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Structural, optical, spectral and thermal studies of nonlinear optical pure and deuterated l -alanine single crystals
ARTICLE IN PRESS
Journal of Crystal Growth 310 (2008) 1182–1186 www.elsevier.com/locate/jcrysgro
Structural, optical, spectral and thermal studies of nonlinear optical pure and deuterated L-alanine single crystals K. Suriya Kumar, Thenneti Raghavalu, V. Mathivanan, M. Kovendhan, B. Sivakumar, G. Ramesh Kumar, S. Gokul Raj, R. Mohan Department of Physics, Presidency College, Chennai 600 005, India Received 19 May 2007; received in revised form 22 July 2007; accepted 21 December 2007 Communicated by S. Veesler Available online 1 January 2008
Abstract L-Alanine single crystals have been grown from H2O and D2O by slow evaporation and temperature lowering methods. The grown crystals were characterized by powder X-ray diffraction analysis and the vibrational frequencies of various functional groups in the crystals have been derived from FTIR spectrum. Optical transparency of the grown crystals was investigated by UV–vis–NIR spectrum. Differential scanning calorimetry (DSC) was carried out to investigate the thermal stability of the grown crystals The NLO property of the grown crystal has also been confirmed by the Kurtz-powder SHG test. r 2008 Elsevier B.V. All rights reserved.
PACS: 77.84.2; 78.20.Nu; 81.10.Dn Keywords: A1. Characterization; A2. Growth from solution; A2. Single crystal growth; B2. Nonlinear optical materials
1. Introduction In recent times, one has witnessed a growing interest in the study of amino acid single crystals. This interest has been stimulated by the perspective of understanding a system where the hydrogen bond plays a fundamental role and as a consequence of this a better knowledge of some important biological molecules (e.g. proteins) can be obtained. The growth of large single crystals of a amino acids has investigation so far, even as regards the simplest acentric member of the family L-alanine (CH3CHNH2COOH) which was first crystallized by Bernal and later by Simpson and Marsh [1] who refined the structure (a ¼ 6.032 A˚, b ¼ 12.343 A˚, c ¼ 5.784 A˚; a ¼ b ¼ g ¼ 901) and assigned it the P212121 space group. Large-size (3 cm3) crystals were reported by Misoguti et al. [2] with very few details on growth and crystal quality. As Corresponding author. Tel.: +91 44 2854 4894; fax: +91 44 2851 0732.
E-mail addresses: [email protected] (T. Raghavalu), [email protected] (R. Mohan). 0022-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.12.049
to the crystal habit, it was systematically made up of faces belonging to the families {0 2 0}, {0 1 1} and {1 2 0}. The largest faces were always found to be {1 2 0}. The {0 2 0} faces were usually the smallest one but they appeared to be the best as to their optical quality and this has been reported by Razzetti et al. [3]. Neutron diffraction studies were also carried out by Jonsson and Kvick [4] for the hydrogen atoms position. A number of studies have been made to determine the vibrational spectra of L-alanine single crystals focusing on the assignment of modes done on the effects of various intermolecular potentials on the crystals vibrations [5,6]. Temperature-dependent study of X-ray and Raman scattering measurements of L-alanine single crystals were also studied earlier [7]. Thermal conductivity measurements of L-alanine single crystal [8] showed that lattice modes of L-alanine are strongly anharmonic. Presently, we are interested to study the growth, structural, optical and thermal properties of pure and deuterated L-alanine single crystals. An attempt has also been made to compare the vibrational assignments of pure
ARTICLE IN PRESS K. Suriya Kumar et al. / Journal of Crystal Growth 310 (2008) 1182–1186
