Effect of the intramolecular hydrogen bond on the spectral and optical properties in chitosan oligosaccharide

• We establish the chitosan oligosaccharide models DP2–DP5 by density functional theory, the model results are agreed with helical structure which obtained in experiment.
• We have analyzed the statistics of relationship between IR spectra and bond lengths and angles of amino, hydroxyl.
• The nonlinear optical properties of chitosan oligosaccharide display a regular oscillation of occurs with DP increasing, the mean dipole polarizability and the polarizability anisotropy invariant are linear increase.

The geometric structures, hydrogen bond types, IR spectra and nonlinear optical properties of chitosan oligosaccharide (degree of polymerization 2–5) are studied by density-functional theory (DFT) at B3LYP/6-31+G(d) level. We have analyzed the statistics of relationship between IR spectra and bond lengths, and angles of amino, hydroxyl. The results show that: (1) the active groups C3–OH, C6–OH and –NH2 can form intramolecular hydrogen bond in chitosan oligosaccharide; (2) the IR spectra of three active groups have size effect in growth process, however, its IR intensity increases significantly and IR frequencies are red shifted obviously when the active hydroxyl form hydrogen bonds, because the bond length of active hydroxyl becomes longer; (3) the effect of hydrogen bond on intensity and frequency of the three vibration mode of amino is the main factor and complication. The paper also provides the nonlinear optical properties of chitosan oligosaccharide. The reason why hydrogen bond can make an appreciable difference to IR spectra, and the nonlinear optical properties of chitosan oligosaccharide are discussed. This research has important significance in the characterization of chitosan oligosaccharide, the properties of chitosan material and hydrogen bond by infrared spectroscopy.

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