In the present paper a theoretical method has been suggested in order to predicate the IR absorption edges of two inorganic-organic NLO crystals, L-Arginine Phosphate Monohydrate (LAP) and its deuterated analog (d-LAP). We first calculated and simulated the IR spectra of LAP and d-LAP crystals by using ab initio calculations of supermolecular models, of which the former agreed well with the experimental values. It is exhibited that quantum chemical calculations can successfully predicate the IR spectra of these crystals. Subsequently, the important fundamental lines have been assigned and the IR intensities and overtones of these fundamentals have been analyzed. Finally, the IR absorption edges of LAP and d-LAP crystals have been estimated according to the method suggested by us, and the calculated values are very close to the experimental ones. In addition, it is shown that if the IR absorption edges of the crystals are determined by the overtone frequency correlated to H stretching, the deuterated method of H is very effective in improving the ranges of optical transparency.
Spectroscopy and Spectral Analysis