Polymorphism and intermolecular interactions in crystalline fluorinated alkylbenzoic acids

of Molecular Structure, 160 (1987) 229-243 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Journal

POLYMORPHISM AND INTERMOLECULAR INTERACTIONS CRYSTALLINE FLUORINATED ALKYLBENZOIC ACIDS

IN

A. K. ATAKHODZHAEV Samarkand

State

University,

70300,

Samarkand

(U.S.S.R.)

L. M. BABKOV Saratov

State

University,

I. M. ZALESSKAYA Institute

of Organic

410000,

Saratov

(U.S.S.R.)

and YU. A. FIALKOV Chemistry,

Academy

of Sciences

of Ukr. S.S.R.,

252660,

Kiev

(U.S.S.R.)

V. P. PRIVALKO

and G. I. KHMELENKO

Institute of Macromolecular Kiev (U.S.S.R.) G. A. PUCHKOVSKAYA Institute

of Physics,

Chemistry,

Academy

of Sciences

of Ukr. S.S.R.,

252660,

and A. A. YAKUBOV

Academy

of Sciences

of Ukr. S.S.R.,

252028,

Kiev (U.S.S.R.)

(Received 11 June 1986; in final form 24 February 1987)

ABSTRACT Thermodynamic and spectral characteristics of fluorinated derivatives of alkylbenzoic acids (FABA) were studied in the temperature range 100-550 K. Experimental values of temperatures, enthalpies and entropies of phase transitions, as well as hydrogen bond energy at different temperatures and in the various physical (phase) states were compared to those for alkyl- and fluoroalkyl-benzoic acids. IR absorption spectra of substances studied were interpreted on the basis of theoretical calculations of frequencies, shapes and intensities of normal modes of molecular vibrations using the published data for related compounds. The information derived was used to construct models of packing of dimerized molecules of FABA in the crystalline state, and possible causes of the absence of mesomorphism on complete fluorination of alkyl radical were discussed.

INTRODUCTION

4-n-Alkylbenzoic acids (hereafter referred to as n-ABA) of general formula 4-CH3(CHz),_ 1C6H4COOH (where n is the number of carbon atoms in the chain), which exhibit nematic-type mesomorphism beginning from IZ = 4 [l] , find wide application in the synthesis of industrially important classes of liquid crystals (LC) [ 2, 31. Experimental data on phase transitions and intermolecular interactions in such acids were reported in our previous publication [4].

0022-2860/87/$03.50

0 1987 Elsevier Science Publishers B.V.

As a natural extension of the last work, it seemed interesting to carry out a similar study of physical properties and structure of a new class of compounds, fluorinated derivatives of n-ABA (i.e., n-FABA), which contain either totally or partially fluorinated alkyl radicals, such as shown below

(compound I, n = 0, 1, 3-7,

9)

(compound

II)

Aside from the academic point of view, synthesis and physical characterization of fluorinated derivatives of liquid crystals might well prove to be of practical importance, since introduction of fluorine into organic molecules is known to increase their photo-, chemo- and thermo-stability. We have undertaken a study of polymorphic transitions in fluorinated n-ABAs (compounds I and II) using differential scanning calorimetry (DSC) and IR spectroscopy techniques. EXPERIMENTAL

