Polarized Raman and hyperpolarizability studies of Hydroxyethylammonium (l ) tartrate monohydrate for quadratic nonlinear optics

Journal of Molecular Structure 988 (2011) 17–23

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Polarized Raman and hyperpolarizability studies of Hydroxyethylammonium (L) tartrate monohydrate for quadratic nonlinear optics R. Nagalakshmi a, V. Krishnakumar b,⇑, Hans Hagemann c, S. Muthunatesan d a

Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India Department of Physics, Periyar University, Salem 636 011, India c Department of Physical Chemistry, University of Geneva, Geneva, Switzerland d Department of Physics, Government Arts College, Kumbakonam, India b

a r t i c l e

i n f o

Article history: Received 20 September 2010 Received in revised form 17 November 2010 Accepted 17 November 2010 Available online 1 December 2010 Keywords: Solution growth X-ray diffraction Vibrational spectroscopy Polarized Raman Hyperpolarizability

a b s t r a c t Single crystals of Hydroxyethylammonium L-tartrate monohydrate [HEALT] have been grown by slow evaporation technique using water as a solvent. The structural and vibrational properties of the crystals were studied. Besides these characterizations ab initio quantum chemical calculations have been performed at HF/6-31G (d) level to derive first order hyperpolarizability. It is shown that the first order hyperpolarizability is found to be 14.2 times more than that of urea. The characteristic vibrational frequencies obtained from polarized Raman spectra in different scattering configurations have been assigned based on the complete factor group analysis. Vibrational analysis of IR and Raman reveals that the charge transfer interaction must be responsible for nonlinear optical (NLO) properties of the present system. The UV absorption measurements have also been carried out to confirm the utility of the material for optical applications. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Accentric bulk materials or polar crystals are found to possess technologically useful properties such as ferroelectricity, piezoelectricity, triboluminescence and nonlinear optical (NLO) function especially second harmonic generation (SHG). These properties are of particular importance for their practical applications in areas such as telecommunications, optical storage, and information processing as well as mechanical energy transfer. The multidirectional hydrogen bonded tartrate anions provide a conformational rigid environment for the incorporation of cations to form accentric crystalline salts, i.e. second harmonic generation materials [1]. The tartaric acid forms a broad family of hydrogen-bonded crystals. Several tartrate compounds deserve special attention because of their many interesting physical properties such as dielectric, piezoelectric, ferroelectric and optical second harmonic generation. These characteristics of tartrate compounds are exploited for their use in transducer sand in several linear and non-linear mechanical devices [2]. Some complexes of the amino acids or similar molecules with organic and inorganic (for example: Hydroxyethylamine or ethanolamine) compounds appears to be promising for optical

⇑ Corresponding author. Tel.: +91 427 2345766; fax: +91 427 2345565. E-mail address: [email protected] (V. Krishnakumar). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.11.056

second harmonic generation and electrooptical properties. Also this family of compounds is receiving significant attention for its favourable nonlinear optical properties. As like melaminium L-tartrate monohydrate [3] and L-lysinium L-tartrate [4], the title crystal seemed to be promising material as a nonlinear optical generator. Ethanolamine forms a very stable salt with tartaric acid. In the present study the bonding energy present in the hydrogen bonds linking organic amine and carboxylic acid group in amino acid, counteract the tendencies of the organic dipoles to form pairs, and drive the formation of crystals with high susceptibility values. In this paper we report in a systematic way regarding the growth and characterization of the title crystal. The spectroscopic technique (FTIR) was carried out to identify the hydrogen bonds which are responsible for its molecular hyperpolarizability and to confirm the formation of the molecular complex. To the best of our knowledge, hitherto, a full analysis of vibrational aspects in the large unit cell has not been performed and published. We report polarized Raman spectra from all three crystal surfaces as well as infrared phonon spectra. Based on a full group theoretical analysis and on comparison to known compounds with similar bonds we are able to assign the modes in the entire characteristic frequency region of the compound. The optimized geometry of the compound and the first order hyperpolarizability were calculated using the Hartee Fock-6-31(G (d)) basis set. It is found to be 14.2 times more than that of urea.

