Density functional theory and Møller-Plesset studies of hindered rotations of acetone

The hindered rotations of acetone were studied density functional theory (B3LYP) and second order Møller-Plesset approaches using 6-31G** and 6-311G** basis sets. One of the CH(3) groups of acetone with fixed heavy atoms was rotated from 0.0 to 120 degrees, and CCH angles were scanned from 90.3 to 130.3 degrees to cover the potential energy surface of interest; a circular valley was obtained with the deepest potential value at a CCH angle equal to 109.3 degrees. Potential energy profiles were then calculated by assuming that the molecular geometry could relax during rotation (i.e., each value of the torsion angle of the molecular geometry was optimized). Next, the two methyl groups were both rotated clockwise, and then one was rotated clockwise and the other counterclockwise. Using the variation method, and utilizing the first 20 harmonic oscillator wave functions, the energy levels, relative transition moment and relative transition intensities of the component of the hindered rotation nu(2) (125.16 cm(-1)) were computed in a one-dimensional Schrodinger equation. The first three energy levels were almost degenerate; the next three were opened up, and the seventh energy level appeared above the level where tunneling can occur.