Study of Intra-Molecular C−H‧‧‧O Bonded Chair Conformer of (R)−(−)−Menthyl Acetate using Combined Spectroscopic and Computational Methods

The intra-molecular C−H‧‧‧O bond has been studied in (R)−(−)−Menthyl Acetate employing both DFT and QT-AIM methods. The B3LYP/6−311+G(d,p) level has predicted a stable chair conformation in agreement with measured IR and Raman modes. This C−H‧‧‧O bonded chair conformer is also supported by a downfield shift at δ 4.68 ppm (computed δ 4.78 ppm) in the measured 1H NMR spectrum and positive values of electron density and its Laplacian obtained from QT−AIM. The computed H···O distance (2.351 Å) in C–H···O bond is shorter than the sum of van der Waals radii of H and O atoms, 2.720 Å. A strong positive Cotton effect at 232 nm in the ECD spectrum corresponds to the medium UV band at 241 nm (other bands: 219 and 276 nm) and is assigned to the n→π* transition. It is argued that the good agreement between computed CD band at 214 nm and its corresponding experimental band at 232 nm support proposed intra−molecular C−H‧‧‧O bonded chair conformer for (R)−(−)−Menthyl Acetate. The charge delocalization in C−H‧‧‧O bond is characterized by overlapping n(2)O→π*(C=O) natural bond orbitals with ~47 kcal/mol stabilization energy. The C−H‧‧‧O bonding and its neighbor atoms are shown in terms of electron localization function and localized orbital locator as electron density distributions and their spatial locations in the interaction.