Two-Dimensional Raman (2D Raman) Correlation Spectroscopy Study of Non-oxidative Photodegradation of β-Carotene

摘要

Photoinduced decomposition reaction of all-trans-β-carotene in the absence of oxygen was studied by 2D Raman correlation spectroscopy. Generalized two-dimensional correlation analysis, coupled with NIR-excited Raman scattering measurements, reveals the presence of both simultaneous and sequential changes of Raman intensities associated with the decomposing β-carotene and the creation of photoreaction products. The largest intensity increase occurs for the Raman band at 1537 cm~(-1), which is likely due to the formation of a photoisomerization product containing one or more cis C=C groups. The formation of this product occurs at an earlier stage of the reaction than the steady decrease of band intensities associated with all-trans-β-carotene at 1522 and 1520 cm~(-1). Smaller intensity increases at 1133, 1569, and 1284 cm~(-1) occur synchronously with the growth of the 1537 cm~(-1) band and prior to the overall decrease in the all-trans-β-carotene bands. Thus, the decomposition product associated with the Raman bands at 1133, 1569, and 1284 cm~(-1) are either due to the same decomposition product as that represented by the 1537 cm~(-1) band, or different products formed at the same rate. The intensities of Raman bands at 1005 and 1000 cm~(-1) decrease in intensity, but this decrease lags behind that observed in the main 1520 and 1520 cm~(-1) all-trans-β-carotene bands. These bands may represent spectral contributions from different decomposition products overlapped with the dominant β-carotene contribution as well as a toluene solvent band.