This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 21479–21486 21479

Cite this: Phys. Chem. Chem. Phys., 2011, 13, 21479–21486

Structure, stability and spectral signatures of monoprotic carborane

acid–water clusters (CBWn, where n = 1–6)w

Muthuramalingam Prakash and Venkatesan Subramanian*

Received 6th August 2011, Accepted 11th October 2011

DOI: 10.1039/c1cp22532d

The gas phase structure, stability, spectra, and proton transfer properties of monoprotic carborane

acid–water clusters [CB11FmH11�m(OH2)1]–(H2O)n (where m = 0, 5, and 10; n = 1–6) have been

calculated using density functional theory (DFT) with the Becke’s three-parameter hybrid exchange

functional and Lee–Yang–Parr correlation functional (B3LYP) using 6-31+G* basis set. Results

reveal that Eigen cation defects are found in CBWn (where n = 2–6) clusters and these clusters are

significantly more stable than the non-Eigen geometry. In addition to the conventional hydrogen

bond (H-bond) the role of dihydrogen bond (DHB) and halogen bond (XB) in the stabilization of

these clusters can be observed from the molecular graphs derived from the atoms in molecules

(AIM) analysis. Spectral information shows the features of Eigen cation and proton oscillation

involved in the proton transfer process. The dissociation of proton from the perfluoro derivatives

with two water molecules is more favorable when compared to the other derivatives.

1. Introduction

Weakly coordinating anions have achieved major commercial

importance in olefin polymerization.1 The usefulness of these

anions in lithium battery technology has also been illustrated.2

Triflate (CF3SO3�) and perfluorinated tetraphenylborate

(F20–BPh4�) anions are some of the important examples with

exceptional characteristics viz., low nucleophilicity, chemical inert-

ness, solubility, leaving group lability and weak coordination.3–7

The applications of these anions in chemistry have been well

documented.7–20

A significantly different class of weakly coordinating anions

was recognized based on the remarkably stable boron cluster

framework on monocarborane anions such as the icosahedral

(Ih) carborane anion ([CHB11X11]�). It was first synthesized by

Knoth at Du Pont in 1960s.21 The synthesis was improved by

Plesek et al.22 It was introduced in 1986 as a new candidate for

weakly coordinating anions.3 Icosahedral carborane anions

([CHB11X11]�, where X = H, OH, F, Cl, Br, I, CH3, CF3, CN

and mixture of these anions) are amongst the least coordinating

and most chemically inert anions known in the literature.3–7

Prior to the discovery of fullerene (C60) in 1985,23 the

[B12H12]2� dianion and its derivatives were the only molecules

with perfect Ih symmetry. Due to the thermal, chemical, and

geometrical advantages of these carborane derivatives, these can

be used as energy and storage materials.24,25 One of the impor-

tant properties of carborane derivatives is its superacidity7–15

which can be employed in the development of materials for fuel

cell technology.

Remarkable stability of the carborane anions is a consequence

of s aromaticity of the CB11 cage.5–8 The acidity of carborane

acids cannot be measured in the conventional manner of Hammett

acidity function because carborane acids are solids not liquids.

Theoretical studies have been devoted to quantify the acidity and

prediction of pKa of these derivatives.26,27 The gas phase Brønsted

acidity of a neutral acid HA is equal to the gas-phase basicity of its

conjugate base, A�.

HA - H+ + A� (1)

