Journal of Fluorine Chemistry 146 (2013) 59–65

Quantum chemical investigations on superhalogen properties of MnFn (n = 1.6)nano-complexes and the consequential possibility of formation of new MnFn–Nasalt species

Tabish Rasheed a, Shamoon Ahmad Siddiqui b,c,*, Nadir Bouarissa b,c

a Department of Applied Sciences, School of Engineering and Technology, Sharda University, Plot No. 32–34, Knowledge Park III, Greater Noida 201 306, NCR, UP, Indiab Advanced Materials and Nano Research Centre, Najran University P.O. Box 1988, Najran 11001, Saudi Arabiac Department of Physics, College of Arts and Science, Najran University P.O. Box 1988, Najran 11001, Saudi Arabia

A R T I C L E I N F O

Article history:

Received 5 October 2012

Received in revised form 7 January 2013

Accepted 8 January 2013

Available online 16 January 2013

Keywords:

DFT

Superhalogen

Supersalt

Electron affinity

Dissociation energy

A B S T R A C T

In the present investigation, we report equilibrium geometric structures of MnFn (n = 1.6) nano-

complexes obtained by an all-electron linear combination of atomic orbitals scheme within the density-

functional theory utilizing the popular B3LYP (Becke, three-parameter, Lee–Yang–Parr) exchange-

correlation functional. The vibrational stability of all the nano-complexes was examined on the basis of

vibrational frequencies. The stability of these complexes was further established by examining their

HOMO-LUMO gaps and applying the principle of maximum hardness. The superhalogen properties of

MnFn nano-complexes have been examined by calculating their electron affinities (EAs). A maximum of

six fluorine (F) atoms were allowed to bind to a single manganese (Mn) atom and the resulting increase

in EAs with increase in the number of F atoms was studied. The binding energies, HOMO and LUMO of the

MnF4–Na complex was calculated in order to perform a case study to determine the stability of the newly

predicted MnFn–Na salt species.

� 2013 Elsevier B.V. All rights reserved.

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Journal of Fluorine Chemistry

jo ur n al h o mep ag e: www .e lsev ier . c om / loc ate / f luo r

1. Introduction

Negative ions are known to play a very crucial role inchemistry, since they act as the building blocks of salts andoxidizing agents. In this context, halogen atoms are particularlyimportant, since they have the ability to attract electrons andreadily form negative ions. These atoms are known to possessthe highest electron affinities (EAs) amongst all elements ofperiodic table, lying in the range 3.0–3.6 eV [1]. The central roleplayed by negative ions has been a source of constantencouragement to the scientific community toward the designand synthesis of new negative ions, which are termed assuperhalogens. These are composed of molecules whose EAsexceed the 3.6 eV limit due to collective effects [2,3]. Suchcompounds may be used for oxidation of counterpart systemswith relatively high ionization potentials (such as O2 and Xe)and for the synthesis of unusual chemical substances (e.g.,compounds involving nobel gases atoms). Furthermore, sincethese molecules possess high EAs, they are widely used in the

* Corresponding author at: Advanced Materials and Nano Research Centre,

Najran University, Najran, Saudi Arabia. Tel.: +966 592119923.

E-mail address: shamoonasiddiqui@gmail.com (S.A. Siddiqui).

0022-1139/$ – see front matter � 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jfluchem.2013.01.010

production of organic superconductors [4]. A number of workershave estimated the EAs of many superhalogens both experi-mentally [5–10] as well as theoretically [11–19]. Since, thesemolecular systems exhibit unique characteristics, hence theyhave been studied extensively in the past, particularly by Gutsevand Boldyrev [2,3]. They proposed a simple formula for thedescription of one class of these compounds, MXk+1, where M is amain group or transition metal atom, X is a halogen atom, and k

is the maximal formal valence of the atom M [2].In the present investigation, another important class of

compounds in the superhalogen series has been studied in detail.The manganese (Mn) atom has an Ar [3d54s2] configuration andhence, a maximal valence of six and is expected to form a series ofsuperhalogen compounds with fluorine (F), having the generalformula MnFn (n = 1.6). Using density functional theory (DFT), wehave shown that, Mn can indeed have oxidation states rangingfrom +1 to +6. In the past, superhalogen properties of anothercompound MnO4 have been studied extensively through the use ofboth experiment and theoretical techniques [20]. In a recent study,Pradhan et al. [21] have studied interactions of Mn atom withhalogen atoms and also elucidated the stability of its half-filled 3d-shell, but our work considers the structure of MnFn in more detail.In the present investigation, we have verified that, MnO4 indeedhas superhalogen properties. EA of this compound has been found

Fig. 1. Optimized geometries of MnFn neutral and anionic clusters.