Acids of type I were synthesized in two ways: by condensation of perfluoroalkyliodides, F(CF,),I (with n = 3, 4 and 6) with 4iodobenzoic acid to obtain 3-, 4- and 6-FABA, while compounds 5, 7- and 9-FABA were the products of reaction between 4-tolylmagnesium bromide and corresponding perfluoroalkylcarbon acids, F(CF,),COOH (with n = 4, 6, and 8), with subsequent fluorination of 4-tolylperfluoroalkylketones obtained with sulfur tetrafluoride and further oxidation of methyl groups from 4-fluoroalkyltoluenes with chromic anhydride [ 51. The presence of highly hydrophobic perfluoroalkyl radicals (PFAR) in these acids of type I inhibits their solubility in aqueous solutions of inorganic bases, whereas solubility in aqueous di(2-oxyethyl)-amine is retained, and this was used for a preliminary purification of these compounds by precipitation. Further purification involved multiple recrystallization from hexane and heptane solutions, and constancy of the melting point was used for purity control. Chemical composition of all acids studied was confirmed by elemental analysis for carbon, hydrogen and fluorine [ 51. The acid of type II containing a partially fluorinated alkyl radical, was synthesized by reaction of the Grignard reagent obtained from 3,3,3-trifluorochloropropane with 4-bromobenzaldehyde, by subsequent reduction of the carbinol obtained to 4-( 4,4,4-trifluorobutyl)-bromobenzene, and by final transformation of the latter into acid type II via carbonization of the corresponding magnesium organic compound. The substance obtained was purified essentially by the same route as for compounds of type I (see above) until constancy of the phase transition temperatures was achieved; its chemical composition was also checked by elemental analysis data [ 51 .

231

Phase transitions in acids types I and II, were monitored visually with the aid of the Reinhert polarizing microscope equipped with a heating Koffler table. Specific heat in the temperature interval from 130 to 500 K was measured with the aid of a home-made differential calorimeter on diathermal ( Ti), enthalpies ( AHi) shell [4] at a heating rate of 2 K min -‘. Temperatures and entropies (AS,) of individual transitions, as well as the integral entropies of all phase transitions (XAS,) for all substances (with the exception of 0-FABA and l-FABA) are reported in Table 1 for initial (powdery) samples (designated as “org”), and for their quenched counterparts (“qnc”) obtained by an instant quench of the melt heated to 450-500 K, in liquid nitrogen. IR spectra of the acids studied were registered with the aid of IR spectrophotometers, models FIS-3 and UR-20, (Carl Zeiss Jena, G.D.R.) in the range 60 to 4000 cm-’ and temperature interval 100-550 K. Spectra of various crystalline modifications, of the melt and gaseous phases, and of the solutions in carbon tetrachloride were obtained. Experimental details are given in refs. 4,6. As an example, in Fig. 1 are shown the IR spectra of 4- and 7-FABA recorded at different temperatures. RESULTS

AND DISCUSSION

As may be seen from DSC and polarizing microscopy studies, at equal values of y1 both initial, as well as quenched samples of n-FABA exhibit a smaller number of polymorphic crystalline modifications compared with the corresponding n-ABA [4]. Temperatures (T,) and enthalpies (AH,) of the solid crystal-isotropic liquid transition are systematically higher for n-FABA-type I (Table 1) than for n-ABA [4]. Finally, in contrast to n-ABA, fluorinated n-FABA-type I compounds appear to be unable to form the LC state. We observed the monotropic phase transition into the nematic phase in the course of cooling the molten, partially fluorinated acid FABA-type II, which contains more flexible radicals than acids n-FABA-type I. The sequence of the phase transitions for the former acid may be schematized, as shown below 383 K SCI-I 1r

389 K sc-II~/7& NLC

(where SC is solid crystal, IL is isotropic liquid, and NLC is nematic liquid crystal). To elucidate reasons for the different abilities of ABA and FABA to form mesogenic phases we have carried out a detailed analysis of IR spectra of these substances, since this technique is known to be one of the most sensitive in studying both molecular and crystalline structure, and the change of intermolecular interactions accompanying polymorphic transitions of crystals.

1

org qnc org qnc org qnc org qnc org qnc

org qnc

3

9

‘Temperatures

7

6

5

4

Specimen

n

Thermodynamic

TABLE

1.3 1.3

-

AH,

6.7 6.7

-

-

AS,

in SC phase

1.3 0.9 K.

