Determining the conformer populations of (R)-(+)-3-methylcyclopentanone using vibrational absorption, vibrational circular dichroism, and specific rotation

Experimental mid-infrared absorption and vibrational circular dichroism (VCD) spectra of (R)-(+)-3-methylcyclopentanone were measured in CCl4 Solution. Theoretical absorption and VCD spectra were obtained with B3LYP functional and large basis sets, namely aug-cc-pVDZ, 6-311++G(2d,2p), and aug-cc-pVTZ. From the temperature dependent absorption spectra, DeltaHdegrees for equatorial <----> axial equilibrium was determined to be 4.84 kJ/mol, and the ratio of percent populations of equatorial-methyl: axial-methyl conformers, to be 87:13. The populations of equatorial-methyl and axial-methyl conformers were also determined using the experimental and predicted vibrational intensities. However, the errors associated with the populations determined from infrared absorption and VCD band intensities are fairly large. Optical rotation of (+)-3-methylcyclopentanone at 589 nm was measured as a function of concentration in CCl4, CH3OH, and CH3CN solvents, and intrinsic rotation was extracted therefrom. Theoretical specific rotations were predicted using the B3LYP functional and the same basis sets as those used for infrared absorption and VCD. The populations determined from predicted specific rotations and experimental intrinsic rotation are within the error ranges of those determined from infrared absorption and VCD intensities. Using the populations determined from temperature dependent absorption spectra, the predicted population weighted specific rotation for an isolated molecule is found to be larger than the observed intrinsic rotation. The incorporation of solvent influence in specific rotation calculation increased the deviation of predicted value from experimental value, indicating the need for improved models for solvent influence.