Molecules with internal hydrogen bond are attracting scientific attention from the beginning of the 20th century. Despite numerous theoretical and experimental studies, a number of structure and dynamic aspects, even for the smallest molecules such as Glycolaldehyde (GA) or Acetylacetone (AcAc), remains under question.
Infrared spectroscopy combined with matrix isolation technique provides a powerful tool to study various properties of molecular systems. Solid molecular hydrogen crystals with all molecules in the J=0 rotational state (parahydrogen, pH2) is recently widely recognized as a very attractive media for the matrix isolation experiments. Hydrogen molecule in the J=0 state has anti-parallel nuclear spins resulting in absence of dipole quadruple or higher moments. Very weak interaction and "softness" of molecular hydrogen crystal results in a very small perturbation on the molecules isolated in such environment.
Our studies of infrared spectra of GA and AcAc isolated in pH2 show similar patterns as those isolated in other matrices. Spectral bands are sharper in pH2 than in other matrices. Very important feature of pH2 is that no notable matrix split was observed. This facilitates more accurate assignment of the spectral bands which is important for the studies of conformational diversity, isomerization processes and possible hydrogen bond complexes. Experiments with added water allow us to observe and assign spectral bands of water-Ga complex. UV irradiation results in isomerization and fragmentation of GA isolated in pH2.
Infrared studies of AcAc revealed that the most stable structure in matrices is enol with Cs symmetry contrary to C2v recently proposed in the gas phase. About 7 % of keto tautomer of AcAc was observed in the pH2 matrices. Photoisomerization experiments using quadruple YAG radiation revealed five additional stable enolic forms of AcAc. No fragmentation was observed in the pure pH2 matrices.