Acta Chim. Slov. 2005, 52, 159–163 159 Scientific Paper Spectroscopic Study of Polyaniline Emeraldine Base: Modelling Approach Medhat Ibranim"* and Eckhard Koglin6 0 Spectroscopy Department, National Research Center Dokki, Cairo, Egypt E-mail: m.ibrahim@gom.com.eg b Institute of Physical Chemistry (ICG-IV), Research Center Juelich, 52425 Juelich, Germany Received 17-12-2004 Abstract The polymerization of aniline by Cu(II) montmorillonite was studied using attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Experimental spectra were compared with that calculated by AM1, PM3, PM5, MINDO, Hartree-Fock, HF/6-31g(d), as well as Density Functional Theory, BLYP/DZVP and B3LYP/6-31g(d,p). Furthermore, the final heat of formation is studied as a function of temperature. Results indicate that, for aniline B3LYP/6-31G(d,p) calculated frequencies are in a good agreement with experimental data, while HF/6-31G(d) is the optimal in čase of polyaniline. The vibrational calculations of a four-ring unit (emeraldine base: EB) are believed to be a good representation of the polyaniline at Cu(II)-montmorillonite. The final heat of formation is a function of polymerization and changed from aniline monomer to five ring repeat unit of polyaniline structure. Key words: aniline, polyaniline, vibrational spectroscopy, Ab initio calculation, heat of formation Introduction Conducting polymers continue to be the focus of active research in many areas and fields.1 Polyaniline is unique among conducting polymers in its wide range of electrical, electrochemical, electroluminescence, optical and anticorrosion applications as well as good stability.2"4 It is typically synthesized by oxidizing aniline monomer either electrochemicalh/.5"6 or chemicalh/.7"9 Aniline form complexes with transition metal ions in the montmorillonite interlayer by coordination of the free electron pair of the amino group to the metal ions.10"12 A reduction in the resistivity from 1013 to 103 il in the polyaniline polymethylmethacrylate film is noticed as a result of FeCl3 oxidation.13 Vibrational spectroscopy was used to investigate the infrared optics application of polyaniline emeraldine base films.14 Furthermore vibrational spectroscopy can be used to study the aggregation and interachain doping in emeraldin base.15 In addition, vibrational spectroscopy was used to study the interac-tion of polyaniline with the platinum (IV) ions.16 and polypyrrole.17 4-(3-(4-((4-Nitrophenyl)azo)phenyloxy)p ropyl) aminobenzene sulfonic acid (C3-ABSA) was de-signed and synthesized as a novel dopant of polyaniline. The molecular structures were characterized by FTIR, UV-Vis absorption and X-ray diffraction, showing that, the main chain and electronic structure are identical to the doped polyaniline, but exhibit partial crystalliniry.18 Ab initio calculations give an accurate description for the structure and vibrational spectra for huge number of molecules.19 Car-Parrinello ab initio molecular dynam-ics MD, was used to investigate polyaniline equilibrium geometries.20 In addition, the polaron lattice and the mechanism of conduction for doped polyaniline were studied by ab initio MO calculations.21 A scaled quan-tum mechanical oligomer force field for oligomers of leucoemeraldine base and for one oligomer of the imine form of polyaniline was established.22 Semiemperical method AM1 was used to investigate the proton effect on the electropolymerization of aniline.23 The corro-sion inhibition by aniline oligomers through charge transfer was studied using Densiry Functional Theory DFT.24 Both vibrational spectroscopy and ab initio calculations were used to study the adsorption and polymerization of aniline on Cu(II) montmorillonite.25 The present work is conducted to study the polymerization of aniline by Cu(II) montmorillonite, using attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. The experimen-tal spectra are