers. This allows for predictions about feasibility and selectivity that may be contrasted with the experiments. In this chapter we shall review a few cases of such theoretical-experimental collaborations with out pretending to be exhaustive but rather as examples of the mutual influence of theory and experiment. Method The method used in our calculations is the ab-initio pseudopotential method of Durand et al. [32-34]. We apply it for the Cu, Pd and Pt metal atoms whose pseudopotentials are also given in the literature: that of Cu in [2£], that of Pd in [M], that of Pt in [3fi]. In every case all of the valence electrons as well as all the electrons of the outermost d-subshell are always treated explicitely and without restrictions. The basis sets used are always of double-zeta quality at least, those for Cu and H are given in [3Z], that of Pt in [3£], that of C by Pacchioni et al. [2£], that of Pd in [24], that of Ν by Daudey (Daudey, J.P. Preprint of the Laboratoire de Physique Quantique, Université Paul Sabatier, Toulouse, France, 1986). The convergence criterion of the SCF iteration energies was set at 10~ . Basis set superposition errors were systematically tested and corrected for by following the counterpoise correction of Kolos [22] when necessary. The basis sets were selected by thoroughly testing their accuracy in the reproduction of the energy splittings between the ground and lowest excited states (eg. the P- S and D- S splittings in Cu, the D- S splitting in Pt, etc.) but this is better seen by reading the original papers [36-37]. One limitation of these basis sets is the lack of /-polarization functions. In some instances it has been shown that their use introduces only marginal improvements in some calculations involving Cu [4Ω] or Pt [H]. The use of /-functions is however an open subject of current interest as is exemplified by several chapters of this book. We use the CIPSI algorithm [1£] which introduces configuration interaction by perturbation with multiconfigurational wave functions selected by an iterative process using the M0ller-Plesset barycentric values as proposed by Malrieu [42]. By this ap proach we then include many more configurations (typically of the order of a million or more) that interact effectively with the original reference states. These reference states correspond to more than one metal atom state, generally we take three or more states of each metal considered, say the ground and the lowest lying excited states. The im portance of this will be evident when discussing the comparison of the specific results with the experimental data. In every case a careful analysis of the configurations that are included in this large CI scheme is carried out trying to determine how their role during the process is to be understood. Such configurations represent very often a polarization of the d-subshell, which in many cases is closed or nearly closed, so that its relaxation substantially lowers the interaction energy between a transition metal atom and a small closed-shell molecule 6

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108

THE CHALLENGE OF d AND f ELECTRONS

(eg. H2, C H or N2). As interesting as this and also many other trends shown by the configurations participating in the CI schemes are, it is not adequate to describe them within the overall fashion in which we shall discuss the results on several different systems in the present paper. The reader is therefore referred to the original articles to review this important aspect [36-37 40.43-44]. Comparisons with other theoretical methods are important. Our Cu calculations are based on the pseudopotential of Péllisier [35], who applied it to the C u system. A controversy arose when CASSCF-CCI calculations on the same system seemed to imply [4Q] that the CIPSI calculations matched the experimental energy too well for a non-relativistic method. Péllisier [45] replied by showing his pseudopotential included relativistic effects. On the other hand a recent CASSCF-CCI calculation on the Pt+H reaction was published [4£]. The author apparently was not aware of our previous work on the same system [25] and yet he obtained potential energy surfaces that are virtually identical to those obtained using CIPSI as is commented elsewhere [47]. He did make some comparisons with GVB-CI calculations [4£] on the same P t H systems concluding that both this method and his agreed well. From all this we must conclude that sometimes CIPSI, CASSCF-CCI and GVB calculations can lead to the same results and quite similar chemical pictures. Other methods also match well in their predictions. In one of our papers both the CIPSI and Monstergauss programs were used in the CuH-f H thermal reaction giving coinciding results in most aspects of the process [18]. 4

