Crystal growth and characterization of L-threoninium ...

Crystal growth and characterization of L-threoninium chloride and L-threoninium bromide, new nonlinear optical crystals V.V. Ghazaryan* Institute of Applied Problems of Physics, NAS of Armenia 25 Nersessyan Str., 0014 Yerevan * [email protected] ; phone +374 10 241106; fax +374 10 281861

ABSTRACT New crystalline salts of L-threonine, i.e. L-Thr.HCl, and L-Thr.HBr have been obtained from aqueous solution by slow evaporation technique. Infrared and Raman spectra of L-Thr.HCl and L-Thr.HBr were recorded and interpreted on the basis of crystal structure of L-Thr.HCl. The similarity of their IR and Raman spectra indicates similar structures. Nonlinear optical properties of the crystals were tested qualitatively by observation of second harmonic generation of Nd:YAG laser. Keywords: L-threoninium chloride, L-threoninium bromide, vibrational spectra, NLO crystals.

1. INTRODUCTION Amino acids and their salts are investigated very intensively due to their possible application as nonlinear optical materials. L-threonine (L-Thr) displays higher nonlinear optical (NLO) properties than KDP [1]. Moreover, salts of amino acids usually display higher NLO properties than the pure amino acids. Therefore it is surprising that there are relatively few investigations on salts of L-threonine. To the best of our knowledge, only the following studies are available: the crystal bis (L-threoninium) sulfate monohydrate (2L-Thr+.SO42-.H2O) was known [2, 3] irrespective of its NLO properties, and very recently the crystal L-threoninium picrate has been obtained and published as a nonlinear optical material [4]. As it was shown in [5], the “L-threoninium acetate” reported in [6] actually is L-threonine. We started to research the systems L-Thr + HCl + H2O and L-Thr + HBr + H2O to obtain new crystals. Two phases (2LThr.HCl and L-Thr.HCl) were obtained in the first system [7]. The present work deals with study of crystallization conditions and characterization of L-Thr.HCl and L-Thr.HBr crystals.

2. EXPERIMENTAL As initial reagents we used L-threonine obtained in the Institute of Biotechnology (Yerevan) and the mineral acids HCl, HBr “chem. pure” grade. From aqueous solution containing L-threonine and hydrochloric acid with 2:1 M ratio a mixture of L-threonine and 2L-Thr.HCl is formed. From 1:1 M ratio a mixture of 2L-Thr.HCl and L-Thr.HCl is formed. In case of 1:2 M ratio only L-Thr.HCl is formed. L-Thr.HBr was obtained from aqueous solution containing Lthreonine and hydrobromic acid with 1:1 M ratio. The crystals of L-Thr.HCl and L-Thr.HBr have been grown from aqueous solutions by slow evaporation technique at room temperature. The as-grown crystal of L-Thr.HCl is shown in Fig. 1a as well as Bravais–Friedel–Donnay–Harker morphology (Fig.1b) calculated by Mercury 2.3 program [8]. To register FT-Raman spectrum a Nicolet 5700 spectrometer with NXP FT-Raman Module was used (number of scans: 256, laser power at sample: 0.44 W, resolution: 4 cm-1, spectral range: 4000–150 cm-1). FTIR-ATR spectrum was registered by the same spectrometer by means of Smart Accessory (ZnSe prism, number of sample scans: 32, spectral range: 4000–550 cm-1, resolution: 4 cm-1). The region of infrared spectrum with 550–400 cm-1 spectral range has been registered with Nujol mull technique (4000–400 cm-1, number of scans: 32, resolution: 2 cm-1). Nonlinear activity has been checked by observation of powder second harmonic generation of Nd:YAG laser [9].

3. RESULTS AND DISCUSSION 3.1. The crystal and molecular structure of L-Thr.HCl

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In a recent study, we have determined the crystal structures of several salts of amino acids [7], including the structure of L-threoninium chloride. The structure determination and refinement was made by means of single crystal Xray diffraction (see details given in [7]). L-threoninium chloride crystallize in the monoclinic space group P21 (Z=2). The unit cell comprises L-threoninium cations and chloride anions (Fig. 2). The L-threoninium cation contains a neutral COOH carboxyl group, a protonated NH3+ group and an alcoholic OH group capable to form hydrogen bonds. There are several hydrogen bonds in the structure. Particularly, the NH3+ group forms three hydrogen bonds, two of them with chloride ion with distances 3.203(1) and 3.253(1) Å and the third bond with oxygen atom of carboxyl group with distance 2.840(1) Å. Besides above mentioned, the chloride ion forms also two hydrogen bonds with carboxylic and alcoholic hydroxyl groups with distances 3.027(1) and 3.150(1) Å.

