The crystal structure of C70S48: the first ${a~priori}$ structure determination of a C70-containing compound

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The crystal structure of C70S48: the first a priori structure determination of a C70-containing compound

G. Roth, P. Adelmann

To cite this version:

G. Roth, P. Adelmann. The crystal structure of C70S48: the first a priori structure determination of a C70-containing compound. Journal de Physique I, EDP Sciences, 1992, 2 (8), pp.1541-1548.

�10.1051/jp1:1992225�. �jpa-00246637�

Classification Physics Abstracts

61.50 61.10

Short Communication

The crystal structure of C70S48: the first

a

priori structure

determination of

_a

C70.contablblg compound

G. Roth and P. Adelmann

Kemforschungszentrum

Karlsruhe, Institut fir Nukleare, Festk6rperphysik, Postfach 3640, W-7500 Karlsruhe, Germany

(Received

3 June 1992, accepted 11 June

1992)

Abstract The preparation and crystal structure of _a novel fullerene-containing compound, C70548, is described. The structure has been determined by direct methods flom single crystal X-ray diffraction data collected at _room temperature and has been refined to atomic resolution

on the C70 molecules. The compound is orthorhombic, _{space group} C 2 _{m m} with _a

= 10.329

(2) A,

b

=

20.420(4)

A, _c

= 38.198 (7) A ^{and four formula units} per cell. The _structure consists of planes ofdosely packed C70 ^molecules perpendicular to the c-axis interleaved by _a complex

array of 58 rings. This first _a priori crystallographic structure determination of

a C70-containing compound confirms to _a remarkable _accuracy the predictions of C-C bondlengths from Hartree- Fock calculations of the molecular structure.

1 Introduction.

The method found

by

Kritschmer et al.

ill

to prepare fullerenes

("C2n")

in

macroscopic

_quan- tities has turned the

study

of these

interesting

molecules from _a

highly special topic

of cluster

physics

into _one of the "hottest"

topics

in solid state

chemistry

and

physics today.

Within _a few

months, compounds

with such remarkable solid state

properties

like

superconductivity

_[2]

and

ferromagnetism

_[3] have been found. Central to _a

deeper understanding

of these

properties

is the detailed

knowledge

of the

crystal

structures

and, primarily

of _course, of the _structure of the fullerene molecules themselves.

Unfortunately,

sound

crystallographic

data _are rather _{scarce even} for the most abundant and well studied fullerene molecule C60 [4, 5]. ^For

Cm,

there is _no such data at

all,

neither for pure

Cm

_nor for _any

compound containing C70.

Molecular disorder

plays

_a

major

role in

both _cases, but the lack of

single crystals

of sufficient

quality

is also _{a severe} restriction _to any decent structure determination and refinement.

Therefore,

all _we know at present ^about the structure of C70 stems from either local

(only semi-quantitative) probes

like NMR [6] ^and

electron diffuse

scattering iii

_or from theoretical considerations [8, 9].

1542 JOURNAL DE PHYSIQUE I N°8

In this short

communication,

we describe _{results of}

an a

priori single crystal X-ray

structure determination of _{a new}

compound, CmS48>

which contains well ordered C70 molecules

along

with

crown-shaped 58 rings

and allows the first direct

crystallographic

determination of the structure of the Cm molecule.

2

Experimental.

C70 _was

prepared

and

separated by

the usual methods: _an electric _arc between

graphite

elec-

trodes, burning

in _a helium

atmosphere

of about 100 torr, was used to

produce

fullerene-

containing

soot

[I].

The fullerene molecules _were then extracted with hot toluene and _sepa- rated into C60>

Cm

and

higher

fullerene fractions

by liquid chromatography (n-hexane

+ 5 9l

toluene)

_on neutral A1203. As

a final step C70 _was heated to 250 °C for 16 h under _vacuum to drive off residual solvents.

Crystals

of the _new

phase

_{were grown} from _a solution of stoichiometric amounts of sulfur and Cm in CS2 under ambient conditions

by slowly evaporating

the solvent.

They

_{occur as}

platelets

with

dominating (001)-faces

and show _a

pronounced anisotropy

of the

light absorption

in this

plane (ruby-red along

b, ^black

along a) pointing

to _a low

dimensionality

of the electronic structure.

Table I.

Experimental

details.

CmS48

Solution

growth (from CS2) Single crystal X-ray

diffraction

Room temperature

Space

_group C 2 _{m m}

(standard setting:

Am _m

2)

a = 10.329

(2) ¹

b = 20.420

(4) I

c = 38.198

(7) ¹

z = 4

4-circle diffractometer

MoKa-radiation

Graphite

monochromator

Crystal

diameter 200 _~Jm

measurement:

+h, +k,

+I

sin(8) IA

₌ 0..0.54

A~~

2621

independent

reflections with 1 _>

3a(i~

Direct methods

(Multan, [10])

+ Differences Fourier

synthesis

Block list _squares refinement Final RF _" ^6.46

$i,

_RWF

" 4.79 $i

(lla~-weights)

Single crystals

^with linear dimensions of _up to 200 _{~Jm were}

investigated

_{on a} standard 4-circle

X-ray

diffractometer under ambient conditions

(see

Tab. I for

experimental details).

