Abdel-Nour, K N

Hanna, F F

English

IACS Institutional Repository

Dielectric relaxation of some large aromatic molecules in benzene from microwave absorption measurementsF. F . H a n n a  and K . N . A b d e l -N our Microwave Laboratory, National Research Centre, Dokki, Cairo, U.A.R.{Received 5 August 1970)T h o  (lie le c tn o  c o n s t a n t  (c ')a n r l d ic le c t n c  lo ss  (c ")  o f  th r e e  la rg e  a r o m a t ic  m o le c u le s  n a m e ly  a ,  a ,  a,', ol', 2 , 3 , 5 ,  6 -o c t a c h lo r o  -^ j-x y le n c , 9 -a c e t y l  a n th r a c e n e  a n d  a c r id a n  h a v e  b e e n  m e a s u r e d  a t  fo u r  w a v e le n g t h s  b e tw e e n  0 .2  c m  a n d  2 5  c m  a n d  a t  t w o  o r  th r e e  t o m p e ia t u r o s  b e tw e e n  2 0 °  a n d  6 0 ° C .  T h e  d a t a  h a v e  b e e n  a n a ly s e d  a n d  t w o  lo la x a t io n  i im e s  h a v e  b e e n  f o u n d  in  e a c h  c a s e . T h e  r e s u lts  o b t a in e d  a re  in te r p r e te d .Measurements ol e' and c" are carried out at wavelengths 0,22, 3.24, 10 and 25 cm. to an accuracy of 2%. The apparatus used have boon described befoie by Garg d a? (19G5), Laquer & Smyth (1949), Pitt & SmjdJi (1959), The static dielectric constant e has been measured at lOKHz using General Radio Low Frequency Bridge Typo J615A.Three large aromatic molecules of grade ‘ Pure’ were measured namely . a, a, a', a', 2, 3, 5, G-octachloro-jo-xylone, acridan (both obtained from Almdrich Chemical Co Tnc.) and 9-acotyl anthracene (obtained from K & K Laboratories Iiio,), Benzene “ Analar” was distilled twice ovei' sodium and tised as a solvent. Dilute solutions of (he substances under tost in benzene were used.£(,, t' and e." Avere measured at. tempeiatures ranging between 20° and 60 C Avitli tile exception of acridan c' and e" for the substances used wore plotted in a coiupl(‘x plane (Colo & Cole 1941) and the dispersion step (e^ —6'oo) was obtained fui’ each substance at the different temperatures from these arc plots. As an example, the arc plot of 9-acetyl anthracene at 20°0 is given in figure 1 Then theIndian J, Phya. 44, 451-460, (1970)l^guro 1. Arc plot for 9-acetyl anthracene in benzene at 20°C (fof concentration 0.0099 molo fraction).461452 F . F . H an n a  a n d  K . N . A b d e l-N o u rreduced ab.sorj)tion t}" =  -------- was calculated for each frequency. The valuesof if  were jilotted against log A. The absorption curves obtained were analysed into two Deybe terms using the equation( 1 - 0 ) . - - ' r +  O wTal +  o> V  1 +  w VThe numerical analysis has been carried out using a computer IBM 1620.In case of acridan the variation of e' with frequency was very small so that it was difficult to draw the arc plots and hence ?/" could not be calculated, t" was plotted against log F  and the results were analysed also into two Debye terms The results of the analyses of all substances are given in table 1.D is c u s sio n1. a, a, a', ol\ 2, 3, 4, G-ootachloro -p-x y^lene : Cl Clra -/S -c iC l-'' i - c iJh/  \ c iClThe measurements Avere carried out at 20° and 40°C only. It was possible to measure c" at a w'avelength 1 25 cm at 20° C and so 7}" for this frequency w^as also calculated. Figure 2 shows the results o f analysis at 20°0 and 40°C. As shown, the analyses represent the experimental data very well and