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JMol Struct 348 91

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JMol Struct 348 91
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  Journal of Journal of Molecular Structure 348 (1995) 91-94 MOLECULARSTRUCTURE A semiempirical approach for the calculation of the vibrationalspectra of conducting polymers: the case of polyselenophene F.J. Ramireza, V. Hernandeza, G. Zottib and J.T. L6pez NavarreteaaDepartamento de Qrumica Fisica, Facultad de Ciencias, Universidad deMglaga, Campus de Teatinos, 29071-MBlaga,Spain.bCNR, Istituto di Polarografia ed Elettrochimica Preparativa, Corso Stati Uniti 4,35020 Padova, Italy.The vibrational dynamics of oligomers of selenophene is treatedtheoretically and the experimental infrared and Raman spectra are studied inorder to derive information for the interpretation of the spectra of pristinepolyselenophene. Both the electronic and geometrical structures as well as thedynamical and spectroscopic properties have been calculated by thesemiempirical PM3 method. For the polymer, k=O phonon frequencies arecalculated and a vibrational assignment in proposed. 1. INTRODUCTION Since the discovery that polyacetylene can be made highly conducting bychemical or electrochemical doping, major efforts have been devoted to thesearch for organic polymers which would have similar properties. Conjugatedheterocyclic polymers such as polypyrrole (PPy), polythiophene (PTh),polyfuran (PFu) and polyselenophene (PSe) constitute an important class ofthese new materials. Among the four mentioned substances, PSe has been lessthorougly investigated than the rest.In the present work both the electronic and geometrical structures as wellthe dynamical and spectroscopic properties have been calculated by the PM3 [l]method for the oligomers of selenophene (Se),, with n = 1,2,3 and 4. Latticedynamic calculations are carried out for PSe using a set of Pulay coordinates [2]. The force field of the polymer was derived from PM3 calculations of thetetramer. The calculated force constants are scaled with the method proposedby Pulay [2], transferring a set of scaling factors from the monomer andpolythiophene [3]. In this work the calculations have been restricted to thevibrational in-plane modes of oligomers and the polymer. 0022-2860/95/$09.50 0 1995 Elsevier Science B.VAll rights reserved SSDI 0022-2860(95)08596-3  92 2AMonomerzSelenophene. To date, the vibrational spectrum of selenophene (Se) has been the subject ofvery little research [4-51. Consequently, we have undertaken the objective ofestablishing the complete assignment of the vibrational spectrum on the basisof our experimental infrared and Raman data as well as of dynamic PM3calculations. The main purpose here is to derive the spectroscopic informationin order to understand the spectra of PSe and to find the scaling factors for theforce constants used to build the F matrix of the polymer.The infrared spectra were recorded with a Perkin Elmer FTIR 1760Xspectrophotometer, and the Raman spectra were obtained with a Jobin YvonRamanor UlOOO spectrometer. The semiempirical PM3 force field wasevaluated in Cartesian coordinates and then transformed to a set of non-redundant Pulay coordinates. Experimental and theoretical details will begiven elsewhere [6]. According to the experimental [7] and the PM3 optimizedgeometries, the molecule belongs to the C,, punctual group and the in-planemodes are distributed as 8 A, + 7 B,. In the Table 1 we report the comparisonbetween the experimental and scaled frequencies. The normal modes aredescribed in terms of the potential energy distribution matrix (P.E.D.) usuallyTable 1Vibrational frequencies (in cm-l) for the in-plane normal modes of selenophenecalculated from unscaled and scaled PM3 force constants, and the comparisonwiht the experimental data.Species Factor ~,a~ v,,al vexp P.E.D. (greater 10%) Al 0.980731770.69211707 0.9197 13841.0408110210341.19007161.0329 