5.0 3.4

-

-

AS,

-

AH,

in C,F,,+

ASi in J mol-’

260 259

-

T,

of phase transitions

in K, A Hi in kJ mol-‘,

193 193

-

T,

Transitions

parameters

290 272

299 298 -

-

-

T,

1.1 0.8

2.3 2.3 -

-

-

AH,

,C,H,COOHa

3.8 2.9

7.7 7.7 -

-

-

AS,

343 339 424 420 440 440

282 -

-

-

T,

1.1 1.1 2.7 1.4 2.4 2.1

5.7 -

-

AH,

3.2 3.2 6.4 3.3 5.4 4.6

20.2 -

-

-

AS, 439 439 439 438 459 454 465 463 473 469 480 480

T,

Melting

20.0 21.6 22.9 21.7 21.7 19.9 25.6 21.0 30.4 22.4 39.8 32.7

45.6 49.2 52.2 49.5 47.3 43.8 55.0 45.4 64.3 47.8 82.9 68.1

AH,,, AS, 45.6 49.2 52.2 49.5 67.5 43.6 65.9 56.3 70.7 51.1 103.8 85.9

IZASi

233

‘E ”

s

234

Preliminary interpretation of IR spectra of FABA, types I and II, involved comparison with corresponding spectra of ABA [4] and n-alkylcarbon acids (n-ACA) for which we have calculated frequencies and modes of normal vibrations [7, 81. For all these substances both position, shape and intensities of characteristic bands for carboxyl group vibrations turned out to be the same. In view of this finding we concluded that molecules of FABA, type I and II, form hydrogen-bonded, cyclic dimers in the crystalline state, as do ABA and ACA, i.e.,

Substitution of hydrogen atoms in alkyl radicals bonded to benzene ring by fluorine leads to the inversion of their electronic structure; namely, weak electron-donor alkyl groups transform into strong electron-acceptor perfluoroalkyl groups [9]. As a result, there occurs a radical change in the influence of such substituents on aromatic systems and other groups bonded to the latter. It seemed natural to expect concomitant changes in properties of a reaction site (in this case, a car-boxy1 group), and a corresponding increase of H-bond energy in FABA, type I. Enthalpies of H-bonds for all substances studied were estimated by the Iogansen rule [lo] from shifts of the frequency of twisting vibration, p(OH), and of the center of gravity of the band corresponding to q(OH) of dimerized molecules with respect to those for monomeric species (Table 2). Frequencies p(OH) and q(OH) of monomeric molecules of FABA as determined from spectra of solutions and of gaseous phase, were 618 cm-’ and 3565 cm-‘, respectively. As can be seen from Table 2, H-bond energy in ABA is slightly higher than in the corresponding ACA, whereas proton substitution by fluorine has little effect both on the energy and pattern of molecular selfassociation. It follows therefrom that the basic structure of molecular dimers in FABA and in ABA is the same. Evidently, one should seek for the origin of different mesomorphic properties of these acids in details of spatial structure of the radicals, as well as in intermolecular interactions which control the molecular packing in the crystalline state. When the temperature of the substance in different phase states was changed, we observed changes of the IR spectra of n-FABA, type I, at 530650; 700-960; 1410-1430 and 1700 cm-‘; location of the band maxima changed, redistribution of the spectra intensities occurred, and new bands appeared (Fig. 1). In order to be able to compare the bands cited above to vibrations of particular atomic groups in molecules of the acids studied (e.g. to vibrations of PFAR), we calculated frequencies and intensities of normal vibrations of n-FABA, type I, with n = 3,5,6, 7 (in calculations the existence of H-bonds was duly accounted for). Such calculations of vibration spectra

235

TABLE 2 H-bond enthalpies (in kJ mall’)

series of carbon and benzoic acid?

