Morphology, Crystal Structure, and Thermal and Spectral Studies of Semiorganic Nonlinear Optical Crystal LAHClBr Tanusri Pal,† Tanusree Kar,*,† Gabriele Bocelli,# and Lara Rigi#
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 4 743-747
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India, and IMEM-CNR Parco Area delle Scienze 37/a, I-43100 Parma, Italy Received September 22, 2003;
Revised Manuscript Received April 9, 2004
ABSTRACT: Solubility and specific heat properties of the semiorganic nonlinear optical crystal, L-arginine hydrocholorobromide (LAHClBr), have been measured as a function of temperature. The compound crystallizes in a rectangular bipyramidal shape with well-developed {100} and {001} pinacoids. X-ray diffraction analysis shows that it belongs to the space group P21 with two independent molecules in asymmetric units and lattice parameters a ) 11.158(2) Å, b ) 8.579(3) Å, c ) 11.235(3) Å, β ) 91.55°(4). The arginine molecules occur as zwitterions, with the amino groups and the guanidyl groups each accepting a proton from the acid groups present in the molecule. The molecules are held together by a three-dimensional network of hydrogen bonds. Introduction Nonlinear optical (NLO) materials which can generate highly efficient second harmonic blue-violet light are of great interest for various applications including optical communication, optical computing, optical information processing, optical disk data storage, laser fusion reactions, laser remote sensing, color display, medical diagnostics, etc. In this context, amino acids are interesting materials for NLO applications.1 L-Arginine is also an amino acid that forms a number of complexes (organic and semiorganic) upon reaction with different acids.2-3 Semiorganic NLO materials are attracting a great deal of attention due to their high NLO coefficients, high damage threshold, and high mechanical strength compared to organic NLO crystals. Detailed studies on two such semiorganic NLO crystals, L-arginine hydrochloride monohydrate (LAHCl) and L-arginine hydrobromide monohydrate (LAHBr), have been done in our laboratory.4-8 In continuation of this project work, we have attempted to grow a mixed crystal of LAHCl and LAHBr to improve the morphology, hardness, and optical properties of these halide systems. The single crystal growth and NLO properties of this mixed crystal (abbreviated as LAHClBr) were reported previously.9 It was observed that it possesses good thermal stability and mechanical strength compared to the parent crystals. Its damage threshold is also high and its transmitted cut-off wavelength goes down to the UV region. To continue this work, in this paper we report the results of X-ray diffraction analysis of the crystal and molecular structure of LAHClBr together with its solubility, morphology, specific heat, and IR spectral properties. Experimental Section Crystals of LAHClBr were grown by mixing an equimolar solution of LAHCl and LAHBr. A large-sized single crystal was grown on a small seed crystal dipped in the aqueous solution * Corresponding author: Tanusree Kar, Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India. E-mail:
[email protected]. † Indian Association for the Cultivation of Science. # IMEM-CNR.