and deuterated L-alanines. Deuteration of many NLO crystals gives rise to many improved optical properties. Deuterated KDP is still considered as one of the best electro-optic modulator with large electro-optic coefficient [9]. Deuteration on zinc tristhiourea sulphate results in considerable enhancement in its electro-optics properties [10]. Reduction in optical absorption and increase in optical transparency have also been observed for deuterated LAP [11]. Vibrational spectra of many deuterated amino acids were thoroughly studied by Krishnan et al. [12]. Similar studies were carried out for the case of DLserine by Jarmelo et al. [13]. For a complete assignment of the bands in the spectra of amino acids, deuteration studies are extremely helpful. In the case of glycine, Tusboi et al. [14], have made complete assignment of observed peaks in the region from 400 to 1800 cm1 by comparing the spectra of glycine, glycine hydrochloride and sodium glycinate and their N-deuterated
1183
analogs. Suzuki et al. [15] have also studied the spectra of Cdeuterated compounds of alanine. All vibrations involving the N atom in DL-alanine were identified by Tsuboi et al. [16] by substituting 14N by 15N and observed the isotope shifts in the spectrum. From the above ideas, an attempt has been made to grow the deuterated crystals of L-alanine. 2. Experimental procedure 2.1. Crystal growth The vapor pressure of L-alanine is low in the lowtemperature regime. Obviously, solution growth technique would appear to be the only feasible method. Hence, Lalanine crystals were grown from aqueous solution saturated at 40 1C, with the well-known slow cooling method. The solubility at this temperature was found to be 18.12 g/100 ml. Optical quality seed crystals were collected from the supersaturated solution and by successive recrystallization. Single crystals of deuterated L-alanine were prepared by the exchange reaction with heavy water (D2O). The degree of deuteration could be improved by successive crystallization of salt with deuterated water (D2O). Thermal strength of both pure and deuterated Lalanine was determined by the differential scanning calorimetry (DSC). Powder SHG efficiency of polycrystalline samples of both compounds has also been found out. The grown crystal of deuterated L-alanine is shown in Fig. 1. 2.2. Characterization studies The unit cell dimensions and X-ray intensity data were obtained using an Enraf-Nonius CAD-4 diffractometer equipped with CuKa radiation (l ¼ 1.5418 A˚). o/2y scan
100 90 80 70 60 50 40 30 20 10 100 2000
1361
20
500
1000
1500
1620
945 1056 1185
40
771
60
2500
2000
2500
Wavenumber
(cm -1)
3000
3500
4000
2937 3084
1500
2603
1000
2032 2112 2227 2275
500 80
3740
Pure L-alanine
535
Transmittance (%)
Fig. 1. As grown crystals of deuterated L-alanine.
3000
3500
Fig. 2. FTIR spectrum of pure and deuterated L-alanine single crystals.
4000
ARTICLE IN PRESS K. Suriya Kumar et al. / Journal of Crystal Growth 310 (2008) 1182–1186
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100 to 400 1C in nitrogen atmosphere at a heating rate of 5 1C/min. The NLO property of polycrystalline samples of pure and deuterated L-alanine of uniform size was tested by Kurtz-powder SHG test using an Nd: YAG laser (1064 nm).
10
Absorption
8
6
Pure L-alanine single crystals Deuterated L-alanine single crystals
3. Results and discussion
4
3.1. Vibrational spectra of pure and deuterated L-alanine single crystals 2
0 200
400
600
800
1000
1200
Wavelength (nm)
Fig. 3. UV–vis–NIR spectrum of pure and deuterated L-alanine single crystals.
1 0 -1
DSC (mW/mg)
-2 -3 -4 -5
Pure L-alanine Deuterated L-alanine
-6 -7 -8 -9 -10 -11 -100
0
100
200
300
400
Temperature (°C)
Fig. 4. DSC curve of pure and deuterated L-alanine single crystals.
