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Solid-state linear-polarized IR-spectroscopic characterization of l -methionine
Vibrational Spectroscopy 44 (2007) 30–35 www.elsevier.com/locate/vibspec
Solid-state linear-polarized IR-spectroscopic characterization of L-methionine Bojidarka B. Koleva * University of Sofia, Department of Chemistry, 1164 Sofia, Bulgaria Received 13 April 2006; received in revised form 2 July 2006; accepted 6 July 2006 Available online 21 August 2006
Abstract The IR-spectral characteristic bands assignment in 1700–400 cm1 IR-spectral region of L-methionine in solid-state has been carried out by means of linear-polarized IR-spectral analysis of oriented solid-samples as a suspension in nematic liquid crystal. The later results are supported by the comparison with the known single crystal X-ray data as far as the presented spectroscopic approach give possibilities not only for obtaining of the experimental IR-bands assignment but a receiving a stereo-structural information. # 2006 Elsevier B.V. All rights reserved. Keywords: L-Methionine; Solid-state LD-IR spectral analysis
A full understanding of the in vivo manner and mechanism of coordination of the Pt(II) and Au(III) metal ions with DNA demands a systematic investigation of their ability to coordinate with di- and tripeptides [1–3]. On account of the potential antitumor activity of some Pt(II) and Au(III) complexes with peptides it was necessary also to carry out a structural and spectral study [4–6]. Of particular interest in this respect are the thioesters and imidazole donor atoms in methionine and histidine side chains. Infrared spectroscopy is powerful methods for investigating the coordination ability of such biological samples. However, in the case of amino acids, their amides and peptides it is difficult to apply conventional IR-techniques on account of the complicated IR-spectral patterns of both the ligands and the corresponding metal complexes that result from the presence of more than one amide fragment. The IR-LD of oriented solid samples as a suspension in nematic liquid crystal combined with the reducing-difference procedure for linear-polarized IRLD spectra interpretation reduce these difficulties to a significant level as has been demonstrated in a series of papers [5–13]. In addition take possibility to obtaining a stereostructural information in solid state. Other advantage of this new technique and the complex approach for IR-LD spectra
* Tel.: +359 2 8161208; fax: +359 2 963458. E-mail address: [email protected]. 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.07.007
interpretation (see Section 1) is the possibilities to prove the origin of the given doublet character in the IR-spectral bands of compound studied in solid state to Davydov’s splitting or is a result of the presence of non-equivalent molecules in the frame of the unit cell (see below). Hence a solid-state LD-IR spectroscopic characterization is presented for the L-methionine (Scheme 1) with a view of IRcharacteristic bands assignment in 1700–400 cm1. The latter data are compared with, confirmed and supported by single crystal X-ray diffraction ones of entitled compound [14]. For methionine in the crystal, two crystallographically independent forms have been identified in X-ray studies [14] Number of spectroscopic studies on methionine analogues and metal complexes have been reported in [15–20], but even fewer for methionine itself [21–24]. One of these studies is on mehionine zwitterions and combined approach of infrared spectroscopy and targeted ab initio molecular orbital calculations. Infrared spectra have been reported for Met in KBr pellets and adsorbed on surfaces [14,25]. 1. Experimental L-Methionine was purchased from Bachem Organics (Switzerland). The 4000–400 cm1 solid-state IR-spectra were recorded on a Bomem Michelson 100 FT-IR spectrometer (resolution 2 cm1, 150 scans) equipped with a Perkin-Elmer wire-grid
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
31
Scheme 1.
polarizer. The oriented solid samples were obtained as a suspension in a nematic liquid crystal of the 40 -cyano-40 alkylbicyclohexyl type (ZLI 1695, Merck), mesomorphic at room temperature. Its weak IR-spectrum permits the recording of the guest-compound bands in the whole 4000–400 cm1 range (Fig. 1(1)). The presence of an isolated nitrile stretching IR-band at 2236 cm1 serves additionally as an orientation indicator. The effective orientation of the samples was achieved through the following procedure: 5 mg of the compound to be studied was mixed with the liquid crystal substance until a slightly viscous suspension was obtained. The phase thus prepared was pressed between two KBr-plates for which, in advance, one direction had been rubbed out by means of fine sandpaper. The grinding of the mull in the rubbing direction promotes an additional orientation of the sample [5–13,25]. The conventional IR-spectra in solid-state are recorded as nujol mull and KBr pellet. IR-LD spectroscopy and the interpretation of the linearpolarized IR-spectra are described in [26–29]. The method consists of subtraction of the perpendicular spectrum (Fig. 1(3)) (IRs, resulting from a 908 angle between the polarized light beam electric vector and the orientation of the sample) from the parallel one (IRp) obtained with a co-linear mutual orientation (Fig. 1(2)). The recorded difference (IRp IRs) spectrum (Fig. 1(4)) divides the corresponding parallel (Ap) and perpendicular (As) integrated absorbencies of each band into positive values originating from transition moments, which form average angles with the orientation direction (n) between 08 and 54.78 (magic angle), and negative ones corresponding to
Fig. 1. Non-polarized IR (1), parallel (2), perpendicular (3) and difference (4) IR-LD spectra of ZLI 1695.
