Final Report Summary - MOLHENANO (Molecular Aggregation and Microsolvation in Ultracold Helium Nanodroplets) The study of the molecular interactions in molecular aggregates has been one of the main objectives of physical chemistry. His practical importance is very broad ranging from molecular processes crucial for the chemical industry to key aspects for the understanding of living organisms. A new experimental approach to the study of molecular aggregates was possible by combining high resolution spectroscopy with superfluid helium nanodroplets. The fast cooling of the molecules embedded by the helium droplet as well as the very weakly interacting helium environment causes that, within this frame, new phenomena appear in the study of molecular aggregates. It is worth commenting, for instance, the observation of structures different from the global minimum energy configurations for the molecular aggregates in this cold environment. This makes possible that local energy minima that are very difficult or impossible to observe experimentally could be stabilised and studied using these techniques.The aim of the present project was to carry out the study of molecular aggregates of biological importance embedded in superfluid helium nanodroplets by means of high resolution infrared spectroscopy. In the first phase, the study of HCl-water complexes was accomplished continuing the work done previously at the Ruhr University Bochum. The observation of unexpected shifts in the spectra upon oxygen isotopic substitution indicates that previous results on this system have to be re-examined.The second part of the project was focused on the study of pyridine aggregation and the water-pyridine interaction. In the case of pyridine dimer, only one non-global energy minimum structure was found. This experimental result is surprising taking into account the very complex potential energy surface in which more than ten different energy minima were found in ab initio calculations. Molecular alignment due to long distance dipole-dipole interaction provides the explanation to the experimental results. This explanation is reinforced in the light of the experimental results for the pyridine-water system. Two different structures were found. One of them, the global energy minimum presented a NH connection between pyridine and water. The other one about 5 kcal/mol higher in energy is also present. The alignment of the electric dipole moments for this structure makes favourable its formation in helium nanodroplets. The application of the methodology used in this project can be extended to other systems. As the energy available in the matrix is very low, long range orientation of these molecules will determine the final conformation. Therefore very shallow minima in molecules can be obtained by considering its permanent dipole moments.