Balanarayan and Gadre have used the molecular electro-

static potential (MESP) to understand the Brønsted acidities

of carborane derivatives.26 It was found that MESP distribu-

tion on the zero-flux surface of the strongest isolable carborane

anion provides a good measure of its acidity. It is well known

that strong Brønsted acidity has been explained in terms of

chemical concepts such as resonance stabilization of the con-

jugate base and conjugative, hyperconjugative, and aromatic

effects. Since all these effects are manifested in the MESP, it acts

as an indicator of Brønsted acidity. Recently, the gas-phase

superacidities of carborane derivatives have been calculated using

DFT(B3LYP) and G3(MP2) methods.27 The predicted intrinsic

gas-phase acidities of these systems vary according to the sub-

stituents (X) in the following order of decreasing strength: CF3 >

F>Cl > Br > I> CN>CH3. They found that it depends on

Chemical Laboratory, Council of Scientific and Industrial Research -Central Leather Research Institute, Adyar, Chennai 600 020, India.E-mail: subbu@clri.res.in, subuchem@hotmail.com;Fax: +91 44 24911589; Tel: +91 44 24411630w Electronic supplementary information (ESI) available: Calculatedvibrational frequencies and optimized geometries of various[CB11FmH11�m(OH2)1]–(H2O)n (where m = 0, 5, and 10; n = 3–6)clusters and full author list in ref. 39. See DOI: 10.1039/c1cp22532d

PCCP Dynamic Article Links

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21480 Phys. Chem. Chem. Phys., 2011, 13, 21479–21486 This journal is c the Owner Societies 2011

the field-inductive and resonance effects of the substituents X

in [CHB11X11]�.27 As shown in a number of previous studies,

electron donor and acceptor groups influence chemical and

physical properties of the molecules.15,26,27 Hence, the intrinsic

gas-phase acidities of [CHB11X11]� have been investigated by

varying X (where X = OH, CF3, F Cl, Br, I, CN, and CH3)

using experimental techniques and computational methods.6–28

Reed and coworkers have made significant contributions

to the understanding of superacidity of the carborane

derivatives.3,7–12,14–16 It has been shown that the protonation

of C60 ([HC60]+) is possible with the aid of carborane deriva-

tives, whereas conventional strong acids (e.g. H2SO4) have

failed to protonate the same.8 They have also developed

qualitative measures of their acidities and basicities using

spectroscopic techniques.9,11,14 Stasko and co-workers have

synthesized mixed halo/hydroxyl derivatives of carborane.28

The X-ray diffraction structures of these derivatives show the

hydration of the carborane anion. It is possible to note the

presence of (CB)H+(H2O)n (where n = 1–6) motifs from

the X-ray diffraction structure. This framework is highly

useful to stabilize and isolate the hydronium ion.28

Although the superacidity of the CB is well established, the

proton transfer capacity of CB has not been investigated in detail.

Thus, a systematic study has been carried out on the structure,

stability, spectra and proton transfer ability of monoprotic

carborane acid–water clusters ([CHB11X10(OH2)1]–(H2O)n,

where X = H and F, n = 1–6) using electronic structure

methods.

2. Computational details

CB contains 12 vertices (1 : 5 : 5 : 1). It has two pentagonal

belts made out of 10 boron atoms. The apical positions are

occupied by CH and BH units. Usually the position which is

antipodal to carbon is substituted first, followed by the lower

pentagonal belt.

Different CB derivatives considered in this study are given

in Fig. 1a–d. They are: (i) hydroxy group substituted CB at

the antipodal position (Fig. 1a), (ii) the protonated form of

hydroxy CB (Fig. 1b), (iii) the lower pentagonal unit of

hydroxy CB substituted with F atoms (Fig. 1c) and (iv) belts

of hydroxy CB substituted with 10 F atoms (Fig. 1d). The

above-mentioned models (Fig. 1b–d) are used for the discrete

hydration studies. The general molecular formula of various

clusters considered is [CB11FmH11�m(OH2)1]–(H2O)n (where

m= 0, 5, and 10; n= 1–6). For brevity, CB11FmH11�m(OH2)1is denoted as CB and hence various clusters considered are

referred to as CBWnFm (where n = 1–6; m = 0, 5, and 10) in

the remaining part of the text.