Phase state

Substance

n-ACA even odd

n-ABA n-FABA-type n-FABA-type

in homologous

I II

SC1

SC II

SC III

IL

27.3 28.6

25.6 29.4

30.2 30.1

30.3 r 0.8

SC

SCM

NLC

IL

34.9 * 1.0 36.1 34.9

33.6 2 1.0

33.6 2 1.0 33.2

32.8 * 1.0 29.0 32.3

34.0

aSC, SC II and SC III correspond to A(A’), B(B’) and C(C’) in notations of ref. 29, SCM is the metastable phase produced by quenching.

of molecules containing perfluorinated alkyl chains, to our knowledge, were carried out for the first time. (Polytetrafluoroethylene, PTFE, and several other compounds containing up to two fluorine atoms or CF,-groups, are the only exceptions. For these substances frequencies and modes of normal vibrations were calculated [ 11-131.) An additional difficulty is caused by the paucity of data on molecular and crystalline structure of fluorinated compounds. It is proved that molecules of PTFE in the solid state are in a helical conformation with 15 carbon atoms per turn, which is a consequence of steric hindrances resulting from substitution of hydrogen atoms in a hydrocarbon chain by fluorine [ll]. It is also established that molecules of 4fluorobenzoic acid in the crystalline state form cyclic dimers (with H-bond distance, 0. * *H-O, 2.618 a), the plane of the latter coinciding with those of benzene rings to within 5” [14]. These sparse pieces of information served as a basis for a geometrical analysis of the model molecule of FABA used in our calculations. In the present paper to calculate the IR spectrum of n-FABA-type I we solved the direct mechanical and electro-optical problems in valence force field approximation. In such calculations we used a set of programs described in ref. 15 and the method of splitting the polyatomic molecule into separate fragments [ 161. This method permits the synthesis of the vibration equations for molecules containing up to 160 vibrational degrees of freedom, from pertinent equations for separate fragments which are stored in the external memory of a computer. Where necessary both atomic masses and bond lengths may be varied at junction points, and excessive vibrational coordinates are rejected. The maximum number of fragments is 10, an equation for a single fragment is of the order of about 80, and the number of atoms in a synthesized molecule should not exceed 52. These are the limitations of

236

computational program which was realized on a digital computer, model ES 1040 (Computers, Minsk, U.S.S.R.). Three model molecules were chosen as library fragments, i.e., benzene, the carboxyl group involved in the H-bond (COOH- a-H), and the CF,-group whose geometrical parameters are well established in structural chemistry. Relevant force constants were taken from refs. 13,17, 18, and electro-optical parameters from refs. 17-19. A vibration equation for the whole n-FABAtype I molecule was synthesized from vibration equations for each of the fragments cited above, so that the CF,-group was coupled a required number (n - 1) of times to the initial group, CF,-C,H,-COOH* * *H, assuming a non-shadowed configuration of the alkyl chain. Initial values of the force constants and electro-optical parameters were then corrected to achieve better correspondence between measured and calculated spectra, in particular at the coupling points. Shown below are the selected values of dipole moment derivatives by vibration coordinates which were used in our calculations apcc/aq,,

= -0.5;

ap,,/aG

= 0.26;

ap,,lap

= 0.25;

apcclay

= -0.4

In Fig. 2 the experimental IR spectrum of crystalline 7-FABA-type I recorded at 300 K is shown together with the theoretical spectrum corresponding to vibrations of the model planar molecule, CF3(CF2)&H4COOH** *H. One observes a satisfactory agreement between both spectra for their shape, intensity distribution and the location of IR bands. Proposed assignments of the IR absorption spectrum of 3-FABA-type I are listed in Table 3. The validity of these assignments for the majority of characteristic bands in the IR absorption spectra is assured by results of calculations, by experimental measurements and their comparison to the spectra of related substances, n-ABA, perfluorododecane C2FZ6, PTFE recorded by ourselves, as well as published data on IR spectra of p-substituted benzenes [ 18, 201.

3600

1800

1600

1400

1200 IO00 Y,

Fig. 2. Theoretical

800

600

400

200

cm-’

(a) and experimental

(b) IR spectra

of ‘I-FABA.

23'7 TABLE

3

IR absorption

LJ(cm-‘)

bands for crystalline

Vibration

Vibration

number

shape

after

ref.