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2. Experimental 2.1. Preparation Single crystals of Hydroxyethylammonium L-tartrate monohydrate [HEALT] have been grown from saturated solution (pH = 3.26) of the synthesized salt of L-tartaric acid and ethanolamine by slow evaporation technique using water as a solvent. The starting compounds L-tartaric acid (Aldrich, 99%) and Ethanolamine, were used as reagents to prepare saturated solutions in the ratio of 1:1. Solubility of the amino acid L-tartaric acid was good in water. Hence the supersaturated solution was prepared in water.

Fig. 1a. Photograph of grown Hydroxyethylammonium L(+) tartrate monohydrate crystals.

Fig. 1b. Morphology of Hydroxyethylammonium L(+) tartrate monohydrate crystal.

Fig. 2. Molecular structure of Hydroxyethylammonium L-tartrate monohydrate optimised at HF/6-31G level.

The dissolved acid was added to the solution of hydroxyethylamine. The solution remained clear, without any precipitate. Then, the solution was filtered to remove the unsaturated materials. This was closed by perforated polythene sheet and kept undisturbed

Table 1 Optimised geometrical parameters of Hydroxyethylammonium L-tartrate monohydrate obtained by HF/6-31G calculations. Parametera

Valueb

R(1-7) R(1-27) R(2-7) R(3-8) R(3-11) R(4-9) R(4-12) R(5-10) R(5-13) R(6-10) R(7-8) R(8-9) R(8-14) R(9-10) R(9-15) R(16-19) R(16-20) R(17-18) R(17-25) R(17-26) R(18-19) R(18-21) R(18-22) R(19-23) R(19-24) A(7-1-27) A(1-7-2) A(1-7-8) A(2-7-8) A(8-3-11) A(3-8-7) A(3-8-9) A(3-8-14) A(9-4-12) A(4-9-8) A(4-9-10) A(4-9-15) A(10-5-13) A(5-10-6) A(5-10-9) A(6-10-9) A(7-8-9) A(7-8-14) A(9-8-14) A(8-9-10) A(8-9-15) A(10-9-15) A(19-16-20) A(16-19-18) A(16-19-23) A(16-19-24) A(18-17-25) A(18-17-26) A(17-18-19) A(17-18-21) A(17-18-22) A(25-17-26) A(19-18-21) A(19-18-22) A(18-19-23) A(18-19-24) A(21-18-22) A(23-19-24)

1.307 0.971 1.189 1.390 0.954 1.380 0.950 1.313 0.953 1.195 1.529 1.539 1.084 1.524 1.080 1.400 0.951 1.467 1.003 1.005 1.524 1.084 1.084 1.084 1.088 115.6 122.9 117.4 119.7 107.7 109.6 107.7 110.2 108.8 111.1 113.6 107.3 108.8 123.1 114.3 122.5 111.4 108.5 109.6 108.5 110.2 105.9 109.8 111.1 106.2 111.4 110.2 107.9 112.7 108.6 109.1 105.9 108.5 110.6 110.5 109.8 107.1 107.7

a R-Bond length; A-Bond angle; For numbering of atoms see Fig. 2. b Bond length are in Å and Bond angles are in degrees.

R. Nagalakshmi et al. / Journal of Molecular Structure 988 (2011) 17–23

geometry is determined by minimizing the energy with respect to all possible geometrical parameters without imposing molecular symmetry constraints. The optimized structure is shown in Fig. 2. The optimized geometrical parameters of HEALT are tabulated in Table 1. The first static hyperpolarizability (b0) and its related properties (b, a0 and a) have been calculated using HF/6-31G(d) based on finite field approach. In the presence of an applied electric field, the energy of a system is a function of the electric field and the first hyperpolarizability is a third rank tensor that can be described by a 3  3  3 matrix. The 27 components of the 3D matrix can be reduced to 10 components because of the Kleinman symmetry [8]. The components of b are defined as the coefficients in the Taylor series expansion of the energy in the external electric field. When the external electric field is weak and homogeneous this expansion becomes

and slowly evaporated for a period of 2 weeks till the crystalline material appeared. Evaporation of solvent yields the good quality transparent colorless crystals having the size of 2 cm  1.8 cm  0.6 cm and is shown in Fig. 1a. 2.2. X-ray diffraction data and morphology X-ray diffraction data were collected using both single crystal and powder diffraction techniques. The corresponding data agree well with published results [5,6]. (Space group P21, a = 7.6031 Å, b = 7.5022 Å, c = 8.7984 Å, b = 92.962°). From the diffraction analysis, it has been found that the investigated compound crystallizes in monoclinic system. The morphology is simulated using WINXMORPH package and it is as shown in Fig. 1b.