2621

independent

reflections with 1>

3a(1)

_up to sin

(8)/J

₌ 0.54

A~~

were measured. The

corresponding

real _space resolution

proved

to be sufficient to locate the 26

inequivalent

sulfur

atoms

by

direct methods

[10].

^{The 42}

independent

carbon atoms were then found

by

successive different Fourier

syntheses.

No reference to the

suspected

molecular structure of Cm _was made

throughout

this _process

so that the

resulting

structure can

really

be considered _as

being

determined

a

priori.

Coordinates and

isotropic

temperature factors

(a

total of 255

parameters)

were then refined

by

the block least _squares method. No non-symmetry constraints had to be

imposed

onto the model. The final

reliability

factors _were

RF

_" 6.46 $i

(unit weights)

and RWF _# 4.79 $i

(lla~ weights), quite

reasonable values for such _a "macromolecular" _structure

at _room temperature. ^Atomic coordinates and

isotropic

temperature factors _are available from the

publisher.

3. Discussion of the structure.

Figure

I shows the structure in two different

perspective projections.

The

Cm

molecules foriu

planes perpendicular

to the c-axis interleaved

by

_a

complex

_array of

crown-shaped 58 rings

The shortest center-to-center distances between C70 molecules _are 10.3

A along

_a and 11.4 ii

along

the

diagonal

of the a-b

plane.

There _are two

independent

C70 molecules in the unit cel>

one in each

plane (Fig. I).

The

long

axes of all

C70

molecules

(the

S-fold molecular rotation

axis) point along

b, the second molecule differs from the first _one

by just

_a 2-fold rotation around this axis

(Note

that this is neither

a symmetry

operation

of the space group nor of the

point

synunetry _group of the idealized molecule

(5/nun)).

Except

for their

orientation,

the two molecules _are identical within the _accuracy of the _exper- iment. Both molecules _are symmetry constrained

by

two

mutually perpendicular

mirror

planes through

their center of

gravity (plus

the

resulting

2-fold axis

along

the line of

intersection,

_per-

pendicular

to the S-fold molecular

axis).

It should be noted that this is in fact the

highest

site symmetry

compatible

with the molecular summetry of C70. This

"matching"

between site- and molecular

point

symmetry ^is

accomplished by

the low symmetry

arrangement

of the

58 rings

and it constitutes the

probable

_reason for this

compound

to be the

orientationally

ordered

exception

_among all the disordered

fullerene-containing compounds

known at present.

For the data

presently available,

the accuracy on individual bond

lengths

is about 0.03

1.

The actual values _range from 1.35

I

_to

1.571.

_The

average C-C bond

lengths (averaged

_over

both

molecules)

amount to 1.45

I

_{for the pentagons and 1.42}

I

_{for the}

hexagons.

The bond

angles

in the pentagons _range ^{from 99.8°} to 119.8°

(average

108.0° those for the

hexagons

from l14.5° to 125.8°

(average 120.0°).

To facilitate the

comparison

with information available for the '~free" Cm molecule

(diffuse,

elastic electron

scattering iii

and Hartree-Fock calculations [8,

9])

all distances and

angles

which would be

equivalent

under the

point

_{group syiuiuetry}

operations

of the free molecule

(5/mm)

and also all

pairs

of

corresponding

distances and

angles

on the two

independent

C70 molecules of

C70548

were

averaged (see

Tab. II and

Fig. 2).

Such

an

averaging,

which further

improves

the _accuracy of these derived

quantities,

is

justified by

the close

similarity

of the individual bonds involved

(not shown).

The agreement ^{between observed}

(this work)

and calculated [8, 9]

values,

with

typical

devi- ations between _one and two estimated standard deviations

(e.s.d.)

for the distances and less than _one e-s-d- for the

angles,

is most remarkable. The

proposed

_{18, 9]} bond

ordering

near the

"poles"

of the molecule, with

long ("single")

^C-C bonds in the top pentagon ^and

alternating long

and short

("double")

bonds in the

adjacent hexagons,

is

fully confiriued,

_as is the "aro- matic" character

(delocalized "single"

and "double" bonds like those in the benzene

ring)

of the

hexagons

around the equator. Note that _none of these bonds is

strictly

_a

single

_or double C-C bond. The

long

_ones

(No.

1, 3, 5 and 8, see

Fig.