Several methods with different levels of theoretical accuracy

can be used for the geometry optimization, prediction of BE

and calculation of vibrational spectral properties. For example,

Lipping et al.27 have used B3LYP/6-311+G**, B3LYP/

6-311++G** and G3(MP2) for the prediction of acidities of

CB derivatives. However, it is found from the earlier studies

on the protonated water clusters that B3LYP/6-31+G* yields

reliable estimates of geometrical parameters, energetics and

vibrational frequencies.29–36 Thus the same method has been

used in the present investigation. Vibrational frequencies were

scaled by a factor of 0.973.36 The geometries of all the clusters

were minima on their respective potential energy surfaces at the

B3LYP/6-31+G* level of theory. The BEs of all clusters were

calculated using the supermolecule approach and corrected for

basis set superposition error (BSSE) using the counterpoise

(CP) procedure suggested by Boys and Bernardi.37

BE ¼ � ECluster �Xni¼1

Ei

!ð2Þ

where Ecluster is the total energy of the cluster, Ei is energy of

the monomer and n is the total number of monomers in the

cluster. Specifically, BSSE was estimated for each monomer

by computing its energy corresponding to the geometry in

the cluster with n-mer basis set. The wave function was gene-

rated from B3LYP/6-31+G* calculation using the optimized

geometries. The AIM analysis was carried out using the

AIM2000 package.38 All calculations were performed using the

Gaussian 09 (revision A 0.2) suite of programs.39

3. Results and discussion

Since proton transfer reactions involve formation and cleavage

of covalent bonds, inclusion of too many water molecules in

the model systems leads to difficulties in the analyses of the

elementary reactions and dynamics. Hence, the number of

water molecules has been restricted to six as followed in the

previous studies.40,41 In fact, earlier experimental and theore-

tical studies have revealed that the first hydration shell of

H3O+ consists of four water molecules and only three of them

are strongly H-bonded to the hydrogen atoms of H3O+.

Hence, discrete solvation of carborane derivatives has been

investigated with six water molecules.

3.1 Geometries of CBWnFm clusters

Fig. 2 illustrates the optimized geometries of different CBWnFm

clusters as obtained from the B3LYP/6-31+G* level of calcula-

tion along with the important H-bonding distances. The calcu-

lated O–H distance in protonated monohydroxy derivatives

(([CB11FmH11�m(OH2)1]) (where m = 0, 5 and 10)) of CB is

0.977, 0.980, 0.981 A, respectively (Fig. 1). The same O–H

distance in CBW1F0, CBW1F5, and CBW1F10 clusters is 1.026,

1.071, and 1.082 A, respectively. It can be seen from Fig. 2 that

the O–H distance in the model systems increases upon inter-

action with water molecules which indicates the dissociation of

proton from the monoprotic carborane acid.

Two isomers are observed for the CBW2Fm (m=0, 5 and 10)

clusters which are designated as CBW2Fma, and CBW2Fmb.

Examination of these structures reveals that two different types

of hydration take place at the protonated hydroxy group. They

are: (i) water molecule interacts with the protonated O–H group

by forming the O–H� � �O hydrogen bond and (ii) both hydrogen

atoms of the protonated O–H group are involved in the formation

of H-bonds with the water molecules (Fig. 2).

Close inspection of the geometrical parameters unveils

several interesting information. The transfer of proton from

the carborane acid is accompanied by the formation of Eigen

(H3O+) cation in the CBW2F10a cluster. The O–H distance in

CBW2F0a which is H-bonded to the water molecule is 1.049 A.

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The same distance in CBW2F5a and CBW2F10a is 1.170 and

1.409 A, respectively. Considerable lengthening of the O–H

distance in CBW2F10a is evident from the above-mentioned

values owing to the formation of Eigen cation. It is interesting

to note from the geometrical parameters of the CBW2F10a

isomer that the excess proton hops from CB to the H-bonded

water molecule. The formation of Eigen cation is not observed

in the CBW2F0b, CBW2F5b, and CBW2F10b clusters due to the

inherent nature of the hydration pattern.

Fig. S1 (ESIw) shows the optimized geometries of various

CBW3Fm (where m = 0, 5, and 10) clusters. Each derivative

exhibits three different hydration patterns. These clusters are

referred to as CBW3Fma, CBW3Fmb, and CBW3Fmc. The geo-

metrical parameters elicit that the Eigen cation is involved in

the hydration of CBW3F0a, CBW3F5a, and CBW3F10a clusters.