E ¼ E0 la F a  1=2aab F ab1=6 babc F a F b F cþ

2.3. Computational details of first order hyperpolarizability

where E0 is the energy of the unperturbed molecules. Fa is the field at the origin and la, aab and babc are the components of dipole moment, polarizability and first hyperpolarizabilities respectively. The total static dipole moment l, and the mean first hyperpolarizability b0, using x-, y- and z-components are defined as

Computational approach allows the determination of molecular NLO properties as an inexpensive way to design molecules by analyzing their potential. Ab initio computations are performed at the HF/ 6-31G (d) level using GAUSSIAN 98 W Program Package [7] to derive the optimized geometry of the title crystal. The optimum

Table 2 Calculated and observed FTIR and polarized Raman bands of Hydroxyethylammonium L-tartrate monohydrate and their assignments. Wavenumbers (cm1) Calculated with HF/6-31G

Observed FTIR

3365 1882 1653

3406 1873 1720

1590 1478

1595

1429 1381 1358

1394

1354 1306 1245

1347 1305 1263

1209 1150

1210 1131

1079 1061 1010 1005

1067

915

903

842 794 688 624 577 540 507 299 343

840 786 681 617 572 522 483

Observed Raman (0° polarization)

Observed Raman (90° polarization)

YY

XY

XX

1661 1566 1498

1481

1400

1009

ZZ

993 978 877 856

Symmetry

Assignments

A+B B A A+B A+B A+B B

NHþ 3 stretching C–H stretch C@O stretch NHþ 3 asym bending, C–C stretching NHþ 3 asym bending, H2O in-plane bending COO asymmetric stretch NHþ 3 sym bending COO sym stretch Coo sym stretch and C–H bending N–H in-plane bend and C–H bend

C–H in-plane bend and C–N stretch C–C stretch C–O and C–N stretch C–O, O–H     O in-plane bending NHþ 3 rocking O–H   O in-plane bend

ZX

1621

1662 1628

1662 1624

1597 1477 1433 1401

1481 1461

1480 1459

1480 1458 1400

1397

1397 1375

1316 1280 1248

1320 1284 1244

1321 1286 1260

1325 1284 1249

1319 1283 1245

1144

1140

1140

1145

1133

A A+B A+B A+B A A+B

1076 1016

1076 1012

1089 1017

1091 1077

A+B A+B

C–N stretch, C–C–O asym stretch and C –H in-plane def

995

995

996

995

A+B

C–H out of plane

887 867 839 795

888 867 839 795 698

888 867 839 795

891

888

C–C stretch

839 795

838 796 698 622

A+B A+B A+B A+B A+B A+B

502

498

499

342

342

342

392 341

222

298 270 218

622

623

498 307

253 234 185 162 141

YZ

1661

1376 1358 1346 1272 1233 1195 1123 1109 1082 1035

161

19

266 185 141

221 181 155

A+B A B

O–H    O out of plane, C–H out of plane def COO in-plane C–C and C@O in-plane def COO– wagging C–CO bend, in-plane C–N bend NH3 twisting C–CO bend in-plane C–C twisting COO Twisting Lattice vibrations

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l01 ¼ ðl2x þ l2y þ l2z Þ1=2 btot ¼ ðb2x þ b2y þ b2z Þ1=2 The calculated magnitudes of the first static hyperpolarizabilities and dipole moment values are reported in Table 3. 2.4. FTIR and polarized raman measurements Vibrational analysis of the grown crystal was done by recording FTIR spectrum and polarized Raman measurements. The FTIR spectrum was recorded in the range of 400–4000 cm1 using Brucker IFS 66V Spectrophotometer and is shown in absorbance scale in Fig. 3. In order to record polarized Raman spectra the FT Raman spectrometer was used in the back scattering arrangement (180° scattering) so the propagation direction of the exciting laser beam and that of the detected scattered radiation is collinear (opposite directions). The measurements were done at 4 cm1 resolution with the laser operating at 100 mW power. The polarized Raman spectra were recorded at room temperature on a Kaiser Holospec monochromator f/1.8 equipped with a Princeton Instruments liquid nitrogen cooled CCD camera, using 488 nm radiation of argon

Table 3 The dipole moment (l) and first order hyperpolarizibility (b) of Hydroxyethylammonium L(+) tartrate monohydrate derived from DFT calculations. 0.4456 0.3120 0.2511 2.5413 0.2592 0.3233 1.055 0.1532 0.5112 1.120 4.2388 0.6133 9.949 11.5396 10.6578

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btot

lx ly lz l

Fig. 4a. Polarized Raman spectra along on-diagonal geometries for 0° polarization.

