Theoretical study of the interaction of CuCO+ with Ar

THEO CHEM Journal of Molecular Structure (Theochem) 362 (1996) 359-363

Theoretical study of the interaction of CuCO+ with Ar M. Bragaa3’, A.L. Almeidab, C.A. Taftb** “Department of Physical Chemistry, Chalmers University of Technology, Goteborg, Sweden bCentro Brasileiro de Pesquisas Fisicas. Rua Dr. Xavier Sigaud, 150, 22290. Rio de Janeiro, R. J., Brazil

Received 15 December 1994; accepted 27 August 1995

Abstract

Ab initio calculations at the Hartree-Fock level and including correlation effects using the modified coupled-pair functional method have been performed in order to investigate the bonding interactions in the ArCuCO+ and CuCOAr’ clusters. Only ArCuCO+ is bound, and the inclusion of correlation effects in this cluster considerably increases the binding energy. Keywords: Argon; Cluster; CuCO+; Rare gas-metal ion interaction

1. Introduction The investigation of transition metals and their compounds isolated in rare gas matrices may provide good prototypes for systems such as carbonyls adsorbed onto transition metal surfaces [l-7]. The interaction of one or more metal atoms with carbon monoxide is an area of great interest in many fields of current research. The study of such complexes may give insight into aspects related to homogeneous and heterogeneous catalysis, surface chemistry and transition-metal ion chemistry. The catalytic reaction of CO with Hz on metal surfaces is an important industrial process for manufacturing synthetic petroleum products. The M/CO systems, where M represents a transition metal, are involved in important reactions in which the * Corresponding author. ’ Departamento de Quimica Fundamental, Universidade Federal de Pernambuco, 50.670-900, Recife, PE, Brazil.

chemisorption of CO on several metals seems to play a fundamental role [7]. The interaction of CO with metal surfaces has thus been studied extensively with a variety of surface sensitive techniques. One method of studying the CO-metal interaction is to prepare carbonyl complexes by matrix condensation reactions of metal atoms with carbon monoxide. In the early 1970s Ogden, as well as Ozin, Moskovits and co-workers showed that mononuclear carbonyls of group 1B metals were formed when the metal atoms and CO molecules were condensed in rare gas matrices at cryogenic temperatures, and reported the infrared as well as EPR spectrum of CuCO [Slo]. The closed 3d shell of the Cu atom makes molecules containing Cu easier to treat theoretically than other transition metals. However, a large amount of correlation energy is associated with the 3d” configuration, which must be carefully considered. Interaction of a copper atom with carbon

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monoxide presents an especially intriguing problem. Some theoretical calculations do not predict a bound complex for the CuCO 2x+ ground state; others have revealed a possible weak van der Waals interaction for this system [l l-131. It is generally accepted that bonding in transition-metal carbonyls occurs via a synergistic mechanism, whereby the CO ligands forms a (Tbond by electron donation from its 5u orbital to empty metal orbitals, and rr back-donation occurs from filled metal d orbitals to the occupied 27r* orbitals. There has also been considerable interest in transition-metal ion chemistry in the past decades. Although experiments on neutral transition-metal systems, especially metal clusters, can be difficult, because of problems in selecting an individual system for study, these problems are considerably alleviated if the analysis is carried out for the corresponding positive ion which is commonly produced by ionization of stable organometallic compounds followed by fragmentation, either by collision-induced dissociation or photodissociation. Copper and Cu+ and their carbonyl complexes formed in matrix isolation experiments have attracted considerable theoretical and experimental interest in EPR spectroscopy as well as transition-metal ion chemistry. The nature of the interactions are however quite different in CuCO and CuCO’. Since the largest interaction comes from the electrostatic interaction it should be expected that the interaction between the rare gas and the molecules should be larger for charged species than for neutral species. Another important point to be considered is the extent to which the rare gas matrix can affect the trapped copper carbonyls. In our previous work [l-4] we used the multiple scattering Xcr model to investigate first row transition metal atoms and ions isolated in Ar matrices. For the neutral species we found very good agreement between calculated and experimental hyperfine parameters for spectroscopies. For the single ionized clusters, however, we found that this model was unsatisfactory for describing the attractive interaction that arises from the polarization of the rare gas by an ion. In a subsequent work [5] we made effective core potential calculations to determine the binding energy trends of the transition metal-argon diatomic positive atoms and

362 (I 996) 359-363

determined that the interaction in these systems is mainly governed by charge-induced dipole forces, with the metal carrying the charge. Configuration interaction calculations indicated the importance of electron correlation in these systems. We subsequently performed ab initio Hartree-Fock calculations on FeAr, FeAr’ and FeCO in order to determine the interaction of both neutral and singly-ionized Fe atoms and FeCO molecules trapped in Ar [6]. In this work we have used the self-consistent field (SCF) modified coupled-pair functional (MCPF) method [14] to perform calculations on ArCuCO+ and CuCOAr+ in order to ascertain the effects of electron correlation as well as the Ar matrix on the singly-ionized species. We show that the binding energies of the transition metal noble gas ion systems are substantially increased by electron correlation.

