Vibrational spectral studies and non-linear optical properties of l -leucine l -leucinium picrate: A Density Functional Theory approach

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 437–444

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Vibrational spectral studies and non-linear optical properties of L-leucine L-leucinium picrate: A Density Functional Theory approach Sameh Guidara ⇑, Habib Feki, Younes Abid Laboratoire de Physique Appliquée (LPA), Université de Sfax, Faculté des Sciences, BP 1171, 3000 Sfax, Tunisia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A detailed interpretation of Infrared

and Raman spectra of LLLLP were reported.  Nonlinear optical properties of LLLLP were studied using DFT calculations.  HOMO–LUMO energy gap explains the charge transfer interactions in the molecule.  Stability of the molecule has been analyzed using NBO analysis.

a r t i c l e

i n f o

Article history: Received 15 April 2013 Received in revised form 11 June 2013 Accepted 19 June 2013 Available online 1 July 2013 Keywords: L-leucine L-leucinium picrate Vibrational spectroscopy DFT NBO First hyperpolarizability

a b s t r a c t Single crystals of L-leucine L-leucinium picrate were grown by slow evaporation at room temperature and were characterized by X-ray powder diffraction study to confirm the crystalline nature of the synthesized compound. The optimized molecular structure, vibrational spectra and the optical properties were calculated by the Density Functional Theory (DFT) method using the B3LYP function with the 6-31G(d) basis set. Good consistency is found between the calculated results and the experimental structure, IR, and Raman spectra. The detailed interpretation of the vibrational modes was carried out. The natural bond orbital (NBO) analysis, confirms the occurrence of intermolecular hydrogen bonds that are responsible for the stabilization of the title compound, leading to high nonlinear optical (NLO) activity. The lowering in the HOMO and LUMO energy gap explains the eventual charge transfer interactions that take place within the molecules. Ó 2013 Published by Elsevier B.V.

Introduction Nonlinear optical (NLO) materials have recently attracted a lot of attention due to their potential use in the fields like laser technology, optical communication, optical data storage and optical signal processing [1–3]. In this respect organic nonlinear optical materials are found to have high nonlinear coefficient equated to those of inorganic materials [4,5]. Extensive literature is available on the organic NLO materials with deep theoretical and

⇑ Corresponding author. Tel.: +216 23865145. E-mail address: [email protected] (S. Guidara). 1386-1425/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2013.06.080

experimental studies [6–9]. Many of these organic molecular crystals owe their nonlinear optical properties to the presence of delocalized p-electron systems linking donor and acceptor groups which enhance the necessary asymmetric polarizability [10]. Recently, considerable efforts have been made to combine amino acids with interesting organic matrices to produce materials having non-centrosymmetric cell, large polarizabilities and non-linear optical coefficient. When the organic acid mixed with the amino acid, NLO properties has been increased due to the zwitterionic nature and high transparency range [11,12]. Picric acid forms stable crystalline picrates salts with various organic molecules through p or hydrogen bonding interaction [13]. Recently, the

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crystal structures of some amino acid picrates have reported [14– 17]. L-leucine L-leucinium picrate (LLLLP) belongs to this large organic NLO family in which L-leucine L-leucinium acts as donor and the picric acid as electron acceptor. The crystal structure of this compound was elucidated by Anitha et al. [18]. Very recently it has also been the subject of infrared investigation by Bhagavannarayana et al. [19]. However, no detailed assignments of the vibrational bands were reported in that paper. Vibrational spectroscopy is an efficient tool for the characterization of crystalline materials. It is effectively used to identify functional groups and determine the molecular structure of synthesized crystals. It can also provide deeper knowledge about the relationships between molecular architecture, non-linear response and hyperpolarizability and support the efforts towards discovery of new efficient materials of technological applications. The combination of infrared Fourier transformation (IR-TF) and Raman spectroscopy with quantum chemical computations have been used as effective tools in the vibrational analysis of complex organic molecular systems [20–23]. The natural bond orbital (NBO) analysis can be employed to identify and substantiate the possible intra- and intermolecular interactions between the units that would form the H-bonded network [24]. A literature survey reveals that to the best of our knowledge no DFT wavenumber and structural parameter calculation of the title compound has been reported so far. Therefore, the present work deals with the growth and detailed vibrational spectral investigation of the crystal LLLLP to elucidate the correlation between the molecular structure and NLO property, charge transfer interaction, hydrogen bonds and first hyperpolarizability, aided by using the scaled quantum mechanical force field (SQMFF) technique based on Density Functional Theory (DFT) computation. Experimental Crystal growth Single crystals of LLLLP were grown by the slow evaporation solution growth technique. L-leucine and picric acid (from Sigma–Aldrich, 99.9%) were dissolved respectively in water and acetone in the ratio 2:1 and mixed well using a magnetic stirrer to ensure homogeneous concentration in the entire volume of the solution. The solution was allowed to evaporate at room temperature a few days until yellow crystalline salt of LLLLP were formed. Repeated recrystallization yielded to good quality crystals with dimensions of 18  1  0.2 mm3 as shown in Fig. 1. The reaction scheme involved in the formation of complex compound is:

