Theoretical and Experimental Vibrational Spectroscopic Investigation

Article pubs.acs.org/JPCA

Theoretical and Experimental Vibrational Spectroscopic Investigation of Two R1R2‑Diphenylsilyl-Containing Monomers and Their Optically Active Derivative Polymer Carmen M. González-Henríquez* Universidad Tecnológica Metropolitana, Facultad de Ciencias Naturales, Matemáticas y del Medio Ambiente, Casilla 9845, Correo 21, Santiago, Chile

Claudio A. Terraza Pontificia Universidad Católica de Chile, Facultad de Química, P.O. Box 306, Correo 22, Santiago, Chile

Mauricio Sarabia Pontificia Universidad Católica de Chile, Facultad de Física, P.O. Box 601, Correo 22, Santiago, Chile ABSTRACT: FT-IR and Raman spectra of bis(4aminophenyl)diphenylsilane (DIA) and a dicarboxylic acid containing the imide function and a L-alanine moiety (L-ALA) and their resultant polymer (PALA) were recorded in the 500−4000 cm−1 and 400−3800 cm−1 regions, respectively. The optically active poly(imide-amide) obtained has two sp3 carbons in the main chain, favoring its flexibility. Raman analysis identifies the fluorescence produced by the electronic conjugation between the aromatic rings and the amidic groups, which affects the molecular fine structure. Thus, the theoretical study of the vibrational patterns has become a support and a complementary technique for the characterization of this fluorescent system. The optimized molecular geometry of the monomers and the polymeric unit using B3LYP and HF methods at the 6-31G(d) level of theory were used for the vibrational assignments. Thus, the small variations between the calculated and experimental vibration values could be related to possible intra- and/or intermolecular interactions or to the existence of a charge transfer phenomena between a donor or acceptor group within the system.

I. INTRODUCTION Poly(imide-amide)s are thermoplastic materials that present good high-temperature resistance, outstanding mechanical properties, and great oxidative stability. Due to these properties, these polymers are widely used in electronic materials, adhesives, composites materials, fibers, and film materials among others.1−8 However, some of their applications are limited by their high rigidity and low solubility in common organic solvent, due to the existence of fully occupied aromatic rings in the polymer backbone. The incorporation of chirality factors in the polymers often adds relevant functions as key basic high-performance materials.9,10 The synthesis of chiral polymers containing amino acid moieties is a subject of much interest due to their harmless, biocompatible, and biodegradable properties, which can be employed in biomedical applications.11,12 On the other hand, the synthesized compounds require characterization of their structural properties. This can be done via the analysis of a detailed molecular vibration spectrum, including molecular stretching, bending, wagging, and scissoring vibrations. Although the fluorescence emitted by some © XXXX American Chemical Society

compounds makes the interpretation of the Raman spectrum difficult, this technique is still suitable for a qualitative and quantitative analysis in order to evaluate the extent of the conformational and configurational isomerism of specific structures and their degree of crystallinity.13−19 In 1991, Yokoyama et al.20 reported the simulation of Raman spectra of formic acid monomer and dimer in the gaseous state, whose parameters were estimated empirically on the basis of the bond polarizability model. Recently, Wang et al.21 studied the resonance Raman spectra of the 4,5-ethylenedithio-1,3dithiole-2-thione compound; herewith, the density functional calculations were performed to elucidate the electronic transitions. Lin et al.22 have focused their investigation on calculation of infrared/Raman spectra of poly(lactic acid) polymorphs by employing density functional perturbation theory (DFT) methods.23a,b However, the results obtained were analyzed in Received: September 13, 2013 Revised: January 28, 2014

A

dx.doi.org/10.1021/jp409178j | J. Phys. Chem. A XXXX, XXX, XXX−XXX

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order to understand the effect of symmetry, molecular packing, and intermolecular interaction on the vibrational spectra, based on the crystalline model. On the other hand, Johannessen et al.24 studied the Raman optical activity (ROA) spectra of both enantiomers of 2-bromo-hexahelicene. Their results were similar using theoretical (DFT) and experimental methods.25 According to previous research,20−25 the novelty of this work is based on the comparison between the experimental and theoretical results derived from the vibrational spectra of the two difunctional silylated monomers (DIA and L-ALA) and the corresponding repetitive unit of the polymer (PALA) in compounds that show a large fluorescence background in the same vibration region of the functional groups that should be characterized by experimental methods. This problem of noise is associated with compounds that present high amorphicity or fluorescence,26 a limitation in the Raman spectroscopy studies. In order to rationalized this, Hartree−Fock (HF) and density functional theory (DFT) calculations based on Becke’s three parameter hybrid model using the Lee−Yang−Parr correlation functional (B3LYP) at the 6-31G(d) level were employed to obtain calculated vibrational frequencies for the monomers and the poly(imide−amide). These values were compared with the experimental results obtained using Infrared and Raman analysis.

