131–132 (2007) 239–245www.elsevier.com/locate/molliq

Journal of Molecular Liquids

On the structure of liquid antimony pentafluoride

S.E. McLain a, A.K. Soper a, J.J. Molaison b, C.J. Benmore c,M.R. Dolgos b, J.L. Yarger d, J.F.C. Turner e,⁎

a ISIS Facility, Rutherford Appleton Laboratories, Chilton, Didcot, OXON OX11 QX, UKb Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA

c Intense Pulsed Neutron Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USAd Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA

e Neutron Sciences Consortium and Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA

Available online 26 September 2006

Abstract

The liquid structure of antimony pentafluoride at room temperature has been investigated using neutron and high-energy X-ray diffraction andsubsequently modelled using Empirical Potential structure refinement. The neutron diffraction measurements show that each antimony centre issurrounded by 6 fluorine atoms; four at a non-bridging distance of 1.86±0.03 Å and two bridging fluorines at a distance of 2.03±0.06 Å. The X-ray data show an additional peak at 3.93±0.03 Å attributed to antimony–antimony contacts. The diffraction data were fit to three models; cis-monomer, isolated tetramer and cis-linked chains. The X-ray data rule out the cis-monomer model but good fits are obtained for the isolatedtetramer and cis-linked chain models. It is argued that the liquid is comprised of chains of cis-linked tetrameric building blocks when these dataand modelling results are considered in light of NMR measurements.© 2006 Elsevier B.V. All rights reserved.

Keywords: Neutron diffraction; High-energy X-ray diffraction; Empirical potential structure refinement; Liquid antimony pentafluoride

1. Introduction

Since the first reported synthesis of antimony pentafluoride(SbF5) [1], the molecular structure, properties and chemicalbehaviour of this material have presented a significant challenge.In condensed phases, SbF5 is highly associated; it also serves asan archetype for associated fluids that are bound by 3 center–4electron (3c-4e) interactions. SbF5 is the strongest simple fluorideacceptor on the Lewis acidity scale of Christe [2] and theassociated nature of SbF5 represents an ‘autoamphoteric’interaction in that in the pure fluid SbF5 acts both as a fluoridedonor and acceptor. Indeed, in 1932, prior to the collection of anystructural data, the associated nature of SbF5 was suggested byRuff, who stated that

As a working hypothesis, it is assumed that the constituentsof molecules not only touch but also interpenetrate both inthe case of the same or different constituents [3]

⁎ Corresponding author.E-mail address: jturner@atom.chem.utk.edu (J.F.C. Turner).

0167-7322/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.molliq.2006.08.042

This hypothesis was confirmed using 19F NMR datarecorded from the liquid [4]. The results of these experimentswere interpreted in terms of a cis-coordinated linear chain,which is shown schematically in Fig. 1. Using the samelabelling scheme as Hoffman et al., the fluorine atoms FC aremutually trans, those labelled FB are mutually cis and FA arethe bridging fluorines. At the lowest temperature of measure-ment, 263 K, when the fluid is supercooled, the resonances forFA, FB and FC are observed, together with internuclear couplingbetween 19F nuclei [4]. That the chain length is long is evincedin these studies by the fact that no observation of resonances dueto the terminal SbF5 units was observed. Interestingly, thisanalysis of the liquid is conducted under the assumption ofpurely ionic bonding between Sb and F.

Theoretical models of the structure of liquid SbF5 are sparse,with only one simulation of the liquid being performed to date,using ab initio molecular dynamics [5]. In this study, cis-linkedoligomers containing up to 8 SbF5molecules were observed alongwith a distribution of shorter chains and dimers. Additionally,SbF5 and related species have also been the subject of thermo-chemical calculations [2,6]. It has also been suggested in previous,

Fig. 1. A schematic representation of the chain structure of liquid SbF5, deduced from 19F NMR data [4].

Fig. 2. Schematic representation of SbF5 in the solid state [12].

