Surface enhanced raman spectroscopy studies on triglycine sulphate single crystals

Accepted Manuscript Surface enhanced raman spectroscopy studies on triglycine sulphate single crystals A. Parameswari, R. Mohamed Asath, R. Premkumar, A. Milton Franklin Benial PII:

S0022-2860(16)30928-0

DOI:

10.1016/j.molstruc.2016.09.012

Reference:

MOLSTR 22924

To appear in:

Journal of Molecular Structure

Received Date: 5 July 2016 Revised Date:

3 September 2016

Accepted Date: 5 September 2016

Please cite this article as: A. Parameswari, R. Mohamed Asath, R. Premkumar, A. Milton Franklin Benial, Surface enhanced raman spectroscopy studies on triglycine sulphate single crystals, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.09.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Surface Enhanced Raman Spectroscopy Studies on Triglycine Sulphate Single

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Crystals

A. Parameswari1, R. Mohamed Asath1, R. Premkumar1, A. Milton Franklin Benial1* 1

PG and Research Department of Physics, N.M.S.S.V.N. College, Madurai-625 019, Tamilnadu,

*Author for correspondence: Dr. A. Milton Franklin Benial Ph. D

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India.

Associate Professor, Department of Physics NMSSVN College, Nagamalai

TEL: +91-9486468945

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Fax: +91-0452-2458356

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Madurai-625 019, Tamilnadu, India

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E-mail: [email protected] (A. Milton Franklin Benial)

Manuscript information Total Word Count

: 6,500

Number of Text pages

: 25

Number of Figures

: 10

Number of Tables

:4 1

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Abstract Adsorption characteristics of triglycine sulphate (TGS) on silver (Ag) surface were

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investigated based on density functional theory calculations and surface enhanced Raman spectroscopy (SERS) technique. The single crystals of TGS were grown by slow evaporation method. Ag nanoparticles (Ag NPs) were prepared by solution combustion method and The calculated and observed structural parameters of TGS molecule were

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characterized.

compared. Raman and SERS spectra for TGS single crystal were studied experimentally and

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validated theoretically. Frontier molecular orbitals (FMOs) analysis was carried out for TGS and TGS adsorbed on Ag surface. The second harmonic generation measurements confirm the nonlinear optical (NLO) activity of the TGS molecule. SERS spectral analysis reveals that the TGS adsorbed as tilted orientation on the silver surface. The theoretical and experimental results

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evidence the suitability of the grown TGS single crystal for optoelectronic applications.

Keywords: Triglycine sulphate; Silver nanoparticles; NLO; Combustion method; Surface

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enhanced Raman scattering; Density functional theory.

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1. Introduction The study of chemical and physical process at interfaces is fundamental to the surface

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sciences and applied sciences. Fleischmann and coworkers discovered the novel phenomenon, surface enhanced Raman spectroscopy (SERS) [1] and reported an extraordinary million-fold enhancement of Raman signal from pyridine molecules adsorbed onto electrochemically

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roughened silver electrode compared to that from free molecules in liquid environment. This enormous enhancement of Raman scattering cross section from molecules adsorbed to metallic

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nanostructures attracted considerable interest. SERS has received much attention in various fields of science including physical chemistry, analytical chemistry, and biomedical sciences [2,3]. It has become a powerful tool to study structures of biological molecules and materials such as amino acids [4,5], nucleic acids [6,7], proteins [8,9], drug-target complexes. Raman scattering by molecules adsorbed on rough metal surfaces has been observed both

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experimentally as well as theoretically is an interesting phenomenon [10]. SERS is accredited to the long-range electromagnetic (EM) enhancement and the short-range chemical enhancement (CE), which has been discussed in many reviews [11]. SERS technique has received increasing

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attention and become a powerful spectroscopic technique due to its narrow bandwidth as well as

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the ability to perform multiplexed analysis using a single laser wavelength [12]. Moreover, SERS possesses other inherent benefits, such as operation over a wide range of excitation wavelengths, reduced photo bleaching and highly resolved spectroscopic bands [13]. SERS phenomenon exhibits only a few metals such as silver, gold and copper and these

metals are considered efficient SERS active surfaces. Generally, on a silver surface chemisorption can take place through the formation of a bond between the Ag surface and an adsorbed molecule, e.g. Ag–N, Ag–O, Ag–S, or Ag–X (X = halogen) bond [14–17] . In recent 3

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years, silver nanoparticles have attracted a lot of attentions due to their good conductivity and chemical stability [18]. Silver is the most widely used substrate due to its broad plasmon

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resonance in the visible region, high stability and easy preparation [19]. The adsorption of biomolecules on the metal substrates has been widely studied by many researchers due to its potential applications. SERS has become a prominent tool for monitoring the changes in the

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chemical nature of the molecule adsorbed on Ag NPs.

Density functional theory (DFT) method provides valuable information regarding the

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molecular reactivity, which plays a key role in the design of new compounds and in understanding the various biological systems [20]. The DFT calculations are established to correlate well with the experimental spectroscopic techniques [21]. DFT provides excellent vibrational frequencies of organic compounds, if the computed frequencies are scaled to compensate for the electron correlation; basis set deficiencies and anharmonicity [22].

