Theoretical Study on the Molecular Mechanism for the Reaction of VO 2 + with C 2 H 4 L. Gracia, ² J. R. Sambrano, ‡ V. S. Safont,* ,² M. Calatayud, ² A. Beltra ´ n, ² and J. Andre ´ s ² Departament de Cie ` ncies Experimentals, UniVersitat Jaume I, Box 224, 12080 Castello ´ , Spain, and Departamento de Matema ´ tica, UniVersidade Estadual Paulista, Box 473, 17033-360 Bauru, Brazil ReceiVed: October 21, 2002; In Final Form: February 26, 2003 The complex reaction between VO 2 + ( 1 A 1 / 3 A′′) and C 2 H 4 ( 1 A g / 3 A 1 ) to yield VO + ( 1 ∆/ 3 Σ) and CH 3 CHO ( 1 A′/ 3 A′′) has been studied by means of B3LYP/6-31G* and B3LYP/6-311G(2d,p) calculations. The structures of all reactants, products, intermediates, and transition structures of this reaction have been optimized and characterized at the fundamental singlet and first excited triplet electronic states. Crossing points are localized, and possible spin inversion processes are discussed by means of the intrinsic reaction coordinate approach. Relevant stationary points along the most favorable reaction pathways have been studied at the CCSD/6- 311G(2d,p)//B3LYP/6-311G(2d,p) calculation level. The theoretical results allow the development of thermodynamic and kinetic arguments about the reaction pathways of the title process. In the singlet state, the first step is the barrierless obtention of a reactant complex associated with the formation of a V-C bond, while in the triplet state a three-membered ring addition complex with the V bonded to the two C atoms is obtained. Similar behavior is found in the exit channels: the product complexes can be formed from isolated products without barriers. The reactant and product complexes are the most stable stationary points in the singlet and triplet electronic states. From the singlet state reactant complex, two reaction pathways are posssible to reach the triplet state product complex. (i) A mechanism in which a hydrogen transfer process is the first and rate limiting step and the second step is an oxygen transfer between vanadium and carbon atoms with a concomitant change in the spin state. The crossing point between singlet and triplet spin states is not kinetically relevant because it takes place at a later stage occurring in the exit channel. (ii) A mechanism in which the first stage renders a four-membered ring between vanadyl cation and the ethylene fragment and an oxygen- carbon bond is formed; on going from this minimum to the second transition structure, associated with a carbon-vanadium bond breaking process, the crossing point between singlet and triplet spin states is reached. The final step is the hydrogen transfer between both carbon atoms to yield the product complex. In this case the spin change opens a lower barrier pathway. The transition structures with larger values of relative energies for both reactive channels of VO 2 + ( 1 A 1 ) + C 2 H 4 ( 1 A g ) f VO + ( 3 Σ) + CH 3 CHO ( 1 A′) present similar energies, and the two reaction pathways can be considered as competitive. 1. Introduction Wide technological applications of metal oxides justify the necessity to understand their physical and chemical properties. 1,2 Numerous studies on the chemical reactions of these systems in the gas phase have been carried out to clarify their catalytic activity as well as the mechanism and intermediates in many important processes. 3 The study of the gas-phase chemistry can provide information about their intrinsic chemical reactivity and can contribute to a better understanding of their behavior in the condensed phase. Due to the recognition of their key role in many reactive processes, chemical reactions between metal oxides and hydrocarbons have received special attention by different research groups. 4-6 While their chemical reactivity has been exploited for many years, a prerequisite for a more extensive understanding of catalytic reactions is to discover the details on an atomic scale. Therefore, the knowledge of the corresponding molecular mechanisms represents a research topic of great interest. The behavior of metal oxides is strongly influenced by the presence of multiple low-lying electronic states in these species. Therefore, during the course of the chemical reaction, the system can access these energetically quasi-degenerate states and adapt to different bonding situations. Then, spin inversions can occur on going from reactants to products. In principle, the reaction rate can be limited by a transition structure (TS) or by the rate to cross between different electronic states. It seems quite difficult to speculate the detailed mechanism of such reactions which may involve complex isomerization and dissociation channels. 7-10 Reactions that involve a change in the spin state and thus occur on two or more potential energy surfaces (PESs) represent a field of growing interest in chemical physics and have received much attention in recent years. 11-18 In these cases, the standard theoretical description of chemical reactivity based on the idea of spin conservation along the reaction path must be abandoned. 19-22 This fact opens the possibility to a nona- diabatic behavior, in which the most favorable reaction pathway does not remain on a single PES as it evolves from rectants to products. Many experimental and theoretical findings provide evidence for the importance of nonadiabatic processes, ranging from photodissociations, 23,24 reactions between metal cations and water, 25-27 and organic, 28-31 inorganic, 32-34 or organometallic 35-40 chemistry in which states of different multiplicities determine * To whom correspondence should be addressed. E-mail: safont@ exp.uji.es. Fax: 34 964 728066. ² Universitat Jaume I. ‡ Universidade Estadual Paulista. 3107 J. Phys. Chem. A 2003, 107, 3107-3120 10.1021/jp0222696 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/03/2003