Journal of Molecular Liquids, 44 (1990) 161-174 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

161

DfE&ECTRIC RELAXATION STUDY OF SOkE HALOANISOLE IN VARIOUS NON POLA2 SOLVEXIS

CHHAVI AGGAB’ilAL, IiASHkI ABYA, J .M. GANDHl AND M.L. SISODIA

I$W&Wt~ent of Physics, University of Rajasthan, JAIPUR-302004 'a.

(Received 21 February 1989)

ABsT,UCT

The permittivity e' and dielectric loss C** have been

measured at 9.93 GRs for 2 bromoanisole, 4 bromo anisole, 3 bromo

anisole and 3 chloro anisole in various non-polar solvents e.g.

Heptane, benzene, cyclohexane carbon tetrachloride,l,4 dioxane and

Decalin at 35'C. The static permittivity Co at 300 KHz and high

frequency limiting permittivity 8, have also been measured at

35'C. The data has been analysed by Higasi's method in terms of

aO9 a', a'* and k. The relaxation time z o, Z(l) and 2 (21,

dipole momentp, distribution parameter a and free energies of

activation A F, and 4F 1 have been evaluated at 35'C. The values

of relaxation times, distribution parameter the difference

(sVaD) anapextra indicated the existence of more than one

relaxation mechanism. This has been interpreted in terms of intra-

molecular rotation of methoxy group occuring simultaneously with

'the overall molecular orientation. Comparative Study of free

-en;er@g of activation for the dielectric relaxation and viscous

flow suggest the presence of solute solvent interactions.

INTRODUCTION

Bdeasurements employing several solvents provide a good

insight into the mechanism of dipole reorientation. The relaxation

162

process in dilute solution of polar solutes in non polar solvents

at microwave frequency depends on many factors such es inter-

molecular and intramolecular rotations, molecular structure, vis-

cosity of the medium and the interaction betvreen the solute and

solvent molecules. This work deals with dielectric relaxation

mechanism of systems with two Debye's dispersion regions since the

molecules contains a freely rotatable methoxy group.

'The problem of metboxy group rotation in the substituted

an-iSoleS is Simple and a straight forward mechanism can be

postulated for *WO separate relaxation times. The me-&ho- group

has a dipole moment of its own and can rotate independently of the

rest of the molecule. The methoxy group rotation should encounter

leSS resistance than the total molecular rotation hence should

have shorter relaxation time. In other words, if a molecule

contains a polar group which is able to rotate independently of

the molecule, this rotation constitutes an alternative mode of

orientation, vhich results in broadening of the absorption peak

and lowering of the apparant most probable relaxation time to a

value leas than that expected for a comparable rigid molecule. In

dilute solution Fischer [l) found a relaxation time of 7.6~1O-~~S

set for anisole as compared to 12.3 x 10 '%for bromobenzene, the

two molecules being of approximately same size. Many other

workers [2,3,4] in pure liquid state have confirmed the findings.

On the basis of present theories given by Krishna31 aA

wansinghl5-J and by Kalman and Smyth L6 J it is expected that the

relation between log z and log 'I should be linear- This has

been verified by many workers 1'7,8] Using a sin&e solvent whose

viscosity was &an@ by varying the temperature- The dependence

of the relaxation time of a polar solute in different nO*Polar

solvents having different viscosities was studied by Srivastava

163

and Crossley L9J , Vij and Srivastava LlO] in a small range of

viscosities. They found that the relation betweenlogzand log? no

longer remains straight line. This may be due to fact that the

viscosity which we are using is not the effective viscosity of

the solution . It may be possible to get a straight line

(logz vs log?) if the d ynamicsl viscosity would have been used.

The present investigations are aimed to investigate:

(i> relaxation behaviour of some polar molecules,

(ii) dependence of relaxation time on the viscosity of solvents,

(iii) solute-solvent interactions, if any.

EXPERIMENTAL DETAlLS

The samples of 2 bromo anisole, 3 bromo anisole, 4 bromo

anisole and 3 chloroanisole were supplied by M/s. Fluka A.G.,

Switzerland. These compounds were used without any further puri-

fication. Among the six non-polar solvents,heptane, benzene and

1,4 dioxane (B.D.H. India) were dried over sodium wire and subse-

quently distilled, cyclohexane (B.D.H. India) was used after

double distillation. Garb on tetrachloride (B.D.H. India) was

dried over anhydrous calcium chloride and then distilled Decalin

(Pure, Fluka Switzerland) was used as such.

