Molecular Structure and Internal Dynamics of 2′-Hydroxyacetophenone by Free-Jet Absorption Millimeter-Wave Spectroscopy

The rotational spectrum of 2'-hydroxyacetophenone has been recorded and assigned for the first time using a Stark-modulated free-jet absorption millimeter-wave (FJ-AMMW) spectrometer in the 59.6-74.5 GHz frequency range. The most stable conformer has been detected and assigned: <i>A</i> = 2277.076(11), <i>B</i> = 1212.113(5) and <i>C</i> = 795.278(5) MHz. It is characterized by a Cs symmetry where a strong hydrogen bond between the acetyl oxygen atom and the hydroxyl atom takes place. The transition lines show a fine structure due to the internal rotation of the methyl group, which allowed the determination of a V3 = 565.1(5) cm-1 barrier. The corresponding tunneling splittings have been estimated to be 51 MHz. Calculations at the B3LYP-D3(BJ)/Def2-TZVP level underestimate the height of the barrier by about 156 cm-1. This value decreases to 25 cm-1 with MP2/aug-cc-pVTZ.