Hydrated Complexes of Atmospheric Interest: Rotational Spectrum of Diacetyl-Water

The rotational spectrum of the molecular complex diacetyl-water has been measured by pulsed jet Fourier transform microwave (FTMW) spectroscopy. The water molecule acts as an electrofore and induces a dipole moment when combined with nonpolar diacetyl. Only one conformer has been observed, corresponding to the global minimum. Its shape is unambiguously established: the water moiety is linked asymmetrically to the organic molecule; it behaves as a proton donor to one of its oxygen atoms and interferes with the internal rotation of adjacent methyl group through a C-H center dot center dot center dot O interaction. Each rotational transition appears as a quintuplet, due to the internal rotation of the two methyl groups, which are nonequivalent in the adduct. From the analysis of the observed splittings, the V(3) barriers to the internal rotation of the two methyl groups have been determined to be 3.81(2) and 4.11(2) kJ/mol, respectively.