of LAHClBr by the temperature lowering method and reported earlier.9 The well-shaped as-grown crystal was then set aside for morphological analysis. Solubility of LAHClBr in water was measured as a function of temperature in the range 33-55 °C. A thermostatically controlled vessel (100 mL) is filled with an aqueous solution of LAHClBr with some undissolved LAHClBr and stirred for 24 h. On the next day, a small amount of the solution is pipetted out, and its composition is determined gravimetrically. Figure 1 shows the solubility curve for LAHClBr. The density of the as-grown crystal was measured by the flotation method in a mixture of carbon tetrachloride (CCl4) and chloroform and found to be 1.568 g/mL, which agrees well with the theoretical value of 1.567 g/mL, calculated from the molecular weight of LAHClBr and the unit cell dimensions as determined by crystal structure analysis. Crystal dimensions and the angle between faces of the crystal were measured using the Zwei Kreis Reflections goniometer (P. Stoe Company, Heidelberg, Germany). These data are then correlated with the X-ray intensity data with the help of the program “MORPHO”,10 and finally the figures for habit faces and orientation of faces were obtained with program “SHAPE”.11 X-ray diffraction data were collected at room temperature on a Philips PW 1100 single-crystal diffractometer with a local program.12 Systematic extinction together with a SHG powder test9 unequivocally determined the space group as P21. The preliminary cell parameters were obtained from least squares of the (θ,χ,φ) angular values of 33 reflections (θ range ) 9.417.4°) accurately well centered on the diffractometer. The intensity of 5062 reflections were recorded in the range 3.48° < θ < 28.01°, of which 3501 reflections with Fo > 4σ(Fo) were regarded as observed. The intensity of one standard reflection, recorded for every 100 reflections, showed no significant changes. The recorded data were corrected for polarization and Lorentz effects. The absorption correction was performed following the empirical method of Walker & Stuart13 with a program written by Gluzinski.14 The crystal structure was solved by a direct method with the SIR97 program15 and refined by full matrix least squares with SHELX9716 to an R value of 0.0372. All the non-hydrogen atoms were refined anisotropically, while the hydrogen atoms, all found in ∆F map were refined with isotropic thermal parameters with a restraint in the N-H distance for N3, N5 atoms. The ORTEP drawing was performed with ORTEP3 program.17 Values of bond distances and angles are listed in Tables 1 and 2.
10.1021/cg0341757 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/22/2004
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Figure 1. Solubility curve of LAHClBr as a function of temperature in aqueous solution. Table 1. Intermolecular Features: Bond Lengths (Å) in LAHClBr, H2O Molecule molecule A O1-C1 O2-C1 C1-C2 C2-N1 C2-C3 C3-C4 C4-C5 C5-N2 N2-C6 C6-N3 C6-N4
1.238(4) 1.258(4) 1.542(5) 1.501(5) 1.536(5) 1.538(6) 1.526(6) 1.451(5) 1.326(6) 1.334(6) 1.322(4)
molecule B O3-C7 O4-C7 C7-C8 C8-N5 C8-C9 C9-C10 C10-C11 C11-N6 N6-C12 C12-N7 C12-N8
1.242(4) 1.248(4) 1.539(5) 1.495(5) 1.519(5) 1.521(5) 1.506(6) 1.467(5) 1.319(5) 1.344(5) 1.326(5)
Table 2. Intermolecular Features: Bond Angles (°) in LAHClBr, H2O Molecule molecule A O1-C1-O2 O2-C1-C2 O1-C1-C2 C3-C2-C1 N1-C2-C1 N1-C2-C3 C2-C3-C4 C5-C4-C3 N2-C5-C4 C6-N2-C5 N4-C6-N2 N2-C6-N3 N3-C6-N4
126.3(3) 114.9(3) 118.7(3) 110.7(3) 110.0(3) 111.0(3) 113.1(3) 109.5(4) 113.8(3) 125.6(4) 120.4(4) 120.7(3) 118.9(5)
molecule B O3-C7-O4 O4-C7-C8 O3-C7-C8 C9-C8-C7 N5-C8-C7 N5-C8-C9 C8-C9-C10 C11-C10-C9 N6-C11-C10 C12-N6-C11 N6-C12-N8 N6-C12-N7 N8-C12-N7
126.0(3) 116.3(3) 117.7(3) 110.8(3) 110.2(3) 110.1(3) 114.6(3) 111.7(3) 113.4(3) 124.9(4) 118.8(4) 121.6(3) 119.6(4)
The specific heat was calculated from the DSC curve obtained using a Perkin-Elmer differential scanning calorimeter (DSC-7) in the range of 30-55 °C. Fourier transform infrared (FTIR) spectra were recorded in the range 400-4000 cm-1 using a nicolet MAGNA-IR 750 (series II) IR spectrophotometer with reference to a potassium bromide pellet.