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The novel and more efficient nonlinear optical (NLO) materials have a wide range of applications in fast data transfer, optical frequency conversion, electro-optical modulation, dynamic holography, optical writing and optical guiding of laser beams [23]. The higher

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molecular first order hyperpolarizability (β) which leads to the higher susceptibility in an organic

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molecule arises due to the transfer of proton from the electron donor group to the electron acceptor group. Triglycine sulphate (TGS) single crystals have shown promising applications in various devices including military systems, astronomical telescopes, laser beam characterization, environmental analysis monitors and infrared detector for scientific studies. TGS crystals have been focused in various aspects such as growth rate, structural modification, pyroelectric, mechanical, optical, ferroelectric properties and to overcome the depolarization effect by a number of authors [24,25]. 4

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Recently, effect of doping an organic molecule ligand on TGS single crystals was studied [26]. Growth and characterization of L-tryptophan-doped ferroelectric TGS crystals was

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reported [27]. Effect of amino acid doping on the growth and ferroelectric properties of triglycine sulphate single crystals was carried out by Raghavan et al [28].

Growth and

characterization of biadmixtured TGS single crystals was analysed [29]. Simple maccroscopic

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model of pyroelectric phenomenon in TGS single crystals in ferroelectric phase was investigated [30]. Electronic structure and related properties of the ferroelectric triglycine sulphate crystal

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was studied by Andriyevsky et al [31]. Studies on the growth, structural, optical and mechanical properties of ADP admixtured TGS crystals were reported [32]. Growth, structural, mechanical, spectral and dielectric characterization of NaCl-added Triglycine sulphate single crystals was presented [33]. Toshio Kikuta et al., reported the growth and ferroelectric properties of L-, Dand DL-methionine-doped triglycine sulphate crystals [34]. Studies on the effect of L- glutamine

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on morphology, structure, optical, mechanical and electrical properties of TGS crystal was carried out [35]. The effect of nitric acid (HNO3) on growth, spectral, thermal and dielectric properties of triglycine sulphate (TGS) crystal was analysed [36].

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In this present study, the effect of Ag NPs on the enhancement of SERS spectra and the

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orientation of TGS on the Ag NPs surface was investigated by the experimental and DFT methods. The present investigation has been developed as a model system to understand the interaction of Ag NPs with TGS single crystals. 2. Materials and methods

2.1. Computational details The molecular structure of TGS was optimized by the DFT/B3LYP method with 6311G(d,p) basis set using Gaussian 09 program [37]. The molecular structure of TGS adsorbed 5

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on Ag surface was optimized by the density functional theory (DFT) level using B3PW91 functional and LANL2DZ basis set. Silver cluster, Ag3, identified to be stable and reactive,

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which has been considered in this study for the theoretical analysis of adsorption characteristics [38]. The structural parameters, Raman vibrational frequencies and frontier molecular orbital calculations were also carried out for TGS and TGS adsorbed on silver surface (TGS-Ag). The

distribution (PED) calculation using the VEDA 4.0

program [39] and also visualized by

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GaussView 05 program [40].

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vibrational modes were assigned with a high degree of accuracy on the basis of potential energy

2.2. Experimental 2.2.1. Materials

Silver nitrate (AgNO3), citric acid (C6H8O7), hydrogen sulphate (H2SO4) and glycine (C2H5NO2) were purchased from Sigma Aldrich Chemical Co, St. Louis, Mo, USA. All glass

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wares were properly washed with deionised water and dried in hot air oven before use. 2.2.2. Preparation of Silver nanoparticles

Stoichiometric amount of silver nitrate (1.6987 g) was dissolved in deionised water (10

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mL) and then citric acid (0.277 g) was added into it. The solution was kept in the furnace at

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300°C. With large amount of fumes, the combustion reaction was completed in 15 mins and loose powder was formed. This was crushed and ground thoroughly [41]. 2.2.3. Synthesis of triglycine sulphate single crystal TGS single crystal was synthesized by taking analar grade glycine and concentrated

sulfuric acid (H2SO4) in the ratio 3:1. The required volume of concentrated sulphuric acid was diluted with deionised water. Then the calculated amount of glycine salt was slowly dissolved in the diluted sulphuric acid. Glycine reacts with sulphuric acid as follows.

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3(NH2CH2COOH) + H2SO4 → (NH2CH2COOH)3 (H2SO4) The solution was subjected to slow evaporation and extreme care was taken while

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crystallization. The solution temperature was maintained below 60˚C to avoid oxidation of the glycine and recrystallization was done to reduce the impurity content in the crystallized salt [42]. Well transparent TGS single crystals were obtained in 27 days.

