Shukla, Jagdish P

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Indian J. Phys. 64A  (2 ). 112-120 (1990)Dielectric and conductivity properties of some amides in solid polymer matricesJagdish P Shukla and Manisha GuptaDepartm ent of P h y s ics , Luckno w  U n iversity , Lu ck n o w -2 2 6  00 7 , India  R ece ived  28 Ju n e  1 9 8 9 , accepted 19 Decem ber 19 89A b s t r a c t  : D ie lectric  and co nd uctiv ity  properties of som e am ides have been  studied in a polystyrene matrix. D ie lectric  absorption stu d ies have been made for benzam ide. The m easurem ents exhib ited  a sing le  absorption peak, w h ich  w as identified  as due to the m olecular relaxation process. A C  co nductiv ity  being  associated  w ith  d ie lectric  d isp ersio n , has a lso  been evaluated for three am ides  nam ely ; benzam ide, N-m ethyl acetam ide and N ,N -dim ethyl form am ide in the frequency range of 0.1 to 100  kH z and in  the varying  tem perature regions. A  correlation of w ith  the frequency has been made. An estim ate of activation  energy for co nd uctiv ity  has a lso  been made.K e y w o rd s  : Relaxation tim e, co nductiv ity , activation  energy, polym er matrix. P A C S . N o s  : 7 7 .4 0  i, 7 2 .2 0  iI . IntroductionNM R studies (Lowe 1968, Binsch 1968, Allan et al 1972) have shown the presence of restricted rotation, about the C -  N bond in amides. The barrier to internal rotation has generally been ascribed to the partial double bond character of the C - N  bond of the amide group. It has been observed by Matsuo and Shosenji(1969) that internal rotation is strongly restricted at low temperatures. Compara­tively high values for energy barriers to the internal rotation about formal single bonds have been observed in a number of molecules (Gutowsky and Holm 1956, Blears 1964, Anet and Ahmad 1964). Such high values of energy barrier is attributed to conjugated interaction between n electron on both sides of the single bond which induces restricted rotations of these groups.Investigations on large number of amides using NMR by several workers yieldedjvarying values for the energy barrier to the internal rotations. Binsch (1968) has made a comprehensive review of intramolecular process and has pointed out the existing disagreement in the energy barrier data reported by various workers using NMR. It has been suggested by earlier workers that the energy barrier of the C — N bond Is influenced by steric hindrance, resonance and inductive effects.112Dielectric and conductivity properties etc 113In view of existing disagreement in the energy barrier data by NMR and that of very little effort persued to study rotational behaviour in amides using dielectric technique, it seemed important to utilize the dielectric absorption method to investigate the hindered rotations in amides. The dielectric method can determine a wide range of energy barriers provided the absorption peaks fall within the frequency range of the equipment for the available temperature region.The polystyrene matrix method for studying the intramolecular processes has been found very useful in many cases as reported by Davies et al (Davies and Swain 1971, Davies and Edwards 1967) and later supported by Walker et ol (Tay and Walker 1975, Tay et al 1976, McLellan and Walker 1977). The method which provides for the separation of the molecular and group processes in most of the systems, was therefore utilized for studying the energy barriers and other relaxation parameters of several amides by Chan et al (1983). The /IHj values for the group process was found to lie in the range 52-75 kJ mol These measurements were extended to cover a cyclic system, benzamide for its dielectric behaviour.Further, since the dielectric investigations account for the dispersion behaviour associated with molecular configuration and its ordering, which in turn affects the conductivity of the system, it was considered interesting to examine the AC conductivity of three such amides, namely, benzamide, N-methyl acetamide, N,N-dimethyl formamide using dielectric data. The dielectric