mode was employed for data collection using the crystal of dimension 0.3 0.2 0.2 mm3. Polycrystalline powders were obtained by grinding in an agate mortar with a pestle. The vibrational measurements were carried out at room temperature. Fourier transform infrared spectrum of pure and deuterated L-alanine was recorded using a Bruker IFS66 V FTIR spectrophotometer in the frequency range 400–4000 cm1 by KBr pellet method is shown in Fig. 2. The optical transmission range of pure and deuterated Lalanine crystals was measured using Varian Cary 5E UV–vis–NIR spectrophotometer in the range 200–1000 nm is shown in Fig. 3. DSC on pure and deuterated L-alanine single crystals shown in Fig. 4 was carried out in the temperature range
The frequencies observed at 3084 and 3087 cm1 of pure and deuterated L-alanine single crystal can be attributed to 1 NHþ 3 stretching vibrations. The peak observed at 2937 cm in the IR spectrum is attributed to C–H stretching of the CH3 group. The peak observed at 2603 cm1 in the IR spectrum is assigned to the N–H stretching in the NH3 group, which is formed after protonation. The peaks at 1620 and 1614 cm1 are assigned to the NHþ 3 asymmetric bending vibrations of pure and deuterated L-alanine compounds. In pure L-alanine, the vibration at 1508 cm1 corresponds to the NHþ 3 symmetric bending. But it is not observed in deuterated L-alanine, which may be due to deuterization of the L-alanine. The peak at 1595 cm1 in the IR spectrum is assigned to the NH2 scissoring mode. Methyl deuteration has little effect on the position of the C–H stretching mode, and apparently deuteration of the methyl group causes no appreciable frequency shift. Bonds at 2275 and 2227 cm1 represent the deuteration of methyl group in L-alanine (CD3) and the peak of 2032 cm1 represents CD3 [17,18] vibration, which is usually very weak. The peak observed at 1185 cm1 in the IR spectrum is attributed to ND3 degree of deformation vibration. The peak at 1056 cm1 in IR spectrum is due to ND3 symmetric deformation vibration. The peak at 945 cm1 is assigned to the ND3 rocking vibration. The strong peaks observed at 1110, 1153 and 1180 cm1 were attributed to symmetric and degenerate deformation vibration of NDþ 3 . The peak at 535 cm1 represents the COO rocking. All other peaks of pure and deuterated L-alanine are listed in Table 1. 3.2. Effect of isotopic shift on the vibrational structure of Lalanine single crystal In the deuterated L-alanine crystals, all hydrogen atoms except the hydrogen atom of the CH group are replaced by deuterium atoms. When deuterium is substituted in the position of hydrogen of the molecule, some shift is observed in the infrared spectrum. This isotopic shift can be calculated theoretically by using basic physical laws [19] such as u ¼ 1=2pC½K=m1=2 ,
(1) 8
where C is the speed of light (3 10 m/s), K the force constant of bond in N/m and m the reduced mass of molecules.
ARTICLE IN PRESS K. Suriya Kumar et al. / Journal of Crystal Growth 310 (2008) 1182–1186
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Basically, frequency of vibration (u) of any molecule depends on the two parameters K and m which depend on the nature of the bond and, therefore, on the electronic arrangement in the molecule. The isotopic shift of molecules mainly differs only in their reduced masses of the elements. For any two isotopic molecules A and B having reduced masses mA and mB, the vibrational frequencies are
Employing Eq. (4), the isotopic shift of deuterated Lalanine for various C–D, N–D vibrational modes are calculated with the corresponding frequencies of pure Lalanine. Calculated frequencies of various vibrations and their frequency shift are presented in Table 2 and theoretically calculated frequencies of vibrational modes of C–D and N–D are in very good agreement with the perspective observed frequencies.
uA ¼ 1=2pC½K=mA 1=2 ,
(2)
3.3. Thermal analysis for pure and deuterated L-alanines
uB ¼ 1=2pC½K=mB 1=2 .
(3)
From Eqs. (2) and (3) uA =uB ¼ ½mB =mA 1=2 , uB ¼ uA =½mB =mA 1=2 .
(4)
Table 1 Comparison of observed vibrational frequencies for pure and deuterated L-alanine single crystals Wave number (cm1)
Pure L-alanine
3084 2937 2603 2275, 2227 and 2032 1620 and 1595 1452 1361
Asymmetric NH3 stretching CH stretching in CH3 NH stretching in NH3
1236 1185 1151
Deuterated L-alanine
Stretchings of CD3 vibration very weak NH2 scissoring
3.4. Transmission and nonlinear optical studies of pure and deuterated L-alanine single crystals
CH3 deformation Weak symmetry stretching vibration of COO– C–H-in plane bending vibration ND3 deformation vibration C–C stretching + CH bending + NH2 bending vibrations ND3 symmetric deformation
1056 771,850 and C–H-out of plane bending 920 vibrations 945 535 COO rocking
The endothermic peaks at 299 and 301 1C were observed for pure and its deuterated analog is shown in DSC analysis. The slight shift towards the high-temperature region reveals that the deuterated L-alanine is thermodynamically stable up to 301 1C whereas the pure L-alanine single crystals decomposes by 299 1C. Slight improvement in the thermal stability is attributed to the deuteration effect of L-alanine. The thermal strength of the compound is comparable to L-histidinium trichloro acetate (251 1C) [20] and L-threonine of 265 1C [21]. Barthes et al. [22] have reported that in L-alanine, there is a small change in the specific heat volume (less than 0.01 w/g) around 30 to 33 1C. The instability in this region was assigned to second-order structural phase transition. But such a slight structural instability is ruled out in the present case as we have not observed any small thermal event in the region 100 to 290 1C for both pure and deuterated L-alanine crystals.