2. Computational method Ab initio calculations at the Hartree-Fock level and using the MCPF method have been performed for the ArCuCO+ and CuCOAr+ clusters. The MCPF method has been previously used extensively to treat transition metal rare gas systems [14] and it is well known that it provides an accurate and computationally simple treatment of the bonding in these systems. Following previous work we correlate the ns and np valence electrons of the Ar atom and CO molecule and all the metal valence electrons. For the Cu+ atom this means that we correlate only the 3d electrons. For the Cu atom Wachter’s [ 151 basis set (14s9p5d) has been extended with the inclusion of two diffuse 4p functions and one diffuse d function given by Hay [16]. In addition we have added two f type functions taken from Bauschlicher and Langhoff [ 171. The final basis set was contracted to (Ss6p4d2f). For argon we have started from the Veillard DZ basis set (12~6~) and we have extended this basis by adding one diffuse p function and two d type and one f type polarization functions [18]. The final basis set for Ar was contracted to (6s5p2dlf). For C and 0 the (1 ls7p) basis set of van Duijneveldt [ 191 increased by one d type polarization function was used and contracted to (5s3pld). In all cases the

M. Braga et al./Journal of Molecular Structure (Theochem)

criteria for the contraction was to obtain a contraction flexible enough to minimize the basis set error and to reproduce accurate atomic polarizabilities and at the same time to keep as low as possible the computational requirements. The importance of a good description of the atomic polarizabilities for an accurate description of the bonding in these ionic systems which are expected to be interacting mainly electrostatically has already been discussed in our previous work as well as by other authors. The MCPF method is better than a CISD treatment and in addition is size-extensive so that systems with different numbers of electrons can be treated with a comparable accuracy. Calculations were performed using the MOLCAS-z program of Roos and co-workers [20] on an IBM 3090 computer. Basis sets superposition errors were estimated using the counterpoise method and in general were found to be a small fraction of the binding energy. To check the suitability of our basis set we have performed a calculation for the CuAr+ system and compared our results with those of Bauschlicher and Langhoff [21] obtained with a large basis set. We obtain a difference that is only 1% larger, which shows that our basis set is good enough to give a reasonable description of the binding energies in these systems.

3. Results and discussion The rare gas-metal ion interaction may be divided into three regions: an attractive van der Waals interaction at long range, a repulsive interaction at short distances and a molecular-orbital overlap effect at intermediate separations. At intermediate distances, the overlap of the molecular orbitals between the transition metal (neutral or single-ionized) and the noble gas may result in a distortion effect which shrinks both the metal and the rare gas orbitals of the same spin to reduce the repulsive interaction and causes a partial electron transfer from Ar to the metal ion. These effects depend strongly on the particular metal ion and its electronic state. At long range, the M-Ar molecule can be viewed as a nearly unperturbed M+ ion bound to Ar by simple charge-induced dipole forces. The form of the classical attractive

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interaction between the ionized transition metal and the Ar electron cloud is given by --iaq2/r4, where the metal is considered to be a point charge and (I: is the Ar polarizability. In general the bonding in transition-metal ionnoble gas systems is predominantly electrostatic [l-7]. As the polarizability of the noble-gas atom increases, it distorts more when it interacts with the metal ion, thereby decreasing the bond length and increasing the binding energy. Although the dominant interaction of a metal ion with a noble gas atom arises from charge induced polarization of the noble gas atom (if this is the only important interaction), then the bond strength of a diatomic molecule, whether it involves a transition metal or alkali atom, for example, would be related to the size of the ion and noble gas atom and the polarizability of the noble gas atom. The size of the atoms determines the internuclear separation where the Pauli repulsion balances the attractive forces. The metal, in addition to distortion of the noble gas atom, has several other mechanisms available to enhance the bonding. If the metal ion has also the s orbital occupied, sp hybridization, sda hybridization, or s to d promotion can reduce the metal-ligand repulsion or increase the strength of a covalent bond [l-7, 17,201. Although the concepts of promotion and hybridization are straightforward and influence most chemical bonding, the hybridization of the metal s and d orbitals is less commonly encountered. The positive and negative combination of 4s and 3da orbitals, for example, yield different charge densities along the z axis and can thus modify the Pauli repulsion. For transition metal ions interacting with noble gas atoms the 3d hole can be an important factor in the ordering of the state as it lowers the Pauli repulsion and decreases the shielding of the nucleus. Those ions with the 4s orbital occupied are more weakly bound, because the larger radial extent of the 4s orbital results in longer bond lengths and, therefore, a reduced electrostatic interaction. Since the bonding is relatively weak, promotion of the s orbital into the d shell will occur only when the s’d”-d”+ ’ separation is quite small. For those metal ions with a 3d”+ ’ occupation, the orientation of the 3d open-shell orbital can affect the metalligand repulsion. The relative importance of these

