vibrational spectral and structural studies of nonlinear ... · vibrational spectral and structural...

Vibrational spectral and structural studies of nonlinear optical

crystal Zinc Salicylate Dihydrate

Meera J Nath1,3,4, B.R. Bijini2, S. D. D. Roy1,4, I. Hubert Joe3,4,* 1Department of Physics, Nesamony Memorial Christian College, Marthandam-629 165, Tamil Nadu, India

2Department of Physics, Mahatma Gandhi College, Thiruvananthapuram-695 004, Kerala, India 3Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram-695 015, Kerala, India

4ManonmaniamSundaranar University, Tirunelveli-627 012, Tamil Nadu, India *Corresponding author E-Mail: [email protected]

Abstract - Nonlinear optical active zinc salicylate dihydrate (ZSD) single crystals were grown by slow evaporation technique. The crystal structure was determined by single crystal X-ray diffraction technique. FT-IR, FT-Raman spectra have been recorded and analyzed. The equilibrium geometry, vibrational wavenumbers and second-order hyperpolarizability have been calculated with the aid of density functional theory method. The O-H bond length of the phenolic O-H group is 0.820 Å, and the calculated O-H bond length is 0.849 Å, which reflects a strong intramolecular hydrogen bond. The natural bond orbital and molecular electrostatic potential analyses confirm the occurrence of strong intermolecular hydrogen bonding responsible for the stabilization of the molecule. UV-Visible spectral analysis has been carried out to identify the various electronic transitions. The intercontact in the crystal structure has been analyzed using Hirshfeld surfaces analysis. Keywords -Hyperpolarizability, DFT, Hirshfeld, NBO, UV-Vis

I.INTRODUCTION Organic materials showing second harmonic generation (SHG) properties are of great interest owing to their wide range of applications in the field of telecommunication, optical information,optical storage devices and frequency conversion of lasers[1],[2]. Hydrogen bonding has a very significant role in the formation of new organic materials with enormous nonlinear optical (NLO) and electro-optic responses. Zinc is usuallytetrahedrally coordinated, but in some catalytic binding sites it is found pentacoordinated and rarely hexacoordinated[3].Salicylate ligand can bind to metal center as a monodentate [4], bidentate chelating (hydroxyl oxygen atoms and one carboxylate oxygen) [5], two oxygens of carboxylate group [6], and bridging bidentate carboxylate ligand [7]. Salicylic molecules exhibit a strong intramolecularhydrogen bond between the hydroxyl group and the neighbouring carbonyl group [8]. There are many theoretical and experimental investigations done on the ground or excited states intramolecular proton transfer of salicylic acid [9]-[13].The experimental geometrical parameters of ZSD in crystal form were determined in a crystallographic study carried out by Bijiniet al. [14].A detailed quantum chemical study will aid in making absolute assignments to the fundamental normal modes and in clarifying the obtained experimental data for the title molecule.

In the present work, we have reported the structural geometry, vibrational spectra of the title compound by B3LYP/6-

311++G(d,p) level of theory. The natural bond orbital (NBO) analysis has been performed to discover interaction between intramolecular charge transfer and hydrogen bonding. Hirshfeld surface have done to understand the various types of intermolecular interactions in themolecule. UV-Visible (UV-vis) spectral analysis hasbeen carried out to identify the various electronic transitions.

II. EXPERIMENTAL METHOD The X-ray diffraction data results of crystallized (ZSD) in a monoclinic crystal system with space group C2 and Z = 2 are

already reported [14]. The unit cell parameters are a = 15.4674Ǻ, b= 5.3431Ǻ, c = 9.1715Ǻ, β = 93.524°. And also studied FTIR spectrum, thermal analyses, nonlinear optical behavior using UV –Vis spectroscopy and SHG measurements were carried out. FT-IR spectrum of the sample was recorded in solid phase in the region from 4000 to 400 cm-1 using Perkin Elmer spectrometer at a resolution of 1 cm-1. FT-Raman spectrum was recorded in the region from 3500 to 50 cm-1 using Bruker RFS 27 with standalone model with excitation wavelength of 1064 nm, the spectral resolution of 2 cm-1. The UV-Vis absorption spectrum of the crystal was recorded in double distilled water solution using Varian Cary 100 B10 UV-vis spectrophotometer in the range of 200- 700 nm.

