Experimental and ab initio theoretical vibrational Raman optical activity (VROA) spectra are presented for N-acetyl-N‘-methyl-l-alaninamide (NANMLA), in order to determine the predominant conformations of this molecule in the solution phase. The experimental spectra were obtained in three different solvents, CHCl3, H2O, and D2O. The ab initio VROA spectra were predicted for nine different conformations of NANMLA optimized with the 6-31G* basis set. The vibrational frequencies and normal modes for all nine conformers were also obtained with the 6-31G* basis set. From a comparision of the predicted and observed VROA spectral patterns, it is suggested that there is reasonable support for the presence of C7,eq−C5, C7,eq, and αR conformers in aqueous solution and for C7,eq−C5 and αR conformers in chloroform solutions.

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This review presents the recent progress towards elucidating the structures of chiral natural products and applications using vibrational optical activity (VOA) spectroscopy.

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Vibrational absorption and circular dichroism spectra of (R)−(−)-2-butanol have been measured in CS2 solutions in the 2000−900 cm-1 region. Experimental spectra obtained at different concentrations have been compared with the ab initio predictions of absorption and VCD spectra obtained with density functional theory using B3LYP/6-31G* basis set for nine different conformers of (R)-2-butanol. The Boltzmann populations, obtained from Gibbs free energies, indicate the presence of all nine conformations for isolated molecule. Vibrational assignments have been proposed with the observed bands assigned mainly for the most stable conformers. The population weighted theoretical spectra are in satisfactory agreement with the experimental spectra obtained at dilute concentrations. The influence of intermolecular hydrogen bonding on the bands originating from C−O−H bending and C−O stretching is observed in the experimental spectra.

Bromochlorofluoromethane is the simplest chiral molecule that is used as an example to illustrate chirality or asymmetric carbon atoms, yet its absolute configuration remains uncertain. The synthesis of bromochlorofluoromethane in a pure form was achieved by Berry and Sturtevant in 1942. [1] They determined its boiling point, melting point, and refractive dispersion, but the individual enantiomers were not resolved. Despite the unavailability of enantiomers and their optical rotation values, Brewster hypothesized that (S)-bromochlor-ofluoromethane would have positive optical rotation, assuming that the polarizabilities decrease in the order Br > Cl > H > F. [2] Hargreaves and Modarai were able to resolve the enantiomers of 1-bromo-1-chloro-1-fluoroacetone and convert them into the corresponding bromochlorofluoromethane enantiomers. [3] They reported the specific rotations to be þ 0.20 and À0.13. In 1973 Applequist predicted, using an atom±dipole interaction model and assuming the polarizabilities to be in the order, Br > Cl > F > H, that (S)-bromochlorofluoromethane would have positive optical rotation. [4] However, the different polarizability orders chosen by Brewster and Applequist would yield opposite conclusions. [5] Collet and co-workers achieved optical resolution of bromochlorofluoromethane by enantioselective inclusion in cryptophan C. [6] Using the NMR resonance signals of (þ) and (À) enantiomers encapsulated in cryptophan, they estimated the enantiomeric excess and maximum rotation values at five different wavelengths. In a different approach, Wilen et al. also resolved the enantiomers of bromochlorofluoromethane using brucine [5] and addressed the ambiguity remaining in the absolute configuration of bromochlorofluoromethane; Brewster and Applequist had arrived at the same assignment but using mutually conflicting polarizability trends. Wilen et al. concluded that ™experimental determination of the absolute configuration of bromochlorofluoromethane is a challenge∫. [5] Using a quantum-mechanical static method, the specific rotation of (R)-bromochlorofluoromethane at the sodium D line was predicted (without the Lorentz factor) to be À6. [7,8] Comparison of this value with the experimental value of À1.78 reported by Collet et al. for enantiopure (À)-bromochlorofluoromethane, and based on the comparison of experimental and ab initio predicted Raman optical-activity (ROA) spectra, it was concluded that the absolute configuration of bromochlorofluoromethane is (S)-(þ) and (R)-(À). [7] Previous conclusions [7] on the absolute configuration were based on quantum-mechanical calculations of both ROA and specific rotation, carried out at the Hartree±Fock (HF) level using smaller basis sets. Since then, evidence has been collected [9] to indicate that the HF level calculations, because of lack of electron correlation, are not quantitatively accurate. DFT, [10] which includes electron correlation using density functionals, has now been widely accepted as the preferred approach for predicting molecular properties a...

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