Surface assisted chemical reactions: Chemistry beyond the test-tube

A variety of approaches was investigated to control the structure and yield of chemical reactions. Stereochemistry, based, e.g. on enantioselective reaction and chirality, and molecular recognition processes which represent a key in all biological self-assembly processes leading to complex multilevel systems, allow to design highly specific chemical products. They are based on reactions which require special geometrical conditions such as multiple bond formation according to the key-lock principle, which are usually not applicable in systems exhibiting mirror symmetry and single bond reactions. While these concepts are already widely used in conventional solvent chemistry two other, however, much less investigated effects are cooperativity/synergy requiring helper systems (the best ones known are catalysts), and the influence of dimensionality/spatial confinement. Even far less investigated is the combination of these two conditions.

Ideally, the two concepts, which will not work in a conventional 3-test-tube environment, may be perfectly studied under well controlled vacuum conditions on clean surfaces. Depending of the local structure of surfaces, such as metallic single crystals with a simple crystallographic structure or exhibiting a nanostructure after having undergone, e.g. a surface reconstruction, may significantly change the reactivity of molecular reactants on the surface. In addition the surface itself might undergo a re-ordering process due to the presence of the molecular system prior to the reaction, resulting in optimal spatial conditions for the molecular educts for driving them from a state of usually unfavored molecular reactions to preferred ones. In this way, for example, chemical reactions within 1-D channels or 2-D patches may occur fully self-consistently in the sense that the whole system (molecule + surface) finds its global minimum in the energetic hyper-plane without local external influence. Kinetic and thermodynamic effects can be used to tune these effects. First experiments demonstrated that the reactivity even in fairly simple, usually non-reactive alkane systems can be dramatically increased at very mild conditions making use of this approach (e.g. CH-activation).

The concept of applying cooperative/synergetic surface chemistry and its study at the single molecular level is just at the beginning and requires intense cooperation between surface physics, allowing to monitor chemical states during and after reactions by spectroscopic and microscopy techniques, and preparative as well as theoretical chemistry.