JASC: Journal of Applied Science and Computations

Volume V, Issue XII, December/2018.

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III.COMPUTATIONAL METHODS

All density functional theory (DFT) computations of ZSD have been performed using the Gaussian 09[15] program and visualize the results using GaussView 3.0 program [16]. The molecular geometry and vibrational wavenumbers were computed by the DFT/Becke-3-Lee-Yang-Parr (B3LYP) [17] function with 6-311++G(d,p) basis set. The calculated harmonic wavenumbers were scaled by a uniform scaling factor of 0.9673 to neglect the vibrational anharmonicity. The vibrational modes were identified by potential energy distribution (PED) by VEDA 4 program [18]. The Raman intensities were calculated using the basic theory of Raman scattering [19]. Natural bond orbital analysis was performed using NBO 3.1 [20] program. Hirshfeld surface map and fingerprint plots were generated using the program of CrystalExplorer 3.1 [21].

IV.MOLECULAR STRUCTURAL GEOMETRY The optimized structure with the atom numbering scheme for the compound is shown in Fig. 1. The selected structural

geometry parameters compared with their experimental XRD data [14] are given in Table 1. Obviously, the optimized bond lengths are slightly deviating than the experimental values. The correlation plot of experimental and theoretical bond length of ZSD is shown in the Fig. 2. The global minimum energy of molecular structure was obtained as -2935.57hartrees. The DFT calculation indicates that the metal coordination bond lengths Zn17-O13 (2.0543 Å) and Zn17- O32 (2.0525Å) in ZSD and the corresponding experimental bond length of Zn17-O13 and Zn17- O32 at 1.9833Å. This difference in the experimental and the theoretical values are due to the intermolecular interactions in the solid state [22]. The O33…H19 and O14…H37 bond lengths are 1.824 and 1.812Å respectively, which are significantly less than the van der Waals radii[23]. This indicates the presence of O-H…O hydrogen bonding with in the molecule. The C-C bond lengths of the molecule are observed in the range 1.369-1.483Å, which shows good agreement with the experimental values. The change in the dihedral angles C10-C1-C2-H3 (178.58º) and C1-C2-C4-H5 (179.47º) are due to the charge transfer interaction by establishing O-H…O hydrogen bonding.

Fig.1. Optimized molecular structure of ZSD Fig. 2. Correlation plot for ZSD

Table 1: Structural parameters of ZSD along with the corresponding experimental data aTaken from reference [15]

Bond length (Å) Bond angle (°) Dihedral angle (°)

Parameters Expt.a Calc. Parameters Expt.a Calc. Parameters Expt.a Calc. O12-Zn17 1.980 2.1953 C1-C10-C8 119.62 119.3472 C2-C1-Zn17-O32 96.83 168.8764

O13-Zn17 2.546 2.1227 C1-C11-O13 121.75 121.4034 C10-C1-Zn17-C30 -102.25 -56.6981

O14-H16 1.830 1.8142 C1-C11-O14 118.61 120.1848 C10-C1-Zn17-O31 14.16 78.1345

O14-Zn17 1.994 2.1333 O13-C11-O14 119.56 118.4107 C10-C1-Zn17-O32 -77.12 -10.318

Zn17-O31 1.980 2.1953 H18-O12-H19 111.73 107.5117 C1-C2-C4-C6 0.56 0.0629 Zn17-O32 2.625 2.1223 C10-O15-H16 109.48 109.6055 H3-C2-C4-H5 0.56 0.0289 Zn17-O33 1.994 2.1332 C11-Zn17-O12 109.81 113.3778 H3-C2-C4-C6 -179.41 -179.883 C27-C29 1.389 1.396 O13-Zn17-O14 55.78 60.6739 H9-C8-C10-C1 -179.46 -179.922 C29-O34 1.354 1.3297 O13-Zn17-C30 88.98 144.7063 H9-C8-C10-O15 -0.43 0.0697 C30-O32 1.246 1.2437 O13-Zn17-O31 147.49 101.7691 C1-C10-O15-H16 0.98 1.2057 C30-O33 1.276 1.2587 O13-Zn17-O32 92.44 172.5448 C8-C10-O15-H16 177.00 -178.786