The measurements of wavelengths in the dielectric and

voltage standing wave ratio (VSWR) were made at 9.93 GHz using

a slotted line and short circuiting plunger. The calculations of

S' and C" were made following the method of Heston et al [ld

adopted for short circuited termination. Ihe accuracy of measure-

ment of C' and C" is + li6and + 5%respectively. The dipolemeter

based on the principle of heterodyne heat method was used for

the measurement of the static permittivity Co at 300 KHz.

Refractive indices were measured by a Abbe's refractometer which,

in turn, gives the optical permittivity.

164

TABLE 1

Values of Relaxation time S and Distribution Parameter (&)

Solute Solvent Solvent Viscosity

~,XlOl2 7(l)x1012 t(2)x1012 o(

CP set set set

2 BrOmO Heptane 0.374 Anisole Benzene 0.525

Cyclohexane 0.750

carbon tetra- chloride 0.797

14 Dioxane 0.987

Decalin 1.831

4 Bromo Heptane Anisole

Benzene

cyclohexane

carbon tetra- chloride

14 Dloxane

Decalin

3 BrOmO Heptane Anisole

Benzene

Cyclohexane

carbon tetra- chloride

14 Dioxane

Decalin

3 chloro Heptane Anisole Benzene

cyclohexane

carbon tetra- chloride

14 Dioxane

Decalin

8.8 7.8 16.5 0.220

12.3 9.8 19.1 0.207

13.4 11.3 17.6 0.139

28.4 17.0 31.4 0.1&l

38.6 31.2 38.4 O-C48

17.6 14.1 21.4 0.132

15.7 10.6 23.7 0.380

10.9 10.8 11.4 0.016

12.1 9.6 19.3 0.216

13.3 9.2 25.2 0.306

19.0 13.6 23.5 0.172

28.1 10.6 38.0 0.150

10.7 8.8 18.3 0.221

8.6 9.1 6.8 -0

10.8 11.7 13.4 0.190

11.3 10.6 14.0 0,087

15.2 14.1 16.6 0.051

15.5 10.7 23.2 0.024

6.7 6.5 14.9 0.231

8.5 7.8 15.5 0.200

10.7 8.5 19.7 0.256

14.3 11.8 18.6 0.144

20.5 12.0 27.8 0.254

15.5 11.8 18.8 0.274

165

The most probable relaxation time 2, and the relaxation

times 7(l) and 7: (2) have been evaluated using the methods

described by Iiigasi [12) and Iiigasi et al [13) respectively. The

dipole moment values hsve been calculated following the method

given by Higasi [14]. The molar free energy of activation, for

both dielectric relaxation as well as the viscous flow processes

have been evsluated Using the equations given by Eyring et al

1151 *

Table (1) reports the values of distribution parameter a,

the relaxation times 7 o, Z (1) and Z(2) for the four anisoles in

the dilute solutions of six non-polar solvents. The values of

dipole moment, si, aD, difference (a_raD) andrextra are listed

in table 2. Table 3 records the molar activation energies for

the dielectric relaxation as Well as viscous flow processes for

the four compounds.

DISCUSSION

There seems to be no available data on dielectric absorp-

tion study in any detail for these haloanisoles - 2 bromo anisole,

3 bromo anisole, 4 bromo anisole and 3 chloroanisole. However,

farmer and Walker [16] have studied 4 bromo anisole in p xylene

solution at different temperatures. At 40°C co value reported

by these workers is 21.9 ps and a = 0.06 which are in good agree-

ment with our investigation where 7, = 20.9 and a = 0.15 at 35'C

in the same solution.