Dipole moment (l) in Debye, hyperpolarizability b(2x, x, x) in 1030 esu.

300

νNH3

250

Absorbance(%)

200 150 100 50 0 4000

3500

3000

2500

2000

1500

1000

500

-1

wave numbers (cm ) Fig. 3. FTIR spectrum of Hydroxyethylammonium L(+) tartrate monohydrate.

Fig. 4b. Polarized Raman spectra along off diagonal geometries for 90° polarization.

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ion laser at 100 mW power in the following orientations viz., y(xx)y, y(xz)y and z(xy)z in 0° and 90° polarization geometries are presented in Figs. 4a and 4b respectively. The characteristic vibrational frequencies were assigned and it is tabulated in Table 2. 3. Results and discussion 3.1. Factor group and vibrational analysis According to the powder X-ray data, the title crystal crystallizes in monoclinic system of space group P21 = C2 (No: 4). For the factor group analysis the unit cell, containing four non-translationally equivalent Hydroxyethylammonium and tartaric acid units (ZB = 2) was used. Based on this information and using the site group analysis proposed by Rousseau et al. [9,10], it is possible to calculate the irreducible representation at the center of the Brillouin zone. A total set of 177 optic vibrations of the title crystal lattice is distributed by symmetry types in the following way: 89A + 88B. The three acoustic modes (A + 2B) were subtracted from the total number of vibrations leaving 177 optical modes. In accordance with the selection rules the vibrations of A and B symmetry are active in the IR and Raman spectra. From the whole set of optically active 177 vibrations 168 may be related to intermolecular and 12 to the crystal lattice vibrations. The complete factor group analysis information is presented in Table 4. In general, the influence of the strong hydrogen bonds on the nonlinear properties of the molecule can be considered, thus the vibrational spectra can be helpful in the elucidation of the role of hydrogen bond in the structure of the crystal exhibiting nonlinear optical properties. From the FTIR spectrum it has been seen that there is large absorption in the entire region extending from 3500 to 400 cm1. The intermolecular hydrogen bonding network formed between amino hydrogen atoms of a cation of an ion pair and carboxyl oxygen atoms of adjacent anions of another tartrate ion pair are interesting features and play an important role to achieve noncentrosymmetric structures in the present crystal. Hence a change in intensity and shift in frequency corresponding to NHþ 3 and COO vibrations have been expected and is identified in different scattering geometries of polarized Raman spectra depending on the modification of charge transfer axis. The tartrate ion consists of two planar halves each having a carboxyl group, a tetrahedral carbon and a hydroxyl oxygen atom. The two halves are oriented in such a manner that the four carbon atoms lie in a plane. The interesting feature in this configuration is the alignment of the carboxyl groups with respect to the CCO(H) plane. Even though the carboxyl groups have restricted freedom of rotation around the C–C bond, they are always found to be coplanar with CCO(H) of the same half [11–13]. The conformation of the tartrate is similar to be found in other tartrates [14–19]. The dihedral angle has been reported to be 63° [5]. The internal vibrations of tartrate and ethanolamine cation anionic pair may be classified  into those arising from functional groups NHþ 3 , CH and COO . However, these vibrations might be strongly coupled among themselves.