O31-H36 0.849 0.9457 O13-Zn17-O33 88.58 114.5431 H18-O12-Zn17-C11 -74.92 -1.6295



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V. VIBRATIONAL SPECTRAL ANALYSIS The experimental and calculated wavenumbers computed at DFT level with B3LYP/6-311++G(d,p) basis set, along with

possible assignments and potential energy distribution (PED) are summarized in Table 2 the FT-IR and FT-Raman. The vibrational modes were assigned on the basis of PED analysis using the VEDA 4 program [24]. For optimal comparison the observed and calculated Infrared and Raman spectra are shown in Figs. 3 and 4, respectively.

The free hydroxyl group absorbs strongly in the 3600–3500 cm-1 region, but intramolecular hydrogen bonding can lower

the O–H stretching wavenumber to 3550–3200 cm-1 region with the increase in IR intensity [25],[26]. The band in the vicinity about 3600 cm-1 is typical for O-H group that does not establish an H bond; it allows us to unambiguously characterize such an O-H group [27],[28]. A very weak band appears at 3579 cm-1 is assigned to the free O-H stretching of the water molecule. Another very broad band at 3286 cm-1 is attributed to the intramolecular hydrogen bonded (O-H…O) phenolic O-H stretching. The O-H bond length of the phenolic O-H group is 0.820 Å, and the length of the O-H hydrogen bond is calculated as 0.849 Å, reflecting a strong intramolecular O-H…O hydrogen bond. The infrared bands are observed at 1467, 1386, 1335 cm-1and Raman bands 1468, 1388, 1341 are attributed to O-H in-plane bending vibrations and the corresponding calculated frequencies were observed at 1467, 1382 and 1361 cm-1. The in-plane bending O–H deformation vibration appears at 1314 cm-1 in IR and at 1309 cm-1 in Raman. The O-H in-planebending and out-of-plane bending vibrations values in ZSD are increasing, because of the hydrogen bonding effect through the carboxyl groups.

The vibrations assigned to aromatic C-H stretching in the range 3066–3144 cm-1 [29] agree with experimental

assignment 3067 cm-1 in both FT-IR and FT-Raman. In aromatic compounds, the C-H in-plane bending wavenumbersappear in the range of 1000–1300 cm-1 and the C-H out-of-plane bending vibration in the range 750-1000 cm-1 [30,31]. The C-H in-plane bending is assigned to the FT-IR and FT-Raman bands at 1314cm-1, 1238cm-1, 1193cm-1, 1153cm-1and 1309cm-1, 1230cm-1, 1188cm-1, 1157 cm-1, respectively which shows well agreement with the calculated values. Some of these vibrations are coupled with C-O stretching vibration. The C-H out-of-plane bending vibrations are observed at 761cm-1in IR spectrum, which is not observed in Raman spectrum. The ring C-C and C=C stretching vibrations bands are observed in the wavenumber range 1625–1280 cm-1 for the salicylic acid derivatives [32],[33],[34]. The actual positions of these modes are determined not so much by the nature of the substituent but rather by the form of the substitution around the ring. In the present work, C=C stretching modes are observed at 1626, 1599 cm-1 in the FT-IR and 1624cm-1, 1597 cm -1 in FT-Raman spectrum. Simultaneous activation of the C=C stretching mode of the salicylate ring in IR and Raman spectra provides evidence for the charge transfer interaction between the donor and acceptor groups and enhances NLO activity. Zn-O stretching and O-Zn-O deformations are probably contained in bands near 450-200 cm-1 [35]. The Zn-O stretching vibration is identified as a weak band at 424, 406 cm-1 in FT- IR,which is in good agreement with the computed results.