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2.2.4. Characterization Techniques

The single crystal X-ray diffraction (XRD) analysis of the grown crystal was carried out

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to identify the structural and cell parameters using Bruker AXS Kappa Apex2 CCD diffractometer. Powder X-ray diffraction (XRD) patterns of Ag NPs were recorded on PANalytical X-ray diffractometer using Cu Ka radiation (λ=1.5406 Å) operated at 40kV and 30mA and the measurements were taken for the diffraction angle range of 2θ = 30-800. High resolution-transmission electron microscope (HR-TEM) images were recorded using a JEOL

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3010, with a lattice resolution of 0.14 nm and a point to point resolution of 0.12 nm, operating at 200kV. The samples were made by depositing the Ag NPs on a carbon coated Cu grid and the size measurements were performed manually on HR-TEM images.

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The optical absorption measurements were carried out using Shimadzu UV-3600 UV-

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Vis-NIR spectrophotometer (Shimadzu Scientific Instruments, Columbia, MD) in the +wavelength region 200-500 nm using ethanol as a solvent. Nonlinear optical property of the grown TGS crystals was studied by Kurtz and perry SHG test [43]. A Q-switched Nd :YAG laser (1064 nm, Quanta ray series) and Coherent Molectron powermeter, USA was used as light source. A laser beam of fundemental wave length 1064 nm, was made fall normally on the sample cell. The sample was crushed into the fine powder and tightly packed in a micro capillary tube. The power of the incident beam was measured using a power meter. The input laser energy 7

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incident on the powdered sample was chosen to be 0.68 J. The Raman spectrum was recorded by micro Raman system using Horiba-Jobin Yvon LABRAM-HR with He-Ne laser and an

was 1cm-1 and dispersive geometry was employed.

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excitation wavelength of 632 nm. The power of the laser source is 5mW. The spectral resolution

The synthesized TGS crystal was compressed and crushed into a fine powder using

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scientific grade mortar and pestle instrument. The mortar and pestle is standard equipment for grinding small quantities of samples to a fine powder [44]. The particle size is also related to

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SERS effect of TGS on AgNPs. Dynamic Light Scattering (DLS) apparatus (B08631AA, Beckman Coulter, California, USA) was used in this study with a 35 mW He-Ne laser of 632.8 nm, which measures the particle size distribution, displayed as intensity histogram using the Delsa Nano UI Software program [45]. The DLS spectrum of TGS fine powder is shown in Fig 1 as the intensity histogram. The average diameter of the particle size was estimated as ~ 393 nm.

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SERS spectrum was recorded by mixing solid form of silver nanoparticle and TGS in the ratio of 1:3. From the mixture, a fraction of the sample was taken for SERS measurement. The experimental conditions were maintained same for both nRs and SERS measurements.

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3.1. XRD Analysis

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3. Results and discussion

The single crystal-XRD is a nondestructive tool to analyze crystal structure of

compounds. Fig 2 shows the photograph of single crystal of TGS. From single crystal-XRD data, the lattice parameters of TGS was found to be, a = 5.712 Å, b = 12.561 Å, c = 9.119 Å, α = 90º, β = 105.69º , γ = 90º with a monoclinic structure and belongs to the space group of P21/m [46]. The crystal data, details of data collection and structure refinement are given in Table 1. 8

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The X-ray diffraction pattern of synthesized Ag NPs was compared with the reflection peak characteristics of Ag NPs and the observed XRD peaks were found to be correlating well

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with the different crystallographic planes. Fig 3 shows the XRD pattern of the synthesized Ag NPs, with four different peaks at 2θ = 37.78º, 43.96º, 64.16º and 77.12º corresponding to the Bragg’s reflection from the (111), (200), (220) and (311) planes respectively. The XRD pattern

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of the Ag NPs, which belongs to the face centered cubic structure with lattice parameters, a = b = c = 4.078 Å; α = β = γ = 90º [47]. The observed data agree well with the experimental values

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(ICDD 04-0783). The crystalline size is calculated as ~ 33 nm using the Debye-Scherrer formula. 3.2. HR-TEM Analysis

The morphology and crystalline structure of the synthesized Ag NPs were further analyzed using the image taken on HR-TEM and are shown in Fig 4. HR-TEM is a high-spatialresolution structural tool and provides exact information about the particle size and shape. HR-

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TEM image indicates that the dispersed Ag NPs are crystalline in nature and almost spherical in shape with the uniform particle size of ~ 35 nm. 3.3. UV-Visible Analysis

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The theoretical and observed UV-Vis spectra of the TGS molecule were shown in Fig

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5(a) and 5(b). From the optical absorption spectrum, the crystal is transparent in the range 300600 nm without any absorption peak and it reveals the good optical quality of the grown crystal. High transmission in the whole visible region for TGS crystal shows the suitability of the crystal for the use of UV tunable laser and second harmonic generation (SHG) device applications. For TGS molecule, electronic transition was calculated at 207 nm with excitation energy values of 6 eV. The transition was observed at 215 nm with excitation energy value 5.78 eV. The calculated parameters show that the peak arises due to the bonding and anti-bonding π electron transitions 9

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of the molecule. The band gap was calculated using the formula, Eg = hc/ λ. The optical band gap is found to be 5.44 eV, which is in good agreement with the reported value in the literature [48].