3-FABA-type

I (at 300 K) Vibration

” (cm-‘)

Vibration

number after

22

ref.

shape 22

3115

w

2oc

dC-H)

1023

m

18a

y(C-C-“C),

3080

w

20a

3070

sh

2

MC-H) q(C-H)

972

sh

17a

P(C!“CH) y(CxC”C)

2820

cg

MO-H)

946

mb

2690

m

905

s

Q(C-C),^I(CCW

2570

m

855

s

y(C”C”C),

1955

w

817

m

1707

VW

Q(C=OL Y(C--c-0) P(C--OH), Y(OCO)

812

sh

805

sh

772

s

750

s

/3(C=CH),

715

s

693

w

673

m

1692

VW

1680

s

1636

sh

1612

w

1575

m

complex

i 8a

Q(C=C),

sh

1523

sh

1516

w

m

1350

s

1324

m

1309

sh

1300

sh

1288

s

1279

s

1227

VW

1206

s

1188

sh

1180

s

1144

m

1129

sh

P(CH) ***H), y(C-C-O), y(C”C”C)

fl(C=CH) w m

Q(C”C), -((C”CKC)

543

sh

535

In

KC--OH), fl(C=CH).

495

w

460

m

19c

P(C=CH)

445

w

P(C=CH)

405

m

14

@(C-H). y( C”CZC

338

sh

314

s

Q(C---0). P(C-CW, Q(C--W QV-F) Q(C--F),y(FCF) Q(C--F),Q(C-C) y(CCF),y(FCF)

290

s

264

m

254

w

228

m

264

m

175

s

P(C=CH)

156

s

Q(C-C),T(FCW Q(C--F),T(CCF)

130

s

108

m

p( C=CH)

s

P(CH).

3 I

9a

)

P(C=CH). Q(C-0). ‘)

1118

VW

1108

w

18~

1075

m

1

1043

wb

Q(C-C),K

Y(OCO). P(C=O

560

s

1412

4 12

593

i 1430

P(CW. 17c

fl(C=CH), 19a

y(CCF)

10a

Q(C=C),

EC

i 1563

y(C-C-C)

P(OW

P(CX) K, y(CCF) P(C=O*.‘H) 11

16a

PGW, K @(C=O***H) -dCCF) y(CCF) y(CCF)

15

P(C-CH) ^((CCF)

y(c”c=c), NC-CH)

80

y(c1’c”c), K

y(C-C-C)

Q(C--F),-/(FCF) QCC-C,,

a~ - strong, m - medium, w - weak, v - very, b - broad, sh - shoulder: cg - center of gravity. Designation of coordinates after ref. 18. b Q(C-C), q(CH) are stretching vibrations; 7 (C=C=C), p( GC-H) are in-plane, and p (CH), K out-of-plane bending vibrations of the benzene ring. “Q*(C-C), Q(C=O), Q(C-0), q(OH), q(O*** H) are stretching vibrations; 7(OCO), 7(OCC) are in-plane, and p(CC),p(OH) are out-of-plane bending vibrations of carboxyl groups. d Q( C-C,), Q(C-C), Q( C-F) are stretching vibrations; 7(CCC), 7(CCF), ol(FCF) are bending vibrations of fluoroalkyl radicals, $ Cl?, are rocking vibrations.