2C6 H13 NO2 þ C6 H3 N3 O7 ! C6 H13 NO2  ½C6 H14 NO2 þ  ½C6 H2 N3 O7 

Characterization Powder XRD measurements were carried out using a Phillips powder diffractometer PW 1710 with Cu Ka radiation (k = 1.54187 Å) at the scan rate 0.03°s1 for the 2h angular range of 4.51–79.96° at room temperature, in order to confirm the crystal identity. The powder XRD pattern was recorded as shown in Fig. 2. The Fourier transform infrared (FT-IR) spectrum of LLLLP was recorded in the range 4000–400 cm1, with samples in KBr pellets using PERKIN-ELMER FT-IR spectrometer. The resolution of the spectrum is ±2 cm1. The Fourier transform Raman (FT-Raman) spectrum of the same compound was recorded using Horiba Jobin Yvon LabRAM HR 800 Dual Spectrophotometer. The incident laser excitation is 632 nm. The scattered light was collected at the angle of 180° in the region 3600–50 cm1 and the resolution was set up to 2 cm1. Due to the poor detector response, the Raman counterparts of the infrared bands located above 3200 cm1 are not observed in the spectrum. Computational methods The molecular geometry optimization and vibrational wavenumber calculations of LLLLP were performed by DFT method using the Gaussian 03 package [25]. The Becke three-parameter hybrid exchange functional and the Lee–Yang–Parr correlation functional (B3LYP) were utilized in the calculation with the 631G(d) basis set. An empirical, uniform scaling factor of 0.967 was used to offset the systematic errors caused by basis set incompleteness, neglect of electron correlation and vibrational anharmonicity [26]. The vibrational modes were assigned on the basis on our DFT calculations. Natural bond orbital (NBO) calculations of LLLLP were performed at the B3LYP/6-31G(d) level using the NBO 3.1 program [27] included in the Gaussian 03 package. The Raman activities (Si) calculated by the Gaussian 03 program have been converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering [28,29]:

Ii ¼

f ðv 0  v i Þ4 Si h i hcv v i 1  e kT i

where m0 is the exciting wavenumber, mi is the vibrational wavenumber of the ith normal mode, h, c and k are universal constants, and f is the suitably chosen common scaling factor for all the peak intensities. It is well known that the nonlinear optical response of an isolated molecule in an electric field Ei(x) can be presented as a Taylor series expansion of the total dipole moment, l, induced by the field:

l ¼ l0 þ aij Ej þ bijk Ej Ek þ    where a is the linear polarizability, l0 is the permanent dipole moment and bijk are the first hyperpolarizability tensor components. The isotropic (or average) linear polarizability is defined as [30]:

1 3

a ¼ ðaxx þ ayy þ azz Þ The first hyperpolarizability or the second-order polarizability b was calculated using B3LYP/6-31G(d) basis set. The components of the first hyperpolarizability can be calculated using the following equation

bi ¼ biii þ

Fig. 1. Grown single crystals of LLLLP.

 1 X bijj þ bjij þ bjji ; 3

ði–jÞ

Using the x, y and z components, the magnitude of the first hyperpolarizability tensor can be calculated by

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10400 (0 1 0)

10200 10000

9600 9400 (0 2 0)

9200 9000

(2 2 6)

(0 5 2)

(0 3 3)

(1 3 1) (0 4 1)

(0 3 0) (1 3 1) (1 2 2) (0 2 4)

(0 2 3) (1 0 3) (1 2 1) (0 2 2)

(1 0 2) (0 1 3)

(0 1 2)

(0 1 1) (0 0 2) (1 0 0)

200

(0 0 1)

400

(1 2 0)

(0 2 1)

600

(1 1 0)

Intensity (counts/s)

9800

0 10

20

30

40

50

60

2θ (°) Fig. 2. Powder XRD pattern of LLLLP.

b¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi b2x þ b2y þ b2z

rate anions (Z = 2). The crystal packing is determined mainly by NAH  O and OAH  O hydrogen bonds.

