Comprehensive Urey-Bradley Force Field for Molecules with Geminal Methyl Groups. I. Acetone

The coupling of motion between methyl groups in gem‐dimethyl molecules is studied by performing a normal coordinate analysis of acetone and acetone‐d₆. A Urey‐Bradley force field (UBFF) is employed, and the usefulness of four‐atom (cis) and five‐atom (supercis) configurations is investigated. Improvement over previous analyses of acetone is found when the in‐plane $\mathrm{C}–\mathrm{H}$ bending and nonbonded force constants are adjusted separately from the other $\mathrm{C}–\mathrm{H}$ terms, and when certain cis configurations are included. Possible physical significance is discussed for the negative value of the cis $\mathrm{H}–\mathrm{O}$ nonbonded force constant obtained; attraction between the in‐plane hydrogens and the carbonyl oxygen is indicated.