238

It is pertinent to clarify here which details of the vibration motion in the substances studied are most characteristic for their spectra. Typical in this case is the delocalization of vibrations of not only aromatic and dimer rings but also perfluoroalkyl chains (vibrations of CF,-group included). Such a delocalization effect originates from replacement of hydrogen atoms by heavier fluorine atoms, which results in the appearance in below 1200 cm-’ of a large number of bands corresponding to stretching and bending vibrations, Q(C-F), y (F-C-F), y (C-C-F), which by numerical values of frequencies closely resemble those for vibrations of the alkyl chain backbone and bending vibrations of benzene rings (both in-plane and off-plane). The higher intensity of these bands as compared to those in corresponding alkylbenzoic acids is very likely the reflection of the much larger dipole moment of a fluorinated alkyl group compound with that of an ordinary alkyl radical. Standard values of the dipole moment of the C-F bond are either 1.39 D or 2.15 D [ 211 depending on the sign of the dipole moment of the C-H bond (0.4 D by magnitude); the dipole moment of the CH3 group (in toluene) is 0.37 D while that of the CH3 group is 2.54 D. Therefore, substitution of hydrogens by fluorine atoms substantially increases the polarity of the resulting perfluorinated alkyl radical (and of the whole molecule as well) which leads to a sharp increase of intensities of IR bands in the corresponding range of the spectrum (Fig. 2). Both location and shape of the bands in the range for C-F stretching vibrations are little affected by the change of PFAR length (that is, in the range 1250-1150 cm-‘), whereas the number of bands corresponding to bending vibrations (below 1000 cm-‘) increases with n (Fig. 1). However, in contrast to the case of ABA, isolation of a succession of bending vibrations of (CF,), chains from spectra of FABA-type I proved impossible due to their intensive overlap with vibrations of a carbon-carbon backbone. However, the intensive band at about 200 cm-’ which according to calculations for PTFE [ 111 may be identified as a rocking vibration J/cF, of an infinite perfluoroalkyl chain with symmetry E. A similar band $cn, for vibrations of a chain of infinite (CH,), is located at 720 cm-’ [6]. As was the case for alkyl radicals, the location of cited bands in spectra of FABA depends on numerical values of n (e.g., 228 cm-’ for n = 3; 218 cm-’ for n = 4; 212 cm-’ for n = 5; 205 cm-’ for n = 6; 204 cm-’ for it = 7; 202 cm-’ for n = 9) and 201 cm-’ for PTFE), while its intensity increases in proportion to the number of CF, groups. It thus appears that this band may be used for determination of the length of PFAR. We also mention a set of narrow intense bands in the range 700-850 cm-’ (Fig. 1). Both our calculations for a planar, completely extended chain of the type (CF,),, and similar calculations for PTFE [ll] suggest that in this range no IR absorption is to be found. Liang et al. [ll] attribute the bands observed to rotational conformers of a perfluorinated polymethylene chain (CF,),. As can be readily seen from Fig. 1, the number of bands in that range increases in 7-FABA compared with 4-FABA, while these bands are partly “frozen out” as the temperature is lowered. These results are thus consistent with the above assignment.

239

On the other hand, increase of the chain length is accompanied by an increase in the number of possible rotational conformers and, correspondingly, should lead to a large number of polymorphic crystalline modifications in n-FABA-type I (Table 1). It appears that the phase transitions in the SC state of n-FABA-type I involve the change of PFAR conformation and perfecting the structure of dimerized molecules. The latter conclusion is consistent with the observed narrowing of bands corresponding to the dimer ring vibrations on cooling to 100 K, as well as with a slight strengthening of H-bonds (to about 1.7 kJ mol-‘). We also draw attention to the specificity of benzene ring vibrations as reflected in the IR spectra of n-FABA. In the majority of cases we managed to find the correspondence of the bands observed with various vibration modes of p-substituted benzenes (see Table 3 with assignments made according to Wilson’s nomenclature [22] ). The specificity referred to above correlates rather well both by frequency and by shape with other data [18,20]. Of particular relevance here is the narrow but quite intense band in the vicinity of 1080 + 7 cm-’ the location of which remains unchanged with change of n. According to our calculations, this band may be attributed to VI vibrations of benzene rings which are specific by frequency in spectra of n-FABA-type I, contrary to claims otherwise [ 181. Similar conclusions may be drawn about ~12 vibrations at 675 f 3 cm-’ and ~16a at 405 cm-‘. The location of IR bands corresponding to stretching vibrations Q(C=C), V19a at 1515 f 1 cm-‘, v8b at 1578 * 2 cm-’ and ~8a at 1615 + 3 cm-’ is the same for all compounds studied. However, one may notice a substantial intensity redistribution for these bands following the substitution of an alkyl radical by its fluorinated counterpart: in ABA and FABA-type II the intensity ratio for these bands is the same at 0.4:0.8:1.0, while linking of electronegative PFAR to a benzene ring increases the intensity of the second band and decreases the intensity of the third, so that the ratio becomes 0.7:1.0:0.3. The latter ratio is essentially independent of the radical chain length and may serve as a tool for rapid identification of substances of the kind FABAtype I. The location of the stretching vibrations band Q(C-H) proves sensitive to the increase of negative electric charge of a substituent and shifts from 3045 cm-’ (~2) and 3070 cm-’ (v20b) for ABA to 3070 cm-’ and 3115 cm-‘, respectively, for n-FABA-type I, while vibration v20a remains unchanged at 3080 cm-‘. Specific for n-FABA-type I compounds is the intense band at 288 f 2 cm-’ in the long-wave range of the spectrum (vibration ~15 in Wilson’s notation), the counterpart of which in IR spectra of ABA and FABA-type II is the band at 255 cm*‘. It should be emphasized that all cited features are observed in ABA and* n-FABA-type I series starting from II = 3 and are independent of the radical chain length. When the distance between the benzene ring and polar CF3 group is sufficiently large as in compound II, the effect of the latter on electrons of the former is virtually nil. On the basis of the above data for thermodynamic properties and spectral characteristics of n-FABA-type I we made an attempt to construct a model