Results and Discussion Large and optically transparent single crystals of LAHClBr (Figure 2a) were obtained by the slow cooling method. The morphology of the LAHClBr crystal as depicted in Figure 2b is a polyhedron with eight developed faces. Out of these eight faces, the bestdeveloped faces are {100} and {001} pinacoids. The macropinacoids {101} and {101 h }, which were present
Figure 2. (a) Optical photograph of LAHClBr crystal grown from aqueous solution by the slow cooling method, with polar axis vertical, mark 0.4 cm. (b) Observed morphology of LAHClBr with the polar axis along the long dimension of the crystal.
in the parent crystals, LAHCl and LAHBr, are seldom found in LAHClBr. All the dihedral planes {110} and {011} are well developed. LAHClBr belongs to the monoclinic space group P21, having four molecules in unit cell and lattice parameters a ) 11.158(2) Å, b ) 8.579(3) Å, c ) 11.235(3) Å, β ) 91.55°(4), V ) 1075.1(5) Å3, Z ) 4, D ) 1.567 g/mL, λ (Mo-KR) ) 0.71069Å, µ ) 2.290 cm-1. The symbols have their usual meanings. The arginine molecule consists of two terminal groups: one is the carboxyl group C1, C2, O1, O2 and the other is a guanidyl group C6, N2, N3, N4 connected together through an aliphatic chain. Crystal structure analysis of the present L-arginine complex shows that the cation (arginine molecule) exists as positively charged zwitterions, in which the guanidyl and amino groups are protonated and the carboxyl group is deprotonated. Thus, π-π* transition occurs in the carboxylate and guanidyl groups, which give rise to the NLO properties in this type of complex of L-arginine. The anionic position is partially occupied by either chlorine or bromine ions with partial occupancy of Cl - 0.63547, Br - 0.36453 and Cl 0.24306, Br - 0.75694 in two independent molecules in an asymmetric unit from which the chemical composition of the mixed system is thus established as C6H14N4O2‚HCl0.44Br0.56,H2O. The bond lengths and bond angles of two crystallographically independent molecules in the asymmetric unit are almost the same and also comparable with the values reported in the literature.18,19 The guanidyl group is attached to the aliphatic chain through a C-N bond of length 1.451 Å
Spectral Studies of LAHClB
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Figure 3. (a) ORTEP diagram of LAHClBr. The displacement ellipsoids are at 50% probability level. (b) Crystal structure of LAHClBr viewed down the b-axis, with hydrogen bonds indicated by dashed lines.
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Figure 4. Variation of specific heat with temperature.
in molecule A and 1.467 Å in molecule B, which are shorter than the usual C-N single bond length but agree well with the value found in the literature.20 The Ortep diagram with 50% probability is shown in Figure 3a. From the packing diagram (Figure 3b), it is observed that the crystal structure consists of alternate layers of L-arginine molecules stacked parallel to the diagonal of the ac plane with chlorine or bromine ions between the layers. The specific heat data of crystalline solids in general could be explained with the help of the Debye lattice theory in terms of harmonic frequency spectrum. But in our case due to the complexity in the structure of LAHClBr it is difficult to correlate the measured specific heat data with the predictions of a lattice theory. The specific heat data of LAHClBr at different temperatures are shown in Figure 4. The figure shows that the specific heat of LAHClBr slightly increases with an increase in temperature, and it is nearly equal to LAP but greater than KDP. In our previous paper,9 we have reported an optical damage threshold of LAHClBr, which is about 29.84 GW/cm2 at 1064 nm and is greater than KDP. Intuitively, one would expect a nonlinear material with a higher specific heat to be more resistant to laser damage since a rise in temperature with laser irradiation is one of the mechanisms by which damage is caused. To analyze the IR spectrum (Figure 5) of LAHClBr, we have to consider the single-crystal structure which shows that in the crystalline state the arginine molecule is deprotonated at the carboxyl group (COO-) and protonated at the guanidyl (+(H2N)2CNH) and amino (NH3+) groups. Thus, the structure of the LAHClBr crystal consists of an L-arginine molecule in the ionized form and a chlorine or bromine ion with partial occupancy. In the IR spectra, the peaks observed at 547, 617, 648, 671 cm-1 are attributed to deprotonation of the carboxyl group, while the absorption peaks at 1094, 1134 cm-1 are associated with protonation of the NH3+ group. There appears an intense absorption band at 1590 cm-1 due to asymmetric stretching of the ionized carboxyl group. The asymmetric and symmetric stretching of water which gives rise to strong absorption bands at 3756 and 3652 cm-1 are shifted to 3490 and 3330 cm-1 due to an extensive system of hydrogen bonding. So with the help of available data on the vibrational frequencies of amino acids,21 we have identified the
Figure 5. Infrared spectra of LAHClBr.
