Infrared Absorption Cut-Off of Molecular Nonlinear Optical Crystals: Theoretical Studies on Vibrational Spectra of MDNB, Urea and MNA Molecules

A theoretical model has been set up for evaluating the infrared absorption cut-off (λ_IR) of molecular nonlinear optical (NLO) crystals on the basis of calculated molecular infrared vibrational spectra. This model is applied to meta-dinitrobenzene (MDNB), urea, and 2-methyl-4-nitroaniline (MNA) crystals. The λ_IR values for MDNB and urea crystals are predicted to be 2.0 and 1.37μm, respectivelx which are in good agreement with experimental measurements. Although the λ_IR of MNA is experimentally unknown yet, we predict it to be 1.50μm. The theoretical analysis indicates that, while the λ_IR of MDNB arises from asymmetric torsion of N-O in the molecule, those of urea and MNA arise from symmetric N-H stretch and C-H stretch, respectively. These results should be helpful for the molecular design of novel infrared NLO crystals.