Electronic structure and first hyperpolarizability of poly(μ ₂-l-alanine-μ ₃-sodium nitrate (I)) crystals

Abstract

Poly(μ ₂-l-alanine-μ ₃-sodium nitrate (I)), p-LASN, crystals have been grown by slow evaporation at room temperature. The nominal size of the crystals obtained by the method was of 500 nm. The UV-Vis spectrum shows a wide range, where absorption is lacking around 532 nm, which is required in order to have the second harmonic emission, when an incident radiation of 1064 nm strikes on the crystal. This guarantees the possible use of the crystal in visible light applications. The transparent nature of the crystal in the visible and infrared regions within the transmission spectrum confirms the nonlinear optical properties of the crystal. Additionally, Fourier transform infrared spectroscopy displays its functional groups which correspond to the poly(μ ₂-l-alanine-μ ₃-sodium nitrate (I)), where the presence of nitrates in the lattice generally can be identified by their characteristic signature within the 1660–1625, 1300–1255, 870–833 and 763–690 cm⁻¹ range. Single crystal diffraction was carried out in order to determine atomic structure and lattice parameter. Structural parameters were a = 5.388(9) Å, b = 9.315(15) Å and c = 13.63(2) Å. The structure of poly(μ ₂-l-alanine-μ ₃-sodium nitrate (I)) shown by single crystal diffraction shows an asymmetric unit consisting of one sodium and one nitrate ion and one l-alanine molecule. The coordination geometry around the sodium atom was trigonal bipyramidal, with three bidentate nitrate anions coordinating through their oxygen atoms and two l-alanine molecules, each coordinating through one carboxyl oxygen atom. Electronic structure was obtained by using the Becke-Lee-Yang-Part and Hartree-Fock approximations with hybrid exchange-correlation three-parameter functional and G-311**G(dp) basis set. Theoretical and experimental results were compared and discussed as having an excellent agreement among them.