Studies of a polymerizable crystal : I. - structure of monomer pts (bis-p-toluenesulphonate of 2,4-hexadiyne 1,6-diol) by neutron diffraction at 120 and 221 k

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Studies of a polymerizable crystal : I. - structure of monomer pts (bis-p-toluenesulphonate of 2,4-hexadiyne

1,6-diol) by neutron diffraction at 120 and 221 k

J.P. Aimé, J. Lefebvre, M. Bertault, M. Schott, J.O. Williams

To cite this version:

J.P. Aimé, J. Lefebvre, M. Bertault, M. Schott, J.O. Williams. Studies of a polymerizable crystal : I.

- structure of monomer pts (bis-p-toluenesulphonate of 2,4-hexadiyne 1,6-diol) by neutron diffraction at 120 and 221 k. Journal de Physique, 1982, 43 (2), pp.307-322. �10.1051/jphys:01982004302030700�.

�jpa-00209398�

Studies of a polymerizable crystal :

I.

Structure of monomer pTS (bis-p-toluenesulphonate ^of 2,4-hexadiyne 1,6-diol) by ^neutron diffraction at 120 and 221 K (*)

J. P. Aimé (1) (2), ^{J. Lefebvre} (2) (4), ^{M. Bertault} (1), ^{M. Schott} (1) ^{and J. O. Williams} (3) (1) Groupe ^de Physique des Solides de l’ENS (**),

Université Paris VII, ^Tour 23, ² place Jussieu, 75251 Paris Cedex 05, France

(2) ^Institut Laue-Langevin, 156X, ^{Centre de} Tri, ^{38042 Grenoble} Cedex, ^France

(3) ^Edwards Davies Chemical Laboratories, University College ^of Wales, Aberystwyth, Wales, ^U.K.

(4) Dynamique des Cristaux Moléculaires (***), Université de Lille I, ⁵⁹⁶⁵⁵ Villeneuve d’Ascq, ^France (Rep ^{le 25} juin 1981, accepté le 15 octobre 1981)

Résumé.

Les structures cristalline et moléculaire du monomère pTS (bis-toluène-sulfonate ^de hexadiyne- 2,4 diol-1,6) ^{ont été} déterminées par diffraction de ^neutrons à 120 K et 221 K. La structure basse température

est monoclinique P21/c

^Z

^{4 et a}

14,745(7) Å, b

5,086(2) Å,

25,738(12) Å, 03B2

91,71(5) degrés

et V

1 929 Å3 à 120 K. La structure haute température ^est également monoclinique P21/c

^Z

^{2 et}

a ⁼

14,630(20) Å, b

5,133(2) Å,

14,845(15) Å , 03B2

^118,55(7) degrés ^{et V}

^{979 Å3 à 221 K.}

Ceci correspond ^{à 295 K à b}

5,178 Å. ^Les quatre molécules de la maille basse température occupent des centres de symétrie ^et appartiennent à deux familles (sites)

^des géométries ^et des environnements diffé- rents. En particulier, ^les groupements diacétylène ^ont

les deux sites des orientations et des contacts intermolé- culaires légèrement différents,

^aucune contrainte intramoléculaire notable. Les groupements latéraux toluène- sulfonate ont peu de contacts atome-atome intermoléculaires et leurs librations sont encore à 120 K de grande amplitude; ceci facilite les mouvements induits par la contraction selon b qui accompagne la polymérisation.

La structure à 221 K, analysée de manière usuelle, ^sans désordre, ^{montre de très} grandes librations des groupes latéraux. Elle

pu être également analysée,

^la supposant désordonnée, chaque groupe latéral occupant ^l’une

ou

l’autre de deux configurations très semblables à celles des sites à 120 K. Cependant, la barrière de potentiel qu’on peut

déduire, ^à partir ^de l’analyse ^de l’agitation thermique, ^est très peu élevée. Pour le reste, les géométries moléculaires et les contacts intermoléculaires sont très semblables à 120 K et 221 K. Le monomère pTS ^{et le} polymère poly-pTS ^ont également ^des structures très semblables, dans les deux phases. On discute aussi briève- ment la signification ^des structures ici déterminées pour la réactivité, ^{et diverses} propriétés électroniques.

Abstract.

The crystal ^{and molecular} structures of

pTS (bis-p-toluenesulphonate ^of 2,4-hexadiyne 1,6-diol) have been determined by ^neutron diffraction at 120 K and 221 K. The low-temperature ^{structure is}

monoclinic P21/e ^{with Z}

^{4, and a}

^14.745(7) Å, b

5.086(2) Å,

25.738(12) Å, 03B2

91.71(5) deg.,

V = 1929 Å3 at 120 K. The high-temperature ^structure is also monoclinic P21/c ^{but with Z}

^{2 and}

a ⁼

14.630(20) Å, b

5.133(2) Å,

14.845(15) Å, 03B2

118.55(7) deg., ^V

⁹⁷⁹ Å3 at 221 K. This corresponds

to

temperature b parameter ^{of 5.178} Å. The four molecules in the low-temperature ^{unit cell}

^unstrained

and all lie at centres of symmetry. They belong ^{to two families} (sites) with different geometries and different envi- ronments. Particular diacetylene ^groups

^{the two} sites have slightly different orientation and intermolecular

contacts. Toluenesulphonate side groups have few ^atom-atom intermolecular contacts and exhibit libration of

large amplitude ^{at 120} K, leaving

for the contraction along ^b brought ^about by polymerization. ^{A «}

tional

analysis of the 221 K structure (i.e. ^without positional disorder) shows very large libration of the side groups. It could be analysed by assuming disorder, ^each side group occupying ^{either of two} configurations ^which

Classification

Physics ^Abstracts

61.12

(*) ^The isotropic ^and anisotropic ^{thermal factors}

^well

as

calculated and observed structure factors for all (hkl)

are

available from

Editions de Physique, Zone Industrielle de Courtaboeuf, ^B.P. 112, 91944 Les Ulis Cedex (France) ».

