Combined Experimental and Theoretical Study on the Raman and Raman Optical Activity Signatures of Pentamethylundecane Diastereoisomers

Abstract

The synthesis and the separation of the four stereoisomers of 2,4,6,8,10-pentamethylundecane (PMU) are described together with their characterization by Raman spectroscopy. In parallel, theoretical calculations of the Raman and vibrational Raman optical activity (VROA) spectra are reported and analyzed in relation with the recorded spectra. A very good agreement is found between the experimental and theoretical spectra. The Raman spectra are also shown to be less affected by the change of configuration than the VROA spectra. Nevertheless, by studying the overlap between the theoretical Raman spectra, we show clear relationships between the spectral fingerprints and the structures displaying a mixture of the TGTGTGTG conformation of the (4R,6s,8S)-PMU (isotactic compound) with the TTTTTTTT conformation of the (4R,6r,8S)-PMU (syndiotactic compound). Then, the fingerprints of the VROA spectra of the five conformers of the (4R,8R)-PMU have been related to the fingerprints of the regular (TG)(N) isotactic compound as a function of the torsion angles. Since the (TT)(N) syndiotactic compound has no VROA signatures, the VROA spectroscopy is very sensitive to the helical structures, as demonstrated here.