Scheme 1. Schematic Representation and Theoretical Geometric Structure Obtained by Using DFT Method for DIA Monomer, L-ALA Monomer, and an Approach to the Repetitive Unit of PALA Polymer

II. EXPERIMENTAL SECTION Materials (Scheme 1). Aromatic diamine (bis(4aminophenyl)diphenylsilane; DIA) and the dicarboxylic acid containing the imide function and the L-alanine moiety (L-ALA) were prepared according to procedures described in the literature.27−29 PALA was synthesized from DIA and L-ALA (1:1 mol/mol) by using the Yamazakiś direct polycondensation methodology.30,31 Equipment. Infrared spectra (FT-IR) of the monomers and polymer were recorded in the region 500−4000 cm−1 on a Perkin-Elmer FT-IR-1310 spectrometer; samples were prepared by grinding with powdered potassium bromide (KBr) and pressing into a disc. The Raman studies were obtained thought film deposition, where DIA and L-ALA were dissolved in acetone and PALA in THF. These solutions were distributed on a glass substrate with a spin coater (Chemat Technology, KW-4A); afterward, the remaining solvent was removed at room temperature in vacuum (1 × 10−3 to 8 × 10−4 Torr) for 2 h. The unpolarized Raman spectroscopy was performed with a LabRam 010 instrument from ISA equipped with a 5.5 mW HeNe laser beam (633 nm) in the 400−3800 cm−1 region. The Raman microscope uses a backscattering geometry; microscopy objectives lenses were Olympus Mplan 50× and Olympus Mplan 10×. The exposure time and accumulations varied within the different samples in order to achieve the optimal peak definition.

and 0.8929 were used in B3LYP and HF calculations, respectively.33 On the other hand, DFT calculations are known to provide a good correlation between the theoretical and experimental values of vibrational frequencies in organic compounds if the calculated energies are scaled to compensate the approximate treatment of electron correlation, for basis set deficiencies and for the anharmonicity effects.34−40 All the vibrational spectrum assignments are based on the respective symmetry point group of the molecule, Ĉ 2 for DIA and L-ALA and Ĉ 1 for PALA.41 Assignments were made through visualization of the atomic displacement representation for each vibration through Gaussview v 3.0.1 and matching the predicted normal frequencies and corresponding intensities with the experimental data.

III. COMPUTATIONAL DETAILS All the calculations were performed using Gaussian 03, rev D.02 program package on a personal computer.32 The theoretical study was performed at HF/6-31G* and B3LYP/6-31G* levels of theory to get the optimized geometry and the corresponding vibrational frequencies of the normal modes of the molecules studied. The DFT hybrid B3LYP functional tends to overestimate the fundamental modes; therefore, scaling factors have to be used to obtain a considerably better agreement with the experimental data. In this work, scaling factors of 0.9613

IV. RESULTS AND DISCUSSION Structurally, DIA contains two kinds of substituents bonded to central silicon atom, i.e., 4-aminophenyl and phenyl groups. The four phenylic rings are not coplanar due to the high steric impediment generated around silicon atom (Scheme 1a). Scheme 1b shows the optimized geometry for the dicarboxylic B



696, 634 two bands (w) 690, 639 two bands (w) 704, 659, two bands (ms) (14,15) 673 (w) 678 (w) 643 (w) (10)

The numbers in parentheses in the Experimental column represent the type of vibration plotted in Figure 1a and b obtained from Raman and FT-IR spectra, respectively. oop = Out of plane. ip = In plane.

−C−H bending (oop and ring torsion), R2C6H4 oop NH2 wagging

(w) (1) 3360 (w) (2) 3045 (mw) (3) 1627 (w)(4) 1570 (w) 1600, 1591 (s) (5) 1194 (w) (6) 1116, 1102 (ms) (7) 1004 (s) (8) 835 (s) (9) asymmetric NH2 stretching symmetric NH2 stretching amine primary (−NH2) stretching C−H stretching ip NH2 scissors, bending Si−C arom. stretching CC stretching C−N stretching (H2N−C arom.) −C−H bending (oop, ip, and ring torsion), R-C6H5

a

theoretical (HF) theoretical (B3LYP)

3510 (w) 3414 (w) 3079−3045, three bands (w) 1628 (s) 1595−1495 (m) 1275 (m) 1178 (m) 1086 (m) 813 (w) (two bands) (m) (1) (s) (2) (w) (3) (mw) (4,5) (ms) (6) (mw), 997 (w) (7), 1504 (s), 1303 (w) (m) (8) (m) (9) (s) (10) (w) (11) (m) (12) (s) (13) 3456−3438 3359 3210 3064−3017 three bands 1619 1561 1596 1270 1186 1111 1026 746 820

experimental (No) theoretical (HF) experimental (No) vibration

theoretical (B3LYP)

