J. Phys. Chem. 1986, 90, 5561-5565
5561
Production of Highly Ordered Organic Conducting Polymers (Poly( 3-methylthiophene)) under Electrochemical Inclusion of Cu2+ Ions: An ESR Study D. Gourier* Laboratoire de Chimie AppliquPe de I'Etat Solide, UA 302, Ecole Nationale SupZrieure de Chimie de Paris, 75231 Paris Cedex 05, France
and G. Tourillon Laboratoire pour 1'Utilisation du Rayonnement Electromagnttique, UniversitP Paris-Sud. 91 405 Orsay Cedex, France (Received: January 24, 1986; In Final Form: April 25, 1986)
ESR studies of poly(3-methylthiophene) (PMeT) doped with S03CF3anions and with various amounts of Cuz+ions are reported. The S03CF3doped PMeT has the characteristics of a metallic state: Dysonian line shape with a Pauli spin paramagnetism. When cathodically polarized in an aqueous copper solution, its conductivity increases by a factor of nearly 3 and is accompanied by an increase of the conduction electron spin resonance (CESR) line width. The features of the Cu2+ spectra depend on the cathodic polarization time. (i) For very short times (1 C t C 50 s), isolated Cu2+ions in a strongly axially distorted octahedral environment are detected, with each ion surrounded by four water molecules and by two methylthiophene units (Cu2+(H20),(MeT),, complexation by sulfur atoms). This configuration leads to the bridging of the neighboring PMeT chains by the Cuz+ions and consequently to the formation of a bidimensional system, which increases the interchain hopping rate. (ii) For a longer cathodic polarization time, Cu2+clusters are formed which do not modify or affect either the ordering or the electrical properties of the polymer.
Introduction A considerable number of experimental' and theoretica12v3 studies have been devoted to the understanding of the conduction processes in doped five-membered polyheterocycles (polypyrrole, polythiophene, etc.). One significant problem encountered with these polymers comes from the presence of structural defects4 and from the lack of long-range order5 which limits both intrachain and interchain conduction. The first parameter (intrachain conduction) can be controlled and modified by varying the structure of the monomer units6*'and the nature of the dopants.' Thus, the substitution of one hydrogen atom in the 0-position by a methyl group on the thiophene molecule leads to a more regular polymer with (i) a metallic-like behavior; (ii) a long-range order? and (iii) a high doping level.1° However, poor interchain contacts limit the macroscopic conductivity and the crystallinity of poly(3-methylthiophene) (PMeT)." Two methods can be considered if one is to increase the organization of such a polymer: (i) by changing the electrochemical parameters ( i , V, T) and the electrolytic medium composition and (or) (ii) by including specific ions in the matrix which can bridge the polymeric chains. On the basis of this approach, we studied the influence of the electrochemical inclusion of metallic ions in a S 0 3 C F c doped PMeT.'* The primary interest of the elec(1) Handbook on Conjugated Electrically Conducting Polymers: Skotheim, T., Ed.; Marcel Dekker: New York, 1985. (2) Duke, C. B.; Paton, A.; Salaneck, W. R. Mol. Cryst. Liq. Cryst. 1982, 81, 177. ( 3 ) Bredas, J. L.; Themans, B.; Fripiat, J. G.; Andre, J. M.; Chance, R. R. Phys. Rev. B Condens. Matter 1984, 29, 6761. (4) Pfluger, P.; Street, G. B. J. Chem. Phys. 1984, 80, 544. ( 5 ) Ford, W. K.; Duke, C. B.; Salaneck, W. R. J . Chem. Phys. 1982,77, 5030. (6) Kobayashi, M.; Colaneri, N.; Boyssel, M.; Wudl, F.; Heeger, A. J. J . Chem. Phys. 1985.82, 5717. (7) Tourillon, G. In Handbook on Conjugated Electrically Conducting Polymers; Skotheim, T., Ed.; Marcel Dekker: New York, 1985; Vol. 1, Chapter 9, p 294. (8) Tourillon, G.; Gourier, D.; Gamier, F.; Vivien, D. J. Phys. Chem. 1984, 88, 1049. (9) Jugnet, Y.; Tourillon, G.; Tran Min Duc Phys. Rev. Letf., in press. (IO) Tourillon, G.; Gamier, F. J . Phys. Chem. 1983, 87, 2289. (1 1) Gamier, F.; Tourillon, G.; Barraud, J. Y.; Dexpert, H. J . Mater. Sci.