Fig. 3. Combined infrared spectra of ZSD

Fig. 4. Combined FT-Raman spectra of ZSD



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Table 2. Experimental and calculated B3LYP level vibrational wavenumbers (cm-1) with potential energy analysis.

ν-streching, γ- out-of-plane bending, τ-Torsion, ρ-rocking, β- in-plane bending, vs-very strong, s-strong, w-weak, vw-very weak, br-broad. aonly PED values greater than 10% are given

VI. NATURAL BOND ORBITAL ANALYSIS Natural bond orbital analysis was performed using the NBO 3.1 program implemented in Gaussian 09 program package

for the DFT method. Some significant orbital interactions and corresponding second-order perturbation energies derived from the NBO computation are listed in Table 3. This analysis has been performed to identify and confirm the possible intramolecular interactions between the units that would form the proper and improper hydrogen bonding [36].The importance of the hyperconjugative interactions and electron density transfer (EDT) from lone electron pairs of the Y atom to the X—H anti-bonding orbital in the X—H…Y system have been analysed thoroughly [37]. In general, such interaction leads to an increase in population of X— H anti-bonding orbital. The increase of electron density in the X—H anti-bonding orbital weakens the X— H bond, which leads to its elongation and concomitant red-shift of the X—H stretching wavenumber. The larger the E (2) values, themore intensive is the interaction between electron donors andelectron acceptors. The intramolecular O-H…O hydrogen bond are formed by the orbital overlap between LP (O14, O33) andσ*(O15-H16& O34-H35), which results in intermolecular charge transfer causing stabilizing of the hydrogen bondedsystem. The significant decrease of electron density in σ*(O15-H16) is due to the large interaction of LP (O14) are significant and can be used as a measure of the intramolecular charge delocalization. The most important intramolecular interaction (n-σ*) energies related to the resonance in the molecules are electron donating from LP (O15) atom to anti-bonding acceptor σ*(C1-C10) atoms. The stabilization energy E(2) associated with the resonance interaction energy 6.84kcalmol-1.Another energetic hyperconjugative interactions and their energiesare σ*(O31-H36), σ*(O34-H35), σ*(O15-H16) are 7.44, 7.82 and 5.66 kcalmol-1, respectively.

Scaled Wavenumber (cm1)

Experimental Wavenumber (cm-1)

Relative intensity Assignment with PED (%)a

IR Raman IR Raman

3812 3574vw - 52.20 15.36 ν O-H (H2O) (100) 3325 3286br - 20.18 9.23 ν O-H (99) 3065 3067vw 3067w 8.79 221.57 ν C-H(90)

1608 1626w 1624w 268.42 514.26 νC=C (53)

1579 1599s 1597w 159.31 9.89 ν C=C (75)

1468 1488w 1491vw 133.58 602.14 β C=C(60), β O-H(20)

1467 1467vs 1468w 188.74 200.08 β O-H(31)

1382 1386s 1388w 43.98 711.24 β O-H(56), β C-C(18)

1361 1335s 1341w 0.24 2600.85 β O-H (86)

1301 1314vw 1309w 10.14 2171.13 β C-H (79)

1240 1238vs 1230w 182.44 421.57 β C-H(62), ν C-O(21)

1144 1193vw 1188vw 3.99 170.74 β C-H(75)

1143 1153s 1157w 40.19 117.29 βC-H(63)

864 876w 876vw 1.82 1262.63 βC-O-O(51), τring(35)

814 820w 822w 11.56 3738.78 τ ring(46)

760 761vs - 66.63 22.79 γ O-H (84)

692 674s 682vw 11.27 10.28 γ C-H (75) 567 570w 571vw 271.14 546.16 ρH2O(62) 539 530s 538vw 22.12 20.45 τ ring(63), τ C-O-H(46) 428 424vw - 0.49 6.33 ν Zn-O (53), γC-H(20)

407 406s - 98.04 16.24 ν Zn-O (40), τC-O-H(13)

401 - 402vw 40.77 274.09 τC-O-H(40), τ H2O(35)



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Table 3.Second-order perturbation theory analysis of Fock matrix in NBO basis

Donor ED(e) Acceptor ED(e) E(2)a

kcal mol-1 E(j)-E(i)b

a.u F(i,j)c

a.u.

