Indian Journal of Pure & Applied Phys ics Vol. 41. January 2003, pp. 46-5 I

Study of dipole moment of 18-crown-6 in CCI4 solutions at 298.15 K K J Patil *, P K Patil , R R Kolhapurkar & D H Dagade

Department of C hemi st ry, Shivaji University, Kolhapur 4 16 004

Received 14 June 2002:

Dielectric constant (£), density (d ) and refrac tive index (n) measurements at 298. 15 K ror solutions or l li -c rown-6 in

CCI4 (0. 5 to 25 x I o··' weigh t rract io n of solute) have been reported. The data , along with partia l mo lar vo lume data ror so lute are subjected to analysis using various eq uations to obtai n solute po larizati on and dipole moment va lues at tlnite and inll nite ly di lute concentrations. The dipole moment value of 3.5 D has been compared wi th gas-phase value and the va lues in solutions of benzene and cyclohexane, repo rted earlier. The unexpected large va lue of dipole moment obtained has been discussed in terms or co nformation adopted by the cycl ic po lyether in a so lvent environment. It is further explained that , in soluti on ph ase if 18C6 molecules ex ist in C; symmetry conformati on (thi s cannot have dipole moment), the atomic polarization via -CH2- chains must be mak ing large contributi on to account for the observed solute polarization.

1 Introduction

Macrocyclic polyethers have been the subject of muc h attention since these cyc li c compounds and their deri vat ives ex hi bit remarkably high specific ity in binding to severa l cationic s pec i es 1 ·~ . Crowr\ ethe r 18-crown-6 ( I ,4,7, 10, 13, 16 hexaoxacyc lo octa­decane) ( 18C6) is the a rchetype of the great famil y of crown e thers. The most prominent feature of 18C6 is, its capability to complex alkali meta l ions in a po lar cage of oxygen by un-directed coul omb forces (spherical recognition) and to transport them eventu a ll y into lipophilic phases. The c rown is able as well to bind neutra l polar guest molecules by H­bonds and direc ted dipo lar forces (molecular recogniti on)". Thus, many theoreti c ians in the ir s imulati on and molecular dynami c studi es have considered 18C6 as the most simple mode l system for understanding the fundamental enzyme properti es.

In recent yea rs, the research work from thi s laborato ry was concerned with the volumetric, viscos ity, NTR spec tral, se lf-di ffusion coeffic ient and NMR re laxation properties of 18C6 in water and in non-aq ueous solutions7

.11

• It has been shown from X-ray diffraction and Raman spectroscopic studies 12

· " that, 18C6 molecules attain D 1d

conformation in the presence of certain guest or solvent mo lecu les in so lid state as we ll as in liquid phase 12

· ". ln contrast, pure 18C6 exis ts in C; conformati on, in the gas and so lid phases a long with lit t le proporti on of C, while, Ha & Chakraborty'"

have inte rpreted the ir Monte-Carlo simulati on results in liquid CCI 4 , in te rms of C; conformati on for 18C6. Kowa ll & Ge iger 15 from s imul ation studies, and the authors of present paper from experimenta l NIR spectral data have shown that, in aqueous 18C6 solutions, at least four water mo lecules are H-bonded with the oxygen atoms of 18C6 (two bridging and two singly H-bonded). These wate r molecules along with hydroph obic inte rac tion with so lvent stabili ze the D3d

conformation. The 0 20 relaxation studies of aqueous 18C6 and KBr so luti ons further indi cated that, the e lec trostatic interact ion of K+ dominates and replaces water molecule during complexation. Vi scos ity studies indicated that, in CC14 so luti ons, the B coefficient of Jones-Do le eq uation remain constant as a function of temperature, while in water, the B coefficient values are high and have -(dB!dD behaviour. Thi s indicated that , in CCI.~

so lutions, 18C6 might be in C; conformat ion w hil e, it is in D1," in aqueous solutions.