trochemical method resides in the possibility of controlling the oxidation state and the concentration of the included ions. We have shown recently that the electrochemical inclusion of copper and platinum species in PMeT leads to new catalytic properties for proton reduction in H2.l3 In this paper we report the ESR study of a S03CF3- doped PMeT containing various amounts of electrochemically included Cu2+ions. When the polymer is cathodically polarized during a short time (1 C t 4 50 s ) in an aqueous CuClz solution, its conductivity increases (u N 50-150 Q-' cm-I for pressed-pellet samples) and is associated with the formation of isolated Cu2+ ions in a strongly axially distorted octahedral environment. In this configuration, four water molecules and two methylthiophene units surround the Cu2+ion, which gives a bidimensionally ordered polymer. For longer polarization times ( t > 50 s), Cu2+clusters are formed, which do not affect these new properties. Thus, we demonstrate that the complexation of ions by an organic conducting polymer is a new approach for increasing the organization of such systems. Experimental Section Poly(3-methylthiophene) (PMeT) samples were prepared by electrochemical synthesis according to a previously published p r ~ c e d u r e . ' ~The polymerization was performed in a singlecompartment cell with the classical three-electrode configuration, in an electrolyte medium composed of acetonitrile (CH3CN), a supporting electrolyte (Bu4NCF3S0,, 5 X 10-'-1 M), and the monomer (3-methylthiophene, 5 X 10-'-1 M). PMeT in its oxidized form was grafted on a Pt electrode with a controlled potential. Acetonitrile (Normapur grade) purchased from Prolabo and the supporting electrolyte (Fluka) were directly used without further purification. The monomer was utilized just after distillation. Argon was bubbled through the solution before electrolysis. Doped PMeT was synthesized by applying an anodic potential of +1.35 V/SCE (saturated calomel electrode) to a Pt electrode. It was rinsed with acetone, dried in a N, stream, and immersed in an aqueous 1 M CuCl, solution. Polarization at -0.2 V/SCE led to a partial dedoping of the polymer from 50% to 30%
1985, 20, 2681.
(12) Tourillon, G.; Dartyge, E.; Dexpert, H.; Fontaine, A,; Jucha, A,; Lagarde, P.; Sayers, D. E. J. Electroanal. Chem. 1984, 178, 357.
(13) Tourillon, G.; Gamier, F. J. Phys. Chem. 1984, 88, 5281. (14) Tourillon, G.; Gamier, F. J . Electroanal. Chem. 1982, 135, 173.
0022-3654/86/2090-5561$01.50/00 1986 American Chemical Society
5562
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Gourier and Tourillon
I 33LmT
I;
3
50 -
0
1m
50
150
250
200
PCCARlZATlON
TIME
( 51
Figure 2. Conductivity of S03CF3doped PMeT vs. cathodic polarization Figure 1. 9.3-GHz ESR spectrum at (a) room temperature and (b) 100 K of the conduction electrons (CESR) of PMeT doped with 50 mol %
time.
1
SO3CF3. (determined by elemental microanalyses) and to the inclusion of copper species: Cu2+, Cu+ for short polarization times (1 s C t < 6 min) and Cu+, Cuo for t > 6 min.I5 The results reported in this work concern only the first step, i.e., formation of Cu2+and Cu+ ions. The treated polymer was then, rinsed with water and dried. The concentration of copper (determined by X-ray absorption spectroscopy for very short polarization times (1 i1 i50 s)) corresponds to 1 copper for 40 monomeric units. The conductivity values before and after copper treatment were determined by the four-probe method using pressed pellets of the polymer. ESR experiments were performed at X band (9.5 GHz) and at Q band (35 GHz) by using Bruker 220 D and Varian CS-E109 spectrometers fitted with variable-temperature accessories (100 < T < 300 K). Measurements of the ESR parameters were performed at X band with a BNM 12 N M R proton probe and a microwave frequency counter.
