Molecular Conformation of Optically Active Five and Six- Membered-Ring Ketones

Conformational analysis of chiral five and six -- membered - ring ketones will be presented. Electronic circular dichroism (CD) and vibrational circular dichroism (VCD) spectra for both $R$- and $S$- enantiomers of optically active carvone (C$_{10}$H$_{14}$O) exhibit marked temperature dependences. Theoretical calculations, using density functional theory (B3LYP with aug-cc-pVDZ basis set), show an equal magnitude but opposite \textit{sign} for the CD and VCD for the two conformers of each $R$- and $S$- enantiomer. The Temperature dependent circular dichroism (CD) measurements of $R$-(+)-3-methylcyclopentanone $R$3MCP in 36 different common solvents is being employed to determine the conformers energy between the equatorial methyl and axial methyl of $R$3MCP and carvone. The results will be compared to the CD in the gas phase, solvent effect on optical rotation of $R$3MCP and carvone will be demonstrated and supported by DFT calculations. Temperature dependent vibrational Raman spectroscopy in the C-H stretch region is used to study conformation over a wide range of temperature (-15-135 \r{ }C) and at liquid nitrogen temperature. Temperature dependent variations of CD and Raman spectra are shown to be a useful technique to study the conformer's populations and energy difference.