transition moments between 54.78 and 908 (Scheme 2). In the reducing-difference procedure, the perpendicular spectrum multiplied by the parameter c, is subtracted from the parallel one and c is varied until at least one band or sets of bands are eliminated. The simultaneous disappearance of these bands in the reduced IR-LD spectrum (IRp cIRs) obtained indicates co-linearity of the corresponding transition moments, thus yielding to information regarding the mutual disposition of the molecular fragments. This elimination method is carried out graphically using a subtraction procedure attached to the program for processing of IR-spectra. The validation of the method about the experimental conditions and the effect of the liquid crystal medium on band position and intensities as well as the determination of the last parameters for each i-band were made by deconvolution and curve-fitting procedures at 50:50% ratio of Lorentzian to Gaussian band functions, x2 factors within 0.00047–0.00039 and 2000 iterations [30,31]. The
Scheme 2.
32
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
Fig. 2. Solid-state IR-spectra of L-methionine as nemalic liquid crystal suspension (1) and KBr pellet (2).
Fig. 3. Non-polarized IR- (1) and difference IR-LD spectra (2) of L-methionine oriented as a suspension in nematic liquid crystal.
means of two treatments were compared by Student’s t-test. The experimental IR-spectral patterns have been acquired and processed by GRAMS/AI 7.01 IR-spectroscopy (Thermo Galactic, USA) and STATISTICA for Windows 5.0 (StatSoft, Inc., Tulsa, OK, USA) program packages.
Fig. 3 illustrates the non-polarized IR- and difference IR-LD spectra of L-methionine oriented as a suspension in nematic liquid crystal. As could be see the significant degree of orientation of crystalline sample leads to adequate application of reducing-difference procedure for polarized IR-spectra interpretation and on the IR-characteristic bands assignment. In the case of L-methionine the comparison with the know single crystal X-ray diffraction data supported the IR-LD analysis of one side make easier the spectroscopic analysis and of the other—confirmed the validity of stereo-structural conclusions obtained by presented new IR-LD spectroscopic approach [25]. According to X-ray data the L-methionine
2. Results and discussion The absence of any interactions between oriented solid samples are concluded examining the band position and integral absorbency ratio of L-methionine in KBr pellet (Fig. 2(2)) and in nematic mesophase (Fig. 2(1)).
Fig. 4. Curve-fitted IR-spectrum of L-methionine in solid-state.
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
Scheme 3.
Fig. 5. Non-polarized IR- (1) and reduced IR-LD spectra of L-methionine after the elimination of the bands at 1411 cm1 (2) and 1416 cm1 (3).
