Conformational diversity in biogenic terpenoid linalool: Liquid phase FTIR spectroscopic and quantum chemical studies

Abstract

The conformational diversity of linalool, a biogenic terpenoid, has been investigated in detail through IR spectroscopy and quantum chemical calculations. Theoretical conformational analysis reveals that energetic stability is guided primarily by the orientation of the tertiary hydroxyl group, the only polar entity, about the extended hydrophobic part of the molecule. The most stable conformers are those in which this group is intramolecularly hydrogen bonded to the π-bond in the δ-position. Natural Bond Orbital analysis demonstrates that the remote hyperconjugative interaction between the filled π-bonding orbital and the vacant anti-bonding σ∗ orbital on the O–H bond is responsible for the intramolecular interaction. The presence of a bond critical point along the O–H … π bond path, as obtained from electron density topology studies using Atoms-in-Molecules analysis confirms the interaction. A full vibrational analysis of the global minimum conformer has been reported. Thermal solvation in CCl₄ solution reveals the co-existence of this conformer with those of higher energy, as obtained from a detailed comparison of the experimental spectrum with theoretically predicted conformationally averaged spectrum. The self-dimers of linalool have also been investigated both experimentally and theoretically. It is evidenced that in solution, the π-hydrogen bond in one or both of the dimer constituents opens up, as it enters into an intermolecular O–H⋯O hydrogen bonded interaction. Also, in the heterodimeric complexes of linalool with proton accepting solvents tetrahydrofuran and deuterated dimethyl sulfoxide, respectively, the open non-π-hydrogen bonded form is energetically much more preferred than the closed π-hydrogen bonded form. Such conformational switching of linalool in its dimeric forms as compared to its monomeric form is attributed to the decrease in the classical energetic barrier of conversion from the closed to the open forms as a result of specific intermolecular association.