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Theoretical Investigations on the Superhalogen
Properties and Interaction of PdOn (n 5 1–5) Species
Ambrish Kumar Srivastava and Neeraj Misra*
Density functional calculations on the ground state geometries
and stabilities of PdOn species (n 5 1–5) are performed in neutral as well as anionic forms. Calculations reveal that Pd can
bind stably with four O atoms indicating the maximum oxidation state of Pd as high as 18. The electron affinities of PdOn
suggest that these species behave as superhalogens for n 2.
The large electron affinities of PdOn species along with stability of their anions point toward the synthesis of new class of
compounds having unusual oxidizing capabilities. This possibility is explored by considering the interaction of PdO2 superhalogen with Ca atom which forms a stable CaPdO2 complex.
In this complex, PdO2 unit closely mimics the behavior of O
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Introduction
and Aln (n 5 1–3, 13) clusters which was attributed to the
charge-dipole interaction generated with an addition of extra
electron.[14] Even more interesting is the case of transition
metal oxides. For example, the maximum oxidation state of
manganese (Mn) can go up to 17. The stability of MnO42 and
the existence of KMnO4, a well known oxidizing agent, support
this fact. The EA of MnO4 was estimated to be 5 eV which is
verified experimentally.[15] This value is quite larger than the
EA of O atom which is only 1.42 eV. Similarly, the EA of FeO4
and CrO4 have been estimated to be 3.8 and 4.96 eV,
respectively.[16]
The present study includes an investigation on superhalogen properties of PdOn (n 5 1–5) species and their interaction
with an alkaline metal (Ca) atom using density functional
theory (DFT). The study is motivated by the fact that superhalogens can be used to synthesize supersalts by reacting with
suitable metal cations.
Superhalogen species have been got much attention since last
three decade. Due to their extremely high electron affinity (EA)
exceeding the maximum atomic EA of the most electronegative halogens (3.0–3.6 eV), superhalogens can be used for oxidation of counterpart systems with relatively high ionization
potentials (such as O2, Xe) and synthesis of unusual chemical
compounds.[1,2] Conventionally, superhalogens are a special
kind of molecular species consisting of central metallic atom
and highly electronegative ligands such as halogens or oxygen. A metal atom surrounded by peripheral F, Cl, or O atoms
causes to increase electronegativity due to delocalization of
electron over electronegative atoms consequently increasing
EA of the species.
Initially, s block metal elements were invited to play the central role.[3] However, due to their fixed valence, they were able
to bind with only a limited number of electronegative atoms.
To overcome this difficulty of fixed valence, transition metal
elements came into play as their valences or oxidation states
vary due to the presence of d orbital electrons. A systematic
study on fluorinated coinage metal (Cu, Ag, and Au) clusters
revealed that Cu and Ag bind atomically with four F while Au
can bind with maximum of six F atoms and EA of these species were estimated as high as 8 eV.[4] Recently, many other
transition metals M such as Pt, Pd, Ru, Ir, Cr, Mn, Co and so
forth are also found to form hexahalide, MX6 (X 5 halogen
atoms) molecules and the EAs of MX6 are very large as compared to X. It simply implies that the oxidation state of M can
go as high as 16 and these species conventionally behave as
superhalogens.[5–11] Interestingly, some non conventional species have also been studied in which either central s or d metals were replaced with p block elements and/or some complex
ligands are used instead of electronegative F or O atoms.[12,13]
Thus, high EA values of such species suggested their superhalogen nature. Moreover, it was also noticed that the superhalogen property can be induced by doping of iodine in pure Lin
DOI: 10.1002/qua.24564
Methodology
All initial geometries are fully optimized without any symmetry
constraint by quadratically convergent self consistent field
(SCF) method in DFT. The total energies are calculated by
hybrid exchange-correlation functional B3LYP[17] with SDD
basis set for all atoms. The present computational scheme has
already been used in some previous studies[5–11]and provided
reliable geometries and energetics of superhalogen species.
