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.

Vibrational (resonance) Raman optical activity with real time time dependent density functional theory

“…For both Raman and ROA spectra, a backscattering geometry is assumed; i.e., the spectra are calculated according to Eqs. (20) and (21).…”

“…The calculation of ROA spectra in the nonresonant regime has appeared at the end of the 1980s 14 and since then has been performed on a variety of systems. After the first calculations at a Hartree-Fock level of theory, 15,16 many density functional theory (DFT) [17][18][19][20][21][22][23][24] and later higher level of theory studies 25,26 followed. Advances for speeding up the calculations include the calculation of analytical ROA intensity differences, 27 density fitting, 28 Cartesian transfer, 29 molecules in molecules fragmentation schemes, 30 and intensity tracking.…”

Localized molecular orbitals for calculation and analysis of vibrational Raman optical activity

“…Recent progress in this area encompasses, for example, the measurement of paramagnetic ROA [12], spectrometer for the deep-ultraviolet region [13], and first ROA spectra of chiral ionic liquids using a combined experimental/computational approach [14]. Whereas proteins [15][16][17][18][19][20][21][22], carbohydrates [23][24][25], and polymers [26,27] have been studied more frequently (see reviews in Refs. [28][29][30][31][32]), the field of chiral metal complexes is a relatively new field of research [22,[33][34][35][36].…”

Raman Optical Activity Spectra from Density Functional Perturbation Theory and Density-Functional-Theory-Based Molecular Dynamics

“…[28][29][30] ) as well as polymers. 31,32 In contrast to that, calculation of Resonance ROA is a rather unexplored, yet promising field, [33][34][35][36] in particular for metal complexes. 35,37 Simulated ROA spectra of the latter are in general still quite rare 11,27,35,38,39 and we refer to Ref.…”

“…ROA computations have been presented for various biomolecules ranging from carbohydrates 17−19 up to complex proteins 20−27 (for reviews, see refs 28−30) as well as polymers. 31,32 In contrast to that, calculation of resonance ROA is a rather unexplored, yet promising field, 33−36 in particular for metal complexes. 35,37 Simulated ROA spectra of the latter are in general still quite rare, 11,27,35,38,39 and we refer to ref 37 for a recent review about the ROA of transition metal-containing coordination compounds and solids.…”