The Microwave Spectrum and Structure of the Argon-Acetaldehyde van der Waals Complex

Abstract

The argon-acetaldehyde van der Waals dimer was studied by Fourier transform microwave spectroscopy. Two tunneling motions were observed in the spectrum, an inversion through a planar configuration and methyl interal rotation. A simple deperturbation technique was employed in order to obtain rotational constants for structural purposes. The structure was found to be a nonplanar skew, with the argon binding on top of the C-C-O triangle. This was determined by assigning the rotational spectrum of two isotopic species: normal and Ar [middle dot] CH3CDO. The argon atom is located 3.592(5) A from the acetaldehyde center of mass, and the distances Ar-Ocarbonyl, Ar-Ccarbonyl, and Ar-Cmethyl are 3.59(1), 3.77(1), and 3.85(1) A, respectively. The dipole moment was determined as [mu] = 2.63(2) D. An induction model was employed to explain the decrease in the dipole moment compared to free acetaldehyde. A dispersion model was used to rationalize the structural data. The binding energy of the dimer was estimated to be 204(1) cm-1 from centrifugal distortion data and a Lennard-Jones potential.