Hydroxyacetone (CH₃COCH₂OH): A combined microwave and millimeter-wave laboratory study and associated astronomical search

A combined laboratory and astronomical investigation has been conducted on the methyl sugar hydroxyacetone (CH₃COCH₂OH). Rotational transitions of this species in the ground torsional state (v_t = 0) were recorded using both millimeter-wave direct absorption techniques and Fourier transform microwave spectroscopy. A total of 1145 lines of CH ₃COCH₂OH were analyzed in the frequency range 4 to 180 GHz, including transitions arising from both A- and E-symmetry species. A modified rho-axis method Hamiltonian was needed for the analysis because of the presence of perturbations resulting from the torsional motion of the methyl group in this molecule. Assignment of the E-species was particularly problematic as a consequence of significant mixing between the ground and torsionally excited levels. The complete data set was fitted using 21 spectroscopic parameters and had a global rms of 90 kHz; the barrier to internal rotation was established to be 65.3560(22) cm^-1. An astronomical search was subsequently conducted for hydroxyacetone at 2 and 3 mm using the 12 m telescope of the Arizona Radio Observatory. Twenty-eight favorable transitions arising from both A- and E-species, each consisting of collapsed quartets, were searched for toward Sgr B2(N). Although credible features were detected at several frequencies of hydroxyacetone, there were a sufficient number of missing lines to rule out an interstellar detection. An upper limit to the column density of N_tot < 5 × 10¹² cm^-2 was derived for CH₃COCH₂OH in Sgr B2(N), indicating that this species is an order of magnitude less abundant than glycolaldehyde (CH₂OHCHO).