(a)

(b)

Figure 1. As-grown crystal (a) and crystal morphology (b) of L-Thr.HCl

Figure 2. Structural unit of L-Thr.HCl

3.2. Vibrational spectra of L-Thr.HCl and L-Thr.HBr IR and Raman spectra of L-threoninium chloride and L-threoninium bromide are shown in Figs. 3, 4. Interpretation of the spectra made on the basis of the structure, vibrational spectra of L-threonine [10] and bis (Lthreoninium) sulfate monohydrate [3]. In L-threoninium chloride alcoholic hydroxyl group forms not strong hydrogen bond with chloride ion O–H···Cl (3.150(1) Å), therefore O–H stretching vibration is slightly shifted towards low frequencies and is observed as a band at 3366 cm-1 and a line at 3373 cm-1 in the IR and Raman spectra respectively. Since in L-threoninium bromide O–H stretching vibration is observed about 80 cm-1 higher (IR-3446 cm-1 and Raman- 3452 cm-1) than that in L-threoninium


Figure 3. Vibrational spectra of L-Thr.HCl


Figure 4. Vibrational spectra of L-Thr.HBr


chloride, one can conclude that in L-threoninium bromide alcoholic hydroxyl group forms weaker H-bond in comparison with L-threoninium chloride. Carboxylic group also interacts with chloride ion and forms stronger hydrogen bond O– H···Cl (3.027(1) Å). Based on correlation between stretching vibration of O–H, N–H bonds and O···Cl, N···O and N···Cl distances [11] one may expect O–H (of carboxylic group) and N–H stretching vibrations near 3000 cm-1. Therefore, the broad, strong absorption in the region ca. 3400-2400 cm-1 is observed due to the stretching vibrations of N–H and carboxylic O–H bonds overlapped with C–H stretching vibrations. The C–H stretching vibrations have higher intensity in the Raman spectrum and are overlapped with N–H and carboxylic O–H stretching vibrations. On the basis of vibrational spectra one can suppose that the same situation is in the spectra of L-threoninium bromide. N–H stretching vibrations of protonated amino group are observed as shoulders at 3195, 3173, 3105 cm-1 and bands at 3079, 3019 cm-1. As a rule this vibrations has low intensity in the Raman spectrum. One can notice the lines at 3193 and 3039 cm-1. In Lthreoninium bromide N–H stretching vibrations are observed as a band at 3159 cm-1 and a shoulder at 3021 cm-1. There are two CH moieties and one methyl (CH3) group in both L-threoninium chloride and bromide. As a result of their stretching vibrations the peaks from shoulder at 2999 cm-1 to the band at 2966 cm-1 are observed. There are corresponding lines between 2999 and 2958 cm-1 in the Raman spectrum. The bands at 3003-2931 cm-1 and the lines at 3008-2935 cm-1 are observed in the IR and Raman spectra of L-threoninium bromide. Deformation vibrations of CH3 group fall in the region ca. 1470-1320 cm-1. Carboxyl group has characteristic stretching vibration of C=O bond which is observed as a very strong absorption band at 1732 cm-1 and a middle intensity line at 1745 cm-1 for L-threoninium chloride. Similarly in the spectra of L-threoninium bromide we find a very strong band at 1725 cm-1 and a middle intensity line at 1738 cm-1. It is known that there is a relation between stretching and bending vibrations of the same moiety. If the position of stretching vibration is shifted towards higher wavenumbers, then the position of deformation vibration moves towards the lower wavenumbers and vice versa. In-plane deformation vibration of carboxylic OH group are coupled with stretching vibration of carboxylic C–OH bond and observed as a band at 1273 cm-1, which in the Raman spectrum corresponds to low intensity line at 1275 cm-1. One can expect in-plane deformation of carboxylic OH of L-threoninium bromide approximately in the same region (1280 cm-1), since stretching vibration of carboxylic O–H bond is observed in the same region in the spectra of both compounds. In-plane deformation vibration of alcoholic OH is observed at lower wavenumbers (at 1225 cm-1 for chloride and at 1177 cm-1 for bromide), because the corresponding stretching vibration is in the higher region in comparison with carboxylic O–H. The corresponding Raman lines have low intensity. The NH3+ group makes two bending vibrations, asymmetric and symmetric, which are presented with strong peaks at 1599 cm-1 and 1504 cm-1 with their corresponding very weak lines at 1581 cm-1 and 1507 cm-1, respectively. In L-threoninium bromide there are strong peaks at 1582 cm-1 and 1504 cm-1 with their corresponding very weak lines at 1599 cm-1, 1585 cm-1 and 1514 cm-1, respectively. Rocking vibrations of NH3+ and CH3 groups are observed in the region ca. 1150-1000 cm-1. The stretching vibrations of C–N, C–C bonds can absorb around 1000 cm-1. The deformation vibration of carboxyl group, torsion oscillations of OH, NH3+ and CH3 groups can be in the lower region.

4. CONCLUSION The crystals L-threoninium chloride and L-threoninium bromide which are formed in the systems L-Thr + HCl + H2O and L-Thr + HBr + H2O are possible to grow in form of single crystals. Vibrational spectra of L-Thr.HCl and LThr.HBr are registered and interpreted. The crystals display NLO activity.

ACKNOWLEDGMENTS This work was made possible in part by a research grant PS1839 from the Armenian National Science and Education Fund (ANSEF) based in New York, USA. The author thanks Dr. A.E. Aghajanyan for providing L-threonine used as initial reagent as well as Dr. A.M. Petrosyan and Dr. M. Fleck for useful discussions.

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