Influence of intermolecular hydrogen bonding on IR-spectroscopic properties of (R)-(−)-1-phenylglycinium hydrogen squarate monohydrate in solid-state. IR-LD, Raman spectroscopy and theoretical study

Available online at www.sciencedirect.com

Journal of Molecular Structure 846 (2007) 139–146 www.elsevier.com/locate/molstruc

Influence of intermolecular hydrogen bonding on IR-spectroscopic properties of (R)-()-1-phenylglycinium hydrogen squarate monohydrate in solid-state. IR-LD, Raman spectroscopy and theoretical study Tsonko Kolev

*

Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str. build. 9, 1113 Sofia, Bulgaria Plovdiv University ‘‘P. Hilendarski’’, Faculty of Chemistry, Department of Organic Chemistry, 24 Tzar Assen Str., 4000 Plovdiv, Bulgaria Received 8 December 2006; received in revised form 17 January 2007; accepted 19 January 2007 Available online 2 February 2007

Abstract Influence of the intermolecular interactions in solid phase on the overlapped IR-spectroscopic pattern of (R)-()-1-phenylglycinium hydrogen squarate monohydrate is studied experimentally by means of a complex approach, including IR-LD spectroscopy of oriented solid-samples as suspension in nematic liquid crystal, reducing difference procedure for polarized spectra interpretation, deconvolution and curve-fitting procedures. Raman ones completes the IR-spectroscopic data. The experimental results are supported with theoretical ones and the calculated frequencies obtained on UHF/6-311++G** level of theory and basis and scaled with a factor of 0.8929 correlated well with experimental observed data, giving a standard deviation of 9 cm1 for so-called non-characteristic bands. Ó 2007 Published by Elsevier B.V. Keywords: (R)-()-1-Phenylglycinium hydrogen squarate monohydrate; Solid state; IR-LD study; Raman spectroscopy; Vibrational analysis; Structurespectroscopic properties

1. Introduction Intermolecular hydrogen interactions are proposed to be the important of the biological activities of molecules involved in biochemical processes in the living cell. These hydrogen bonds, with low binding energies of typically 10–30 kJ mol1, allow the biomolecules to interact with their targets before breaking free. Nearly all biochemically relevant molecules contain intramolecular hydrogen bonds. Therefore, the in vivo biochemical processes involve a combination of intra- and intermolecular hydrogen bonding interactions, the details of which are not always well understood.

*

Tel.: +3592 9606 106. E-mail address: [email protected]

0022-2860/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.molstruc.2007.01.057

The interest on phenylglycine and especially of its hydrogen squarate is provoked by the following reasons. The discovery of genotoxic amino acids derived from phenylglycine, and possessing halogen substituents, has been reported [1]. Gabapentin and pregabalin have been demonstrated to be effective analgesics particularly for the treatment of naturopathic pain. The precise mechanism of action for these two drugs is unknown, but they are generally believed to function via initially binding to the a2d-subunit of voltage-gated Ca2+ channels. The anti-allodynic effects of gabapentin and pregabalin, along L-phenylglycine shown to be potent a2d ligand [2]. The biological role in enzymic process of production of D- and L-phenylglycine as well as the function of dipeptide D-phenylglycine-L-dopa has been reported [3–5]. A large number of medications based on squaric acid and its derivatives as hydrogen squarates and ester amides have

