Conformational Landscape of the 1/1 Diacetyl/Water Complex Investigated by Infrared Spectroscopy and ab Initio Calculations

The complexes of diacetyl with water have been studied experimentally by Fourier transform infrared (FTIR) spectroscopy coupled to solid neon matrix and supersonic jet, and anharmonic ab initio calculations. The vibrational analysis of neon matrix spectra over the 100–7500 cm^–1 infrared range confirms the existence of two nearly isoenergetic one-to-one (1/1) diacetyl-water S₁ and S₂ isomers already evidenced in a previous argon matrix study. A third form (S₃) predicted slightly less stable (J. Mol. Mod. 2015, 21, 214) is not observed. The correct agreement obtained between neon matrix and anharmonic calculated vibrational frequencies is exploited in several cases to derive band assignments for the vibrational modes of a specific isomer. Thereafter, theoretical x_ij anharmonic coupling constants are used for the attribution of combination bands and overtones relative to the 1/1 dimer. Finally, the most stable isomer of the one-to-two (1/2) diacetyl–water complex is identified in the OH stretching region of water on the grounds of comparison of experimental and calculated vibrational shifts between water dimer and the three most stable 1/2 isomers.