(**) Laboratoire associe

C.N.R.S. No 17.

(***) Equipe de Recherche associee

C.N.R.S. No 465.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01982004302030700

are

very similar to the two low-temperature ^site configurations. However, the barrier height between the two

configurations, deduced from TLS analysis of thermal motions, is very low. Except ^for this, the overall molecular

geometries ^and intermolecular contacts

very similar to those fo~nd ^{at 120 K. pTS}

^{and poly-pTS-}

polymer have similar structure both at low- and high-temperatures. The relevance of the structure to reactivity

and various electronic properties ^is briefly ^discussed.

,1. Introduction.

Recent interest in the diacety-

’lenes is due, among other reasons, to the fact that several of them polymerize in the solid state, ^to yield

pure, macroscopic crystals ^of conjugated polymers [1].

The polymerization process conforms with the topo- chemical principle enunciated by ^Schmidt [2] ⁱⁿ

which the crystal ^{structure of the reactant controls} completely ^{the nature of the} product ^{formed. In such}

systems, ^accurate knowledge of the detailed structure of the reacting ^monomer crystal ^is essential, together

with information about structural changes brought

about by progressive polymerization. For the final

product ^{to be a} polymer single crystal ^both polymer

and monomer must belong ^{to the same} space group, and _possess closely ^related structures. However reac-

tive monomers polymerize fairly rapidly ^under X-ray irradiation, ^{and a detailed} X-ray study is, ^{in these} cases,

extremely ^{difficult or} impossible. ^Neutron diffraction,

therefore offers a solution to the determination of the

crystal ^{structure of these monomers. The} only way for a thermal neutron to deposit enough energy in a monomer crystal ^{to initiate} polymerization ^{would be}

a nuclear scattering ^event

^a very ^{rare event} in

crystals containing only H, C, N, 0, ^{S atoms. Some}

·X-ray studies of reactive monomers are, however,

available (see below). Reactivity ^{is not a} problem ^for polymer crystals ^{which can be} routinely ^studied by X-rays.

In this _paper we report ^the study ^{of a} diacetylene

which is known in the literature as pTS, ^{or alternati-} vely ^{as TSHD or TS. The} gefleral formula of the dia-

cetylene molecules is R--C=---C--C=---C-R’ where R and R’ _may be various ^« side-groups ^{». In} pTS, ^a symmetrical diacetylene, ^{R and R’ are}

a para-toluene sulphonate moiety.

pTS ^was chosen for study ^{since it} is, by far, ^the

most thoroughly investigated diacetylene. ^The polymer

structure at 300 K is known [3]. ^It belongs ^to space group P21 /c ^{with two} repeat units in the cell, ^Z

2,

and the one-dimensional polymer ^{backbone chains} elongated along ^{the b} crystal axis. The low-tempera-

ture polymer ^{structure has been} explored ^{at 120 K} [4].

The space group remains P21/c but the unit cell doubles with Z

4 and two inequivalent sites. The

phase transition ^{occurs near} 190 K [5].

It was known that the monomer exhibits a similar

phase change [6] ^and recently ^an incommensurable

phase, ^which is absent in the polymer, has been dis- covered between about 160 and 200 K [7]. ^{A low}

accuracy structure at 120 K was available [6], together

with several unit-cell measurements at various tempe-

ratures. The polymerization ^of pTS ^is homogeneous,

and polymer ^{and monomer are} considered to form

a continuous series of solid solutions extending ^from

a perfect ^{monomer to a} perfect polymer crystal.

However, this does not necessarily ^{mean that mono-} mer-polymer ^mixed crystals ^are untrained and, indeed, the kinetics of single crystal polymerization [8]

take account of the lattice strain. Several methods for

determining, ^{at least} approximately, ^the polymer ^con-

tent in reacting crystals ^{have been} proposed [9-10].

This paper is part ^{of a} series of investigations ^on pTS which include structural studies (reported here),

inelastic neutron scattering [11], ^Raman scattering [12]

and calorimetry [13] ^{to be} reported elsewhere. The present paper is organized ^{as follows.} Experimental

details are given in section 2. In section 3, ^{we discuss} the low-temperature structure, ^{which serves as a}

starting point since it is the only perfectly ^ordered phase. ^{In section 4, we} discuss the high-temperature phase. Finally, in section 5, ^a comparison ^{is made}

between monomer and polymer structures. We shall

keep in mind the relation between structure and reac-

tivity throughout temperature ^and pressure depen-

dences of lattice parameters, ^and preliminary ^results

on the incommensurable phase ^{and the} correspond- ing transitions, ^{will be} given ⁱⁿ following papers.

2. Experimental (general).

pTS monomer was

prepared by ^{reaction of} tosylchloride (Fluka) ^with 2,4-hexadiyne 1,6-diol, ^or HDD (K ^and K) ^{at 4 °C} [14].

