Thermal stabilities, packing coefficients and molecular packings in a series of homologous liquid crystals

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Thermal stabilities, packing coefficients and molecular packings in a series of homologous liquid crystals

J. Shashidhara Prasad, P.K. Rajalakshmi

To cite this version:

J. Shashidhara Prasad, P.K. Rajalakshmi. Thermal stabilities, packing coefficients and molecular packings in a series of homologous liquid crystals. Journal de Physique, 1979, 40 (3), pp.309-313.

�10.1051/jphys:01979004003030900�. �jpa-00209110�

Thermal stabilities, packing coefficients and molecular

packings ^{in a series of} homologous liquid crystals

J. Shashidhara Prasad and P. K. Rajalakshmi

Department ^of Physics, University ^of Mysore, Mysore 570006, ^India

(Reçu ^{le 2 août} 1978, révisé le 13 octobre 1978, accepté le 23 novembre 1978)

Résumé.

L’étude systématique ^des structures cristallines et moléculaires des composés mésogènes ^{est d’un} grand ^intérêt pour l’interprétation d’un certain nombre de propriétés physiques de base des cristaux liquides.

Dans cette optique,

déterminé a priori ^{la structure des} arrangements moléculaires des matériaux liquides-

cristallins para-heptanoate ^de phénylazo-para’-éthoxyphényl (P1) ^et undécylénate correspondant (P1 ^et P21/c

pour les deux modifications)

utilisant les données

la structure moléculaire de molécules semblables, para-valérate ^de phénylazo-para’-éthoxyphényl (P1) ^et para-hexanoate correspondant (P21/c), ^suivant l’approche

du modèle de Kitaigorodsky. ^Les coefficients d’empilements ^estimés correspondent ^bien

stabilités thermiques

dans les séries homologues.

Abstract.

A systematic study of the molecular and crystalline structures of the mesogenic compounds ^{is of}

considerable interest in order to interpret

number of basic physical properties ^of liquid crystals. ^In view of this

an a

priori determination of the molecular packings have been made for the liquid crystalline ^materials p-p-ethoxy- phenylazo phenyl heptanoate (P1) ^and undecylenate (P1 ^and P21/c ^{for the two} modifications) ^based

the earlier results of detailed molecular structure data for the similar molecules, p-p-ethoxyphenylazo phenyl ^valerate (P1)

and hexanoate (P21/c) and the model approach ^of Kitaigorodsky. ^{The estimated} packing coefficients correlate well with thermal stabilities in the homologous ^series.

Classification Physics ^Abstracts

61.30

64.70E

An investigation ^of single crystals ^of mesogenic

materials (that ^is compounds which exhibit smectic, nematic or cholesteric liquid crystalline phases

heating) ^{shows the nature of} packing of molecules in the solid state and provides other molecular characte- ristics which could give insight ^{into the nature of} liquid crystals. ^A systematic study of the molecular and crystalline ^{structure of the} mesogenic ^materials

is of considerable interest for

number of basic _pro- blems, ^for example, ^to help interpret ^{the bulk} pro-

perties ^{of the} liquid crystalline phase, ^such

optical anisotropy, diamagnetic anisotropy ^{etc. It may also} provide

^{to the} problem

^{to why only}

^few organic substances exhibit liquid crystallinity ^and why

some of these exhibit more than one phase. ^{In view}

of the _very little work [1-23] ^{that has} been done since the first diffraction studies

liquid crystalline ^mate-

rials by ^{Bernal and Crowfoot} [24],

project ^{has been}

started to study ^{the state} of molecular packing ⁱⁿ

different homologous series in order to achieve

greater insight regarding imbricated packing ^and

hence to increase knowledge ^{of the} variation in

melting points, thermal stabilities etc., within homo-

logous series and between _groups of different homo-

logous series. As a part ^{of this} project detailed mole- cular structures of crystals ^of p-p-ethoxyphenylazo phenyl valerate and hexanoate have been obtained [ 19, 20].

Herein

have tried ^to obtain the molecular struc- ture of the higher homologs ^viz p-p-ethoxyphenylazo phenyl, heptanoate ^and undecylenate using ^{the results}

available for the lower members of the homologous

series and the model approach ^of Kitaigorodsky [25].

Also packing coefficients for all the members of the

homologous series of p-p-ethoxyphenylazo phenyl

alkanoate have been estimated, these well explain

the thermal stability variations within the homologous

series.

l. Molecular packing ⁱⁿ heptanoate ^and undecy-

lenate.

The geometrical analysis technique ^or

model approach ^of Kitaigorodsky ^{has been used for}

the a priori determination of the molecular packing

for undecylenate ^and heptanoate ^{without detailed} X-ray diffraction studies. Cell dimensions, ^which

are essential for this approach,

determined by Weissenberg ^and Precession methods (Table I). ^Unde- cylenate crystallizes ^{in two} space groups ^viz Pl and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01979004003030900

310

Table I.

Crystallographic ^data for ^the homologous ^series p-(p-ethoxyphenylazo) phenyl ^alkanoates

P21/c. ^The molecular models

constructed using

the intermolecular radii Rcarbon

^1.80 À,

and Rnitrogen

^{1.58 A, and were arranged}

^{that the}

distances between their centres corresponded ^{to unit}

cell dimensions while taking ^{care that the} projection

of one molecule falls into the hollows of adjacent

molecules with the closest possible packing. By ^means

of the symmetry ^{elements of the} particular space group, the initial orientation of the molecules ^were fixed by ^the angles ({Jl’ ({J2 and _p3

defined in figure ^1.

