Two-Dimensional Correlation Spectroscopy (2D-COS) Studies of Solution Mixtures in the Low Frequency Raman Region.

Raman spectra of a series of binary solution mixtures, including chloroform (CHCl), ethanol (EtOH), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), were analyzed using the two-dimensional correlation spectroscopic (2D-COS) technique in the low frequency region. Numerous asynchronous cross-peaks ubiquitously appeared in the concentration-dependent Raman spectra of these organic solvent mixtures. The result clearly demonstrated a deviation from ideal solution behavior, reflecting the presence of specific molecular interactions causing a subtle nonlinear spectral intensity response of Raman bands to the concentration changes. Furthermore, the combination of 2D-COS and low frequency Raman spectroscopy was extended to poly[(R)-3-hydroxybutyrate--(R)-3-hydroxyhexanoate] (PHBHx) copolymer solutions in CHCl-HFIP co-solvents. The results suggest the existence of hydrogen bonding interaction between the PHBHx and HFIP, which is consistent with the previous infrared spectroscopic study of PHBHx solutions.