T. Rasheed et al. / Journal of Fluorine Chemistry 146 (2013) 59–6560

to be �5 eV. Theoretical possibility of formation of new MnFn–Nasalt species has been explored, taking MnF4–Na complex as a testcase to investigate its stability and other properties in detail. Webelieve that the electron binding energies we provide in this workwould be very useful for experimental chemists who are involvedin the design and synthesis new materials in which strong electronacceptors are involved.

2. Results and discussions

The optimized geometrical structures of neutral and anionicMnFn (n = 1.6) complexes are shown in Fig. 1. The geometriescorresponding to the neutral and anionic forms of MnFn complexesare almost similar, except for the case of MnF2, where adistinguishable difference may be observed. Geometry of the

1 2 3 4 5 6

1.70

1.71

1.72

1.73

1.74

1.75

1.76

1.77

1.78

1.79

1.80

1.81

1.82

1.83

1.84

1.85

1.86A

ve

rag

e b

on

d le

ng

th o

f

MnF

n c

luste

rs (

Angstr

om

)

Number of F atoms

Neutral

Anion

Fig. 2. Average bond length between Mn and F in neutral and anionic MnFn clusters.

T. Rasheed et al. / Journal of Fluorine Chemistry 146 (2013) 59–65 61

anionic form appears to be linear, whereas, the neutral formexhibits a slightly bent structure and its shape corresponds to atriangle (Fig. 1). Variation of the average bond lengths (ABLs) ofMn–F bonds in MnFn complexes is presented in Fig. 2. It may beobserved from this figure (Fig. 2), that, ABLs of the neutral forms ofMnFn complexes is smaller than that of anionic forms, which is inaccordance with previous investigations [22,23].

In Table 1, the relative energies for two lowest spinmultiplicities of MnFn neutral and anionic complexes are given.With the exception of neutral MnF2, MnF4 and MnF6, all thecomplexes, both in neutral and anionic form have the lowestpossible spin state. The relative stabilities of these complexesagainst fragmentation to F atoms and F2 molecules are studied bycalculating the energy DEn needed to dissociate these complexesinto MnFn�1 + F, and MnFn�2 + F2, which are given as:

DEn ¼ � E MnFn½ � � E MnFn�m½ � � E Fm½ �f g; n ¼ 1; ::; 6; m

¼ 1; 2 (1)

DE�n ¼ �fE½MnF�n � � E½MnF�n�m� � E½Fm�g; n ¼ 1; ::; 6; m

¼ 1; 2 (2)

The DEn and DE�n values are plotted in Fig. 3(a) and (b)respectively, for different channels of MnFn complexes. We canconclude from these figures that energy costs decrease assuccessive F atoms and F2 molecules are detached. Vibrationalfrequencies of all MnFn complexes were calculated and were

Table 1Energy difference (in eV) between different multiplicities (M = 2S + 1) for anion and

neutral MnFn cluster.

No. of F atoms Neutral Anion

M DE M DE

1 1 0.00 2 0.00

3 2.09 4 1.79

2 2 0.00 1 0.00

4 0.86 3 1.95

3 1 0.00 2 0.00

3 1.38 4 2.22

4 2 0.00 1 0.00

4 0.84 3 0.53

5 1 0.00 2 0.00

3 1.57 4 1.86

6 2 0.00 1 0.00

4 0.28 3 1.74

found to be positive irrespective of their charge state. Thisimplies that, all these complexes are at local minima and areprotected by energy barriers. More specifically, it may be statedthat, all these complexes are stable against dissociation toMnFn�1 + F and MnFn�2 + F2. Thus, neutral and anionic complexesup to n = 6 in the MnFn series can be formed if atomic F ormolecular F2 are used in the experimental synthesis. Fig. 3(a) and(b) also show that anionic forms of these complexes are alsostable against same dissociation channels. The stability of thesecomplexes can be further established by examining their HOMO–LUMO gaps. Pearson [24], had given the principle of maximumhardness, which states that, in terms of molecular orbital theory,‘‘the highest value of h reflects the highest possible energy gap

between the lowest unoccupied and highest occupied molecular

orbitals; this value correlates with the stability’’ [25]. Here, his defined as the absolute hardness and is given as twice theenergy gap between the HOMO and LUMO [26]. Within thevalidity of Koopmans’ theorem [25], the frontier orbital energiesare given as

