Dielectric Properties of N-Methyl Acetamide in Carbon Tetrachloride Solution BY MUSA M. OMAR* Department of Chemistry, Faculty of Science, University of Tripoli, Tripoli, Libya Received 10th June, 1976 The dielectric relaxation parameters have been measured for N-methyl acetamide in carbon tetrachloride solution (0.0016 to 1.5 mol dm-9, at 30.1"C and a frequency range of 450 to 1000 MHz. The static permittivities have been also measured at 100 kHz for twelve different concentrations. The apparent dipole moment and the relaxation times of the amide as functions of concentration have been evaluated. N-methyl acetamide is probably the simplest molecule which can be taken as an adequate model of the units in polypeptide and protein structures. This, in addition to its large permittivity values has caused extensive study of its molecular properties. The studies of this compound have been principally concerned with the dielectric properties of the pure 1iquid.l Little attention has been given to the dielectric study in solution for smaller amide molecules.The dielectric properties of pure liquids or concentrated solutions are attributed to the properties of molecular aggregates. In this work we tried to trace the molecular dielectric parameters of a solution at various concentrations hoping to obtain a complete spectrum of properties, starting from a highly associated medium to a very low or negligible associated system. This was done by utilising the available infrared spectroscopic studies of N-methyl acetamide in carbon tetrachloride solution. - EXPERIMENTAL N-methyl acetamide was purified by fractional distillation under reduced pressure.The solvent carbon tetrachloride was of spectroscopic grade. The amide and the solvent were checked for purity, using infrared spectroscopy and refractive index measurement. Static permittivity measurements were made using a W.T.W. Diplometer (Model 06) with MFL 1 cell for concentrations up to 0.4 mol dm-3, and MFL 2 for high concentrations. The cells were thermostatted through the outer water jacket using a water thermostat providing a temperature control to +O.l"C. The static readings were taken at 100 KHz, which is considered to be below the dielectric absorption frequency range. The other dielectric properties were studied in the frequency range 450 to 1000 MHz, using a General Radio slotted line which was designed by the General Radio Company, Massachussetts, U.S.A.This apparatus has been described fully by William~.~ The material studied was contained in a coaxial line. This was achieved by dipping the line into a vessel containing the liquid. The liquid enters through small holes at the end of the line. The liquid depth was measured using a cathatometer. The vessel including the line was then immersed in a thermostat at a fixed temperature of 30.1"C. RESULTS The dielectric absorption of five different concentrations of the N-methyl acetamide in carbon tetrachloride solution (1.5 to 0.0016 mol dm-3) was studied at a constant 115116 DIELECTRIC STUDY OF METHYL ACETAMIDE TABLE DI DIELECTRIC PARAMETERS OF N-METHYL ACETAMIDE IN CCI, AT 30.1"C onc./mol dm-3 1.5 0.8 0.1 0.02 0.0016 fIMHz 450 650 850 1000 450 650 850 1000 450 650 850 1000 450 650 850 1000 450 650 850 1000 d 3.1 8+ 0.01 2.98+ 0.01 2.91 kO.01 2.79& 0.08 2.87k0.04 2.67+0.00 2.642 0.02 2.55k0.07 2.48+ 0.05 2.45k0.01 2.47+ 0.02 2.44+ 0.07 2.29k0.01 2.28 & 0.03 2.26k0.01 2.23 + 0.01 2.24k0.04 2.23k0.03 2.20+ 0.01 2.1 8 _+ 0.02 E *' 1.700k0.020 1.11 5+ 0.031 0.857 & 0.025 0.774k 0.050 1.018+0.007 0.570+0.001 0.5 I2+ 0.007 0.366k0.045 12.1 & 0.5 x 11.8+0.4x 9.8k0.7 x 8.4k0.5 x 6.0k0.3 x 6 .9 k 0 . 6 ~ 7.2+ 0.9 x 6.92 0.4 x 1.5k0.2~ 2.8+ 0.3 x 6.1k0.5~ 5.6k0.5 x temperature of 30.1"C and at four different frequencies (450,650,850 and 1000 MHz). The measured permittivity of the carbon tetrachloride of 2.22 + 0.02 is in good agree- ment with the literature value of 2.218 at the same temperature. The losses observed for the solvent are too small to have much significance in the losses for the concentrations above 0.02 mol dm-3.The relaxation parameters as deduced are given in table 1. The reported dielectric parameters are the mean values of two or three determina- tions using different sample depths. It is found that the agreement between different TABLE 2.