Optical nonlinearity and electric conductivity origin study on sucrose crystal by using IR, Raman, INS, NMR, and EPR spectroscopies

• Sucrose rigid structure and internal rearrangement models agree with NMR results.
• Hyperpolarizability DFT calculations for sucrose clusters were carried out.
• Proton transfer couples with –CH2OH group rearrangements.
• Proton transfer is the most important in SHG and conductivity origin.
• OH group vibrations participating in hydrogen bonds were assigned to bands.

The supposed importance of hydrogen bonds toward the origin of second harmonic generation (SHG) and electric conductivity in crystalline sucrose was investigated by IR (4000–10 cm−1), INS (2000–10 cm−1, at 35 K), polarized Raman (3600–50 cm−1) spectra, and 1H NMR second moment line records in the temperature range 450–80 K. The temperature dependence of NIR (7000–5500 cm−1) polarized spectra gave information about –CH2 motions complementary to NMR results concerning –CH2OH group rearrangements. The EPR spectra were applied to study the generation of radical ions by exposure to NIR radiation. Density functional theory quantum chemical calculations were performed to reproduce the vibrational spectra in order to complete as far as possible the assignments of bands observed by us and in the literature in sucrose crystals, and to throw more light on the possible reasons of sucrose electric conductivity and optical nonlinearity by the knowledge of theoretical values of dipole moments, polarizabilities, first order hyperpolarizabilities of sucrose molecule and clusters as well as ionization energy and electron affinity. The proton transfer in one specific hydrogen bond parallel to the helical axis b is proposed to be the most important in SHG and conductivity origin.

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