2

following

the

labelliiig

iii Ref. [9] are

intermediate between the

single

C-C bond in diamond

(1.5445 (1) I, ill], sp~-hybrid)

and the

1544 JOURNAL DE PHYSIQUE I N°8

b

c

b

u

Fig. I. -Structure of C70S48 Projected

a)

along _a,

b)

along b: small black circles: carbon; large, outlined circles: sulfur [13].

"partial

double bond" of

graphite (1.4210 (1) 1, _[11], sp~-hybrid).

The short bonds

(No.

2 and

4)

_are ^still

considerably longer

than the

typical "simple

double bond"

(1.337 (6) 1, ₍₁₁₎₎

_while

the "aromatic" bonds

(6

and

7)

are not far from the

typical

C-C bond distance in aromatic

compounds (1.395 (3) I, [iii).

Also _as

predicted,

the bond _on the

"equator>' (No. 8)

is

clearly

a

"single"

C-C

bond,

in contrast to what has been derived from electron diffuse

scattering

data

[ii.

The

only discrepancy remaining

between _our

experimental

and the theoretical data

concerns bond No. 6 in the "aromatic"

hexagon

which

happens

to be

slightly longer (by

about

three

e-s-d-)

^than

predicted.

Table II.

Average

^C-C bond

lengths (I) _{and-angles (°).}

~°~~ ~~~~~°~~

~$~~~l _{~~~i~~~ i~~j~~~(~}

i 1.45816) 1.4608 1.451 1.464(9)

2 1.380(4) 1.3788 1.375 1.37 Ii)

3 1.459 (5) 1.4553 1.446 1.47 (1)

4 1.370 (4) 1.3643 1.361 1.37 Ii)

5 1.460 (4) 1.4702 1.457 1.46 Ii)

6 1.43014) iA174 1.415 1.4712)

7 lA07(7) lA136 1.407 1.39 II)

8 1.476(5) 1.4859 1.475 1.41(2)

Angle

a I 19.8 (4) ii 9.59

b i in.] (4) 120.34 120.3

c 107.1 (7) 106.79

d 120.0 (4) 120.07

e 108,1(4) 108.38 108A

i I 19.8 (4) 120.04

g 108.3 (4) 108.22

h 121.3(4) 121.24 121.4

121A(5) 121.57

j 18.6 (4) 118.53 l18.4

k 115.6 (7) 115.78

Labelling according to Baker et al. [9].

[8, 9]: SCF-Hartree-Fock calculations.

[7]: ^{Electron diffuse} scattering.

Estimated standard deviations in parentheses.

The excellent

agreement

^between

theory

and

experiment

does not

merely

illustrate the

qual- ity

of the present diffraction data but _{even more so} the

predictive

_power of the ab-initio self consistent field Hartree-Fock calculations of references [8, 9]. Furthermore it demonstrates that the C70 molecules in

CmS48

_{are, on an average,}

only

_very

weakly

disturbed

by

the

predomi- nantly

v-d- Waals

bonding

to their

surrounding.

A

complete

list of individual C-C bond

lengths

and

angles

is not included in this short contribution both for the _reason of

brevity

and of the limited _accuracy of the present room

temperature ^data. With

potentially

even more

precise

low temperature diffraction data

(which

is

currently being collected)

it should be

possible

to discuss _{on a} sound basis also the deviations of the individual bond

lengths

from their _average values.

Also,

it could be of interest to discuss

the small deviations flom the

predicted

values in terms of electron-electron correlations _on the C70-molecule which _{are not} included in the

simple

Hartree-Fock type ^of calculations.

The

comparison

with the electron diffuse

scattering

data in reference [7] shows two

major

discrepancies

for bonds No. 6 and 8. We attribute these to the limited _accuracy of the bond

1546 JOURNAL DE PHYSIQUE I N°8

c

Fig. 2. C70 molecule in the center of the unit cell, projected approximately along _{a [13].} Numbers and letters refer to the labelling of bond distances and angles in reference [9]

(see

Tab.

II).

lengths

derived from such type of measurements. The method suffers from two

major problems:

first of

all,

it is not an a

priori

method in the _sense that it

requires

in advance considerable

knowledge

about the molecular _structure to be refined.

Secondly,

with the electron diffuse

scattering

data alone, there is

hardly

_any overdetermination of the least _squares

problem:

the number of

independent

observations _at best

equals

the number of parameters to be determined

(positions

of the

independent

carbon _{atoms on} the idealized

molecule).

Additional soft obser- vations

(or

soft

constraints)

in the form of

hypothetical

interatomic

potentials

have therefore to be introduced _to force convergence and the

meaning

of the standard deviations derived for the refined parameters ^is not

completely

obvious.

