The structure, vibrational spectra and nonlinear optical properties of neutral melamine and singly, doubly and triply protonated melaminium cations - theoretical studies

The melaminium cation can occur in three form: singly, doubly or triply protonated. The geometry and nonlinear optical properties (NLO) for all forms were computed by ab initio method. The restricted open Hartree-Fock (ROHF) computational method with 6-311 + G basis set was used. Comprehensive studies of electronic and molecular structures have been performed. The theoretical vibrational frequencies and potential energy distribution (PED) of melaminium cations were calculated by B3LYP method. The calculated frequencies were compared with measured one for newly obtained compounds where melaminium cations are common component. The assignment of the bands has been made on the basis of the calculated PED. The restricted Hartree-Fock (RHF) methods were used for calculation of the hyperpolarizability for each ion. For comparison, analogous calculations were carried out for the closed-shell neutral melamine. The theoretical results can be used in design of new molecular compounds with nonlinear optical properties. (c) 2004 Elsevier B.V. All rights reserved.