3563 (m) 3454 (s) 3016−2995 (m) 1602 (m) 1276 (m) 1181 (w) 1078 (m) 1005 (w) 806 (ms)

Infrared spectroscopy (ν, cm‑1) Raman spectroscopy (ν, cm‑1)

Table 1. Comparison of the Experimental and Theoretical Wavenumbers of DIA Monomera C

3510 (w) 3414 (s) 3079−3045 (w) 1625 (w) 1595 (mw) 1275 (w) 1178 (w) 1086 (w) 1014 (m) 813 (s)

acid. The monomer contains a silicon atom bonded to four aromatic rings, with a chiral center contributed by an α-amino acid moiety. On the other hand, Scheme 1c shows an approach to the repetitive unit derived from the reaction between DIA and L-ALA. In this structure, the explicit backbone flexibility obtained from the incorporation of the amino acid residue is combined with the silicon atoms and the symmetry breaking presented by the aromatic rings. Based on these theoretical geometric structures, IR and Raman bands were estimated and compared with the experimental data. These results with their relative intensities and assignments are shown in Table 1, Table 2, and Table 3 and summarized in Figure 1a,b, Figure 2a,b, and Figure 4a,b, respectively. Experimental Raman and FT-IR spectra are showed in the Figure 1c,d, Figure 2c,d, and Figure 4c,d, respectively. In addition, the most representative theoretical signals were obtained by DFT (B3LYP) and HF with its respective intensity. DIA Monomer. Figure 1a,b compares the vibrations obtained from experimental Raman (Ramanexp) and FT-IR (FT-IRexp) spectra and the calculated data, respectively. These values are summarized in Table 1. For the NH2 group, two well-defined peaks were observed due to the asymmetric stretch at higher frequencies, separated by 18 cm−1, absorbing moderately in the 3456−3438 cm−1 range obtained by FT-IRexp. DFT and HF calculations predict this mode at 3510 cm−1 and 3463 cm−1, respectively. Moreover, these bands were predicted in 3510 cm−1 and 3563 cm−1 in the Raman spectrum according to DFT and HF calculations. These values are similar to those obtained experimentally at 3480 cm−1. The symmetric NH2 stretching appears strongly at 3359 cm−1 in FT-IRexp and as a weak band at 3360 cm −1 in the Ramanexp spectrum. Nevertheless, the calculated frequency values are 3414 cm−1 and 3454 cm−1 by using DFT and HF for Raman absorption, respectively. Besides model limitations, the differences are also related to uncertainties in the assignment of the vibrational frequencies due to low signal-to-noise ratio, and the possible interference with vibrations of the aromatic ring and CC stretching modes. The experimental overtone band near 3200 cm−1 was better resolved in the FT-IR than in Raman, and it corresponds to the Fermi resonance of the NH2 stretching. In addition, the in-plane (ip) vibrations of the NH2 groups appear at 1619 cm−1 showing a medium-strength FT-IR intensity. However, these modes were obtained at 1628 cm−1 and 1625 cm−1, calculated for DFT and HF, respectively. It was obtained at 1627 cm−1 with a weak intensity in the Ramanexp spectrum. A band observed at 1270 cm−1 in the experimental FT-IR spectrum was assigned as the C−N stretch of arylamine. On the other hand, DFT and HF model calculations predict this frequency at 1275 cm−1 and 1276 cm−1, for both IR and Raman spectra. The out-of-plane (oop) NH2 wagging mode was obtained by FT-IRexp spectroscopy at 704 cm−1 and 659 cm−1. This signal was calculated at 690 cm−1 and 639 cm−1 by using DFT and 696 cm−1 and 634 cm−1 at the HF methods. In addition, this frequency was obtained at 643 cm−1 in the Raman spectrum, whose band calculations were predicted at 678 cm−1 and 673 cm−1 by using DFT and HF methods, respectively. The existence of one or more aromatic rings was determined from their related C−H and CC ring vibrations. The aromatic compounds exhibit a multiple medium-weak band in the 3064− 3017 cm−1 range due to aromatic C−H stretching vibrations. In the present case, DFT and HF calculations predict vibrations in the 3079−3045 cm−1 and 3022−2987 cm−1 range, respectively.