Yet, the measurements

reported

in reference

iii have, given

the _severe

problenls

with disorder in

"pure"

C70> been the

only experimental

data available for the C70 molecule for

quite

_a while

and have

"historically"

been the first confirmation of the

shape

of the

Cm

molecule.

The second molecular

species

in

CmS48,

the 58

rings,

_are in the usual

crown-shaped config-

uration

found,

for instance, in orthorhombic

sulfur,

with S-S bond

lengths

within the

rings

all very close _to the average of 2.045

1 (Tab. III)

and bond

angles

_near 108°

(not shown).

There

are four different 58

rings

in the structure

(Tab. III),

two of them _are symmetry restrained

by

a mirror

plane.

The closest S-S contacts between sulfurs in different

rings

_are around

3.41, slightly larger

than next nearest

neighbour

distances within the

rings (3.3 I).

As _a final remark _we wish to add that the title

compound C70548

_appears to be

only

_one

out of _a whole _new

family

of similar _ones which combine fullerene molecules with 58

rings

and have the

general

formula

C2n (58)m (with

_n and _m

integer,

here: _n

= 35, _{m =}

6).

There is evidence for at least three other

compounds containing

C60 and 58. The structure of two of them

(C60S16

and

C60S8CS2)

^has

already

been determined [12]. ^We propose to call this _new

family

of

compounds

"sulfo-fullerites" for

brevity, using

the convention to call

fullerene-containing compounds

'~fullerites" and

notwithstanding

the _erroneous association of

sulfur-oxygen

bonds with the

prefix

"sulfo". These _new

substances,

_as well _as

possible doped

derivatives of

them,

could be of interest not

only

for their structural but also for their

physical

properties. Eventually,

sulfc-fullerites

might

offer the chance to

crystallize

also the

higher

Table III. S-S bond

lengths (1).

Bond: Distance(A): Bond: Distancell):

SVI-SV2 2.02919) SWI-SW2' 2.029(9)

SV2-SV3' 2.05419) SW2-SW3 2.06819)

SV3-SV4' 2.047(8) SW3-SW4 2.054(9)

SV4-SVS 2.05817) SW4-SWS' 2.028(9)

Average: ^2.047 Average: 2.045

Bond: Bond:

SXI-SX2 2.071 (8) SYi-SY2 2.061 (9)

SXI-SXB 2.028 (7) SYI-SYB 2.038 (9)

SX2-Sx3 2.042 (81 SY2-SY3 2.019 (lo)

Sx3-Sx4 2.059 19) SY3-SY4 2.053 (11)

SX4-Sx5' 2,037 18) SY4-SYS' 2.007 (9)

Sx5-Sx6' 2.045 18) SYS-SY6' 2.067 18)

SX6-Sx7 2.04218) 2.044(8)

SX7-SXB 2.06418) SY7-SYB 2.042 (8)

Average: 2.048 Average: 2.041

Primed symbols denote transformed atoms.

fullerenes in _an ordered fashion and

thereby

get

quantitative

_access to the atomic structures of these molecules _too.

4. Conclusion.

We have described the first _a

priori

structure determination and refinement of _a

C70-containing compound,

C70548> to atomic resolution. The C70 molecules form

planes perpendicular

to _c with

58 rings

between these

planes.

Theoretical

predictions

of the

shape

of the

Cm molecule, including

subtle details of the bond

ordering

into

long

and short bonds and the frustration of

this

ordering

in the

hexagons

close to the equator _are

fully

confirmed.

References

[ii

KRiTSCHMER W. et al., Nature 347

(1990)

354-358.

[2] ^{HEBARD A,} et al., Nature 350

(1991)

600-601.

[3] ALLEMAND P-M-, Science 253

(1991)

301-303.

[4] HAWKINS J-M-, Science 252

(1991)

312-313.

[5] DAVID W.I.F., Nature 353

(1991)

147-149.

1548 JOURNAL DE PHYSIQUE I N°8

[6] ^{TAYLOR R.} et al., J. Chem. Soc. Chem. Commun.

(1991)1423.

[7] MCKENZIE D.R, et al., Nature 355

(1992)

622-624.

[8] SCUSERIA G.E., ^Chem. Phys. Lett. 180

(1991)

451-455.

[9] BAKER J. _et al., Chem. Phys. Lett. 184

(1991)

182-186.

[10] ^WooLfsoN M-M-, Methods and Applications in Crystallographic Computing, S-R- Hall, T.

Ashida Eds.

(Clarendon

Press Oxford, 1984) _pp. 106-l19.

[11] ^{R-C- Weast} Ed., CRC Handbook of Chemistry and Physics 59 F215

(CRC

_press, Boca Raton,

1979).

[12] ^{G. ROTH} et al., submitted, Chem. Phys. Lett.

(1992).

[13] ^Structural plots _were prepared with the _program "Schakal 88", ^E. Keller,

Freiburg (1988).