AB INITIO POTENTIAL ENERGY SRFACE FOR THE Xe - OH INTERACTION 64 th OSU International Symposium on Molecular Spectroscopy June22-26’2009 COLUMBUS Vipin Bahadur Singh* and Michael C Heaven Emerson Center For Scientific Computation EMORY UNIVERSITY ATLANTA * Present address: U P Autonomous College Varanasi India64

1. INTRODUCTION The hydroxyl radical (OH) is an important constituent of terrestrial atmosphere, interstellar space and of combustion gases. It is involved in many hydrogen abstraction reactions. The H2+OH  H2O+H reaction is one of several possible models for such reactions. A useful probe of the entrance channel of this process is provided by the interaction of OH radical bound to a rare gas atom. Van der Waals complexes of a OH radical bound to a rare gas atom (Rg) offer unique opportunities for investigations of weak bonding interactions and predissociation dynamics. Rare gas radical systems (Rg-X) such as Rg-OH are a good model system for studying weak, long range intermolecular interactions. There is a growing interest in the open-shell Van der Waals complexes since their interactions are viewed as intermediate between the nonbonding vander Waals interactions and chemical bonding[1,2] and many of them are prerequisite complexes formed in the entrance valleys of reactive potential energy surfaces. In spite of the considerable interest in the Rg.OH systems, much less is known about Xe.OH complex. It is known that when Rg=Xe, the strength of the interaction is much larger. In the ground state, X 2 Π, the interaction of Xe-OH has been characterized by J J Gilijames et al [3] experimently and theoretically.Recently the electronic spectra (A 2 Σ -X 2 Π ) was observed and reported by Christopher and M.C.Heaven [4]. Excited state A 2 Σ of Most of the Rg-OH complexes, have been thoroughly characterized, however, the excited state PES of Xe-OH are not characterized by the ab initio computation. 1. Michael C Heaven, Int.Rev.Phys.Chem.24,375 [2005] 2. C.C. Carter, H.-S.Lee, A.B. McCoy and T A Miller, J. Mol.Struct. 525, 1, [2000] 3. J J Gilijames et al, SCIENCE 313,1617 [ 2006] 4. J. Lue, Christopher and Michael C Heaven, OSU Symposium Columbus June [2007]J.Lue, Christopher and

Continued…. The ground state of OH has 2 Π electronic symmetry and the electronic configuration is 1σ 2 2 σ 2 3 σ 2 1л 3.There is a low lying excited state, the A 2 Σ state, in which an electron is promoted from the 3σ bonding orbital to the 1л nonbonding orbital. When OH is incorporated into a complex with a rare gas atom, the X 2 Π states are, in general, very weakly bound, with dissociation energies of approximately 5, 25,100, 117 and `224 cm -1 for He.OH, Ne.OH, Ar.OH, Kr.OH and Xe.OH respectively [2,3,5]. In contrast, the A 2 Σ states, of these complexes have the interesting property that the interaction range from several cm -1 for the He.OH to 1800 cm -1 for the Kr.OH. For the Xe.OH it was found to be much more larger. 5. Hee-Seung Lee and Anne B.McCoy, J.Chem.Phys 113,5736 [2000]

Aim of the present work: The aim of the present work is to provide a High level ab initio potential energy surface for Xe+OH interaction/complex in the ground state (X 2 Π ) and its first (low lying) excited state (A 2 Σ). The calculation have been performed by the internally c ontracted multi-reference-configuration interaction(MRCI) approach with reference wave functions obtained from complete active-space-self- consistent-field (CASSCF) calculations using the state averaged procedure as implemented in MOLPRO 6.0 program. A large one-electron basis consisting of the augmented-correlation consistent polarized valence five zeta (aug-cc-pV5Z) set is used. Interaction energies were obtained as the difference between the energy of the complex and energies of the fragments [with Basic Superposition Error (BSSE)Correction]. We computed the fragments in the same basis set as complex to avoid so called basis set superposition error. The Xenon atom has 54 electrons; the 28 inner shell electrons are described by a relativistic pseudo potential. The work has been carried out at the Emerson center for Scientific Computation, Department of Chemistry, Emory University, Atlanta during the summer of 2007.

2. Methodology/Xe - OH Interaction The interactions of Xe with OH in a Xe.OH complex were clearly expressed in the atom-diatomic Jacobi coordinates. In these coordinates, the OH bond length is represented by r, the distance between the rare gas atom and the center of mass of the OH radical is given by R and the angle between R vector and OH bond axis is denoted by θ. In the present work θ =0 corresponds to the Xe-H-O collinear arrangement. The O—H interatomic separation in the state has been kept rigid at the value r = 0.97 A. Since in the X 2 Π state, the 1π orbital's of OH have different occupations, in the presence of a perturbing rare gas atom the cylindrical symmetry of the radical is lowered to the Cs point group. If the singly occupied π-orbital is in the plane of the complex, an A’ state is obtained, while, if the doubly occupied π orbital is in the plane of complex, the state has A’’ symmetry. By contrast, in the A 2 Σ state, both of the π orbital's are doubly occupied, and only one surface needs to be considered in this case.