3463 (m) 3454 (m) 3022−2987, three bands (w) 1625 (s) 1603,1501 (m) 1276 (ms) 1175 (ms) 1078 (m) 1001 (w) 746 (w) 823 (w)

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1592 (s), 1586 (w)

1383 (w) 1263 (w) 1176 (m) 800 (w) 730 (w) 706 (w)

2650, 2624 (1417w) (4) 1780 (s) (5), 1723 (w) 1614, 1592 (s) (6) 1571 (w) 1417 (w) (7) 1386 (w) (8) 1317 (w) (9) 1201 (m) (10) 1109 (w),1002 (s) (11) 920 (w) (12) 713 (w) (13) 682 (w)

CH3 and CH stretching CO cyclic imides (ip and oop) and CO stretch, monomer and dimers H-bonded of carboxylic acid CC in ring, stretching

Si−C aromatic C−O−H ip bend of carboxylic acid CH3 bending deformation

C−N stretching C−O stretching of carboxylic acid −C−H bending (ip and ring torsion) mono-substituted R-C6H5 O−H oop bending −C-H bending (oop) and ring torsion, tri-substituted R3-C6H3 −C−H bending (oop), monosubstituted R-C6H5

D

-

765 (w) 712 (w)

1360 (w) 1213 (m) 1134 (w)

1528 (w) 1404 (w) 1460, 1462 (w)

1618−1603 (m)

2881 (w) 1839 (w)

3675 (m) 3544 (m) 3040 (w), 3028 (s)

theoretical (HF)

744 (ms) (21)

1611, 1590, (mw), two bands (8,9) 1546 (10), 998 (w) 1451 (m) (11) 1415 (w),1383 (m) (12,13) 1316 (m) (14) 1220 (m) (15) 1154 (16), 1108 (17), 1065 (m) 930 (m) (18) 849 (m), 701 (s) (19,20)

3474 (broad) (1) 3219 (broad) (2) 3070−3001 (three bands) (m) (3,4) 2946 (two bands) (m) (5) 1775 (m), 1719 (s), two bands (6,7)

experimental (No)

670 (w)

785 (w) 836, 697 (w)

1329 (s) 1263 (w) 1081, 1054 (w)

1574 (ms) 1383 (w) 1474−1459 (w)

1592 (s), 1586 (w)

3005 (w) 1739 (m), 1696 (ms), two bands

3569 (w) 3309 (s) 3087−3077 (w)

theoretical (B3LYP)

infrared spectroscopy (ν, cm‑1) theoretical (HF)

695 (w)

765 (w) 844, 683 (w)

1348 (w) 1213 (m) 1133, 1085, 1080 (w)

1522 (s) 1404, 1281 (w) 1404 (m), 1395 (ms)

1618−1603 (w)

2942 (w) 1756 (s), 1618 (m) two bands

3675 (s) 3544 (w) 3028−3019 (w)

a The numbers in parentheses in the Experimental column represent the type of vibration plotted in Figure 2a and b obtained from FT-Raman and FT-IR spectra, respectively. oop = Out of plane. ip = In plane.

1396 (w) 1474−1459 (w)

3569 (w) 3309 (m) 3087 (s), 3070 (w), 3068 (w) 2951 (m) 1781 (mw)

theoretical (B3LYP)

3478 (w) (1) 3290 (w) (2) 3057 (w) (3)

experimental (No)

O−H stretch H-free OH-stretch monomer H bonded C−H aromatic, stretching

vibration

Raman spectroscopy (ν, cm‑1)

Table 2. Comparison of the Experimental and Theoretical Wavenumbers of L-ALA Monomera

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The numbers in parentheses in the Experimental column represent the type of vibration plotted in Figure 4a and b obtained from Raman and FT-IR spectra; respectively. oop = Out of plane. ip = In plane.