140

T. Kolev / Journal of Molecular Structure 846 (2007) 139–146

been obtained. Some of them are effective monoanionic inhibitors of protein tyrosine phosphatases [6] or selective inhibitors of DNA polymerases from several viruses [6]. The squaric acid amides of antracycline glycoside-type antibiotics as daunomycin, adriamycin, epirubicin and carminomycin are potential antitumor agents [7,8]. Other products are applied to investigate an NMDA antagonist that regulated the activation of glutamate receptors. A novel series of benzylamine derivatives of squaric acid are potential potassium channel openers for treatment of urge urinary incontinence [9]. Replacement of the amino carboxylic moiety in thioproline CT5219, which is a VLA-4 antagonist, with squaric acid, give a potent medication for treatment of asthma, multiple sclerosis and rheumatoid arthritis [10]. Some peptide conjugates are evaluated as inhibitors of matrix metalloproteases [11]. Some derivatives of b-lactam antibiotics such as penicillin and cephalosporins are with potential usage for interfering with the biosynthesis of the peptidoglycan layer of bacterial cell like non-substituted antibiotics [9]. The neurocemical activity of some derivatives is also reported. IR-and Raman spectroscopic characterization and the influence of solid-state interactions in the unit cell on characteristic frequencies of (R)-()-1-phenylglycinium hydrogen squarate monohydrate (Scheme 1) are studied by means of solid-state linear polarized IR-method, Raman spectroscopy and theoretical calculations. The results obtained give correlation structure/spectroscopic properties of L-phenylglycinium cation and can help the complete understanding of the vibrational characterization of oxocarbons in general. 2. Experimental 2.1. Synthesis Titled compound was synthesised using the scheme described in [12]. Elemental analysis and FAB mass-spectrometric data showed a confirmation for obtaining of the compound studied. Found: C, 50.88; H, 4.62; N, 4.95%; [C12H13NO7] calcd.: C, 50.89; H, 4.63; N, 4.95%. The most intensive signal in the mass spectrum is a peak at m/z 152.15, corresponding to singly charged cation [C8H10NO2]+ with a molecular weight of 152.17. TGV analysis in the range 350–500 K showed a loss of molecule weight of 6.0% and DSC revealed an enthalpy effect of 12.16 kcal mol1, corresponding to inclusion of one H2O molecule in the structure obtained.

O H3N

OH x

O

OH

O

O

x H2 O

Scheme 1. (R)-()-1-Phenylglycinium hydrogen squarate monohydrate.

2.2. Methods The 4000–400 cm1 solid-state IR-spectra were recorded on a Bruker 113v FT-IR spectrometer (resolution 2 cm1, 250 scans) equipped with a Specac wire-grid polarizer. The oriented solid samples were obtained as a suspension in a nematic liquid crystal of the 4 0 -cyano-4 0 -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. 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 KBrplates 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. The validation of this new orientation solid-state method used in linear-dichroic infrared (IR-LD) spectroscopy for accuracy, precision and the influence of the liquid crystal medium on peak positions and integral absorbances of the guest molecule bands have been presented [13]. Optimization of experimental conditions and an experimental design for quantitative evaluation of the impact of four input factors has been shown [14,15]. The number of scans, the rubbing-out of KBr-pellets, the amount of studied compounds included in the liquid crystal medium and the ratios of Lorentzian to Gaussian peak functions in the curve-fitting procedure on the spectroscopic signal at five different frequencies has been studied [14,15]. It has been found that the procedure for the position (mi) and integral absorbancies (Ai) determination for each i-peak have been carried out by deconvolution and curve-fitting procedures at 50:50% ratio of Lorentzian to Gaussian peak functions, v2 factors within 0.00066–0.00019 and 2000 iterations [14,15]. The means of two treatments were compared by Student t-test. The applicability of the last approach for experimental IR-spectroscopic band assignment as well as an obtaining of stereo-structural information has been demonstrating in series of organic systems and coordination complexes as heterocyclic [16,17], Cu(II) complexes [18], polymorphs [19–22], codeine derivatives [23], peptides their Au(III) complexes, hydrochlorides and hydrogen squarates [24–31]. IR-LD spectroscopy and the interpretation of the linearpolarized IR-spectra are described in [32–36]. The method consists of subtraction of the perpendicular spectrum, (IRs, resulting from a 90° 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. The recorded difference (IRp–IRs) spectrum divides the corresponding parallel (Ap) and perpendicular (As) integrated absorbencies of each band into positive values

T. Kolev / Journal of Molecular Structure 846 (2007) 139–146

originating from transition moments, which form average angles with the orientation direction (n) between 0° and 54.7° (magic angle), and negative ones corresponding to transition moments between 54.7° and 90°. 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. Solid-state Raman spectra are recorded on Jobin-Yvon 6400 Spectrometer as polycrystalline powder samples, introduced in capillary tubes. The excitation was performed with 514.50 nm laser line (Spectra Physics) in the 3500–50 cm1 region, using 100 mW powers. The intermolecular interactions in solid-state resulted to a deviation of corresponding peak positions in experimental and theoretical predicted modes. Moreover, its is typical for so-called characteristics IR-bands. Quantum chemical calculations were performed with GAUSSIAN 98 program package [37]. The output fails are visualized by means of ChemCraft program [38]. The calculation of vibrational frequencies and infrared intensities were checked for which kind of calculations performed agree best with the experimental data for the non-affected of intermolecular interactions groups in the molecule. UHF method provides more accurate vibrational data, as far as the calculated standard deviations 8 cm1 (B3LYP), 9 cm1 (UHF) and 11 cm1 (UMP2) are obtained. So, the UHF/6-311++G** data are presented for above discussed model, where for the obtaining of the better correspondence between the experimental and theoretical values, a modification of the results using the empirical scaling factor (0.8929) is made. The calculation of the vibrational data is carried out by preliminary optimization of the molecular geometry of the titled compound and conformational analysis data. The geometry was optimized at levels of theory: second-order Moller–Pleset perturbation theory (MP2), Unrestricted Hartree–Fock (UHF) and density functional theory (DFT) using 6-311++G** basis set. DFT method employed is B3LYP, which combines Becke’s three-parameter non-local exchange functional with the correlation function of Lee, Yang and Parr [39,40]. Molecular geometries of the studied species were fully optimized by the force gradient method using Bernys’ algorithm [41]. For every structure the stationary points found on the molecule potential energy hypersurphases were characterized using standard analytical harmonic vibrational analysis. The absence of the imaginary frequencies, as well as of negative eigenvalues of the second-derivative matrix, confirmed that the stationary points correspond to minima of the potential energy hypersurfaces. Comparing with crystallographic data the UHF/6-311++G** approach give the better