240 S.E. McLain et al. / Journal of Molecular Liquids 131–132 (2007) 239–245

unpublished neutron diffraction measurements that cyclic tetra-mers are present in the liquid [7].

In the gas phase, equilibria between trimers and tetramers ofSbF5 are observed [8] and electron diffraction measurementsalso show that at lower temperatures, trimeric and tetramericspecies exist [9]; we are not aware of an electron diffractionmeasurement of the structure of molecular SbF5, in contrast toother binary pentafluorides [9,10]. Association between SbF5molecules also determines the structure in the solid state. Themolecular structure of tetrameric SbF5, though not the crystalstructure, is very similar to that of NbF5 and TaF5 [11] andconsists of corner-sharing octahedra, shown schematically inFig. 2. A change in intermolecular connectivity and structure isto be expected in SbF5; the crystallographically determineddensity of SbF5 is 4.07 g cm

−3 at 279 K, whereas the density ofthe liquid is 3.16 g cm−3 at 281.65 K [12] and such a largechange in density, ∼22% is indicative of a large structuralchange in the fluid.

In this paper, we report the results of neutron and high-energy X-ray diffraction experiments on liquid SbF5 giving adetailed picture of the structure of the liquid at the total radialdistribution function level for the first time. Additionally thedata have been modelled using empirical potential structurerefinement (EPSR) which creates a three-dimensional model ofthe liquid which is consistent with the measured diffractiondata.

2. Experimental

2.1. Sample handling and preparation

Antimony pentafluoride (Aldrich Chemical Co.), a clear,viscous liquid, is quite aggressive and will slowly attack glass-ware unless the liquid is extremely pure and the glassware is dry.All manipulations of SbF5 were handled under vacuum on a highvacuum line (pminb10

−4 mbar) or under an inert atmosphereusing a Schlenk line (pminb10

−2 mbar) in standard borosilicateglassware which was flame dried under vacuum. The purchasedliquid was transferred to a Schlenk flask and vacuum distilledfour times. The resulting liquid was much more viscous than theoriginal. The non-ideal viscosity before distillation results fromthe presence of dissolved HF and other impurities in the liquid,and although the process described above significantly de-creases the impurity concentration, to obtain ultra-pure an-

timony pentafluoride, further methods were employed. Thequadruply distilled SbF5 was subsequently fractionally distilledon a glass high-vacuum line. SbF5 was condensed at 250 K witha CCl4/N2 bath and the subsequent trapping of other volatiles at77 K. Since HF condenses at ∼293 K, some remain in solution.Therefore, it was necessary to repeat this process at least fourtimes. The resultant liquid is noticeably more viscous than thatobtained from vacuum distillation alone. Neutron diffractionsamples were prepared by cryogenic distillation of ∼0.75 ml ofSbF5 into quartz tubes (5.0 mm ID×7.0 mm OD) and flamesealed under vacuum. Samples consisting of ∼0.35 ml SbF5 in4.08 mm ID×4.09 mm OD quartz tubes were prepared in anidentical manner for X-ray diffraction experiments. Using 1Hand 19F NMR the purity of the sample with respect to hydrogencontamination (e.g. HF) was N 99.999% [13].

2.2. Diffraction measurements

2.2.1. Neutron diffractionNeutron diffraction experimentswere performed on0.0110mol

of SbF5 on the Glass Liquid and Amorphous Materials Diffrac-tometer at the Intense Pulsed Neutron Source facility of ArgonneNational Laboratory (ANL) at 296 K±2 K. The neutron datahave been corrected for detector efficiency, empty cell scattering,attenuation, inelastic scattering, and multiple scattering usingstandard analysis methods [14]. In addition separate diffractionexperiments were performed on the SiO2 container at 296 K to

241S.E. McLain et al. / Journal of Molecular Liquids 131–132 (2007) 239–245

ensure that container normalization was correct and that the con-tainer could be subtracted successfully.

