NEAR‐INFRARED FOURIER‐TRANSFORM RAMAN STUDY OF CHLOROPHYLL a IN SOLUTIONS

Abstract— The 1064 nm excited Fourier‐transform (FT) Raman spectra have been measured for chlorophyll a (Chl‐a) in various solutions. Features of the obtained spectra are largely different from those of Soret‐resonant Raman spectra of Chl‐a. For example, a band due to the C=0 stretching mode of the 13‐keto group is much stronger in the former than in the latter. Although the excitation wavelength is rather far from that of the Qy absorption of Chl‐a, notable enhancements of bands due to the C=O stretching mode of the 131‐keto group and to the C=C stretching mode of the vinyl group and those in the 1250–600 cm‐1 region suggest that the 1064 nm‐excited Raman spectra of Chl‐a are preresonance enhanced via the Qy band. Some of the marker bands for the coordination number of the central Mg atom, which have been used in the visible‐excited resonance Raman spectra, are not valid in the 1064 nm‐excited Raman spectra. Instead, new marker bands are found in the latter. The 1064‐nm excited Raman spectra of Chl‐a in a hydrated hexane solution reveal that the dehydrated oligomer of Chl‐a as well as Chl‐a‐water micelles coexist in the solution. They also suggest that the size of the oligomer is considerably larger and the hydrogen bonding of the 131‐keto group in the oligomer is weaker than that in the micelles.