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3.4. Structural properties Fig 6 depicts the optimized structure of TGS before and after adsorption on a three atom silver cluster (TGS and TGS-Ag). Table 2 shows the optimized structural parameters such as

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bond length, bond angle and dihedral angle of TGS and TGS-Ag calculated by DFT method and compared with the experimental values obtained from the single crystal XRD data. The unit cell

G3 are in the form of glycinium ions.

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of TGS contains three types of glycines G1, G2, and G3. G1 is in the form of Zwitter ion. G2 and

The bond length between carbon and oxygen (C1-O20) atom is calculated as 1.324 Å, which is increased as 1.335 Å when adsorbed on silver cluster. The increase in bond length is

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due to the interaction between the silver (Ag40) and oxygen atom (O20) of TGS molecule. The increase in bond length was obtained between the oxygen and hydrogen atom (O20-H21), which is due to the interaction between the silver and oxygen atom. The bond angle between carbon,

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oxygen and hydrogen (C1-O20-H21) in TGS is calculated as 109˚. The computing bond angle (C1-O20-H21) is also increased as 116˚ in the presence of silver cluster, which indicates that the

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TGS molecule adsorbed on silver cluster. An intramolecular hydrogen bond is formed between the O19 and H21 atom in G3 molecule with the bond length of 2.336 Å, which is also increased as 2.507 Å when adsorbed on silver cluster. The optimized minimum energy is calculated as 1553.79 a.u for TGS and -1602.39 a.u when adsorbed on silver cluster. The dipole moment of TGS is calculated as 16.82 D which is also increased as 25.33 D when adsorbed on silver cluster. A large dipole moment plays an important role in stabilizing the structure. The calculated 10

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vibrational frequencies for both TGS and TGS-Ag were real values, which imply that the optimized geometry of TGS and TGS-Ag are located at the local minimum on the potential

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energy surface. 3.5. Frontier Molecular Orbitals (FMOs) analysis

The highest occupied molecular orbital (HOMO) energy and lowest unoccupied

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molecular orbital (LUMO) energy are called as FMOs, which is used to determine the way in which a molecule interacts with other species. HOMO and LUMO of TGS and TGS-Ag are

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presented in Fig 7. In TGS, HOMO is located mainly around the zwitterionic form of glycine (G1) and the carboxylic group of glicinium ion (G2), while the LUMO tends to form specific anti bonding orbitals around the hydrogen atoms, which belongs to methylene and the carboxylic group of glicinium ion (G3). The band gap of TGS was found to be 5.632 eV, which agrees well

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with the previously reported value [48]. The calculated band gap indicates the kinetic stability of the compound. However, in the case of TGS-Ag, the anti-bonding LUMOs extend to the silver cluster, whereas the HOMOs are mainly spread over silver cluster and the oxygen atoms of

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glicinium ion (G3). The shift of molecular orbitals from TGS to the silver cluster in the LUMO of TGS-Ag accounts for the intramolecular charge transfer. It is also interesting to note that,

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the band gap of TGS (5.632 eV) is significantly reduced after adsorption on silver surface (1.748 eV) and the shifting of MOs can be correlated to the transfer of charge associated with the process of adsorption. The theoretical results shows that the HOMO energy of the molecule ~ (-7.673 eV) which is energetically much lower than the Ef of silver (~5.48 eV) [49]. Hence, the metal to molecule charge transfer interaction is more preferred.

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3.6. First order hyperpolarizability The first order hyperpolarizability (

) was calculated based on the finite field

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approach. In the presence of an applied electric field, the energy of a system is a function of the electric field. The first order hyperpolarizability is a third rank tensor, which can be described by a 3×3×3 matrix. The 27 components of the 3D matrix can be reduced to 10 components because

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of the Kleinman symmetry [50]. The matrix can be given in the lower tetrahedral format, which is obvious that the lower part of the 3×3×3 matrices is a tetrahedral. The components of β are

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defined as the coefficients in the Taylor series expansion of the energy in the external electric field. When the external electric field is weak and homogeneous, the Taylor series expansion becomes

…….

is the energy of the unperturbed molecules,

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where

is the field at the origin,

,

and

are the components of the dipole moment, polarizability and first order hyperpolarizability,

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defined as

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respectively. The mean first order hyperpolarizability βtot using the x-, y- and z-components are

The first order hyperpolarizability (βtot) of TGS molecule was calculated by the DFT

/B3LYP method with 6-311G(d,p) basis set. The calculated first order hyperpolarizability components of the TGS molecule were listed in Table 3. The potential application of the molecule in the field of nonlinear optics demands the investigation of its structural and bonding features contributing to the hyperpolarizability enhancement. The calculated first order 12

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hyperpolarizability of TGS molecule was obtained as 4.443 x 10-30 esu, which was 0.719 times that of KDP (6.174 x 10-30 esu). The TGS sample with a second harmonic signal energy of 5.2

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mJ was obtained. The reference sample KDP gave a SHG signal energy of 8.8 mJ for the same input beam energy. Thus the SHG efficiency of the TGS crystal has the relative SHG efficiency of 0.59 times that of KDP. This result confirms the NLO activity of the TGS molecule.