Pure liquid study on 2 bromo anisole and 4 bromo anisole is

reported by Ghatak et al [17]. It is interesting to note that the

rele.xation time is higher in liquid state then in solution of

different solvents. This is not surprising since relaxation

166

TABLE 2

values of Dipolemoment_r, a_ , aD (a&, - aD) andpextra

Solute Solvent P a,, in Debye "D (a*, '"D Jextra 1

in Debye

2 BrCanO Heptane Anisole Benzene

Cyclohexane

carbon tetra- chloride

14 Dioxane

Decalin

4 BrclmO Heptane Anisole Benzene

Cyclohexane

carbon tetra- chloride

14 DiOXane

DeCalin

3 Braso Heptane Anisole

Benzene

~yclohexane

carbon tetra- chloride

14 Dioxane

Decalin

3 chloro Beptane Anisole Benzene

Cyclohexane

carbon tetra- chloride

14 Dioxane

Decalin

3.02 1.51 0.31 1.20 1.79

2.35 0.97 0.12 0.85 1.24

2.43 0.73 0.22 0.51 1.09

2.50 1.22 0.31 0.91 0.96

2.59 0.42 0.28 0.14 0.48

2.34 0.56 0.14 0.42 0.93

2.54 0.92 0.25 0.67 1.34

2.03 0.22 0.13 0.08 0.39

2.2s 0.83 0.18 0.65 1.23

2.27 1.99 0.31 1.68 1.31

2.18 0.81 0.25 0.56 0.94

2.41 0.91 0.12 0.79 1.28

2.37 0.91 0.24 0.67 1.37

1.86 -0.4 0.12 -0.42 0

2.18 0.36 0.19 0.17 0.62

1.92 0.87 0.31 0.56 0.76

1.69 0.39 0.25 0.14 0.47

2.12 0.73 0.13 0.60 1.11

2.41 1.36 0.19 1.17 1.54

1.98 1.12 0.16 0.95 1.15

2.25 1.17 0.18 0.99 1.32

1.94 1.29 0.34 0.95 0.86

2.05 1.14 0.28 0.85 1.02

1.90 0.56 0.11 0.45 0.84

167

TABLE 3

values of thermodynamic parameters.

toxlol* AFc in A F,., in *Fe x=-

2 BrOmo AniSOh s Kcal/mole Kcal/mole AF 7

Heptane 8.8

Benzene 12.3

Cpclohexane 13.4 Carbon Tetra- Choride 28.4

14 Dioxane 38.6

Decalin 17.6

4 BromO AniSOle

Heptane 15.7

Benzene 10.9

cyclohexane 12.1 carbon tetra- chloride

13.3

Dioxane 19.0

Decalin 28.1

3 Bromo Anisole

Heptane 10.7

Benzene 8.6

cyclohexane 10.8

carbon tetra- chloride 11.3

14 DiOXane 15.2

Decalin 15.5

3Chloro Anisole

Heptane

Benzene

cyclohexane

carbon tetra- chloride

14 Bioxane

Decalin

6.7

8.5

10.7

14.3

20.5

15.5

2.46 3.02 0.81

2.66 2.93 0.91

2.72 3.26 0.83

3.17 3.23 0.98

3.36 3.29 1.02

2.88 4.03 0.71

2.81 3.02 0.93

2.59 2.93 0.88

2.65 3.26 0.81

2.71 3.23 0.84

2.93 3.29 G.89

3.17 4.03 0.79

2.58 3.02 0.85

2.44 2.93 0.83

2.58 3.26 0.79

2.61 3.23 0.81

2.79 3.29 0.85

2.80 4.03 0.69

2.29 3.02 0.76

2.44 2.93 0.83

2.58 3.26 0.79

2.75 3.23 0.85

2.97 3.29 0.90

2.8 4.03 0.69

168

time deduced from measurement on pure liquids are almost &ways

longer than those from dilute solution% It seems that the diver-

gence is to be sought in the additional factors which operate in

the pure 1iqUid state, such as the much appreciable dipole-dipole

in&era&ion, which is negligibly small in dilute solution. Small

relaxation time in dilute solutions suggest that there is a

significant contribution of solute solvent interaction.

Since the polar molecules used in present investigation

are not spherical, therefore, the Viscosity of the surrounding

medium should have considerable effect on the dielectric relaxa-

tion time. !Pheoretically it is expected that as the Vi6CO6ity

increases it become6 difficult for the molecule to reorient,

resulting in the lengthening of the relaxation time. However, a6

the Vi6CO6ity increases beyond a certain value, a6 in the case of

Deealin, the effects other than the Vi6cO6ity i.e. solute_sOlvent

interaction6 become relatively more effective resulting in the

shortening of the _c, value [10,18,193.

