The dimers of glyoxal and acrolein with H2O and HF: Negative intramolecular coupling and blue-shifted C–H stretch

The structures and the vibrational spectra of the hydrogen-bonded complexes: glyoxal-H2O, glyoxal-HF, acrolein-H2O, and acrolein-HF, are investigated within the MP2/aug-cc-pVTZ computational approach. It is demonstrated that the calculated blue shifts of the C–H stretching frequencies in the glyoxal-H2O complexes are only indirectly pertinent to hydrogen bonding to the C–H group. The comparison with the glyoxal-HF and the acrolein-HF complexes reveals that these blue shifts are a direct consequence of a negative intramolecular coupling between vicinal CO and C–H bonds in the aldehyde groups of isolated glyoxal and acrolein molecules. To support this interpretation, the halogen-bonded complexes glyoxal-BrF and acrolein-BrF are discussed.