Results The conductivities of PMeT doped with 30-50 mol % S03CF3ions lie in the range 50-80 $2-' cm-' at 25 OC.'O This doped polymer exhibits a single ESR line characteristic of the existence of a metallic state8 (Figure 1): (i) The line shape is asymmetrical (Dysonian) due to the limited penetration of the high-frequency field in the conducting particles. The asymmetry of the ESR signal is obviously not due to an anisotropy of the g factor since the line shape is independent of the klystron frequency (9 and 35 GHz). (ii) The ESR intensity is independent of the temperature between 300 and 200 K and decreases significantly below 200 K.* This behavior was explained by a Pauli spin paramagnetism and indicates the presence of a metallic state above 200 K. The phase transition at 200 K is not yet understood, and its origin is presently under investigation. (iii) The small g shift Ag = 5 X lo4 excludes the contribution of the heteroatom to the ground state. This result agrees with recent theoretical calculations of the band structure of polyt h i ~ p h e n ewhich ~ . ~ shows that the valence band has a purely carbon pT character. Electrochemical inclusion of copper ions in PMeT leads to an increase of the polymer conductivity (Figure 2). The highest conductivity value ( N 150 Q-' cm-') is obtained after a cathodic polarization time of about 50 s and remains constant after longer polarization times. Two modifications in the ESR spectra are observed when copper ions are incorporated in the polymer: (i) a broadening of the (15) Tourillon, G.; Dartyge, E.; Fontaine, A,; Jucha, A. Phys. Rev. Lerf.,
to be
published.
0
50
loo
150
200 FOLARIZATDN TIWE
250 (SI
Figure 3. Variation at room temperature of the CESR line width AB with the cathodic polarization time.
I
y
CESR
Figure 4. 35-GHz (Q band) ESR spectrum at (a) room temperature and (b) 100 K of S03CF, doped PMeT polarized for 65 s.
conduction electron spin resonance (CESR) signal and (ii) the observation of ESR signals due to Cu2+ species. The variation of the CESR line width (AB) with the polarization time is shown in Figure 3. The broadening is independent of the klystron frequency, indicating that it is not due to a distribution of g factors produced, for example, by an inhomogeneous distribution of copper species in the polymer. In addition to the CESR signal at g = 2.0028, the spectra of cathodically polarized PMeT are characterized by the appearance of several signals with positive g shifts due to Cu2+ions. Figure 4 shows the ESR spectra at Q band recorded at room temperature and at 120 K for a S03CF3doped PMeT polarized for 65 s. At
Highly Ordered Organic Conducting Polymers
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5563
20mT
Figure 5. 9.3-GHz ESR spectrum at (a) room temperature and (b) 100 K of S03CF3doped PMeT polarized for 65 s.
TABLE I: ESR Parameters of Polarized PMeT signal
8
A , mT
CESR
-pgll ==2.003 2.402
All = 13.3
D2+1 I
g, = 2.09
[Cu2+I*
g = 2.184
this frequency, the different ESR signals are well separated. The spectrum is typical of Cuz+ in an axially distorted environment with gll = 2.042 and g, = 2.09 (see Table I). This species will be hereafter referred to as [Cuz+] Its signal intensity decreases upon increasing the temperature, in contrast to the CESR intensity. The spectra are more complicated at X band, as shown in Figure 5. The signal observed a t high magnetic field corresponds to the CESR transition while all the other signals at lower field, which partially overlap, are due to two Cu2+species. These two species can be studied separately due to the fact that the variation of their intensities with temperature is different. The broad signal (AB = 12 mT) at g N 2.184 increases with temperature, as opposed to the other transitions of Cuz+ at high and low field. This new species will be hereafter referred to as [Cu2+I2. The transition at g 2.09 and the weak ones at g = 2.042 and A = 13 mT correspond to the perpendicular and the parallel components, respectively, of the [Cu2+] species. The assumption that these ESR signals belong to two different copper species is reinforced by the evolution of their intensity changes with the polarization time. The contribution of [ C U ~ + ] ~ to the spectrum is significant only after long polarization times (Figure 6 ) .