π(C6–C8) 1.97791 π*(C1–C10) 0.43394 23.82 0.27 0.075

π(C6–C8) 1.97791 π*(C2–C4) 0.29510 15.55 0.29 0.060

LP1(O14) 1.93807 σ*(O15-H16) 0.04548 10.12 1.11 0.095

LP1(O15) 1.97497 σ*(C1–C10) 0.03651 6.84 1.11 0.078

σ(C20-C29) 1.97436 σ*(C21–C23) 0.29510 23.07 0.29 0.075

σ(C20-C29) 1.97436 σ*(C25–C27) 0.01348 13.68 0.29 0.058

σ(C21–C23) 1.97938 σ*(C20–C29) 0.03588 15.16 0.27 0.059

σ(C21–C23) 1.97938 σ*(C25–C27) 0.01348 22.63 0.29 0.072

σ(C25–C27) 1.97810 σ*(C20–C29) 0.03588 23.79 0.27 0.075

LP2(O14) 1.83524 σ *( O15-H16) 0.04548 5.66 0.79 0.061

σ(C25–C27) 1.97810 σ*(C21–C23) 0.29510 15.55 0.29 0.060

σ(O31-H36) 1.99507 σ*(O34-H35) 0.04547 7.82 3.31 0.145

LP2(O32) 1.81871 σ*(C20–C30) 0.06276 14.19 0.84 0.100

LP2(O31) 1.91505 σ*(O31-H36) 0.00113 7.44 1.09 0.082

σ(O31-H37) 1.99557 σ*(O34-H35) 0.04547 83.45 3.29 0.473

LP1(O33) 1.93809 σ*(O34-35) 0.04547 3.52 3.20 0.095

aE(2) means energy of hyperconjugative interactions bEnergy difference between donor and acceptor i and j NBO orbitals cF(i,j) is the Fock matrix elements between i and j NBO orbitals

VII.NATURAL HYBRID ORBITAL ANALYSIS The natural hybrid orbitals (NHOs) result from a symmetricalyorthogonalized hybrid orbital, which is derived from natural

atomic orbital (NAO) centered on particular atoms through a unitary transformation. The direction of each hybrid is specified in terms of the spherical polar angles theta (θ) and phi (Ψ) from the nucleus as well as the deviation angle Dev. from the line of centers between the bonded nuclei. The angular properties of the natural hybrid orbitals are very much influenced by the type of substituent that causes conjugative effect or steric effect.Thedirection of geometry changes due to the geometrical optimizationcan be predicted using the data attained from Table 4. Oxygen of σ(O-H) is more bent away from the line of O34–H35 centres by 4.8° as a result of lying in the strong charge transfer path, whereas the hydrogen NHO is approximately aligned with the O–H axis.



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Table 4.NHO directionality and ‘‘bond bending’’ (deviations from line of nuclear centers) of H-bonded NBOs in ZSD