141

geometry parameters, and then other two levels of theory used. The FAB mass spectra were recorded on a Fisons VG Autospect instrument employing 3-nitrobenzyl alcohols as the matrix. The elemental analysis was obtained according to the standard procedures for C and H (as CO2, and H2O) and N (by the Dumas method).. The thermogravimetric study was accomplished using Perkin-Elmer TGS2 instrument. The calorimetric ones are performed on DSC-2C Perkin-Elmer equipment in argon.

3. Result and discussion 3.1. Theoretical data Calculations identified a series of conformational minima for protonated phelylgycine molecule, but only one (Scheme 2) is characterized with Erel less then 5 kJ mol (Erel = 0.2 kJ mol). Backbone conformation with intramolecular N+H3  OH(C@O) hydrogen bond was found to be more stable than structures with bifurcated amine (N+H3) to OH interaction. The former bond is a conventional interaction commonly accepted to play the dominant role in determining the relative stability of the conformation. N+H3  OH bond length and (H3)N+  OAH angle ˚ and 98.9(0)° are obtained, respectively. The of 2.561 A

Scheme 2. Calculated geometry of protonated phenylglycine.

T. Kolev / Journal of Molecular Structure 846 (2007) 139–146

geometry parameters i.e. bond lengths and angles (Scheme 1) correlated well with the experimental refined ones, ˚ and due to the values do not differ then 0.041 A 5.2°, respectively (Scheme 1 and [12]). It could be concluded for dihedral angle values, excepting the dihedral O@CACHAN(H3), (O@)CACHACACH and O@CACHAC ones. The obtained difference within 5°– 52° is explained with the different types of intermolecular interactions in solid-phase. According to crystallographic data [12] the series of intermolecular hydrogen bonds are ˚ ), refined (Scheme 3): NH3 þ . . . OðH2 Þ (2.887 , 2.797 A þ ˚ NH3    O@CðSqÞ (2.780 , 2.957 A), (Sq)OH. . .O@C(Sq) ˚ ), (Sq)OH. . .OH2 (2.556 A ˚ ) and HOH  O@C (2.510 A ˚ (2.637 A). The calculated IR-spectrum of protonated phenylglycine (Fig. 1) is characterized with highest absorption band at 3526 cm1 corresponding to stretching mOH mode. The 0 mas NH3 þ , mas NH3 þ and ms NH3 þ bands are calculated at 3326, 3305, and 3219 cm1. The bands between 3040 and 2996 cm1 belonging to in-plane (i.p.) modes of phenyl ring and the 2954 cm1 – mCH, respectively. The characteristic maxima and their assignment in 1800–1400 cm1 are following: 1761 cm1 (mC@O), 1650 cm1 das NH3 þ , 1633 cm1 0 das NH3 þ , 1606 cm1 (8a), 1593 cm1 (8b), 1540 cm1 s d NH3 þ , 1498 cm1 (19a) and 1461 cm1 (19b). The maximum at 1318 cm1 corresponds to mCHO. The characteristic out-of-play maxima of monosubstituted benzene between 1000 and 600 cm1 are calculated at 714 and 690 cm1 corresponds to 11-cCH and 4-cAr o.p. modes (Scheme 4A and B). The calculated frequencies for aromatic system correlated well with the experimental ones due to they does not differ less then 9 cm1, because of the corresponding

25

20 IR Intensity (KM/Mol)

142

15

10

5

0 4000

3500

3000

2500 2000 1500 Wavenumber (cm-1)