2.2.2. High-energy X-ray diffractionHigh-energyX-ray diffractionmeasurements on 0.005125mol

of SbF5 were performed on the 11-IDC line at BESSRC-CAT atthe Advanced Photon Source facility of ANL at 296 K±2 K. Inaddition, a separate experiment was performed on the blank SiO2

container, again to assure accurate subtraction. The high-energyX-ray data were corrected for detector efficiency, instrumentalgeometrical effects and polarization using standard analysisprocedures [15].

3. Theory and simulation

3.1. Neutron and X-ray scattering theory

The quantity measured in a neutron diffraction experiment isthe differential scattering cross-section, dσ/dΩ where

dr=dX ¼ dr=dXself þ dr=dXdistinct

¼Xa

cab2a þ PðQ; hÞ þ FN ðQÞ; ð1Þ

P(Q,θ) is the inelastic contribution and FN (Q) is the inter-ference function arising from the ‘distinct scattering’ contribu-tion, cα is the atomic concentration and bα the scattering lengthof isotope α. FN (Q) is related to the Faber–Ziman partialstructure factors, Sαβ (Q) by the following equation

SN ðQÞ ¼ FN ðQÞ=Xa;b

cacbbabb

!

¼Xab

cacbbabbðSabðQÞ−1Þ=Xa;b

cacbbabb

!ð2Þ

Similarly in a high-energy X-ray experiment, the quantitymeasured is

dr=dX ¼ dr=dXCompton þ dr=dXself þ dr=dXdistinct

¼ CX ðQÞ þXa

ca f2a þ IX ðQÞ; ð3Þ

where CX (Q) is the Compton scattering function, IX (Q) is themeasured scattering intensity arising from the ‘distinct scatter-ing’ contribution and fα(Q) is the atomic form factor for speciesα. The X-ray total pseudo-nuclear structure factor, SX (Q) can beobtained from the differential scattering cross-section by

SX ðQÞ ¼ IX ðQÞ−CX ðQÞ−X

ca f2a ðQÞ

h i=Xa;b

cacb faðQÞfbðQÞ

ð4Þ

Independent of the radiation source used in a diffractionexperiment, the Fourier transform of any structure factor yieldsthe associated radial distribution function, G(r), which is thesum of the respective site–site radial distribution functions,

gαβ(r), each weighted by concentration and the intensity factorsof atomic species, α and β, present in the sample. For neutrons,this intensity factor is the bound coherent scattering length, band for X-rays the electronic form factor, f(Q). S(Q) is relatedto the total radial distribution function G(r) via

SðQÞ ¼ 1þ 4pq=QZ

r tGtotalðrÞ−1bsinðQrÞdr; ð5Þ

where ρ is the number density (atoms/Å−3) of the sample.Measurement of the total structure factor yields the total radialdistribution function, while each partial structure factor yieldsthe corresponding site–site radial distribution function.

In order to understand the average local structure of a liquid,integration of gαβ (r) gives the coordination number of atoms αaround β atoms between two distances, r1 and r2 as

nbaðrÞ ¼ 4pcbqZr2r1

gabðrÞr2dr ð6Þ

where ρ corresponds to the atomic number density of the sam-ple and cβ is the concentration of atom β.

3.2. The empirical potential structure refinement (EPSR)method

Empirical potential structure refinement (EPSR) was used tomodel both the neutron and high-energy X-ray diffraction datacollected for pure SbF5. EPSR is a computational methodcreated for modelling disordered materials such as liquids andglasses [16], which allows a three-dimensional model to beconstructed that is consistent with a set of one-dimensionalstructure factors. EPSR begins with a standard Monte Carlosimulation using an initial reference potential consisting of anintramolecular harmonic potential to define the geometry of themolecules being modelled, and an intermolecular potential,which in this case consisted of Lennard–Jones 12-6 potentialsfor the site–site interactions. This reference potential is used togenerate a starting configuration of molecules. EPSR theniteratively adjusts a perturbation to this reference potential toobtain the best possible agreement between the computed andexperimental S(Q) [16].

