Interactions in Open-Shell Clusters: Ab Initio Study of Pre-reactive Complex O( 3 P) + HCl † Joanna E. Rode, ‡ Jacek Klos, ‡,§ Lukasz Rajchel, ‡,# Malgorzata M. Szczesniak, ‡ Grzegorz Chalasinski,* ,# and Alexei A. Buchachenko | Department of Chemistry, Oakland UniVersity, Rochester, Michigan 48309, Faculty of Chemistry, UniVersity of Warsaw, Pasteura 1, 02-093 Warszawa, Poland, and Department of Chemistry, Moscow State UniVersity, Moscow 119992, Russia ReceiVed: June 23, 2005; In Final Form: October 18, 2005 Van der Waals interactions between the ground-state triplet O( 3 P) atom and the closed-shell HCl molecule are investigated in the pre-reactive region. Three adiabatic (two of A′′ symmetry and one of A′ symmetry) and four non-relativistic diabatic potential energy surfaces are obtained by combining a restricted open-shell coupled cluster approach with the multireference configuration interaction method. The lower A′′ adiabatic potential surface has a single minimum (D e ) 589 cm -1 ) for a linear O‚‚‚HCl configuration. The upper A′′ potential has a weak (D e ) 65 cm -1 ) minimum for a linear HCl‚‚‚O configuration. The A′ adiabatic potential has a weak (124 cm -1 ) T-shaped minimum. Adiabatic potentials intersect once in the O‚‚‚HCl linear configuration and twice in the linear HCl‚‚‚O geometry. The role of electrostatic interactions in shaping these potentials is discussed. The effects of spin-orbit coupling on this interaction are also investigated assuming a constant value of the SO parameter. I. Introduction Reactions involving atoms with nonzero angular momentum proceed on multiple, coupled potential energy surfaces. 1 If such reactions involve potential barriers, the reactants form van der Waals complexes in the reactant valley. These van der Waals minima, which support bound and quasibound states, may trap the reactants before they have a chance to engage in reactive encounters. For example, Skouteris et al. 2 observed that in the Cl( 2 P) + HD reaction the presence of a tiny van der Waals well (of ca. 0.5 kcal/mol) in the entrance valley ahead of the 8.5 kcal/mol barrier strongly affects the products distribution. They state, “The fact that such weak van der Waals forces can actually affect the outcome of the reaction is quite remarkable.” A frequent occurrence in the pre-reactive region is the crossing of Born-Oppenheimer potential surfaces, which gives rise to nonadiabatic effects. The presence of spin-orbit coupling provides further opportunities for nonadiabatic coupling. 3,4 We have examined a number of pre-reactive complexes involving 2 P halogens interacting with closed-shell diatomics. 5-9 Ab initio calculations have been employed to characterize the Cl( 2 P) + HCl, 5 Cl( 2 P) + H 2 , 6 F( 2 P) + H 2 , 7 and Br( 2 P) + H 2 8 complexes. The results were also reviewed recently. 9 The adiabatic and diabatic potentials derived in these studies were employed in bound-state calculations of Zeimen et al. 10,11 The subject of this paper is the O( 3 P) + HCl van der Waals complex formed in the reactant valley of the reaction between the ground-state O( 3 P) and HCl. The O + HCl f OH + Cl reaction is of great interest to atmospheric chemistry and serves as a classic example of reaction with heavy-light-heavy dynamics. For these reasons it attracted a great deal of attention from the reactive scattering community (for a recent review see Althorpe and Clary 12 ). Recent quantum scattering calculations (on the lowest surface) 13,14 revealed the presence of sharp resonances below the reaction threshold which have been attributed to the presence of the O( 3 P)-HCl van der Waals wells. The ground-state potential energy surface (PES) for the O( 3 P) + HCl f OH + Cl reaction has been the subject of theoretical investigations for a long time. The first reasonable PES of the ground state 3 A′′ was advanced by Koizumi, Schatz, and Gordon (KSG). 15 This PES, which was based on the MP2/6-31G** calculations, was used in numerous quantum calculations of the rate constants (see ref 16 for references). In 1999 Ramachandran et al. developed the multireference configuration-interaction with the approximate correction for quadruples, MRCI+Q/cc-pVTZ, representation of this state. 17 The quasiclassical trajectory calculations on this surface revealed much improvement over the KSG surface, but more accurate quantum calculations suggested that some details were still inaccurate, especially concerning the barrier region. 13,18 Xie et al. used this surface in quantum reactive scattering calculations. 14 They observed a number of low-energy resonances in the cumulative reaction probability of this reaction and concluded that they are related to the quasi-bound states due to the entrance-channel van der Waals complexes. More recently MRCI+Q with a sequence of aug-cc-pVnZ (n ) 2-4) basis sets by Ramachandran and Peterson 19 generated the lowest energy surfaces for both 3 A′′ and 3 A′ symmetries. A high level of theory, CBS extrapolation combined with scaling, resulted in what appeared to be the most accurate to date surfaces for the reaction O( 3 P) + HCl f OH + Cl. Their potential contains van der Waals wells on both the product and reactant sides of the barrier. All the potential energy surfaces mentioned thus far map the total Born-Oppenheimer energy of a dimer as a function of geometry. In this representation, the energy does not approach † Part of the special issue “Jack Simons Festschrift”. * Corresponding author. E-mail: chalbie@tiger.chem.uw.edu.pl. ‡ Oakland University. § Present address: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742-2021. # University of Warsaw. | Moscow State University. 11484 J. Phys. Chem. A 2005, 109, 11484-11494 10.1021/jp053419q CCC: $30.25 © 2005 American Chemical Society Published on Web 12/15/2005