Thus, FT-IR spectroscopy showed a triplet, whereas the Ramanexp showed only one band at 3045 cm−1. This same vibration is obtained in the 3079−3045 cm−1 and 3016−2995 cm−1 ranges by using DFT and HF methods. On the other hand, weak overtones and the convolution of tones were found only by FT-IR in the 1965−1772 cm−1 range. Out of plane (oop) and (ip) bending of −C−H and the ringtorsion reported three bands in the 1194−1004 cm−1 range and one strong band at 835 cm−1 in the Ramanexp spectrum. Moreover, this vibrational mode was obtained at the 1178− 1014 cm−1 range and 813 cm−1 by the DFT method, and in the 1181−1005 cm−1 range and 806 cm−1 using HF. In the FTIRexp, a phenyl group showed the characteristic multiplet. Three bands at 1186, 1111, and 1026 cm−1 were obtained. In addition, this spectrum presents strong and medium bands at 820 cm−1 and 746 cm−1, respectively. Moreover, the same bands were predicted in the 1178−1086 cm−1 region, and 813 cm−1 using DFT; and in the 1175−1001 cm−1 and 823−746 cm−1 regions using HF methodologies. On the other hand, the benzene showed two degenerate modes at 1596 and 1504 cm−1, and one nondegenerated mode at 1303 cm−1 due to the skeleton stretching of single C−C bonds. In addition, DFT and HF calculations predicted these modes in the 1595−1495 cm−1 and 1603−1501 cm−1 ranges, respectively. All these values were obtained in the FT-IR spectrum. Thus, these bands were observed at 1600 cm−1 and 1591 cm−1 in the Ramanexp, and predicted at 1595 cm−1 (DFT) and 1602 cm−1 (HF). Likewise, vibrations of Si−C aromatic showed two experimental bands at 1561 and 997 cm−1, that were assigned to the stretching and scissoring deformation, respectively. This type of vibration also is observed at 1570 cm−1 by the experimental Raman spectrum. The calculated vibrations for the structures were performed in the gas phase, while the experiments were obtained in solid state. The anharmonicity is neglected in the real system for the calculated vibrations, and therefore, disagreements between the calculated and observed vibrational wavenumbers could happen. In addition, the highest discrepancies between the theoretical and experimental values are in the 3000 to 3500 cm−1 range, which is related to the asymmetric and symmetric NH2, C−H, and CC stretching (Figure 1c,d). Probably, this behavior is related with the difficulty to assign these bands that can be associated with other absorptions bands. Also, the energy associated to these vibrations is lower, and therefore, it is very difficult to detect and define them. On the other hand, the calculated DFT values showed a better correlation with the experimental values (FT-IR and Raman) due to the flexibility of the model, characteristic of the ab initio methods that take into account the optimized molecular geometry and the resonance of the π-electrons in the aromatic moiety. L-ALA Monomer. Figure 2a and b present comparisons of the vibrations obtained for experimental and calculated IR and Raman spectra. The results are summarized in Table 2. Figure 2c and d show the experimental Raman and FT-IR spectra, respectively, associated with the intensity of certain theoretically bands (DFT and HF). FT-IRexp spectra of L-ALA monomer is rich in bands in the 3000−3500 cm−1 region, and from the strength and broadening of these bands, we suggest that intermolecular hydrogen bonds occur by means the carboxylic acid groups (Scheme 1). O−H stretching experimental FT-IR vibrations present a very broad band in 3474 cm−1 and one possible intermolecular bond at 3219 cm−1. In this region, the experimental frequencies present strong and broad bands in the FT-IR spectra, but smaller in the

a

746, 695, 682, 609 (w) 782 (w) 729, 696, 688, 675 (w) 835 (w) 743 (m), 700 (s) (12,13) 822 (w) (11) 725 (w) (12) −C−H oop bending and ring torsion, R2C6H4

748, 607 (w) 690, 550 (w)

1406 (m), 1377 (ms), 1177, 1087, 1080, 1070, 1000 (w) 1592, 1578 1477 (w) 1382 (m) 1266 (m), 1232, 1088, 1050, 991 (w) 1593 (m) (6) 1511 (m) 1492 (mw) 1429, 1412 (w) (7) 1377 (m) 1110 (m) 1188 (w), 1156 (w), 1067 (m) (8,9,10)

1592 (m), 1564 (w) 1382 (m) 1266 (w), 1178 (m), 1088, 1047 1014, 976 (w) 729, 706, 690, 675 (w) 829 (w) 1593 (s) (6) 1414 (w) (7) 1110 (w) 1189, 1159, 1030 (m), 999 (s) (8,9,10, 11)

3028 (s), 3011, 3010 (w) 2881 (m) 1838 (m) 3069, 3068, 3051 (w) 2950 (m) 1780 (ms), 1695 (m) 3054 (w) (1,2,3) 2950 (w) (4) 1777 (w) (5)

aromatic C−H stretching (sym and anti-sym) CH3 and CH stretching CO cyclic imides (ip and oop) and amide CO stretching CC ring stretching Si−C aromatic stretching C−NH stretch−bend CH3 bending O−H ip bending C−O stretch −C−H oop and ip bending, ring torsion, R-C6H5

1577, 1398 (w) 1288, 1177, 1154, 1132, 1080, 1005, 970 (w)

3019, 3011, 3010 (w) 1838 (m), 1754 (s) 3087, 3076, 3051 (w) 2950 (w) 1777, 1734 (s)

3301 (s)

theoretical (B3LYP) experimental (No) theoretical (HF)

3301 (s)

theoretical (B3LYP) experimental (No)

3478, 3465 (w)

vibration

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N−H stretch and OH

Raman spectroscopy (ν, cm‑1)

Table 3. Comparison of the Experimental and Theoretical Wavenumbers of PALA Polymera

3428 (broad), one band for 2° amide (1) 3069 (w), three bands (2) 2937 (w) (3) 1774 (m), 1719 (s) (4,5)

infrared spectroscopy (ν, cm‑1)

-

theoretical (HF)


E



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Figure 1. Comparison between the observed and calculated spectra of DIA monomer: (a) Raman wavenumber values; (b) IR wavenumber values; (c) Raman spectra; and (d) FT-IR spectrum.

