An Experimental Comparison of Vibrational Circular Dichroism and Raman Optical Activity with 1-Amino-2-propanol and 2-Amino-1-propanol as Model Compounds

Mid-IR vibrational circular dichroism (VCD) and the corresponding Raman optical activity (ROA) spectra of 1-amino-2-propanol and 2-amino-1-propanol in neat solution are compared to yield insight into the dominant structural sensitivity of each technique. The ROA spectra for these isomeric compounds are quite similar while their VCD spectra are substantially different. The contrast between the results with these two techniques can be empirically interpreted to imply that VCD is more sensitive to the overall chirality of a molecule, conformation plus configuration, while ROA is more dependent on the nature of the local environment, or the configuration, of the functional groups. This observation would correlate with VCD having a significant dipolar coupling contribution that is highly dependent on conformation. This distinction between VCD and ROA sensitivities would be expected to be most appropriate for high dipole strength transitions in conformationally unconstrained, open-chain molecules. These observations directly reflect the contrast between current applications of VCD and ROA to biomolecular conformational analyses.