ND3 rocking vibration
The transparency range of these compounds normally lies in the range 250–1300 nm. However, significant absorption feature exists in both the wavelength ranges 800–1200 and 200–300 nm of the spectrum. The UV cutoff wavelengths of both crystals were found to be same and it is shown in Fig. 3. Kurtz–Perry powder SHG efficiency of the crystal has been measured for the deuterated and pure L-alanine polycrystalline powders using intense laser source of wavelength 1064 nm with a pulse width of 10 ns, and repetition rate of 10 Hz. SHG efficiency was found to be 0.23 times that of
Table 2 Isotopic shift comparison for pure and deuterated L-alanine single crystals [12,13] Observed frequency in cm1 L-alanine A
Observed frequency in cm1 deuterated L-alanine B
Isotopic shift in cm1 (experimental) Du A–B
Calculated frequency in cm1 C
Assignment
2362 2337
1740 1720
617 610
1745 1727
1595 1508 1305
1180 1110 945
425 401 347
1170 1107 958
ND3 (stretching vibration ND3 (asymmetric stretching vibration) ND+ 3 (deformation) CD3 ND3 (rocking)
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KDP for deuterated L-alanine while for pure L-alanine it was found to be 0.33 times. Same range of efficiency was observed for L-alanine by many research groups [2,3]. The decrease in the SHG output intensity of deuterated Lalanine may be due to the exchange of deuterium in NHþ 3 and CH3 sites that causes less perturbation of the molecular dipoles on the incidence of high power laser beam. The shift in the stretching frequencies of N–D–O and C–D–O, as well as the carboxylate groups towards the lower energy side, accounts for the decrease in the second-order nonlinear polarizability of the deuterated crystals [23]. 4. Conclusion Pure and deuterated L-alanine crystals have been grown by slow evaporation as well as by slow cooling technique. Grown crystals were subjected to FTIR analysis, to confirm the presence of a various functional groups in pure and deuterated L-alanine single crystals. UV–vis–NIR spectrum found to be same cutoff wavelengths of both crystals. Increased thermal stability of deuterated L-alanine has been confirmed by DSC analysis. The NLO property of the grown crystal was also studied by Kurtz–Perry SHG test. The exact sites of the deuteration have also been identified through IR spectra. Acknowledgment One of the author KSK would like to thank Dr. M. Vijaya Raj, Germany, for providing deuterated water and for fruitful discussions. References [1] H.J. Simpson, R.E. Marsh, Acta Crystallogr. 20 (1966) 550. [2] L. Misoguti, A.T. Varela, F.D. Nunes, V.S. Bagnato, F.E.A. Melo, F.J. Mendes, S.C. Zilio, Opt. Mater. 6 (1996) 147.
[3] C. Razzetti, M. Ardoino, L. Zanotti, M. Zha, C. Paorici, Cryst. Res. Technol. 37 (2002) 456. [4] P.G. Jonsson, A. Kvick, Acta Crystallogr. B 28 (1972) 1827. [5] R. Adamowicz, E. Fishman, Spectrochim. Acta: Part A 28 (1972) 889. [6] K. Machida, A. Kagayama, Y. Saito, T. Uno, Spectrochim. Acta: Part A 34 (1978) 909. [7] C.H. Wang, R.D. Storms, J. Chem. Phys. 55 (1971) 3291. [8] R.S. Kwok, P. Moxton, A. Migliori, Solid State Commun. 74 (1990) 1193. [9] D.N. Nokogosyan, A Hand Book on Properties of Optical and Laser Related Materials, Wiley, New York, 1998, 55pp. [10] U.B. Ramabadran, A.L. Mcpherson, D.E. Zelman, J. Appl. Phys. 76 (1994) 1150. [11] Atsushi Yokotani, Takatomo Sasaki, Kunio Yoshida, Sadao Nakai, Appl. Phys. Lett. 55 (1989) 2692. [12] R.S. Krishnan, V.N. Sankaranarayanan, D. Krishnan, J. Ind. Inst. Sci. 55 (1973) 66. [13] S. Jarmelo, I. Reva, M. Rozenberg, P.R. Carey, R. Fausto, Vib. Spectrosc. 