-
Interpretation of the Spectra of Divalent Copper [ Cu2+] Species. The ESR spectrum of [Cu2+] is typical of a Cu2+ion in an axially distorted octahedral environment. The lack of splitting at Q band of the perpendicular component of the spectrum indicates that there is no orthorhombic distortion. The observed g values gll> g, > g, are characteristic of an unpaired electron in a purely dX2-,,2orbital. In contrast, dz2 ground state should lead to g, > gll = g,.l6 The axial symmetry of [ C U ~ +and ] ~ its electron ground state d2-9 show that the coordination polyhedron of Cu2+is a distorted octahedron with D4,, symmetry. [Cu2+I1may thus be written C U ~ + ( L ~ ) , (where L ~ ) ~L1 and L2 are the ligands, L1 being located in the plane perpendicular to the C, axis and L2 along this axis. (16) Wertz, J. E.; Bolton, J. R.In Electron Spin Resonunce; McGraw-Hill: New York, 1972.
Figure 6. Evolution of the ESR spectra of S03CF, doped PMeT with the cathodic polarization time ( v = 9.3 GHz, T = 120 K).
ESR alone does not allow one to determine the nature of the ligands. However, considerable insight concerning the nature of the ligands can be obtained from EXAFS measurements. A recent study made on copper doped PMeT with this technique confirmed the divalent state of the copper ions and revealed that three or four oxygen atoms are present in the first shell of Cuz+ within 1.95 A.17 This bond length is close to the Cu-0 distance in aqueous CuClZsolution (six Cu-0 bonds at 1.95 A).17 In addition to the nearest-neighbor shell of oxygen, the Fourier transform clearly shows other neighbors at 2.32 A. This distance should correspond to a Cu-Cl or to a Cu-S bond.’* Owing to the axial symmetry of [Cu2+I1,ESR is consistent with EXAFS only if we assume that the ligands L1are water molecules. The reasons are the following: (i) The presence of only three oxygen ligands is impossible symmetry. Thus, there are necessarily four because of the D4,, first-neighbor oxygens located in the q, plane of the polyhedron. (ii) The g values obtained by ESR are similar to that of partially dehydrated C U ’ + ( H ~ Ocomplexes )~ trapped in cages of zeolites.19 (iii) The ligands cannot be attributed to oxygen atoms of S03CF< ions because steric effects would be expected with such large molecules, subsequently leading to rhombic distortion of the coordination polyhedron. Such distortions are not detectable in the ESR spectra at X or Q bands. Consequently, one single species of the type C U ~ + ( H ~ O ) ~satisfies ( L ~ ) ~both ESR and EXAFS results. Based upon the EXAFS measurements, the L2 ligands could be Cl- ions or S atoms from the methylthiophene rings. However, Cl- ions appear unlikely since these ligands produce less crystal field stabilization energy than S2- ions. In fact, estimations of crystal field splitting 10 Dq for C1- and S2-give 16 000 and 26 000 cm-I respectively.2o [Cu2+] is therefore more likely attributed (17) Dexpert, H.; Tourillon, G.; Lagarde, P. In EXAFS and Near Edge II& Hodgson, K.O., Hedman, B.,Penner-Hahn, J. E., Eds.; Springer-Verlag: West Berlin, 1984; Springer Ser. Chem. Phys. (18) Structural Inorganic Chemistry; Wells, A. F., Ed.; Clarendon: Oxford, 1962; pp 337, 882. (19) Conesa, J. C.; Soria, J. J . Chem. SOC.,Faraday Trans. I 1979, 75, 406.