3.2. Vibrations of NHþ 3 group (hydrogen bonding) The broad band observed in high wavenumber region indicates the presence of hydrogen bonding in the title HEALT crystal. The hydrogen bonds unite a cation of an ion pair to the adjacent anion of another ion pair. Thus it is formed between carboxylate group of amino acid cation and the NH2 group of ethanolamine anion. Therefore, the hydrogen bonding network forms infinite ion-pair arrays. In many cases, specific intermolecular hydrogen bonding networks have been used as a steering force for forming noncentrosymmetric crystal structure in crystal engineering, particularly for second-order nonlinear optics in works of Etter’s group [20,21] and Gunter’s group [22–24]. In the present crystal the infinite hydrogen bonds that are perpendicular to polar orientation of the ionic species play an important role in construction of the noncentrosymmetric crystal structure. The hydrogen bonding networks seem to tailor the molecular dipoles of the ionic species in similar direction. The presence of the hydrogen bond besides the columbic interactions between the ionic species helps in building up stable structures and is also one of the favorable factors to have high melting point and to grow single crystals with relative ease. In the case of amino acids [25] and amino sulphones [26] the broad bands in this region 3400–3000 cm1 corresponds to X–H stretching vibrations. Usually N–H and O–H bands participate in hydrogen bonding which is clearly seen in FTIR spectrum than in Raman spectra. In the present crystal the stretching vibrations have been shifted 6 cm1 to higher wave number side and also they are asymþ metric (Vas) NHþ 3 stretching vibrations. But the asymmetric NH3 bending vibrations around 1621–1661 cm1 with moderate to low intensity is clearly seen in almost all the orientations of Raman spectra, having A + B symmetry, which is absent in FTIR spectrum. Hence this band position may serve as a sensitive measure of the strength of interaction between the COOH and NH2 group. In this way this vibration favours the intermolecular charge transfer and gives rise to a large change in dipole moment, thus gaining strong infrared activity. 3.3. Carboxyl vibrations Carboxyl group vibrations give rise to intense characteristic bands due to conjugation or formation of hydrogen bonds. These stretching and bending vibrations of acid group are generally expected in the region 1400–1200 cm1. Further hydrogen bonds formed between carboxylic oxygens of the L-tartrate anion and hydrogen atom of the NH2 group of ethanolamine link the cationic and anionic chains. In the presence of such type of conjugation the asymmetric stretching mode is shifted to 1597 cm1 in IR spectrum. But unfortunately our Raman signal is very weak along the off diagonal geometries (xy, yz, zx) having 90° polarization in this region. The reason for the above observation could be the in phase displacement of hydrogen atoms changes the electronic environment for the donor and acceptor of the chromophores. In Raman spectra the carboxyl stretching mode has prominently split into two components around 1480–1460 cm1 in off diagonal

Table 4 Results of factor group analysis Hydroxyethylammonium L(+) tartrate monohydrate. Factor group species

C4H6O6:C2H7NO. H2O

C

H

N

O

C2

C1 site

C1

C1

C1

C1

A B Total modes

Internal modes

External modes

75 75 150

6T, 6R 9T, 9R 15T, 15R

Optical modes

Acoustic modes

Spectral activity Raman

IR

axx, ayy, azz, axy ayz, axz

Tz Tx, Ty

HEALT C1 site 18 18 36

45 45 90

3 3 6

24 24 48

90 90 180

01 02 03

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(xy, yz, zx) geometries recorded in 90° polarization with shift in frequency and twist in intensity. This splitting of the carboxyl mode which is assigned to COO symmetric stretching might be attributed to intermolecular association based on C@O  H type of hydrogen bonding in the molecules. When this group is participating in the hydrogen bond a resonance can occur which puts a negative charge on the oxygen atom accepting the hydrogen band and positive charge on the atom denoting the hydrogen which shifts the carboxyl stretching vibration to high wave number region. NHþ 3 bending vibrations also got mixed in this region. Thus the activation of carboxyl vibration in both IR and Raman clearly shows the charge transfer interaction between carboxyl group Ltartaric acid and amino group of ethanolamine makes the molecule highly polarized which in turn makes the molecule more NLO active and is reflected in large hyperpolarizability value. Thus the modification in vibrational features gives a correlation to NLO activity.

Fig. 5a. HOMO monohydrate.

representation

of

Hydroxyethylammonium

L(+)

tartrate

Fig. 5b. LUMO monohydrate.

representation

of

Hydroxyethylammonium

L(+)