(Methodology/Xe - OH Interaction) Continued.. Ab initio Potential Energy surfaces for the X and A states of Xe.OH are computed with the MOLPRO 2006 program package at the MRCI( Multi Reference Configuration Interaction) level. The molecular orbital's are obtained from state averaged Complete active self consistent field (SA-CASSCF) calculations. A large one-electron basis consisting of the augmented-correlation consistent polarized valence five zeta (aug-cc-pV5Z) set is used. The Xe atom has 54 electrons; the 28 inner shell electrons are described by effective core potential (ECP). 21molecular orbital's are selected which are in corresponding with the Xe -14 (4s,4p,4d,5s,5p,6s) and OH-7.The thirteen (4p,4d,5s,5p,6s) outermost orbital's of Xe and six outermost orbital's of OH are placed in the active space, while the rest are frozen. The resulting wave function was then used for the MRCI calculations in which all valence electrons were correlated. The interaction energies were computed for about 200 geometries in the Jacobi Angle and atomic-molecule separation 10 A 0.The OH bond length is kept fixed at r=0.97 A 0.

Ground state (A’) Geometry of Xe-OH Interaction

3. Results and Discussion. The X 2 Π state Complexes: The interaction energies were obtained using the aug-cc-pV5z basis set, for a wide range of intermolecular distances, from 2 A 0 to 10 A 0 and for angles from θ = 0 0 to 180 o.The PESs for the A’ and A’’ states are calculated. In the ground state the global minimum of 218 cm -1 occurs (at R~3.5 A 0 ) on the A’ potential for a T-shaped geometry. This potential has a local minimum for a linear OH-Xe geometry(θ =0 0).The PES for the A’’ state has two minimum for the two collinear forms: Xe-OH and Xe-H-O. Our results are found in consonance, with the earlier reported value [obtained by CCSD method by] J J Gilijames et al [2].

The SA-CASSCF –MRCI Interaction Energies( in cm -1 ) obtained with the Aug-CC-pV5Z basis set for the A’- ground state of Xe-OH R(A 0 )θ = 0 0 θ = 20 0 θ = 40 0 θ = 60 0 θ = 80 0 θ = θ = θ = θ = θ =

Potential Energy Curve for the Ground X-State of Xe-OH [With ECP=46 ]Complex at θ = 90 0

Excited State The A 2 Σ state Complexes: The electron configuration of the A 2 Σ + state of OH is 1 σ 2 2 σ 2 3 σ 1 1 л 4. Since the 3 σ orbital is singly occupied the intermolecular repulsive exchange interaction is significantly reduced. This has a profound effects on the depths of the linear minima. Our calculation predicts the global minima of cm -1 for Xe.OH in the first excited state ( A 2 Σ ) for a linear Rg-OH geometry( θ = and R~2.25 A 0 ). The interaction energy (of cm -1 )in the first excited state of this (Rare Gas-OH) Van der Waals interaction behaves just like a chemical bond. Also this value supports the large red shift observed in the electronic spectra of Xe.OH as reported by Lue, Christopher and Michael C Heaven, OSU Symposium Columbus June [2007]. A local minima ofLue, Christopher and ~2324 cm -1 for a linear Rg-OH geometry( θ = 0 0 and R~2.75 A 0 ) was determined. The 2-D PES for A 2 Σ + state Xe.OH given in the following showing the results.

Excited State Geometry of Xe-OH Interaction {at θ = 180 o }

Excited state A 2 Σ -State PES for Xe + OH Interaction:

The SA-CASSCF –MRCI Interaction Energies( in cm -1 ) obtained with the Aug-CC-pV5Z basis set for the Excited A 2 Σ state of Xe-OH R(A 0 ) θ = 0 0 θ = 20 0 θ = 40 0 θ = 60 0 θ = 80 0 θ = θ = θ = θ = θ =

Potential Energy Curve for the Excited A 2 Σ -State of Xe-OH Complex at θ = 180 0

Potential Energy Curve for the Excited A 2 Σ -State of Xe-OH Complex at θ = 0 0

4. Conclusion: Our high level ab initio calculation predicts that the interaction energy for Xe+OH complex in the first excited state, A 2 Σ, is found to be cm -1 for the linear geometry [θ =180 o and R= 2.5 A 0 ], which is about 50 time greater than its ground state interaction. This (Rare Gas-OH) Van der Waals interaction behaves just like a chemical bond. This value is about ten times and five greater the Ar.OH and Kr.OH complexes respectively. In the ground state (A’ )the interaction is weak and geometry of the complex is T-shaped which is in the consonance to the earlier reported value.

Acknowledgement: One of the authors Dr.Vipin Bahadur Singh U P Autonomous College Varanasi India gratefully extends his thanks to Emerson Center, Department of Chemistry Emory University Atlanta and Prof Michael Heaven Department of Chemistry Emory University for providing the Visiting Fellow Award We would also like to extend our thanks to Dr. Jeremy Merritt, Department of Chemistry Emory University for the initial help in the MOLPRO calculations.