These chemicals, especially ^{HDD, were first} recrystal-

lized several times in toluene to remove all traces of the polymer. ^{Care was taken to} minimize side reac-

tions. pTS thus obtained ^was recrystallized ^several

times from CH2Cl2--CH30H ^{and the} crystals were

grown ^at 4 ~C from acetone solution under a slow flow of nitrogen [15]. Except during mounting ^{on the} goniometer head, ^{and of course} during heat tr,eat- ments, the crystals ^were kept ^{at 255 K or below in}

the dark, ^{in order to avoid} polymerization. ^{In all cases,}

the colour of a crystal ^{at the} beginning ^{of an} experi-

ment was pink, indicating ^{an initial} polymer ^content of less than 0.5 %, judging ^{from the} spectroscopic

studies on other crystals. ^{No further} polymerization

was detected after crystals ^had remained in the neutron beam for _up to a week. Indeed, ^the y-contamination ^at

the sample ^{in the neutron beam was} always ^{less than} 10-1 rad., ^{whereas 4 x 105 rad. are needed to achieve}

a 0.01 polymer content at room temperature [9].

’

The structure determinations at 120 K and 221 ^K

were made using ^{data collected at ILL} high-flux

reactor (Grenoble) ^{on D8 4-circle} diffractometer,

using ^neutron wavelengths ^{0.891 A and 1.266 ± 0.001} A,

selected respectively by ^{a Cu} (220) ^{and Cu} (200) ^mono-

chromator. The wavelengths ^{were measured} by ^refin- ing the lattice parameter ^{of a} standard KCI crystal.

A/2 contamination was measured to be "" 2 x 10- 3 Á

at 0.891 A. An Air-Products Displex ^He compressor

was used, ^{with a} temperature stability - 0. I ^{K and an}

absolute accuracy - 1 K.

Data were treated using program COLL5N written

by ^M. S. Lehmann and F. K. Larsen [16] ^{with correc-}

tions for cryostat absorption. ^For crystal absorption corrections, ^real absorption ^was unimportant, ^and only isotropic incoherent scattering ^was considered,

with _~H

30 barn, using ^{ABSCOR in XRAY} pro- gram. This correction plays ^a r5le for the crystal ^used

at 120 K only, ^{which was} plate-shaped

Program ^XRAY [17] ^{was used for structure refine-}

ment, with Fermi lengths ^{0.665 for} C, - ^{0.374 for} H, 0.580 for 0 and 0.28 for S [18]. ^The quality ^{of the}

structure was estimated using R (y ^{Fo - Fc I)IFO}

and Rw

(Y(Fo - F~)2)/Fo.

3. Structure of the low temperature phase.

^3.1

EXPERIMENTAL.

The crystal used for this structure determination was a plate ^of perfect morphology [19], light pink ⁱⁿ colour, ^{with a volume 8 mm3. The}

measurement temperature ^{was chosen to be the same}

as the one chosen for polymer ^{structure determina-}

tion [4]. ¹⁸⁸¹ independent reflections were used. They

were collected at 1.266 A, ^{with 0 50~. No} 1/Q(l~

condition was applied. ^{To avoid} applying ^extinction corrections, ^{the 12 most intense} reflections were

rejected, all the others ^were given equal weight.

’

Refinement ^was started from the known 120 K

polymer ^{structure [4]. A first series of runs in which} only isotropic thermal factors on H atoms were used

yielded ^{a structure with R}

^7.3 %. Introduction of

anisotropic ^{factors on these atoms} decreased R to 4.9 % ^{in two} cycles. The final values are R

3.7 %

and Rw

^3.8 %.

3.2 R.ESULTS.

The structure is P2i/c ^{with Z}

⁴ (using ^{the same} setting ^as previous ^workers [3, 4, 6]).

The monomer molecules are centrosymmetric ^{and lie}

at centres of symmetry. ^{There are therefore two sets} of molecules with different geometries, occupying ^two

sites I and II, ^{as in the} polymer ^{at 120 K} [4], ^{and as}

in trans-stilbene for instance [20].

A cell parameters refinement on 16 intense lines with 0 _up to 39 deg. ^and distributed throughout reciprocal space yielded ^{the data} given ^{on table I.}

The two sets of lattice parameters ^{at 120} K, ^avai- lable from X-ray measurements [6, 21] ^{are also} given

in table I. As mentioned above, they certainly ^suffer

from the fact that the samples change during ^data acquisition, ^since polymerization occurs, albeit more

slowly ^{than at room} temperature, ^and therefore lattice parameters change [21, 22].

The discrepancy ^with present data is outside the stated accuracies. Note in particular ^{the much} larger

Table I.

Low-temperature ^{unit cells} of pTS ^mono-

mer.

(*) ^{The standard errors stated do not} include the uncer-

tainty ^{on the neutron} wavelength, which may add

syste- matic error ³

10-4.

(**) Calculated by

^{from the lattice} parameters given.

value of b which cannot be explained by X-ray ^induced polymerization. The value of b found here is smoothly

connected to the room temperature value of about 5.178 A [42] which has been confirmed independently by X-rays [23].

The lattice contraction along ^b upon complete polymerization is therefore only ^{0.17 A at 120} K, compared ^{to 0.27 A at 300 K,} using ^the quasi tempera- ture-independent polymer ^{value of b}

4.917 A [3, 24]. Despite this smaller contraction, polymer ^chain propagation ^is blocked below 105 K [25]. ^Lattice

mismatch along ^b is therefore not the only ^structural

feature important ^{in the} polymerization ^of pTS.

, -, ’-I~’

Fig. ^1.

Projection

plane (101) ^of the site I molecule in the 120 K structure and labelling ^{of the atoms. The labels}

of the corresponding ^{site II atoms are in} parenthesis.

Final atomic positions ^are given ^{in table II. Thermal} agitation ^is expressed ^as U-equivalent. ^{Atoms are}

labelled as in figure 1, which shows a projection ^of

site I geometry ^{on the} (101) plane. ^Bond lengths ^are.

given ^{in table} III, ^{columns 1 and} 2, and bond angles

in table IV, ^{columns 1 and 2.}

Table II.