Table (1) shows that the behaviour of z. vdLue6 in the

c-e of 2 bromo anisole and 3 chloro anisole is in accordance with

the theoretical expectation6 because a. the Vi6COSity increases,

it becomes difficult for the molecules t0 reorient, which result6

in the lengthening of the relaxation time. But in the case of

4 bromo anisole and 3 bromo anlsole the 2, value in heptane

come6 out to be 15.7 and 10.7presgectively which are longer than

the corresponding value in benzene (10.9 and 8Wrespectively)

despite the fact that the Vi6CO6ity of benzene is greater than

that of heptane. Since the molecule6 have been studied in quasi

isolated state, where dipole dipole interaction i6 negligible,

the only reason left to explain this anomalous behaviour is Some

kiti of interaction between solute molecule and heptane which

increases the effective viscosity of the solution subsequently

169

lengthening the relaxation time. Krishnaji and hlanSin& [5)

reported that at Constant temperature the log z and logy should

be linear. But this is not true in our case. The existing

theories ~20,21,223 can not explain the surves of log2 and log rl'

A critical perusal of table (1) shoves that relaxation

times are not much affected by the macroscopic viscosities.

Relaxation time does not increase in that proportion in Ehich

viscosity increases Grubb and Smyth 1231 and Forest and Smyth

1241 have found the similar results. lhey have analysed the

dielectric data of anisole into contributions from molecular

relaxation and reorientation of methoxy group and have obtained

values, for group rotation, of 7.2 ps (in Nujol), 7.6 ps (in

Decalin) and 6.5 ps (in Densene) at 2C°C. Where macroscopic

viscosities are 211 cp, 2.6 cp and 0.65 cp respectively. These

vahes indicates that group reorientation relaxation time is some-

what insensitive to the macroscopic viscosities. However, in 2

bromo anisole the most probable relaxation time _Co is appreciably

higher in carbon tetrachloride than in cyclohexane despite the

fact both have almost similar viscosities. This is possibly due

to the fairly large short range forces between the molecule of the

solute and carbon tetrachloride. Also, the enera value para-

meter ApC (table 3) is some&at larger in carbontetrachloride,

indicating hinnerence in the rotation of Solute dipoles in this

solution.

A close probing of tablelljreveals that where the most

probable relaxation time y. is very much different from

r(2) values the same behaviour is reflected in the values of

distribution parameter a, e.g. _co value of 2 bromo anisole in

heptane is 8.8 ps and Z(2) is 16.5 ps;the difference is quite

large,this suggeststhat more than one relaxation mechanism is

170

present in this molecule. This is confirmed by the large value

of ~~(0.22). The same molecule i $4 dioxane has z. = 38.6 ps

and Z(2) = 38.4 ps which are almost equal hence as expected the

a is very small (0.048). The similar behaviour is exhibited by

all the other systems. In the distinct case of 3 bromo anisole

in benzene solution 7. value is higher than Z(2) indicating

single relaxation process. This is confirmed by zero a value in

this case.

The finite value of a, given in table (I), for these substi-

tuted snisoles, provides at least qualitatively the direct

evidence of the existence of more than one relaxation mechanism.

Such large'values can not merely be due to the distribution of

activation energies. From the same table it can also be conclu-

ded that distribution parameter is not directly proportional to

viscosity.

A noteworthy feature of table (2) is the unusually high

values of (a.._i-aI) ) which should be negligibly small if the system

has a single relaxation time. The value of a-i should be slightly

higher than the corresponding aI, values of the system, because

a -1 involves some contributions from the atomic polarization as

well. If this difference is anomalously large, we shsll suspect

whether or not there is another dielectric absorption in the

region of higher frequencies. The extra component of the molecular

dipole moment which takes part in extra absorption is estimated

byp extra [25],Table 3 shows thatpextra has appreciable value in

all cases (indicating double relaxation mechanism) except in 3

bromo anisole in benzene solution wherepextra comes out t0 be

imaginary (this imaginary value may be due to some experimental

errors otherwise it should be zero) indicating a single relaxation

mechanism in the molecule.

171

It is evident from table (1) that the most probable relaxa-

tion time is maximum in 2 bromo anisole in almost all solvents

and h&v e separate dispersion regions. This behaviour of 2 ‘bromo

anisole may be due to steric hinderence of rotation, each gmu?

influences the rotation of the other. Perhaps their rotation

becomes partly a cooperative motion becoming similar to oscilla-

tionsabout an equilibrium configuration. Such an oscillator would

be expected to be different from the essentially free rotation

shown by other molecules of this series. Similar observations

have been made by Roberte and Smytn [2].