5564
Gourier and Tourillon
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
times of the conductivity should thus be the intrachain scattering , the interfibrillar time T , , , the interchain hopping time T ~ and hopping time qnter. We have recently shown, by an in situ X-ray absorption study, that several electrochemical steps are observed when a cathodic polarization is applied to a S 0 3 C F 3doped PMeT immersed in an aqueous CuCI, s o l ~ t i o n : ' Cu2+ ~ c u + CUO. The first step (Cu2+ Cu') is obtained for a short polarization time t (1 s < t < 6 min) and leads to the complexation of Cut by the polymer, this ion being unstable in aqueous solution. The ESR results reported in this work only concern this first stage. Additional ex situ ESR and EXAFS experiments have indicated the formation of Cu2+species in PMeT. In fact, a redox process occurs between the doped polymer P+ and the Cu+ ions and leads to a partial dedoping of the polymer: P+ Cu+ Po + Cu2+ as observed with a Fe(CN)64- doped p o l y p y r r ~ l e . ~In~the present case the concentration of Cu ions is rather low (one Cu+ for 40 monomer units) as determined by the absolute absorption coefficient value,*swith the doping level in the range of 30%. It is thus expected that this transformation does not affect significantly . the increase of the intrachain scattering time T ~ ~Moreover, conductivity occurs for short polarization times ( t < 20 s) when only the [Cu2+I1species are formed. Thus, Cu2+clusters [Cu2+I2, which are produced at longer polarization times, do not modify the transport properties. The formation of isolated CuZt ions ([Cu2+I1)enables the bridging of neighboring PMeT chains to occur and consequently leads to a bidimensional ordering of the polymer. Such ordering should result in an increase of the interchain hopping frequency T ~ - ' . The evolution of the CESR line width AB with the polarization time t confirms this assumption. Comparison of Figures 2 and 3 shows that AB closely follows the variation of conductivity. Such dependence of AB on the conductivity in a metallic material indicates that the CESR line width is dominated by spin-phonon interactions through spin-orbit coupling.24 AB is thus determined by the magnitude of the g shift Ag and the electron-phonon scattering time (Elliott me~hanism).,~Furthermore, Weger has shown that, in quasi-one-dimensional systems, AB is determined by the interchain hopping rate T ~ - ~ : ~ ~
- -
-
CH,
+
C H3 CH3 Figure 7. Suggested structure of the [Cu*+], complex.
to CU~+(H,O)~(M~T),, where MeT represents a methylthiophene unit of a PMeT chain. A possible structure is suggested in Figure 7 . It should be emphasized that the existence of C U ~ + ( H , O ) ~ linking the different PMeT chains produces a bidimensional ordering of the chains. This conclusion is moreover reinforced by the transmission electron microscopy and X-ray diffraction data obtained on PMeT cathodically polarized in the aqueous CuC1, solution during a short time ( t < seconds). The studies indeed reveal the existence of well-defined crystalline patterns which do not fit with the formation of any mineral phase (hydrated CuCl,, crystallized Cu+ salt). These results clearly indicate that such treated PMeT evolves from an amorphous state to an organized one. A complete crystallographic description of this new material will be given in a forthcoming paper and is not the purpose of the present study.21 [Cu2+I2Species. The ESR spectra of [Cu2+I2exhibit no apparent structure, indicating that both the hyperfine structure and the g tensor anisotropy are suppressed by other interactions. It appears likely that exchange and dipolar interactions between Cu2+ ions could be responsible for this effect. Thus, [Cu2+I2should be attributed to clusters of Cu2+ions. In this case the decrease of the ESR intensity which was observed upon decreasing the temperature could be the result of an antiferromagnetic coupling between the electron spins. The nearly total disappearance of the [Cu2+I2signal at Q band could originate from a line broadening produced by a g-factor distribution. The evolution of the ESR spectra with the polarization time is consistent with this interpretation (Figure 6). At very low copper content (short polarization time), Cu2+ is essentially represented by isolated [Cu2+] complexes, and Cu2+clusters contribute significantly to the spectra only when more important concentrations of copper are introduced in the polymer. Discussion The doping of PMeT with S03CF3by electrochemical methods allows concentration levels as high as 50 mol %" to be reached. The resulting polymer exhibits a metallic behavior as shown by ESR,8 UV-visible absorption,I0 XPS, and UPS spectrosc~pies.