NBO Line of centers Hybrid 1 Hybrid 2 θ Ψ θ Ψ Dev. θ Ψ Dev. σ (C1 - C10) 136.4 240.9 137.5 242.3 1.5 47 56.4 4.7 σ (C2 - C4) 86.8 201.8 -- -- -- 94.2 21.2 1.2 σ (C6 - C8) 141 338.2 140.3 339.9 1.3 -- -- -- σ (C 10 - O15) 142.4 334.9 140.7 338.7 2.9 -- -- -- σ (C11 - O13) 43.8 59 45.3 57.5 1.8 134.8 236.8 2 σ (C11 - O 14) 141 340.7 139.6 343.6 2.3 39.9 162.3 1.3 σ (O15 - H 16) 84.1 26 88 23.3 4.8 -- -- -- σ (C20 - C29) 43.6 299.1 42.3 297.4 1.8 133 123.5 4.5 σ (C21 - C23) 93.2 338.2 -- -- -- 85.8 158.8 1.1 σ (C25 - C27) 39 201.8 39.8 200.1 1.3 -- -- -- σ (C29 - O34) 37.6 205.2 39.3 201.2 3 -- -- -- σ (C30 - O32) 136.1 121 134.7 122.5 1.7 45.2 303.2 2 σ (C30 - O33) 39 199.3 40.4 196.4 2.3 140.1 17.8 1.3 σ (O34 - H35) 95.9 154 91.9 156.7 4.8 -- -- --

VIII.NATURAL POPULATION ANALYSIS The natural population analysis (NPA) was performed at B3LYP/6-311++ G(d,p) level of theory in order to assess the

distribution of electron density of the molecule. The population analysis is a mathematical way of partitioning the wave function or electron density. It shows an improved numerical stability and portrays the distribution of electrons in a better way [38]. Fig. 5 reveals the atomic charge distribution of ZSD.

Fig. 5. Natural atomic charge distribution chart of the ZSD molecule

In NPA atomic charges show that the all carbon atoms are negative except C10, C11, C29 and C30; those are bonded with

oxygen atoms. This finding suggests that the O atoms are the preferred site for protonation. Zn17 is more protonated and O12 is more deprotonated, indicating the charge delocalization in the molecule. The atoms C11 and C20 shows more positive charge (0.7856e) and atoms C11 and C30 shows more negative (-0.2397e) , which suggest extensive charge delocalization in the entire molecule through C1-C11 and C20-C30. The hydrogen atom attached to the carbon atoms possesses similar values of positive charges. Among the hydrogen atoms H16, H18, H19, H35, H36 and H37 shows the highest positive charges, being involved in the intramolecular hydrogen bonding.

IX. SECOND-ORDER HYPERPOLARIZABILITY ANALYSIS

The nonlinear optical (NLO) properties of molecules have beeninvestigated using quantum chemical computation by calculating the second-order hyperpolarizability. The computational approachpermits to determine the efficiency of NLO properties of the molecule as an economical way to design the molecules by evaluating their potential before synthesis. The average second-order

hyperpolarizability (γ) is calculated using the relation

<γ>= γXXXX +γYYYY+γZZZZ +2γXXYY+γXXZZ +γYYZZ

The calculated second-order hyperpolarizability is -2.62x10-34 e.s.u. Thus, the theoretically predicted value indicates that the title compound has third-order nonlinear optical property.



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X. FRONTIER MOLECULAR ORBITAL ENERY ANALYSIS

HOMO and LUMO are called the frontier orbitals since they determine the way the molecule interacts with other species. HOMO-LUMO energy gap was calculated at the B3LYP/6-311++G(d,p) level of theory. Both the HOMO and the LUMO are the main orbitals taking part in a chemical reaction. The HOMO energy characterizes the ability of electron giving, whereas the LUMO characterizes the ability of electron accepting, and the energy gap between HOMO and LUMO characterizes the molecular chemical stability. The effect on the LUMO levels is stronger when an electron accepting group is present. The HOMO (-6.420 eV) and LUMO (-1.62 eV) energies indicate a charge delocalization taking place within the molecule. The HOMO–LUMO orbitals are shown in Fig. 6. From the figure, HOMO is scattered over the water moiety and zinc atom and LUMO is scattered over the salicylate moiety, which shows ICT takes place within the entire molecule. The lowering of HOMO–LUMO energy gap (4.8eV) enhances this result. Due to the presence of intramolecular charge transfer interaction thehyperpolarizability value is increased and hence enrich the NLO activity of the molecule.