Dipole moment is a mo lecul nr property and depends upon the chnrge distribution and shape of molecule. S i. milarly , the die lectri c properties are to so me ex tent structure-sensiti ve, that is, they depend upon the corre lation or interact ion between the e lectri c dipoles, both pe rmanent and induced. In order to gain additi ona l informat ion about C; conformat ion, n lite rature survey of' dipo le moment properties of 18C6 is made and found that, the d ipole moment value is 2.6 D in ga · phnse wh il e, it is 2.76 D in cyc lohexane and the temperature

PATILet a /.: DIPOLE MOMENT OF 18-CROWN-6 47

vari ation has been inte rpre ted in te rms of the presence of mixtures of conformers 1r,·17

• The molecul ar dynamics s imulation studies in cyc lohexane have indicated shifts in the spectrum of conformations, and the va lue of average root-mean square dipo le moment of 18C6 is in agreement with the experiment , on the bas is of two pote ntia l mode ls. However, thi s was achieved by studying modifi ed c harge-de nsity di stributi on for 18C6 molecules 1x. Since the potentia l assumed or derived should ta ke into account the e nvironmental effects as we ll , the re is a need to have s imila r studies in vari e ty of solvents. The refore, it was thought inte resting to measure dipo le moment prope rty which can be re la ted to confo rmations of 18C 6 in solu tions. In thi s paper, the measure ments of die lectri c constant , refracti ve index and de nsity pa ramete rs in so luti ons of 18C6 in CC I~ have been reported . The de ri ved dipo le moment value is di scussed on the bas is of conformation and the prope rti es o f 18C6.

2 Experimental Details

18C6 (99% pure) procured from M e rck­Schuc hardt was used w ithout further purificati on. The contac t w ith atmosphe ri c mo isture was avo ided by handling 18C6 in a dry box, fa bricated in our labo rato ry. CCI~ (HPLC grade, M e rck) was used after di still at ion . A ll the solutions in CCI4 we re prepared on mo la lity bas is in g lass-stoppered bottles u sing M ettle r Tol edo (AB 204-S mode l) , we ighing

ba lance hav ing ±0. 1 mg readability. The concentrati ons were converted into mola rity,

weight-fracti on (0.5 to 25 x I o-' range) and mole­fract ion sca les using de nsity data and u sua l express ions.

The refrac ti ve index (n) measure ments were carri ed out, us ing a Ru ss ian the rmostat

re fractomete r, at 298. 15 ± 0 .02 K . The va lues of (n) were obta ined in day light and have the accuracy of

± 5 x I0-4• It was observed that , in the stud ied concentrati on range, n did not vary muc h and re ma ined more or less constant w ithin the limits of expe rime nta l uncertainty.

The de nsity measure ments were carri ed out by us ing Anton Paar osc illa ting tube d ig ita l

de nsitometer (mode l DMA 60/602), a t 298. 15 ±0.02 K. The re produc ibilty in the de nsity va lues was of

the o rde r of± l x I0-2 Kg m·1. The de ta il s about the calculati ons of the partial mo lar vo lumes usi ng the de nsity data, a t finite concentrati ons and at infini te dilution are described e l sewhere 1 ~.

The re la ti ve permittivity(£) measure me nts were

made at 298 . 15 ±0.02 K and at a freque ncy of I .8 MHz using Toshniwal (T ype N o RL 09, S r No 5243) dipo le-mete r. The cell was a two te rm ina l body ha ving e lectrodes made up o f brass and then pl ated w ith go ld (to avo id corros io n) a nd hav ing capac ity for air, equa l to 19 .1 pF. The ce ll was thermostated through the outer jacket, by c ircul ating wate r from a constant tempe rature c ryos tat (Julabo­F25) hav ing the accuracy in ma inta tntng

te mpe rature constancy of ± 0 .02 °C. The ce ll was initi a lly calibrated using the puri fied liquids li ke, be nzene, to luene and CCI ~, as well as mi xtures of CHClr C r,H r, a nd p-nitro to luene-CC I ~ . The d ipo le mome nt va lues fo r c hlo roform and p -nitroto luene were found to be 1.15 and 3 .9 D at 298. 15 K w hich agreed very well w ith the best lite rature data availab le 211