Atomic positions ⁱⁿ monomer pTS ^{at 120 K.}

Atoms S(I) ^to H(9) belong ^to site I molecules and atoms

S(2) ^to H(19) ^to site II. Note that there is

atom labelled

H(10). Coordinates in reduced units. U-equivalent, express- ed in 10-2 A2.

Figure ^{2 shows a} projection ^{of a} whole unit cell

on (101) plane.

3 , 3 DlscussloN.

3 . 3 .1 Bond lengths ^and angies.

-

The pTS ^{monomer molecule} comprises ^several

~

1.

Fig. ^2.

Projection of the low-T unit cell on plane ^(101).

fairly rigid ^units (diacetylene, phenyl) ^connected by non-rigid series of bonds. It can then take different overall shapes ^without strongly distorting any indi- vidual bond. Indeed, ^on neither site is _any bond _very different from the _average length corresponding ^{to its} type [26, 27]. ^For instance, ^the phenyl rings ^are only slightly ^distorted regular hexagons ^{and bonds} C(6)C(7j

and C( 16)C( 17) only differ from the ^« average » ^aro- matic C-C bond length ^{of 1.395 A} [26] by ^{more than}

the standard deviation of 0.003 A - they ^measure

1.405 A. The rings ^have approximate ^threefold sym- metry ^{and differ} only slightly ^{on the two} sites. Three CCC angles ^{are 118.9} (resp. 119.1) degrees; ^{the three}

others 121.2 (resp. 120.8), ^{with a} standard deviation of about 0.2 degree.

The phenyl ^{C and H atoms are almost} exactly coplanar ^{in site I, but site II shows a} slight ^deviation

from planarity (Table V). ^Atoms H(6) ^and H(16) are somewhat farther out of plane. ^As they ^{seem to have}

some specific interaction with the diacetylene moiety

of a neighbouring ^molecule (see § 3.3.3), they ^were

not used in the calculation of _average plane ^coordi-

nates.

Differences in the bonds connecting ^{the more or}

less « rigid )) ^{units are absent or} barely significant :

the only ^ones larger than twice the standard deviation

C(1)C(3)-C(11)C(13) ^{= 8 x} 10-3 A and

C(3)0(1)-C(13)0(4) ^{= 7 x 10-3 A.}

No angular difference is larger ^{than 0.6} degree.

3.3.2 Relative position of parts of the molecule.

In a non-rigid molecule, ^molecular geometries ^can

differ by the relative displacement ^of parts ^{of the}

molecule, ^{such as} that caused by rotation around

Table III.

Bond lengths ^{obtained in the} different refinements.

u(X- Y) ^{is the standard}

^{for all bonds not} involving ^H atom, between different

identical atoms. The

is about the same in all these

Bond lengths corresponding ^to

rigid group

^not given.

single ^bonds. This indeed occurs in pTS ^monomer.

First of all, ^{the two} sites differ by ^the angle ^between

the plane ^defined by the axis of the diacetylene group and the b axis (equivalent ^to the backbone plane ^{in the} polymer) ^{and the average} plane ^{of the} group S---C6H4-C. ^This angle ^{is 60.0} degrees ^{on site I and}

74.8 degrees ^on site II. The small uncertainty ^{in the}

definition of the _average phenyl plane, implies ^that

it is not useful to specify ^these angles ^{to less than}

0.4 degree. Similarly, ^{in the} polymer, ^the correspond- ing angles ^are 64.2 and 79.3 degrees [4, 6].

The atoms S( 1)C(4)C(7)C( 10) define the « axis » of the rigid phenyl group, and ^are almost exactly ^collinear.

The maximum distance from the least-square straight

line is 3.5 ^x 10- 3 A on site I and 2 x 10- 3 on site II

(atoms S(2)C( 14)C( 17)C(20)). ^{The angle} between this axis and the corresponding diacetylene ^axis C(1)C(2)

is 82.86 degrees ^{(site 1) and 88.52} degrees ^{on site II.}

One difference between the molecular geometries ^on

the two sites is therefore a difference in position ^of

the unit SC6H4CH3 ^{as a} whole, ^{which can} be described

(although ^{it is not an actual} movement) ^{as a} rocking

of its (approximate) ^« twofold axis » to and from the

diacetylene axis, by ^{about 6} degrees, ^{and a rotation}

of the unit around its o twofold axis » leading ^{to a}

15 degrees difference of the angle between the planes

of the phenyl ring and the backbone.

The deformation does not extend _very far toward

C(1)C(2) : ^the angle ^between planes C(2)C(1)C(3)0(1)

and C(3)0(1)S(1) ^{is 9.9} degrees, and between the cor-

responding planes C(12)C(11)C(13)0(14) ^and C(13)0(4)S(2), ^11.9 degrees. ^{All other} pairs ^of planes

have the same relative orientation on both sites within 0.5 degree.

As mentioned above (§ 3.1), ^a significant improve-

ment of the structure is produced by introduction of

anisotropic ^thermal agitation factors for the H atoms.

This is especially ^{true of the} methyl ^H atoms, whose temperature ^{factors are} very large (see ^table I) ⁱⁿ comparison with others which ^are normal. In addi-

tion, ^the largest principal ^axes of the 3 thermal ellip-

soids are in the planes ^{of the 3} hydrogens. ^This suggests

Table IV.

Bond angles ^{obtained in the} different refinements.

that fast rotation of methyl groups around the

C(7)C(10) ^and C(17)C(20) ^{axes still occurs at 120 K.}

3.3.3 Intermolecular packing.

^{Extensive dis-}

cussion of the intermolecular packing will be deferred until comparison with calculations using ^atom-atom potentials ^is possible. The overall impression ^{is one}

of fairly efficient space filling by ^the non-rigid pTS

molecules. Most atoms are in close contact with one

or several atoms of neighbouring molecules. Only ^a

few salient points will be mentioned here, including ^the packing around the diacetylene ^{itself, relevant to the} polymerization reaction, and differences between the two sites.