440 % 3173314330731467132610347644553140177515111189ll8210491058807 81857562231083074142213431078101375845631081514107882062398v(C-H),%v(C-H)B7ov(c=c)+49v(c-c)24v(C-C)+30v(C=C)+26v(C-H)708(C-H)64&C-H)72V(C&)+22Sring 71s i,,+22V(CSe)99W-H),99v(C-IDB86V(C=C)92&C-H)706(C-H) 83sring86v(C-se)  93 adopted in molecular dynamics. The fitting between the experimental andscaled frequencies can be considered very good in the light of the fact that noleast-square adjustment of the force constants has been made after scaling.The scaling factors obtained are reported in the same Table.2d.OlQome~~ (Se),, n=2,3 and 4.In the Table 2 we show the calculates geometries, bond orders, ionizationpotentials and HOMO-LUMO bandgaps for the oligomers considered as anti-coplanar. The changes of these parameters with increasing the length fullysupport the existence of n-electronic delocalization. From the Table 2 it is clearthat, within the PM3 method, the calculated geometries do no substantiallychange with the conjugation length. On this basis, we decide to use in ourdynamical calculation on the polymer the optimized geometry calculated forthe tetramer.Polyselenophene was electrochemical prepared as described elsewhere [8].Before the study of the polymer we have transfered the scaling factors derivedfrom the work on selenophene to the case of the tetraselenophene. The PM3scaled force field for the inner part of (Se), has been used for the calculation ofthe phonon dispersion curves of the polymer by the solution of the k-dependent GF equation. When the polymer is taken as an one-dimensional lattice ofselenophene units, all anti-coplanars, the factor group is isomorphous with thepunctual group Da. In-plane phonons with k=O are distributed as 7 A, + 7 B Ig+6B,+6B,.In the Table 3 we propose a vibrational assignment for the in-plane modes. Moreover, to improve the fitting of the modes, we have used thescaled PM3 force field as a starting set for a refinement process, which resultsare included in the last Table. From those results on derive an importantconclusion: the anti-coplanar structure assumed for PSe in this work issupported by the good agreement between the calculated and experimentalspectra.Table 2Chain length dependence of PM3 geometries of the central structural unit andelectronic properties of anti-coplanar (Se)n, n=2,3 and 4 (bond lengths inAngstrom and energies in eV; in parenthesis the corresponding bond orders) se (se), (Se>,(SW4 r(C=C) 1.344(1.797)1.352(1.705)1.354&687)1.355(X82)W-C)l&47(1.118)l&2(1.136)1.4380.157)l/438(1.159)W-C*) - 1.410(1.066)1.409(1.069)1.4090.072)r(C-Se) 1.887c1.054)lNW.024) 1.888(1.024) l&38(1.024)IP 9.50 9.198.86 8.70Egap 8.94 7.987.27 6.91  94 Table 3Vibrational assignment for the k=O in-plane modes of polyselenophene derivedfrom lattice dynamic calculations. Species V,,l Vref Vexp(ir) Ve,&Ra) P.E.D. (greater 10%) Bl, 1615*, 1580B3U 1518B2u 1492*, 1389B2u l288Bl, 1277*,I211Bl,IL74B3U lx3B3U Ixx*fZ 1060B3U 868B2u 802Big 660B2ll 632*, 608BQ! 524B2u 447*g 226BQ! 204B3ll8362v(C=C)+46v(C-C*)63v(c-c*)+49v(c=c)84v(C=C)66v(c=c)+5ov(c-c)87v(C-C)%v(c=c)+33v(c-c)38v(C-C*)+266 . 46v(c=c)+13v~:~*)67&C-H)80&C-H)65&C-H)+lSv(C-C)64&C-H)886 * 73vi%e)+216 - 43v(c-Se)+376:;;;82v(C-Se)39v(C-Se)+18&C-c*1216 i,g+ZO&C-C*)736 * 41vi%e)+33&C-c*131&C-C*)+21v(C-Se)100&c-c*>15711518151614921388610 _ 1.2.J. J.P Stewart, J. Comp. Chem., 10 (1989) 209.I? Pulay, G. Fogarasi, G. Pongor, J.E. Boggs and A. Vargha, J. Am.Chem. Sot, 105 (1983) 7037.3.4.J.T. Lopez Navarrete and G. Zerbi, J. Chem. Phys, 94 (1991) 957,965.V. Alekranyan, M. Kimelfeld and N. Magdesieva, Y. Yurev, Opt.spectrosc., 3 (1967) 168.5. A. Santucci, G. Paliani and R.S. Cataliotti, Spectrochim. Acta, 41A (1985)679.6.F.J. Ramfrez, V. Hernandez and J.T. Lopez Navarrete, J. Phys. Chem., (tobe published).7. R.D. Brown, F.R. Burden and J.F?Godf%ei, . Mol. Spectrosc, 25 (1968) 415.8.G. Zotti, (to be published).
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