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 (λIR) 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 λIR 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 λIR of MNA is experimentally unknown yet, we predict it to be 1.50μm. The theoretical analysis indicates that, while the λIR 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.