Apparent conflicting indications on the conformation of dimethylether-argon from the rotational spectra of the d6 and 13C species

The rotational spectra of dimethyl ether-d6-Ar and dimethyl ether-13C-Ar have been investigated by molecular beam Fourier transform microwave spectroscopy. The obtained rotational constants, when compared to those of the normal species, give contrasting indications on the conformation of the complex. We believe the indications given by the dimethyl ether-Ar / dimethyl ether-13C-Ar pair to be the correct ones. They suggest the Ar atom to lie in the v symmetry plane of dimethyl ether perpendicular to the C-O-C plane, shifted towards the oxygen atom. This conformation is in agreement with several kinds of theoretical calculations. Probably isotopic substitution effects make the data from dimethyl ether-d6-Ar not directly compatible with those of the other species.