compared with AM1, PM3, PM5, MINDO, HF as well as DFT calculated spectra. Furthermore, the final heat of formation is studied as a function of temperature at semiemperical PM3 level. Experimental Adsorption experiment: Batch adsorption experi-ments in the presence of oxygen were performed to Ibrahim and Koglin Polyaniline Emeraldine Base 160 Acta Chim. Slov. 2005, 52, 159–163 study the interlayer reactions of aniline on Cu(II)-montmorillonite in aqueous solutions. At concen-trations below a critical value of Cc = 2.6 mmol dm"3 only a coloured Cu(II)-aniline complex is formed, characterized by a stability constant of log (Kass /dm3mor1) = 1.5. At concentrations beyond Cc aniline polymerizes yielding a dark brown product.25 ATR-FTIR spectroscopy: Spectra were recorded interferometrically with a BRUKER EQUINOX-55 spectrometer equipped with a DLATGS detector. Single-beam IR spectra were the result of about 1000 co-added interferograms and ranged from 400 to 2000 cm"1 with a spectral resolution of 0.5 cm"1. A ZnSe ATR optical accessory set at 45° was used as a reflectance medium. Calculations details: Both Hartree-Fock and Den-sity Functional Theory calculations were performed with the Gaussian 98.26 suite of programs on the CRAY supercomputer at Juelich Research Center, Germany. The geometries were fully optimized without imposing external symmetry constraints. The geometry of aniline was optimized using both DZVP and 6-31G(d,p) basis set, while polyaniline is optimized using DZVP and 6-31G(d). Semiempirical AM1,27 PM3,28 PM5 and MINDO29 calculations were carried out on a personal computer, performed using quantum mechanics pack-age, MOPAC 2002 as implemented with the version 1.33 CAChe Program (by Fujitsu), at Spectroscopy Department, National Research Center, Egypt. A force constant calculation was then performed to obtain the vibrational frequencies. Furthermore, vibrational spectra were performed for each level of theory in the harmonic ap-proximation. The errors within this type of calculations including a significant part of anharmonicity correc-tions, it can be avoided using empirical scale factor.30"31 Results and discussion For utilizing ATR-FTIR to study the IR spectra of both aniline and polyaniline, we need precise assignements of vibrational wavenumbers. This is achieved by a comparison of the band positions and intensities observed in IR spectra with wavenumbers and intensities from molecular modelling calculations of both aniline and polyaniline. Aniline is optimized at semiemperical AM1, PM3, PM5 and MINDO in addition to Density Functional Theory methods, BLYP/DZVP and B3LYP/6-31G(d,p) respectively. Frequencies were calculated at the same level of theory and no negative frequencies are obtained which indicate that the calculations were done upon the optimized structure. Both experimental and calculated frequencies of aniline are tabulated in Table 1. There are 36 calculated genuine vibrations cor-responding to this molecule. The general formual for "\ //—^'=\ /^^—\ ^—^^—\ ^—^^—•" /x 12 Figure 1. General formula for both aniline and the base form of polyaniline emeraldin base and their calculated structures. aniline beside its calculated structure is mentioned in Figure la. Aniline has an empirical formula C6H7N4 and its molecular point group Cs is equal to 1. Table 1 presents the experimental vibrational spectrum of aniline in liquid phase. The spectrum of aniline can be assigned as: NH2 bending or scissors mode which is attributed to 1626 cm"1, a ring stretch-ing with a contribution of the NH2 scissoring band is appeared at 1604 cm"1. A band at 1498 cm"1 is characterized as typical ring stretching. The mode at 1265 cm"1 is assigned as parth/ to C-N stretching and parth/ to the ring stretching vibration. Comparing betvveen both calculated and experimental frequencies indicate that DFT methods give a good agreement with experimental