Raman spectra at 3478 and 3290 cm−1, while predicted in infrared and Raman spectrum at 3309 and 3544 cm−1 using DFT and HF models, respectively. Table 2 showed that these bands are assigned to the O−H stretch monomer H-bonded. Thus, we also calculate H-free vibrations associated to the COOH group at 3569 cm−1 using DFT and 3675 cm−1 using HF, in the IR and Raman spectra. In the imide-aromatic moiety, three hydrogen atoms participate in the three C−H stretching, three C−H ip bending, and three C−H oop bending. Thus, the C−H stretching frequencies are observed at the 3070−3001 cm−1 range assigned in the FT-IRexp. This band was predicted at the 3087−3077 cm−1 and 3028−3019 cm−1 ranges using DFT and HF methods, respectively. On the other hand, this vibration is observed at 3057 cm−1 in the Ramanexp, and at 3087−3068 cm−1 using DFT and 3040−3028 cm−1 using HF methods. The ip and oop C−H bending vibrations in the FT-IRexp are observed at the 1154−1065 cm−1, 849−701 cm−1, and 744 cm−1 ranges, respectively. The calculated values of these vibrations are obtained at 1081−1054 cm−1, 836−697 cm−1, and 670 cm−1 ranges using DFT and 1133−1080 cm−1, 844−683 cm−1, and 695 cm−1 ranges using HF methods. Ramanexp spectrum shows certain bands to 1109−1002 cm−1 and 713−682 cm−1 assigned to the C−H bending ring-torsion of the mono-substituted RC6H5 and tri-substituted R3C6H3, respectively. Moreover, these values were predicted at 1176, 730, and 706 cm−1 using DFT and 1134 and 712 cm−1 using HF methods. These bands were assigned at 2946 cm−1 and 1415, 1383 cm−1 as stretching and bending deformation modes of the alkyl groups (CH3 and CH). Theoretical frequencies for these vibrations were

predicted at 3005 cm−1, 1474−1459 cm−1 ranges by using DFT and 2942 cm−1, 1404−1395 cm−1 ranges in the IR spectrum using HF methods. On the other hand, the methyl deformation was observed at ∼2650 and 1386 cm−1 with a weak intensity in the Ramanexp spectrum, and the bending deformation was obtained using theoretical methods at 2951 cm−1 and 1474− 1459 cm−1 range by DFT, and at 2881 cm−1 and 1460− 1462 cm−1 range by HF. In the FT-IRexp spectra, the stretching of the carbonyl group (CO) of the cyclic imides and of the carboxylic acid showed a doublet at 1775 and 1719 cm−1 of medium and strong intensities; respectively. These vibrations modes were found simultaneously in the Ramanexp spectra at 1780 and 1723 cm−1. As in the present case, and even in the absence of inversion of symmetry, the FT-IR and Raman spectra are complementary. In fact, usually strong IR bands show weak intensity in the Raman spectra and vice versa. The corresponding theoretical frequencies in the IR spectrum were assigned at 1739, 1696 cm−1 and 1756, 1618 cm−1 by using DFT and HF methods, respectively. Moreover, in the theoretical Raman spectra they were predicted at 1781 (DFT) and 1839 (HF) cm−1. The C−O−H ip bending and C−O stretching of the carboxylic acid were observed at 1451 and 1220 cm−1, respectively, in the FT-IRexp spectrum, while the same signals appear at 1417 and 1201 cm−1 in the Ramanexp spectrum. Theoretical calculations predicted these vibrational modes in the infrared spectra at 1383 and 1263 cm−1 using DFT, and at 1404, 1281, and 1213 cm−1 using HF methods. In addition, the corresponding theoretical Raman bands were assigned at 1396 and 1263 cm−1 by using DFT and 1404 and 1213 cm−1 using HF methods. On the other F



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Figure 2. Comparison between the observed and calculated spectra of L-ALA monomer: (a) Raman wavenumber values; (b) IR wavenumber values; (c) Raman spectra; and (d) FT-IR spectrum.