41 (2006) 73. [14] M. Tsuboi, T. Onish, I. Nakagawa, T. Shimanouchi, S. Mizushima, Spectrochim. Acta 12 (1958) 253. [15] S. Suzuki, T. Ohshima, N. Tamiya, K. Fukushima, K. Shimanouchi, R. Mizushima, Spectrochim. Acta 15 (1959) 969. [16] M. Tsuboi, T. Takenishi, A. Nakamura, Spectrochim. Acta 19 (1963) 271. [17] M. Barthes, A.F. Vik, A. Spire, H.N. Bordallo, J. Eckert, J. Phys. Chem. A 106 (2002) 5230. [18] D.M. Byler, H. Svsi, Spectrochim. Acta Part A 135A (1979) 1365. [19] K. Veera Reddy, Symmetry and Spectroscopy of Molecules, Academic Press, Hyderabad, 1997. [20] S. Gokul Raj, G. Ramesh Kumar, Thenneti Raghavalu, P. Kumar, R. Mohan, R. Jayavel, Spectrochim. Acta: Part A 65 (2006) 1161. [21] G. Ramesh Kumar, S. Gokul Raj, R. Sankar, R. Mohan, S. Pandi, R. Jayavel, J. Crystal Growth 267 (2004) 213. [22] M. Barthes, H.N. Bordallo, F. Denoyer, J.E. Lorenzo, J. Zaccaro, A. Robert, F. Zontone, Eur. Phys. J. B 37 (2004) 375. [23] C. Castiglioni, M. Del Zoppo, G. Zerbi, Phys. Rev. B 53 (1996) 13319.
Journal of Crystal Growth 310 (2008) 1182–1186 www.elsevier.com/locate/jcrysgro
Structural, optical, spectral and thermal studies of nonlinear optical pure and deuterated L-alanine single crystals K. Suriya Kumar, Thenneti Raghavalu, V. Mathivanan, M. Kovendhan, B. Sivakumar, G. Ramesh Kumar, S. Gokul Raj, R. Mohan Department of Physics, Presidency College, Chennai 600 005, India Received 19 May 2007; received in revised form 22 July 2007; accepted 21 December 2007 Communicated by S. Veesler Available online 1 January 2008
Abstract L-Alanine single crystals have been grown from H2O and D2O by slow evaporation and temperature lowering methods. The grown crystals were characterized by powder X-ray diffraction analysis and the vibrational frequencies of various functional groups in the crystals have been derived from FTIR spectrum. Optical transparency of the grown crystals was investigated by UV–vis–NIR spectrum. Differential scanning calorimetry (DSC) was carried out to investigate the thermal stability of the grown crystals The NLO property of the grown crystal has also been confirmed by the Kurtz-powder SHG test. r 2008 Elsevier B.V. All rights reserved.
PACS: 77.84.2; 78.20.Nu; 81.10.Dn Keywords: A1. Characterization; A2. Growth from solution; A2. Single crystal growth; B2. Nonlinear optical materials
1. Introduction In recent times, one has witnessed a growing interest in the study of amino acid single crystals. This interest has been stimulated by the perspective of understanding a system where the hydrogen bond plays a fundamental role and as a consequence of this a better knowledge of some important biological molecules (e.g. proteins) can be obtained. The growth of large single crystals of a amino acids has investigation so far, even as regards the simplest acentric member of the family L-alanine (CH3CHNH2COOH) which was first crystallized by Bernal and later by Simpson and Marsh [1] who refined the structure (a ¼ 6.032 A˚, b ¼ 12.343 A˚, c ¼ 5.784 A˚; a ¼ b ¼ g ¼ 901) and assigned it the P212121 space group. Large-size (3 cm3) crystals were reported by Misoguti et al. [2] with very few details on growth and crystal quality. As Corresponding author. Tel.: +91 44 2854 4894; fax: +91 44 2851 0732.