-cOMPARISON OF THE ESTIMATED DIPOLE MOMENTS USING DIFFERENT VALUES FOR Em concl mol dm-3 1.5 1 .o 0.8 0.6 0.4 0.2 0.1 0.08 0.04 0.02 0.01 0.001 6 EO 22.15 12.66 11.86 9.56 5.97 3.78 2.78 2.65 2.38 2.29 2.25 2.23 PD 2 ea = ND 10.71 9.51 10.21 10.31 9.09 8.44 7.52 7.54 7.47 8.54 10.96 8.02 PD Em = E0* 10.67 9.46 10.61 10.24 8.97 8.18 6.93 6.79 5.86 5.43 5.86 7.32 PD C + O Em = EO 10.67 9.46 10.15 10.24 8.96 8.15 6.89 6.74 5.75 5.24 5.40 4.63 * PD 10.68 10.01 - - - 1 5.63 - - 5.07 - -M. M.OMAR 117 depths varied from 1 to 3 % in permittivity (8') and from about 2 to 10 % in the loss ( E " ) values. The static permittivity of twelve different concentrations (including the one mentioned above) was also measured at the same temperature (30.1"C) and the results are reported in table 2. ANALYSIS OF THE DATA The experimental results for the four higher concentrations (1.5, 0.8, 0.1 and 0.02 mol dm-3) were analysed in terms of a distribution of relaxation times, using the Fouss-Kirkwood equation (see fig. 1) cosh-I ~ ~ , ~ / s " [ 2 +(l/skax)2/2 +(I/s')~] - - pin f!i where B is a measure of deviation from a single relaxation time, 0 < p < 1, and E k a x is the permittivity obtained at maximum absorption.This equation was used 1.4 \ FIG. 1 .-Fuoss-Kirkwood plots for (c) 1.5 (O), (b) 0.1 (9) and (a) 0.02 mol dm-3 (9) NMA solution at,30.1 "C. x = 2+('/&rnax)*/2+('/~~ax)* E" 22 - ---__ -- El FIG. 2.-Cole-Cole plots for (a) 1.5 (0) and (&) 0.8 mol dm-3 (0) NMA solution at 30.1OC.118 DIELECTRIC STUDY OF METHYL ACBTAMIDE for the 1.5 and 0.8 mol dm-3 solutions, which have large losses. For 0.1 and 0.02 mol dm-3 solutions where E” 4 E’ and E’ = &&ax, the following reduced equation was used The results were checked against a Cole-Cole plot (see fig. 2) to ensure that the permittivities were consistent with the relaxation parameters obtained.In all cases a distribution of relaxation times was observed, and the Fuoss-Kirkwood distribution parameter (p) was converted to the Cole-Cole distribution parameter (a) (p = I for single relaxation times) and shown in table 3. TABLE 3 .-COLE-COLE AND FUOSS-KIRKWOOD DISTRIBUTION PARAMETERS B B &Ax Emax conc/mol dm-3 Cole-Cole Fuoss-Kirkwood Cole-Cole Fuoss-Kirkwood 1.5 0.87 0.90 7.10 7.10 0.8 0.90 0.92 4.00 4.00 0.1 0.89 0.98 0,155 0.155 0.02 - 0.97 - 7.21 x 10-3 The results for the 0.0016 mol ~ l m - ~ solution indicate two overlapping absorptions, but it was not possible to analyse them according to the Fuoss-Kirkwood distribution of relaxation times. These results were analysed, assuming two overlapping simple Debye absorptions and using a suitable IBM 1620 computer program (see fig.3). log f FIG. 3.-Analysis of dielectric absorption of the 0.0016 mol dm-3 NMA solution, 0 experimental points. The relaxation parameters are listed in table 4. The distributed relaxation times The dipole moments associated with these relaxation processes were calculated are estimated to be correct to between 8 and 12 %. from the absorption intensities using the Onsager equation :M. M. OMAR 119 where S is the number of the polar molecules per cm3 of solution, p2 is the dipole moment of the solute, gal and em2 are the static and the optical permittivities of the solvent and solute respectively, and c0 and em are those for the solutions. The calculated dipole moments are listed in table 2.TABLE 4.-RELAXATION PARAMETERS FOR N-METHYL ACETAMIDE SOLUTION IN CARBON TETRA- CHLORIDE AT 30.1"C , conc/mol dm-3 t x 1012/s Emsx B EO Em PD 1.5 3 690 7.10 0.90 22.15 2.65 10.68 0.8 3650 4.00 0.92 11.85 2.50 10.01 0.1 546 0.155 0.98 2.78 2.41 5.63 0.02 187 7 . 2 1 ~ 0.97 2.29 2.23 5.07 0.001 6 62.3* - - 2.23 * 71 I - For the 0.0016 mol dm-3 solution, the dipole moment associated with each process is calculated from the absorption intensities using the Cole treatment : P: = A&;I(&o -&m)P2 (80 -Em) where ,ul and p2 are the dipole moments associated with the low and high frequency absorptions respectively. 