results, specialh/ B3LYP/6-31G(d,p). Figure 2. ATR-FTIR spectrum of adsorbed aniline (2.0 mmol g-1) on Cu(II) montmorillonite. The polymerization of aniline in aqueous solu-tion was already studied.32 Tochima et al found that, aniline is oxidized by oxygen and subsequently polym- a i Ibrahim and Koglin Polyaniline Emeraldine Base Acta Chim. Slov. 2005, 52, 159–163 161 Table 1. Comparison betvveen calculated and experimental vibrational frequencies (cm v) of aniline in liquid phase. AM1 PM3 PM5 MINDO DFT1 DFT2 Exp. Assignment25 1733 1776 1631 1832 1652 1627 1626 NH2 sciss, ring str 1665 1678 1538 1632 1637 1608 1604 ring str., NH2 sciss 1588 1580 1490 1560 1515 1503 1497 C-H bend, ring str 1334 1369 1341 1294 1313 1271 1264 C-N str., ring str 1252 1239 1244 1270 1160 1176 1175 C-H bend, ip 1198 1170 1168 1204 1139 1156 1153 C-H bend 1109 1049 1108 1067 1028 1020 1030 ring def, ip 943 971 902 963 901 994 Ring breathing 670 643 617 823 813 800 792 C-N str., ring str. DFT1: BLYP/DZVP, DFT2: B3LYP/6-31G(d,p), Exp: Experimental frequencies. Table 2. Comparison between calculated and experimental vibrational frequencies (cm r) of the polvaniline emeraldine base. AM1 PM3 PM5 MINDO HF DFT Exp. Assignment25 1931 1857 1770 1943 1667 1680 Ring 2; C=N 1914 1827 1762 1939 1647 1643 1659 Ring 2; C=N 1733 1679 1543 1829 1624 1628 NH2 1695 1616 1514 1671 1602 1622 Ring 2; C=N 1637 1465 1491 1647 1599 1586 1599 Ring 2; C=N 1564 1441 1440 1605 1576 1573 1573 Ring 1, 3, 4 1560 1418 1402 1540 1500 1528 1531 Ring 4; C-N 1470 1387 1336 1505 1490 1515 1493 Ring 3; C-N 1245 1204 1258 1488 1373 1365 1341 Ring 4 1093 1091 1176 1387 1246 1327 Ring 3, 4, C-N 903 985 994 1356 1259 1263 1285 Ring 3, 4; C-N 798 804 816 1198 1186 1164 1166 ncies. Ring 1, 2, 3, 4 HF: HFLY P/6-31G(c ), DFT: VWN/DZVP, Ex p: Experim ;ntal freque erizes radically. The general formula and structure of polyaniline is described in Figure 1b. The empirical formula corresponding to polyaniline is C24H20N4, the molecular point group is corresponding to C1. There are 138 calculated genuine vibrations corresponding to this molecule. Both Table 2 and Figure 2 are presenting the IR spectra of polyaniline. The vibrational frequencies of C–N are obtained at 1285 cm–1 corresponding to ring 3, 4. Again, at 1493 cm–1, this band corresponding to C–N of the ring 3. Finally the C–N of the ring 4 is observed at 1531 cm–1. C=N of the ring 2 has two vibrational modes at both 1599 cm–1 and 1659 cm–1 respectively. Another two vibrational modes were noticed for the ring 4 and the ring 1, 2, 3 and obtained at both of 1341 cm–1 and 1573 cm–1. The last band which appeared at 1166 cm–1 is assigned to the ring 1, 2, 3, 4. The obtained results in-dicate that, both ab initio calculation HF/6-31G(d) and Density Functional method, VWN/DZVP gives good agreement with the experimental results. Moreover, both are considered to be more accurate as compared with that of semiempirical calculations AM1, PM3, PM5 and MINDO. This is due to the effect of electron cor-relation which is included in both types of calculation rather than semiempirical methods. It is worth to men-tion that, this effect of electron correlation is responsible for bringing the semiempirical calculations of aniline in fair agreement with experimental results while that of polyaniline is far from experimental results. The effect of electron correlation for aniline monomer is consid-ered as a little source of error in the calculations while polyaniline has four aniline ring unit so that the error increased and as a result its effect is clear. From the above results one can observe that, the vibrational calculations of a four-ring unit (emeraldine base:EB) are believed to be a good representation of the polyaniline at Cu(II)-montmorillonite. Furthermore, the presented model could be used to study the possible adsorption of polyaniline on the surface of other metals (mainly transition metals). Final heat of formation: Heats of formation are relative to the element in their standard state at 298 K. Final heat of formation is calculated at PM3 level Ibrahim and Koglin Polyaniline Emeraldine Base 162 Acta Chim. Slov. 2005, 52, 159–163 A-------!