This computational scheme yields the ionization potentials for
Pd and Ca atoms as 8.7 and 6.2 eV consistent with experimental values of 8.33 and 6.11 eV, respectively.[18] The calculated
bond lengths for CaO and O2, 1.83 and 1.27 Å agree well with
corresponding experimental values, 1.822 and 1.207 Å,
A. K. Srivastava, N. Misra
Department of Physics, University of Lucknow, Lucknow, Uttar Pradesh
226007, India
E-mail: [email protected]
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respectively.[19] The bond dissociation energy of CaO molecule is
calculated to be 3.39 eV which agrees satisfactorily with the
experimental value of 3.97 eV (383.3 KJ/mol).[20] These numerical
figures further suggest the reliability of results as well as validity
of the scheme. Normal mode frequencies are calculated for all
structures to ensure that they belong to at least a minimum in
the potential energy surface. Calculations are also repeated for
higher spin states to determine the preferred spin multiplicity of
the neutral and anionic species. Gaussian 09 code[21] is used to
perform all computations. It is widely accepted that the natural
bonding orbital (NBO) analysis gives insights into chemical bonding and interactions.[22] NBO-based partial atomic charges are
more reliable due to its low basis set dependency.[23] NBO analyses are performed with the help of NBO 3.1 program[24] as
implemented in Gaussian 09 package.
Results and Discussion
In order to locate the ground state structures, we have started
with various initial geometries of PdOn species from n 5 1 to 5
in their neutral and (mono) anionic forms as shown in Figure
1. We have considered all possible geometries in which all O
atoms bind atomically to central Pd atom. In addition, we
have also included structures in which two O atoms bind
molecularly to Pd for n > 3.
Structures and stabilities
The equilibrium (ground state) geometries of PdOn (n 5 1–5)
neutral and anionic species are shown in Figure 2. The bondlengths, symmetries, and spin multiplicities (M) are also displayed. The bond length, PdAO in neutral PdOn is smaller
than that of corresponding anions. All PdOn species energetically favour higher spin states except for n 5 1 and 5 in PdOn2.
For n 5 1 and 5, the higher spin states of PdOn2 species are
0.49 and 0.42 eV higher in energy than corresponding lower
spin states, respectively. Neutral PdO2 takes a bent (C2v ) structure while its anion becomes linear. The geometry of neutral
PdO3 closely resembles to that of its anion which is a trigonal
planar (C2v ). Neutral PdO4 assumes a square planar structure
but its anion deviates from planarity. For n 5 5, both neutral
and anionic PdOn becomes (PdOn22)O2 complex in which O2
moiety is very weakly bound to the central Pd atom. The distance of O2 moiety from Pd atom in anion is greater than that
Figure 2. Ground state geometries of PdOn (n 5 1–5) species bond-lengths
(in Å), symmetries, and spin-multiplicities (M) are also shown.
in neutral PdOn. The bond length, OAO in O2 moiety 1.26–1.28
Å is in accordance with that in free O2 molecule which is 1.27 Å.
Normal mode analysis reveals all real frequencies for structures given in Figure 2. Thus, they belong to at least a local
minimum in the potential energy surface implying that they are
thermodynamically stable. In order to analyze further the stability of these structures, we have considered their dissociations to
O atom and O2 molecule. The corresponding dissociation energies for neutral as well as anions are calculated as follows,
De ðPdOn ! PdOn21 1 OÞ 5 E½O
1 E½PdOn21 2 E½PdOn De ðPdOn ! PdOn22 1O2 Þ 5 E½O 2 1 E½PdOn22 2 E½PdOn ; n51–5
Figure 1. Initial geometries of PdOn (n 5 1–5) species considered in this
study. Central dark spheres represent Pd atom and peripheral light spheres
represent O atoms.
2
where E[..] represents the electronic energy of respective species excluding zero point energy. Figures 3 and 4 plot the dissociation energies for PdOn21 1 O and PdOn22 1 O2 fragments,
respectively, as a function of n. All PdOn species are found to
be stable as all De values are positive. The dissociation energies decrease as the successive O atoms are attached to Pd.
PdOn anions are more stable than their neutrals due to high
De values for both dissociation channels up to n 5 4. Conversely for n 5 5, dissociation energy of neutral PdOn is higher
than its anion against fragmentation to O atom but same in
case of dissociation to O2 molecule.
