FTIR study of acetylacetone, D2-acetylacetone and hexafluoroacetylacetone–water complexes in argon and nitrogen matrices

Abstract

Association of acetylacetone molecules with water was studied by means of infrared absorption spectroscopy aided by matrix isolation technique. The spectra of acetylacetone–water mixtures isolated in low temperature argon and nitrogen matrices revealed additional spectral bands, not observed in the spectra of pure substances, thus confirming the formation of hydrogen bonded complexes. The precise assignment of the spectral bands was performed by varying the sample concentration, performing annealing experiments and DFT B3LYP 6-311++G(3df, 3pd) calculations. Positions of the associated water bands indicate a medium strength hydrogen bond comparable to the one observed in the water trimers. The effect of hydrogen bond formation is rather minimal for the acetylacetone molecule and our experiments confirm no significant influence on an internal hydrogen bond structure or dynamics in the acetylacetone molecule. Similar conclusions are valid in the case of the D₂O D2-acetylacetone complex. Different situation is observed when CH₃ groups in acetylacetone are replaced with CF₃ groups. The calculated energy of the complex is twice as small. This is also confirmed by a very small bounded OH stretch shift. This observation confirms that the electronic structure of the molecular groups even relatively far away from the hydrogen bond accepting atom has a large influence on its possibility to form a hydrogen bond.