While EPSR may provide an ensemble of molecules con-sistent with the diffraction data, it does not necessarily provide adefinitive model for the structure of the liquid in question. Theremay be several distinct structures which give equally reasonableagreement between data and simulation. Therefore, it is impe-rative that the simulation box be constrained from the outset withas much prior information regarding the properties of the liquidin question as is possible.

The purpose of EPSR analysis is to explore the validity ofvarious structural models against a set of diffraction data. Asdiffraction data only provide a measure of the structure of thefluid at the pair correlation function level and not thermody-namic or dynamic properties, EPSR only gives a model of thestructure itself. Detailed descriptions of EPSR are given else-where in the literature [16,17].

Fig. 3. (a) S(Q) for X-ray and neutron diffraction where the X-ray data have beenshifted by 1.5 for clarity. (b)G(r) from the X-ray and neutron experiments wherethe X-ray G(r) have been shifted by 6 for clarity.

242 S.E. McLain et al. / Journal of Molecular Liquids 131–132 (2007) 239–245

In order to provide a good fit to the data and, therefore, a morereliable model of the structure of liquid SbF5, it is necessary tomake an assumption about the structure. From NMR evidence[4], the structure of the liquid is likely to be comprised of cis-linked chains as are shown in Fig. 1. In an effort to assess thevalidity of this model given the diffraction data, three separateEPSR simulations were performed using ensembles of SbF5molecules related to the cis-linked structure, where details ofeach of these models are described below (Section 4.2). In eachcase, the total number of atoms (500 Sb atoms and 2500 Fatoms), density (ρ=0.05271 atoms/Å3) and temperature (298 K)were the same. The reference potentials used for fluorine weredeveloped for methyl fluoride [18] and because Lennard–Jonespotentials for antimony have not been reported, to our know-ledge, potentials developed for a iodine atom [19] were used forthe initial reference potential for the antimony site. The valuesfor both potentials are listed in Table 1. These three simulationsare named cis-monomer model, tetramer model and cis-linkedchain model hereafter and details of each model are described indetail in Sections 4.2.1–4.2.3.

4. Results and discussion

4.1. Diffraction data

The measured neutron diffraction data for liquid SbF5, SN(Q), are shown in Fig. 3a. SN(Q) can be written in terms of theweighted partial structure factors as

SN ðQÞ ¼ 0:698SFFðQÞ þ 0:276SSbFðQÞ þ 0:027SSbSbðQÞ ð7ÞFrom Eq. (7), it is evident that the total structure factor in the

case of neutrons is dominated by SFF(Q) and to a lesser extentby SSbF(Q). The corresponding radial distribution function, GN

(r), for SN(Q) is shown in Fig. 3b.The measured high-energy X-ray total structure factor for

SbF5, SX(Q) is also shown in Fig. 3a. SX(Q) can be written interms of the weighted partial structure factors, using the elec-tronic form factors at Q=0 Å−1 as

SX ðQÞ ¼ 0:282SSbSbðQÞ þ 0:498SSbFðQÞ þ 0:220SFFðQÞ ð8ÞIt is clear from Eq. (8) that the diffraction data measured

using high-energy X-rays are dominated by the SSbF(Q) partialstructure factor. The corresponding radial distribution function,GX(r), for SX(Q) is shown in Fig. 3b. It should be noted that Eq.(8) gives values at Q=0. However, with X-rays the form factorsvary with Qwhich are not easily expressed in real space, and forthis reason, this equation only represents a relative comparisonof the intensity observed in the radial distribution function.

Table 1Reference potential parameters for EPSR simulations of SbF5 [11,12]

Atom σ/(Å) ε/(kJ mol−1) qe re

Sb 4.989 0.4184 0.0 0.0F 2.94 0.25522 0.0 0.0Fch 2.94 0.25522 −0.5 0.1Q 0.0 0.0 0.5 0.1