Around the diacetylene itself, packing ^is fairly ^dense.

Each of the C(l), C(2), C(3) carbons is associated with

two kinds of non-bonded interactions : with carbon

atoms of translationally equivalent molecules dis-

Table V.

Distance of the benzene ring ^atoms (in A)

from ^their least-square plane.

Site I qitt- 11

(*) ^{Atoms not included in the} definition of the planes.

placed by ^± b and with carbon and hydrogen ^atoms

of the benzene group of neighbouring ^{molecules on}

the other site. The first type ^{of contacts is} obviously

of prime importance ^{in the} polymerization ^reaction

itself. The second type ^{of contacts would be} of impor-

tance, ^{if it} could hinder the large amplitude displace-

ment of atom C(l) ^{in the reaction. In this case it would}

communicate the reaction of molecules in one stack

parallel ^{to b to molecules in} neighbouring ^{stacks, so}

that reactivity ^{of monomers vicinal to a} polymer ^chain

would be affected

Fig. ^3.

Contacts between neighbouring diacetylenes ^relat-

ed by

^b translation, ^with

der Waals radii indicated.

The

shown is site I in the low-T phase. First values of

C(l)-C’(1) ^and C(1)-C’(2) ^distances correspond ^{to site I of}

the low-T phase, ^{values in} parenthesis ^to site II and values in brackets to the high-T phase (independent ^{atoms refine-} ment).

Figure ^{3 shows a} projection ^{on the} plane ^defined by ^{b and site I} diacetylene axis. There is close contact between the reacting ^atoms C(I) and C’(1). ^{Table VI}

shows there is a noticeable difference between sites I and II, ^{the HT} phase resembling the latter.

Table VI.

Interatomic distances corresponding ^to

intermolecular contacts near the diacetylenes (in A).

(*) ^In Polymer C(1)-C’(1) ; ^1.36 A (Bond).

Figure ^{4 shows a} projection ^{on the} (a, c) plane ^of

molecules at site I and site II. The benzene rings ^of

site II molecules of the ^same unit cell define a cage, and the only ^{hindrance to} diacetylene motion in the

plane ^defined by ^its axis and b is the _very close

C’(2)H(16) ^contact. Table VI shows the significant

intermolecular distances. To our knowledge only

pyrene [28] ^{has such a} short C... H distance. Since

C(19), ^{the C atom to which} H(16) ^is bonded, ^{is in}

contact with C’(2), the situation is reminiscent of that

occurring ^{in weak} hydrogen bond-like A-H... B

bonds, although ^no such C-H... C bond is known to us. Some of the _necessary ingredients ^are present :

a somewhat polar ^{C-H bond - on a benzene} ring nearby S03 - ^{as H-donor, and the} triple ^{bond 1t}

electrons as acceptor. ^{The excess} energy correspond- ing ^to this short C... H distance, calculated using ^the

parameters given by Kitaigorodskii [27], ^is only

0.5 kcal./mole ^{or 2 kT at 120} K, ^{and can} be compen- sated locally by ^{even a} very weak bond. Such interac- tion does not occur on site II since the distance

C’(12)H(6)

^{2.84 A is} almost normal. There is ^no hindrance to polymerization ^{on site II due to} neigh- bouring ^atoms, but the situation is not so clearcut

on site I. This difference _may be reflected in different

polymerizabilities between the two low-T sites.

Here again, ^the high-T situation is similar to that

of site II.

Fig. ^4.

Projection ^on plane (101) ^{of the} vicinity ^of

diacetylene ^with

der Waals radii. Case of site I in the low-T phase.

A region ^{of close} intermolecular ^contact is also found near the diacetylene itself, ^{around atom} 0(3)

on site I, 0(6) ^on site II. The salient distances are

given ^{in table} VII. Note that, ^{if the} oxygen ^van der Waals radius is taken as 1.40 A [29], rather than 1.52 A

as preferred by Kitaigorodskii [27], ^no distance is

anomalously ^{short. In addition to} these intermolecular contacts, there its also close intramolecular contact between 0(3)H(2)

0(6)H(12)

^2.62 A. Therefore,

the molecules related by successive b translations are

connected by ^a continuous chain of non-bonded contacts between atoms C(3)H(1)0(3)H(2)C(3), ...

which _may help ^{in the} arrangement ^{of the} diacetylenes

in positions favourable to reaction. This configuration

Table VIIA.

Intermolecular contacts around 0(3) (in A).

is also present ^{in the} high-T phase, ^and probably ⁱⁿ

the polymer, although ^{of course} C(I) ^{has now moved}

away, and H positions ^{are unknown.}

The packing along b of the side _groups is loose : table VIIB shows the corresponding shortest intera- tomic distances.

There is only ^one C-H contact, of H(8) with either

C(8), ^{on site} I, ^or C(9), ^{on site} II, displaced by ^{b :}

distance 2.90 A. The real significance ^{of such contacts}

is dubious, ^since H(8) belongs ^{to the,} presumably rotating, methyl group. Contacts around this _group

are such that they allow almost free rotation, ^{on both} sites. There is no C-C contact, the shortest distance

being ^{3.76 A between} C(9) ^and C(10) displaced by ^b

on site I. The only ^C-0 near-contact is C(5)-0(2).

Therefore, ^more than half of all contacts along ^{b are}

between diacetylenic ^{carbons. This,} plus ^the flexibility

of the pTS molecule around C(3), helps ^{to understand}

how the structure can accommodate a large change

of parameter ^{b with} slight side-group reorientations.