1000

500

Fig. 1. Calculated (UHF/6-311++G**) IR-spectrum of protonated phenylglycine.

groups are not influenced from intermolecular interaction. However in the cases of C@O, OH and NH3 þ ones the experimental data indicated the following differences. Like in the cases of zwitterions in amino acids and peptides the stretching NH3 þ bands are observed as broadband between 3310 and 2200 cm1 in solid state, overlapped in this case with the bands of mOH(Sq). The strong hydrogen bonding (see above) resulted to an observation of the broad band to 1860 cm1. The hydrogen bond interaction in solid state in the system studied resulted as well on the significant low frequency shifting of mOH with 109 cm1 due to refined intermolecular interaction OH  O(H2) in crystalline

Scheme 3. Hydrogen bonding in the unit cell of (R)-()-1-Phenylglycinium hydrogen squarate monohydrate.

T. Kolev / Journal of Molecular Structure 846 (2007) 139–146

143

Scheme 4. The disposition of the transition moment vectors of 11-cCH (a) and 4-cAr (b) o.p. modes.

1514

1498

1575

1808

1645

1725

1671

1621

1606

1695

phase. The presence of the H2O molecule in the crystal structure of compound studied leads to an observation of the peak at 3509 cm1 corresponding to mOH(H2O). The complicated IR-spectroscopic patterns in 1800– 1450 cm1, where are overlapped the bands of bending NH3 þ band, i.p. modes of benzene ring and mC@O as well as these for hydrogen squarate species i.e. msC@OðSqÞ , s mas C@OðSqÞ and mC@CðSqÞ . The curve-fitting procedure (Fig. 2) applied on the IR-curve arise the series of maxima (Fig. 2) assigned as following: 1808 cm1 ðmsC@OðSqÞ Þ, 1 1725 cm1 (mC@O), 1695 cm1 ðmas (8a), C¼OðSqÞ Þ, 1621 cm

1800

1750

1700

1650

1600

1550

1500

Absorbance / Wavenumber (cm-1)

Fig. 2. 1850–1480 cm1 Curve-fitted IR-spectrum of (R)-()-1-phenylglycinium hydrogen squarate monohydrate.

0

1606 cm1 (8b), 1671 cm1, 1644 cm1 (das NH3 þ , das NH3 þ ), 1550 cm1 ðds NH3 þ Þ and 1498 cm1 (19a), respectively. The deconvolution shows doublet character of first band, which could be described with the different types of intermolecular hydrogen bonding with participation with hydrogen squarate anion (HSq) in the crystal structure of compound studied. The assignment stated above is experimentally confirmed by the IR-LD spectroscopic characterization using the reducing difference procedure. The adequate vibration assignment follows from the obtained significant degree of macro-orientation of crystalline sample using the criteria in [13,14] (see Fig. 3.2). The obtained negative peaks at 723 and 692 cm1 assigned these maxima to 11-cCH and 4-cAr o.p. modes (B1) of benzene ring. Their multiple characters could be assigned with presence of four different molecules in the unit cell of (R)-()-1-phenylglycinium hydrogen squarate monohydrate. Independently of the number of the molecules the four benzene rings in the unit cell are disposed mutually closing at an angles of 19.3(4)°, 32.1(8)°, 19.3(2)° and 37.1(0)° (Scheme 5). The resulted direction of the B1 o.p. modes are shown in Scheme 6. However, the elimination of the bands at 723 and 692 cm1 (Fig. 3.3) in same dicroic ratio confirms the assignment stated above, but in the reduced IR-LD spectrum are observed bands at 719 cm1 and 689 cm1 with same character of the other, different disposed molecules in the unit cell (Scheme 3). According to literature data of rubidium hydrogen squarate [42], different squarate salts [43] and theoretical vibrational analysis of hydrogen squarate anion [44] in this region could be observed the o.p. maximum of HSq- species. However in the IR-spectrum are observed also the B1-bands of aromatic system. Contrarily, in the Raman spectrum of (R)-()-1-phenylglycinium hydrogen squarate monohydrate (Fig. 4) is found a very strong band at 736 cm1 with doublet character, which could be assigned to discussed o.p. mode of HSq anion. Its high frequency shifting with about 10 cm1 and multiple characters could be explained as well with the participation of HSq anion in two types strong intermolecular hydrogen bonding.

144

T. Kolev / Journal of Molecular Structure 846 (2007) 139–146

692

723

(3)

(2)

Scheme 6. Calculated resulting direction of B1 modes of benzene rings included in the unit cell of (R)-()-1-phenylglycinium hydrogen squarate monohydrate.