4-bromo snisole may be compared with anisole and other

para substituted anisoles e.g. p dimethoxy benzene, p-methyl-

anisole and p-phenyl anisole. Grubb and Smyth [23] have studied

p dimethoxy benzene in Nujol and Decalin and they have found that

most probable relaxation time is small indicating the dominence

of internal rotation only, since fixed dipole moments are equal

and oppositely directed. However, probability of molecular

relaxation can not be neglected suggesting that there may be

me:,omeric moments Klong the major axis or that the potential

ener@y barrier hindering rotation of the polar groups cause the

molecule to exist in unstable cis and trano form. The cis-form

contributing to dielectric relaxation by molecular rotation. p

dimethoxybenzene was found to have ?So/, of internal rotation at

20°C (as compared to 60% for anisole where the fixed dipole

moment raises the overall molecular contribution [23]. The case

of p phenylanisole is similar to anisole in regards to the fixed

moment component. In p methyl anisole the moment due to the

methoxy group is in opposite direction to that of fixed moment of

methoxy group. So this molecule has 706/,of internal rotation. But

the analysis of para substituted hsloanisoles as a pure liquid and

in solution has shown the predominence of molecular rotational

172

process although the internal relation also occur. Grlibb and

Smyth's [23] results show the molecular rotation contribution to

be 79% of the total relaxation process at 2O'C. Forest and Smyth

reported 877,at 60°C. Due to our ex?erimental constraints

we are not able to find the cl and c2 values. But it is evident

from table (1) that the contribution from molecular rotation

(Z (2)) is much largerin 4 bromo anisole. This may be explained

on the basis of larger fixed dipolemoment of p bromosnisole which

is 2.03 D in benzene solution in comparison to the 1.45 D for

p-aimethoxy benzene in liquid state [23.

Not only in p bromo anisole but in all the other anisoles

in our series it is evident that molecular rotation is predominent

this may be due to higher fixed dipole moment as compared to their

corresponding aimethoxy benzenes (2-j. Another factor may be

their tendency to exist in all the possible rotational isomers,

the most stable are cis and trans structures, which satisfy the

condition of maximum overlapging of p-electron cloud of the hetero-

atom with the R cloud of the double bond. The percentage of s trans

form is S@/,in 2 bromo anisole. These rotational forms contribute

to the dielectric relaxation due to molecular rotation.

The Velues of dipolemomentJI for each molecule in different

solvents given in table (2) are nearly equal. The small difference

may be due to solvent effect. These values do not show any direct

relationship with the relaxation times, althoughJu is one of the

various factors involved in influencing the relaxation mechanism.

These experimentally determined values of dipole moments agrees

well with the values calculated on the basis of vectorial addition

of group moments [26-j.

The difference between the calculated and experimental

values of dipolemoment might be due to the non-consideration of

inductive and meromeric effects.

173

By a critical examination of table (3) it is noticed that

the energy value parameter AFc is somewhat larger in carbon

tetrachloride ano 1,4 Dioxane solution of all solute molecules

a_M in some cases viz. 4 bromo anisole and 3 bromo anisole A Fg

in decalin is also high. One may say from Tunis that carbon

tetrachloride, 14 diOxs.ne,ancf in some cases .deCalin,kinders the

rotation of the solute dipoles much more effectively than

other solvents.

It is evident from table (3) that the molar free energy of

activation for viscous flow AF7 is greater than nF,, the free

energy of activation for dielectric relaxation. This is in agree-

ment with the fact that tke process of viscous flow involves

&rester interference by neighbours than does dielectric relaxa-

tion, as the latter tckes place by rots.tion only whereas the

viscous flop involves both the rolztional and translational forms

of motion. The ratio of the two energies AFG/AF '1

is less than

one and approaches unity for some of the compounds investigated,

which suggests that the lrroving units participating in the two

processes are identical and that the activation takes place in

same degree of freedom because the bonds have to be broken before

either motion is possible.

The authors are grateful to Prof. Y.P. Saksena, the Head

of Department of Physics, University of Rajasthan, for providing

laboratory facilities.

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