Vibrational Circular Dichroism study of chiral food additives: γ-valero- and γ-caprolactone.

γ-valerolactone and γ-caprolactone are commonly used as flavor additives in the food industry. In the present work we fully explore the molecular structure and conformational distribution of enantiopure γ-valerolactone and γ-caprolactone in solution state by using Vibrational Circular Dichroism (VCD) spectroscopy assisted by quantum chemical calculations. In order to establish the most accurate DFT method for this type of samples a set of methods and basis sets have been implemented and their performances have been compared. Subsequently, we have performed a complete vibrational assignment, which allowed to detect certain key vibrational features related to specific solution-state conformational speciation. In spite of the rigidity of the samples being studied, our results point to the incidence of conformational mixture in CCl4 solution in both samples.

Conformational properties of chiral tobacco alkaloids by DFT calculations and vibrational circular dichroism: (-)-S-anabasine.

A thorough DFT and MM study of the conformational landscape, molecular and electronic structures of (-)-S-anabasine is reported aimed to reveal the mechanism controlling its conformational preference. Although the conformational flexibility and diversity of this system is quite extensive, only two structures are populated both in gas-phase and solution (CCl4 and DMSO). NBO-aided electronic structure analyses performed for the eight conformers representing minima in the potential energy surface of (-)-S-anabasine indicate that both steric and electrostatic factors are determinant in the conformational distribution of the sample in gas phase. Nonetheless, hyperconjugative effects are the key force tipping the balance in the conformational equilibrium between the two main rotamers. Increasing the polarity of the medium (using the IEF-PCM formalism) barely affect the conformational energy profile, although a slight increase in the theoretical population of those structures more affected by electrostatic interactions is predicted. The validity of the theoretical models and calculated conformers populations are endorsed by the accurate reproduction of the IR and VCD spectra (recorded in pure liquid and in CCl4 solution) of the sample (that have been firstly recorded and assigned in the present work) which are consistent with the occurrence of a 2:1 conformational ratio.