2 A 4 I 2 P2 = - P DISCUSSION As has been shown earlier, the results for the 1.5, 0.8, 0.1 and 0.02moldm-3 solutions of N-methyl acetamide (see table 4) are adequately described in terms of a distribution of relaxation times.In all these cases the distribution parameter (fl) increases, as expected, with decreasing concentration and tends to unity (corresponding to a Debye single relaxation time). Bass, Meighan and Cole have measured the dielectric parameters for pure liquid N-methyl acetamide in the frequency range 1-250 MHz. Their results have been described by a simple Debye function with a single relaxation time. This does not follow the trend which we expected from our solution results and for such strong hydrogen bonded molecules. The values obtained hare for the relaxation time of N-methyl acetamide solution are much larger than expected for such small molecules. The estimated value for the relaxation time of the 1.5 mol dm-3 solution is 3690x 10-l2 s.This is much higher than the value (7,) obtained for the pure liquid (N-methyl acetamide) of 74 x 10-l2 s at 31.4"C. At the same time the relaxation time obtained for 1.45 rnol dm-3 solution of N-methyl palmitamide in par& wax at 91°C is 6 8 0 2 ~ s. Taking into consideration the molecular size of the latter, our value for N-methyl acetamide is in fair agreement with that for N-methyl palmitamide solution. The large values for N-methyl acetamide could be attributed to the molecular aggregation formed by chain association. Decreasing concentration decreases the molecular association of such agglomerates which leads to a decrease in the relaxation times (see table 4 and fig. 4). This does not seem to be true at higher concentrations (0.8 and 1.5 mol dm-3) where the relaxation times kept almost constant.Taking into consideration the small values obtained for the pure liquid, one could suggest that at much higher concentration the apparent relaxation times may start to decline120 DIELECTRIC STUDY OF METHYL ACETAMIDE with increasing concentration. This behaviour may be explained by the formation of some agglomerates (cyclic association) in pure liquids and highly concentrated solutions which are different from those formed at low concentrations (chain association) and rotate faster than the latter. The dielectric behaviour of the 0.0016 mol dm-3 solution which is described by two overlapping Debye processes should not be taken as giving unique values, because the values obtained carried a large uncertainty due to the small values of the dielectric losses.However, the values obtained for low and high frequency relaxation times are z1 = 62.3 x 10-l2 s and z2 = 21.5 x 10-l2 s respectively. For pure liquid n-ethyl alcohol, which is associated in a linear form similar to mono-substituted amides, Saxton l1 resolved the overall absorption into two separate Debye relaxation times, z1 = 170 x 10-l2 s and z2 = 1.6 x 10-l2 s at 20°C. Garg and Smyth l2 have observed for n-propyl alcohol three Debye relaxation processes with single relaxat ion times. I 1 . - - -- as 0.5 1.0 1.5 concentration/mol dm-3 FIG. 4.-Variation of NMA static permittivity (0) and relaxation time (9) with its molar concmtra- tion at 30.1 "C. Similar results have been obtained lo for a dilute solution of N-methyl palmitamide (0.097 and 0.049 mol dm-3) in paraffin wax, where the dielectric absorption has been resolved into two overlapping Debye relaxation processes.The values obtained for the 0.049 mol dm-3 solution are z1 = 208 x 10-l2 s and z2 = 30 x 10-l2 s, at 91°C. Our relaxation time values (2,) for 0.0016 mol dm-3 solution of N-methyl acetamide seem to be in good agreement with the N-methyl palmitamide solution. Since the two relaxation times were not followed up at lower concentrations or at different temperatures, because of the experimental difficulties due to the small values of the observed losses, it is not possible to establish any general pattern or conclusion, but some idea could still be formed from these results.Davies and Evans have studied the infrared spectroscopy of N-methyl acetamide solution in carbon tetrachloride. They have shown that the (N-H) non-bonding stretching vibration absorption dominates at the 0.001 6 mol dm-3 concentration. Therefore the assignment of the two relaxation times could be suggested as follows : the low frequency relaxation time (zl) could be attributed to the rotation of the free moleculesM. M. OMAR 121 (monomeric), and the high frequency relaxation time ( T ~ ) could be associated with the rotation of the whole polar head group around the R-C1 