33
34
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
crystallized in P21 space group and Z = 4. The unit cell contains four different oriented molecules as shown in Scheme 2. However, their mutual disposition supposed the near to colinear orientation in the frame of the neighboring molecules of the following transition moments of: asymmetric, symmetric 0 stretching bending vibrations NH3+ (daNH3 þ , daNH3 þ , dsNH3 þ ) and s (ii) asymmetric stretching vibration nas COO , nSC and dðSÞNH3 (Scheme 2). In the frame of one molecule the corresponding transition moments are co-linear oriented: (i) nas COO and rCOO , (ii) nsCOO and dCOO and (iii) nSC and dsðSÞCH3 (Scheme 2). The presence of the pairs of molecules supposed a doublet character of all the characteristic IR-spectroscopic maxima, which is confirmed by the preliminary applied deconvolution and curve-fitting procedure to non-polarize IR-spectrum of the compound studied. Similar behavior has been obtained as well for other system studied [5–13]. The data are presented in Fig. 4. The bands about 1655 cm1, 1615 cm1 and 1517 cm1 as0 s could be assigned to das NH3 þ , dNH3 þ and dNH3 þ . The typical values of zwiterionic systems are in the ranges 1610 25 cm1, 1585 25 cm1 and 1505 25 cm1 [32], while the observed (Fig. 4) relatively high-frequency values are explained with the crystallographically established strong intermolecular hydrogen NHO bonds (Scheme 3). The values correlated well with the known ones of L-methionine, studying theoretically and by H/D isotopic effects on IR-spectra of the crystalline compound [33]. It is interesting to note that the different NH3+-groups (Scheme 3) interacted with different manner. The corresponding bond ˚ ), NO (2.733 A ˚ and 2.825 A ˚ ) and lengths are NO (2.854 A ˚ ˚ ˚ NO (2.976 A, 2.849 A and 2.862 A) [14], which could be explained the relatively higher frequency differences between 0 1 the pairs of das (Fig. 4). NH3 þ maxima with 6 cm
Table 1 Wavenumbers (cm1) of the bands, observed in the IR-spectrum of the polycrystalline L-methionine sample in the 1700–400 cm1 region Assignment
n (cm1)
daNH3 þ
1655 1615
0 daNH3 þ dsNH3 þ nas COO nsCOO
dSðCH3 Þ dCOO vCOO rCOO nSC Twisting NH3+
1517 1587 1415 1340 721 686 559 779 555
The bands about 1587 cm1 and 1415 cm1 (Fig. 4) respond 1 s to nas could be COO and nCOO , while the bands about 1340 cm s assigned to dðSÞNH3 . The last value is typical for SCH3 molecule fragment and usually is observed at 1320 20 cm1 [32]. The IR-spectral region 800–500 cm1 (Fig. 3(1)) show series of pairs of maxima about 780 cm1, 755 cm1, 720 cm1, 686 cm1 and 555 cm1. In this region should be to observed the characteristic IR-bands of dCOO , vCOO , rCOO , nSC and NH3+ twisting modes with typical ranges of 739 90 cm1, 625 55 cm1, 530 60 cm1 and 480 50 cm1. However, the overlapped IR-spectral regions difficult in significant level the adequate vibrational assignment with conventional techniques. The next IR-LD analysis shown experimentally the corresponding origin of these and all above state IR-spectral frequencies and the data are summarized in Table 1. The reducing-difference procedure leads to the following results: (i) The consequently elimination of nsCOO maxima at 1411 cm1 and 1416 cm1 (Fig. 5(2 and 3)) of the different oriented pairs of molecules in the unit cell (Scheme 2), leads to elimination of each of the pairs of bands at 721 cm1 and 717 cm1 belonging to dCOO . (ii) The simultaneously disappearance of 1580 cm1 (nas COO ) with 1340 cm1 (dsðSÞCH3 ) and bands at 779 cm1 and 559 cm1 (Fig. 6(2)) assigned last maxima to nSC and rCOO , respectively. The rest bands at 686 cm1 and 555 cm1 belong to vCOO and NH3+ twisting modes, respectively. (iii) The reduction of the bands for about 1659 cm1, 1615 cm1, 1517 cm1 in same dichroic ration confirms 0 their assignment as daNH3 þ , daNH3 þ and dsNH3 þ (Fig. 7(2)) and the obtained values correlated well with some ones published in [33] (see Table 1 as well).
Fig. 6. Non-polarized IR- (1) and reduced IR-LD spectra of L-methionine after the elimination of the band at 1580 cm1 (2).
All the eliminations of separate bands leads to an observation of the inflex point curve due to Davydov’s splitting effects. On the reduced linear-polarized IR-spectra has last phenomenon has been studied in details on out-of-plane aromatic modes in the co-planar disposition of the aromatic
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
35
obtaining of supramolecular structural information of other compounds that could not be characterized by X-ray diffraction or IR-LD spectral analysis of monocrystals. References
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 7. Non-polarized IR- (1) and reduced IR-LD spectra of L-methionine after the elimination of the band at 1515 cm1 (2).
[19] [20] [21] [22] [23]
fragment in the unit cell of antipyrine an 4-aminopyridine, where the pairs of maxima of different oriented molecules is impossible to be observed [34].