�eHOMO ¼ IP and; � eLUMO ¼ EA; (3)

where, eHOMO and eLUMO are the orbital energies of HOMO andLUMO respectively. IP and EA are respectively, the ionizationpotential and electron affinity of the chemical system underconsideration. Then, using Eq. (3), the absolute hardness h can bewritten as

h ¼ IP � EA

2¼ eLUMO � eHOMO

2¼ � eLUMO � eHOMO

2(4)

Now, from Eq. (4), we find that, greater the HOMO–LUMOgap, greater is the value of h, which refers to a more stablesystem. In quantum theory, optical polarizability results from amixing of suitable excited state wave functions with the groundstate wave function. The mixing coefficient is inverselyproportional to the excitation energy from the ground to theexcited state. A small HOMO–LUMO gap implies small excitationenergies to the manifold of excited states [27]. Hence, wecan state that, hard molecules, with a large HOMO–LUMO gap will

be less polarizable as compared to soft molecules, with a small

HOMO–LUMO gap. In Fig. 4, the HOMO–LUMO gaps are plottedas a function of the number of F atoms. These figures clearlyindicate that the HOMO–LUMO gaps of MnFn complexes rangesbetween 1.91 and 4.70 eV, in both neutral and anioniccomplexes. According to the maximum hardness principle,MnF4 is expected to be most stable in both the neutral andanionic forms.

Another important point which, we tried to understand wasthat, how a Mn atom which has a nominal valence of 2 was ableto bind with six halogen atoms? In order to understand this, wehave analyzed the HOMOs and LUMOs and investigated thepossibility of involvement of inner shell 3d-electrons in bonding.It was studied by using natural bond analysis. The d-character-istics of the HOMO and LUMO are clearly seen confirming theirinvolvement in bonding (Fig. 5). In ligand field theory, thevalence band edge orbitals (HOMOs of closed shell complexes)are ligand orbitals (bonding with Mn), HOMOs are non bondingand LUMOs are Mn d orbitals (antibonding). It may be recalled atthis point that, the electronic configuration of Mn atom is givenas [Ar] 4s2 3d5. In the MnF2 molecule, the 4s2 electrons of Mninteract with the 2p electrons of F, leaving the 3d5 half-filledshell untouched. As more F atoms are attached to the Mn atom,its 3d electrons begin to participate in the bonding and all the 3dand 4s valence electrons of Mn hybridize and mix with ligandorbitals (F 2p). Gutsev et al. [20] had observed similar orbitalmixing in the case of MnO4 explaining the origin of its unusualstability. In Fig. 6, the number of 3d electrons participating in

1 2 3 4 5 6

0

1

2

3

4

5

6

7

8

Fra

gm

en

tatio

n e

ne

rgy (

eV

)

Number of F atoms

Neutral

Anion

(a)

2 3 4 5 6

0

2

4

6

8

10

12

Fra

gm

en

tatio

n e

ne

rgy (

eV

)

Number of F atoms

Neutral

Anion

(b)

Fig. 3. (a) Fragmentation energies of neutral and anionic MnFn clusters for fragmentation channel MnFn = MnFn�1 + F (b) fragmentation energies of neutral and anionic MnFn

clusters for fragmentation channel MnFn = MnFn�2 + F2.

1 2 3 4 5 6

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

HO

MO

-LU

MO

gap (

eV

)

Number of F atoms

Neutral

Anion

Fig. 4. HOMO–LUMO gap of neutral and anionic MnFn clusters.

T. Rasheed et al. / Journal of Fluorine Chemistry 146 (2013) 59–6562

bonding in MnFn complexes has been plotted as a function of n.We can see that the number of 3d electrons participating inbonding increases with n in the neutral as well as anionic formsof MnFn complexes. The involvement of 3d electrons is in terms

Fig. 5. (a) HOMO picture of MnF6 cluste

of hybridization between Mn 3d and F 2p states rather than acomplete charge transfer from the Mn to F atoms.