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3.7. SERS Analysis 3.7.1. Vibrational Assignments

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TGS has 37 atoms and 105 normal modes of vibrations and TGS-Ag has 40 atoms and 114 normal modes of vibrations, which belong to the same symmetry species (A). All vibrational modes of TGS are both IR and Raman active because of its C1 point group symmetry. In this section, the assignment and analysis of vibrational frequencies of functional groups, NH3+, CH2,

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COO- and SO42- are discussed. The calculated and observed vibrational frequencies were assigned on the basis of potential energy distribution (PED) calculation and listed in Table 4. Fig 8 shows the simulated nRs and SERS spectra of TGS. Fig 9 shows the experimental nRs and

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SERS spectra of TGS. The calculated vibrational frequencies of TGS and TGS-Ag wereobserved as real values, which imply that the optimized geometries located at the local minimum

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potential energy surface. The calculated vibrational frequencies were scaled in order to account for the anharmonicity in DFT calculations. The scaling factors of TGS were calculated by the formula [51].

C=

∑γ *ω ∑ω i

i

2

i

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Where, C is the scaling factor, γi and ωi are experimental fundamental frequency and theoretical harmonic frequency respectively. The uniform scaling factors used for nRs and SERS

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simulated spectra of TGS are 0.989 and 0.986 respectively. 3.7.2. NH3+ vibrations

In general, the amino acids in the zwitterion form exhibit NH3+ stretching vibrational

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frequencies in the region of 3100-2600 cm-1 as a strong broad band [52,53]. In the present study, the calculated NH3+ stretching frequency appeared at 3030 and 3005 cm-1 in nRs and at 3032

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and 3002 cm-1 in SERS spectra. The corresponding observed NH3+ stretching frequency appeared at 3030 and 2992 cm-1 as a strong band in nRs, which is not observed in SERS spectra. Usually, the amino acids show weaker Raman band in the region1660-1590 cm-1 due to the asymmetric deformation mode of the NH3+ group [54]. The asymmetric NH3+ deformation mode was

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calculated at 1651 and 1620 cm-1 in nRs spectrum and SERS spectrum respectively. The same mode was observed at 1678 and 1653 cm-1 as a weak peak in both nRs and SERS spectra. Normally, the NH3+ symmetric deformation mode of amino acid appears in the region 1550-1485

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cm-1 with variable intensity [54]. In nRs spectrum, the calculated symmetric NH3+ deformation was appeared at 1570 and 1512 cm-1 and in SERS, which was appeared at 1568 and 1508 cm-1

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respectively. The symmetric NH3+ deformation was observed at 1578 cm-1 as a weak band in nRs and as a medium band at 1555 and 1501 cm-1 in SERS spectrum. The calculated NH3+ rocking vibrational mode of the TGS molecule was obtained at 1162 and 1173 cm-1 in nRs and SERS respectively, and the corresponding mode was observed at 1156 as a weak band in nRs and at 1172 cm-1 as a strong band in SERS spectra [55].

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3.7.3. CH2 vibrations Generally, the CH2 stretching vibrational modes occur in the region 2926-2853 cm-1 [56].

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The calculated CH2 symmetric stretching vibrational mode was appeared at 2968 cm-1 in nRs and raised at 2965 cm-1 in SERS spectra. The observed CH2 symmetric stretching vibrational mode was appeared at 2969 cm-1 in nRs and at 2963 cm-1 as a strong band along with a substantial

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band broadening in SERS spectrum. Normally, the CH2 scissoring vibrational modes fall in the region 1450-1390 cm-1 [54]. The calculated CH2 deformation was appeared as a coupled

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vibration with COO- symmetric stretching at 1409 cm-1 in nRs and found at 1405 cm-1 in SERS spectra. The observed CH2 deformation was appeared at 1421 cm-1 in nRs and found at 1434 cm1

in SERS spectra as a medium band [57]. The calculated CH2 in plane bending was appeared at

1327 and 1347 cm-1 in nRs and SERS respectively. The observed CH2 in plane bending was

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coupled with SO42- symmetric stretching and appeared at 1320 and 1350 cm-1 as a medium band in both nRs and -SERS spectra respectively. The rocking CH2 vibrational modes generally occur in the region 950-750 cm-1. In the present investigation, the calculated CH2 rocking vibrational

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mode was coupled with COO- deformational mode, which was obtained at 885 and 918 cm-1 in nRs and SERS respectively [55]. The coupled CH2 rocking vibrational mode was observed at 884

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cm-1 as a medium band in nRs and as a very strong band at 890 cm-1 in SERS spectra respectively.