•21

• T he instrume nt is o ld but w ith carefu l experimentati on, one could determine the d ipo le

Table I- Dens ity, refract ive index and re lative permitt ivity data for I RC6 in CC I ~ at 298. 15 K

Ill IOOOw d 10-.1 X Vm £ N 10·1 X Po D '~liD / Kg mor 1 /kg m·1 /mm1mol·1 /mm.1 mo-r 1

0.0000 0.000 1584.35 97.09 2.2279* 1.4590* 332.3 * 0 .0 170* 0.0 11 7 3.093 1582.30 97.35 2.2969 1.4591 330.9 0.02X4 5.44 0.01212 5.576 1580.70 97.55 2.303 1 1.4593 329.7 0.0293 4.22 0.0289 7.588 1579.40 97.71 2.303 1 1.4594 328.8 0.0293 3.62 0.0497 12.974 1575.95 98.15 2.3282 1.4596 326.3 0.0332 3. 19 0.0600 15.686 1574.26 98.36 2.3407 1.4597 325.1 0.0351 3 07 0.0760 19.689 1571.66 98.69 2.4222 1.4599 323 .3 0.0478 3.57 0.0824 2 1.3 15 1570.62 98.83 2.4473 1.4600 322.6 0 .05 15 3.66

*Values at zero concentration

48 INDIAN J PURE & APPL PHYS, VOL 41 , JANUARY 2003

moment va lues in non-aqueous so lvents with suffi c ient precision and accuracy .

The data of re lat ive pe rmitti vity, density and re fractive index for various concentrations at 298.15 K are g iven in Table I .

3~ ,--------------------------------.

320 c__ ____ _,_ ____ ___._ ______ _.__ ____ _._ ____ ___J

0 000 0 005 0 010 0.015 0.020 0.025

Fig. I - Vari ation of polarizat ion of 18C6 in CCI4 as a function of weight fraction o f 18C6 at 298. 15 K

3 Results and Discussion

The calcul ati ons o f the average dipole moment

(Jl) we re made by applying three diffe rent equations as g iven below. It is assumed that, 18C6 is accommodated in a sphe ri ca l cavity of a cont inuum

of liquid CCI 4 although in final ca lculati ons of Jl , a shape factor correc tion, as carri ed ou t by Buckingham"" , can be made. However, the authors deferred w ith such a correction , as it was diffi cult to visua li ze the exact conformation and shape of 18C6 in soluti ons . In the first me thod, the apparent dipole moment value (P 2) for 18C6 was calculated by using mi xture law and Eq . ( I ) :

{ (P -~)l

P1 =M 1P2 =M 1 ~+ w J . . . ( I )

where P 1 and P a re the specific polarization of so lvent and soluti on, respective ly, whil e wand M 2,

respect ive ly represent weight fraction and the

molecular weight of 18C6. P 1 and P were estimated using the Eqs (2) and (3), respective ly as:

P. _ ( c1 - I )v1 ,-

(c, + 2)

p = _( c_-_l_)_v

(c + 2)

. .. (2)

.. . (3)

where £1 and£ are the diel ectri c constants of so lvent

and so lution , respective ly, while U 1 and u represent the corresponding spec ific vo lumes. In these

equat ions, .small e tTOrs in £ and u cause di sproportionate ly large e rror in P2, in dilute soluti ons. To avoid the consequent di sto rtion of the Pr w plot in it he region of dilute solu t ions , smoothed

values of£ and u were ca lculated , using Eqs (4) and (5) and were substituted in Eq. (3) fo r soluti ons contai ning on ly monomers" ~ .

v = v1 + f3w

. .. (4)

. .. (5)

The resulting Prw pl ot shown in F ig. has the

same form as that of expe rimenta ll y de te rmined £-U plot (Fig. 2) , and is thought to justify this approach.