the absorption curves are found to be composed of two Debye terms having relaxation times Ti and Tg. The availability of the measured values at 2 mm wavelength assure these analyses The longer relaxation time Tj is that which may be due to the rotation of a molecule of this size as a whole The short relaxation time Tg is due to a complete rotation of the CHClg groups. From Stuart & Briegleb model, it is found that no complete rotation of the group is permissible. So, it is concluded that the rotation of the CHClg-groups is restricted to a slight rotating oscillation by the adjacent Cl-groups. The value of Tg (5.18 pscc at 20‘*C) is comparable with the value 3.72 p sec-for the CHgCl-group in a, a', dichloro-p-xylene (Purcell et al I960), is vej-y temperature dependent as would be expected for the oscil­lation of the CHClg groups while Q does not change much,D idectric rdaxation of some, large aromatic molecules etc. 453pa.SmIaoap^'B1C05S. tfO UShpd 1G5 ©(N KS(N ©«M ©CO . 04■Hh s§O © d d d ducuOD »£9CO © © 04H*. ©©CO id ■oi Hf. dwilOa>ItsCOMS© usCO©hh'©©©©1 B11.5*IIS f : § g SO  i-t CSJ e*3 CQ©  00 PH00 in ©  ©  (N «  <M©  ©  tP US ^  OT pH ^©  04 tH COCO ©  ©  QO ©  ©  ©  QO ©  ©  CO PH 1 1 1 1o  ©  o  o  o ' O  O  CO ©' d  d  d  d ©  d  d  d I I  I I 1 1 1w dau1<N© CO©oCO©o 1 1 1 1 1 1 1<0* © o d d 1 1 1 1 1 1 1S  t -  N  rt w  ^  l>  i-H o  00 O  PH CQ COO  O  O  o  ©©  ..H PH ^tr l> O  M  CO (N O  ©  ©  ©04 Cp ©  ©  ©  ©  US5  2J ^o  ©  ©  ©l>  pH ©  CO M CO 04 r- O  «  CO pH ©  ©  ©  ©04 ©  ©  W ^  04 CO 04 ©  ©  ©  ©  ©  ©  o  ©^  ©  ©  W  04 ©  04o  ©  o  ©  ©  ©  ©  ©© ' © © © © © d o © ©  d  d  d d  ©  d  d o  d  d  d d  d  d  d'.u©  |» ©  N  m  M  W  CO ©  ©  CO CO CO eol> ©  N  CO W  ©  CO tp CM CO CO COCO Hj. (M CO CO CO hCI©  CO ©  tp 04 ©  ©  i-H 04 04 04 ©©  ©  ©  ©  00 ©  ©  ©  04 ©  ©  Mip tp ©  ©  ©  ©  ©  ©  04 04 04 04<N (N N  N  (N d  (N d  d 04 04 04 04 d  d  d  d 04 04 04 04 04 04 d  dg©^  ©  -^ ©  ©  (N M (N O  ©©pH "it ©  ©(N (N ©  © N  ^  ©  ©©^  -^ ©  ©  04 04 ©  O©PH ©  ©04 W ®  o< O  pH CO C> idPH (M d  CO ©  idPH (N o  CO d  USpH 04 o  CO d  ©’pH 04 ©  n  ©  ©pH 04 ©  ©  d  mpH 04ouw■Hf.l "co©04CO§©©©04(N O'! 04 04 04N S § § ©hC<1: - i l» : i - |  «  6 , 01| lI fg4^1£03CQ« -g o9 §P©  '3  ®IIt f S  H 11 «  ©  « 1  II<  H454 F . F . H a n n a  an d  K .  N . A b d e l-N o u rFigure 2. AbBOT'ption of a, a, a', a', 2, 3, 5, 0— octaohloro- p-xylone in benzene at 20" and 40'>C (or coneontratiori 0,0336 mole fraction obtamod from the Hiun of two Debye termK having relaxation times Ti and TgThe dipole moment (//,) was calculated at the two temperatures and was found to be 1.43 D.2, 9-Acetyl AnthraceneOH3i . or A oThe measurements were carried out in this case at 20° and 60°. It was found in the case of 1-acetyl naphthalene (Fong & Smyth 1963) that the molecule possesses only one relaxation time due to the blocking of the acetyl group by the 8-hydrogenatom, the acetyl group in 2-acetyl naphthalene is able to rotate having a relaxa­tion time 7.3 psec. l^'ollowing this interpretation, it is expected that the acotyl- i»roup in the 9-acetyl anthracene will be more blocked by the 1- and 8-hydrogen atom Anyhow, the relation between c* aaid e" plotted in a complex plane could not be roprosonted by one semicircle. Both absorption curves at 20"C and G0“C arc composed of two Debye terms. Figure