data of the latter two species were taken from an earlier paper of Chan et al (1983). The molecules chosen were such that they represent the constituents from formamide to amide group substituted to benzene ring.The present study therefore reports the dielectric and conductivity behaviour on the above systems in the frequency range of 0.1 to 100 kHz and at varying temperatures.2. Experimental detailsThe dielectric measurements have been made on GR bridge in the frec.uency range of 0.1 to 100 kHz. The apparatus and measurement technique have been describedby Tay and Walker (1975).We have adopted the-Davies and Swain (1971) procedures both experimentally and in the evaluation of the relaxation time and the distribution parameter by means of the Fuoss-K irkwood equation. The polystyrene had a weight average molecular weight of 230 000. The matrix solutions were prepared by dissolving weighed quantities of polystyrene and the desired solute in a non-polar so vent,trans 1,2-dichloro-ethylene, which was subsequently removed by heating to 360followed by treatment in a heated vacuum oven. The resultant matrix mass was pressed, in a cylindrical die heated to 380 K to produce a sample disk 2.0 inch m114 Jagdish P Shukla and Manisha Guptadiameter and approximately 0.065 inch in thickness. Prior to pressing, the matrix mass containing the solute was returned to the vacuum oven at 360 K for 24 hours with no resultant weight loss, indicating that this preparation procedure did not produce any loss of solute from the matrix.To obtain the relaxation time t, Fuoss-Kirkwood equation (Fuoss and Kirkwood 1941) as given below was used.c o s h - M w ^ " )  =iSln(f.„„f)  T - 1  2nf„, ( 1)where jS, r and f„ are dielectric loss, distribution function, relaxation time and frequency maxima respectively.Analysis of relaxation time (x) values as a function of temperature were carried out using Eyring relationship (Glasstone et al 1941) to yield /IH, and .(2)1A  plot of log (tT) vs will give a straight line with slope equal to fromwhich can be calculated.The value of free energy of activation JG^ can be evaluated at various tempera­ture using the following equationJ G ,  - RT ( v ) (3)Knowing the values of JH  and JF, the JS  is evaluated with the help of following equation(4)where JG,, /IH* and dS* are the free energy, the enthalpy and the entropy of activation respectively, h, k and R are the plank's constant, boltzmann constant, and gas constant respectively.The conductivity (a) has been evaluated using the relationCT'-tnC <0 (5)where t o ' s  permittivity of free space.3. ResultsThe results of Eyring rate equation analysis along with the relaxation times for benzamide are reported in Table 1. The conductivity data for benzamide, N-methyl acetamide and N,N-dimethyl formamide are given in Tables 2 and 3 respectively.Tem pe­rature(K)_1 6 2 .3164.3177.8  184.61 9 2 .8T a b le  I. Relaxation  data and Eyring activation  param eter for benzam ide  in a polystrene matrix.Dielectric and conductivity properties etc 115Tem pera­ture (K )TV 10"secJ G ,  kJ  mol^*JH c kJ  mo!~*JSc J  mol deg~*1 6 2 .3 17 .9 27 .3 31 .2 24 .2164.3 14 .9 27 .4 2 3 .217 7 .8 2.6 2 7 .2 22 .51 8 4 .6 1.1 2 6 .9 23 .01 9 2 .8 0.5 2 6 .9 22 .4T a b le  2. C ond uctiv ity  ( t x 1 0 '* ) 0 “ > cm~* of benzam ide at frequencies.kH z kH z^ 100 kH z2 .7 9 5 .796 .47  6.466 .4 8  6 .2 615 .0  16 68 17.31  1 7 .2 0  16 .863 0 .2 43 3 .6 93 5 .7 0  36 .0 8  3 5 .8 81 4 3 .2162.7183.119 1 .21 9 6 .22 7 5 ,231 4 .936 6 .45 2 6 .9  6 0 9 .27 2 0 .7773 .78 2 6 .9407.1T a b le  3. C o n d u ctiv ity  (o rx IO *) cm * at frequencies.1 1 6 6 .8  1 8 9 2 .214 46.4  26 00 .017 65 .9  3 3 2 6 .01 8 87 .2  3 7 62 .22 0 70 .4  4054.7Tem pera­ 0 .2 0 .5 1 2 5 10 20 50 100ture (K ) kHz kH z kH z kH z kH z kH z kHz kH z kH zN - m e t h y l  a c e t a m i d e2 9 9 .6 1.26 3 .3 2 6 .63 1 2 .8 5 31 15 115.5 267 .83 0 3 .4 1 .25 3 .27 6 .60 12 .9 0 31 .2 3 - 268.1 507.53 0 7 .5 1 .15 3 .16 6.57 - 3 1 .4 0 6 0 .8 2 11 7 .8 270 .6 508.0310 .4 1.10 3 .1 0 6.47 1 2 .9 5 31 ,6 0 61 .1 3 118.3 2 7 0 .8 508.53 1 2 .5 