[24] [25]
3. Conclusion
[26] [27] [28]
Using the possibilities of the reducing-difference approach to the investigation of polarized IR-spectra, a correct assignment of IR-characteristic frequencies of L-methionine in 1700– 1400 cm1 range is accomplished. The results obtained by the IR-LD spectral study confirm the X-ray data for the discussed compound in the solid-state. The results obtained by the new technique for orientation of the solid sample as a nematic liquid crystal suspension demonstrate its applicability for the
[29] [30] [31] [32] [33] [34]
J. Reedjik, Chem. Commun. (1996) 801. D.B. Zambe, S.J. Lippard, Trends Biochem. Sci. 20 (1995) 435. J. Reedjik, Inorg. Chim. Acta 198 (1994) 873. K.J. Barnham, M.I. Djuran, P. Del Soccoro Murrdoch, P.J. Sadler, Chem. Commun. (1994) 721. B.B. Ivanova, M.J. Mitewa, J. Coord. Chem. 57 (2004) 217. B.B. Ivanova, J. Coord. Chem. 58 (2005) 587. B.B. Ivanova, M.G. Arnaudov, St.T. Todorov, J. Coord. Chem., in press. B.B. Ivanova, J. Mol. Struct. 782 (2006) 122. B.B. Ivanova, M.G. Arnaudov, Spectrochim. Acta, Part A, in press. B.B. Ivanova, M.G. Arnaudov, St. Todorov, W.S. Sheldrick, H. MayerFigge, Struct. Chem., in press. B.B. Ivanova, Spectrochim. Acta, Part A, in press. B.B. Ivanova, T. Kolev, S.Y. Zareva, Biopolymers, in press. T. Kolev, B.B. Ivanova, S.Y. Zareva, J. Coord. Chem., submitted for publication. K. Torii, Y. Iitaka, Acta Crystallogr. 29B (1973) 2799. B. Onoa, V. Moreno, Trans. Met. Chem. 23 (1998) 485. P. Tandon, V. Gupta, O. Prasad, S. Rastogi, V. Gupta, J. Polym. Sci. B: Polym. Phys. 35 (1997) 2281. C.L. Gatlin, F. Turecek, T. Vaisar, J. Mass Spectrom. 30 (1995) 1617. T. Grochowski, K.L. Samochocka, J. Chem. Soc. Dalton Trans. (1992) 1145. A. Isab, Inorg. Chim. Acta 153 (1988) 209. T. Kowalik, H. Kozlowski, Inorg. Chim. Acta 67 (1982) L39. S. Stewart, P. Fredericks, Spectrochim. Acta 55A (1999) 1641. P. Faller, M. Vas ak, Inorg. Chim. Acta 272 (1998) 150. E. Cooper, F. Krebs, Mc.D. Smith, R.J. Raval, Electron Spectrosc. Relat. Phenom. 64/65 (1993) 469. A. Grunenberg, D. Bougeard, J. Mol. Struct. 160 (1987) 27. B.B. Ivanova, M.G. Arnaudov, P.R. Bontchev, Spectrochim. Acta 60A (2004) 855. B. Jordanov, R. Nentchovska, B. Schrader, J. Mol. Struct. 297 (1993) 401. B. Jordanov, B. Schrader, J. Mol. Struct. 347 (1995) 389. J. Michl, E.W. Thulstrup, Spectroscopy with Polarized Light. Solute Alignment by Photo-selection, in Liquid Crystals, Polymers and Membranes, VCH Publishers, New York, 1986. E.W. Thulstrup, J.H. Eggers, Chem. Phys. Lett. 1 (1996) 690. B.B. Ivanova, D.L. Tsalev, M.G. Arnaudov, Talanta, in press. B.B. Ivanova, V.D. Simeonov, M.G. Arnaudov, D.L. Tsalev, Specrochim. Acta, Part A, in press. N.P. Roeges, A Guide to the Complete Interpretation of Infrared Spectra of Organic Structures, Wiley, NY, 1993. X. Cao, G. Fischer, J. Phys. Chem. A 106 (2002) 41. M.G. Arnaudov, Bull. Chem. Commun. 37 (2005) 230. !
