Novel Materials for Second Harmonic Generation - Salts of L-Valine and Selenic Acid
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Novel Materials for Second Harmonic Generation - Salts of L-Valine and Selenic Acid Ivan Němec1, Robert Gyepes1, Zdeněk Mička1 and František Trojánek2 1 Department of Inorganic Chemistry, Charles University of Prague, Hlavova 2030, Prague 2, 128 40, Czech Republic. 2 Faculty of Mathematics and Physics, Charles University of Prague, Ke Karlovu 3, Prague 2, 121 16, Czech Republic. ABSTRACT The X-ray structural analysis of di-L-valine selenate monohydrate has been carried out (P21, a=6.181(2), b=18.675(3), c=15.342(4) Å, β=90.14(2)°, V=1770.8(7) Å3, Z=4, R=0.0532 for 3057 observed reflections). The crystal structure is formed by selenate anions, L-valinium cations and water molecules (in the ratio of 1:2:1) interconnected by a system of hydrogen bonds. The FTIR (298-90 K temperature range) and FT Raman spectra were recorded and interpreted. DSC measurements were carried out in the temperature range 95-363 K. Quantitative SHG measurements of di-L-valine selenate monohydrate and tri-L-valine selenate were carried out on powdered samples. INTRODUCTION Recently, materials possessing non-linear optical properties, especially those exhibiting second harmonic generation (SHG) have received considerable attention due to their wideranging technical applications. A novel class of SHG active compounds based on hydrogenbonded salts of inorganic oxyacids and polarizable organic cations seems to be very promising [1] because of the higher SHG efficiency and better chemical and/or physical properties compared to the traditional classes of SHG active materials (i.e. inorganic salts or oxides and organic molecules). Di-L-valine selenate monohydrate (DVSe) was synthesized and intensively studied within the frame of searching for new SHG active salts. The methods of X-ray structural analysis and vibrational spectroscopy were used to explain bonding conditions in the crystals of the compound prepared. The crystal structure of DVSe, FTIR and FT Raman spectra are reported and discussed. The results are compared to previously described tri-L-valine selenate (TVSe) [2]. Quantitative SHG measurements of DVSe and TVSe were carried out on powdered samples. The results are discussed with respect to crystal structures of both compounds. EXPERIMENTAL DETAILS Crystals of DVSe were prepared by slow spontaneous evaporation of a solution of L-valine (99%, Aldrich) in selenic acid (Fluka, 2 mol.l-1) (in a molar ratio of 1:1) at laboratory temperature. The colorless crystals obtained were collected under vacuum on an S3 frit, washed with ethanol and dried in the air. Elemental analysis results for C, H and N (30.0 % C, 6.4 % H, 6.8 % N) agree well with theoretical content (30.2 % C, 6.6 % H, 7.1 % N). The N,O-deuterated compound ((CH3)2CHND3+CHCOOD)2.SeO42-.D2O was prepared by repeated recrystallization of natural DVSe from D2O (99%) in a desiccator over KOH.
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