1 .06 3 .0 2 6 .3 6 12.81 31 .64 55 .64 118.1 2 6 8 .9316 .0 1 .00 2 .9 2 6 .2 4 1 2 .79 28.91 1 1 9 .6 27 1 .3 50 9 .33 1 9 .0 0 .9 5 2 .7 9 6.07 12 .4 8 31 .60 6 2 .1 0 119 .7 27 1 .5 509.73 2 8 .0 0 .8 2 2 .44 5 .49 11 .7 8 3 0 .9 0 6 1 .70 12 1 .9 274.4 - -3 3 2 .0 0 .76 2 .26 5.13 1 1 .20 3 0 .4 0 61 .10 1 2 0 .2 27 4 .8 - -N . N ^ d i m e t h y l  f o r m a m i d e29 2 .4 0 .8 9 2 .33 4 .6 3 8 .9 3 2 0 .9 8 3 9 .7 5 74 .17 156.4 —2 9 7 .9 0 .8 3 2 .24 4 .54 8.91 21 .3 5 40 .6 8 7 6 .2 9 1 6 3 .9 —3 0 2 .6 0 .7 6 2 .13 4 .4 5 8.87 21 .48 4 1 .07 7 5 .7 6 16 6 .2 286.330 9 .6 0 .67 1 .9 5 4.21 8 .55 21 .38 41 .4 6 8 0 .4 2 175 .9 3 1 2 .031 6 .9 0 .5 8 1 .73 3 .8 3 8.14 2 0 .95 4 1 .4 5 8 0 .4 2 172 .6 299.1323,7 0 .4 9 1 .53 3 .4 9 l . b l 20 .2 0 4 0 .6 6 8 0 .4 2 1 7 5 .9 3 1 2 033 3 .9 0 .4 0 1 .2 3 2 .8 7 6.51 1 8 .37 38.91 7 9 .0 2 1 7 9 .5 325.7116 Jagdish P Shukla and Manisha GuptaFigure 1 shows the plot of log tT versus 1/T for benzamide. The graphical representation of log a versus log f for benzamide at 162.3 K and 164.3 K is givenFigure 2. Log o versus log f  for benzamide. •  164.3 K, O 162 3 KIn Figure 2. Figure 3 represents plot of log a versus log f for N rmthwiand N,N-din»thyl formamida. Fiaure 4 shows plot of log a vatLs 1 /T to  benzamide, N-methyl acetamide and N,N-dimethy| formamide.  ^ '  ^dielectric and conductivity properties etc 117F i g u r e s .  L o g  o  v e rs u s  log  f  fo r  N -m e th y l a c e ta m id e  a t 3 1 0  K O /  a n d  N ,N -  d im e th y l fo rm a m id e . at»298 K  •( a ) B e n za m id e-60( b) N-methyl acetamide-9-5®-mo- 1 0 5 -7.55l0 5k  S e  6 l0  1/T«10^z e  56  152 W(c)N.N-dlincthyl formamide-6.5-7.0 ~0 OTi) Q -0 0 -0 ~X  < rO —O-O-O- .------- a T - t Sl/T*10^Figure 4. Log a versus 1 /T. # 1 0 0  KHz, O  50 KHz, O  20 KHz. - 6 - 1 0  KHz4. Discussion4.1. Dielectric behaviour :The absorption exhibited by a molecular system having both molecular and intramolecular processes may appear at different temperatures for a given frequency range. Amides could give rise to molecular relaxation process, group relaxation, a nitrogen inversion or a cooperative process. The intramolecular process is influenced by various factors such as steric hindrance, inductive effects and conjugation while the molecular process depends upon the shape and size of the molecule. A  cooperative process is characterized by energy barrier which is appreciably higher compared with that of the molecular and intramolecular processes.Benzamide examined in a polystyrene matrix gave rise to a single absorption peak in the temperature range of 162-193 K, yielding an enthalpy of activation dH, of 31.2 kJ m ol"'. The free energy of activation at 193 K (d G d  1,3) has been found to be 26.9 kJ mol“ .^ This value of is compared with the zlH, of 60 kJ m ol"’ and 66 kJ mol~^ for the group rotation around C - N  bond of molecules namely N,N-dimethyl formamide and N,N-dimethyl acetamide reported by Chan et al (1983). Thus the possibility of rotation of group around C - N  bond is ruled out in view of higher enthalpy values. The cooperative phenomena in a system if present yields much higher enthalpy. Therefore, the process exhibited by benzamide appears to be due to the molecular rotation of the entire molecule. The value of 31.2 kJ m ol"’ and zlG, at 193 K of 26.9 kJ m ol"’ obtained here were therefore compared with zlH< of 29 kJ m ol"’ and idG, at 200 K of 30 kJ m ol"’ for molecular process of almost a similar sized molecule benzyl iodide (Shukla et al 1980). For further strengthening our argument, the enthalpy obtained was also compared with the dH, of 29 kJ m ol"’ again for the molecular process of benzyl chloride (Shukla et al 1980), which is almost of similar size and shape. Since these values of and are in close agreement with the values obtained for molecular process, it may be suggested that the dielectric absorption observed here in benzamide is due to molecular process. The enthalpy for the group rotation, which would be expected to be much higher in the range of 50-75 kJ m ol"’ as observed earlier by Chan et 