Fig. 6. (a) HOMO and (b) LUMO plots of ZSD

XI. ELECTRONIC ABSORPTION SPECTRAL ANALYSIS Electronic transitions in ZSD have been investigated by UV-visspectrum taken in water phase is compared with simulated UV-visspectrumof ZSD in water using DFT-B3LYP/6-311++G(d,p) level of theory. The calculated three lowest energy transitions of the molecule with their vertical excitation energies, oscillator strength (f), wavelength, transition assignments and their percentages are summarized in the Table5. Table 5. Calculated absorption wavelengths, energies and oscillator strengths of ZSD using DFT-B3LYP/6-311++G(d,p) level of basis set.

The UV-visible spectrum of LLS taken in the wavelength range 800-200 nm is shown in the Fig. 7. The spectrum shows

that the maximum absorption (λmax) is found to be at 240 nm. It is worth mentioning that the band gap (4.955 eV) calculated by Tauc′s plot method [39], which is extrapolating the straight line portion of (αhν)2 versus hν graph shown in the Fig. 7, obtained from the UV-vis spectrum shows good consistency with DFT calculated HOMO-LUMO energy gap (4.79 eV). The broadening and intensity enhancement of the UV-visible absorption spectra gives the evidence for intramolecular charge-transfer interactions in the title molecule.

Excitation Wavelength (nm) Energy kJ mol-1

Oscillator strength (f)

Transition Assignments

96-97 290.07 4.2743 0.1748 H→L+1 (67%)



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Fig. 7. UV-visible absorption spectrum of ZSD in the region 800-200 nm (insert Tauc’s plot)

X. HIRSHFELD SURFACE ANALYSIS Hirshfeld surface and their associated two-dimensional (2D) fingerprint plots [40] have been used describe the surface

characteristics of the molecules and to quantify the various intermolecular interactions in the title compound. The Hirshfeld surface of a molecule is mapped using the descriptor dnorm which encompasses two factors: one is de, representing the distance of any surface point nearest to the internal atoms, and the other one is di, representing the distance of the surface point nearest to the exterior atoms and with the van der Waals radii of the atoms [41]. Corresponding regions are visible in the 2D fingerprint plots where one molecule acts as donor (de>di) and the other as an acceptor (de<di) [42]. The molecular Hirshfeld surface (shape index) of ZSD is shown in Fig. 8.

Fig. 8. Hirshfeld surface mapped with shape index for the ZSD

The shape index is the most sensitive to very delicate changes in surface shape, the information conveyed by shape index are consisted with the 2D fingerprint plots. The 2D fingerprint plots can be decomposed to highlight particular atom pair close contacts. This decomposition enables separation of contributions from different interaction types, which overlap in the full fingerprint. The Hirshfeld surface analysis of molecule showed H---H interactions(37.3%), which revealed that the main intermolecular interactions were H---H interactions. The intermolecular interactions of the title compound are shown in the 2D fingerprint plots shown in Fig. 9. O-H/H-O (30.4%), interactions are represented by left-side blue spikes, top and bottom. The C-H regions while the green C-H/H-C interactions (24.5%) are between the C…O and H…H regions.



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Fig. 9. Fingerprint plots of ZSD

XI. CONCLUSION The structural parameters, vibrational assignments and NBO were carried out using DFT computation at B3LYP/6-

311++G(d,p) level of basis set.FT-IR and FT-Raman spectra were compared with the computational wave numbers. The experimental values agreed well with the theoretical results.The simultaneous occurrence of IR and Raman bands explain the presence of intramolecular charge transfer within the molecule, which is responsible for the optical nonlinearity of the crystal. The Hirshfeld surface analysis with fingerprint plots and electrostatic potential map reveals the percentage of intermolecular interactions and distribution of electrostatic potential of the title compound.NBO and NPA analyses reveal theintramolecular O-H…O hydrogen bonding and charge distribution takes place within the molecule.The calculated value of second-order hyperpolarizability and low value of HOMO-LUMO energy gap exposes the nonlinear optical properties of the title compound.

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