In Fig. 7, the adiabatic electron affinities (EAs) of the MnFn

complexes have been plotted as a function of n. These arecalculated by taking the energy difference between the neutral andcorresponding anionic complexes, taking both of them in theirground state configuration. The EA values increase steadily withincrease in the number of fluorine atoms reaching a peak value of6.65 eV for the MnF5 complex. This value is approximately twotimes larger than the EA of F atom, which is considered to behighest among all elements of periodic table. Hence, we mayconclude that MnFn (n � 3) complexes are superhalogens.

According to superhalogen theory, Mn with a nominal valenceof 2 permits the formation of MnF6. The fact that MnFn (n � 3)complexes are superhalogens is further evidence that inner shellelectrons contribute to its valence, as has been discussed above.Thus, it may be summarized that, MnFn complexes have similarproperties as AuFn complexes.

Certain other important issues also need a discussion. We needanswers to the following questions to further understand thesuperhalogen behavior of MnFn. Up to what extent a superhalogencomplex behaves like a halogen atom? We know that halogenatoms such as F and Cl combine together to form F2 and Cl2

molecules. Does a superhalogen complex form a dimer? Secondly,It is a well known fact that, alkali atoms readily react with halogenatoms to form stable compounds (generally salts). Does a

r (b) LUMO picture of MnF6 cluster.

1 2 3 4 5 6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Nu

mb

er

of 3

d e

lectr

on

s in

bo

nd

ing

Number of F atoms

Neutral

Anion

Fig. 6. Number of 3d electrons participating in bonding in neutral and anionic MnFn

clusters.

1 2 3 4 5 6

0

1

2

3

4

5

6

7

Ele

ctr

on a

ffin

ity (

eV

)

Number of F atoms

Fig. 7. Electron affinities of MnFn clusters as a function of n.

T. Rasheed et al. / Journal of Fluorine Chemistry 146 (2013) 59–65 63

superhalogen bind to an alkali atom as strongly as halogen atom?MnFn, which are superhalogens, must also react with Na atom toform stable compounds. In order to verify this, we have consideredthe test case of interaction between Na atom and MnF4 super-halogen.

Let us first of all consider the interaction between two MnF4

superhalogens. Based upon the charge distribution in MnF4, threeinitial configurations may be chosen in order to study theformation of MnF4 dimer. In the first configuration, the two unitsare placed parallel to each other but shifted so that the Mn site inone unit can be closer to two F atoms in the other. In the second andthird configurations, the two complexes were placed perpendicu-lar to each other with Mn and F atoms close to each other in orderto promote interaction. However, after full geometry optimization,it was found that these two units bind only in the firstconfiguration to form a stable dimer, suggesting that the super-halogen complex MnF4 can form a dimer. Stability of these dimerswas checked through frequency calculation, which clearly shows apositive result, since all the calculated frequencies are real. Thesedimers along with their HOMO–LUMO picture are shown in Fig. 8.The binding energy of MnF4 dimer was found to be 1.80 eV, whichis quite high in comparison to the binding energy of F2 molecule,which is 1.21 eV. The HOMO–LUMO gap for MnF4 dimer is found tobe 0.99 eV, which is quite small as compared to the HOMO–LUMOgap in F2 molecule, which is 6.19 eV. These results clearly suggestthat MnF4 dimers are chemically more reactive as compared to F2

molecules.

Fig. 8. (a) Optimized structure of MnF4 dimer, (b) HOMO pict

The study of interactions between MnFn superhalogens and anatom of alkali metals such as, sodium (Na) is the other importantpart of this study. Initially, the Na atom was placed on top of the Mnatom in MnF4 molecule. After full geometry optimization, weobtained a structure where the Na atom binds with two F ionsforming a planar structure (Fig. 9(a)). The binding energy of Na–MnF4 is found to be 3.95 eV. This is a bit small value as compared tothe value of binding energy between a Na atom and a F atom, whichis 4.31 eV. The binding of Na atom with MnF4 complex increasesthe HOMO–LUMO gap by 0.18 eV, which increases the stabilityfurther. The kinetic stability of the MnF4–Na complex is furtherconfirmed through frequency calculations. All the frequencies arereal, which implies that this structure is stable. Fig. 9(b) and (c)show the HOMO and LUMO pictures of MnF4–Na, where it may beobserved that, the main contributions to HOMO are from MnF4

molecule, but in LUMO Na atom also contribute. This is inagreement to the NaF molecule, in which, the Na site does notcontribute to HOMO but contributes to LUMO.

