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Top-top internal rotation coupling in the microwave spectrum of pinacolone



Jon T. Hougen, Yueyue Zhao, Ha Vanh L. Nguyen, Wolfgang Stahl


The molecular beam Fourier-transform microwave spectrum of pinacolone (methyl tert-butyl ketone) has been measured in several regions between 4 and 40 GHz. 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 six-fold barrier. It seems likely that rotational transitions within the 1 E and 1 A 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.
Journal of Chemical Physics


internal rotation, least-squares fit, methyl top, microwave spectrum, pinacolone, two-top potential surface


Hougen, J. , Zhao, Y. , L., H. and Stahl, W. (2015), Top-top internal rotation coupling in the microwave spectrum of pinacolone, Journal of Chemical Physics, [online], (Accessed May 24, 2024)


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Created October 8, 2015, Updated November 10, 2018