Figure 3. Options to the Si−C aromatic bond vibrations (ν, cm−1): Stretching at 1546 (FT-IRexp), 1574 (DFT); Scissoring at 998 (FT-IRexp), 1522 (HF).

hand, the Ramanexp spectrum shows two bands at 1614 cm−1 and 1592 cm−1 that correspond to CC stretch bands. These modes were predicted in the 1592−1586 cm−1 range by DFT and in the 1618−1603 cm−1 range by HF. These modes showed weak intensities in the FT-IRexp spectra, at 1611 and 1590 cm−1. DFT and HF calculations predicted these modes in the 1592− 1586 cm−1 and 1618−1603 cm−1 ranges, respectively. The FT-IRexp spectrum presents two vibrations for the Si−C aromatic bond at 1546 and 998 cm−1 attributed to stretching and scissoring deformations, respectively.42−45 These vibrations were predicted at 1574 and 1522 cm−1 by DFT and HF methods, respectively (Figure 3). At 1571 cm−1 was observed a weak experimental band using Raman spectroscopy, but one vibration at 1528 cm−1 was predicted by HF calculations. The assignment of the C−N stretching is a rather difficult task since there are problems identifying these wavenumbers

between the other vibrations. Thus, the bands observed in the experimental FT-IR and Raman at 1316 and 1317 cm−1, respectively, might correspond to these modes. Moreover, DFT and HF calculations predicted these vibrations at 1383, 1360 (Raman) cm−1 and 1329, 1348 (FT-IR) cm−1. In the Ramanexp spectra, O−H oop bending group appears as a weak band in 920 cm−1 and in 930 cm−1 in the case of the FT-IRexp. These modes were predicted in 800 cm−1 and 785 cm−1 (DFT) and 765 cm−1 (HF) for theoretical Raman and FT-IR. All the modes calculated using DFT or HF methods showed small variations with respect to the experimental results due to the possible intra- and/or intermolecular interactions such as −OH bridges, dimers formed by strong hydrogen bonding, and the existence of charge transfer from donor to acceptor groups. This last one could be produced through a single−double bond path that induces large variations of both molecular dipole G



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Figure 4. Comparison between the observed and calculated spectra of PALA polymer: (a) Raman wavenumber values; (b) IR wavenumber values; (c) Raman spectra; and (d) FT-IR spectrum.

N−H stretching at 3428 cm−1 assigned to the secondary amide, which also is detected by Ramanexp analysis (3478 and 3465 cm−1) and calculated using DFT at 3301 cm−1 in the IR and Raman spectra, respectively. Stretching of C−N and C−N−H ip bending were obtained at 1492 cm−1 by experimental FT-IR technique. The O−H ip bending was observed in FT-IRexp at 1377 cm−1, while that is not observable in Ramanexp. This evidence suggests that the polymer might present a free OH as terminal group. Aromatic groups exhibit a weak band at 3069 cm−1 due to the aromatic C−H stretching vibration. In addition, three bands were obtained by DFT calculation and HF methods at 3087, 3076, 3051 cm−1 and 3019, 3011, 3010 cm−1, respectively. The fluorescence observed in the Ramanexp spectra hinders the assignment of the corresponding frequencies; however, a band at 3054 cm−1 is evidenced. Nevertheless, the calculations assigned three bands at 3069, 3068, 3051 cm−1 and 3028, 3011, 3010 cm−1 using DFT and HF methods, respectively. Three C−H ip bending and three C−H oop bending vibrations were observed in the FT-IRexp spectrum at 1188, 1156, 1067 cm−1 and 822, 743, 700 cm−1, respectively. Their calculated frequencies using DFT were assigned at 1266, 1232, 1088, 1050, 991 cm−1 and 835, 729, 696, 688, 675 cm−1. These same bands were assigned at 1177, 1087, 1080, 1070, 1000 cm−1 and 782, 746, 695, 682, 609 cm−1, using HF. Likewise, these modes were observed in the Ramanexp spectrum at 1189, 1159, 1030, 999 cm−1 and 725 cm−1, and calculated by DFT and HF methods at 1266, 1178, 1088, 1047, 1014, 976, 829, 729, 706, 690, 675 cm−1 and 1288, 1177, 1154, 1132, 1080, 1005, 970, 748, 690, 607, 550 cm−1; respectively.

