Investigating (R)-3-Methylcyclopentanone Conformers Using Temperature-Dependent Raman Spectroscopy

Recorded temperature-dependent Raman spectra of neat (R)-3-methylcyclopentanone (R3MCP) over the Raman active C-H stretch region (2850-3000 cm^-1) are being employed to determine conformer energy difference (ΔH° = 4.83 ± 0.45 kJ/mol) between R3MCP equatorial-methyl and axial-methyl isomers. Upon comparison with spectra obtained at room temperature, crystalline R3MCP Raman spectra recorded at liquid nitrogen temperature (~77 K) are being utilized to assist identifying Raman vibrational modes a rising due to R3MCP equatorial and axial conformers. Correspondingly, density functional theory calculations (correlation function type B3LYP using a moderate 6-31G* and large aug-cc-pVDZ basis sets) are also manipulated to obtain highly resolved Raman spectra for the optimized geometries of equatorial and axial conformers, which are also used to help identify vibrational modes a rising due to each conformer. Reported calculated spectra of the individual R3MCP conformers are shown to have good agreement with corresponding experimental Raman spectra.