Sulfur-Containing Flavors: Gas Phase Structures of Dihydro-2-methyl-3-thiophenone

Dihydro-2-methyl-3-thiophenone was investigated using a combination of quantum chemical calculations and molecular beam Fourier transform microwave spectroscopy. The substance is present in coffee, roasted peanuts, and whiskey. The microwave spectrum was recorded under molecular beam conditions in the frequency range from 9 to 14 GHz. We report on the two main conformers of dihydro-2-methyl-3-thiophenone, for which highly accurate rotational constants and centrifugal distortion constants were obtained. No splittings due to internal rotation of the methyl group could be observed in the microwave spectrum. This is in agreement with the theoretical predictions of the barrier heights, which have been determined to be more than 1000 cm^–1 at the MP2/6-311++G­(d,p) level of theory. In addition to the most abundant ³²S-isotopologue of the main conformer, also the ³⁴S-isotopologue was assigned, which occurs with a natural abundance of about 4%. Using the experimental rotational constants, different quantum chemical calculations were validated for the two observed conformers. To complete the theoretical investigation of dihydro-2-methyl-3-thiophenone, different transition states were optimized to understand the intramolecular conversion between the two conformers at the MP2/6-311++G­(d,p) level. The transition states were optimized using the Berny algorithm