E-mail addresses: [email protected] (T. Raghavalu), [email protected] (R. Mohan). 0022-0248/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.12.049
to the crystal habit, it was systematically made up of faces belonging to the families {0 2 0}, {0 1 1} and {1 2 0}. The largest faces were always found to be {1 2 0}. The {0 2 0} faces were usually the smallest one but they appeared to be the best as to their optical quality and this has been reported by Razzetti et al. [3]. Neutron diffraction studies were also carried out by Jonsson and Kvick [4] for the hydrogen atoms position. A number of studies have been made to determine the vibrational spectra of L-alanine single crystals focusing on the assignment of modes done on the effects of various intermolecular potentials on the crystals vibrations [5,6]. Temperature-dependent study of X-ray and Raman scattering measurements of L-alanine single crystals were also studied earlier [7]. Thermal conductivity measurements of L-alanine single crystal [8] showed that lattice modes of L-alanine are strongly anharmonic. Presently, we are interested to study the growth, structural, optical and thermal properties of pure and deuterated L-alanine single crystals. An attempt has also been made to compare the vibrational assignments of pure
ARTICLE IN PRESS K. Suriya Kumar et al. / Journal of Crystal Growth 310 (2008) 1182–1186
and deuterated L-alanines. Deuteration of many NLO crystals gives rise to many improved optical properties. Deuterated KDP is still considered as one of the best electro-optic modulator with large electro-optic coefficient [9]. Deuteration on zinc tristhiourea sulphate results in considerable enhancement in its electro-optics properties [10]. Reduction in optical absorption and increase in optical transparency have also been observed for deuterated LAP [11]. Vibrational spectra of many deuterated amino acids were thoroughly studied by Krishnan et al. [12]. Similar studies were carried out for the case of DLserine by Jarmelo et al. [13]. For a complete assignment of the bands in the spectra of amino acids, deuteration studies are extremely helpful. In the case of glycine, Tusboi et al. [14], have made complete assignment of observed peaks in the region from 400 to 1800 cm1 by comparing the spectra of glycine, glycine hydrochloride and sodium glycinate and their N-deuterated
1183
analogs. Suzuki et al. [15] have also studied the spectra of Cdeuterated compounds of alanine. All vibrations involving the N atom in DL-alanine were identified by Tsuboi et al. [16] by substituting 14N by 15N and observed the isotope shifts in the spectrum. From the above ideas, an attempt has been made to grow the deuterated crystals of L-alanine. 2. Experimental procedure 2.1. Crystal growth The vapor pressure of L-alanine is low in the lowtemperature regime. Obviously, solution growth technique would appear to be the only feasible method. Hence, Lalanine crystals were grown from aqueous solution saturated at 40 1C, with the well-known slow cooling method. The solubility at this temperature was found to be 18.12 g/100 ml. Optical quality seed crystals were collected from the supersaturated solution and by successive recrystallization. Single crystals of deuterated L-alanine were prepared by the exchange reaction with heavy water (D2O). The degree of deuteration could be improved by successive crystallization of salt with deuterated water (D2O). Thermal strength of both pure and deuterated Lalanine was determined by the differential scanning calorimetry (DSC). Powder SHG efficiency of polycrystalline samples of both compounds has also been found out. The grown crystal of deuterated L-alanine is shown in Fig. 1. 2.2. Characterization studies The unit cell dimensions and X-ray intensity data were obtained using an Enraf-Nonius CAD-4 diffractometer equipped with CuKa radiation (l ¼ 1.5418 A˚). o/2y scan
100 90 80 70 60 50 40 30 20 10 100 2000
1361
20
500
1000
1500
1620
945 1056 1185
40
771
60
2500
2000
2500
Wavenumber
(cm -1)
3000
3500
4000
2937 3084
1500
2603
1000
2032 2112 2227 2275
500 80
3740
Pure L-alanine
535
Transmittance (%)
Fig. 1. As grown crystals of deuterated L-alanine.
3000
3500
Fig. 2. FTIR spectrum of pure and deuterated L-alanine single crystals.
4000
ARTICLE IN PRESS K. Suriya Kumar et al. / Journal of Crystal Growth 310 (2008) 1182–1186
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100 to 400 1C in nitrogen atmosphere at a heating rate of 5 1C/min. The NLO property of polycrystalline samples of pure and deuterated L-alanine of uniform size was tested by Kurtz-powder SHG test using an Nd: YAG laser (1064 nm).
10
Absorption
8
6
Pure L-alanine single crystals Deuterated L-alanine single crystals
3. Results and discussion
4
3.1. Vibrational spectra of pure and deuterated L-alanine single crystals 2
0 200
400
600
800
1000
1200
Wavelength (nm)
Fig. 3. UV–vis–NIR spectrum of pure and deuterated L-alanine single crystals.