Indeed, ^as shown in table VIIB, the short distances

are almost the same in monomer and polymer, ^the

maximum difference being ^{less than 0.1 A.}

3.3.4 Structural consequences for reactivity ^and

electronic properties.

The discussion of § 3. 3. 3 has Table VIIB.

Shorter distance between atoms of ^one side-group ^{and those} of ^{the same} one, displaced by ^b (in A).

Distances within 0.05 A of the

of

der Waals radii are underlined. Displayed

distances shorter than

C-C

3.7 A, ^C-0

^3.3 A, ^C-H

^3.1 A, ^C-S

^3.85 A, ^0-H

^{2.8 A and H-H}

^{2.5 A.}

revealed packing differences between the sites which,

albeit small, may affect the reactivity. Study ^{of such} reactivity differences would be interesting.

The packing also shows close proximity ^{of the sul-} fonyl part ^{or one} molecule and of the diacetylene ^unit

of another molecule. This _may be important ^{in the} polymerization ^{processes in which an} excited elec- tronic state is a kinetic intermediate. The short distance

implies ^efficient energy transfer from the _upper lying tosyl singlet ^{and even} triplet ^{excited states to the}

lower lying diacetylene ^{excited states of the same multi-} plicity. ^Hence photopolymerization ^{even for} light predominantly ^absorbed by ^the tosyl group may be facilitated. Even if the polymerization actually ^occurs

from a vibrationally ^excited ground state, excited by

internal conversion, ^{the close contact} observed _sug- gests ^the possibility of efficient vibrational (multi- quantum C-H) energy transfer from the benzene _group to the diacetylene. ^{In such a situation «} photopolyme-

rization » will occur even if the side-group ^excited

states are below those of the diacetylene ^unit.

Another consequence of the existence of two sites may be a difference in electronic excitation energies.

In the polymer, the existence of two sites in the low-T

phase manifests itself spectroscopically by ^a

~

250 cm-1 splitting (at ⁴ K) ^of the vibronic bands

corresponding ^to the first excitonic transition, ^near

620 nm [1, 30]. ^{This can be} interpreted ^{as site} split- ting [31], ^as observed in the _very similar crystal ^struc-

ture of trans-stilbene [32]. Broadly speaking, ^{such a} splitting may be of intramolecular and/or ^intermole-

cular origin. ^{The same} phenomenon certainly ^occurs

in pTS ^{monomer as well. The monomer UV} absorption

bands have not been studied at high enough ^resolu-

tion. However, ^the absorption ^{due to} polymer ^chains

near 600 nm in slightly polymerized crystals ^{show a} larger splitting : ^{500 cm-’}

^{at 4 K} [33].

Since the molecules on both sites seem largely unstrained, and since the geometrical differences are

mainly ^{confined to the outer} parts ^{of the} molecules,

it is unlikely ^{that a} splitting ^{of a monomer electronic}

transition would correspond ^to changes in intramo- lecular through-bond interaction between benzene and

diacetylene. ^{The same} probably applies ^{to the} pure

polymer, ^since X-ray ^data [4] ^{seem to allow to rule}

out difference in such interactions. The situation of isolated polymer chains in the almost _pure monomer

matrix is not clear, since these chains are known to be extended [34], ^and might ^be differently ^{strained on}

the two sites. Another, ^{and more} likely, ^{source of site} splitting ^{is a} site-dependent contribution to exciton

energies ^{of the} change ^{in van der Waals and} multipolar

interactions [35], ^{as in} 1.4-dibromonaphthalene ^for

instance [36]. ^In pTS ^monomer, diacetylene ^and tosyl

moieties can be approximated ^as independent ^chro- mophores. Differences in relative positions imply ^diffe-

rence in angles ^made by dipole and transition dipole

moments of neighbouring chromophores (of ^{the same}

or different molecules) with the transition dipole

moment of a given diacetylene, yielding differences in the corresponding ^{energy terms. The same will} apply

to an isolated polymer ^chain.

Considering ^{that the amount of} splitting ^{of the}

electronic transition of an isolated polymer ^{chain in}

the monomer crystal reflects the difference between

sites, the maximum difference _may not yet ^{be reached}

at 120 K : the splitting ^{of the} principal peak ^near

600 nm is, ^{at 120} K, only 2/3 ^{of its 4 K value} [33],

and the low _energy optical line shows a red shift of 200 cm-1 from 120 to 4 K. Therefore, ^{a 4 K structure}

would _very likely ^{show a} larger difference between sites than that found at 120 K.

4. Structure of the high-temperature phase.

^4.1

EXPERIMENTAL.

Data were collected at 221 K, ^i.e.

about 20 K above the onset of incommensurable modu- lation [7]. Data collection took about a week and

a relatively low-temperature ^{was chosen to minimize}

thermal polymerization.

A different crystal ^{from the one used at 120 K was} used, ^since polymerization might have occurred during

the several months which elapsed between the two

experiments. ^The crystal ^{used at 221 K was not far}

from cubic in shape ^{and had a} volume ~17 mm3.

Absorption corrections were minimal. 1895 indepen-

dent reflections with 0 550 were collected using ^a

neutron wavelength A

^{0.891 A. 1696} reflections with 1/ 6(~ > 1.5 were used in the subsequent ^calcu- lations, ^all being given equal weighting. ^{The 3 most}

intense lines were rejected ^{and no} extinction correc-

tion was applied.