The va lues of coefficie nts a and f3 are 9.4296 and 0 .2588, respec tively . In reality , a non-linear behav iour was observed due to some stacking type interaction , in the studied concentration region (weak solute-so lute interac tion s). Suc h stacking type of inte rac ti ons are a lso revea led from osmotic and act ivity coeffic ient study of I 8C6 in CCI 4

so luti ons 1 ~ . The P1 , value was obtained by ex trapo lating P2 va lues to infinite di luti on. The same can be obtained by use of Hal ve rstadt-Kuml er equati on24

:

p = 3M 1v~+ M l v1 + (J)(c1 -I ) 2oo ')

(c1 +2t (c1 +2)

... (6)

where a and~ defined as (oflow)w=o and (au/ow)"'='" respectively, were calcul ated from the equati ons a= L(£-£ 1) /I,w and ~ = I,(u-u 1)/ I,w. The P2 , value obtained is -344, while the ex trapo lated value is 332.34 cm1 mo le·1

• The same also can be obta ined by using mole fraction statis ti cs. E lect ron po lari zati on, EP2 were obta ined from refracti ve index values and a ll owing I 0 % of tP 2 fo r the atom polarization, the molar o ri entati on po larization of 18C6, ,,P1 was ca lculated from Eq . (7) .

PATILet al.: DIPOLE MOMENT OF 18-CROWN-6 49

0 p2 = p2~ -J.I Ep2 ... (7)

and the dipole moment:

11 =0.0 128 I( 0 P2 T)~ ... (8)

30 ~---------------------------,

2.9

28

2.7

26

"' 2.5

24

23

2.2

2.1

2.0 L_ _ _.J.... __ _,__ __ ._ _ _.J.... __ _,__ _ ___l

0 000 0 005 0.010 0.015 0.020 0 025 0.030

Fio 2- Vari ation of dielectric constant (E) as a functi on of o· weight fracti on of 18C6 in CCI ~ at 298 . 15 K

The dipole moment value of 3.51 0 has been calculated.

In the second method, the authors applied Guggenheim25 for Eq. (9).

. .. (9)

where

c--1 n2 - l 3(c- -n 2)

D=-----= ?

c-+2 n 2 +2 (c-+2)(n- +2) .. . (I 0)

and Cis the concentration of 18C6 in moles per unit volume. Hence, if D is plotted against C, the inte rcept yields the value of D,, while the initia l s lope gives the value of (A P2-EP2+1.P 2) or [(APz-EP2 + (4rtN!il9kT)]. The variation of D parameter as a function of C of 18C6, is shown in Fig. 3. Using the slope value and Eq. ( II ), the dipole moment value of 3.49 D for 18C6 has been calculated, assuming that, Ap2 = EP2.

J..l= slope x 9kT

4nN . . . ( II )

0.10

0 09

0.08

007

0 06

Cl 0.05

0.04

003

0.02

0.01

0.00 L__..___~-~--'---~-~-~--

0.02 0.04 0.06 0.08 0. 1 0.12 0.14 0 16

C x 1000 /mole cm· 3

Fig. 3-Vari ation of D parameter (Guggenheim equ ati on) as a function of concentration of 18C6 in CCI4 at 298. 15 K

Of the various modern developments of the Onsagar & Kirkwood treatment in the field of dielectrics, that of Frohlich26 is most generally accepted and used successfully by Stokes & Marsh27

for calculation of dipole moment of ethano l di ssolved in non-polar solvents. Eq. ( 12) is used in

the present work as a third method to obtain 11 for 18C6. The Eq . ( 12) is :

