engleski

FT Infrared and NIR FT Raman Spectra of Salicylideneaniline Melt

Salicylideneaniline, 2-OH-C_6H_4CH=NC_6H_5, undergoes both photochromic and thermochromic transformations and hence was the subject of numerous spectroscopic studies in the past few decades. We recorded FT infrared and NIR FT Raman spectra of crystalline salicylideneaniline and compared them with the spectra of its melt. High-quality FT Raman spectra excited in the NIR region (Nd-YAG laser) are obtained and in comparison with infrared spectra of crystalline samples only a few bands (1278, 825 and 692 cm(-1)) are missing. No changes in FT-IR spectra were obtained on heating the sample up to the melting point, while the spectrum of the melt (50 degrees C) revealed a thermal isomerisation. The melt was further heated up to 100 degrees C without any changes in the spectrum. The general trend of the increase of bands by 1-4 cm(-1) is observed, with several cases of pairs of closely spaced bands (1579/1572, 1456/1452, 1239/1224 etc.) fused into one. Changes in intensities are not always the consequence of this fusion. Thus the broad band at 824 cm(-1) (tentatively assigned to the OH out-of-plane bending) is very reduced in intensity and shifted to approximately 815 cm(-1). A pair of ring deformation modes is separated by 19 cm(-1) (434/415 cm(-1) in IR and Raman) in the crystal and 30 cm(-1) (440/410 cm(-1)) in the melt. Another example is a pair of skeletal modes found in the low frequency region of Raman spectra, where the separation increases from 57 to 70 cm(-1). Infrared and Raman spectra of the melt are identical to those obtained by the UV-phototautomerization process. It is difficult to discuss the structure of the isomerisation, since it seams that products contain two tautomeric forms.

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