1 0 -1
DSC (mW/mg)
-2 -3 -4 -5
Pure L-alanine Deuterated L-alanine
-6 -7 -8 -9 -10 -11 -100
0
100
200
300
400
Temperature (°C)
Fig. 4. DSC curve of pure and deuterated L-alanine single crystals.
mode was employed for data collection using the crystal of dimension 0.3 0.2 0.2 mm3. Polycrystalline powders were obtained by grinding in an agate mortar with a pestle. The vibrational measurements were carried out at room temperature. Fourier transform infrared spectrum of pure and deuterated L-alanine was recorded using a Bruker IFS66 V FTIR spectrophotometer in the frequency range 400–4000 cm1 by KBr pellet method is shown in Fig. 2. The optical transmission range of pure and deuterated Lalanine crystals was measured using Varian Cary 5E UV–vis–NIR spectrophotometer in the range 200–1000 nm is shown in Fig. 3. DSC on pure and deuterated L-alanine single crystals shown in Fig. 4 was carried out in the temperature range
The frequencies observed at 3084 and 3087 cm1 of pure and deuterated L-alanine single crystal can be attributed to 1 NHþ 3 stretching vibrations. The peak observed at 2937 cm in the IR spectrum is attributed to C–H stretching of the CH3 group. The peak observed at 2603 cm1 in the IR spectrum is assigned to the N–H stretching in the NH3 group, which is formed after protonation. The peaks at 1620 and 1614 cm1 are assigned to the NHþ 3 asymmetric bending vibrations of pure and deuterated L-alanine compounds. In pure L-alanine, the vibration at 1508 cm1 corresponds to the NHþ 3 symmetric bending. But it is not observed in deuterated L-alanine, which may be due to deuterization of the L-alanine. The peak at 1595 cm1 in the IR spectrum is assigned to the NH2 scissoring mode. Methyl deuteration has little effect on the position of the C–H stretching mode, and apparently deuteration of the methyl group causes no appreciable frequency shift. Bonds at 2275 and 2227 cm1 represent the deuteration of methyl group in L-alanine (CD3) and the peak of 2032 cm1 represents CD3 [17,18] vibration, which is usually very weak. The peak observed at 1185 cm1 in the IR spectrum is attributed to ND3 degree of deformation vibration. The peak at 1056 cm1 in IR spectrum is due to ND3 symmetric deformation vibration. The peak at 945 cm1 is assigned to the ND3 rocking vibration. The strong peaks observed at 1110, 1153 and 1180 cm1 were attributed to symmetric and degenerate deformation vibration of NDþ 3 . The peak at 535 cm1 represents the COO rocking. All other peaks of pure and deuterated L-alanine are listed in Table 1. 3.2. Effect of isotopic shift on the vibrational structure of Lalanine single crystal In the deuterated L-alanine crystals, all hydrogen atoms except the hydrogen atom of the CH group are replaced by deuterium atoms. When deuterium is substituted in the position of hydrogen of the molecule, some shift is observed in the infrared spectrum. This isotopic shift can be calculated theoretically by using basic physical laws [19] such as u ¼ 1=2pC½K=m1=2 ,
(1) 8
where C is the speed of light (3 10 m/s), K the force constant of bond in N/m and m the reduced mass of molecules.
ARTICLE IN PRESS K. Suriya Kumar et al. / Journal of Crystal Growth 310 (2008) 1182–1186
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Basically, frequency of vibration (u) of any molecule depends on the two parameters K and m which depend on the nature of the bond and, therefore, on the electronic arrangement in the molecule. The isotopic shift of molecules mainly differs only in their reduced masses of the elements. For any two isotopic molecules A and B having reduced masses mA and mB, the vibrational frequencies are
Employing Eq. (4), the isotopic shift of deuterated Lalanine for various C–D, N–D vibrational modes are calculated with the corresponding frequencies of pure Lalanine. Calculated frequencies of various vibrations and their frequency shift are presented in Table 2 and theoretically calculated frequencies of vibrational modes of C–D and N–D are in very good agreement with the perspective observed frequencies.
uA ¼ 1=2pC½K=mA 1=2 ,
(2)
3.3. Thermal analysis for pure and deuterated L-alanines
uB ¼ 1=2pC½K=mB 1=2 .
(3)
From Eqs. (2) and (3) uA =uB ¼ ½mB =mA 1=2 , uB ¼ uA =½mB =mA 1=2 .