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Activation of Small Molecules by Closed-Shell Transition Metal Atom States The matrix isolation experiments using epr, ir, uv-visible and other spectroscopic tech niques on transition metal-olefin complexes [8,49] have naturally attracted the attention of theoretical chemists and calculations on the Ni-C H4 system were reported in one of the first theoretical-experimental papers mentioned in the introduction [15]. These results were later supplemented with a larger (double-zeta) basis set [5Q] and also [51] extended for a N i ( C H ) system. The main conclusions are that a net charge transfer of almost 1/5 of an electron from the metal to the ethylene is evident and that a dona tion and back donation mechanism consistent with a classical Dewar-Chatt-Duncanson model exists. The Ni-ethylene binding energy is 12.8 kcal/mol. Another system that has been studied theoretically is Cu-C H4 where, in con trast [54], it was suggested that a weak charge transfer from the olefin to the metal (0.164e) without the participation of the carbons and the unpaired electron remaining in the 45 - 4p hybridized orbital exists. This is indeed very far from the Dewar-ChattDuncanson model [52-53]. We shall now report our results on Pd-C H4 which are much more in coincidence with those of Ozin et al. [Ifi] and Siegbahn and coworkers [50-51]. 2

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Palladium-Ethvlene Interaction The case of the interaction of Pd with C H is interesting because the Palladium ground state has a closed shell, d , configuration. This notwithstanding, the existence of a stable P d C H complex was established experimentally [&] and a net charge transfer from Palladium to the olefin carbons was reported. They also showed that the πbonded Pd-C H4 complex had very similar stretching modes to those observed for ethylene adsorption on Palladium surfaces [55], thus concluding that the complex is an acceptable model for this adsorbed species. This system is then interesting enough to justify a theoretical study and this was done by us using the methods described above [56-57]. The main conclusions of the matrix isolation experiments were confirmed by us as depicted in Fig. 1 where the Pd-fC H interaction energy curve as well as the geometrical and charge transfer properties of the complex are given. The binding energy of our complex, 47 Κ Joules/mole, was close to the value (54 Κ Joules/mole) of the desorption energy on Pd surfaces reported in [55], fulfilling the expectations of Huber, 2

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8. NOVARO



f Energy (Kcal/mol) 9,0 \-

Figure 1. Potential energy curve and geometrical and charge transfer parameters of the Pd-ethylene complex.


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THE CHALLENGE OF d AND f ELECTRONS

Ozin and Power [g] that the P d - C 2 H complex is an acceptable model of the adsorbed olefin species. We also see from Fig. 1 that the C-C distance is lengthened and the C-H2 planes are rotated by 15° to the original plane of the ethylene, thus explaining the red shifts in the respective vibrational modes reported in [fi]. Also, their ultraviolet results show a charge transfer from the metal to the olefin, which is also evident in Fig. 1. The value of this donation is similar to that reported for the N 1 - C 2 H 4 complex [52] and we also find that a reasonable consistency of the Dewar-Chatt-Duncanson model exists for Palladium as for Nickel [52] but apparently not for C u C H [54] although the fact that the latter study included the cf-electron subshell in the pseudopotential may completely falsify this aspect of their results. At least our own studies of Cu reactions, to be described later on, systematically showed the need of having a flexible and explicit description of the d-subshell in order to obtain the very important avoided crossings and activation barriers and in general the multiple-well potential energy surfaces that will be discussed later on. 4


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Ho Capture by Palladium Atoms Several calculations on the P d H 2 system exist [58-60] showing that a weakly bound A i ( C 2 v ) Palladium dihydride can be stable. However no attempts to depart from this C2V symmetry were carried out except for some analyses of the H2 positions around a Palladium dimer [fil]. We have studied both the side-on ( C 2 ) and the headon approaches of Pd to H2 showing that both present attractive curves without any activation barriers [12]. More recently we have studied a substantial part of the potential energy surface of the Pd(4 Cu + H

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which produced high yields of ground state Copper atoms and hydrogen. Theoretical calculations using both the Monstergauss and CIPSI programs showed an energetically downhill reaction coordinate for the H+CuH addition reaction [IS]. The addition re action implies a linear approach of the H atom towards the Cu moiety of CuH. Also an abstraction reaction was studied, which implied a linear approach of H now towards the H moiety of HCu. The latter process does have an activation barrier of less than 7 kcal/mol [IS]. The CuH complex formed by either of these reactions spontaneously dissociates into the final products (H and ground state Copper atoms) thus explaining the experimental results of [64]. 2