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Figure 3. Dissociation energies (De) of neutral and anionic PdOn (n 5 1–5)
into PdOn21 1 O fragments.
Figure 5. HOMO-LUMO gap of neutral and anionic PdOn for n 5 1–5.
In order to compare chemical reactivity of PdOn species, we
have calculated the energy gap between highest occupied
molecular orbital (HOMO) and lowest unoccupied MO (LUMO).
These orbitals are responsible for chemical reaction and interaction with other species. In Figure 5, we have plotted HOMOLUMO gap as a function of n. The higher gap for PdO3 suggests
that it is relatively more stable, whereas smaller gap for PdO2
indicates that it is chemically more reactive and can interact easily with other species. Thus, it is possible for PdO2 to react easily
with appropriate metal atoms and form a complex compound
with high oxidizing properties. This possibility will further be
examined in detail in a later section. As usual, an opposite trend
of HOMO-LUMO gap is observed in case of PdOn anions.
Evidently, Pd can bind with a maximum of four O atoms successively and form stable neutral and anionic PdOn species up
to n 5 4. Thus, the maximum oxidation state of Pd can be as
high as 18 at least in case of bonding with O atoms. Note
that Pd possesses a main oxidation state of 12 which is
reflected by the existence of PdO and palladium acetate. Such
a high oxidation state of Pd can be possible due to involvement of inner shell d electrons in bonding. We have performed NBO analyses on these species to explore the
participation of d electrons in bonding. In Figure 6, we plot
the contribution of d electrons as a function of n. It is apparent that the number of 4d electrons participating in the
bonding increases with the increase in O atoms up to n 5 4.
Now, we discuss the EAs of these species and their origin.
The adiabatic EA is calculated by the difference of energies
between neutral species and their anions both in their ground
state configurations. The calculated EAs of all these species are
plotted in Figure 7 as a function of n. We can see that the EA
value increases remarkably as the successive O atoms are
attached to Pd and reaches at its maximum of 4.57 eV for
Figure 4. Dissociation energies (De) of neutral and anionic PdOn (n 5 1–5)
into PdOn22 1 O2 fragments.
Figure 6. Contribution of d electrons in bonding of neutral PdOn for n 5 1–5.
Bonding and electron-affinity
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Interaction of PdO2 superhalogen with Ca
Figure 7. EA of PdOn (n 5 1–5) as a function of n.
n 5 4. This EA value is slightly lower than that of PdF3 calculated with the same level of theory[6] which is obvious due to
relatively high electronegative nature of fluorine. The EA of
PdO2, 3.63 eV is also larger than fluorine atom and very large
as compared to oxygen. The calculated EA values suggest that
PdOn species behave as superhalogens for all n 2. These
large EAs result due to more positive charge localization on Pd
atom. We analyze the distribution of electrons in PdOn species
referring to NBO charges plotted in Figure 8. In PdO, charge
concentrated on Pd is 10.41 e. As the number of O atoms
increases, charge on Pd increases but saturates at 10.95 e in
PdO3. Moreover, in PdO2, about 75% of extra negative charge
is located on Pd. As successive O atoms are attached to PdO2,
extra charge starts to delocalize over several O atoms. Moreover, for n 5 4, only 10% of extra electron is contained by Pd.
This explains the reason for high EA values of PdOn species for
n 5 2 to n 5 4.
Figure 8. NBO charges on Pd atom in neutral and anionic PdOn for n 5 1–5.
Charge difference, DQ (in e) is shown for n 5 1 and n 5 4.
4
The above discussion reveals that PdOn species behave as
superhalogens for n 2. One may be interested in the interaction of a PdOn superhalogen with s block metals. In order to
study the interaction of PdO2 superhalogen with Ca atom, we
put a Ca atom on the top of PdO2. After optimization, we find
a planar (Cs) structure in which Ca binds with two O atoms
and also interacts weakly with Pd as shown in Figure 9. The
thermodynamic stability of CaPdO2 complex is confirmed by
frequency calculations. Apparently, PdO2 fragment in CaPdO2
bends to form a closed mesh. The bond-lengths, CaAO and
PdAO are found to be 2.06 and 1.95 Å, respectively.