3.7.4. COO¯ vibrations

In general, the COO¯ asymmetric and symmetric stretching Raman vibrational modes occur in the re-gion 1600-1570 and 1415-1400 cm-1 respectively [56,54]. The calculated symmetric COO¯ stretching is appeared as a coupled vibration with CH2 deformation mode at 15

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1409 and 1405 cm-1 in nRs and SERS respectively. The observed symmetric COO¯ stretching is appeared as a medium band at 1421 cm-1 in nRs and at 1434 cm-1 as a strong band in SERS

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respectively. The calculated COO¯ deformation vibrational mode of the TGS molecule was coupled with the CH2 rocking mode, which was obtained at 885 and 918 cm-1 in nRs and SERS respectively, and the corresponding peak was observed at 884 cm-1 as a medium band in nRs and as a very strong band at 890 cm-1 in SERS. Usually, the COO¯ scissoring vibrational modes

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appear in the wavenumber region of 750-650 cm-1[58]. In TGS molecule, the calculated COO¯

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deformation vibrational mode was coupled with C-C stretching vibration, which was obtained at 666 and 653 cm-1 in nRs and SERS respectively, and the corresponding vibrational mode was observed at 663 as a weak band and 677 cm-1 as a medium band in nRs and SERS respectively. The amino acids in the zwitterion form exhibit COO¯ rocking vibrational mode in the region 550-500 cm-1 [59,60]. The calculated COO¯ wagging vibrational mode was appeared at 613 and

coupled with SO4

2-

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608 cm-1 in nRs- and SERS respectively. The observed COO¯ wagging vibrational mode was asymmetric in-plane bending at 616 cm-1 as a weak band in nRs and as a

strong band at 603 cm-1 in SERS spectra. The calculated rocking COO¯ vibrational mode of the

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TGS molecule was coupled with C-N deformational mode, which was obtained at 512 and 517

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cm-1 in nRs and SERS respectively. The coupled COO¯ rocking vibrational mode was observed at 508 as a weak band in nRs and as a strong band at 502 cm-1 in SERS [57-59]. In the TGS molecule, the calculated rocking COO¯ vibrational mode was coupled with SO42- symmetric inplane bending vibration, which was obtained at 449 and 429 cm-1 in nRs and SERS respectively. The same mode was observed at 455 cm-1 as a weak band in nRs and as a strong band at 448 cm-1 in SERS spectra.

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3.7.5. SO42- ion vibrations The sulfate group has four fundamental vibrational modes of SO42-, (i) nondegenerate ν1

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symmetric stretching at 981 cm-1 , (ii) doubly degenerate ν2 symmetric bending at 451 cm-1 , (iii) triply degenerate ν3 asymmetric stretching at 1104 cm-1 , and (iv) triply degenerate ν4 asymmetric bending at 613 cm-1. Reduction in symmetry in the crystal structure of sulfates will cause the

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splitting of these vibrational modes [61]. The asymmetric stretching mode of SO42- ion was coupled with CH2 in-plane bending vibration which was observed at 1320 and 1350 cm-1 in nRs,

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SERS respectively [62]. In our case the calculated symmetric stretching vibrational mode of SO42- ion was appeared at 994 and 976 cm-1 in nRs and SERS spectra respectively. The observed symmetric stretching vibrational mode of SO42- ion was appeared at 978 and 965 cm-1 as a very strong band in both nRs and SERS spectra respectively. The calculated asymmetric bending

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mode of SO42- ion was appeared as a coupled vibration with COO- wagging vibration at 613 and 608 cm-1 in nRs and SERS spectra. The same mode was observed as a weak band at 616 cm-1 in nRs and as a strong band at 603 cm-1 in SERS spectra. The calculated symmetric in-plane

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bending mode of SO42- ion was coupled with COO- rocking vibrational mode, which was appeared at 449 and 429 cm-1 in nRs and SERS spectra. The same mode was observed as a weak

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band at 455 cm-1 in nRs and as a strong band at 448 cm-1 in SERS spectra. 3.8. Orientation studies

There are two possible orientations that the molecule may adsorb through a

chemisorption process, i.e. face-on and stand-on orientations. The TGS molecule has three different binding sites, i.e., NH3+, SO42- and COO¯ groups. These binding sites may lead to the adsorption of the molecule on the silver surface. The orientation of an adsorbed molecule has 17

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been studied from different points of view. The orientation/adsorption mechanism of TGS can be deduced from its SERS spectrum through a detailed analysis of the peak intensity, peak shift and

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peak broadening caused by the surface adsorption. In SERS of TGS molecule, the NH3+ and CH2 stretching vibrational modes were not much enhanced compared with the COO¯ vibrational modes, which indicate that the NH3+ and

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CH2 groups in TGS molecule were adsorbed on parallel orientation to the Ag surface whereas the COO¯ group was adsorbed on perpendicular orientation to the Ag surface [59]. The

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orientation of TGS adsorbed on Ag surface was shown in Fig 10. The enhancement factor was calculated by taking the ratio of ISERS and InRs, which corresponds to signal intensity of SERS and normal Raman intensity. The enhanced coupled COO¯ in plane bending vibrational modes were observed at 448,502 and 677 cm-1 in SERS and the corresponding enhancement factors were

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calculated as 200, 120 and 60. These enhanced intensities confirm that the COO¯ group of TGS molecule is directly adsorbed on the Ag surface in perpendicular orientation. The enhanced coupled COO¯ deformational mode was observed at 890 cm-1 with the enhancement factor of 80,

on orientation [63].