In the earlier parts of this study, we had observed how twomolecules of MnF4 combine to form dimmers. It was also verifiedthat, binding of Na atom with MnF4 results in increase of theHOMO–LUMO gap by a considerable amount, in turn increasingthe overall stability. We further investigated whether the MnF4–Nacomplex is able to form dimer with itself like the MnF4 moleculesare able to do. On the basis of charge distributions, the most stablegeometry is given in Fig. 10(a). In the MnF4–Na dimer, bindingtakes place between the Na and F sites with a distance of 2.13 A,

ure of MnF4 dimer and (c) LUMO picture of MnF4 dimer.

Fig. 10. (a) Optimized structure of MnF4–Na dimer, (b) HOMO picture of MnF4–Na dimer, (c) LUMO picture of MnF4–Na dimer, (d) optimized structure of NaF dimer, (e) HOMO

picture of NaF dimer and (f) LUMO picture of NaF dimer.

Fig. 9. (a) Optimized structure of MnF4–Na complex, (b) HOMO picture of MnF4–Na complex and (c) LUMO picture of MnF4–Na complex.

T. Rasheed et al. / Journal of Fluorine Chemistry 146 (2013) 59–6564

which is larger than the distance between two NaF dimers (2.07 A)(Fig. 10 (d)). The binding energy of MnF4–Na dimer is found to be1.91 eV, which is 0.96 eV lower than the binding energy of NaF–NaF dimer. It is evident from the optimized geometry, that, in theMnF4–Na dimers, the distortion occurs at the Na sites, while theplanar structures of MnF4 are kept intact. Thus, it is conclusivelyproved that, MnF4 serves as the building block of new salts.Moreover, the main contributions to HOMO (Fig. 10(b)) and LUMO(Fig. 10(c)) in MnF4–Na dimer is from MnF4, which is differentfrom NaF–NaF (Fig. 10(e) and (f)) dimer. These results illustrate thesimilarities and differences between a superhalogen complex and ahalogen atom.

3. Conclusions

In the present investigation, we have shown that a Mn atom canbind with upto six F atoms if atomic F is used in their synthesis.MnFn complexes in both neutral and anionic forms are stableagainst all dissociation channels, ensuring that Mn can exist inhexavalent state. The EAs of these MnFn complexes are higher thanthe most electronegative atom F in the periodic table and can reacha value as high as 6.65 eV for MnF5 complex. The interaction of

superhalogens with alkali atoms was modeled by considering thetest case of MnF4 and Na. The binding energy of Na–MnF4 is a bitsmaller than that of NaF suggesting that a new class of salts can besynthesized by reacting MnFn superhalogens with appropriatemetal cations. The resulting salts with high oxidizing propertiescan have potential applications in combating biological agents.Properties of materials derived from these superhalogens are alsocapable of giving a new dimension to materials science in years tocome.

4. Computational details

Theoretical calculations were carried out using the self-consistent field-linear combination of atomic orbital-molecularorbital (LCAO-MO) approach. The total energies were calculatedusing density functional theory (DFT) [28,29] and the B3LYPfunctional [30–32]. The atomic orbitals are represented by aGaussian basis and we have used the SDD basis set in ourcalculations. Various geometries were optimized without anysymmetry constraint using the Gaussian 03W code [33]. Normalmode frequencies were calculated for all geometries to ensure thatthey belong to minima in the potential energy surface. Several

T. Rasheed et al. / Journal of Fluorine Chemistry 146 (2013) 59–65 65

initial molecular structures were used to confirm the ground statestructure. These calculations were also repeated for higher spinstates to determine the preferred spin multiplicities of the neutraland anionic complexes. The convergence for energy and force wasset to 0.00001 eV and 0.001 eV/A. This numerical procedureyielded the EA value for the F atom as 3.48 eV respectively andan ionization potential for the Mn atom as 7.30 eV, which agreevery well with the corresponding experimental values of 3.40 eV[34] and 7.43 eV [35] respectively. The calculated bond length andbinding energy of F2 comes out to be 1.461 A and 1.37 eV, whichagrees well with the corresponding experimental values [36,37].

Acknowledgements

One of the authors Dr. Shamoon Ahmad Siddiqui is thankful tothe Deanship of Scientific Research, Najran University, Najran,Kingdom of Saudi Arabia for all financial support. AdvancedMaterials and Nano Research Centre (AMNRC), Najran University,Najran is highly acknowledged.

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