Similarly, the presence of Eigen cation is observed in CBW3F5b

and CBW3F10b clusters. The CBW3F0c, CBW3F5c, and CBW3F10c

clusters are extension of branch type which is found in CBW2F0c,

CBW2F5c, and CBW2F10c clusters. Although these clusters do

not contain the Eigen cation, noticeable changes are observed

in the O–H bond distance. Specifically, O–H distance is influ-

enced by the number of H-bonded water molecules. It can be

noted from Fig. S1 (ESIw) that the O–H distance in CBW3Fmc

clusters undergoes significant elongation due to the H-bonding

interaction and associated proton transfer. Similar findings have

been found in the hydration patterns of protonated carbonic acid

water clusters.36

Different motifs of water clusters can be seen in the clusters

which are hydrated with Wn (n Z 4) due to the floppy nature

of H-bonding interaction. The optimized geometries of CBW4Fma,

CBW4Fmb, and CBW4Fmc (where m = 0, 5, and 10) isomers

are provided in Fig. S2 (ESIw). Examination of these geo-

metries elicits that most of the clusters have the Eigen cation

except CBW4F0b and CBW4F0c isomers.

The optimized geometries of different CBWnFm (where n= 5

and 6;m=0, 5, and 10) clusters are displayed in Fig. S3 (ESIw).All these clusters contain the Eigen cation. It is observed from

results that formation of the Eigen cation requires two/three

water molecules when m = 0 and 5. When m = 10, two water

molecules are sufficient to form the Eigen cation. It is observed

from the X-ray diffraction structures of the halo/hydroxyl

derivatives of CB with protonated water clusters that the O–O

distance of the H7O3+ moiety varies from 2.498–2.603 A. Results

illustrate that the calculated O–O distance at the B3LYP/

6-31+G* level for the same is in close agreement with the

above-mentioned range.28 Therefore, fluorination plays a signi-

ficant role in the transfer of protons from CB to water molecules.

This study demonstrates that a lower level of hydration is

sufficient to transfer protons from CB to water when compared

to the TA moiety in the Nafion model.42

Fig. 1 Optimized geometries of carborane and monoprotic carborane derivatives at the B3LYP/6-31+G* level. Distances are in A.

(a) [CHB11H10(OH)1]�; (b) [CHB11H10(OH)1]

�H+; (c) [CHB11F5H5(OH)1]�H+ and (d) [CHB11F10(OH)1]

�H+.

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3.2. AIM analysis

The AIM approach is a useful tool to quantify the various types

of noncovalent interactions in molecular clusters.43–49 The

calculated values of electron density (r(rc)) and its Laplacian

(r2r(rc)) values of H-bond, dihydrogen bond (DHB) and

halogen bonds (XB) are depicted in Fig. 3. These values are

similar to the standard values stipulated for weak interactions.44

3.2.1 Dihydrogen bond (DHB). In addition to H-bonding,

the presence of B–H�� � �H–OW and B–F�� � �O–H interactions is

observed in the formation of cyclic structure in CBWnFm clusters.

In fact these types of interactions have been found in biological and

chemical sciences.45,46,50,51 The B–H�� � �H–O type of interaction is

known as a dihydrogen bond (DHB) because the link between the

two molecules within the complex is realized through the H� � �Hcontact. It has been pointed out that one of the H atoms acting as

the proton acceptor differs from typical acceptors such as oxygen

and nitrogen atoms where the lone electron pairs are responsible

for the existence of H-bonding. The presence of DHB in different

X-ray structures has been reported.50 The distance d(H� � �H) for

such systems ranges from 1.7–2.2 A which is significantly less than

the sum of the vdW radii for two hydrogen atoms (i.e. 2.4 A). The

same range in CBWnFm clusters is 1.553–2.060 A.

3.2.2 Halogen bond (XB). The B–F�� � �O–H interaction

found in these clusters can be considered as halogen bond-

ing (XB). This interaction can be schematically described as

Y–X� � �D, where X represents the electron-deficient halogen

atom (Lewis acid/XB donor), D is a donor of electron density

(Lewis base/XB acceptor), and Y is any suitable atom such as

carbon, nitrogen, halogen, etc.52,53 In addition to crystal

engineering, this novel interaction has lately been applied in

other fields of material science, such as supramolecular separa-

tions, liquid crystals, organic semiconductors, and paramagnetic

materials technologies.54 Recently, the role of halogen bonding in

biological systems and its potential in drug development have

also been recognized.55,56 Although F is not frequently observed

in the halogen bonding, it is interesting to note the prevalence

of B–F�� � �O–H interaction in the CBW clusters. The presence

of this interaction is clearly seen with the help of the molecular

graphs presented in Fig. 3.