tartrate

3.4. Water molecules vibrations Basically, water has two OH stretching modes (around 3600 cm1) and one bending mode around 1630 cm1. The corresponding bands are strong in the IR and weak in the Raman spectra, but in our case, hidden within the broad bands of the NH3 group. Lower frequency IR modes of water (below ca. 600 cm1) could arise from librational modes, but these overlap also with other bands and cannot be clearly assigned here. 3.5. Optical external modes There are only few reports concerning the investigation on the assignment of lattice vibrations in amino acids [27–29]. It is obvious that, in this region the translational and librational modes of anions appear. It is very difficult to specify exactly the appearance of any mode in this region. In the present investigation the translational and librational modes of cations and anions can be distributed as 6A + 6B and 6A + 6B respectively. In addition to that the rotational modes of the water molecules are given as 3A + 3B have been added to external modes since the translational symmetry is absent. All these modes are more or less found with weak intensity in the recorded polarized Raman and IR spectra. 3.6. First order Hyperpolarizability and molecular orbital calculations The force exerted by the molecular packing and intermolecular interactions in the crystal will play as a decisive factor for the determination of macroscopic properties of these materials. So, understanding these intermolecular interactions should help towards understanding the nature of the macroscopically produced effects. The first computed hyperpolarizability (btot) of HEALT is found to be 4.238  1030 esu, which is 14.2 times more than that of urea (0.2991  1030 esu). The ab initio calculated non-zero l value shows that this compound might have microscopic first static hyperpolarizabilities with non-zero values obtained by the numerical second-derivative of the electric dipole moment according to the applied field strength. There is a rather strong relationship between the calculated l and btot values. Therefore, in this study, the l values may be responsible for enhancing and decreasing the btot value. The value of first order hyperpolarizibility is dominated in yyy, yyz and zzz directions. This indicates that the charge transfer mechanism is more in those particular directions. The magnitude of molecular hyperpolarizability, presence of the number of chromophores and the degree of noncentrosymmetry are the deciding criteria of the second order susceptibility v(2) values in an NLO system. The NLO responses can be understood by investigating the frontier molecular orbitals calculations [19]. It

is found that there lies an inverse relation between hyperpolarizability and the energy gap which is given by the difference between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy gap. These calculations have been carried out using Gaussian output.

LUMO energy gap ¼ 0:062 a:u HOMO energy gap ¼ 0:234 a:u HOMO—LUMO energy gap ¼ 0:172 a:u Fig. 5 shows the highest occupied molecule orbital (HOMO) and lowest occupied molecule orbital (LUMO) of title molecule. Larger value of molecular polarizability is due to this smaller value of HOMO–LUMO gap due to increased charge transfer [20,21]. This study reveals the suitability of the material for frequency conversion applications.

3.8. UV–visible optical spectra The UV–visible spectra of the synthesized compound have been measured by PerkinElmer Lambda 35 spectrophotometer in the region 190–1100 nm using 2 mm thickness crystal. The title crystal is active in the entire UV–vis region and the compound material could be a viable alternative for a possible material for the entire region. The lower cut off absorption edge is observed at around 210 nm.

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4. Conclusions Single crystals of HEALT have been grown by slow evaporation solution technique. Vibrational spectral analysis was carried out using FTIR and polarized Raman spectra. The resemblance between IR and Raman spectra in carboxyl group vibrations reveals the existence of Inter molecular Charge Transfer (ICT) which makes the molecule NLO active. Water molecule can be considered as a glue element in crystal structure stabilized by a set of intermolecular hydrogen bonds involving both N–H  O type. The first hyperpolarizability of HEALT calculated using quantum chemical calculations performed at HF/6-31G (d) level is found to be 4.238  1030 esu, which is 14.2 times that of urea. The title crystal may possesses potential application as nonlinear and electrooptical material as supported by optical absorption studies. Acknowledgements The author R.N is thankful to Council of Scientific and Industrial Research, New Delhi for the financial assistance under major research project. The author V.K is thankful to Department of Science and Technology, New Delhi for the fellowship awarded under Indo Swiss Joint Research Program and to Department of Physical Chemistry, University of Geneva, Geneva, Switzerland for providing the spectral facilities. References [1] M.K. Marchewka, S. Debrus, A. Pietraszko, A.J. Barnes, H. Ratajczak, J. Mol. Struct. 656 (2003) 265–273. [2] I. Quasim, A. Firdous, B. Want, S.K. Khosa, P.N. Kotru, J. Cryst. Growth 310 (2008) 5357–5363. [3] M.K. Marchewka, J. Baran, A. Pietraszko, A. Haznar, S. Debrus, H. Ratajczak, Solid State Sci. 5 (2003) 509–518. [4] A. Pietraszko, M. Marchewka, A. Haznar, M. Drozd., XXV International School and IV Polish–Ukrainian Ferroelectric Physics Meeting, Cracow, Poland, 18–22 September 2000. [5] M. Akkurt, I. Celik, S. Ozbey, E. Kendi, Z. Kristallogr, NCS 215 (2000) 71.

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