[1] [2] [3] [4]
Solid-state linear-polarized IR-spectroscopic characterization of L-methionine Bojidarka B. Koleva * University of Sofia, Department of Chemistry, 1164 Sofia, Bulgaria Received 13 April 2006; received in revised form 2 July 2006; accepted 6 July 2006 Available online 21 August 2006
Abstract The IR-spectral characteristic bands assignment in 1700–400 cm1 IR-spectral region of L-methionine in solid-state has been carried out by means of linear-polarized IR-spectral analysis of oriented solid-samples as a suspension in nematic liquid crystal. The later results are supported by the comparison with the known single crystal X-ray data as far as the presented spectroscopic approach give possibilities not only for obtaining of the experimental IR-bands assignment but a receiving a stereo-structural information. # 2006 Elsevier B.V. All rights reserved. Keywords: L-Methionine; Solid-state LD-IR spectral analysis
A full understanding of the in vivo manner and mechanism of coordination of the Pt(II) and Au(III) metal ions with DNA demands a systematic investigation of their ability to coordinate with di- and tripeptides [1–3]. On account of the potential antitumor activity of some Pt(II) and Au(III) complexes with peptides it was necessary also to carry out a structural and spectral study [4–6]. Of particular interest in this respect are the thioesters and imidazole donor atoms in methionine and histidine side chains. Infrared spectroscopy is powerful methods for investigating the coordination ability of such biological samples. However, in the case of amino acids, their amides and peptides it is difficult to apply conventional IR-techniques on account of the complicated IR-spectral patterns of both the ligands and the corresponding metal complexes that result from the presence of more than one amide fragment. The IR-LD of oriented solid samples as a suspension in nematic liquid crystal combined with the reducing-difference procedure for linear-polarized IRLD spectra interpretation reduce these difficulties to a significant level as has been demonstrated in a series of papers [5–13]. In addition take possibility to obtaining a stereostructural information in solid state. Other advantage of this new technique and the complex approach for IR-LD spectra
* Tel.: +359 2 8161208; fax: +359 2 963458. E-mail address: [email protected]. 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.07.007
interpretation (see Section 1) is the possibilities to prove the origin of the given doublet character in the IR-spectral bands of compound studied in solid state to Davydov’s splitting or is a result of the presence of non-equivalent molecules in the frame of the unit cell (see below). Hence a solid-state LD-IR spectroscopic characterization is presented for the L-methionine (Scheme 1) with a view of IRcharacteristic bands assignment in 1700–400 cm1. The latter data are compared with, confirmed and supported by single crystal X-ray diffraction ones of entitled compound [14]. For methionine in the crystal, two crystallographically independent forms have been identified in X-ray studies [14] Number of spectroscopic studies on methionine analogues and metal complexes have been reported in [15–20], but even fewer for methionine itself [21–24]. One of these studies is on mehionine zwitterions and combined approach of infrared spectroscopy and targeted ab initio molecular orbital calculations. Infrared spectra have been reported for Met in KBr pellets and adsorbed on surfaces [14,25]. 1. Experimental L-Methionine was purchased from Bachem Organics (Switzerland). The 4000–400 cm1 solid-state IR-spectra were recorded on a Bomem Michelson 100 FT-IR spectrometer (resolution 2 cm1, 150 scans) equipped with a Perkin-Elmer wire-grid
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
31
Scheme 1.
polarizer. The oriented solid samples were obtained as a suspension in a nematic liquid crystal of the 40 -cyano-40 alkylbicyclohexyl type (ZLI 1695, Merck), mesomorphic at room temperature. Its weak IR-spectrum permits the recording of the guest-compound bands in the whole 4000–400 cm1 range (Fig. 1(1)). The presence of an isolated nitrile stretching IR-band at 2236 cm1 serves additionally as an orientation indicator. The effective orientation of the samples was achieved through the following procedure: 5 mg of the compound to be studied was mixed with the liquid crystal substance until a slightly viscous suspension was obtained. The phase thus prepared was pressed between two KBr-plates for which, in advance, one direction had been rubbed out by means of fine sandpaper. The grinding of the mull in the rubbing direction promotes an additional orientation of the sample [5–13,25]. The conventional IR-spectra in solid-state are recorded as nujol mull and KBr pellet. IR-LD spectroscopy and the interpretation of the linearpolarized IR-spectra are described in [26–29]. The method consists of subtraction of the perpendicular spectrum (Fig. 1(3)) (IRs, resulting from a 908 angle between the polarized light beam electric vector and the orientation of the sample) from the parallel one (IRp) obtained with a co-linear mutual orientation (Fig. 1(2)). The recorded difference (IRp IRs) spectrum (Fig. 1(4)) divides the corresponding parallel (Ap) and perpendicular (As) integrated absorbencies of each band into positive values originating from transition moments, which form average angles with the orientation direction (n) between 08 and 54.78 (magic angle), and negative ones corresponding to
Fig. 1. Non-polarized IR (1), parallel (2), perpendicular (3) and difference (4) IR-LD spectra of ZLI 1695.
