AB INITIO POTENTIAL ENERGY SRFACE
FOR
THE Xe - OH INTERACTION
64th 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




1.
2.
3.
4.
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, X2Π, the interaction of Xe-OH has been characterized by J J
Gilijames et al [3] experimently and theoretically .Recently the electronic spectra
(A 2Σ -X 2Π) was observed and reported by Christopher and M .C.Heaven [4]. Excited state A
2Σ 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.
Michael C Heaven, Int.Rev.Phys.Chem.24,375 [2005]
C.C. Carter, H.-S.Lee, A.B. McCoy and T A Miller, J. Mol.Struct. 525, 1, [2000]
J J Gilijames et al, SCIENCE 313,1617 [ 2006]
J. Lue, Christopher and Michael C Heaven, OSU Symposium Columbus June [2007]
.
Continued…
The ground state of OH has 2Π electronic symmetry and the electronic
configuration is 1σ2 2 σ2 3 σ2 1л 3.There is a low lying excited state, the
A 2Σ 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 X2Π 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 2Σ 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 (X2Π )
and its first (low lying) excited state (A 2Σ).
The calculation have been performed by the internally contracted multi-reference-configuration
interaction(MRCI) approach with reference wave functions obtained from complete active-space-selfconsistent-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 X2Π 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 A2Σ 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 A0.The OH bond length is kept fixed at r=0.97 A0.
Ground state (A’) Geometry of Xe-OH Interaction
3. Results and Discussion.

The X2Π state Complexes:
The interaction energies were obtained using the aug-cc-pV5z basis
set, for a wide range of intermolecular distances, from
2 A0 to 10 A0 and for angles from θ = 00 to 180o .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 A0 ) on the A’ potential for a T-shaped
geometry. This potential has a local minimum for a linear OH-Xe
geometry(θ =00).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(A0)
θ=
00
θ=
200
θ=
400
θ=
600
θ=
800
θ=
1000
θ=
1200
θ=
1400
θ=
16 00
θ=
1800
2.0
-
86214.
1
-
-
-
-
15365.
5
18324.
4
22217.
5
24460.
0
2.25
51973.
7
39927.
2
-
12724.
2
-
7186.5
8278.1
11056.
7
15233.
7
19059.
2
2.5
22466.
6
18182.
1
-
6258.1
-
3744.2
4620.4
6281.4
8147.3
9065.8
3.0
4511.7
3878.2
-
1797.0
1402.8
1398.7
1633.7
1964.4
2246.6
2363.5
3.5
1409.2
1336.5
-
-
-
-
-
1158.3
1158.3
1216.2
3.75
-
1098.8
-
-
-
-
-
1098.9
1112.9
-
4.0
1041.2
1046.0
1057.5
1069.0
1078.2
1085.7
1092.0
1097.0
1100.6
1101.9
5.0
-
1131.1
1141.8
1151.2
1156.3
1157.8
1157.3
1155.9
1154.8
1154.3
6.0
1169.9
1171.3
1174.2
1176.9
1178.2
1178.7
1178.4
1177.8
1177.4
1177.2
8.0
1185.3
1185.4
1185.8
1186.1
1186.4
1186.4
1186.3
1186.2
1186.1
1186.1
10.0
1187.3
1187.3
1187.3
1187.4
1187.5
1187.5
1187.4
1187.4
1187.4
1187.2
Potential Energy Curve for the Ground
X-State of Xe-OH [With ECP=46 ]Complex at θ = 900
-91.003
-91.004 0
2
4
6
8
10
12
-91.005
-91.006
-91.007
-91.008
-91.009
-91.01
-91.011
-91.012
-91.013
Series1
Excited State

The A 2Σ state Complexes:
The electron configuration of the A2Σ+ state of OH is 1σ2 2 σ2 3 σ1 1л 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 11900 cm-1 for Xe.OH in the first excited state ( A 2Σ ) for a linear Rg-OH
geometry(θ = 1800 and R~2.25 A0) .The interaction energy (of 11900 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 of
~2324 cm-1 for a linear Rg-OH geometry(θ = 00 and R~2.75 A0) was determined.
The 2-D PES for A2Σ+ state Xe.OH given in the following showing the results.
Excited State Geometry of Xe-OH
Interaction {at θ =180o}
Excited state A 2Σ -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 2Σ state of Xe-OH
R(A0)
θ = 00
2.0
-
θ=
200
-
θ = 400
θ = 600
θ=
800
θ=
1000
θ=
1200
θ=
1400
θ=
16 00
θ=
1800
-
-
-
-
-
-
8425.5
5421.6
2.25
24352.
9
14100.8
-
13338.6
-
13688.
5
7903.0
1203.5
-5634.6
10484.2
2.5
2.75
4918.8
-636.2
3740.1
-
-
6807.3
-
7656.6
3911.0
-725.5
-4779.3
-6500.3
-1872.9
3.0
-908.8
-423.5
-
2215.2
2775.0
2561.1
1813.1
836.8
11.21
-308.3
3.5
235.9
480.6
-
-
-
-
-
1182.1
1043.8
990.3
3.75
-
813.6
-
-
-
-
-
1246.8
1192.8
-
4.0
951.7
1032.0
1189.0
1299.8
1339.2
1336.4
1316.7
1292.9
1273.8
1266.4
5.0
-
1344.5
1362.6
1375.6
1380.6
1381.4
1381.9
1382.2
1383.7
1385.0
6.0
1396.0
1397.0
1400.5
1403.4
1405.1
1405.0
1405.1
1406.4
1406.9
1407.1
8.0
1413.9
1414.1
1414.5
1414.9
1415.0
1415.0
1415.0
1415.1
1415.1
1415.1
10.0
1416.1
1416.1
1416.2
1416.3
1416.3
1416.3
1416.3
1416.3
1416.3
1416.3
potential (cm -1 )
Potential Energy Curve for the Excited
A 2Σ -State of Xe-OH Complex at θ = 1800
35000
30000
25000
20000
15000
10000
5000
0
-5000 0
-10000
-15000
2
4
6
Distance (Å)
8
10
Potential Energy Curve for the Excited
A 2Σ -State of Xe-OH Complex at θ = 00
35000
30000
25000
20000
15000
10000
5000
0
-5000
0
2
4
6
8
10
12
4. Conclusion:




Our high level ab initio calculation predicts that the
interaction energy for Xe+OH complex in the first
excited state, A 2Σ , is found to be 11900 cm-1 for the
linear geometry [θ =180o and R= 2.5 A0], 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-07-08 . 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.