3.3 Energetics of CBWnFm clusters (where m=0, 5, and 10;

n= 1–6)

The calculated BSSE corrected BEs of various CBWnFm clusters

at the B3LYP/6-31+G* level are listed in Table 1. The binding

energies (BEs) of CBW1F0, CBW1F5, and CBW1F10 clusters are

Fig. 2 Optimized geometries of CBWnFm (where n=1 and 2;m=0, 5, and 10) clusters at the B3LYP/6-31+G* level. The blue color indicates the

Eigen (H3O+) cation. Distances are in A.

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18.58, 27.23, 28.67 kcal mol�1, respectively. It can be seen from

Fig. 2 that CBW2Fm clusters form cyclic structures. The BEs of

CBW2F0a, CBW2F5a, and CBW2F10a clusters are 32.34, 52.36

and 140.44 kcal mol�1, respectively.

The difference in the BE (DBE) of penta- and non-fluorinated

clusters is 9.98 kcal mol�1. The DBE of deca- and penta-

fluorinated clusters is 88.0 kcal mol�1. The same for the deca-

and non-fluorinated clusters is 108.0 kcal mol�1. It can be noted

from the DBE values that maximum (m = 10) fluorinated

CBs have higher BEs when compared to the other clusters.

Thus, fluorination influences the stability of the H-bonded

clusters. Further, complete fluorination of CB facilitates the

formation of Eigen cation in the CBW2F10a isomer.

Among the CBW3Fma (where m = 0, 5 and 10) clusters,

stability of CBW3F0a is higher than that of other isomers in

this category due to the formation of ion pairs. The BEs of

CBW3Fmb, where m = 0, 5 and 10, clusters are 44.02, 163.16,

and 152.64 kcal mol�1, respectively. The predicted BEs of

CBW3Fmc (where m = 0, 5 and 10) clusters are found to be

44.75, 60.75, and 63.33 kcal mol�1, respectively. The BEs of

these clusters are lower than the other clusters in this category

owing to the absence of Eigen cation.

A similar trend has also been observed in the CBW4Fm

clusters. The BEs of clusters with five water molecules CBW5F0a

and CBW5F5a are almost similar. On perfluorination, the BE

decreases by B10.0 kcal mol�1 as evident from the BE of

CBW5F10a due to the water repelling nature of the fluorin-

ated systems. The BEs of CBW6F0, CBW6F5, and CBW6F10

clusters are 196.04, 196.64, and 188.41 kcal mol�1 respectively.

It is interesting to observe from the variations in the BEs that

the stability of clusters with n = 1–2 water molecules pre-

dominately depends on fluorination. Overall, it is clear that

the stability of the clusters depends on the following factors:

(i) fluorination, (ii) the location of proton and (iii) structural

arrangement of water molecules in the first coordination

sphere.

Fig. 3 Molecular graphs of carborane acid–water clusters along with the r(rc) and r2r(rc) values in black and blue colors, respectively. Arrows

show the presence of different noncovalent interactions in the clusters. Values are in a.u.

Table 1 Calculated BEs (in kcal mol�1) of various derivatives ofCBWnFm (where n = 1–6) clusters at the B3LYP/6-31+G* level

CBWnFm (where n = 1–6; m = 0, 5, and 10)

BEs

m = 0 m = 5 m = 10

n = 1 CBW1Fm 18.58 27.23 28.67n = 2 CBW2Fma 32.34 52.36 140.44n = 2 CBW2Fmb 33.17 45.26 46.48n = 3 CBW3Fma 167.14 165.18 154.28n = 3 CBW3Fmb 44.02 163.16 152.64n = 3 CBW3Fmc 44.75 60.75 63.53n = 4 CBW4Fma 177.02 176.99 166.65n = 4 CBW4Fmb 53.60 171.56 173.30n = 4 CBW4Fmc 50.61 176.74 167.43n = 5 CBW5Fma 186.17 186.79 177.10n = 5 CBW5Fmb 183.96 185.65 176.35n = 6 CBW6Fm 196.04 196.64 188.41

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3.4 Vibrational analysis

Recent advancements in spectroscopic techniques and theore-

tical calculations have successfully unveiled structures of

various types of water clusters.36,57–64 It is evident from these

studies that the H-bonding of each water molecule in a given

cluster can be characterized as A (single acceptor), D (single

donor), AA (double acceptor), DD (double donor), AD (single

acceptor–single donor), AAD (double acceptor–single donor),

ADD (single acceptor–double donor) or AADD (double

acceptor–double donor).

