Rotational spectra of argon acetone: A two-top internally rotating complex

The rotational spectra of the argon acetone weakly bound complex was studied by pulsed jet Fabry-Perot Fourier transform microwave spectroscopy. Over 500 transitions of the complex were measured between 5.5 and 26 GHz from J = 2-1 to J = 12-11. The two methyl groups undergo hindered internal rotation resulting in four or five internal rotation states. The microwave transitions are within these states, resulting in a splitting of each rotational transition into four and sometimes five distinct transitions. The three-fold barrier to internal rotation is determined to be 260 cm(-1), 2% less than the 266 cm(-1) barrier in acetone itself. The structure of the complex has the argon atom above the heavy atom plane of the acetone, 3.52 A from the C=O bond and approximately in the C-s plane, which is perpendicular to the C-C-C plane of acetone. (C) 2002 Elsevier Science (USA).