Introduction

The problem of molecular structure elucidation is that of identifying a molecule from a given set of (mostly spectroscopical) data. Thus, in order to make that a little bit more precise, we have to make up our mind according to what we do mean by molecule. This means we first of all have to tell which mathematical models of a molecule we would like to use. So lets agree upon that we usually use approximative models for molecules, and that the first three approximations are the following ones:

• The first approximation is that of the chemical formula, for example, CH for benzene.
• The second step of approximating reality is the introduction of constitutional formulae, for example, there are exactly 217 constitutional isomers that correspond to the same chemical formula as benzene, and so this is a considerable refinement compared with the first step of approximation, where there was exactly one model of benzene only.
• The third step of approximation takes the spatial arrangement into account. To a constitutional formula there may correspond several configurational isomers, which contain besides the constitution also geometrical descriptors. For example, in the case of benzene, there are 958 such isomers for the chemical formula; again this step provides a considerable refinement of the second approximation.