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which also confirms that the COO¯ group of TGS molecule adsorbed on the Ag surface in stand-

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The CH2 stretching, bending and wagging vibrational modes were observed with lower

enhancement, which implies that the CH2 group has a face-on orientation to Ag surface. In the SERS of TGS molecule, the NH3+ group vibrational modes also appeared with show lower enhancement, which indicates that the NH3+ group in TGS molecule is located far away from the Ag surface in face-on orientation [64]. In SO42- group, the symmetric stretching and in plane

18

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bending vibrational modes were observed with higher enhancement factor, which suggests that the molecule adsorbed to the metal surface via oxygen atom of sulfate ion [65,66].

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4. Conclusion The single crystal of TGS was grown by slow evaporation method and characterized by single crystal XRD technique, which reveals that the grown crystal belongs to monoclinic

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structure. Ag NPs were synthesized by solution combustion method using citric acid as a fuel. The XRD result reveals that the prepared Ag NPs has FCC crystalline structure with the particle

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size of ~ 33 nm. The morphology of the silver nanoparticles was also studied by HR-TEM. The calculated structural parameters of TGS compared with the single crystal XRD results. The calculated structural parameters of TGS after adsorption on silver surface show the slight deviation, which indicates the interaction between the TGS and Ag3 cluster. Raman and SERS

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studies were carried out for TGS single crystal. Raman frequencies were calculated for TGS and TGS adsorbed on Ag surface and analyzed on the basis of PED calculation and compared with the experimental values. FMOs analysis was carried out for TGS and TGS adsorbed on Ag

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surface. The experimental SHG measurements confirm the NLO activity of the TGS molecule, which was further validated by the calculated first order hyperpolarizability value. SERS spectral

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analysis reveals that the TGS adsorbed as tilted orientation on the silver surface. The theoretical and experimental results evidence the suitability of the grown crystal for optoelectronic applications.

Acknowledgement

The authors thank the college management for encouragement and permission to carry out this work and also thank the Department of Physics, N.M.S.S.V.N.College, Nagamalai, Madurai19

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19, for providing the Gaussian 09 program package. This work was supported by the UGC

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Figure Captions

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Fig. 1. DLS spectrum of TGS powder. Fig. 2. Photograph of TGS single crystal. Fig. 3. X-Ray diffraction pattern of Ag NPs.

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Fig. 4. HR-TEM image of Ag NPs in the magnification of (a) 100 nm (b) 50 nm.

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Fig. 5. UV-Vis absorbance spectra of TGS molecule (a) Theoretical (b) Experimental. Fig. 6. (a) The optimized structure of TGS based on DFT / B3LYP method with 6-311G(d,p) basis set (b) The optimized

structure of TGS-Ag based on DFT / B3PW91 method with

LANL2DZ basis set.

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Fig. 7. Frontier Molecular Orbitals of (a) TGS (b) TGS-Ag. Fig. 8. The theoretical nRs and SERS spectra of TGS.

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Fig. 9. The experimental nRs and SERS spectra of TGS.

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Fig. 10. The possible orientation of TGS on silver surface.

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ACCEPTED MANUSCRIPT Table 1. Crystal data and structure refinement for TGS C6 H17 N3 O10 S

Formula weight

323.29

Temperature

296(2) K

Wavelength

0.71073 Å

Crystal system, space group

Monoclinic, P21

Unit cell dimensions

a = 5.7224(2) Å, α = 90 deg. b = 12.6256(4) Å, β = 105.569(2) deg. c = 9.1587(3) Å, γ = 90 deg.

Volume

637.43 A3

Z, Calculated density

2, 1.684 Mg/m3

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Absorption coefficient

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Empirical formula

0.312 mm-1

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Table 2

TGS-Ag 122.94 112.25 116.09

Dihedral angle (degrees) Parameter TGSa O19-C1-C2-N13 6.71 O19-C1-O20-H21 0.38 O20-C1-C2-N13 -174.28

TGSb 1.45 -0.70 -179.11

TGS-Aga 8.41 -0.50 -172.32

108.04

107.66

H21-O20-C1-C2

-178.62

179.89

-179.75

109.26 113.59

111.06 111.77

111.07 113.43

O24-C5-C6-N14 O24-C5-O22-H23

-154.63 9.32

161.18 -1.27

-166.72 12.06

O24-C5-C6

122.31

121.62

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The optimized and observed structural parameters of TGS and TGS-Ag.