The complete equation for calculating the magnitude of the first hyperpolarizability from Gaussian 03 output is given as follows Optimized geometry

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi btot ¼ ðbxxx þ bxyy þ bxzz Þ2 þ ðbyyy þ byzz þ byxx Þ2 þ ðbzzz þ bzxx þ bzyy Þ2 The calculated values of the polarizabilities and the hyperpolarizabilities from Gaussian 03 output have been converted from atomic units (a.u) into electrostatic units (esu) [a: 1 a.u = 0.1482  1024 esu; b: 1 a.u = 8.6393  1033 esu]. The electronic dipole moment li (i = x, y, z), the polarizability aij and the first hyperpolarizability bijk are shown in Table S1 (Supplementary information). The calculated first hyperpolarizability of LLLLP is 46  1031 esu, which is 8 times that of KDP. The comparison of the NLO properties of L-leucine L-leucinium picrate with the already available amino acid is given in Table S2 (Supplementary information). Table S2 shows that the estimated SHG efficiency for L-leucine L-leucinium picrate is larger than the measured SHG values for bis L-alanine picrate, glycine picrate and L-valinium picrate [31–33].

In order to take into account the effect of intermolecular interactions on geometrical parameters and vibrational spectroscopy, we have considered the cluster built up from L-leucine, L-leucinium cation and one picrate anion linked by NH  O and OH  O hydro-

O4

C4 C5 O3

C3

C6

C2

N1

N3

C1

O2

Results and discussion

O7

O6

O1

H11B

Characterization X-ray powder diffraction was used for the identification of the synthesized LLLLP crystal. The LLLLP crystal belongs to the triclinic crystal system with the non-centrosymmetric space group P1 and confirms the crystalline nature of the synthesized compound. The XRD powder pattern has been indexed using CELREF programs and the lattice parameters are evaluated as: a = 7.132(5) Å, b = 11.799(9) Å, c = 15.372(2) Å, a = 106.61(6)°, b = 95.32(6)°, c = 90.97(7)° and V = 1233(2) Å3. The calculated lattice parameters from the powder XRD analysis agree well with the reported values [18]. From the single crystal XRD data [18], it is observed that the asymmetric unit of the title compound contains two unprotonated leucine residues, two protonated leucinium cations and two pic-

O5 N2

H21B O21B

O21A

H11A N21

C21 C22

H21A O11A

H21C

C11 C23

C26 C24

O11B

N11

C12

O11B C15

C13

C14 C25

C16

Fig. 3. Optimized molecular structure of LLLLP.

H11C

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gen bonds. The molecular structure of LLLLP with atom numbering scheme adopted in the computations is given in Fig. 3. All the parameters allowed relaxing and all the calculations converged to an optimized geometry which corresponds to a true energy minimum as revealed by the lack of imaginary values in the calculated vibration frequencies. The computed structural parameters combined with experimental data [18] are listed in Tables S3–S5 (Supplementary Information). The slight deviations of theoretical parameters of optimized geometry from the experimental values are probably due to the intermolecular interactions in the crystalline state. The phenyl ring appears to be distorted as seen from contraction and lengthening of internal angles of 111.868°, 124.212°, 119.307°, 120.858°, 120.141° and 123.482° (C2AC1AC6, C1AC2AC3, C2AC3AC4, C3AC4AC5, C4AC5AC6 and C5AC6AC1) that leads to elongation and shortening of, C1AC2, C2AC3, C3AC4, C4AC5, C5AC6 and C6AC1 bond lengths of 1.471 Å, 1.382 Å, 1.392 Å, 1.396 Å, 1.380 Å and 1.467 Å, respectively associated to the charge transfer interaction. The average differences of the calculated geometrical parameters from the experimental ones were found to be about 0.014 Å (C@O), 0.027 Å (C@C), 0.043 Å (CAC), 0.041 Å (NAO) and 0.019 Å (CAN) bond lengths while for bond angles 2.932° (CACAC), 2.434° (OANAC) and 4.027° (NACAC). The C12AC11AO11A and C12AC11AO11B angles are 117.185° and 114.083°, respectively. As seen, the calculated geometric parameters represent good approximation and can be used as a foundation to calculate the other parameters for the title compound.

NBO analysis The natural bond orbital (NBO) calculations [27] were performed using NBO 3.1 program implemented in the Gaussian 03 package at the DFT/B3LYP/6-31G(d) method in order to understand various second order interactions between the filled orbitals of one subsystem and vacant orbitals of another subsystem, which is a measure of the intermolecular delocalization or hyper-conjugation. In this context, hyper-conjugation may be given as a stabilizing effect that arises from an overlap between an occupied orbital with another neighboring electron deficient orbital when these orbitals are properly oriented. A useful aspect of the NBO method is that it gives information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra and inter molecular interactions. The hyper-conjugative interaction energy was deduced from the second-order perturbation approach. 2

Eð2Þ ¼ DEij ¼ qi

Fði; jÞ ej  ei

where qi is the donor orbital occupancy, ej and ei are diagonal elements and F(i, j) is the off diagonal NBO Fock matrix element. The NBO analysis clearly explains the evidences of the formation of strong H-bonded interaction between oxygen lone electron pairs and r (NAH) anti-bonding orbitals. The importance of hyper-conjugation and electron density transfer from lone electron pairs of the Y atom to the XAH anti-bonding orbital in the XAH  Y system has been reported [34–36]. In general such interaction leads to an increase in population of XAH anti-bonding orbital. The second order perturbation theory analysis of Fock matrix in NBO shows strong intermolecular hyper-conjugative interactions, which are presented in Table 1. The stabilization energy E(2) coupled with hyper-conjugative interactions n2 (O1) ? r (N21AH21B), n2 (O6) ? r (N11AH11B), n2 (O11A) ? r (N21AH21A) and n2 (O11A) ? r (N11AH11A) are obtained as 13.32, 16.06, 8.63 and 13.38 kcal mol1, respectively. This larger energy provides the stabilization to the

Table 1 Second order perturbation theory analysis of Fock matrix in NBO basis. E(2) (kcal mol1)

E(j)  E(i) (a.u)

F(i, j) (a.u)

From unit 1 to unit 2 n1 (O1) r(N21AH21B) n2 (O1) r(N21AH21B) n1 (O2) r(N21AH21B) r(N21AH21B) n2 (O2)

8.86 13.32 2.57 3.18

1.07 0.69 1.21 0.71

0.088 0.087 0.05 0.043

From unit 1 to unit 3 n1 (O6) r(N11AH11B) n2 (O6) r(N11AH11B) n3 (O6) r(N11AH11B)

7.07 16.06 5.32

1.14 0.73 0.67

0.081 0.098 0.061

From unit 3 to unit 2 r(N21AH21C) n1 (O11B) n2 (O11B) r(N21AH21C) n1 (O11A) r(N21AH21A) n2 (O11A) r(N21AH21A) n3 (O11A) r(N21AH21A)

3.01 5 6.86 8.63 2.78

1.07 0.64 1.10 0.66 0.65

0.051 0.052 0.078 0.069 0.041

Within unit 3 n1 (O11A) r(N11AH11A) n2 (O11A) r(N11AH11A)

5.78 13.38

1.04 0.60

0.069 0.082

Donor (i)

Acceptor (j)

molecular structure and quantifies the extend of intermolecular NAH  O hydrogen bonding. Vibrational analysis The vibrational spectral analysis of LLLLP crystal is performed based on the characteristic vibrations of amine group, carbonyl group, methyl, methylene and phenyl ring modes. The calculated vibrational wavenumbers and the atomic displacement corresponding to the different normal modes are used for identifying the vibrational modes unambiguously. The computed wavenumbers and their IR and Raman intensities corresponding to different modes are listed in Table 2 along with detailed assignments. For visual comparison, the observed and simulated FT-IR and Raman spectra are presented in Figs. 4 and 5, respectively. Picrate anion vibrations For the phenol group, the CAH ring stretching and bending modes occur in the regions 3100–3000 and 840–750 cm1, respectively [37]. In LLLLP, the CAH ring stretching vibration appears in the IR and Raman spectra as a shoulder at 3085 cm1 and 3035 cm1, respectively. The CAH ring bending mode is observed as a weak intensity band at 820 cm1 in the IR spectrum and at 822 cm1 as a strong band in Raman spectrum. The band that arises from CAH out of plane bending is observed as a weak band at 912 cm1 in Raman spectrum and as a medium band in IR spectrum at the same wavenumber. As seen in Table 2, CAH ring vibrations mode are well predicted by our DFT calculations. The CAC stretching vibrations of picrate anion are observed at 1604 cm1 as a strong intensity band in the FT-IR spectrum. In Raman spectrum, this mode appears as a medium band at 1555 cm1. The DFT computation predicts this vibrational mode at 1590 cm1. Two other functional groups exist in the picrate ion. They are the NO2 nitro groups and the CAO phenoxy group. The asymmetrical and symmetrical stretching vibrations of the NO2 group absorption results in a strong band in the region 1660–1500 cm1 and 1390– 1260 cm1, respectively [37]. In the LLLLP crystal, the asymmetric stretching mode of NO2 is observed as shoulder at 1517 cm1 in the IR spectrum, and the corresponding Raman band as weak band at 1488 cm1. This mode appears in our DFT calculations at 1509 cm1. Therefore one can recognize that the shift to the lower wavenumber for asymmetric vibration of nitro group in LLLLP compared with the free picric acid (1601 cm1) is due to the large electron density on the picrate as a result of charge transfer inter-

S. Guidara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 437–444

441

Table 2 Calculated vibrational wavenumbers (scaled), measured infrared and Raman band position (cm1) with the proposed assignments. vIR

vRaman

Calculated vcal

IR intensity

Raman intensity

Assignments

3400 w 3216 sh

– –

3565 3391

78.83 61.15

145.9 86.45

v(OAH) vasym ðNHþ 3Þ

3188 sh

–

3207

260.19

70.34

3085 sh 2960 w

3035 sh 2980 w

3035 3029

1318.86 9.32

419.67 24.06

2948 2917 – 1747 1654

2930 w 2917 w 2874 w – –

2940 2934 2900 1795 1650

6.49 25.26 21.59 220.03 463.10

260.31 4.8 67.63 6.62 10.43

vasym ðNHþ 3Þ v(CAH) (ring) vsym ðNHþ 3Þ vasym ðCH3 Þ vsym ðCH3 Þ vasym ðCH2 Þ v(C@O) dasym ðNHþ 3Þ

w vw sh sh

1627 sh

–

1628

603.32

20.61

1604 s 1560s 1537 sh

1555 m – –

1590 1569 1536

581.30 21.5 107.047

4.91 125.24 9.96

1517 sh 1458 sh 1429 m 1413 w sh 1400 sh 1355 sh 1312 m 1263 s 1161s 1079 s 988 w 933 sh 912 m 876 w 820 w 768 m 710 s 585 w 526 w

1488 w – 1430 vw – 1362 s 1332 s 1311 vs 1266 m 1164 m 1100 vw 1081 w 943 m 912 w – 822 s – 717 vw – 545 vw 442 vw 337 w 163 w

1509 1463 1439 1398 1374 1320 1303 1269 1155 1075 942 936 904 857 822 773 698 583 520 438 347 196

112.53 14.08 359.99 20.15 5.23 686.44 84.68 4.97 70.81 22.60 5.33 11.33 28.09 24.36 1.18 22.5 27.01 89.67 97.87 50.33 30.24 20.68

15.87 5.91 5.28 5.44 16.67 306.95 4.75 22.39 8.91 4.39 6.13 6.45 5.38 5.67 3.95 1.99 4.43 5.07 3.46 5.33 2.25 0.96

– –

dasym ðNHþ 3Þ v(CAC) vsym(COO) dsym ðNHþ 3 Þ , c(NO2) vasym(NO2), v(CAC) vsym(COO) dasym(CH3) b(OAH) dasym(CH3) vsym (NO2), x(CH2), b(CAC) dsym(CH3) v(CAO), t(CH2) qðNHþ3 Þ v(CAN), b(CAH) c(OAH) q(CH3), c(CAH) c(CAH) (ring) v(CAC), c(CAOAH) d(CAH) (ring) x(NO2), d(NO2), b(CACAC) s(HAOACAC) q(NO2) s(NHþ3 ) b(CACAN), b(CACAC), c(CACAC) b(CACAC), b(OACAC), b(CACAN), s(CACACAN), s(CACACAC)

698 583

936

1320

3035

3500

3000

2500

2000

1000

526

710

1263

1079

1429

1500

912

1747 1654

2960

3188

1604

4000

1155

1439

1795 2989

1628

2940

3207

3566 3580

(a)

3400

Transmittance

(b)

3392

v, stretching; b, in-plane bending; c, out of plane bending; q, rocking; x, wagging; t, twisting; d, scissoring; s, torsion; Subscripts: asym, asymmetric; sym, symmetric; w, weak; vw, very weak; s, strong; vs, very strong; m, medium; sh, shoulder.

500

Fig. 4. (a) FTIR spectrum of LLLLP. (b) Simulated IR spectra of LLLLP computed at B3LYP/6-31G(d) basis set.

actions in the title compound. The symmetric stretching mode of NO2 groups is seen in the IR spectrum at 1355 cm1 as a shoulder and the corresponding Raman band is observed as a strong band at 1332 cm1. The deformation vibrations of NO2 group (wagging, rocking, scissoring and twisting) contribute to several normal modes in the low frequency region [38,39]. The medium intensity band in the IR spectrum at 768 cm1 is attributed to the NO2

wagging mode. The NO2 in plane bending is observed at 710 and 717 cm1, respectively in the FT-IR and Raman spectra. The NO2 rocking mode appears as a weak band in IR at 526 cm1. The strong band observed in IR spectrum at 1263 cm1 is assigned to CAO stretching mode. In Raman spectrum, this mode appears at 1266 cm1 as a medium band. As seen in Table 2, the vibrational modes related to the picrate anion are well estimated by our DFT calculations. On the other hand, it is interest to note that our assignment agree well with those proposed by Briget Mary et al. for b-alanine b-alaninium picrate, DL-phenylalanine DL-phenylalaninium picrate [40], DL-valine DL-valininium picrate, DL-methionine DL-methioninium picrate [41] and also by Senthilkumar et al. for L-valininium picrate [42]. L-leucine

and L-leucinium vibrations Numerous functional and skeletal groups such as NHþ 3 , CH3, COO, COOH, CH2, CH, NH, CCC, CCN, CC@O and CCCC are present in L-leucine and L-leucinium cation. These groups are manifested in IR and Raman spectra in different range with different intensity. A careful inspection of the IR spectrum shows a medium intensity broad band extending from 3450 to 2850 cm1 with peaks at 3400, 3382, 3216, 3188, 3085, 2960 and 2917 cm1, whereas due to the poor detector response, the Raman counterparts of the infrared bands located above 3200 cm1 are not observed. The NHþ 3 and CH3 groups have C3v symmetry in the free state with pyramidal structure. Their normal modes of vibrations are v1(A1), v2(A1), v3(E) and v4(E). All these modes are both infrared and Raman active with the asymmetric stretching and bend-

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3200

2800

2400

2000

1600 1600

336

520

698

57

163

545

717

800

337

822 943 912

1164

1200

1081

1555 1488

2980 2930 2874

3600

1266

1362

(a)

4000

802

1311

935

1160

1489

1569

2940

3208

3392

3566

Raman Intensity

(b)

1356 1324

3035 2989

442

400

Wavenumber (cm-1) Fig. 5. (a) FT-Raman spectrum of LLLLP. (b) Simulated Raman spectra of LLLLP computed at B3LYP/6-31G(d) basis set.

ing modes being doubly degenerate [43]. In the title compound the doubly degenerate asymmetric stretching and bending modes v3(E) and v4(E) are observed in the split form. The two bands at 3216 and 3188 cm1 are due to the NHþ 3 asymmetric stretching mode. The shoulders at 1654 and 1627 cm1 in IR spectrum are þ attributed to NHþ 3 asymmetric deformation mode. The NH3 symmetric stretching v1(A1) and bending v2(A1) modes are identified in IR spectrum as a weak intensity band at 2960 cm1 and as a shoulder one at 1537 cm1 respectively. A carful inspection of the IR spectrum reveals that NHþ 3 group occupies a lower symmetry than in the free state, with the degeneracy of the modes being lifted. This indicates the strong symmetry related interaction of this group with its environment through the formation of NAH  O hydrogen bonding. In fact, the X-ray data reveals that in LLLLP crystal, the cohesion of the crystal is assured essentially by NAH  O hydrogen bonds ranging from 2.7 to 3.2 Å [18]. On the other hand, the presence of NAH  O hydrogen bonds is evidenced from NBO analysis. Regarding the bending vibration of the NHþ 3 moiety, it is interesting to look for the bands that could be ascribed to rocking and torsional modes. On the basis on our calculations as a primary source of attribution and on the basis on the vibrational spectra of the similar compounds, we have assigned the strong intensity band at 1161 cm1 in the IR spectrum and the corresponding medium Raman band at 1164 cm1 to the þ NHþ 3 rocking vibration. The NH3 torsional modes appear in Raman spectrum at 442 cm1 as a very weak band and coincide with the theoretically computed value 438 cm1. The symmetric CH3 stretching vibration in aliphatic compound absorbs near 2960 and 2870 cm1 [37]. The weak band at 2948 cm1 in IR spectrum is attributed to the asymmetric stretching vibration and the Raman counterpart is located as a weak band at 2930 cm1. The symmetric stretching mode of CH3 is assigned to the weak band at 2917 cm1 in Raman spectrum. In IR spectrum this mode appears as a very weak intensity band at the same value. Two bands with slightly different intensity appear in IR spectrum near 1429 and 1400 cm1 are assigned to CH3 asymmetric bending mode. In Raman, their corresponding bands appear as a very weak band at 1430 cm1 and a strong one at 1362 cm1. The symmetric bending mode of the CH3 group is seen at 1312 cm1 as a medium band in the IR spectrum and the corresponding band in Raman as a very strong at 1311 cm1. The methyl rocking vibration is observed as a shoulder at 933 cm1 in IR spectrum and at 943 cm1 as a medium band in Raman spectrum. As seen in Table 2, all calculated wavenumbers related to methyl group agree well with

experimental values. It is interest to note that, in contrast of the NHþ 3 cation, the vibrational modes of the CH3 do not deviate much from their expected values, suggesting that the interaction of the group with the environment is not strong. The wavenumber of the CH2 vibrational modes depend on its immediate environment. The stretching modes of the CH2 group usually occur in the region 3100–2800 cm1 [44]. In LLLLP crystal, the CH2 asymmetric and symmetric stretching modes are not identified in IR spectrum. These bands are probably masked by the broad bands of NHþ 3 and CH3 stretching vibrations. The weak band located at 2874 cm1 in Raman spectrum is assigned to the CH2 symmetric stretching mode. The deformation modes of this group lie in the same region as the deformation modes of CH3. Since the deformation modes of methyl group are intense, the deformation modes of CH2 are also not identified. However the wagging and the rocking modes of the CH2 group were observed and assigned. It is interesting to note, that similar situation was found in the L-leucine nitrate [45]. The protonated carboxyl group, RACOOH, is characterized by three main IR bands: OH stretching (3500 cm1), C@O stretching at 1700–1780 cm1 and OH bending at 1200–1300 cm1 [46]. A weak band observed at 3400 cm1 in the IR spectrum is assigned to the OH stretching mode and it shifted down only by 100 cm1 from the expected free ion value 3500 cm1. This is due probably to the presence of OH  O hydrogen bonds in the crystal and the relatively change environment. The in plane OH deformation occurs at 1413 cm1 as weak shoulder intensity in IR spectrum. His counterpart is not identified in Raman spectrum. In the IR spectrum a weak band appears at 988 cm1 and the corresponding Raman band also is observed as a weak band at 1081 cm1. These bands are attributed to the OH out of plane deformation. The band observed at 1747 cm1 as a shoulder is assigned to the C@O stretching mode. The deprotonated carboxyl group COO has two important characteristic absorption bands around 1600 and 1450 cm1 which are assigned to the asymmetric and symmetric stretching modes respectively [44,47]. The bands observed at 1560 and 1458 in the IR spectrum suggest the presence of COO group. The bands that arise from CN and CN stretching modes are observed in the wavenumber range 1150–850 cm1 [44]. The strong intensity band observed at 1079 cm1 in IR spectrum and the corresponding very weak Raman band at 1100 cm1 are assigned to the CN stretching mode. The CC stretching vibrational mode is attributed to the weak bands appearing in IR at 876 cm1. The bands at 337 cm1 in Raman spectrum correspond to the in phase and

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out of phase vibration of skeletal carbon. The detailed assignment of all observed bands is presented in Table 2. Hydrogen bonding It is well known that hydrogen bonding brings a remarkable downward wavenumber shifts. The intermolecular hydrogen bonds give rise to broad bands, whereas bands arising from intra-molecular hydrogen bonds are sharp and well resolved. By knowing the bond length, the strength of the hydrogen bond can be determined as very strong (below 2.5 Å), strong (2.5–2.7 Å), normal (2.7–2.9 Å) and weak (above 2.9 Å). X-ray diffraction analysis of LLLLP, reveal that the asymmetric unit contains two unprotonated leucine residues, two protonated leucininum cations and two picrate anions. The structure is stabilized by an extensive network of OAH  O and NAH  O hydrogen bonds. The OAH  O bonds in the unit cell are two in number and their bond length are 2.454 and 2.471 Å while the NAH  O bonds are fourteen with bond length being between 2.7 and 3.4 Å. The first type of hydrogen bonds OAH  O is reflected in the IR spectrum as medium broad band at 3400 cm1 indicating a strong hydrogen bond with the OAH stretch lowered from the expected value 100 cm1. The OAH out of plane deformation wavenumber also is up shifted around 40 cm1 from the expected range 960–870 cm1. The bending wavenumbers, however, are not much shifted indicating that the linear distortion is much greater than the angular distortion. The second type of hydrogen bonds NAH  O bond affects the various NHþ 3 vibrational modes. The rocking mode and the asymmetric stretching mode are the most sensitive to hydrogen bonding. The rocking mode is 200 cm1 higher than the expected range and the stretching modes are considerably broadened owing to the hydrogen bonding. The detailed assignment of all observed hydrogen bonds is presented in Table 2. The above results are well correlated with the previous works proposed by Briget Mary et al. [40,41].

from the donor to the acceptor and gives rise to a large variation of the dipole moment, thus gaining a strong infrared activity. As can be seen in LLLLP crystal, the bands in IR spectrum when compared with their Raman counterparts show that the relative intensities in IR and Raman spectra result from the electron cloud movement through the p-conjugated frame work from the electron donor to the electron acceptor groups. Recently, several studies have provide that certainly hydrogen bonds create and stabilize the crystal structure but more evidently that they also contribute considerably to the enhancement of hyperpolarizability of hydrogen bonded molecular systems or to the enhancement of the second order susceptibility of the crystals [49]. The calculated first hyperpolarizability (btot) and the ground state dipole moment (l) of the title compound are computed to be 46  1031 esu and 11.016 Debye respectively. Table 3 shows that the static hyperpolarizability btot is larger for LLLLP cluster than the btot for all constituents of the LLLLP crystal. As can be seen, the compound having the higher dipole moment results in the higher btot value and the corresponding highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) energy gap is quite low compared with L-leucine L-leucinium cation. This clearly indicates that the strong hydrogen bonding between the charged species reduced the energy gap considerably with the formation of charge transfer axis [50]. The analysis of the wave function indicates that the electronic absorption corresponds to the transition from the ground to the first excited state and it is mainly described by one-electron excitation from the HOMO to the LUMO. The atomic