In Fig. 3b, the first peak in the neutron radial distributionfunction is located at rSbF=1.86±0.03Å and in the X-ray atGX(r)at rSbF=1.83±0.03. This correlation corresponds to the intramo-lecular SbF peak. This peak is not symmetric but rather contains asmall shoulder on the large r side of the peak. The shoulder isconsistent with the longer SbF intramolecular ‘bridging’ distanceobserved in the structure of crystalline SbF5 [4,20]. From theneutron data, this shoulder is located at ∼2.03±0.05 Å in goodagreement with previous studies [7]. The coordination number ofthis peak along with the shoulder with respect to number offluorines around the antimony atom is cSb

F =6.34±0.14 at r-min=2.28 Å showing that each Sb atom is sixfold coordinate. Thesecond prominent peak visible in the neutron data at 2.70±0.05Åcorresponds to the intramolecular FF distances. This peak is alsovisible in the X-ray data but has much lower intensity, which isexpected given that SFF(Q) has a smaller contribution to the totalstructure factor (Eq. (8)) compared to the neutron curve (Eq. (7)),and consequently in the corresponding radial distribution func-tion. The FF coordination number of this peak at rmin=3.12 iscFF=5.58±0.38. These distances give an average F–Sb–F angle ofθFSbF=93.7°±0.8°, which corresponds to octahedral coordina-tion of Sb in the liquid.

The most remarkable difference between neutron radialdistribution function, GN (r), and the X-ray radial distributionfunction, GX (r), is the presence of a peak at 3.93±0.03 Å in GX

(r) which is not detectable in the GN (r). This peak can beattributed to an additional Sb–Sb distance given that it appearsprominently in the Fourier transform of the X-ray data but not inthe neutron data. This is due to the much lower weighting of theSb–Sb partial structure factor in the neutron diffraction data(Eq. (7)) compared to the X-ray data.

4.2. EPSR

4.2.1. Cis-monomer modelFor the cis-monomer model, an ensemble containing 500

SbF5monomers was constructed. Themolecular structure of this

Fig. 5. (a) EPSR fits to the diffraction data using the cis-monomer model and (b)resultant from the model compared with the data derived G(r). The X-ray S(Q)and G(r) have been shifted for clarity.

243S.E. McLain et al. / Journal of Molecular Liquids 131–132 (2007) 239–245

monomer is shown in Fig. 4a. For the molecule, four differentatom types were used: a central Sb atom (black), non-bridgingfluorine atoms (white), a negatively charged fluorine (light grey,Fch) and a positively charged bonding site, Q site, (dark grey).The bonds from the Sb to the ‘non-bridging’ fluorines wereassigned a bond length of rSbF=1.86 Å whilst both thecharged fluorine and the positively charged site (white bondand grey bond, respectively) have an Sb–X (X=Fch,Q) dis-tance of rSbX=2.03 Å. In order to allow bonding betweenindividual SbF5 molecules, the bridging fluorine has a chargeof −0.5qe where the radius of this charge is 0.1 Å. The Sb–Qbond is not a true bond but rather a distance between thecentral antimony and the positively charged ‘Q’ site which hasa charge of +0.5qe and a charge radius of 0.1 Å. The ‘Q’ sitemoves with the molecule and is used to calculate interactionenergies, but does not count as a scattering centre whichcontributes to the neutron or X-ray data and therefore has noscattering length or form factor associated with it. The chargeon both sites is assumed to be uniformly distributed within asphere of this radius. The use of a spherical charge distributionas opposed to a point charge ensures that with the oppositelycharged sites, the interaction potential does not diverge as itwould with a point charge, thus helping the molecules formflexible links that are not easily broken once formed. In thisway chains of different lengths of individual SbF5 molecules

Fig. 4. Representative molecules from each EPSR simulation.

are allowed to form in the simulation. The parameters for thisEPSR simulation are summarized in Table 1.