TGS 123.89 111.38 109.21

TGS 120.69 113.21 109.46

1.278

1.335

H3-C2-H4

108.39

C1-O19 O20-H21

1.212 0.972

1.232 0.820

1.257 1.027

C1-C2-N13 C5-C6-N14

C2-N13

1.480

1.472

1.482

119.18

O22-C5-C6-N14

26.56

-19.80

13.91

N13-H32 N13-H33

1.057 1.026

0.871 0.832

1.050 1.044

O22-C5-C6 H7-C6-H8

113.28 108.53

113.18 107.91

116.09 107.89

N18-C10-C9-O25 N18-C10-C9-O26

-24.19 155.84

1.05 179.76

-27.26 151.62

N13-H34

1.032

0.866

1.066

O24-C5-O22

124.40

125.20

124.72

C1-O19- Ag39- Ag40

-0.229

C5-C6 N14-C6 C5-O22 C5-O24 O22-H23 C6-H7 C6-H8 N14-H15 N14-H16 N14-H17 C9-C10 C9-O25 C10-H11 C10-H12 C10-N18 N18-H35 N18-H36 N18-H37

1.519 1.491 1.342 1.202 0.986 1.087 1.090 1.029 1.021 1.074 1.538 1.261 1.089 1.086 1.497 1.039 1.020 1.028

1.511 1.466 1.302 1.196 0.820 0.970 0.970 0.852 0.872 0.852 1.508 1.205 0.971 0.971 1.452 0.863 0.845 0.867

1.516 1.496 1.331 1.253 1.226 1.092 1.094 1.035 1.025 1.087 1.533 1.297 1.093 1.089 1.501 1.071 1.023 1.031

C5-O22-H23 C9-C10-N18 O25-C9-O26 H11-C10-H12 H35-N18-H36 O29-S31-O28 O29-S31-O27 O27-S31-O30 O30-S31-O28 C1-O19-Ag39 O19-Ag39-Ag40 H21-O19-Ag39 O20-H21-Ag40 Ag38-Ag39- Ag40 Ag39- Ag40- Ag38 Ag40- Ag38-Ag39

109.71 108.94 127.94 109.40 108.95 110.87 110.35 111.93 113.02

109.46 111.57 125.95 107.91 106.84 109.57 108.21 110.32 110.01

118.64 108.21 126.24 109.08 109.54 112.32 111.81 107.62 107.14 120.28 108.82 67.61 174.33 65.34 55.98 58.68

C1-O19- Ag39- Ag38 O20-H21-Ag40-Ag39 O20-H21-Ag40-Ag38

-18.50 162.36 165.28

a

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1.324

b

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C1-O20

a

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TGS 1.499 0.970 0.969

a

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TGS-Ag 1.513 1.092 1.094

Bond angle (degrees) Parameter C2-C1-O19 C2-C1-O20 C1-O20-H21

b

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Bond length (Å) Parameter TGSa C1-C2 1.515 C2-H3 1.089 C2-H4 1.090

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b

- Obtained Experimental data

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a

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1.697 1.644 1.649 1.622 2.467 2.413 2.741 2.659 2.659

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1.482 1.460 1.476 1.465

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1.517 1.476 1.473 1.543

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S31-O30 S31-O27 S31-O28 S31-O29 O19-Ag 39 H21-Ag40 Ag39-Ag40 Ag39-Ag38 Ag38-Ag39

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Table 3

Value (a.u)

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83.427 235.280 79.499 136.066 -75.436 -129.554 -152.864 -1.160 85.516 61.259 514.32

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β components βxxx βxxy βxyy βyyy βxxz βxyz βyyz βxzz βyzz βzzz βtot

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The calculated first order hyperpolarizability components of the TGS molecule.

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SERS Experimental

Theoretical

Assignments with PED (%)

Experimental

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nRs

νs NH3+ (100)

3030

3030 s

3032

-

3005

2992 s

3002

-

2968

2969 s

2965

2963 sb

νs CH2 (99)

1651

1678 w

1620

1653 w

δ asNH3+ (84)

1570

1578 w

1568

1555 m

δs NH3+ (87)

1512

-

1508

1501 m

δs NH3+ (65)

1409

1421 m

1405

1434 s

νs COO¯ (48), δ CH2 (35)

1327

1320 m

1162

1156 w

1055

1039 m

994

978 vs

885

884 m

666

663 w

613

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νs NH3+ (98)

1350 m

β CH2 (52), νas SO42- (43)

1173

1172 s

ρ NH3+ (53)

1040

1032 m

ν C-N(76)

976

965 vs

νs SO42- (79)

918

890 vs

δ COO¯ (67), ρ CH2(32)

653

677 m

δ COO¯ (45), ν C-C (18)

616 w

608

603 s

βas SO42- (62), ω COO¯ (24)

512

508 w

517

502 s

γ C-N(54), ρ COO¯ (37)

449

455 w

429

448 s

βs SO42- (59), ρ COO¯ (33)

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w– weak, m– medium, s– strong, sb– strong broad, vs– very strong, ν– stretching, νs– symmetrical stretching, β– in-plane bending, βs– symmetric in-plane bending, βas– asymmetric in-plane bending, δ–deformation, ρ– rocking, ω– wagging.

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ACCEPTED MANUSCRIPT Highlights
Single crystals of triglycine sulphate (TGS) were grown by slow evaporation method
Silver nanoparticles were prepared by solution combustion method
Structural parameters of TGS were studied

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SERS analysis reveals that the TGS adsorbed as a tilted orientation on the silver surface

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NLO activity of the molecule was also studied by experimental and theoretical methods