I shows the arc plot obtained at 20'’ (y as an example. The measuremeuts at 2 mm wavelength helped much in obtaining this result. From Stuart & Brieglob model, it is found that the acetyl i-j oiip may possess a restricted rotation. It is throfore suggested that the longer relaxation time is to be attributed to the rotation of the molecule as a whole and the shorter relaxation time is due to a restricted orientation of the acetyl ••roup. The values of shown in table 1, are reasonable for a molecule of that size The value of (5.10 psec at 2C0) obtained for the restricted rotation of the aeetyl group in 9-acetyl anthracene is smaller than the value obtained (7.3 prtf'u at 20"C) for the acetyl-group in 2-acetyl naphthalene which is supposed to have free rotation G, the weight of on the total absoiptioii is not much affected ith temperature /x is also calculated and is found to be 2.75DDielectric relaxation of some large aromatic molecules etc, 4553 AcridanH H\ /cII II I\ / \ / \ ^NIHThe measurements were taken at 20“^ and 40°C. No values of (t^—e^) could be obtained due to the very small variation of e' with frequency as men- I ionod before, hence the reduced absorption i f  could not be calculated. The values of c" at each temperature are plotted against log /.  The measurements of e" at at 2 mm wavelength (specially at 40°C) show that the absorption cannot be re­presented by one Debye term. The analysis is carried out as shown in figure 3 using two Debye terms having relaxation times and To. The longei' relaxation tune Tj, which is about the same as that for 9-acetyl anthracene, (molecule being about the same size) could be attributed to the rotation of the whole molecule. This is a reasonable value for a molecule of that size. While the short; relaxation time Tg may be due to an internal motion within the molecule. From jStuart & Brieglob model it is seen that the molecule can have an internal motion, This internal motion could not be an orientation but may be due to a tipping over cf one of the outer benzene rings. This type of motion, as expected is very temperature dependent.456 F. F. Hanna and K . N. Abdel-NourFigure 3. Absorption of acndan m bcnzeiio at 20“ and 40“C for concentration 0.0087 mole fraction obtained from the sum of two Dobyo terms having relaxation times ti and Ta-A c k n o w l b d g e m b n tWe thank Prof. Dr. M. Mokhtar, Cairo University, for Jiis kind interest in this work. We also thank Prof. Dr. C. P. Smyth, Princeton University, for allow­ing Mr. K. N. Abd-El-Nour to take the measurements in his laboratory.R b f b b b n o e sCole K, B. & Cole R  H. 1941 J. Ch&m, Phya. 9. 431.Fong F. K. & Smyth, C P. 1963 J, Am. Ohem Soc. 85, 1566.Garg S, K., Klip H. k  Smyth C. P. 1905 Okem. Phya. 43, 2341.Laquer H L. & Smyth C. P. 1949 J. Am. Chem. Sop., 70, 4097.Pitt D. A. & Smyth C. P. 1959 J. Phya. Ohem. 63, 582.Purcell, W. P., Fish, K. & Smyth, 0. P. 1060 J. Am. Ohem. Soc. 82, 6299.

Dielectric Relaxation of Some Large Aromatic Molecules in Benzene from Microwave Absorption Measurements

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/2907/1/Dielectric%20relaxation%20of%20some%20large%20aromatic_F%20F%20Hanna.pdf

Dielectric Relaxation of Some Large Aromatic Molecules in Benzene from Microwave Absorption Measurements

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository