Growth, molecular structure, NBO analysis and vibrational spectral analysis of l -tartaric acid single crystal

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 127–141

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Growth, molecular structure, NBO analysis and vibrational spectral analysis of L-tartaric acid single crystal V. Sasikala a, D. Sajan a,⇑, N. Vijayan b, K. Chaitanya c, M.S. Babu Raj d, B.H. Selin Joy d a

Department of Physics, Bishop Moore College, Mavelikara, Alappuzha, Kerala 690 110, India Crystal Growth and X-Ray Analysis Section, CSIR-National Physical Laboratory, New Delhi 110 012, India c Molecular Spectroscopy Laboratories, Department of Physics, Andhra University, Visakhapatnam, India d Department of Physics Christian College, Kattakada, Kerala 695572, India b

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

 DFT computations have been

performed on L-tartaric acid molecule.  FT-IR, FT-Raman and UV–Vis spectra are simulated and compared with those of experimental spectra.  ICT interactions and stability of the molecule have been discussed.  Quantum chemical molecular orbital properties for LTA were calculated.  Open aperture Z-scan technique was used for the third-order non-linear absorption study.

a r t i c l e

i n f o

Article history: Received 19 September 2013 Received in revised form 28 November 2013 Accepted 4 December 2013 Available online 18 December 2013 Keywords: L-tartaric acid NBO NLO First hyperpolarizability Z-scan

a b s t r a c t Single crystal of L-tartaric acid (LTA) has been grown by slow evaporation technique. The experimental and theoretical studies on molecular structure, vibrational spectra, electronic absorption spectra and non-linear optical property of the crystal are studied. The FT-IR, FT-Raman and UV–Vis–NIR experimental spectra of LTA crystal have been recorded in the range 400–4000 cm1, 100–3700 cm1 and 190– 1100 nm, respectively. Density functional theory calculations with B3LYP/6-311++G(d,p) basis sets was used to determine ground state molecular geometries, vibrational frequencies, ICT interactions, Mulliken population analysis on atomic charge, HOMO–LUMO analysis, non-linear optical response properties and thermodynamic properties for LTA and the results were discussed. Vibrational analysis confirms the formation of intramolecular OAH  O hydrogen bonding. The stability of the molecule has been analyzed using NBO analysis. The results of electronic absorptions in gas phase and water phase LTA were calculated using TD-DFT method. The third-order nonlinear absorption behaviour of LTA was studied using open aperture Z-scan technique, with 5 ns laser pulses at 532 nm and the nonlinear absorption coefficient of the grown crystal was measured. The predicted NLO properties, UV absorption and Z-scan studies indicate that LTA is an attractive material for laser frequency doubling and optical limiting applications. Ó 2013 Elsevier B.V. All rights reserved.

Introduction The first description of tartaric acid crystals was given in 1841 by De la Provostaye and the relationship between characteristic ⇑ Corresponding author. Tel.: +91 9495043765; fax: +91 479 2303230. E-mail addresses: [email protected], [email protected] (D. Sajan). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.045

crystalline form and optical rotation of its aqueous solution was established by Pasteur in 1848 [1]. The crystalline structure and properties of dextro and leavo forms of tartaric acid have been satisfactorily explained [2]. Also, the crystal structure of the optically active form of tartaric acid is well-known [3,4]. Tartaric acid is an attractive monomer or comonomer for synthesis of functional polymers [5]. The carboxyl COOH and OH functional groups are

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both good groups in forming organic crystals through non-covalent interactions [6,7]. Tartaric acid is generally chosen to synthesize nonlinear materials because it has ability to enhance the macroscopic nonlinearity in a synergistic mode and to form multidirectional hydrogen bonds. L-tartaric acid (LTA), a dihydroxy dicarboxylic acid is an enantiomorph of tartaric acid having simple organic molecular chiral structure (C4H6O6) [8,9]. The crystal belongs to the monoclinic P21 symmetry and has higher solubility in water, could be crystallized into bigger sizes and costs much less in comparison with the amino acid-NLO materials. Its SHG efficiency is comparable to that of the standard KDP crystal. Also, LTA has much higher value of laser damage threshold [10] compared to the values reported for some known organic, semiorganic and inorganic NLO materials [11]. Crystal growth techniques such as hanging seed solution and submerged seed solution techniques [10], unidirectional solidification [12] and slow evaporation methods [13] have been employed for the preparation of LTA. The experimental characterization studies such as single crystal XRD [10,13], powder XRD [12] high-resolution XRD [10], FTIR and UV–Vis–NIR spectral [10,12,13], Kurtz–Perry powder SHG and laser damage threshold measurements [10], thermal and microhardness [12], and dielectric properties [13] of LTA crystal have already been studied and explored the potentially useful non-linear optical behaviours of the crystal [10]. Fundamental parameters like plasma energy, Penn gap, Fermi energy and electronic polarizability of LTA crystal have been theoretically calculated [13]. Vibrational Raman optical activity spectra of (2R,3R)-(+) tartaric acid-d0 in H2O and (2R,3R)-(+) tartaric acid-d4 in D2O between 300 and 1800 cm1 have been measured and ab initio theoretical Raman optical activity intensities were calculated [14]. Recently, the experimental and theoretical studies on the stability of gas-phase tartaric acid anions were investigated by Ralf and co-workers [15]. The molecular structure and spectroscopic properties of some of the L-tartaric acid complexes have been studied at the DFT/B3LYP level of theory [16– 19]. NLO properties of L-tartaric acid complexes have been widely investigated by means of powder second harmonic generation conversion efficiency measurement technique [20–25]. The unidirectional Sankaranarayanan–Ramasamy crystal growth method [26], powder XRD, HRXRD, UV–Vis–NIR absorption/transmission, and Kurtz–Perry powder SHG, crystalline quality by rocking curve studies and also mechanical strength by Vickers hardness test for nonlinear optical amino acid single crystal of L-tartaric acid have been reported [27]. The theoretical vibrational frequencies, potential energy distribution (PED) and hyperpolarizability values of L-lysine  tartaric acid were calculated by quantum chemical method [28]. Tartrate compounds such as L-histidinium–L-tartrate hemihydrates [29], L-prolinium tartrate [30], Hydroxyethylammonium (L) tartrate monohydrate and Hydroxyethylammonium (D) tartrate monohydrate [31], 2,4,6-triamino-1,3,5-triazin-1,3-ium tartrate monohydrate [32], L-cysteine tartrate monohydrate [33], L-alaninium tartrate [34], Picolinium tartrate monohydrate [35], 2-carboxypyridinium hydrogen (2R,3R)-tartrate monohydrate [36] are of considerable interest because of their NLO properties. Because of the simplicity and high sensitivity in measurements, the Z-scan technique has been extensively used to investigate the third-order nonlinear optical properties in crystals. The third-order nonlinear properties in a number of organic crystals [37–39] have been investigated using Z-scan method. In spite of these literatures, a systematic quantum chemical DFT study was not reported yet for isolated LTA molecule. Recent interests in NLO properties of L–tartaric acid and its derivatives have increased the need for the prediction of molecular structure since non-linear optical properties of the crystals are directly linked to the molecular structure and to both inter and intra molecular charge transfer interactions. A detailed knowledge about the

relationships between molecular structure, hydrogen bonding, hyperpolarizability and non-linear response is important, as it can be used to support the effort towards discovery of new efficient materials for technological applications, especially, to the field of nonlinear optics, by the design strategy for the engineering of crystals with predesigned architecture [40–45]. Intermolecular interactions are responsible for crystal packing and gaining an understanding of them allows us to comprehend collective properties and permits the design of new crystals with specific physical and chemical properties [46]. The aim of this study was to investigate the relationship of spectral, non-linear and quantum chemical properties to the molecular structure as well as to the intramolecular interactions of LTA based on the combined results of experimental study and theoretical structural investigation using the DFT level quantum chemical computation. Density functional theory at B3LYP/6-311++G(d,p) level was performed for vibrational and electronic spectral information (TDDFT) on the optimized geometry, frontier molecular orbital analysis, molecular electrostatic potential, charge transfer interactions by NBO analysis owing to elucidate the NLO property, determination of first order hyperpolarizability, vibrational wavenumbers by NCA, assignment of normal modes by SQM, and thermodynamic properties of the molecule. The third-order non-linear absorption behaviour of LTA has been studied using open aperture Z-scan technique. LTA crystal has been grown; FT-IR, FT-Raman and UV–Vis–NIR spectra and the two-photon absorption coefficient for the title crystal have been measured as a part of the present study. The theoretical and experimental techniques are analyzed and reported here.

Experimental Preparation of the crystal The commercially available raw material (L-tartaric acid from 98% Sigma–Aldrich) was procured and its purity was increased by repeated recrystallization processes by using double distilled water as the solvent. The concentrated solution was prepared according to the solubility diagram reported in the literature [10]. The filtered solution was kept in the constant temperature bath by setting the temperature of 32 °C. After a period of 27 days, the nucleation was observed in the glass beaker. Then it was allowed to grow further. The grown single crystal was harvested from the mother solution. The cut and polished single crystal of LTA is shown in Fig. 1. Then the crystal was subjected to different characterization analyses. Crystal packing of LTA is shown in Fig. 2. The crystal data of LTA is presented in Table 1. The lattice parameters of the unit cell of LTA show that its monoclinic crystal structure with P21 symmetry.

Fig. 1. Photograph of LTA single crystal.

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of the sample Tnorm (transmission normalized to the linear transmission of the sample) is known as the Z-scan curve. The nonlinear absorption coefficient of the sample can be numerically calculated from the Z-scan curve. In the present work, water solutions of the sample taken in 1 mm cuvettes were irradiated by plane polarized laser pulses with duration of 5 ns at 532 nm wavelength obtained from a frequency doubled Q-switched Nd: YAG laser (MiniLite, Continuum Inc.). The sample was mounted on a precision translation stage with a step resolution of 100 lm. The sample was moved along the travelling Z-direction of the laser beam, and a transmittance change was recorded as a function of sample position z. The focusing lens is chosen such that the Rayleigh range (z0) larger than 1.5 mm, so that samples taken in 1 mm cuvettes will satisfy the thin sample approximation [48]. The lasers were essentially run in the singleshot mode using appropriate triggering, with an approximate interval of 3–4 s between successive pulses. This low repetition rate prevents sample damage and cumulative thermal effects in the medium. Fig. 2. Crystal packing of LTA.

Computational details Table 1 Crystal data of LTA. Cell lengths (Å)

Cell angles (°)

Cell volume (Å3)

Space group

a = 7.7290(1) b = 6.0004(1) c = 6.2126(1)

a = 90

283.61

P21

b = 100.153(2) c = 90

Spectroscopic measurements FT-IR spectrum of the title crystal was recorded on Thermo Nicolet Magna 760 FT-IR spectrometer by DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) technique. The sample was mixed with KBr and scanned in 400–4000 cm1 wavenumber range (Happ–Genzel apodization, 2 cm1 resolution) using Pike Technology Easi Diff accessory. The FT-Raman spectrum of crystalline sample and water solution was collected on the same spectrometer equipped with Thermo Nicolet Nexus FT-Raman module. The measurements were carried out in the range of 100–3700 cm1 (Happ–Genzel apodization, 2 cm1 resolution, 1064 nm Nd: YVO4 laser excitation, 450 mW power at the sample). Both IR and Raman spectra were processed using the OMNIC software [47]. The UV–Vis–NIR absorption measurements of the title crystal were recorded in water solution using the Perkin Elmer Lambda 35 spectrophotometer in the range 190–1100 nm. Non-linear optical measurements Open aperture Z-scan method was employed for the determination of non-linear transmission of laser light through the sample. The Z-scan is a standard technique developed by Sheik-Bahae et al. [48] to measure optical nonlinearity of materials, and open aperture Z-scan gives information about the non-linear absorption coefficient. Here laser beam is used for sample excitation. The beam is focused using a convex lens and passed through the sample. The beam’s propagation direction is taken as the Z-axis, and the focal point is taken as z = 0. The beam will have maximum energy density at the focus, which will symmetrically reduce towards either side of it for the positive and negative values of z. The experiment is done by placing the sample in the beam at different positions with respect to the focus (different values of z), and measuring the corresponding light transmission. The graph plotted between the sample position z and the normalized transmittance

GAUSSIAN 09 software [49] was used for all quantum chemical calculations. The molecular structure of L-tartaric acid crystal was optimized by Berny’s optimization algorithm using redundant internal co-ordinates at the DFT level using the closed-shell Becke–Lee–Yang–Parr hybrid exchange–correlation three-parameter functional (B3LYP) in combination with 6-311++G(d,p) basis set to derive the complete geometry optimizations and normal mode analysis on isolated entities [50–52]. The optimized geometry corresponding to the minimum on the potential energy surface has been obtained by solving self-consisting field equations iteratively. In order to reduce the overestimation of the computational frequencies, scaling of the force field was performed according to the scaled quantum mechanical procedure (SQM) [53,54] using selective scaling in the natural internal coordinate representation [55,56] to obtain a better agreement between the theory and the experiment. Normal coordinate analysis of the monomer of the sample crystal has been carried out to obtain a more complete description of the molecular motions involved in the fundamentals. Raman and IR wavenumbers of the normal vibrations and intensities of the bands were calculated at the same DFT level. For vibrational assignments, symmetry coordinates in terms of potential energy distribution (PED) have been computed from a normal coordinate analysis in the MOLVIB program version 7.0 written by Sundius [57,58]. The vibrational frequencies are calculated by the MOLVIB with the help of Gaussian force constants. The Raman activities (Si) calculated with the GAUSSIAN 09W program was converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering [59,60]:

Ii ¼

f ðm0  mi Þ4 Si mi ½1  eðhcmi =kTÞ 

ð1Þ

where v0 is the exciting wavenumber, vi is the vibrational wavenumber of the ith normal mode, h and k are universal constants and f is the suitably chosen common scaling factor for all the peak intensities. The extent of hyperconjugation or delocalization of various second order interactions between filled orbitals of one subsystem to vacant orbitals of another subsystem [61] is computed at the DFT/ B3LYP/6-311++G(d,p) level implemented in the GAUSSIAN 09 software package. The natural bond orbital interaction analysis has been performed by using the NBO 3.1 program [62]. The second-order Fock matrix was used to evaluate the donor–acceptor

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interactions in NBO analysis [63]. The hyperconjugative interaction energy, E(2) associated with the i ? j can be estimated from the second-order perturbation approach [64] as

Eð2Þ ¼ nr

F 2ij

rjFjr 2 ¼ nr er  er DE

ð2Þ

where hrjFjri or Fij is the Fock matrix element between the donor (i) and acceptor (j) NBOs, er and er are the energies of r and r⁄ NBOs and nr is the population of the donor r orbital [61]. The electron densities of donor and acceptor NBOs, energy of hyperconjugative interactions (stabilization energy) and energy difference between donor and acceptor NBOs have been computed at the DFT level. The possible intramolecular and hydrogen bonded interactions have been investigated. The frontier molecular orbitals (HOMO and LUMO) are generated at DFT/B3LYP/6-311++G(d,p) level. The HOMO–LUMO analysis has been carried out to explain the electronic and optical properties, UV–Vis spectra [65], kinetic stability and chemical reactivity of the molecule [66]. The important quantum chemical molecular properties, i.e., global reactivity descriptors such as ionization potential (IP), electron affinity (EA), electronegativity (v), electrophilicity index (x), chemical hardness (g), softness (S), chemical potential (l), total energy change (DET) and overall energy balance (DE) have been calculated using the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [67–80]. The first hyperpolarizability (b), the mean polarizability (a), the anisotropy of the polarizability (Da) and the total static dipole moment (l) of LTA crystal have been predicted by using the DFT/ B3LYP/6-311++G(d,p) method. First hyperpolarizability is a third rank tensor that can be described by 3  3  3 matrix. The 27 components of 3D matrix can be reduced to 10 components due to the Kleinmann symmetry [81,82]. The output from Gaussian 09 provides 10 components of this matrix as bxxx, byxx, bxyy, byyy, bzxx, bxyz, bzyy, bxzz, byzz, bzzz, respectively. The components of the first hyperpolarizability can be calculated using the following equations.

  btotal ¼ b2x þ b2y þ b2z

ð3Þ

where

bx ¼ bxxx þ bxyy þ bxzz by ¼ byyy þ byzz þ byxx bz ¼ bzzz þ bzxx þ bzyy The equations for calculating the magnitude of mean polarizability, anisotropy of the polarizability and total static dipole moment are defined as follows:

1 3

a ¼ ðaxx þ ayy þ azz Þ

ð4Þ

Results and discussion Optimized geometry The optimized geometries of monomer of L-tartaric acid with symbols and numbering scheme for the atoms are shown in Fig. 3. Table 2 lists their main structural parameters optimized at DFT/B3LYP/6-311++G(d,p) level corresponds to the gas-phase isolated structure with XRD. The global minimum energy calculated by the DFT structure optimization method is found to be 1.5952  106 kJ/mol. DFT predicted the carbon–oxygen bond lengths, 1.35 Å for CAO and 1.20 Å for C@O and also, the hydrogen–oxygen bond lengths were calculated to be 0.97 Å for both carboxyl groups whereas these distances of non-carboxyl groups were predicted to be 1.41 Å for both CAO bonds and also, 0.97 Å for O6AH14 and 0.96 Å for O10AH16 bonds. The computed CAO bond lengths show good agreement with X-ray data. The influence of neighbouring oxygen atoms affects the CAO bonds that lead to the lengthening of the respective bonds. The weakening of O6AH14 and O10AH16 and the strengthening of C4@O5 and lengthening of C4AO12 indicate the presence of intramolecular O6AH14  O5 (2.677 Å) and O10AH16  O12 (2.835 Å) hydrogen bonds, respectively in the molecule. The bond angles of O6AH14  O5 and O10AH16  O12 were found to be 116.6° and 131.7°, respectively. The geometry of hydrogen bonds in LTA molecule is listed in Table 3. The predicted CAH bond lengths are found to be 1.10 Å, which are larger than the observed values. The bond lengths of C1@O9 (1.20 Å) and C4@O5 (1.20 Å) and the bond angles of C2AC1@O9 (124.3°) and C3AC4@O5 (123.5°) calculated with the DFT/B3LYP/6-311++G(d,p) level indicates the strengthening of C@O bonds and expansion of CAC@O angles over the crystalline bond angle values. This is due to the hyperconjugative interactions between lone pairs of oxygen atoms and the antibonding orbitals of CAC bonds. The predicted C1AC2, C2AC3 and C3AC4 bond lengths are at 1.53 Å, 1.55 Å and 1.52 Å, respectively. The variation in CAC bond lengths are due to the presence of dicarboxyl groups which involved in hydrogen bonding interactions in the molecule. The dihedral angles of C2AC3AC4@O5 and C3AC2AC1@O9 are found to be 122.8° and 60.4°, respectively. The differences in CACAC@O dihedral angles are due to strong charge delocalization. The four carbon atoms of the molecule constitute a chain and are arranged in a coplanar zig-zag pattern with a dihedral angle of 172.9°. Furthermore, the B3LYP predicted the coplanar nature of C3AC4AO12AH13 (179.9°) and C2AC1AO8AH15 (179.0°) dihedral angles. A set of internal force constants for LTA are calculated from scaled quantum mechanical force field and presented in Table 4. The CACAH and CAC vibrations have the lowest force constant values of 0.64 m Dyne/Å and 3.36 m Dyne/Å, respectively. The highest force constant values computed to be of C@O stretching (11.42 m Dyne/Å) and OACAO bending (7.44 m Dyne/Å) vibrations are due

i1=2 1 h Da ¼ pffiffiffi ðaxx  ayy Þ2 þ ðayy  azz Þ2 þ ðazz  axx Þ2 þ 6a2xy þ 6a2yz þ 6a2xz 2 ð5Þ



l ¼ l2x þ l2y þ l2z

1=2

ð6Þ

The time dependent density functional theory (TD-DFT) with the B3LYP/6-311++G(d,p) level has been used to study the electronic absorptions in gas phase and water phase LTA. The thermodynamic properties such as specific heat capacity, entropy, thermal energy, rotational temperatures, rotational constants, and zero point vibrational energy (ZPVE) of the title crystal have also been computed at DFT–B3LYP levels using 6-311++G(d,p) basis set.

Fig. 3. Optimized molecular structure of LTA calculated at DFT level.

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V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 127–141 Table 2 Optimized geometric parameters of LTA by using B3LYP/6-311++G(d,p). Bond lengths

B3LYP (Å)

XRD (Å)

Bond angles

B3LYP (°)

XRD (°)

Dihedral angles

B3LYP (°)

XRD (°)

C1AC2 C2AC3 C3AC4 C4@O5 C3AO6 C2AH7 C1AO8 C1@O9 C2AO10 C3AH11 C4AO12 O12AH13 O6AH14 O8AH15 O10AH16

1.53 1.55 1.52 1.20 1.41 1.10 1.35 1.20 1.41 1.10 1.35 0.97 0.97 0.97 0.96

1.53 1.55 1.53 1.20 1.42 0.94 1.31 1.21 1.40 1.05 1.31 0.79 0.88 0.84 0.84

C1AC2AC3 C2AC3AC4 C3AC4@O5 C4AC3AO6 C3AC2AH7 C2AC1AO8 C2AC1@O9 C1AC2AO10 C4AC3AH11 C3AC4AO12 C4AO12AH13 C3AO6AH14 C1AO8AH15 C2AO10AH16

107.5 111.9 123.5 109.6 109.4 111.5 124.3 106.0 108.5 113.3 108.2 108.1 107.4 108.7

106.7 110.5 124.5 112.2 106.0 110.8 123.5 108.5 108.3 109.1 110.6 110.1 112.1 109.5

C1AC2AC3AC4 C2AC3AC4@O5 O5@C4AC3AO6 O6AC3AC2AH7 C3AC2AC1AO8 C3AC2AC1@O9 O10AC2AC1@O9 O5@C4AC3AH11 O12AC4AC3AH11 C3AC4AO12AH13 C4AC3AO6AH14 C2AC1AO8AH15 C3AC2AO10AH16

172.9 122.8 1.2 66.7 120.1 60.4 60.7 119.4 61.0 179.9 0.0 179.0 54.4

75.3 119.5 4.9 174.2 62.7 115.2 4.60 126.4 53.1 171.6 71.1 178.8 81.6

Table 3 Hydrogen bonding geometry. XAHY

XAH length (Å)

HY length (Å)

XY length (Å)

XAHY angle (°)

O6AH14O5 O10AH16O12

0.9680 0.9646

2.100 2.101

2.677 2.835

116.6 131.7

Table 4 Scaled force constants for monomer of LTA. Description

Force constants (m Dyne/Å)

f(tCAC) f(tCAO) f(tC@O) f(tCAH) f(tOAH) f(dCACAC) f(dOACAO) f(dCAOAH) f(dCACAO) f(dHACAC)

3.36 5.50 11.42 4.78 6.29 1.13 7.44 0.88 2.16 0.64

to the p conjugation and hydrogen bonding interactions in the molecule. NBO analysis The stabilizing interactions between donor and acceptor NBOs and their stabilization energies in LTA molecule obtained from the second-order perturbation theory analysis of NBO Fock matrix have been tabulated and presented in Table 5. The result of NBO analysis shows a large number of stabilizing interactions in LTA. The two such strong (n ? p⁄) hyperconjugative interactions, i.e., n2(O8) ? p⁄(C1AO9) and n2(O12) ? p⁄(C4AO5) have maximum stabilization energies 186.65 kJ/mol and 181.62 kJ/mol respectively, which manifests the intensive interactions between the lone pairs of oxygen atom and the CAO antibonding orbitals. It is also evident from Table 5 that the p⁄ antibonding orbital C4AO5 possesses the highest occupancy of 0.2062e and the p⁄ antibonding orbital C1AO9 has maximum occupancy of 0.1914e, which shows the strong p electron delocalization in the respective bonds. The interaction between n1(O8) and r⁄(C1AO9) results in a low stabilization of 30.65 kJ/mol with low occupancy (0.0238e) of antibonding orbital, whereas the charge transfer from n1(O12) to r⁄(C4AO5) causes low stabilization of 29.85 kJ/mol. The increased occupancy (0.07e) of the antibonding orbitals for the interactions n(O) ? r⁄(CAC) elongates the corresponding bonds and thereby

stabilize the system in the energy range 2.18–84.41 kJ/mol. The hyperconjugative interactions between n2(O5) and r⁄(C3AC4) and between n2(O9) and r⁄(C1AC2) increases the CAC@O bond angles. The stabilization energies, i.e., 138 kJ/mol for n2(O5) ? r⁄(C4AO12) and 5.23 kJ/mol for n1(O5) ? r⁄(C4AO12) interactions, revealed that the hyperconjugative interaction of (C4AO12) increases the electron density (0.097e) that weakens the respective bond and thereby causing a stabilization to the structure. The charge transfer from n1(O9) to r⁄(C1AO8) demonstrates a less stabilization of 6.53 kJ/ mol whereas the orbital overlap between n2(O9) and r⁄(C1AO8) shows a strong delocalization of 139.21 kJ/mol. In addition the increased electron population (0.0994e) of the antibonding orbital C1AO8 results in the lengthening of the bond. These (n ? r⁄) interactions are related to the resonance in the molecule, i.e., the negative charge resonates between the two oxygen atoms, because of the electron donation from the atom of the electron-donating orbital to the anti-bonding acceptor orbital [83]. Consequently, the absorption due to the CAO stretching mode is shifted to low wavenumber region. Appreciably less electron population (0.015e) on the antibonding orbitals for the interactions between n2(O6) and r⁄(C2AO10) as well as between n2(O10) and r⁄(C3AO6) shows the strengthening of the respective CAO bonds, which manifests the blue-shift of the IR (CAO) stretching wavenumber. NBO analysis clearly describes the possible intramolecular interactions in terms of natural atomic hybrid orbitals and thereby elucidates the proper and improper hydrogen bonding within the molecule [61]. The existence of intramolecular OAH  O hydrogen bonds due to the n2(O5) ? r⁄(O6AH14), n1(O12) ? r⁄(O10AH16) and n2(O12) ? r⁄(O10AH16) interactions have been confirmed with stabilization energies 8.96 kJ/mol, 6.53 kJ/mol and 2.64 kJ/mol, respectively. The electron densities of these interactions are respectively 0.0156e, 0.0121e and 0.0121e. The delocalization energies and occupancies due to the formation of OAH  O bonds reveal that the two oxygen atoms of one of the two carboxyl groups participated as donors in the intramolecular hydrogen bond formation. The occurrences of n(O) ? r⁄(CAH) interaction suggested that the bonds are stabilized not only by the presence of intramolecular interactions but also by the attractive electrostatic dipole–dipole interactions [84] caused by the antiparallel CAH and OAC dipoles. The respective optimized bond lengths of both (C2AH7) and (C3AH11) are 1.10 Å while these values observed from XRD are 0.94 Å and 1.05 Å. The observed CAH bond lengths are shortened over the calculated values, which can be attributed to the increase in wavenumbers of the stretching modes. This indicates the lengthening of CAH bonds associated with the antibonding acceptor orbital in the n(O) ? r⁄(CAH) interactions in molecule and might suggest the possibility of blue-shift in the IR stretching frequency in the crystalline state.

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Table 5 Second order perturbation theory analysis of Fock matrix in NBO basis including the stabilization energies using DFT at B3LYP/6-311++G(d,p) level. Donor (i)

r(C1AC2) r(C1AC2) r(C1AC2) r(C1AC2) r(C1AO8) r(C1AO9) p(C1AO9) p(C1AO9) p(C1AO9) r(C2AC3) r(C2AC3) r(C2AC3) r(C2AC3) r(C2AC3) r(C2AH7) r(C2AH7) r(C2AH7) r(C2AO10) r(C2AO10) r(C2AO10) r(C3AC4) r(C3AC4) r(C3AC4) r(C3AO6) r(C3AO6) r(C3AH11) r(C3AH11) r(C3AH11) r(C3AH11) r(C3AH11) r(C4AO5) p(C4AO5) p(C4AO5) p(C4AO5) r(C4AO12) r(C4AO12) r(O6AH14) r(O6AH14) r(O6AH14) r(O6vH14) r(O8AH15) r(O8AH15) r(O10AH16) r(O12AH13) r(O12vH13) n1(O5) n1(O5) n2(O5) n2(O5) n2(O5) n1(O6) n1(O6) n2(O6) n2(O6) n2(O6) n1(O8) n2(O8) n1(O9) n1(O9) n2(O9) n2(O9) n1(O10) n1(O10) n1(O10) n2(O10) n2(O10) n2(O10) n1(O12) n1(O12) n1(O12) n2(O12) n2(O12) a b c

ED(i) (e)

Acceptor(j)

ED(j) (e)

E(2)a (kJ/mol)

E(j)  E(i)b (kJ/mol)

F(i, j)c (kJ/mol)

1.9700

r (C1AO9) r⁄(C3AC4) r⁄(O8AH15) r⁄(O10AH16) r⁄(C1AO9) r⁄(C1AC2) p⁄(C1AO9) r⁄(C2AC3) r⁄(C2AO10) r⁄(C1AO8) p⁄(C1AO9) r⁄(C4AO5) p⁄(C4AO5) r⁄(O6AH14) r⁄(C1AO8) r⁄(C1AO9) r⁄(C3AH11) r⁄(C1AO8) p⁄(C1AO9) r⁄(C3AO6) r⁄(C1AC2) r⁄(C4AO5) r⁄(O12AH13) r⁄(C2AO10) r⁄(C4AO12) r⁄(C2AH7) r⁄(C2AO10) r⁄(C4AO5) p⁄(C4AO5) r⁄(O6AH14) r⁄(C3AC4) r⁄(C2AC3) r⁄(C3AH11) p⁄(C4AO5) r⁄(C3AO6) r⁄(C4AO5) r⁄(C2AC3) r⁄(C3AC4) r⁄(C3AH11) r⁄(C4AO12) r⁄(C1AC2) r⁄(C1AO9) r⁄(C1AC2) r⁄(C3AC4) r⁄(C4AO5) r⁄(C3AC4) r⁄(C4AO12) r⁄(C3AC4) r⁄(C4AO12) r⁄(O6AH14) r⁄(C3AC4) r⁄(C4AO5) r⁄(C2AC3) r⁄(C2AO10) r⁄(C3AH11) r⁄(C1AO9) p⁄(C1AO9) r⁄(C1AC2) r⁄(C1AO8) r⁄(C1AC2) r⁄(C1AO8) r⁄(C1AC2) r⁄(C2AC3) r⁄(C2AH7) r⁄(C2AC3) r⁄(C2AH7) r⁄(C3AO6) r⁄(C3AC4) r⁄(C4AO5) r⁄(O10AH16) p⁄(C4AO5) r⁄(O10AH16)

0.0238 0.0736 0.0114 0.0121 0.0238 0.0791 0.1914 0.0475 0.0168 0.0994 0.1914 0.0211 0.2062 0.0156 0.0994 0.0238 0.0337 0.0994 0.1914 0.0155 0.0791 0.0211 0.0118 0.0168 0.0970 0.0332 0.0168 0.0211 0.2062 0.0156 0.0736 0.0475 0.0337 0.2062 0.0155 0.0211 0.0475 0.0736 0.0337 0.0970 0.0791 0.0238 0.0791 0.0736 0.0211 0.0736 0.0970 0.0736 0.0970 0.0156 0.0736 0.0211 0.0475 0.0168 0.0337 0.0238 0.1914 0.0791 0.0994 0.0791 0.0994 0.0791 0.0475 0.0332 0.0475 0.0332 0.0155 0.0736 0.0211 0.0121 0.2062 0.0121

4.40 7.70 9.96 9.00 2.22 4.94 2.81 2.34 6.45 4.98 8.33 6.70 11.85 3.18 3.10 16.62 8.42 2.97 5.11 5.40 4.40 4.23 9.76 6.28 8.21 9.92 2.09 7.58 21.36 3.27 4.86 4.77 5.40 3.06 3.68 2.13 3.60 5.78 2.55 2.26 14.82 3.60 7.96 13.69 3.68 8.25 5.23 74.49 138.00 8.96 11.85 2.18 25.37 4.23 29.98 30.65 186.65 8.46 6.53 84.41 139.21 8.25 4.48 3.94 27.55 31.11 3.60 2.18 29.85 6.53 181.62 2.64

3308.1 2573.0 2704.3 2835.5 4095.8 3780.7 1102.7 1890.4 1785.3 2573.0 1706.6 3203.1 1601.6 2756.8 2310.4 3019.3 2363.0 3150.6 2284.2 2993.1 2678.0 3334.4 2730.5 2993.1 3071.8 2363.0 2126.7 2914.3 1312.8 2494.2 3859.5 1969.1 2100.4 1050.2 3334.4 4095.8 2756.8 2809.3 2888.1 2730.5 2888.1 3596.9 2809.3 2966.8 3623.2 2835.5 2756.8 1706.6 1627.8 1916.6 2494.2 3150.6 1680.3 1575.3 1811.6 3281.9 945.2 2730.5 2756.8 1601.6 1627.8 2494.2 2441.7 2599.2 1654.1 1811.6 1575.3 2651.8 3281.9 2888.1 945.2 2126.7

86.6 99.8 115.5 112.9 68.3 97.1 42.0 47.3 76.1 81.4 86.6 105.0 102.4 65.6 60.4 157.5 99.8 70.9 78.8 89.3 78.8 84.0 115.5 97.1 115.5 107.6 47.3 105.0 123.4 63.0 99.8 68.3 76.1 42.0 78.8 65.6 70.9 91.9 60.4 57.8 149.7 81.4 107.6 144.4 81.4 110.3 86.6 257.3 338.7 97.1 123.4 57.8 144.4 57.8 165.4 223.2 296.7 110.3 97.1 265.2 341.3 102.4 73.5 70.9 152.3 168.0 52.5 55.1 220.5 97.1 294.1 55.1

1.9956 1.9968 1.9915

1.9623

1.9814

1.9881

1.9764

1.9893 1.9603

1.9967 1.9925

1.9950 1.9950 1.9870

1.9865 1.9851 1.9872 1.9793 1.8402

1.9757 1.9485 1.9485 1.9772 1.8126 1.9791 1.8391 1.9759

1.9500

1.9728

1.8297 1.8297

⁄

E(2) means energy of hyperconjugative interactions (stabilization energy). Energy difference between donor i and acceptor j NBO orbitals. Fock matrix element between i and j NBO orbitals.

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The intramolecular charge transfer interactions p(C1AO9) ? p⁄(C1AO9) and p(C4AO5) ? p⁄(C4AO5) are responsible for the conjugation of respective p bonds in the molecule of LTA crystal. These p conjugative interactions increase the electron density of antibonding orbital and thereby causes red-shift in the C@O stretching frequency which stabilize the system by stabilization of 2.81 kJ/ mol and 3.06 kJ/mol, respectively. The electron density of conjugated double bond (0.2e) clearly shows the strong delocalization in the molecule. The negative hyperconjugative interactions are formed by the orbital overlaps such as p(C1AO9) ? r⁄(C2AC3), p(C1AO9) ? r⁄(C2AO10), p(C4AO5) ? r⁄(C2AC3) and p(C4AO5) ? r⁄(C3AH11), which results in the electron stabilization of the molecule of LTA crystal. These interactions induce covalent character to the respective bonds. The positive hyperconjugative interactions like r(C2AC3) ? p⁄(C1AO9), r(C2AC3) ? p⁄(C4AO5), r(C2AO10) ? p⁄(C1AO9) and r(C3AH11) ? p⁄(C4AO5) have also been identified as electron stabilizing interactions with stabilization energies of 8.33 kJ/mol, 11.85 kJ/mol, 5.11 kJ/mol and 21.36 kJ/mol, respectively. The interactions r(C2AO10) ? p⁄(C1AO9) and r(C3AH11) ? p⁄(C4AO5) induces a partial p character to the respective bonds. Of all the r ? r⁄ sigma conjugative interactions for monomer LTA, r(CAO) ? r⁄(CAO) interaction gains much importance because of the relatively great donor ability of oxygen lone pairs. The electron population of the antibonding orbital for the interaction r(C3AO6) ? r⁄(C2AO10) is lower (0.0168e) than that (0.097e) for the interaction r(C3AO6) ? r⁄(C4AO12), which is consistent with the weakening of C4AO12 bond. These interactions leading to stabilization of 6.28 kJ/mol and 8.21 kJ/mol, respectively. The occupancy of antibonding orbital C1AO8 for the interaction r(C2AO10) ? r⁄(C1AO8) is (0.0994e) higher than that (0.0155e) of antibonding orbital C3AO6 for the r(C2AO10) ? r⁄(C3AO6) interaction, as a result the C1AO8 bond elongates. However, the latter interaction resulting a high stabilization of 5.40 kJ/mol compared to that (2.97 kJ/mol) of former. The second-order hyperconjugative interactions r(C1AO8) ? r⁄(C1AO9), r(C4AO12) ? r⁄(C3AO6) and r(C4AO12) ? r⁄(C4AO5) stabilize the monomer within the range 2.13–3.68 kJ/mol. The interactions r(C2AH7) ? r⁄(C3AH11) and r(C3AH11) ? r⁄(C2AH7) are bidirectional and the orbital of same CAH moiety serves as both a r-donor and a r-acceptor, resulting strong stabilization to the structure with energy 8.42 kJ/mol and 9.92 kJ/mol, respectively. It can be seen from Table 5 that the r(CAH) ? r⁄(CAH) interaction causes high stabilization to the system compared to r(CAC) ? r⁄(CAC) interaction. The sigma conjugative interactions arose from (CAC) to (CAO), to (CAC) and to (OAH) causing stabilization to the system in the energy range 4.23–6.70 kJ/mol, 4.40– 7.70 kJ/mol and 3.18–9.96 kJ/mol, respectively. It is noted that the interaction from r(OAH) to r⁄(CAC) leads to higher stabilization of the system than r(OAH) ? r⁄(CAO) and r(OAH) ? r⁄ (CAH) interactions. The r(O6AH14) ? r⁄(C4AO12) interaction increases the electron density (0.097e) of antibonding (C4AO12) orbital which elongates the CAO bond. The significant sigma conjugative interactions giving stronger stabilization to the system are the interaction between the bonding of O8AH15 and the antibonding of C1AC2 leading to stabilization of 14.82 kJ/mol and the interaction between the bonding of O12AH13 and the antibonding of C3AC4 causing stabilization of 13.69 kJ/mol as well as the interaction between the bonding of C2AH7 and the antibonding of C1AO9 which leads to stabilization of energy 16.62 kJ/mol. The orbital overlaps such as r(C1AO9) ? r⁄(C1AC2), r(C2AH7) ? r⁄ (C1AO8), r(C3AH11) ? r⁄(C2AO10), r(C3AH11) ? r⁄(C4AO5), r(C3AH11) ? r⁄(O6AH14) and r(C4AO5) ? r⁄(C3AC4) results in intramolecular charge transfer causing stabilization of the system in the energy range 3.10–7.58 kJ/mol. All the mentioned intramolecular charge transfer (ICT) interactions can make the molecule more polarized and thereby induce the NLO activity to the LTA crystal.

Mulliken atomic charges are calculated at the DFT/B3LYP/6311++G(d,p) method. All oxygen atoms of LTA molecule have negative charges and all hydrogen atoms have positive charges. The carbon atoms (C2 and C3) attached to the hydrogen atoms exhibit negative charges and atoms C1 and C4 shows positive charges. It shows that the presence of the carboxyl group induces a strong polarity in the CAC bonds along the chain such that induced polarity progressively decreases with increasing distance from the carboxyl group. These charge distribution shows the presence of intramolecular interactions in the molecule. Natural population analysis for LTA is presented in Table 6. The natural charge distribution on each atom is evaluated in terms of natural atomic orbital occupancies. The strong negative charges on all the oxygen atoms and strong positive charges on the remaining atoms of LTA molecule demonstrates the electrostatic attraction or repulsion between the atoms that can result significant contribution to the intramolecular charge transfer interactions leading to the stabilization of the molecule. Frontier molecular orbital analysis The HOMO and LUMO energies of LTA molecule (at the gas phase) calculated by DFT/B3LYP/6 311++G(d,p) method are

HOMO energy;EHOMO ¼ 8:0687 eV LUMO energy;ELUMO ¼ 1:3978 eV HOMO–LUMO energy gap [85–88], DEGAP = ELUMO  EHOMO = 6.6709 eV. The plots of HOMO and LUMO are shown in Fig. 4. Molecule with large HOMO–LUMO gap has been shown to be stable and unreactive; those with small gaps are chemically reactive [89– 93]. The computed high value of HOMO–LUMO energy gap (6.6709 eV) in LTA confirms the chemical stability of the molecule. The interpretation from the HOMO–LUMO analysis is that the HOMO is concentrated over the inner skeletal CAC bonds, symmetrical hydroxyl groups and one of the two carboxyl groups which is not involved in intramolecular hydrogen bond formation as well as the LUMO is spread over the entire molecule except O6AH14 hydroxyl group. Therefore, electron transfers from the HOMO to the carboxyl group in LTA. It is indicated that, carbon and oxygen atoms of one of the carboxyl groups of the molecule acts as electrophilic centres and are the most favourable site for nucleophilic attacks while the other carboxyl group, carbon atoms and associated

Table 6 Natural population analysis (NPA) of LTA calculated with DFT/B3LYP/6-311++G(d,p) method. Atoms

Natural charge

C1 C2 C3 C4 O5 O6 H7 O8 O9 O10 H11 O12 H13 H14 H15 H16

0.8025 0.0398 0.0126 0.7888 0.5950 0.7270 0.1848 0.6749 0.5760 0.7194 0.2210 0.7008 0.4963 0.4851 0.4845 0.4776

Total

0.0000

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V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 127–141 Table 7 Calculated quantum chemical molecular orbital properties for LTA at DFT/B3LYP/6311++G(d,p) method.

Fig. 4. HOMO and LUMO plot of LTA.

hydroxyl group are nucleophilic centres. Both HOMO and LUMO energies indicate the charge delocalization within the molecule. The energy gap between HOMO and LUMO manifests the charge transfer interaction which influences the NLO activity of the molecule. The chemical reactivity and site selectivity of the molecular systems have been determined by the conceptual density functional theory highly successful in predicting global reactivity trends [67]. According to the Koopman’s theorem [68], the HOMO energy (EHOMO) can be used to relate the ionization potential (IP) and the LUMO energy (ELUMO) can be used to estimate the electron affinity (EA) [69]. If EHOMO  IP and ELUMO  EA, then the electronegativity (v) [70] can be defined as, v = (IP + EA)/2. The HOMO–LUMO energy gap is related to the chemical hardness (g) [71–74] by the relation, g = (IP  EA)/2. The extent of chemical reactivity can be expressed by the term global softness [75,76] which can be related to hardness as, S = 1/2g. A large value of HOMO–LUMO energy gap indicates the chemical hardness of molecule and small HOMO– LUMO energy gap means a soft molecule. The chemical potential can be defined as; l = (IP + EA)/2 Parr et al. [77] proposed a new global reactivity descriptor of molecule which measures the energy lowering due to maximal electron flow between donor and acceptor and defined as electrophilicity index, x = l2/2g. The total energy change is defined as, DET = g/4 [78,79]. The overall energy balance (DE) which determines the energy gain or lost, in an electron donor–acceptor transfer [80] is given as, DE = EA  IP. The calculated reactivity descriptors are presented in Table 7. A molecule or atom that has a positive electron affinity is often called an electron acceptor and may undergo charge transfer reactions. The computed electron affinity value of 1.3978 eV for LTA shows its electron accepting behaviour. The chemical hardness and electronegativity provides information about chemical binding, stability, reactivity and selectivity of a molecule by describing the response of a molecule to the variation of the number of electrons at constant external potential [94,95]. The calculated electronegativity value for LTA is found to be 4.7333 eV, which is high and predicts the less nucleophilic behaviour of the molecule. A high value of chemical hardness (3.3355 eV) for LTA indicates that it has not much chemical reactivity and hence a higher chemical stability because of its high HOMO–LUMO energy gap. The calculated

Parameters

B3LYP/6-311++G(d,p)

HOMO energy, EHOMO (eV) LUMO energy, ELUMO (eV) HOMO–LUMO energy gap, DEGAP (eV) Ionisation potential, IP (eV) Electron affinity, EA (eV) Total energy change, ,DET (eV) Overall energy balance, DE (eV) Electronegativity, v (eV) Chemical hardness, g (eV) Global softness, S (eV1) Chemical potential, l (eV) Electrophilicity index, x (eV) SCF energy (kJ mol1)

8.0687 1.3978 6.6709 8.0687 1.3978 0.8339 6.6709 4.7333 3.3355 0.1499 4.7333 3.3584 1.5952  106

electrophilicity index of 3.3584 eV indicates the electron promotion from the nucleophile (HOMO) to the electrophile (LUMO) which contributes the chemical reactivity to the molecule. The direction of the charge transfer is completely determined by the electronic chemical potential of the molecule because an electrophile is a chemical species capable of accepting electrons from the environments; its energy must decrease upon accepting electronic charge, consequently its electronic chemical potential must decreases and tends to be negative. The high value of the electrophilicity index compared to the low value of chemical potential (4.7333 eV) for the molecule of LTA approves its electrophilic character. Total electron density, electrostatic potential and molecular electrostatic potential The electrostatic potential (ESP), total electron density (TED) and molecular electrostatic potential (MEP) plots of LTA molecule are shown in Fig. 5. It is obvious from the ESP plot that the negative electrostatic potential is concentrated over the entire molecule except the one carboxyl group which took part in the intramolecular hydrogen bonded interactions. The ED plot of LTA molecule shows uniform charge distribution. The MEP surface is an important descriptor in predicting and interpreting reactivity of sites for electrophilic and nucleophilic attacks as well as hydrogen bonding interactions. The MEP plot is the depiction of electrostatic potential mapped onto the total electron density surface. Since MEP is related to the total charge distribution of the molecule, it provides correlation between molecular structure with its physiochemical property such as partial charges, dipole moment, electronegativity and chemical reactivity [96– 101]. The different values of the electrostatic potential at the electron density surface are represented by different colours. The red, blue1 and green colours represent the regions of most negative, most positive and zero electrostatic potential, respectively. The colour scheme for the MEP surface is that the red region denotes the electron rich and maximum negative charge behaviour, the blue region shows the slightly electron deficient and partial positive charge behaviours, the light blue indicates the slightly electron deficient region while the yellow portion shows the slightly electron rich region, respectively. The colour code of these maps is in the range between 0.0805 (the deepest red) and 0.0805 (the deepest blue) for LTA. The negative potential region of MEP plot is related to the region of electrophilic attack and positive potential region are related to the nucleophilic attack. It is obtained from the MEP plot that the maximum positive charge is concentrated over H13, C4, O12, H15 and H11 as well 1 For interpretation of colour in Fig. 5, the reader is referred to the web version of this article.

V. Sasikala et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 127–141

135

Fig. 6. (a) Simulated UV–Vis absorption spectrum of LTA in water. (b) Experimental UV–Vis–NIR absorption spectrum of LTA in water. (c) Simulated UV–Vis absorption spectrum of LTA in gas phase.

as the partial positive charge on O5 and these atoms acted as electrophilic centres whereas the partial negative potential region is spread over O8, O9, O10, O6, C1, C2, C3, H14 and H16 atoms in addition, these atoms are favourable site for electrophilic attacks. The hydrogen atoms of carboxyl groups possess maximum positive charges while the two oxygen atoms of one of the two carboxyl groups has maximum negative charges which reflect the most electronegative region. The absence of maximum negative potential region in MEP plot confirms the low chemical reactivity of the molecule and the presence of partial negative potential region (yellow colour) indicates the electron rich region which is the favourable site for electrophilic attack.

LUMO, oscillator strengths, cut-off wavelengths and NBO transitions for the three lowest excited states of the gas phase and water phase LTA calculated by the TD-DFT/B3LYP/6-311++G(d,p) level are listed in Table 8. It can be observed from Table 8 that the oscillator strengths associated with the longest wavelength transitions are small, which is consistent with the electronic absorptions. The TD-DFT calculation predicts intense electronic transitions at 217.04 nm with an oscillator strength f = 0.0302 for gas phase LTA, and at 211.96 nm with an oscillator strength f = 0.0297 for water phase LTA. The observed and simulated cut-off wavelengths positioned at the lower wavelengths. This hypsochromic shift is due to the weak n ? p⁄ transitions responsible for the UV absorptions in LTA at 200 nm with the high excitation energies, nearly 6 eV. The polar solvent (water) effect stabilizes the n state through intramolecular charge transfer and hydrogen bonded interactions. The absence of intensely absorbing p ? p⁄ transitions, correlating with the high HOMO–LUMO gap, provide a qualitative measure of extent of conjugation in neutral LTA molecule.

UV–Vis spectral studies

Vibrational analysis

The extent of p-electron delocalization within the conjugated structure is conveniently probed by UV–Vis spectroscopy. On the basis of DFT calculation and with the use of NBO method; it is possible to determine the origin of the first three electronic transitions for LTA. The experimental and simulated UV–Vis–NIR absorption spectra of LTA are shown in Fig. 6. The observed wavelengths corresponding to the UV absorption maximum of LTA in water is 199 nm. The experimental spectra (Fig. 6 and Fig. SF1, Supporting information) show that the crystal is transparent in the entire range (240–1100 nm) without any absorption peak. This indicates the contribution of the crystal resistance to laser-induced damage, which is an essential requirement for crystals having NLO properties. As a result the crystal can be used as a potential candidate for the applications in the visible region down to blue and violet region, particularly for laser frequency doubling and related opto-electronic applications. The energies for the promotion of electron from the HOMO to the

The vibrational spectral assignments were carried out with the aid of normal co-ordinate analysis (NCA) followed by force field calculation with the same level of theory as was employed for the geometry optimization of the molecule. A non-redundant set of internal coordinates for LTA was defined which was similar to the ‘natural coordinates’ recommended by Pulay et al. [54]. The calculated wavenumbers were selectively scaled according to the scaled quantum mechanical (SQM) procedure, incorporating a set of ten transferable scale factors suggested by Rauhut and Pulay [53]. The full description of the observed IR and Raman spectra is presented in Table 9, along with detailed assignments as stipulated by the PED calculation. The observed FT-IR and Raman spectra and simulated theoretical spectra refined by the SQM procedure implemented at DFT theory are given in Figs. 7 and 8 for visual comparison. The theoretical and experimental wavenumbers are in fair agreement, and assignments of wavenumbers for different functional groups are discussed below.

Fig. 5. ESP, TED and MEP maps for gas phase LTA.

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Table 8 Calculated absorption wavelengths, electronic excitation energies and oscillator strengths of LTA at TD-DFT/B3LYP/6-311++G(d,p) method. Phase

Excited state

Wavelength (nm)

Excitation energies (eV)

Oscillator strengths (f)

Assignment

Gas phase

S1(H ? L) S2(H-1 ? L) S3(H-3 ? L)

225.49 217.04 208.24

5.4985 5.7124 5.9538

0.0042 0.0302 0.0045

n2 ? p⁄ n1 ? p⁄ n1 ? p⁄

Water phase

S1(H-1 ? L) S2(H ? L) S3(H-2 ? L)

220.23 211.96 207.06

5.6298 5.8495 5.9877

0.0027 0.0297 0.0045

n1 ? p⁄ n2 ? p⁄ n2 ? p⁄

Table 9 Observed FT-IR, FT-Raman and computed wavenumbers (in cm1), IR intensities, Raman activities, Vibrational assignments and PED contributions of L-tartaric acid by NCA based on SQM force field calculations. IIRa (km/mol)

Theoretical

Experimental

Scaled

FT-IR

FT-Raman

3393 3344 3342 3339 2956 2944 1726

3404 s 3333 ms 3219 w 3111 w 2965 w 2932 w 1740 s 1735 s 1724 s 1719 s 1447 w 1400 vw 1363 w 1340 vw 1318 w 1288 vw 1254 s 1222 ms 1189 s 1133 s

3405 vw 3332 vw

1087 vs 992 w 899 vw 875 w 831 w 795 w 740 ms 735 ms 669 s 619 ms 606 ms 578 ms 518 w 484 w

1087 w 993 ms 898 s 880 ms 831 w 800 vw

1706 1444 1420 1366 1353 1327 1300 1256 1214 1194 1133 1128 1080 1008 904 849 802 757 688 659 638 593 535 477 386 361 335 305 277 268 203 133 68 54 36

2968 ms 2934 ms 1740 ms

1695 s 1449 w 1427 w 1361 w 1340 vw 1315 vw 1292 vw 1258 ms 1213 w 1189 w 1130 w

737 vs 666 w 644 vw 599 w 533 w 485 w 378 w 346vw 336 vw 299 vw 270 vw 227 w 203 w 156 vw

IRamanb (a.u.)

Characterization of normal modes with PED (%)

24.2 16.1 29.2 35.4 8.26 1.35 79.1

4 12 3 11 1 20 5

m(O10H16) (100) m(O8H15) (100) m(O6H14) (100) m(O12H13) (100) m(CH) (99) m(CH) (98) m(C1@O9) (67), b(O8C1@O9) (14)

100

10

m(C4@O5) (65), b(O12C4@O5) (16)

11.2 3.05 6.48 16.6 11.8 2.5 21 13.3 20 94.5 62.3 28.2 10.1 8.97

2 3 9 2 3 2 7 3 4 1 2 2 12 16

b(C2O10H16) (45), x(CAH) (19) b(C3O6H14) (59), x(CAH) (18) x(CAH) (59), b(COH) (10) b(CAOH)carboxyl (37), b(OC@O) (23), b(CCO) (18) b(CAOH)carboxyl (40), b(OC@O) (21), b(CCO) (19), m(C@O) (10) x(CAH) (72), b(COH) (15) b(CACAH) (49), b(COH) (28) b(CACAH) (62) x(C4O12) (37), m(C4AO12) (30), b(CCO) (12) m(C1O8) (36), b(OC@O) (21), b(CCO) (19), b(C1O8H15) (12) m(C3O6) (38), b(OC@O) (21), b(CCO) (16), b(C3O6H14) (10) m(C2O10) (73), b(CACAO) m(C2AC3)(asym) (33), m(CO) (28), b(C2AC3AC4) (18), x(CO) (16) m(C1AC2)(asym) (57), d(CCO) (16)

2.06 17.1 5.92 6.82 17.7 22.4

2 1 17 12 8 4

m(C3AC4)(asym) (29), x(CO) (23), d(OC@O) (14), d(CCO) (10) d(OC@O) (23), m(C2AC1) (sym) (21), x(C1AO8)(19), s(C1AO8) (11), m(CO) (10) d(OC@O) (32), x(C4O12) (22), d(CCO) (12), d(CACAH)

33.2 9.32 3.5 1.78 30.1 14.3 11 0.69 2.45 0.81 0.08 1.05 0.4 0.57

5 8 5 5 4 3 3 6 3 3 9 13 20 100

d(OC@O) (48), d(CCO) (32) d(OC@O) (35), s(C1O8H15) (32), d(CCO) (25) d(OC@O) (42), d(CCO) (35)

s (C4O12H13) (61), d(CCO) (10) d(OC@O) (38), m(CAC)(sym) (24), x(CO) (14), m(CO) (10) x(CO) (35), m(C2AC3)(sym) (22), d(OC@O) (13) d(CCO) (49), m(C2AC3)(sym) (12), x(C3O6) (11), s(O6H14) (10) s(O10H16) (67), d(CCO) (11) s(O6H14) (52), d(CCO) (22) d(CCO) (44), x(CO) (20) d(CCO) (80) d(CCO) (64), q(O8C1@O9) (11), x(CO) (11) d(CACAC) (43), d(CCO) (32) d(CACAC) (67), d(CCO) (14) C(CAC) (23), s(C2AC3) (22), s(C3AC4) (20), s(O6H14) (17) s(C3AC4) (44), s (C3O6) (23), s(C2AC3) (12), s(C2O10) (10) s(C1AC2) (34), s(C2AC3) (17), s(C3AC4) (12), d(CCO) (10), s(C3O6) (10)

m: stretching, x: wagging, s: torsion, q: rocking, C: twisting, b: in-plane bending, d: out-of plane bending, sym: symmetric, asym: antisymmetric, vs: very strong, s: strong, ms: medium strong, vw: very weak, and w: weak. a Relative absorption intensities normalized with the highest peak absorption equal to 100. b Relative Raman activities calculated by Eq. (1) and normalized to 100.

OAH group vibrations The non-hydrogen-bonded or free hydroxyl group absorbs strongly in the 3700–3584 cm1 region, whereas the existence of intermolecular hydrogen-bond formation can lower the OAH stretching wavenumber to the 3550–3200 cm1 region with

increase in IR intensity and breadth [102,103]. The intramolecular OAH  O hydrogen bond formation lowers the OAH stretching wavenumber in the region 3200–2500 cm1 [104]. In IR spectrum the hydroxyl stretching bands splits into two bands at 3404 cm1 and 3219 cm1 corresponding to O10AH16 and O6AH14. The DFT

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137

identified in the region 1340–1087 cm1 as strong to very weak bands in both IR and Raman spectra. Since these modes coupled with other prominent vibrations the modes are not pure as evident from the PED. The strong to weak peaks observed in the IR region 899–484 cm1 and in the Raman region 898–156 cm1 are associated with the CACAO out-of-plane bending modes. The CAO wagging and torsional bands have been identified and assigned with the support of DFT computations.

Fig. 7. (a) FT-IR spectra of LTA. (b) Simulated IR spectra of LTA.

Fig. 8. (a) FT-Raman spectra of LTA. (b) Simulated Raman spectra of LTA.

computations give the wavenumber of these bands at 3393 cm1 and 3342 cm1 for the O10AH16 and the O6AH14 stretching vibrations respectively with 100% PED contributions. The appearance of the red shift of the O10AH16 and O6AH14 stretching wavenumber is clearly due to the formation of a OAH  O hydrogen bond. Interaction of lone pairs of oxygen (electron donor) with the OAH antibonding r⁄ orbital leads to an increase of electron populations in this orbital, followed by a weakening of the OAH bond which is accompanied by a lowering of the OAH stretching wavenumber. The absorption bands arising from the OAH in-plane bending modes expected in the region 1440 cm1–1260 cm1 [105,106]. The OAH in-plane bending vibration is observed at 1447 cm1 and 1400 cm1 as weak band in IR and weak band at 1449 cm1 and 1427 cm1 in Raman coupled with CAH in plane bending vibration. The weak peaks observed at 378 cm1 and 336 cm1 in the Raman spectra are attributed to O6AH14 torsional modes. The very weak peak observed in the Raman spectra at 346 cm1 is assigned to O10AH16 torsional mode. These torsional modes are also affected by the presence of hydrogen bonding. The existence of this mode is an evidence for an effect of hydrogen bonding that measures the strength of the interaction between the OAH group and the neighbouring lone pair electron of oxygen atom in LTA crystal. The non-carboxylic CAO stretching wavenumbers are expected around 1094 cm1 [107]. The C2AO10 stretching mode appears as strong IR band at 1087 cm1 and in Raman spectrum at 1087 cm1 (weak). The CACAO in-plane bending modes have been

Carboxyl group vibrations The hydroxyl stretching vibrations of carboxyl group are generally observed in the region around 3500 cm1 [108]. In bonded form, a broad and intense band appears in the region 3550– 3200 cm1 [102]. Observed medium band at 3333 cm1 in IR and weak band at 3332 cm1 in Raman are attributed to O8AH15 stretching mode of the carboxyl group which is calculated at 3344 cm1. The medium band observed at 3333 cm1 in IR is assigned to O12AH13 stretching mode. The DFT computations give the wavenumber at 3339 cm1 for O12AH13 stretching vibrations. In solid phase, the C@O group of saturated aliphatic carboxylic acid absorbs strongly in the region 1740A1715 cm1 [109]. The medium intense band in Raman spectrum at 1740 cm1 is assigned to the C1@O9 and C4@O5 stretching modes, respectively. In the IR spectra, the carbonyl stretching mode is prominently split into four components, 1740 cm1, 1735 cm1, 1724 cm1 and 1719 cm1. The results of computations give the wavenumbers of these modes to be 1726 cm1 and 1706 cm1 respectively for C1@O9 and C4@O5 stretching modes. The splitting of the carbonyl mode may be attributed to Fermi resonance and molecular association [110], which are the reasonable alternatives to explain the splitting of this mode. However, Fermi resonance may be excluded by the nature of the intensity ratio of the two components. The doublet of the C@O mode band with the traces of crystal splitting originates from two differently associated CO groups, and the splitting might be due to intermolecular association based on C@O  H type hydrogen bonding in the crystal. The conjugation and influence of intermolecular hydrogen bonding (C@O  H type) network in the crystal results in lowered C@O stretching wavenumber. Hydrogen bonding affects carbonyl wavenumbers, but the effects are more pronounced when hydrogen bonding is combined with mesomeric effects. When a carbonyl is hydrogen bonded and resonance cause a positive charge to appear on the proton donor atom and a negative charge to appear on the acceptor atom, which tends to encourage the hydrogen bond. The increased association of the proton with the acceptor atom tends to encourage the resonance [C@O  HAX M CAO  HAX+@]. Thus hydrogen bonding and resonance are mutually enhanced by the so called ‘‘transfer of allegiance’’ and weaken the carbonyl bond and lower C@O stretching the wavenumbers [111]. The in-plane OAH deformation vibration usually appears in the region 1440–1200 cm1 [104,112]. The weak bands observed in the IR at 1340 cm1 and 1318 cm1 and very weak bands in the Raman at 1340 cm1 and 1315 cm1 are attributed to the CAOAH in-plane bending modes. The corresponding DFT calculated modes are at 1353 cm1 (37% PED) and 1327 cm1 (40% PED). The OAH out-of-plane bending vibrations of carboxyl group are generally expected in the region 700–600 cm1 [102,113–116]. A medium strong band observed in the IR at 578 cm1, weak Raman band at 599 cm1 and a theoretical band at 593 cm1 are assigned to C4AO12AH13 torsional mode with a PED contribution of 61%. The strong IR band at 669 cm1 and a weak Raman band at 666 cm1 are attributed to the C1AO8AH15 torsional mode with PED 32%. In dicarboxylic acid, the absorption due to the CAO stretching mode appears at frequencies >1200 cm1 [117]. A strong band in the IR spectrum of LTA observed at 1189 cm1 and a weak Raman band at the same wavenumber are assigned to the C4AO12

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stretching vibration. The corresponding theoretically calculated value is at 1194 cm1. The C1AO8 stretching mode is observed as strong IR band at 1133 cm1, weak Raman band at 1130 cm1 and calculated mode at 1133 cm1. The bands corresponding to the OAC@O in-plane bending modes have been identified in the IR region 1726–1133 cm1 and in the Raman region 1695– 1130 cm1. The bands arose due to the OAC@O out-of-plane bending vibrations are observed in the IR region 831–484 cm1. The CAO wagging modes and OAC@O rocking modes have been identified and assigned with the help of DFT calculations. The experimental results show good agreement with the simulated results. CAH group vibrations The CAH stretching vibrations of LTA are observed in IR at 2965 cm1 and 2932 cm1 as weak bands and in Raman at 2968 cm1 and 2934 cm1 as medium, as expected [118]. The CACAH in-plane bending modes are observed as strong band at 1254 cm1 and medium band at 1222 cm1 in IR. In the Raman spectrum, the corresponding mode is assigned to a medium band at 1258 cm1 and a weak band at 1213 cm1, which is in agreement with the computed value. The CAH out-of-plane bending vibrations are usually appeared in the region 1000–675 cm1 [119]. The medium band observed in IR at 740 cm1 and weak band in Raman at 771 cm1 are assigned to the CACAH out-ofplane bending mode, which is a mixed mode coupled to OC@O and CCO bending as well as to CAO wagging modes. The absorption bands arising from CAC stretching vibrations are generally measured in the region 1150–850 cm1 [120]. The CAC stretching mode appears as weak IR bands at, 899 cm1, 992 cm1 and 831 cm1 and Raman bands at 898 cm1 (strong), 993 cm1 (medium) and 831 cm1 (weak). Low-frequency vibrations of hydrogen bonds The lattice vibrations usually appear in the region below 300 cm1. About five bands are observed in the region, which is much fewer than predicted by theory. These modes are due to the rotational and translational vibrations of the molecules and vibrations involving hydrogen bonds. The low wavenumber bands of the hydrogen bond vibrations are generally found to be weak, broad and asymmetric in the Raman spectrum. The modes in the region 265–160 cm1 could be associated with hydrogen bond vibrations. The lattice vibrations of rotatory type are generally stronger in intensity than the translatory type [121]. The lattice modes in LTA are found to be very intense in the Raman spectrum compared with other modes in the high wavenumber region. Thermodynamic studies On the basis of the statistical thermodynamics principle and DFT level theory, thermodynamic parameters such as zero point vibrational energy, rotational temperatures and rotational constants, specific heat capacity at constant volume, entropy and thermal energy for gas phase LTA at standard temperature and pressure are evaluated. The temperature dependence of the thermodynamic  functions such as specific heat capacity at constant volume C 0v , entropy (S0) and thermal energy (E0) of LTA crystal in the temperature range 100–900 K are listed and presented in Table ST1 (Supporting information). It is obvious from Table ST1 that C 0v ; S0 and E0 increase with increasing temperature. This is because at low temperature, the main contributions to the thermodynamic functions are from the translation and rotation of molecules; however, at higher temperature, the amplitudes of vibrations are intensified and therefore make more contributions to the thermodynamic properties and lead to the increase in the thermodynamic functions. The second

order polynomial equations are used to fit the temperature dependence of various thermodynamic properties of the LTA. The correlation equations between thermodynamic properties and temperatures are presented below with corresponding fitting factors (R2). The corresponding temperature (Kelvin) dependent correlation graphs are shown in Fig. SF2 (Supporting information).

C 0v ¼ 5:3043 þ 0:1226T  5:9902  105 T 2 ;

ðR2 ¼ 0:9997Þ

ð7Þ

S0 ¼ 57:1425 þ 0:1632T  5:2867  105 T 2 ;

ðR2 ¼ 0:9997Þ

ð8Þ

E0 ¼ 70:4965 þ 0:0184T þ 3:1143  105 T 2 ;

ðR2 ¼ 0:9996Þ

ð9Þ

It can be observed from correlation graphs that all the three thermodynamic properties increase with rise of temperature, which is directly due to the enhancement of molecular vibrations with the increase of temperature [122]. These equations and the predicted data in Table ST1 will be helpful for further studies on the other physical, chemical, and explosive properties of the title crystal. NLO studies First order hyperpolarizability The calculated values of nonlinear optical response properties of LTA molecule such as the first hyperpolarizabiliy, polarizability, anisotropy of polarizability and total static dipole moment are presented in Table ST2 (Supporting information). Molecules with high values of hyperpolarizability, molecular polarizability, and dipole moment display significant NLO properties. According to the DFT calculations, the first order hyperpolarizability value of LTA is 1.1868  1030 esu, the mean polarizability value is 10.4646  1024 esu and the anisotropy of polarizability is found to be 3.2371  1024 esu. The calculated first hyperpolarizability value is referred to corresponding one for other molecules of organic NLO crystals, i.e. L-lysine  tartaric acid (3.047  1030 esu) [28], 30 L-prolinium tartrate (0.7203  10 esu) [30], Hydroxyethylammonium (L) tartrate monohydrate (4.238  1030 esu) [123], L-glutamine picrate (7.36  1030 esu) [109], L-alaninium formate (2.84  1030 esu) [124], and 2,4,5-trichloroaniline (1.1186  1030 esu) [125]. Also, the predicted first hyperpolarizability value is 3.2 times that of urea (0.3728  1030 esu) and the calculated mean polarizability is 2.7 times that of urea (3.8312  1024 esu). Considerably high hyperpolarizabilty values of bzzz, byzz, bzxx, bxzz and bxyy and polarizability values of axx, ayy and azz components indicate the strong delocalization of electron cloud in these directions. Of these components, bzzz has the highest hyperpolarizability value of 0.9414  1030 esu and azz, ayy as well as azz components have polarizability values 12.1704  1024 esu, 10.0090  1024 esu and 9.2141  1024 esu, respectively which shows that the charge delocalization is perpendicular to the bond axis and also the substantial involvement of p orbitals in the charge transfer interaction. Thus the high value of hyperpolarizability is attributed to the intramolecular charge transfer interactions between donor and acceptor substituents, especially the extent of n ? p⁄ conjugation, HOMO–LUMO energies and symmetry of the molecule, which support the NLO activity of the crystal. Another parameter used as a descriptor to depict the charge movement across the molecule is the dipole moment. Dipole moment is an important molecular electronic property which reflects the charge distribution and is given as a vector in three dimensions. Direction of the dipole moment vector in a molecule depends on the centres of positive and negative charges. In monomer of LTA, the total dipole moment vector is pointed from the centre of C2AC3 bond to the direction of intramolecular hydrogen bond forming carboxyl group. The component lx has

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the highest value of dipole moment (2.7019 Debye). The large value of the total dipole moment indicates the strong polarity of the bond and it is computed for LTA to be 3.3319 Debye, which is 2.4 times that of urea (1.3732 Debye). These high values of molecular dipole moment and polarizability relative to urea show the large NLO property of LTA crystal. Z-scan studies The experimental Z-scan data obtained for LTA are numerically fitted to the nonlinear transmission equation for a two-photon absorption (TPA) process [126], given by

T¼

ð1  RÞ2 eaL pffiffiffiffiffiffiffiffi pq0

!Z

þ1

h i 2 ln 1 þ q0 et dt

ð10Þ

1

where T is the net transmission of the sample, L and R are the length and surface reflectivity of the sample, respectively and a is the linear absorption coefficient. The parameter q0 in Eq. (10) is given by b(1  R)I0Leff, where b is the two-photon absorption coefficient, I0 is the on-axis peak laser intensity and Leff is given by (1  eaL/a). Fig. SF3 (Supporting information) shows that as the sample were moved close to the focus, the transmittance through decreased as the irradiation of laser is increased. At the focus (z = 0) where the laser irradiance is maximum, indicated an obvious reverse saturated absorption (RSA). The focusing effect is attributed to a thermal non-linearity resulting from absorption of radiation at 532 nm. The effective two-photon absorption coefficient (2PA) obtained for LTA from the best numerical fit to the data is 3.6  1011 m/W. In comparison, with literature we had obtained values of 23  1011 m/W in a polyaniline–porphyrin nanocomposite [127], and 30  1011 m/W in a copolymer containing oxadiazole and substituted thiophene [128,129], under similar excitation conditions. The major application of RSA materials is in optical limiting. Fig. SF4 (Supporting information) shows the optical limiting behaviour (i.e. the sample transmission decreases as the input intensity is increased) of the sample at 532 nm using 5 ns laser pulses. Optical limiting is an application useful for the protection of human eyes and sensitive optical detectors from accidental exposure to intense light beams [130]. Another interesting application of optical limiting is in laser stabilization [131–133]. The above Z-scan studies indicate that LTA is a potential candidate for optical limiting applications in the ultrafast and short pulse excitation regimes. Conclusion Single crystal of LTA has been grown by slow evaporation technique. Molecular structure optimization and vibrational spectral studies are accomplished perfectly by using DFT computation. The spectral characterization studies such as FT-IR, FT-Raman and UV–Vis–NIR for LTA have been carried out. Making use of the computed data, the complete vibrational assignments are made and analysis of the observed fundamental bands of the molecule is carried out with the help of normal co-ordinate analysis following the scaled quantum mechanical force field methodology. The potential energy distribution (PED) values obtained reflect the correctness of the vibrational assignments. The observed vibrational frequencies have been compared with those obtained theoretically and the simulated infrared and Raman spectra show good agreement with observed spectra. The NBO analysis reveals the possible intramolecular charge transfer interactions in LTA. The stability of monomer was investigated by means of NBO. Vibrational and NBO analysis confirms the existence of intramolecular OAH  O hydrogen bonding. The principal force constants for stretching and deformation modes in LTA are determined. The

139

MEP study confirms LTA as an electron rich species and explains its electrophilic nature. The first-order hyperpolarizability, polarizability, static dipole moment, quantum chemical molecular orbital properties and thermodynamic properties of LTA are computed at the DFT level and the results are discussed. The excitation energies, cut-off wavelengths, oscillator strengths and NBO transitions of the three lowest excited states at the gas phase and water phase LTA are calculated. The simulated UV–Vis absorption studies predicted that the forbidden n ? p⁄ transitions are responsible for the optical absorption in LTA at low cut-off wavelength with high excitation energy and also reveals the absence of an extended strong p conjugation along the length of the molecule. The high value of HOMO–LUMO energy gap reflects the low chemical reactivity, high chemical stability and hardness of LTA molecule. The thermodynamic properties of LTA crystal at the gas phase were studied and the properties were found to have direct relationships with temperature. The third-order nonlinear absorption behaviour of LTA is studied using open aperture Z-scan technique. Reverse saturated absorption optical process is observed for LTA. The effective two-photon absorption coefficient of LTA has been numerically estimated and compared with those values of other reported crystalline organic systems. The predicted NLO response properties, vibrational, UV–Vis absorption, NBO and Z-scan studies confirm the potentially useful NLO behaviour of LTA and indicate the possibility of using it as an attractive material for laser frequency doubling and optical limiting applications. Acknowledgements D.S. (D. Sajan) thanks the Council of Scientific and Industrial Research (CSIR), New Delhi-110 012, India for the financial support (No. 03(1247)/12/EMR-II. The author is highly grateful to Prof. T. Sundius for the MOLVIB program. 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.2013.12.045. References [1] D. Mucha, K. Stadnicka, W. Kaminsky, A.M. Glazer, J. Phys. Condens. Matter 9 (1997) 10829–10842. [2] W. T Astbury, Proc. R. Soc. Lond. A 102 (1923) 506–528. [3] F. Stern, C.A. Beevers, Acta Crystallogr. 3 (1950) 341–346. [4] Y. Okaya, N.R. Stemple, M.I. Kay, Acta Crystallogr. 21 (1966) 237–243. [5] Sunil Dhamaniya, Josemon Jacob, Polymer 51 (2010) 5392–5399. [6] G.R. Desiraju, Crystal Engineering, The Design of Organic Solids, Elsevier, Amsterdam, 1989. [7] (a) P. Metrangolo, H. Neukirch, T. Pilati, G. Resnati, Acc. Chem. Res. 38 (2005) 386–395; (b) T.R. Shattock, K.K. Arora, P. Vishweshwar, M.J. Zaworotko, Cryst. Growth Des. 8 (2008) 4533–4545; (c) K. Biradha, G. Mahata, Cryst. Growth Des. 5 (2005) 61–63; (d) B.Q. Ma, P. Coppens, Chem. Commun. 9 (2003) 504–505; Y. Mizobe, N. Tohnai, M. Miyata, Y. Hasegawa, Chem. Commun. (2005) 1839– 1841. [8] J. Gawron´ski, K. Gawron´ska, Tartaric and Malic Acids in Synthesis, Wiley, New York, 1999. [9] E.P. Serjeant, B. Dempsey, Ionization Constants of Organic Acids in Aqueous Solutions, Pergamon Press, Oxford, 1979. [10] S.A. Martin Britto Dhas, M. Suresh, G. Bhagavannarayana, S. Natarajan, J. Cryst. Growth 309 (2007) 48–52. [11] N. Vijayan, G. Bhargavannarayana, R. Ramesh Kumar, R. Gopalakrishnan, K.K. Maurya, P. Ramasamy, Cryst. Growth Des. 6 (2006) 1542–1546. [12] J. Mary Linet, S. Jerome Das, Mater. Chem. Phys. 126 (2011) 886–890. [13] S. Suresh, D. Arivuoli, J. Optoelectron. Biomed. Mater. 3 (2011) 63–68. [14] L.D. Barron, A.R. Gargaro, L. Hecht, P.L. Polavarapu, H. Sugeta, Spectrochim. Acta, Part A 48 (1992) 1051–1066. [15] Ralf Tonner, Peter Schwerdtfeger, Amanda L. May, Jeffrey D. Steil, Giel Berden, Jos Oomens, Shawn R. Campagna, Robert N. Compton, J. Phys. Chem. A 116 (2012) 4789–4800.

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