bond.1° 0 // \ / \ R-C1 CH3 N H The rotation round the C-N bond is much faster l 3 and cannot be expected to be observed at this frequency range. The other possibility is that the z1 is attributed to the rotation of the associated molecules ; this is supported by the large estimated value of the dipole moments; while the z2 is associated with the non-associated molecules.I 12 I a2 0.6 1.0 L.4 1.6 concentrationlmol dm-3 FIG. 5.Variation of NMA dipole moment with its molar concentration at 30.1"C. The variation of static permittivity with concentration is shown in fig. 4; it is found, as expected, that eo is a linear function of the molar concentration except at the lower concentration (below 0.1 mol dm-3) where eo values hardly varied. The choice of a value for 8, presents a problem in the calculation of the apparent dipole moment, since if we select E , = n& for each solution we find that the calcula- tions are quite meaningless.Using E , = E' (solvent) improves the apparent dipole moment. However it seems that using the extrapolated value of E~ (solution) at zero concentration as gives better results. In all the above cases the ga2 was taken to be l4 1.05 &. In table 2, four values of the apparent dipole moment (p) are listed. It is observed [even using E , (observed), obtained at high frequency or from Cole-Cole plots] that the dipole moments are much larger than those obtained for N-methyl acetamide l5 gases (3.71 D). This confirms that mono-substituted amides do associate in linear chain and that the expected dipole moments of the associated molecules must be larger than the monomer. Similar results have been obtained by Cole l * lS and122 DIELECTRIC STUDY OF METHYL ACETAMIDE coworkers for pure liquids and a benzene solution of N-methyl acetamide.The large, strongly concentration dependent (see fig. 5 ) dipole moment shows large association effects, a phenomenon which is, as also explained by Cole,I a result of chainwise association by hydrogen bonding with the single amino-hydrogen such that adjacent molecular dipole moments are nearly parallel. This phenomenon seems to exist even in highly diluted solution (0.0016moldm-3) where a value of - 1.3 times that of the dipole moment of a monomer was obtained. However, the reported low concentration values of the dipole moment are very sensitive to (E,, - E,) values, i.e. any small error in go or 8, leads to a large difference in the calculated dipole moment. Thus the figures reported for 0.01 and 0.0016 mol dm-3 solutions are exposed to a large uncertainty.The dipole moments p l , associated with the low frequency relaxation mechanism and p2, which is associated with high frequency one, were calculated from the absorption intensities and values of 2.00 and 4.17 D were obtained respectively. I thank the Faculties of Science, Engineering and Education at the University of Tripoli for use of facilities and equipment during this research. I am also indebted to Dr.-Eng. M. Wanas and Eng. E. El-Ghazzawi for their valuable assistance in installing the equipment and apparatus. S. J. Bass, W. I. Nathan, R. M. Meighan and R. H. Cole, J. Phys. Chem., 1964, 68, 509. M. Davies, J. C. Evans and (in part) R. L. Jones, Truns. Furahy SOC., 1955,51,761. (a) G. Williams, Ph.D. Thesis (University College, Wales, 1966) ; (6) A. Von Hippel, Dielectric Materials and Applications (NIT Press, Boston, 1961). Table of Dielectric Constants of Pure Liquids, ed. A. A. Maryott and E. R. Smith (National Bureau of Standards Circular 514, 1951). R. M. Fuoss and J. G. Kirkwood, J. Amer. Chem. SOC., 1941,63,385. K. S. Cole and R. H. Cole, J. Chem. Phys., 1941, 9, 341. S. R. Gough, Ph.D. Thesis (University College, Wales, 1964). S. L. Onsager, J. Amer. Chem. SOC., 1936,58, 1486. R. H. Cole, J. Chem. Phys., 1965,42,637. lo M. M. Omar, Ph.D. Thesis (University College, Wales, 1967). l1 Dielectric Properties and Molecular Behaviour, ed. N. Hill, W. E. Vaughan, A. H. Price and l2 S. K. Garg and C. P. Smyth, J. Phys. Chem., 1965, 69, 1294. l3 T. Drakenberg, K. Dahlqvist and S. Forskn, J. Phys. Chem., 1972, 76, 2178. l4 C. Campbell, G. Brink and L. Glasser, J. Phys. Chem., 1975, 79, 660. l5 R. M. Meighan and R. H. Cole, J. Phys. Chem., 1964, 68,503. M. Davies (Van Nostrand, 1969), p. 352. (PAPER 6/1099)