0/(1983) thus could not be detected, probably due to overlap of cooperative and group process falling in much higher range.4.2. Conductivity behaviour:The conductivity has been found to increase with frequency in all the three amides under investigation. A  slight increase with temperature has been observed for benzamide in the lower frequency region, which enhances in the region of higher118 Jagdish P Shukla and Manisho GuptaDiefectric and conductivity properties etc 119frequency. In the case of N-methyl acetamide and N,N-dimethyl foimamide, with the increase of temperature conductivity decreases slightly in the lower frequency region whereas it increases slightly in the higher frequency region. The decrease in conductivity can be explained due to increased scattering by thermally excited lattice vibrations. The change in conductivity with varying frequency at a fixed temperature has been remarkably high. Conductivity <r varies from (31.15-30.37) X 10~* cm“  ^ at 5 kHz in the temperature range of 299.6-332 Kfor N-methyl acetamide, while at a fixed temperature 310.4 K, a varies from l . lO x  10“® -5 .08x lO "’ cm~^ for the frequency change of 0.2-100 kHz. Similar results are obtained for N,N-dimethyl formamide. It has been observed that the a values for benzamide in the temperature region 162-193 K vary from (1.43-1.96) X 10"^^ cm~* at 5 kHz.Analysis of conductivity data shows that log <r exhibits a linear behaviour with log f (Figures 2 , and 3)-for these amides. It is thus seen that conductivity depends on frequency as characterized by the relation of the type or^A(w)®, where A and S are frequency independent constants. S has been found to vary from 0.95-1.0 in the temperature range 162-193 K for benzamide, 0.97-1.0 for N-methyl acetamide in the temperature range 299.6-320.0 K and, for N,N-dimethyl formamide from 0.94 .o 1.0 in the temperature range 292.4-333.9 K. The values of S exhibit only slight change with temperature. This may be due to a small range of temperature variation observed for dispersion in the present systems.The activation energy for the conductivity process has been evaluated using the relation of the type a where A is a constant and k is the Boltzmann'sconstant. From a linear plot of log a vs 1 T (Figure 4) the activation energy (J£) obtained was found to be in the range of .025-.039 eV, .022-.033 eV and •037-.066 eV for the frequency range of 10-100 kHz for benzamide, N-methyl acetamide and N,N-dimethyl formamide respectively. No substantial change in the activation energy has been observed for the amides investigated here.AcknowledgmentWe are grateful to Prof S  Walker of University of Lakehead, Thunder Bay, Canada for providing the facilities.ReferencesAllan E A , H o b so n  R F , R eev es L  W  and S h a w  N K 1 9 7 2 J .  Am. Chem . Soc . 9 4 1 9  Anet F A  I and  A h m ad  M 1 9 6 4  J . Am. Chem , Soc. 86 1 1 9  Binsch G  1 9 6 8  Topics Stereochem . 3 9  Blears D J  1 9 6 4 ) .  Chem. Soc. 6 2 5 6Chan B , Sh u k la  J  P and W alker S  1 9 8 3 ) .  M ol. Struct. 102 1 6 5  Cavies M and E d w a rd s  E  1 9 6 7  Trans. Fa rad . Soc. 63 2 1 6 3  Cavies M and S w a in  J  1971 Trans. Fa rad . Soc. 67 1 6 3 7120 Ja gd ish  P  Shu k la  and  M a n ish a  G uptaFuoss R M and Kirkwood J  G 19 4 1J . Am. C/iem. Soc. 63 385Glasstone S, Laidler K J  and Eyring H 1941 The Theory of R ate  frccesses (New York: McGraw Hill)Gutowsky H S and Holm C H 1956 J .  Chem. Phys. 25 228Lowe J  P 1968 Progress irt Physical Organic Chemistry (New York: Wiley-Interscience) 6Matsuo T and Shosenji H 1969 Chem. Commun. iO 501McLellan C K and Walker S 1977 C a n .) . Chem. 55 583Shukla J  P, Warren J  and Walker S 1980 Adv. Mol. Relax. In t. Proc. 1 7 107Tay S P, Kraft J  and Walker S 1976 J .  Phys. Chem. 82 303Tay S P and Walker S 1 9 7 5 J. Chem. Phys. 63 1 634

Dielectric And Conductivity Properties of Some Amides in Solid Polymer Matrices

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Dielectric And Conductivity Properties of Some Amides in Solid Polymer Matrices

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