Refinement was started from the published polymer high-temperature ^structure [3]. ^{For reasons discussed} below, three calculations were performed : using

XRAY [17], ^an independent ^atoms calculation, ^and using program ORION [37], ^two calculations using rigid groups. The ten C and H benzene atoms were

taken as one rigid group and thermal group. The

methyl ^{was taken as another} rigid group. In ^one

approach, ^{ORION was used, as usual, with one} posi-

tion _per atom. The corresponding geometry ^chosen

was that of site I, the less intramolecularly ^distorted.

In the other approach, ^{we tried to} analyse ^the high

T-structure as a disordered one, and atoms were given

two positions, ^{each with} probability 1/2, ^{as if each}

molecule could be either in site I or in site II.

These approaches ^were tried since the exact nature of the phase transition, and whether it is displacive

or order-disorder is not clear. The refinements cor-

respond ^{to either of these} assumptions. This is further discussed in paragraph ^4.3.

4. 2 REsULTS.

The structure (the average ^struc- ture if the crystal is assumed disordered) ^is P21/c ^with

Z

2. The two monomer molecules lie at centres of

symmetry ^{and are now related} by ^the interchange

operations usual in that structure [38]. ^{The situation}

is, again, ^{the same as in} the pure polymer [3]. ^{At 221 K,}

since it has been claimed that p 1 o polymer ^is pyro- electric [39], ^{a more detailed} investigation ^{of some}

lines absent in P21~~ ^{but which could be present in the}

related non-centrosymmetric groups P21 ^{or Pc was} performed. The intensities at (010) ^and (030) ^were

found to be (2 ± 1) .10- 3 ^{those of} (020) ^and (060) ^and

could be entirely ^{accounted for} by h/2 contamination transmitted through the monochromator. Some (hOl)

reflections with I odd were tested and none was found.

Atoms being ^{labelled as} those of site I in figure 1, the final atomic positions and thermal agitations expressed ^as U-equivalent ^are listed in table VIII, ^bond lengths ^{in table III, and bond} angles in table IV. The results of all three refinements are listed in tables III and IV, ^since they ^are used in the following discussion.

The atomic positions ^obtained by ^{ORION are avai-}

lable with the (hkl) ^data.

Table VIII.

Atomic positions ^{in monomer} pTS ^at

221 K obtained by independent ^atoms refinement.

U-equivalent

expressed ^{in 10-2 A’.}

4. 3 DISCUSSION.

4.3. .1 Independent ^atoms refi-

nement.

This XRAY refinement yields final values R

8.9 ~/~, R~

^9.2 ;’%. Figure ^{5 shows a} projection

of a whole ^unit cell on (101) plane.

Thermal agitation ^{factors are} large. ^{This is not sur-} prising ^{for the} methyl hydrogens, already rotating ^at

120 K. But all the atoms of the benzene _group, as well

as the methyl ^carbon, also show large anisotropic

motion. Consider for instance the atoms C(7), C(8),

Fig. ^5.

Projection ^{of the} high-T ^{unit cell}

plane (101).

C(9) ^and H(5), H(6). Their motion can be analysed

as due to a rotation around the C(4)-C(7) ^{axis and}

rotation of this axis around a point ^near C(4), ^which

is the type ^of displacement relating, in the low-tem- perature phase, ^the geometry ^{of site I to that} of site II.

In addition, ^{one finds a} fairly large distortion of the

phenyl group : bonds C(4)-C(5) ^and C(7)-C(8) ^for

instance are shorter by ^{more than 0.03 A} than in the low temperature structure. This is more than 3 ^stan- dard deviations (see ^table III).

One is then led to suspect that the refinement _pro- cedure misses a significant feature of the structure, such as disorder. The room temperature ^{structure of}

poly-pTS has been described as disordered [5] ^and pTS

and poly-pTS structural properties ^are usually ^consi-

Fig. ^6.

Comparison ^of

high-temperature ^confi- guration ^{at 221 K} (a) (independent ^atoms refinement) ^and

of polymer high-temperature configuration ^{295 K from refe-}

rence

[3] (b).

dered _very similar, ^not only ^the geometries

except for the polymer ^backbone

^{but even the} anisotropic

thermal agitation ^factors (Fig. 6).

4.3.2 Refinements ^using rigid groups.

a) ^Non- disordered structure.

To analyse ^{the structure as}

’disordered, ^{we must allow at least some atoms to}

occupy several positions ^with probabilities ^{1. Since}

this is not possible ^{with XRAY as we use} it, program ORION was used instead [37]. ^{As a first} step, ^{a refine-}

ment similar to the previous one, i.e. without disorder,

must be performed, ^{to allow} comparison. Using C6H4

and CH3 ^as rigid groups reduces the number of inde-

pendent coordinate parameters ^{from 60 to} 39, ^and using in addition C6H4 ^{as a thermal} group (but ^not CH3 ^{since the H atoms are} rotating) reduces the num-

ber of thermal parameters ^{from 138 to 98. Since the} benzene ring ^is nearly perfectly planar ^{on site I} (table V) ^{but not on site} II, ^{site I} geometry ^{was chosen.}

Final values are R

10.2 %, R,

^10.4 %. ^{The non-}

constrained bonds and angles ^are normal, except C(7)-C(10) ^{which is} slightly ^shortened (table III).

Thermal motion.

Since rigid and thermal _groups

are used, ^the analysis of thermal motion could not be done in terms of independent ^atomic ~~~ ^{as in para-} graph ^3. Instead, ^we shall consider here the motion of the benzene _group C6H4, ^{which is} important ^{in the} understanding ^{of the nature of the} phase ^transition

and high-temperature structure. This benzene group motion ^was studied through ^{a TLS} analysis [40] ^{of its}

translation T, ^libration L, ^and coupling ^{S tensors.}

There is of course some arbitrariness in analysing ^the

normal modes of that _group alone, since it is covalently

linked to the diacetylene carbons, ^but this link is only through ^a flexible series of bonds. The phenyl group

is at a general position in the unit cell and no symme-

try consideration can be invoked to simplify ^{or elimi-}

nate the S tensor [40]. ^{T and L were referred to the} phenyl ^{inertial axes :} e1

perpendicular ^{to the} phenyl plane, ^the C(4)-C(7) ^direction e2 and _e3 perpendicular

to el and _e2.