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Photoexcited Cu Activation of Ho In their study of the activation of H by photoexcitation of Cu atoms deposited in rare gas matrices, Ozing and coworkers [65-66] irradiated with 320nm photons to produce the 3d 4p ( P) «- 3d 4s ( S) transition. This was sufficient and necessary for the reaction of eq. (1) to take place. The efficiency of this photochemical process was originally attributed by them [££] to the radiationless transition of the Ρ state to the lower excited D state which hypothetical^ was the one that reacted with H . Our first calculations showed that the Ρ state itself is also capable [37] of capturing H effectively. Furthermore it was established that while the photoactivation by the 320nm was a sine qua non condition for the process to be triggered (without it Cu simply does not activate H ) right after it the process of eq. (1) proceeds regardless of whether Cu* suffers a radiationless transition to the lower excited state D or eventually to the ground state S of Cu. This was demonstrated [4£] from the relatively moderate (~ 20 kcal/mol) activation barriers that the potential energy curves of Cu^S), Cu( D) and Cu( P) present to H capture. These barriers are easily overcome by the great energy gain from the transition from the Ρ state (whose own potential curve is initially downhill in energy) to the lower D and S states. This process implies a very interesting mechanism involving Landau-Zener-Stuckelberg transitions and Herzberg-Teller couplings between the main A\ and P symmetry potential energy surfaces of the C Cu*+H system. These potential energy surfaces [44] are reproduced in Fig. 6 but the details of the reaction mechanisms envolving the three S, P and D states of Copper in activating H and eventually leading from the CuH energy minima of Fig. 6 towards the final products of eq. (1) cannot, for reasons of space, be reproduced here. For this the reader is referred to the original papers [37 43-44]. It is worth mentioning here however that the restriction to C symmetry in Fig. 6 is not a limitation for the actual chemical process of eq. (1) because, as was shown in [43], any deviation from C symmetry would only enhance the scission of H and the liberation of the CuH-|-H products because the activation barrier (~ 20 kcal/mol) would necessarily be lowered even more considering that both the Αχ and the B surfaces belong to a single representation A' of the C group when C symmetry is broken [21]. In conclusion: the activation of H by Cu is only feasible if the Ρ fc(abstraction). This of course would be disregarding any tunneling effects of an H moiety into the abstraction pathway barrier. However, such tunneling would not be present if D is used in the reaction. At this stage in conclusion we would necessarily infer from our theoretical results (M.E. Ruiz; G.A. Ozin and 0. Novaro, work in progress) that this reaction would indeed lead to no products of the type CuD or D, in complete agreement with the observed facts. 2



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Nitrogen Matrix Hindrance of Ho Activation by Copper An amazing discovery (Gracie, C , M.Sc. Thesis, University of Toronto, 1985) was made when the photoexcitation reaction 2

Cu( P) + H + D ^ CuH + CuD + H + D 2

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previously mentioned and which had a reaction rate ratio of about kjj/kj) ~ 1.5 [55] was carried out replacing the rare gas matrix by a N molecular matrix, with all other conditions (temperature, reactants, etc.) kept equal. The new results showed the reaction products 2

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Cu( P) + H + D ^ CuH + H + D Downloaded by COLUMBIA UNIV on July 23, 2013 | http://pubs.acs.org Publication Date: June 8, 1989 | doi: 10.1021/bk-1989-0394.ch008

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with the CuD and D subproducts being much scarcer, by a factor of about one thousand (in effect, kjj/kj) « 10 ). The suggestion was made based on infrared, optical and epr spectroscopical observations (Ozin, G.A., Gracie, C. and Mattar, S.M., Toronto University, unpublished data, 1984) that a CuN complex existed of the type 3

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( Ν ΞΞ N ) „ 2

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When a Copper atomic resonance line Ρ

The Challenge of d and f Electrons - ACS Publications - American

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