transition moments between 54.78 and 908 (Scheme 2). In the reducing-difference procedure, the perpendicular spectrum multiplied by the parameter c, is subtracted from the parallel one and c is varied until at least one band or sets of bands are eliminated. The simultaneous disappearance of these bands in the reduced IR-LD spectrum (IRp cIRs) obtained indicates co-linearity of the corresponding transition moments, thus yielding to information regarding the mutual disposition of the molecular fragments. This elimination method is carried out graphically using a subtraction procedure attached to the program for processing of IR-spectra. The validation of the method about the experimental conditions and the effect of the liquid crystal medium on band position and intensities as well as the determination of the last parameters for each i-band were made by deconvolution and curve-fitting procedures at 50:50% ratio of Lorentzian to Gaussian band functions, x2 factors within 0.00047–0.00039 and 2000 iterations [30,31]. The
Scheme 2.
32
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
Fig. 2. Solid-state IR-spectra of L-methionine as nemalic liquid crystal suspension (1) and KBr pellet (2).
Fig. 3. Non-polarized IR- (1) and difference IR-LD spectra (2) of L-methionine oriented as a suspension in nematic liquid crystal.
means of two treatments were compared by Student’s t-test. The experimental IR-spectral patterns have been acquired and processed by GRAMS/AI 7.01 IR-spectroscopy (Thermo Galactic, USA) and STATISTICA for Windows 5.0 (StatSoft, Inc., Tulsa, OK, USA) program packages.
Fig. 3 illustrates the non-polarized IR- and difference IR-LD spectra of L-methionine oriented as a suspension in nematic liquid crystal. As could be see the significant degree of orientation of crystalline sample leads to adequate application of reducing-difference procedure for polarized IR-spectra interpretation and on the IR-characteristic bands assignment. In the case of L-methionine the comparison with the know single crystal X-ray diffraction data supported the IR-LD analysis of one side make easier the spectroscopic analysis and of the other—confirmed the validity of stereo-structural conclusions obtained by presented new IR-LD spectroscopic approach [25]. According to X-ray data the L-methionine
2. Results and discussion The absence of any interactions between oriented solid samples are concluded examining the band position and integral absorbency ratio of L-methionine in KBr pellet (Fig. 2(2)) and in nematic mesophase (Fig. 2(1)).
Fig. 4. Curve-fitted IR-spectrum of L-methionine in solid-state.
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
Scheme 3.
Fig. 5. Non-polarized IR- (1) and reduced IR-LD spectra of L-methionine after the elimination of the bands at 1411 cm1 (2) and 1416 cm1 (3).
33
34
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
crystallized in P21 space group and Z = 4. The unit cell contains four different oriented molecules as shown in Scheme 2. However, their mutual disposition supposed the near to colinear orientation in the frame of the neighboring molecules of the following transition moments of: asymmetric, symmetric 0 stretching bending vibrations NH3+ (daNH3 þ , daNH3 þ , dsNH3 þ ) and s (ii) asymmetric stretching vibration nas COO , nSC and dðSÞNH3 (Scheme 2). In the frame of one molecule the corresponding transition moments are co-linear oriented: (i) nas COO and rCOO , (ii) nsCOO and dCOO and (iii) nSC and dsðSÞCH3 (Scheme 2). The presence of the pairs of molecules supposed a doublet character of all the characteristic IR-spectroscopic maxima, which is confirmed by the preliminary applied deconvolution and curve-fitting procedure to non-polarize IR-spectrum of the compound studied. Similar behavior has been obtained as well for other system studied [5–13]. The data are presented in Fig. 4. The bands about 1655 cm1, 1615 cm1 and 1517 cm1 as0 s could be assigned to das NH3 þ , dNH3 þ and dNH3 þ . The typical values of zwiterionic systems are in the ranges 1610 25 cm1, 1585 25 cm1 and 1505 25 cm1 [32], while the observed (Fig. 4) relatively high-frequency values are explained with the crystallographically established strong intermolecular hydrogen NHO bonds (Scheme 3). The values correlated well with the known ones of L-methionine, studying theoretically and by H/D isotopic effects on IR-spectra of the crystalline compound [33]. It is interesting to note that the different NH3+-groups (Scheme 3) interacted with different manner. The corresponding bond ˚ ), NO (2.733 A ˚ and 2.825 A ˚ ) and lengths are NO (2.854 A ˚ ˚ ˚ NO (2.976 A, 2.849 A and 2.862 A) [14], which could be explained the relatively higher frequency differences between 0 1 the pairs of das (Fig. 4). NH3 þ maxima with 6 cm