240

of possible molecular packing in the SC state. As can be seen from Fig. 3, a plot of integral entropy of transitions EASi in the function of the number of carbon atoms n in the chain backbone of PFAR seems to be split into two approximately equal sections with discontinuity at n = 5 and 6. Similar discontinuities were reported for homologous series of cholesterol esters [23], ABA [ 41, alkoxybenzoic acids (AOBA) [24], the location of discontinuity being dependent on relative dimensions of molecular backbone and alkyl radical. Transition from one section into another through discontinuity is known to be accompanied by change of molecular packing in the crystalline unit cell [ 251. Thus, while at the first section of ~ASi vs. rz plot the major factors are the interaction forces between stiff molecular backbones, with increasing length of alkyl radical interactions between alkyl chains become dominating. The most favourable conformation for the dense packing in the crystal is the extended, planar trans-isomer; therefore, at a definite chain length a new type of molecular packing becomes optimum, this effect being responsible for a change of crystal structure as well as for differences in mesomorphic properties of these compounds [ 251. It follows from these and similar considerations that in the case of n-FABAtype I the first (i.e., below n = 6) and the final (above II = 6) members of the series must have different patterns of molecular packing in the crystal. We have carried out the model calculations of the shape of dimerized molecules of n-FABA-type I taking into account the intermolecular radii of all atoms. The central, stiff fragment of a molecule consists of two benzene rings and of a cyclic dimer lying in the same plane (as shown by Colapietro et al. [ 141,

100 -

90 -

80-

a” w

70 -

60-

I

23456789 " Fig. 3. Dependence type I.

