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1 2009

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Effects of Strong Inter-hydrogen Bond Dynamical Couplings in the Polarized IR Spectra of trans-beta-Hydromuconic Acid Crystals

Flakus H. T., Jabłońska M., Tyl A.

Polish Journal of Chemistry

2009

Vol. 83, nr 5

869-886

This paer presents the re sults of in ves ti ga tions of po lar ized IR spec tra of trans-beta hydromuconic acid (HMA) crystals. The spectra were measured by a transmission method using polarized light at room temperature and at 77 K for three different crystalline faces. Theoretical analysis of the results concerned linear dichroic effects, H/D iso topic and temperature effects observed in the spectra of the hydrogen and deuterium bonds in HMA crystals at the frequency ranges of the ni O–H and the ni O–D bands. The basic crystal spectral properties could be sat is factorily interpreted in a quantitative way for a centrosymmetric cyclic hydrogen bond dimer model. Model calculations, performed within the limits of the “strong-coupling” model, al lowed for quantitative interpretation and for the under standing of the basic properties of the hydrogen bond IR spectra of HMA crystals including H/D isotopic, temperature and dichroic effects. Such a model explains not only the two-branch structure of the ni O–H and ni O–D bands in crystalline spectra, but also some essential linear dichroic effects in the band frequency ranges measured for isotopically diluted HMA crystals. In the scope of our studies the mechanism of H/D isotopic “self-organization” processes, taking place in the crystal hydrogen bond lattices, was also studied. It was proved that for isotopically diluted crystalline samples of HMA, a non-random distribution of protons and deuterons occurs exclusively in the hydrogen bond dimers. Never the less, these dynamical co-operative inter actions between the hydrogen bonds do not in volve of the adjacent hydrogen bond dimers in each unit cell. The two-branch fine structure pattern of the ni O–H and ni O–D bands was ascribed to the vibronic mechanism of vibra tional di pole se lec tion rule break ing for IR tran si tions in centrosymmetric hy dro gen bond dimers. The lin ear dichroic ef fect, dif fer en ti at ing properties of the two op po site spec tral branches of the nO–H and nO–D bands, was also discussed.