We first discuss the nature of interaction between Ca and
PdO2. We analyze the geometry PdO2 fragment in CaPdO2
complex. The 12 oxidation state of Ca leaves PdO2 fragment
in dianionic form. To confirm this fact, we optimize dianionic
PdO2 and find a bent (C2v) structure with the bond-length of
1.86 Å. The bent structure of PdO2 moiety along with
increased PdAO bond-length in CaPdO2 indicates that, after
being alkalinized, PdO2 tends to behave as its dianionic counterpart, that is, PdO222. This may suggest the ionic character
of the interaction between Ca and PdO2 superhalogen. Furthermore, the NBO charge on Ca atom in CaPdO2 complex is
found to be 1.43 e while the same in CaO is 1.24 e. Moreover,
the occupancy of CaAO bond in Ca-PdO2, 1.97 is very close to
2 (that in CaO). This may suggest that the interaction of Ca
atom with PdO2 superhalogen is similar to that between Ca
and O atom, that is, ionic.
The HOMO-LUMO gap of CaPdO2 complex, 2.93 eV is
slightly larger than that of PdO2 cluster. This increased gap
due to binding with Ca atom increases the chemical stability
of CaPdO2 as compared to PdO2. Furthermore, the HOMOLUMO gap of CaO is found to be 2.43 eV at the same level
which is smaller than that of CaPdO2. The relative stability of
the complex is a consequence of more positive charge localization on Ca atom in CaPdO2 (as compared to that in CaO)
due to charge transfer from Pd to Ca. Note that the NBO
Figure 9. Equilibrium geometry and possible dissociations of CaPdO2 complex with Cs symmetry. Selected bond-lengths (in Å, italicized) and dissociation energies (in eV) are also given.
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charge on Pd is only 0.61 e in contrast to 0.92 e in PdO2. We
must say that a stable neutral PdO2 cluster is further stabilized
by interacting with Ca and that the resulting complex compound Ca21 PdO222 is more stable than Ca21O22. To further
explain this fact, we consider the dissociations of CaPdO2 into
various possible fragments as shown schematically in Figure 9.
The binding energy of CaPdO2 against dissociation to Ca and
PdO2 is 6.12 eV, whereas that of CaO dissociating to Ca and O
is 6.78 eV. The energies of CaPdO2 against dissociation to O
atom and O2 molecule are 7.70 and 5.96 eV, respectively, indicating further the stability of CaPdO2 complex. Thus, it may be
possible to form a stable CaPdO2 compound by interaction of
PdO2 superhalogen with Ca atom. To examine whether the
formation of CaPdO2 is exothermic or endothermic, we calculate the energy required to fragment CaPdO2 to two stable
salts, namely CaO and PdO. This fragmentation energy is 4.04
eV, advocating the possibility to synthesize CaPdO2.
Conclusions
In summary, we have shown that a Pd atom can bind with a
maximum of four O atoms. It may be attributed to the participation of inner shell d electrons in bonding. The resulting species, PdOn are stable in the neutral as well as anionic states
against dissociations either to O atom or to O2 molecule. The
electron affinities of neutral PdOn for n 2 are found to be
larger than that of halogens, allowing them to behave as
superhalogens. The interaction of PdO2 superhalogen with an
alkaline metal Ca is found to be ionic and leads to the formation of a stable CaPdO2 complex. This opens an opportunity
to synthesize a new class of compounds by interaction of
PdOn superhalogens with appropriate metal atoms having
very high oxidizing properties.
Acknowledgments
AKS acknowledges Council of Scientific and Industrial Research
(CSIR), New Delhi, India for providing a research fellowship.
Keywords: superhalogen electron-affinity palladium oxide interaction density functional theory
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How to cite this article: A. K. Srivastava, N. Misra, Int. J. Quantum Chem. 2013, DOI: 10.1002/qua.24564
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Received: 22 August 2013
Revised: 20 September 2013
Accepted: 23 September 2013
Published online on Wiley Online Library
International Journal of Quantum Chemistry 2013, DOI: 10.1002/qua.24564
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