(1)

4. Conclusions 800

750 700 650 Absorbance / Wavenumber (cm-1)

600

Fig. 3. Non-polarized IR-(1), difference (2) and reduced (3) IRLD spectra of (R)-()-1-phenylglycinium hydrogen squarate monohydrate after elimination of the band at 721 cm1.

Scheme 5. Benzene planes in the unit cell of (R)-()-1-phenylglycinium hydrogen squarate monohydrate.

The influence of the intermolecular interactions on the IR-characteristic bands in solid state of (R)-()-1-penylglycinium hydrogen squarate monohydrate is studied by means of a complex approach, including the linear-polarized IR-spectroscopy of oriented solid samples as suspension in nematic liquid crystal, reducing-difference procedure, deconvolution and curve-fitting procedure. The presence of the H2O molecule and hydrogen squarate ions in the unit cell of compound studied giving different possibilities for intermolecular interactions resulted to complex and strong overlapped IR-spectroscopic pattern. The correlation crystal structure – spectroscopic properties is drown comparing as well with the Raman spectroscopic data. The experimental structural and spectroscopic results are supported with theoretical ab initio ones at UHF level of theory and 6-311++G** basis set. The calculated frequencies are scaled with factor of 0.8929 giving a good theoretical approximation due to the predicted frequencies. In the cases of modes, corresponding to groups, which is not intermolecular interacting the difference between theoretical and experimental values, is less then 9 cm1. The clearance of the IR-spectroscopic properties and their influence of intermolecular interaction in vitro, using model system, giving a possibil-

T. Kolev / Journal of Molecular Structure 846 (2007) 139–146

145

Fig. 4. Raman spectrum of (R)-()-1-phenylglycinium hydrogen squarate monohydrate.

ity for in vivo examination by means of IR-and Raman spectroscopic tools. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.molstruc.2007.01.057. References [1] A. Boto, J.A. Gallardo, R. Herna´ndez, F. Ledo, A. Mun˜oz, R.J. Murguı´a, M. Menacho-Ma´rquez, A. Orjales, C.J. Saavedra, Bioorg. Med. Chem. Lett. 16 (2006) 6073. [2] J.J. Lynch III, P. Honore, D.J. Anderson, W.H. Bunnelle, K.H. Mortell, C. Zhong, C.L. Wade, C.Z. Zhu, H. Xu, K.C. Marsh, C.H. Lee, M.F. Jarvis, M. Gopalakrishnan, Pain 125 (2006) 136. [3] H.-P. Wang, Y.H. Chuang, C.Y. Lee, C.L. Wang, W.H. Hsieh, J. Food Drug Anal. 14 (2006) 225. [4] K.G. Zbinden, U. Obst-Sander, K. Hilpert, H. Kuhne, D. Banner, H. Bohm, M. Stahl, J. Ackermann, L. Alig, L. Weber, H.P. Wessel, M.A. Riederer, T.B. Tschopp, T. Lave´, Bioorg. Med. Chem. Lett. 15 (2005) 5344. [5] G.D.C. Machado, M. Gomes Jr., O.A.C. Antunes, E.G. Oestreicher, Proc. Biochem. 40 (2005) 3186. [6] J. Xie, A.B. Comeau, C.T. Seto, Org. Lett. 6 (2004) 83. [7] A. Tevyashova, F. Sztaricskai, G. Batta, P. Herczegh, A. Jeney, Bioorg. Med. Chem. Lett. 14 (2004) 4783. [8] F. Sztaricskai, A. Sum, F. Roth, I. Pelyvas, S. Sandor, G. Batta, P. Herczegh, J. Remenyi, Z. Miklan, F. Hudecz, J. Antibiotics 58 (2005) 704. [9] A. Gilbert, M. Antane, T. Argentier, J. Butera, G. Francisco, C. Freeden, E. Gundersen, R. Craceffa, D. Herbst, B. Hirth, J. Lennox, G. McFarlane, N.W. Norton, D. Quaglito, J. Sheldon, D. Warga, A. Wojdan, M. Woods, J. Med. Chem. 43 (2000) 1203.