characteristic IR bands for different molecular groups present in LAHClBr,H2O. Conclusions A new crystal was grown by mixing of two NLO crystals L-arginine hydrochloride monohydrate and Larginine hydrobromide monohydrate. All the dihedral planes are well developed in this mixed crystal compared to its parent crystals. Crystal structure analysis reveals that in this mixed system the parent molecules retain their identity with the chlorine and bromine ions occupying the same position with partial occupancy values. From the value of the occupancy factor the chemical composition of the mixed crystal is found to be C6H14N4O2‚HCl0.44Br0.56,H2O. From crystal structure analysis, the π-π* transition in the carboxylate and guanidyl group is established, which accounts for the nonlinearity of the mixed crystal system. From the FTIR study, it is confirmed that all the vibrational frequency lines of L-arginine are also present in LAHClBr with a shift in frequency due to the presence of Cl and Br ions. The IR study coupled with density measured and structural analysis have established the hydrated form of LAHClBr and enabled us to determine the various molecular vibrations in the LAHClBr crystal. Specific heat measurements are in accordance with the value of damage threshold observed for this mixed system. Supporting Information Available: Crystallographic information file is available free of charge via the Internet at http://pubs.acs.org.
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Spectral Studies of LAHClB (10) Nardelli, M. MORPHO, an Utility Programs for Correlating X-ray Data with the Morphology of Crystals. Presented at the sixth European Crystallographic Meeting, Barcelona, Spain, 1980. (11) Dowty, E. SHAPE, Version 6.0.1; Kingsport, TN, USA, 2000. (12) Belletti, D. EFBO, a new Hardware and Software system for Controlling a Philips PW1100 Single-Crystal Diffractometer, Centro di Studio per la Strutturistica Diffratometrica del CNR, Parma- Italy, Internal Report 1-96, 1996. (13) Walker, N.; Stuart, D. Acta Crystrallogr. 1983, A39, 158166. (14) Gluzinski, P. Set of Programs for X-ray Structural Calculations. Icho, Polish Academy of Sciences, Warszawa, Poland, 1989. (15) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Burla, M. C.; Polidori, G.; Camalli, M.; Spagna, R. J. Appl. Cryst. 1999, 32, 115-119.
Crystal Growth & Design, Vol. 4, No. 4, 2004 747 (16) Sheldrick, G. M. SHELX 97, Program for the Crystal Structure Refinement, University of Go¨ttingen, Germany, 1997. (17) Farrugia, L.J. ORTEP-3 for windows, University of Glasgow, Scotland, U.K. 1999. (18) Saraswathi, N. T.; Vijayan, M. Acta Crystallogr. 2001, B57, 842-849. (19) Gomes, E.de Matos; Nogueira, E.; Fernandes, I.; Belsley, M.; Paixao, J. A.; Beja, A. Matos; Ramossilva, M.; MartinGil, J.; Mano, J. F. Acta Crystallogr. 2001, B57, 828-832. (20) IUPAC Commission on the Nomenclature of Organic Chemistry and IUPAC-IUB on Biochemical Nomenclature, Biochem. J. 1975, 149, 1. (21) Krishnan, R. S.; Sankaranarayanan, K.; Krishnan, K. J. Indian Inst. Sci. 1973, 55, 66-116.
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