The experimentally measured asymmetric (as) and symmetric

stretching (ss) frequencies for the gas-phase water molecule are

observed at 3756 and 3657 cm�1, respectively.57 It is found from

earlier experimental studies that the asymmetric (Eas) and

symmetric (Ess) stretching frequencies of bare H3O+ are 3530

and 3390 cm�1, respectively.58 The corresponding values calcu-

lated from the present investigation using the B3LYP/6-31+G*

method are 3508 and 3414 cm�1 which are in good agreement

with the above-mentioned experimental values. In protonated

water clusters, the Eas and Ess modes occur at 2665 and 2420 cm�1

due to H-bonding.61 The partially hydrated stretching

frequency of the Eigen cation is observed at B1900 cm�1.61

Asmis et al. have reported that spectral features of Zundel

(H2O� � �H+� � �H2O (H5O2+)) cations in protonated water

clusters appear in the range of 600 to 1900 cm�1.62 They

found the O–H+–O stretching mode in protonated water

clusters to be at 1317 cm�1. Recently, very weak O–H+–O

bending frequencies (B1510 and B1370 cm�1) have been

reported.63 The same study provides evidence for existence of

(O–H+–O)as stretching frequency which appears at B1000 cm�1.

Similar Eigen and proton oscillation modes are also observed

in various hydration patterns of protonated carbonic acid

water clusters.36

The scaled vibrational frequencies of all the hydrated clus-

ters calculated at the B3LYP/6-31+G* level of theory using

harmonic approximation are listed in Table 2 and Tables S1

and S2 (ESIw). Both experimental and theoretical vibrational

frequencies of protonated water clusters from previous studies58–63

have been used as reference to analyze CBWnFm clusters. The

vibrational frequencies above 3000 cm�1 correspond to the

symmetric and asymmetric stretching modes of individual

water molecules and O–H stretching in CB, whereas those

between 1000 and 2000 cm�1 are associated with the charac-

teristic vibrational frequencies of the transferring protons.

The correlation between the O–H stretching frequency and

the proton transfer in CBWnFm clusters can be seen from the

results.

Only the important shifts occurring in the O–H stretching

modes of protonated hydroxy carborane (O–HCB), Eigen

cation (Eas, Ess) and surrounding water molecules (A, AD,

AAD, and ADD types) have been considered for the analysis

(Table 2). The asymmetric stretching frequencies of O–HCB in

CBW0F0, CBW0F5, and CBW0F10 clusters are 3635, 3584,

and 3574 cm�1, respectively. The same in monohydrated

clusters (CBW1F0, CBW1F5, and CBW1F10) are 2701, 2060,

and 1933 cm�1. The comparison of values reveals that O–HCB

undergoes a substantial red shift upon H-bonding with water

molecules. In addition, fluorination favors the formation of

strong H-bonding interaction with the water as evident from

these red shift values. Similar observations have also been

observed for other clusters.

It can be seen from Table S1 (ESIw) that most of the stretching

frequencies of CBWnFm (n = 3 and 4) clusters range from

1100–3000 cm�1 with higher intensity. From n = 3 onwards,

most of the frequencies correspond to the oscillation of H+ and

Eigen stretching modes (Eas and Ess). The calculated Eas and Ess

modes of CBWnFm clusters are in good agreement with the

Table 2 Calculated O–H stretching frequencies of CBWnFm (where n = 1 and 2) clusters at the B3LYP/6-31+G* level along with red, blue shiftsand experimental values (cm�1)

CBWnFm (where n = 1 and 2; m = 0, 5, and 10) Description of O–H stretching ncalc and red or blue shiftd nexpta