(4)
Table 1 Comparison of observed vibrational frequencies for pure and deuterated L-alanine single crystals Wave number (cm1)
Pure L-alanine
3084 2937 2603 2275, 2227 and 2032 1620 and 1595 1452 1361
Asymmetric NH3 stretching CH stretching in CH3 NH stretching in NH3
1236 1185 1151
Deuterated L-alanine
Stretchings of CD3 vibration very weak NH2 scissoring
3.4. Transmission and nonlinear optical studies of pure and deuterated L-alanine single crystals
CH3 deformation Weak symmetry stretching vibration of COO– C–H-in plane bending vibration ND3 deformation vibration C–C stretching + CH bending + NH2 bending vibrations ND3 symmetric deformation
1056 771,850 and C–H-out of plane bending 920 vibrations 945 535 COO rocking
The endothermic peaks at 299 and 301 1C were observed for pure and its deuterated analog is shown in DSC analysis. The slight shift towards the high-temperature region reveals that the deuterated L-alanine is thermodynamically stable up to 301 1C whereas the pure L-alanine single crystals decomposes by 299 1C. Slight improvement in the thermal stability is attributed to the deuteration effect of L-alanine. The thermal strength of the compound is comparable to L-histidinium trichloro acetate (251 1C) [20] and L-threonine of 265 1C [21]. Barthes et al. [22] have reported that in L-alanine, there is a small change in the specific heat volume (less than 0.01 w/g) around 30 to 33 1C. The instability in this region was assigned to second-order structural phase transition. But such a slight structural instability is ruled out in the present case as we have not observed any small thermal event in the region 100 to 290 1C for both pure and deuterated L-alanine crystals.
ND3 rocking vibration
The transparency range of these compounds normally lies in the range 250–1300 nm. However, significant absorption feature exists in both the wavelength ranges 800–1200 and 200–300 nm of the spectrum. The UV cutoff wavelengths of both crystals were found to be same and it is shown in Fig. 3. Kurtz–Perry powder SHG efficiency of the crystal has been measured for the deuterated and pure L-alanine polycrystalline powders using intense laser source of wavelength 1064 nm with a pulse width of 10 ns, and repetition rate of 10 Hz. SHG efficiency was found to be 0.23 times that of
Table 2 Isotopic shift comparison for pure and deuterated L-alanine single crystals [12,13] Observed frequency in cm1 L-alanine A
Observed frequency in cm1 deuterated L-alanine B
Isotopic shift in cm1 (experimental) Du A–B
Calculated frequency in cm1 C
Assignment
2362 2337
1740 1720
617 610
1745 1727
1595 1508 1305
1180 1110 945
425 401 347
1170 1107 958
ND3 (stretching vibration ND3 (asymmetric stretching vibration) ND+ 3 (deformation) CD3 ND3 (rocking)
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KDP for deuterated L-alanine while for pure L-alanine it was found to be 0.33 times. Same range of efficiency was observed for L-alanine by many research groups [2,3]. The decrease in the SHG output intensity of deuterated Lalanine may be due to the exchange of deuterium in NHþ 3 and CH3 sites that causes less perturbation of the molecular dipoles on the incidence of high power laser beam. The shift in the stretching frequencies of N–D–O and C–D–O, as well as the carboxylate groups towards the lower energy side, accounts for the decrease in the second-order nonlinear polarizability of the deuterated crystals [23]. 4. Conclusion Pure and deuterated L-alanine crystals have been grown by slow evaporation as well as by slow cooling technique. Grown crystals were subjected to FTIR analysis, to confirm the presence of a various functional groups in pure and deuterated L-alanine single crystals. UV–vis–NIR spectrum found to be same cutoff wavelengths of both crystals. Increased thermal stability of deuterated L-alanine has been confirmed by DSC analysis. The NLO property of the grown crystal was also studied by Kurtz–Perry SHG test. The exact sites of the deuteration have also been identified through IR spectra. Acknowledgment One of the author KSK would like to thank Dr. M. Vijaya Raj, Germany, for providing deuterated water and for fruitful discussions. References [1] H.J. Simpson, R.E. Marsh, Acta Crystallogr. 20 (1966) 550. [2] L. Misoguti, A.T. Varela, F.D. Nunes, V.S. Bagnato, F.E.A. Melo, F.J. Mendes, S.C. Zilio, Opt. Mater. 6 (1996) 147.
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