The EPSR fit to the diffraction data is shown in Fig. 5a.While the fit to the neutron data is reasonable, the fit to the X-ray diffraction pattern is poor, from inspection of the radialdistribution functions shown in Fig. 5b. While both the intra-molecular Sb–F and F–F distances are well fit in each G(r)function, confirming the intramolecular configuration withregard to these distances, the peak at 3.93 Å in the GX (r)function is not present in the EPSR model of the diffractiondata. The length of this correlation obviously precludes anintramolecular origin, and it is clear that this model does notaccount for this correlation in the liquid. In this EPSR model,there was evidence of chain formation within the simulation boxby virtue of an overlap with the Q site and the charged fluorineatom (Fch). Although chains of various lengths (up to ∼5molecules in length) were formed, they were not constrainedto link to each other in a fixed configuration, and therefore,this model could not account for the longer distance present inGX(r).

4.2.2. Tetramer modelBecause the individual cis-monomer model was unsuccess-

ful, a model based on the tetramer found in the crystal structure(Fig. 2) was used [20]. The tetrameric unit consists of four SbF5molecules linked by four Sb–F–Sb bridges, where each SbF5molecule provides one bridging fluorine. Additionally, theSb–F–Sb angles were constrained to the values found in thesolid state, namely, θ=170° and θ=141° in an alternatingmanner in the tetramer. In this model, the Sb centre was naturallysix coordinate. This model also introduces additional distances,namely, three Sb–Sb distances of rSbSb=3.96 Å, rSbSb=3.75 Åand rSbSb=5.55 Å. For this simulation, an ensemble of 125(SbF5)4 tetrameric units, at the appropriate density for SbF5, wasconstructed. A representative (SbF5)4 tetramer from the box ofmolecules is shown in Fig. 4b where the white atoms representthe non-bridging fluorines, and the grey atoms represent thebridging fluorines, which have bond lengths of rSbF=1.86 Å and

Fig. 7. (a) EPSR fits to the diffraction data using the cis-linked chain model and(b) resultant G(r) from the model compared with the data derived G(r). The X-ray S(Q) and G(r) have been shifted for clarity.

244 S.E. McLain et al. / Journal of Molecular Liquids 131–132 (2007) 239–245

rSbF=2.03 Å, respectively. Fig. 6a shows EPSR fits from thismodel to the diffraction data and Fig. 6b shows the fit to the radialdistribution functions for each data set.

From Fig. 6a, it is clear that the tetramer model provides amuch better fit to the data than the cis-monomer model. Thoughthe amplitude of the fit for both the X-ray and neutron S(Q) dataare not in exact agreement with the measured diffraction pat-terns, the oscillations in both data sets are at the same positionsshowing an improvement from the cis-monomer fit shown inFig. 5a. Also, the constraint provided by the tetramer con-figuration ensures that the peak at 3.93 Å appears in the EPSRgenerated X-ray radial distribution function (Fig. 6b).

4.2.3. Cis-linked chain modelThough the tetramer model provides a good fit to the data, a

fluid consisting of isolated tetrameric units is not consistentwith previous NMR measurements [4]. Moreover, the volumechange on melting implies a substantial reorganization and it isnot clear that the tetrameric connectivity is conserved. For thesereasons, the cis-linked chain model was employed. This modelconsists of an ensemble of 125 (SbF5)4+Q site molecules and arepresentative molecule is shown in Fig. 4c. Here, all of the Sb–Fbonds in the chain are white with the same distances for bridgingand non-bridging fluorines as described in Section 4.2.2.However, there are two additional atoms in the chain, namely,a charged fluorine (white bond to a grey atom) and a positivelycharged Q site (dark grey atom with a grey bond to the central Sbatom). As in the case of the SbF5 monomer model (Table 1), thecharged fluorine has a charge of −0.5qe with a charge radius of0.1 Å and the Q site has an opposite charge of the samemagnitude and the same charge radius. In each case, the bonddistance from the central antimony atom is rSbX=2.03 Å.