~ However, the macroscopic conductivity, in the range 50-80 9-1 cm-', is much less than could be expected from spectroscopic data. Scanning (SEM) and transmission (TEM) electron microscopy studies have revealed that a 50 mol % S0,CF3 doped PMeT polymer has a fibrillar structure with a fibril diameter of about 1500 A.22 The poor contact between fibers is thus the principal factor limiting the conductivity. The intrafibrillar conductivity should also be limited by the hopping between the different polymeric chains constituting the fibril. The three characteristic
AB
0:
-
(Ag),T,-'
Consequently, both the macroscopic conductivity and the CESR - ~ by the inclusion line width reflect the increase of T ~ produced of copper. Alternatively, if AB would be dominated by spin-spin interaction, or more precisely by the magnetic field produced by the localized spins of Cu2+ions, its magnitude should vary as AB 0: n1/2,where n is the concentration of Cu2+ ions.26 In this case both [Cu2+I1and [Cu2+I2contribute to the magnetic field, and the increase of AB with t should be effective for all polarization times. Based on this mechanism, the variation with t of AB and u should be very different. Furthermore, the signal, homogeneously broadened at t = 0, should become inhomogeneously broadened at t > 0. The fact that these two features were not observed in our experiments supports the concept that AB is controlled by T ~ - ' . Finally, it is important to note that no modification in either the conductivity value or the crystallinity is observed when the doped PMeT is immersed in the aqueous CuCl, solution. That is, without any cathodic polarization, no copper species are detected by EXAFS and no ordering is observed by X-ray diffraction.*' Conclusion Poly(3-methylthiophene) doped with 50 mol % S 0 3 C F 3possesses a metal-like behavior. Inclusion of Cu2+ions in the polymer produces an increase of the conductivity correlated with an increase
(20) Figgis, B. N. In Introduction to Ligand Fields; Interscience: New
York, 1966. (21)~Tourillon,G.; Barraud, J. Y.; Dexpert, H. J . Electrochem. Soc., to be published. (22) Tourillon, G.; Gamier, F. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 33.
(23) Calvo, E. J.; Daroux, M. L.; Yeager, E. B. Proc.-Electrochem. SOC. 1985, 85-1, 125. (24) Elliott, R. J. Phys. Rev. 1954, 96, 266, 280. (25) Weger, M. J . Phys. Colloq. 1978, 1456. (26) Gordon, D. A. Phys. Rev. B: Solid State 1976, 13, 3738.
J. Phys. Chem. 1986, 90, 5565-5567 of the CESR line width. It is proposed that these two features result from an increase of the interchain hopping frequency resulting from the bridging of neighboring PMeT chains by isolated Cuz+ ions. The other copper species, most probably clusters of Cu2+ ions, do not modify the transport properties.
5565
Acknowledgment. The technical assistance of D. Simons is gratefully acknowledged. Registry No. PMeT, 84928-92-7; S03CFg-, 3718 1-39-8; Cu2+, 15158-11-9.
Analysis of the Direction of the Dipole Moments of Some Substituted Amides Bearing a Second Polar Group Maria M. Rodrigo, Maria P. Tarazona, and Enrique Saiz* Departamento de Quimica Fisica, Facultad de Ciencias, Universidad de Alcalii de Henares, Alcalii de Henares, Madrid, Spain (Received: January 28, 1986)
The direction of the dipole moment in N-substituted amides has been deduced by analysis of the experimental values of the dipole moments reported in the literature. The compounds studied have a second polar group in addition to the substituted amide group. The results indicate that the dipole moment of the amide group is directed at an angle (3 = 114 2" from the direction of the R-C*O bond, in very good agreement with previously reported results.