Subsequently geometrical calculations were made with the contacts of molecules related by ^different symmetry conditions using ^{the bond} lengths ^of

various bonds that were obtained for the crystals ^of

hexanoate and valerate by X-ray diffraction methods and refined values of _{(pl, (P2} and _(P3

estimated.

With these refined values of _{(Pl, (P2} and _(P3 for the molecular orientation, ^{the molecular} packing ^dia-

grams have been obtained and

shown in figures ²

to 7 for the ^two compounds. ^Several reflections observ- ed on X-ray diffraction photographs ^of heptanoate

and undecylenate ^{are in} very good agreement ^with the intensities of several major reflections which determine the arrangement of molecules in the case of valerate and hexanoate in their respective space groups, justifying the molecular geometry ^{and arran-} gement.

Fig. ^l.

^The designation ^{of the} angles for molecular orientation.

Fig. ^2.

Projection of the molecular packing ^of heptanoate along

the z-axis.

Fig. ^3.

Projection of the molecular packing ^of heptanoate along

the y-axis.

2. Packing coefficients and thennal stabilities. - The molecular packing coefficient defined as the ratio of the geometrical volume of the molecule to that of the volume of the molécule as determined by X-ray diffraction experiments, ^has been estimated for all the members of the series by using ^{the bond} length

valence angles ^and intermolecular contacts obtained

Fig. ^5.

Projection of the molecular packing ^of undecylenate (Pl) along ^the y-axis.

Fig. ^5.

Projection of the molecular packing ^of undecylenate (Pl) along ^the y-axis.

for crystals of valerate and hexanoate from detailed

crystal ^structure analysis and the formula for volume increments

where R is the intermolecular radius of the atom

concerned, Ri ^{is the} intermolecular radii of atoms that are valence bonded with this atom and

at

positions distance di from this atom; ^the height ^of

the cut off segment ^is

Fig. ^6.

Projection of the molecular packing ^of undecylenate (P21/c) along ^{the x-axis.}

Fig. ^7.

Projection of the molecular packing ^of undecylenate (P2,/c) along ^the y-axis.

The intermolecular radii used for hydrogen, carbon,

oxygen and nitrogen

respectively 1.17, 1.80, ^1.52 and 1.58 Á. The volumes of the unit cells of different members, the number of molecules _per unit cell, ^the densities, ^the melting points, ^the geometrical ^mole-

cular volumes and the packing coefficients are given

in table II.

312

Table II.

The volumes of ^{the unit cells} of different members, the number of molecules per unit cell.Zl ^{the den-}

sities, ^the melting points, ^the geometrical molecular volumes and the packing coefficients

3. Discussion.

It was shown earlier by ^molecular

structure studies that the change ⁱⁿ stability ^within

the homologs valerate and hexanoate could be accôunted for by ^the varying difference in the angles

between the benzene planes. ^{As the} angle ^between

the benzene planes increases thermal stability

decreases. The angle between the benzene planes goes up from 4.84 to 10.16 to 23 .2°

we pass from valerate to heptanoate. This trend should have been exhibited for undecylenate, but the thermal stability

has increased again, ^{due to an} additional dipole ^at

the end of the chain. These results could be explained

more

elegantly ^and easily ^{from the} knowledge ^{of the} packing coefficients which have been estimated. As

go up the homologous series the packing coefficient of successive members gradually decreases and the

packing coefficient itself shows odd-even phenome-

non

similar to orientational order parameter, ^dielec- tric anisotropy, relaxation frequency ^etc. [26-28].

The molecules gradually spread apart ^{in the} crystal-

line state for higher members of the homologous

series. The varying differences between the packing

coefficients of neighbouring ^{members could be} explain-

ed simply

^the tendency ^{of the} packing coefficient ^to increase when

system prefers having ^lower sym- metry. The difference in packing coefficient between

valerate and hexanoate is more than that between hexanoate and heptanoate, instead of being very

ly equal. ^{This can be} explained ^{in terms} of the addi- tional increment in packing coefficient that arises for valerate and heptanoate due to ^their preference ^for having ^lower symmetry i.e., Pl space group. As such, the difference in the packing coefficient between valerate and hexanoate is more than that for hexa- noate and heptanoate

though ^{the number of}

carbon atoms in the alkyl ^{chain of} neighbouring

members changes by ^one.

In the

of undecylenate ^the packing coefficient increases for one of the two forms which is not compa- tible with the increase in chain length

^{for the other} members, accounting ^{for the} comparably higher stability ^of undecylenate. ^{There is}

slight ^difference

in packing coefficient for the two types ^of space groups which it exhibits. The differences in packing coefficients, melting points and densities are all in agreement ^{for the two} crystalline ^{forms of} undecyle-

nate. The unidirectional orientation of molecules in the molecular packing ^{of the two} crystalline ^forms

of undecylenate ^{and the} fact that the crystallographic angles ^{of its} triclinic form

close to 900 makes it

possible ^{for the substance to} go very easily into either space group.

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