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effects depends on the separations between the lowest states of the metal ion. The loss of atomic d-d exchange energy in the molecule is also an important factor for determining the binding energies and the diversity of transition metal bonding which arises. For metal atoms with a sizable ionization potential there may be some charge donation from the ligand to the metal which will depend on the difference in the metal and ligand ionization potential. The ab initio calculation, given that several atomic states typically contribute to the bonding, must be able to describe the separation between the low-lying dn-‘s*, d”s’ or d”+’ atomic states of the ion to be able to correctly account for the mixing that occurs in the molecular system [l-7,17,20]. Even though the CuCO and CuCO+ states are energetically close, the nature of the interactions is quite different: n bonding and mainly electrostatic, respectively. The neutral metal-CO interaction is a balance of g and r backbonding, whereas the driving force for the positive ion metal-CO interaction is essentially electrostatic. The CuCO+ ‘C+ state may be described in terms of a carbonyl ligand, CO (‘C’) and Cuf d” metal atom. Thus bonding in the CuCO+ complex is quite strong due mainly to the electrostatic interaction as well as 0 bonding and 7r back-donation. The positive charge causes a strong electric field on CO in a region where pure electrostatic interactions are dominant, mainly charge-dipole and charge-induced dipole

-0.163-

I

r(Ar-Cu) Fig. 2. Hartree-Fock potential Energies are relative to -2278.00

5.0

46

4.6

4.4

(0.u.)

energy a.u.

curve

for ArCuCO+.

interactions. The binding in CuAr+ also has a significant contribution from charge-induced dipole interactions. For systems with a 3d”+ ’ configuration, such as found in CuAr+ and ArCuCO+, compared with 3d”4s’ systems, one should find smaller binding energies and smaller bond lengths since the spatial extent of the 4s orbital is larger than the 3d orbital which leads to a larger repulsion interaction with Ar. In some cases however, such as NiAr’, the hole in the 3da orbital will contribute to an additional reduction of the transition metal ion-noble gas repulsion [5,7,12-13,171. The MCPF potential energy curve for the IX+ state of CuCOAr+ is shown in Fig. 1. There are no qualitative differences between the HF and the

-0.3ot

5.0

6.0

rKD-Ar)

1.0

(a.~.)

Fig. 1. Hartree-Fock potential energy Energies are relative to -2778.46a.u.

curve

L

4.30

4.40

4.50

4.60

r(Ar-Cd(a.u) for CuCOAr’.

Fig. 3. MCPF potential are relative to -2278.69

energy curve for ArCuCO+. a.u.

Energies

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MCPF results. The total energy of the molecule increases monotonically and no potential energy minimum is found within the range of Ar-CO distances studied. There is no binding interaction between Ar and COCu. In Fig. 2 we show the Hartree-Fock potential energy curve for ArCuCO’. Unlike the CuCoAr” configuration, we note that the ArCuCO+ HF curve indicates a minimum at 4.477a.u. with a binding energy of 204 meV. There is thus a binding interaction between Ar and CuCO in the ArCuCO+ cluster. In Fig. 3 we give the MCPF potential energy curve for ArCuCO+ which indicates a minimum at 4.462 a.u. and a binding energy of 372meV. The inclusion of correlation thus increases considerably (by nearly 100%) the binding energy in this cluster. The MCPF method, which includes electron correlation, yields large binding energies, since the large ionization potentials of the transition metals induce a component of neutral transition metalionized noble gas species character in addition to the dominant ionized transition metal-neutral noble gas contribution [17]. The transition metal ions can reduce the metal-ligand repulsion both by changing the orientation of the d holes and by undergoing either s-d or s-p hybridization or promotion taking into consideration that the reduction in repulsion due to these effects must be balanced against the energetic cost of changing the ion state. The inclusion of electron correlation allows other atomic asymptotes to mix into the wavefunction. As in CuAr+ there is also in ArCuCO+ some 3do-4s hybridization which occurs to reduce the Pauli repulsion and increase the binding energy. All these effects are more well described using correlation effects in the MCPF method which thus yields significantly larger binding energies. Conversely, as expected, there is no interaction between CO and Ar and thus there is no binding in the CuCOAr+ cluster.

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