*
Introduction The orientation of the dipole moment in the amide group has been studied recently' through analysis of the experimental dipole moments of four N,N'-dimethylamides and their corresponding N-methylamides, following a semiempirical procedure used before2 to determine the orientation of the dipole moment in the ester group. The results obtained were consistent and indicated that the dipole moment makes an angle' /3 = 116 f 4 O with the direction of the R-C*O bond (see Figure 1). The disubstituted diamides previously studied' had one or two degrees of rotational freedom, and therefore, a conformational analysis was required in order to evaluate the average (p2)'l2, whose magnitude was then compared with the experimental results. Moreover, the two polar groups required for the procedure to be applied were both N-substituted amide groups. Thus, it seemed interesting to compare our previous results with the values obtained for other molecules which have only one N-substituted amide group and, besides, have a dipole moment which is independent of the conformation adopted by the molecule. The molecules studied in this paper are p-chloroacetanilide, p-bromoacetanilide, and N-ethyl-p-chlorobenzamide;for all these molecules one of the polar groups is a N-substituted amide group while the second dipole comes from a spherically symmetrical group, namely a halogen atom. The last molecule studied was succinimide, for which its cyclic structure does not allow any rotational freedom, and, therefore, the dipole moment of this molecule does not depend on any conformational parameter. Analysis of Results Table I summarizes the values used in the present work for the dipole moments of the amides studied and those of the model compounds representing their polar groups. All these values, taken from the l i t e r a t ~ r ewere , ~ determined in two different solvents. When more than one result was found for the same molecule, the value shown in Table I and used in this work was the average of the values reported in the same solvent and at the same temperature. p-Bromoacetanilide. Figure 2 shows the structure of this molecule in its planar conformation. Analysis of X-ray diffraction (1) Rodrigo, M. M.; Tarazona, M. P.; Saiz, E. J . Phys. Chem. 1986, 90, 2236. (2) Saiz, E.; Hummel, J. P.; Flory,P. J.; Plavsic, M. J . Phys. Chem. 1981, 85, 3211. (3) McClellan, A. L. Tables of Experimental Dipole Moments, Vol. I; Freeman: San Francisco, 1963. Vol II; Rahara entrp.: El Cerrito, CA, 1974.
0022-3654/86/2090-5565$01.50/0
data has shown4 that the plane containing the amide group makes a dihedral angle of 5.8" with the aromatic ring; however, given the spherical symmetry of the bromide atom providing the second dipole group, the dipole moment of the whole molecule does not depend on the rotation over the C-C*O bond and the conformation used for the present analysis is therefore irrelevant. The dipole moment in benzene solution, shown in Table I, was determined by Smith;s however, the value indicated for p-dioxane solution is the mean of the results reported by Thompson and Hallberg6 ( p = 4.47 D) and Aroney et al.' ( p = 4.66 D). Taking the dipole moment of the p-bromoacetanilide molecule as the vectorial sum of the dipole moment attributable to the Br-phenyl bond pBrc, and that of the amide group pA, one obtains cos
= (pZ- KBrC'
- PA2)/(2pBrCpA)
(1)
where p is the dipole moment of the p-bromoacetanilide molecule and 6 the angle between the dipole moments of the polar groups pBc and pA. Identifying pBrC and pA with the mean of the values reported for the dipole moment of bromobenzene and acetanilide, respectively (seeTable I), and relating 6 to the angle that the dipole moment of the amide group pA makes with the direction of the C-C*O bond /3 (see Figure 2, through the values of the valence angles 0CeNC = 128.9 f 2.3' and OC.CN = 117.7 f 2" obtained from X-ray analysis? we calculated the values of (3 shown in Table 11. p-Chloroacetanilide. The structure of this molecule is similar to that of p-bromoacetanilide (Figure 2). Its dipole moment was measured by Smith5 in benzene solution and by Aroney et al.' ( p = 4.64 D) and Gomel et ale8(k = 4.55 D) in p-dioxane solution for which the value used was, as before, the average between the two results reported in the literature (see Table I). Equation 1 can be used to calculate the value of 6 and, from this, the angle /3 can be obtained from the geometry of the molecule: namely, 6C*NC= 127 f 3" and OCeCN = 114 f 3". Taking the dipole moment of the p-chloroacetanilide molecule as the vectorial sum of the dipole moments attributable to the C1-phenyl bond, identified as the mean of the values reported for chloroben~ene,~ and the dipole moment of the acetanilide3 for the amide group we (4) Andreatti, G. D.; Cavalca, L.; Domiano, P.; Musatti, A. Acta Crystallogr., Sect. B 1968, 24, 1195. (5) Smith, J. W. J . Chem. SOC.1961, 4700. (6) Thompson, H. B.; Hallberg, K. M. J . Phys. Chem. 1963, 67, 2486. (7) Aroney, M. J. Le Fevre, R. J. W.; Singh, A. J . Chem. SOC.1963, 51 1 1 . (8) Gomel, M.; Lumbroso, H.;Peltier, D. C. R. Acad. Sci. 1962,254, 3857. (9) Subramanian, E. Z . Krystallogr. 1966, 123, 222.
0 1986 American Chemical Society
