Cálculo ab initio de propriedades optoeletrônicas da L-alanina

Abstract

Ab initio calculations for the molecule and crystal of L-alanine are presented. Geometry optimizations were performed for three L-alanine molecular conformers using three different methods: Hartree-Fock approximation with a 6-31++G(d,p) basis, Hartree-Fock approximation with a 6-311++G(3d,3p) basis and Density Functional Theory (B3LYP exchange-correlation functional) with a 6-31++G(d,p) basis. From the geometry optimizations, total energies, dipole and quadrupole moments, polarizabilities, thermodynamical properties, normal modes, Raman, infrared and VCD spectra, energy levels and frontier molecular orbitals were obtained and comparisons with experimental results were carried out. For the L-alanine crystal, geometry optimization was performed within the framework of DFT (LDA and GGA), and band structures, effctive masses, dielectric function, absorption, reflectivity and refraction index were calculated. Those results were employed to investigate the photoluminescence spectra of undoped and Mn-doped L-alanine crystals. The ab initio results allow assignment of the photoluminescence peaks in the visible region to lattice-related processes of exciton nature associated with polaron levels and defect trapping centers for carriers. By evaluating the vertical and adiabatic transitions between the ground state and excited states of a single L-alanine molecule in the zwitterion form, the very thin photoluminescence peak in the ultraviolet region is assigned to intramolecular transitions in the weakly interacting L-alanine molecules forming the crystals, being a signature of the type of the crystalline amino acid. In the case of Mn-doped alanine crystal, the integrated photoluminescence intensity is shown to be dramatically quenched by the intersticial manganese, with a decrease greater than 65% for higher doping. Preliminar ab initio calculations indicate that this quenching is related to the change of exciton energy levels due to the charge redistribution in the unit cell when manganese ions are present.