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Molecular structure, First-order hyperpolarizability and HOMO-LUMO studies of L- Histidinium Dinitrate

The geometric parameters and hyperpolarizabilities of a nonlinear optical material L- Histidinium Dinitrate (LHDN) was investigated by density functional theory and presented. The compound crystallizes in the noncentro symmetric space group P212121 of orthorhombic system. The first order hyperpolarizability (β) of this molecular system is calculated using B3LYP/6-31G (d,p) method on the finite-field approach. The calculation results also show that the LHDN molecule might have microscopic nonlinear optical behavior with non zero values. The Optical absorption spectrum and second harmonic generation were investigated. The calculated HOMO and LUMO energies showed that charge transfer occurs within the molecule.


Introduction
Nonlinear optical (NLO) phenomena have been extensively studied over the last decades; molecules exhibiting large hyperpolarizabilities have a strong NLO potential and could be used, under conditions for optoelectronics and a variety of optical devices [1,2]. In the present work, LHDN was synthesized by slow evaporation method and the structural properties were calculated by DFT calculation. The experimental spectra are compared with calculated findings. Literature survey also reveals that to the best of our knowledge no DFT calculations have been reported yet. Therefore, the present investigation was undertaken to study the vibrational spectra of this molecule and to identify the various normal modes with greater wavenumber accuracy. In this context, the hyperpolarizability of the title compound was calculated in the present study.

Synthesis of LHDN
The starting materials were AR grade L-histidine (99 % Merck) and nitric acid. The solution was prepared by dissolving 1 mol of L-histidine and 2 mols of nitric acid in deionised water. The chemical reaction is as follows: The synthesized material was then purified by repeated crystallization process.

Powder XRD Studies
The structural property of the grown LHDN is studied by X-ray powder diffraction technique. Powder X-ray diffraction studies were carried out, using Siemens D500 X-ray diffractometer with Cu K α (λ = 1.5406Ǻ) radiation. The samples were scanned for 2θ values from 10° to 55° at a rate of 2° /min. The diffraction pattern ( Figure 1) was indexed by least square fit method. The grown LHDN crystal belongs to orthorhombic system with the centrosymmetric P2 1

Hyperpolarizability
The β values of LHDN compound was calculated by the DFT-B3LYP functional with the 6-31G (d, p) basis set [4]. In this context, the dynamic first hyperpolarizability of the title compound is also calculated in the present study. The first hyperpolarizability (β o ) of this novel molecular system is calculated using B3LYP/6-31G(d) method, based on the finite field approach. In the presence of an applied electric field, the energy of a system is a function of the electric field. First hyperpolarizability is a third rank tensor that can be described by a 3×3×3 matrix. The 27 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry [5].
A large value of the first hyperpolarizability (=4.187944 x 10 -30 esu) is a prerequisite to behave as a good NLO material, and the important parameters influencing β generally are (i) donor-acceptor system, (ii) nature of substituents, (iii) conjugated  system and (iv) the influence of planarity. The results also show that the compounds studied in our research are good NLO materials due to that their β values.

HOMO -LUMO
The calculations were done with the commonly used exchange-correlation functional B3LYP followed by a comprehensive analysis of the calculated highest-occupied and lowest-unoccupied Kohn-Sham orbital (HOMO and LUMO) energies. The basis set dependence of the DFT results shows that the economical 6-31+G basis set is generally sufficient for calculating the HOMO and LUMO energies (if the calculated LUMO energies are negative) for use in correlating with molecular properties. The HOMO-LUMO energy gap of LHDN (Figure 3) was calculated at theB3LYP/6-31(d,p) level reveals that the energy gap reflect the chemical activity of the molecule and found to be -0.04453a.u. LUMO as an electron acceptor represents the ability to obtain an electron, HOMO represents the ability to donate an electron. All of the calculations were performed by using the Gaussian 03.

SHG Efficiency
The SHG efficiency of the grown crystal was measured by the standard Kurtz and Perry powder technique [6] using a fundamental beam of a Q-switched Nd:YAG laser with a wavelength of 1064 nm. To make relevant comparison with known SHG materials, KDP was also powdered and sieved into the same particle size range. The results obtained by this method shows that the SHG efficiency of LHDN sample is nearly 2.9 times more than that of KDP.

UV-Vis NIR Spectrum
The UV-Vis-NIR transmission spectrum of the crystal was recorded in a wavelength range of 200 -1200 nm and is shown in Figure 3. From the spectrum, it is seen that the UV transparency cutoff occurs at 250 nm and there is no remarkable absorption in the entire region of the spectra. This is a favorable character for a NLO material.  Figure 3. UV Vis NIR Spectrum of LHDN

Conclusions
Single crystals of LHDN were synthesized and grown from aqueous solution. The grown crystals were characterized by Powder XRD analysis. The study reveals that the grown crystals belong to orthorhombic system. SHG measurement shows that the crystal has superior properties than KDP crystal. HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule. Furthermore, first-order hyperpolarizability of the molecule shows that the title molecule is an attractive object for future studies of nonlinear optical properties.