[10] J. Porter, S. Archibald, K. Child, D. Critchley, J. Head, J. Linsley, T. Parton, M. Robinson, A. Shock, R. Taylor, G. Warrellow, R. Alexander, B. Langham, Bioorg. Med. Chem. Lett. 12 (2000) 1051. [11] M.B. Onaran, A.B. Comeau, C.T. Seto J. Org. Chem. 70 (2005) 10792. [12] O. Angelova, R. Petrova, V. Radomirska, T. Kolev, Acta Crystallogr. C 52 (1996) 2218. [13] B.B. Ivanova, M.G. Arnaudov, P.R. Bontchev, Spectrochim. Acta 60A (2004) 855. [14] B.B. Ivanova, D.L. Tsalev, M.G. Arnaudov, Talanta 69 (2006) 822. [15] B.B. Ivanova, V.D. Simeonov, M.G. Arnaudov, D.L. Tsalev, Spectrochim Acta Part A (2006) in press. [16] B.B. Ivanova, Spectrosc. Lett. 38 (2005) 635. [17] B.B. Ivanova, Spectrochim. Acta Part A 62 (1–3) (2005) 5. [18] B.B. Ivanova, H. Mayer-Figge, J. Coord. Chem. 58 (2005) 653. [19] B.B. Ivanova, J. Mol. Struct. 738 (1–5) (2005) 233. [20] B.B. Koleva, J. Mol. Struct. in press. [21] B.B. Ivanova, Centrl. Europ. J. Chem. 4 (2006) 111. [22] B.B. Koleva, Vibr. Spectrosc. (2006) in press. [23] B.B. Ivanova, J. Coord. Chem. 58 (7) (2005) 587. [24] R. Bakalska, B.B. Ivanova, Ts. Kolev, Centrl. Eur. J. Chem. 4 (3) (2006) 533. [25] B.B. Ivanova, J. Mol. Struct. 782 (2006) 122. [26] B.B. Ivanova, Spectrochim. Acta Part A 64 (2006) 931. [27] B.B. Ivanova, T. Kolev, S.Y. Zareva, Biopolymers 82 (2006) 587. [28] B.B. Koleva, T.S. Kolev, S.Y. Zareva, M. Spiteller, J. Mol. Struct. (2006) in press. [29] B. Koleva, Spectrochim. Acta Part. A (2006) in press. [30] T.S. Kolev, S.Y. Zareva, B.B. Koleva, M. Spiteller, Inorg. Chim. Acta. (2006) in press. [31] B. Koleva, T. Kolev, M. Spiteller, Biopolymers (2006) in press. [32] Ts. Kolev, Biopolymers 83 (2006) 39. [33] B. Jordanov, B. Schrader, J. Mol. Struct. 347 (1995) 389. [34] B. Jordanov, R. Nentchovska, B. Schrader, J. Mol. Struct. 297 (1993) 292. [35] J. Michl, E.W. Thulstrup, Spectroscopy with Polarized LightSolute Alignment by Photoselection, in Liquid Crystals, Polymers, and Membranes, VCH Publishers, NY, 1986.

146

T. Kolev / Journal of Molecular Structure 846 (2007) 139–146

[36] E.W. Thulstrup, J.H. Eggers, J.H. Chem. Phys. Lett. 1 (1996) 690. [37] M.J.Frisch,G.W.Trucks,H.B.Schlegel,G.E.Scuseria,M.A.Robb,J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant,S. Dapprich,J.M. Millam, A.D. Daniels, K.N. Kudin,M.C. ¨ . Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Strain, O Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, P. Salvador, J.J. Dannenberg, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Koma´romi, R. Gomperts, R. L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L.

[38] [39] [40] [41] [42] [43] [44]

Andres, C. Gonzalez, M. Head-Gordon, F.S. Replogle, J.A Pople, Gaussian 98, Gaussian, Inc., Pittsburgh, PA, 1998. G.A. Zhurko, D.A. Zhurko, ChemCraft: Tool for treatment of chemical data, Lite version build 08 (freeware), 2005. D. Backe, J. Chem. Phys. 98 (1993) 5648. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B37 (1988) 785. C.Peng,P.Y.Ayala,H.B.Schlegel,M.J.Frisch,J.Comp.Chem.17(1996)49. S.L. Georgopoulos, R. Diniz, B.L. Rodrigues, L.F.C. De Oliveira, J. Mol. Struct. 741 (2005) 61. S.L. Georgopoulos, R. Diniz, M.I. Yoshida, L.N. Speziali, H.F.D. Santos, G.M.A. Junqueira, L.F.C. de Oliveira, J. Mol. Struct. 794 (2006) 63. B.B. Koleva, Ts. Kolev, S.Y. Zareva, M. Spiteller, Biopolymers (2006) submitted for publication.