------A____L_ Monomer i —&----- I I I I -30- -60- Dimer - *--------------^________ -------^____ -------^ -90 - Trimer •—_ ~—•¦—_ -120- -150- 5 Ring Polyaniline D-—^^ Emeraldine Base ~n^^ ~—-•—__ -•-—___ ^* i ' i ' l l l l i 200 250 300 350 400 450 500 Temperature, K Figure 3. Final heat of formation as a function of temperature which calculated at PM3 level of theory for aniline monomer, dimer, trimer, four ring unit (emeraldine base) and five ring unit polyaniline. of theory for aniline monomer, dimer, trimer and two structures of polyaniline namely: four ring unit and five ring unit. The final heat of formation is studied as a function of temperature in the range 200 up to 500 K. As seen in Figure 3, and starting with aniline monomer the heat of formation is changed with temperature from –19.2 kcal/mol up to –27.8 kcal/mol in the studied temperature range. Regarding aniline dimer the calculated final heat of formation is changed from –82.1 kcal/mol up to –94.6 kcal/mol. The same behavior is noticed in case of aniline trimer. The heat of forma-tion corresponding to emeraldine base polyaniline which as a function of temperature is changed from –138.1 kcal/mol to –167.7 kcal/mol. Finally, five ring polyaniline shows a change in its final heat of formation and changed from –127.8 kcal/mol to –162.7 kcal/mol. It is clear that, the final heat of formation is a function of polymerization and decrease from aniline monomer to five-ring unit. Conclusion From the comparison of both experimental and calculated frequencies it is concluded that, both scaled Hartree-Fock, HF and Density Functional Theory, DFT, results considered being more accurate than other methods according to electron correlation effect which is of concern in these kinds of calculation rather than semiempirical calculations. Both calculated and experi- mental data are in good agreement with each other. It is a kind of challenge to perform vibrational analysis for this polymer. So that, the presented data of polyaniline suggest that IR data calculated ab initio on relatively short oligomers (quantum-mechanical oligomer ap-proach) may provide valuable information regarding the interpretation of vibrational spectra of polymers. References 1. G. Liu, M. S. Freund, Macromolecules 1997, 30, 5660-5665. 2. Handbook of Organic Conductive Molecules and Polymers, Volumes 1-4, Wiley, New York, 1997. 3. P. Novak, K. Mueller, K. S. V. Santhanam, O. Has, Chem. 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Frisch, Exploring chemistry with electronic structure methods”, 2nd ed., Gaussian Inc., 1996. 31. P. Pulay, G. Fogarasi, J. E. Boggs, A. Vargha,/.^4w. Chem. Soc. 1983, 105, 7037-7047. 32. N. Toshima, H. Yan, M. Ishiwatari, Buli. Chem. Soc. Jpn. 1994, 67, 1947-1953. Povzetek Polimerizacijo anilina s Cu(II) montmorilonitom smo raziskovali s spektroskopijo ATR-FTIR. Experimen-talne spektre smo primerjali s spektri, dobljenimi z metodami AM1, PM3, PM5, MINDO, Hartree-Fockovo HF/6-31g(d), kot tudi teorijo gostotnih funkcionalov BLYP/DZVP in B3LYP/6-31g(d,p). Proučevali smo tvorbeno entalpijo kot funkcijo temperature. Rezultati za anilin kažejo, da se frekvence, izračunane z metodo B3LYP/ 6-31G(d,p), dobro ujemajo z eksperimentalnimi vrednostmi, medtem ko je za polianilin ujemanje optimalno pri uporabi HF/6-31G(d) metode. Menimo, da so vibracijski računi za verige, sestavljene iz štirih obročev, dober približek za polianilin v Cu(II) montmorilonitu. Pokazali smo, da je tvorbena entalpija odvisna od polimerizacije in se spreminja od monomere anilina do petčlenske verige polianilina. Ibrahim and Koglin Polyaniline Emeraldine Base