V,(c - I)= [(1- x)V1 (c-1 -I) l + [ xV~ ( c-~ -1)1 3c- c-1 + 2c- c2 + 2c

. .. ( 12)

where V,, x, V1, V2, £2' , g represent mo lar volume of solution, mole fraction of 18C6, molar volume of solvent , molar vo lume of so lute and square of refractive index of solute by adding I 0 % to the molar refrac tion to take account of vibrational modes, not excited by visible radiation and Kirkwood correlation factor , respective ly. The data used for the application of Eq . ( 12) are summarized in Table I . In the last co lumn of Table I , the individual values of the parameter 11 are collec ted at vari ous concentrati ons of 18C6. If K is assumed to be unity and the lowest concentration value is

so INDIAN J PURE & APPL PHYS, VOL 4 1, JAN UARY 2003

neglec ted ( it may in volve large error), the average va lue of the dipo le moment for l 8C6 is found to be 3.54 D . It is estimated that, the e rror in vo lved in the

11 va lue is of the order of ± 0.05.

Structure A

Structure B

The ~l va lues for 18C6 obta ined us ing three methods, outlined above, a re in excellent agreement

w ith eac h othe r and ass igning the average va lue o f 11 = 3.5 1 D. O n compari son with the va lues obta ined in the gas-phase (3.02 D), in the cyc lohexane (2.94 D) by s imul at ion technique and ex perimenta ll y dete rmined va lue in cyc lohex ane (2.76D), it has been found th at, va lue in CCI4 is quite high. A much bette r compou nd which can serve as a model fo r inte rpretat io n o f these results is, I ,4-d ioxane

(6-c rown-2) which has a sma ll 11 value of 0.45 D . T he a rguments based on symmetry considerati ons point out that, fo r 18C6 in D3" o r in C confo rmati on shoul d have 11 = 0. But , the high va lue of the di po le moment probabl y can be attributed to the presence of flex ibil ity of the c rown ring, which leads to the ex iste nce of confo rmati ona l dynamic equilibri a. Orhe r inte rpretati on of hi gh va lue of dipole moment

is the ex istence of 18C6 in C. ( i.e . hav ing a mean plane o f reflecti on) symmetry po int group (o r of C, type), which leads to a finite va lue of 11· T he dynamic behav ior of pure 18C6 has been stud ied by NMR re laxation, neutron-scatte ring technique~·.~~

and shows that, the inte racti ons in the liqu id state are governed by fast conformati ona l dynamics of -CH 2- groups. The C; confo rmat ion does not have a cav ity and two -CHr groups occupy the inte rstiti a l space (s tructure A) hav ing a rotat ional moti on contro lled y the rig idity of C-0-C li nkage and have the potenti a l barri er.

However, the hi gh va lue of 11 can be att ri buted to the presence of unequal d istribu tion of conformers and high pola ri zability of -CH~- chains affec ting the overa ll charge d istri bu t ion. Caswe ll & Suvannunt 17 have ca lcul ated dipo le moment va lues with the ass istance of a compute r program and us ing bond angles, bond length , d ihed ral ang les (from crysta llography data) and Pauling e lec tro-negativ ity values for va ri ous conformers of 18C6. They have calcul ated the dipo le mo ment va lue o f 3.38 D fo r the confo rmer (as shown in structure B) wh ich

agrees well w ith 11 values in CCI4 , in whi cb a ll the s ix dipo les in the mo lecule are poin ting towa rds the same side of mo lecule and a re hav ing a larger angle

A lternati ve ly, the results of 11 may indica te the importance of atomic po lari zati on effects in -CH~ ·

chains, whi ch effect cannot b'~ neglected 0 1

appropriate ly subt racted in obta in ing the ori entat io t

po lari zati on va lue and hence the 11 va lue.

Acknowledgement

The authors are thankful to P rof M V Kaul gllf fo r hi s gu idance regarding the des ign of d ie lect ri ce ll.

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