Unusual internal rotation coupling in the microwave spectrum of pinacolone

Abstract

The molecular beam Fourier-transform microwave spectrum of pinacolone (methyl tert-butyl ketone) has been measured in several regions between 2 and 40GHz. Fits of the assigned spectrum using several computer programs based on different models for treating torsion–rotation interaction lead to the conclusion that no existing program correctly captures the internal dynamics of this molecule. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory indicate that this molecule does not have a plane of symmetry at equilibrium, and that internal rotation of the light methyl group induces a large oscillatory motion of the heavy tert-butyl group from one side of the Cs configuration to the other. This effect has been modeled for J=0 levels by a relatively simple two-top torsional Hamiltonian, where the magnitudes of the strong coupling terms between the tops are determined directly from the ab initio two-dimensional potential surface. A plot of the resultant 0A, 0E, 1E, 1A torsional levels on the same scale as a one-dimensional potential curve along the zig-zag path connecting the six (unequally spaced) minima bears a striking resemblance to the 1:2:1 splitting pattern of the A, E, E, B levels of an internal rotation problem with a sixfold barrier. It seems likely that rotational transitions within the 1E and 1A torsional levels are the cause of the roughly 50% of the spectrum that remains unassigned after all predicted transitions within the 0A and 0E torsional levels are removed. However, a much more complete measurement campaign and some new torsion–rotation theory will be needed to verify this hypothesis.