The data thus obtained are not directly comparable

to those of the 120 K structure, ^since they ^{have been} processed differently. ^{To assess the} feasibility ^and degree ^of confidence of such a comparison, ^{the 120 K}

data were analysed using ^ORION using ^{the same} rigid and thermal _groups as those used ^at 221 K, starting from the atomic positions already ^determined (§ 3), ^and refining only thermal motions. The final R value is 5.6 %. The increase over the value found in

paragraph ^{3 is} comparable ^to the difference found between the two refinements of the 221 K structure, treated as non-disordered.

The eigenvalues ^{of the T and L tensors} of the low-T

phase ^obtained, either from the atomic ~i~~ independent

atoms refinement using ^XRAY (§ 3), ^or directly by ORION, ^are compared ⁱⁿ table IX. Results are similar,

but differences are obvious. Using ^ORION L22 ^is

smaller and L eigenvectors ^have significant compo- nents along ^{at least two} ~/s. ^{Still, the} eigenvectors

are not much rotated, ^{and the} larger ^libration ampli-

tude is in both cases found to have its larger compo- nent along e2. The TLS analysis may therefore be used,

but for qualitative discussion only : the benzene group is not really rigid.

Table X shows a comparison ^{of T and L data ob-}

tained in different refinements of the high-T phase. ^In

the one discussed here, L33 ^is anomalously ^small (smaller ^{than at 120} K) ^and gives another indication of the imperfection of the TLS approach ^{since the}

benzene ring is almost planar. ^Some non-diagonal

Table IX.

Benzene group T and L tensors at 120 K. Eigenvalues ^and components of ^the eigenvectors ^{on the} e j ^vectors. (Inertial ^axes of the benzene _group, see text.)

Tij ⁱⁿ Angstroms squared, Lii ⁱⁿ degrees squared.

Table X.

Benzene group TL ^{tensors at} 221 K.

(*) Standard deviations about 6 degrees squared, except L33, double-well, position ^I, where it is large : ²⁷ degrees squared.

components of L, ^{which are not zero} by symmetry, ^are ill-defined.

The eigenvectors obtained in the TLS analysis, although predominantly along ^one ej, often have

significant components along ^others. However, ^{in all}

cases, the eigenvalue L22 ^is large ^and corresponds ^to

an eigenvector predominantly along e2. This is the

motion by which the molecule can go from site I to site II (around ^{the same inversion} centre) ^and gives

another indication of a disordered structure. The libration centre is displaced by ^0.76 A from the ben-

zene

symmetry)) ^centre along C(4)-C(7), ^towards C(4).

b) Disordered structure.

The same type ^{of refi-}

nement was attempted assuming that the 221 K struc- ture is disordered, and started with the ^two low-T

geometries being equally probable for each molecule.

The two positions ^of many ^{atoms are} then so close

as to be indistinguishable, and this introduces _pro- blems in the refinement. Thus only ^{the atoms of the} C6H4-CH3 group could be given ^{two different} posi- tions, forcing ^the S( 1)-C(4) ^{bond to be} slightly (a ^few degrees) ^{out of the} average plane of the benzene ring.

The other atomic starting positions ^were those obtain- ed in the previous refinement. The final values

R=8.8%, ^R, =9.1%.

Bond lengths ^are given ⁱⁿ table III and bond angles

in table IV.

Due to the constraints of the rigid group, the

C(7)-C( 10) ^bond lengths ^differ significantly (by ^0.07 A)

in the two configurations.

It is therefore possible ^to refine the high-T pTS

structure as a disordered one, with even an improve-

ment in R over that of the similar ORION non-

disordered one (but ^this may be due at least in part

to the larger ^{number of} parameters introduced in the

refinement). ^{The two} positions introduced initially ^do

not tend to converge towards a single ^{one. We know}

of no example ^{where a} non-disordered structure with

large amplitude motions could be forced ^to refine into a disordered one.

The relative orientations of the benzene rings ^{on the}

two positions ^can be defined by ^the angles ^made by ^the principal ^{inertia axes as} given ^{in table XI. If the two} positions ^{were related} by ^a rotation around a common

C(4)-C(7) ^axis (plus ^a translation), ^{one would have}

e , e"

^{0 and} e;, e"

e3, e". ^{Table XI shows that}

the actual relation between the two positions ^{is not}

far from this. Comparison with the low temperature data also shows that the disordered positions ^are slightly ^less different than are the two low T sites, and the displacement connecting ^{them nearer to a}

rotation around _e2 plus ^a translation (plus ^{the sui-}

table operation ^{of the} space group).

Table XI.

Angles between the principal ^{inertia axes} of the ^benzene groups ^on the two sites (in degrees).

Column 1 : high-temperature structure, treated

disor- dered.

Column 2 : low-temperature structure, . both site confi-

gurations put ^{at the}

position by the suitable operation

of the high-T ^structure interchange group.

e 1 : perpendicular ^to the benzene plane.

e2 : along C(4)-C(’~ (or C(14)-C(17)).

e3 : along el

e2.

Thermal motion.

Results of the TLS analysis ^of

the benzene _group motion on both sites are given ⁱⁿ

table X. The T tensors are similar in both ORION

refinements, and comparison with table IX shows that

they scale with T approximately ^as expected ^on going