LUMO PLOT (Excited State) E LUMO = - 2.84256 eV

HOMO–LUMO analysis Arising from the complexation of organic molecules based on acid–base interaction, highly polarizable cations, responsible for NLO properties, are linked to anions through hydrogen bond networks that generate a non-centrosymmetric structural organization. It is generally recognized that the organic molecules that contain conjugated p electrons are characterized as hyperpolarizabilties and were analyzed by means of vibrational spectroscopy [20,48]. In most of the cases, even in the absence of an inversion center, the strongest bands in the Raman spectrum are weak in IR and vice versa. The normal vibrations modes reveal that the relevant modes can be described as symmetric stretching vibrations of single CAC bonds, C@O bonds and shrinking of CAC double bonds. These vibrations spread over the whole p-conjugated path with relevant amplitudes from almost all the constituent parts of the molecules, which involves the intramolecular charge transfer

Table 3 Comparison of static first hyperpolarizability, polarizability, dipole moment, and HOMO–LUMO energy gap for the constituents of the title compound calculated with DFT//B3LYP/6-31G(d). Compounds

Picrate anion L-leucine L-leucinium

l (D)

atot (1024 esu)

btot (1031 esu)

DEH–L (eV)

2.18 5.13

17.16 11.057

0.852 18.06

1.95 5.06

11.016

30.38

46

3.46

ΔE = 3.46789 eV

E HOMO = - 6.31046 eV

HOMO PLOT (Ground State)

cation L-leucine L-leucinium

picrate

Fig. 6. Molecular orbital surfaces for the HOMO and LUMO of LLLLP.

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orbital compositions of the frontier molecular orbital are sketched in Fig. 6. The HOMO of p nature is located mainly over the phenyl ring and the oxygen atom of picric acid. By contrast, LUMO is located only on the two nitro groups of picric acid, and consequently the HOMO–LUMO transition implies an electron density transfer to the two nitro groups from the phenyl ring and the lone pair on oxygen atom. This explains a significant degree of intra-molecular charge transfer from the electron-donor groups to electron-acceptor groups through conjugated path. The HOMO–LUMO energy gap calculated at the B3LYP/6-31G(d) method reveals that the energy gap reflects the chemical activity as shown in Fig. 6. Moreover, the lowering in the HOMO and LUMO energy gap explains the eventual charge transfer interactions that take place within the molecules. Conclusion This work presents the experimental and theoretical vibrational IR and Raman spectra of the title compound. All observed vibrational bands have been discussed and assigned on the basis on our DFT calculations. The molecular geometry in the ground state has been calculated and compared with the experimental one. Furthermore, the first-order hyperpolarizabilities, total dipole moment, NBO and HOMO–LUMO energy gap have been calculated by using B3LYP methods with 6-31G(d) basis set in order to get an insight into the compound. The lowering in the HOMO and LUMO energy gap explains the eventual charge transfer interactions that take place within the molecules which is responsible for the very high optical nonlinearity of the crystal. In acid–base hybrid crystal hydrogen bonds play an important role not only in the creation of the structure and its stability, but also in the enhancement of the second order susceptibility of the crystal due to the perturbation of the electronic structure of the organic partner. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.06.080. References [1] H.O. Marcy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.C. Catella, Appl. Opt. 31 (1992) 5051–5060. [2] D.S. Chemla, J. Zyss, Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987. [3] H.O. Marcy, M.J. Rosker, L.F. Warren, P.H. Cunningham, C.A. Thomas, L.A. DeLoach, S.P. Velsko, C.A. Ebbers, J.H. Liao, M.G. Kanatzidis, Opt. Lett. 20 (1995) 252–257. [4] H.A. Petrosyan, H.A. Karapetyan, M.Yu. Antipin, A.M. Petrosyan, J. Cryst. Growth 275 (2005) 1919–1925. [5] D. Xue, H. Ratajczak, J. Mol. Struct. Theochem. 716 (2005) 207–210. [6] B.B. Koleva, T. Kolev, R.W. Seidel, M. Spiteller, H. Mayer-Figge, W.S. Sheldrick, J. Phys. Chem. A 113 (2009) 3088–3095. [7] T. Kolev, B.B. Koleva, R.W. Seidel, H. Mayer-Figge, M. Spiteller, W.S. Sheldrick, Spectrochim. Acta A 72 (2009) 502–509. [8] T. Kolev, R.W. Seidel, M. Spiteller, H. Mayer-Figge, W.S. Sheldrick, B.B. Koleva, J. Mol. Struct. 919 (2009) 246–254. [9] T. Kolev, B.B. Koleva, R.W. Seidel, H. Mayer-Figge, M. Spiteller, W.S. Sheldrick, Amino Acids 36 (2009) 29–33. [10] D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, Academic, Orlando, FL, 1987. [11] M.N. Bhat, S.M. Dharmaprakash, Growth of nonlinear optical-glycine crystals, J. Cryst. Growth 236 (2002) 376–380.

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