moment and molecular polarizability, affecting the intensity, width, and energy of the vibrational modes obtained by experimental FT-IR and Raman (Figure 2a,b). Additionally, the planarity loss produced by certain functional groups (aromatic rings or amidic groups) in the chain affects the fluorescence. This characteristic is related to the possible steric hindrance and the high molecular weight of the system that affects the vibration assignment and therefore the difference between the experimental values (Figure 2c,d) and theoretical FT-IR and FT-Raman spectra. PALA Polymer. Figure 4a and b show the comparisons between the vibrations obtained by experimental and calculated IR and Raman spectra. The values are summarized in Table 3. On the other hand, Figure 4c and d show the experimental Raman and FT-IR spectra associated with intensity of the theoretical bands, respectively. In general, the loss of symmetry is accompanied by a large charge-transfer variation in the system that affects the vibrational modes description in the polymer. This behavior also is affected by the influence of the high molecular weight. Thus, the vibrational frequencies obtained by DFT calculations (B3LYP/6-31G*) showed a better agreement with the experimental values obtained by experimental FT-IR and Raman techniques (Figure 4a,b), in comparison with the HF/6-31G* method. Moreover, absorption bands theoretically calculated at the 2800−3000 cm−1 range were not experimentally obtained (Figure 4c), mainly due to the high fluorescence emitted by the polymer. FT-IRexp spectra showed several bands in the 3500− 2900 cm−1 region, which are described as follows: Asymmetric H


The Journal of Physical Chemistry A The asymmetric CH3 and CH stretching were observed at 2937 cm−1 with a weak intensity only by FT-IRexp, and calculated at 2950 cm−1 using DFT method. This same signal is shown by experimental Raman at 2950 cm−1 and theoretical methods at 2881 and 2950 cm−1 using HF and DFT, respectively. However, the presence of C−H stretch bands is often obscured by the much stronger CH3 stretching band in both experimental IR and Raman spectra. On the other hand, the asymmetric CH3 bending showed two weak bands at 1429 and 1412 cm−1 in the FT-IRexp spectrum, and calculated at 1382 cm−1 using the DFT method. These vibrational normal modes were observed at 1414 cm−1 by Ramanexp spectroscopy and calculated at 1382 cm−1 by the DFT method. The CO groups usually absorb in the 1740−1670 cm−1 range;46 thus, the cyclic imides in five-member rings generally have two bands in the carbonyl region. The CO ip mode band was observed at 1774 and 1777 cm−1 with a mediumweak in the experimental FT-IR and Raman, respectively, while the CO oop mode appears at 1719 cm−1 in the experimental FT-IR spectrum. The corresponding IR frequencies were calculated at 1777, 1734 cm−1 (DFT) and 1838, 1754 cm−1 (HF), while the Raman frequencies were assigned at 1780, 1695 cm−1 (DFT) and 1838 cm−1 (HF). Moreover, the C−O vibration was observed at 1110 cm−1 in experimental FT-IR (medium) and Raman (weak) spectrum, respectively. The band observed at 1593 cm−1 by experimental Raman associated to the CC ring stretching mode was assigned at 1592, 1564 cm−1 and 1577, 1398 cm−1 by using DFT and HF methods, respectively. The same signal was observed in the FT-IR spectrum at 1593 cm−1, showing a medium intensity, and calculated at 1592, 1578, 1477 cm−1 and 1406, 1377 cm−1 using DFT and HF methods, respectively. The Si−C aromatic stretching modes show medium intensity only in experimental FT-IR spectrum around at 1511 cm−1.

ACKNOWLEDGMENTS

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REFERENCES

Authors acknowledge the financial support provided by FONDECYT (grant 11121281 and 1095151). C. M. ́ González-Henriquez acknowledges a Scholarship from VRI (PUC) No 10/2010 and The Attraction and Insertion of Advanced Human Capital Program (PAI) by CONICYT (grant 7912010031). Theoretical calculations were realized by Dr. Jose Vicente Correa, Laboratory of Theoretical and Computational Chemistry, Pontificia Universidad Católica de Chile. Raman spectroscopy supported by FONDEF project No D97F1001 (PUC). M. Sarabia acknowledges the financial support given by CONYCYT through the Magister Scholarship Grant.

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V. CONCLUSIONS The experimental and theoretical IR and Raman vibrational spectra of DIA and L-ALA silylated monomers and an approach to the repetitive unit of their derivative poly(imide−amide) PALA were studied. The calculations were performed using DFT (B3LYP) and HF theoretical methods at the 6-31G(d) theory level. The vibrational modes, infrared intensities, and Raman activities were calculated, and a complete vibrational analysis of the compounds was carried out. The best agreement with the experimental results was found when DFT B3LYP/6-31G(d) calculations were used, mainly due to the inclusion of electronic exchange energies that show a high correlation in these systems. Finally, we claim DFT/ 6-31G(d) as a reliable method to perform systematic studies in systems that show such a great level of fluorescence as ours. However, the most important objective of this research is to highlight that the theoretical results could be a strong support for the clarification of experimental research, but in no case could the theoretical data replace the experiments.

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*E-mail: [email protected]. Phone: 56-2-27877188. FAX: 56-2-27877127. Notes

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Theoretical and Experimental Vibrational Spectroscopic Investigation

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