Table 1 Wavenumbers (cm1) of the bands, observed in the IR-spectrum of the polycrystalline L-methionine sample in the 1700–400 cm1 region Assignment
n (cm1)
daNH3 þ
1655 1615
0 daNH3 þ dsNH3 þ nas COO nsCOO
dSðCH3 Þ dCOO vCOO rCOO nSC Twisting NH3+
1517 1587 1415 1340 721 686 559 779 555
The bands about 1587 cm1 and 1415 cm1 (Fig. 4) respond 1 s to nas could be COO and nCOO , while the bands about 1340 cm s assigned to dðSÞNH3 . The last value is typical for SCH3 molecule fragment and usually is observed at 1320 20 cm1 [32]. The IR-spectral region 800–500 cm1 (Fig. 3(1)) show series of pairs of maxima about 780 cm1, 755 cm1, 720 cm1, 686 cm1 and 555 cm1. In this region should be to observed the characteristic IR-bands of dCOO , vCOO , rCOO , nSC and NH3+ twisting modes with typical ranges of 739 90 cm1, 625 55 cm1, 530 60 cm1 and 480 50 cm1. However, the overlapped IR-spectral regions difficult in significant level the adequate vibrational assignment with conventional techniques. The next IR-LD analysis shown experimentally the corresponding origin of these and all above state IR-spectral frequencies and the data are summarized in Table 1. The reducing-difference procedure leads to the following results: (i) The consequently elimination of nsCOO maxima at 1411 cm1 and 1416 cm1 (Fig. 5(2 and 3)) of the different oriented pairs of molecules in the unit cell (Scheme 2), leads to elimination of each of the pairs of bands at 721 cm1 and 717 cm1 belonging to dCOO . (ii) The simultaneously disappearance of 1580 cm1 (nas COO ) with 1340 cm1 (dsðSÞCH3 ) and bands at 779 cm1 and 559 cm1 (Fig. 6(2)) assigned last maxima to nSC and rCOO , respectively. The rest bands at 686 cm1 and 555 cm1 belong to vCOO and NH3+ twisting modes, respectively. (iii) The reduction of the bands for about 1659 cm1, 1615 cm1, 1517 cm1 in same dichroic ration confirms 0 their assignment as daNH3 þ , daNH3 þ and dsNH3 þ (Fig. 7(2)) and the obtained values correlated well with some ones published in [33] (see Table 1 as well).
Fig. 6. Non-polarized IR- (1) and reduced IR-LD spectra of L-methionine after the elimination of the band at 1580 cm1 (2).
All the eliminations of separate bands leads to an observation of the inflex point curve due to Davydov’s splitting effects. On the reduced linear-polarized IR-spectra has last phenomenon has been studied in details on out-of-plane aromatic modes in the co-planar disposition of the aromatic
B.B. Koleva / Vibrational Spectroscopy 44 (2007) 30–35
35
obtaining of supramolecular structural information of other compounds that could not be characterized by X-ray diffraction or IR-LD spectral analysis of monocrystals. References
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 7. Non-polarized IR- (1) and reduced IR-LD spectra of L-methionine after the elimination of the band at 1515 cm1 (2).
[19] [20] [21] [22] [23]
fragment in the unit cell of antipyrine an 4-aminopyridine, where the pairs of maxima of different oriented molecules is impossible to be observed [34].
[24] [25]
3. Conclusion
[26] [27] [28]
Using the possibilities of the reducing-difference approach to the investigation of polarized IR-spectra, a correct assignment of IR-characteristic frequencies of L-methionine in 1700– 1400 cm1 range is accomplished. The results obtained by the IR-LD spectral study confirm the X-ray data for the discussed compound in the solid-state. The results obtained by the new technique for orientation of the solid sample as a nematic liquid crystal suspension demonstrate its applicability for the
[29] [30] [31] [32] [33] [34]
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