Molecular Systems

The main goals of this section can be grouped together into the following categories:
   • Pressure as a probe on unsaturated NOCH systems
   • Pressure-induced reactivity on NOCH materials

a. Molecular Systems: Pressure as a probe on unsaturated NOCH systems

As pressure increases, the range ofstability of a molecular solid in a given crystalline structure is finite and, usually, a transformation towards another structure having a more effective packing of the atoms -a pressure induced phase transition- takes place. It can be said that polymorphism is the fundamental phenomenon associated with structural changes suffered by solids as a consequence of pressure and temperature effects. Frequently, these changes are accompanied by a reduction of the band gap (as in the metallization of some binary semiconductors), an increase of the hardness (as in many ceramic nitrides and oxides). Of course, all these changes modify the bonding between the atoms or molecules in the solid and, in some cases, as in many NOCH molecular solids, the change is so drastic that specific reactive processes are induced.

Therefore, the study of polymorphism has special relevance to the understanding of essential scientific problems (cohesion of molecular solids as a balance of different interactions, conformational changes packing effects, etc), but also to the development of novel molecular materials for technological applications. Since the study of these transformations opens definitely a wide spectrum of scientific cases of interest, in MALTA we shall focus on those physico-chemical phenomena involving NOCH molecular systems which are in close relationship to other topics of this proposal. These are listed in the corresponding Work Plan table.

b.  Molecular Systems: Pressure-induced reactivity on NOCH materials

The relevance of studying the reactivity of pure molecular species lies in the possibility of a complete transformation of these materials in condensed phase, without using additional reactive compounds. Nevertheless, in general, the reaction mechanism under pressure may differ substantially from that at room conditions. This is a consequence of the changes induced by pressure in the electronic structure of the molecules, and becomes more important when the reaction involves excited states, as increasing the pressure decreases the energy difference between the excited states and the ground state. However, high pressure reaction mechanisms are not yet sufficiently understood and there are little detailed studies on this matter. Then, a relevant objective of this proposal will be not to provide new reactive processes but to deep in the understanding of the factors affecting high pressure reactivity. To achieve this objective, it is necessary to perform a detailed study of the first steps of the reaction by combining experiments and theoretical calculations.