of integral

entropy

of phase transitions

on PFAR

length of n-FABA-

241

for 4-fluorobenzoic acid the angle of rotation of the rings about the C-C bond does not exceed 5”), and its dimensions are 3.4 X 6.4 X 16.1 (a”). The PFAR fragment may be modelled as a cylinder of diameter 6.4 A, the length of which varies according to n, viz.: 5.2 a for 3-FABA and 12.8 A for 9-FABA (for illustrative purposes we remark that the cross-section of PFAR is about twice as large as that of the alkyl radical, at the same chain length). Therefore, dense packing of molecules of n-FABA-type I with a parallel arrangement of neighboring PFAR appears to be unlikely, since in such a case central fragments of the molecules would be too far apart. Strongly-bent conformers of PFAR in the SC phase are also very improbable; from one band, this would require a still larger cross-section while from the other hand, the potential energy barrier for rotation about C-C bonds increases almost two-fold in fluorinated alkanes [19]. Taking into consideration the negative electric charge of PFAR, one has to conclude that the optimum packing would require that benzene rings be surrounded by PFAR. Consistent with the above arguments are the two following models of molecular packing in the SC phase of n-FABA-type I (Fig. 4) for two values of the number of difluoromethylenic units n corresponding to two sections of the plot in Fig. 3. As follows from model calculations, in the case of shortchain n-FABA-type I (n = 3-5) the space between main molecular backbones may be occupied by PFAR of two neighboring molecules in the interlayer, whereas for longer-chain members (n = 6-12) the space available is limited to only one PFAR. Dumbbell-shaped molecules can be easily interlocked with the ensuing possibility of formation in the SC state of weak hydrogen bonds of the type, C--F*** H-C [26]. The longer the PFAR, the higher would the overall contribution of such interactions to the potential energy of the system. This effect is likely to account for the observed increase of both temperatures and enthalpies of melting (i.e., those properties which correlate with energy of intermolecular interactions) with increase of PFAR chain length in the series of n-FABA-type I. This is to be contrasted with the case of the ABA series, in which chain-length increase in the more flexible

Fig. 4. Models state.

of molecular

packing

of 4-FABA

(a) and 7-FABA

(b) in the solid crystal

242

alkyl radical leads to a looser molecular packing in the crystal and to lower T, [4]. On the other hand, different molecular packing involving close proximity of PFAR from neighboring molecules, like molecular packing in PTFE, would require lower levels of interaction energy in crystalline n-FABA type I than in corresponding ABA: as shown elsewhere 1271, melting enthalpy of perfluorinated chains (CF,),, or PTFE, is approximately twice as low as that for methylenic units in the polyethylene crystal. As can be readily understood, the proposed models of molecular packing for the n-FABA-type I series (Fig. 4) can be used to explain the observed dependence of ZAS, on n (Fig. 3) in the following way. Both straight sections on the latter plot have similar slopes (that is, the same increment, AXAS, = 11.2 + 1.5 J mol-’ K, per single CF, group) which is typical for isostructural crystals. (We may recall that the same increment AY,ASi was found for isostructural crystals of cholesterol esters, AOBA and ABA [4] .) It can be suggested that in the case of sufficiently long PFAR (e.g., it = 12) in crystalline n-FABA-type I a new type of crystal packing should be expected, wherein the major factor would be the interactions between adjacent PFAR. In such case a further increase of PFAR length should lead to lower melting temperatures, as was the case with ABA [4]. Direct checking of this hypothesis would require structural investigations by means of X-ray diffraction studies. Based on the proposed model of crystal structure of FABA and on the temperature dependence of recorded IR spectra, we may speculate on the possible causes for the absence of mesomorphic properties in the series, n-FABA-type I. First, bulkier and intrinsically stiffer PFAR are more effectively interlocked with neighboring molecules. This must stabilize molecular dimers of n-FABA-type I in comparison to those of ABA and thus arrest the positional disordering of molecules in the vicinity of T, for the SC phase. It is also probable that formation of weak bonds of the type C-F* * -H-C increases the overall intermolecular energy and thus hinders the intermolecular slippage during solid-melt transition. Next, hydrogen bonds of the type 0-H. **0 are slightly stronger in n-FABA-type I than in ABA (Table 2). Enthalpy of H-bonds decreases on heating; however, we did not observe the appearance of opened molecular associations and monomeric molecules in crystalline n-FABA-type I up to T,. This is an indication of absence of dynamic equilibrium between these structural species which usually accompanies the formation of a mesophase [4,281. These effects are nonetheless relatively unimportant for a partially fluorinated compound, FABA-type II; as follows from the analysis of its IR spectra, locations of characteristic bands for vibrations of benzene ring, enthalpy of H-bonds, etc., are very similar to those in spectra of ABA. It is quite natural therefore that for this compound a mesophase was found (see above). We are planning to extend our studies on related compounds with various degrees of fluorination, including both benzoic and cyclohexanoic acids, with the aim of discovering new, stable liquid crystals.

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