Water O–Has and O–Hss 3757, 3635 3756, 3657b

H3O+ Eas and Ess 3508, 3414 3530, 3390c

CBW0F0 O–Has and O–Hss 3635, 3546CBW0F5 O–Has and O–Hss 3584, 3506CBW0F10 O–Has and O–Hss 3574, 3498CBW1F0 O–HCB stretch to AD type water 2701 (934)

Free O–HCB stretch 3628 (7)O–H stretching of A water 3697 (60)

CBW1F5 O–HCB stretch to AD type water 2060 (1524)Free O–HCB stretch 3607 (�23)

CBW1F10 O–HCB stretch to AD type water 1933Free O–HCB stretch 3603 (�29)

CBW2F0a O–HCB stretch to AD type water 2324 (1311)O–HW (AD) stretch to AD water 3156 (601)

CBW2F5a H+ oscillation between CB and AD water 1181O–HW (AD) stretch to AD water 2724 (1033)

CBW2F10a H+ oscillation between CB and AD water 1884E stretching to AD water 2193 1880

CBW2F0b O–HCB stretch to A type water 2923 (712)CBW2F5b O–HCB stretch to A type water 2561 (1023)CBW2F10b O–HCB stretch to A type water 2491 (1083)

a Experimental O–H stretching frequencies of protonated water clusters taken from ref. 58–62. b Taken from ref. 57. c Taken from ref. 58.d Positive and negative values in parentheses indicate the red and blue shifts. CB—hydroxycarborane: O–Has and O–Hss asymmetric and

symmetric stretching frequencies; Eas—Eigen core asymmetric stretch; Ess—Eigen core symmetric stretch; E—Eigen cation.

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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 21479–21486 21485

available experimental values for protonated water clusters.61

Similar to water clusters, AD and ADD types of water molecules

are found in these clusters. The O–H stretching modes ofAD and

ADD types of water molecules in CBWnFm (n=3 and 4) clusters

vary from 2735–3317 cm�1. The red shift values of these modes

range from 600–1000 cm�1. The important stretching frequencies

of CBWnFm (n = 5 and 6) clusters are listed in Table S2 (ESIw)which show the spectral signatures corresponding to the Eigen

cation in the CBW5Fm and CBW6Fm clusters. The stretching

frequency of O–H groups of water molecules exhibits larger red

shift with higher intensities due to their involvement in the

H-bonding interaction.

4. Conclusions

Structural, energetic and spectral information obtained from the

calculations illustrates that CB derivatives can transfer protons to

water molecules in the immediate neighborhood. In addition, the

following important observations emerge from the electronic

structure calculations on the [CHB11X10(OH2)1]–(H2O)n (where

X = H and F, n = 1–6) clusters using the B3LYP/6-31+G*

method. Results illustrate that the stability of CBWn clusters is

influenced by fluorination. It can be observed that the stability of

perfluorinated clusters with less water molecules (n = 1–2) is

higher than that of pentafluoro and carborane derivatives. On

the other hand, for the clusters with n=3–6 water molecules, the

stability of non-fluorinated derivatives is higher than that of the

corresponding fluorinated counterparts. The role of conventional

H-bond, DHB and XBs in the stabilization of these clusters is

evident from the geometrical parameters. The information

derived from the AIM analysis supports these observations.

The existence of Eigen cation in the hydrated clusters is

apparent from the geometrical and spectral details. The higher

stability of various clusters can be attributed to the presence of

ion-pairs. The calculated vibrational frequency corresponding

to the proton oscillation in these clusters is comparable to

that of protonated water clusters. These results confirm the

presence of Eigen and Zundel cations in the proton transfer

process. It is observed that the calculated vibrational frequencies

of Eigen cation and water molecules at the B3LYP/6-31+G*

level are in close agreement with the previous experimental and

theoretical reports. Overall, these entire findings highlight that

monoprotic carborane acid derivatives can be useful in proton

exchange membrane fuel cell applications.

Acknowledgements

This study has been supported by grants from the DST India-

European Union sponsored project (HYPOMAP) and

High performance computational facility provided by DST

and Council of Scientific and Industrial Research (CSIR),

New Delhi, India.

Notes and references

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