This model retains the Sb–Sb distance constraints seen in thetetramer whilst providing an avenue, via the charged Fch and qsites, for formation of longer chains. The EPSR fit to thediffraction data using this model are shown in Fig. 7a with thecorresponding fits in real space shown in Fig. 7b. This EPSR

Fig. 6. (a) EPSR fits to the diffraction data using the tetramer model and(b) resultant G(r) from the model compared with the data derived G(r).The X-ray S(Q) and G(r) have been shifted for clarity.

model provides a similar fit as the tetramer model (Fig. 6) toboth measured diffraction data sets. Of the three attemptedEPSR models, this model is consistent with the liquid beingcomprised of long cis-linked chains as suggested by NMR. Theradial distribution functions (Fig. 7b) are well reproduced bythis model fit, specifically with regard to the distance at 3.93 Åin the X-ray G(r).

Fig. 8 shows a representative chain from the EPSR generatedconfiguration after fitting to the data for this model. It is clearfrom this figure that chains are forming within the simulationbox, though no chain more than 8 SbF5 molecules in length was

Fig. 8. SbF5 chain generated from the cis-linked chain EPSR fit to the diffractiondata.

245S.E. McLain et al. / Journal of Molecular Liquids 131–132 (2007) 239–245

formed. This observation is consistent with the ab initio si-mulations of this liquid which show a similar result [5]. Themeasured diffraction data presented here are consistent withboth the tetramer and the cis-linked chain models of the liquid.The two models differ in the cyclic nature of the former and theabsence of a single ring-forming 3c-4e interaction in the latter;apart from this difference, the local geometry of the two modelsis very similar. Both models are consistent with the diffractiondata and accordingly afford good fits to both X-ray and neutrondiffraction sets; in particular, the correlation present in the X-rayat 3.93 Å is well reproduced by both models. Further dis-crimination between the two models is not supported by thediffraction data alone. However, previous NMR measurements[4] together with the diffraction and modelling study presentedhere clearly show that the structure of liquid SbF5 is bestdescribed by chains of octahedrally coordinate antimony atomswith cis-coordination.

The diffraction data provide measurements of the averagelocal structure of the liquid with atomic resolution on a lengthscale of approximately 0–8 Å and as such chains of very longlength will be difficult to observe. This does not necessarilymean that correlations on longer lengths scales do not exist butsuch structures on a longer length scale will lie outside theavailable Q range of these experiments and as such, only theaverage, local structure is available. This is also true of theEPSR fits to the diffraction data.

5. Conclusions

From neutron diffraction data, it is clear that SbF5 is a highlyordered fluid, with on average ∼4 short Sb–F bonds at 1.86 Åand ∼2 bridging Sb–F bonds at 2.03 Å. The X-ray data alsoshow Sb–Sb interactions which occur at a distance of 3.93 Å.Subsequent modelling to the data shows that these distances areconsistent with Sb–Sb distances and that the liquid cannot justbe described in terms of simple monomeric SbF5 units. Ourmeasurements and subsequent EPSR modelling are consistentwith the liquid being comprised of chains of cis-linked tetra-meric building blocks of four linked SbF5 molecules, though theprecise longer range order of the liquid is not distinguishable inthis study. The average Sb–F–Sb angle from the EPSR mea-surements is θSbFSb=150.9° which is slightly less than solid-state Sb–F–Sb angle which is θSbFSb=155.5°.

It is likely that the cis-monomer model would indeed providean adequate fit to the data given enough time. However, thecomputational time required for this complex system is far toolong to provide an adequate model using the cis-monomermolecules. That the other models in fact are more successful isevidence that more information must be put into the startingbox in order to provide a starting structure which more closelyrepresents the measured data, thus providing a reduction incomputational time. Furthermore, this study reinforces the needfor combined neutron and high-energy X-ray studies as in this

case the neutron data alone are insufficient to provide a com-plete characterization of the local structure.

Acknowledgements

The authors thank the U.S. National Science Foundation forfinancial support for J.F.C. Turner under CAREER award CHE0349010, J.L. Yarger under award CHE-612553 and DMR-555159 and for S.E. McLain under award OISE-0404938. Theexperiments performed at Argonne National Laboratory weresupported under U.S. DOE Contract No. W-31-109-ENG-38.We also thank Qiang Mei for help with the neutron experimentsand D.T. Bowron for useful discussions.

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