Foreword
Loosely speaking the structure of molecules is determined by the forces between
the atoms. In practice, the potential energy surface of a polyatomic molecule is
calculated by ordering the atomic nuclei in the field created by the electrons. This
basic concept is known as the Born–Oppenheimer approximation applied to
molecular physics. As a result, the molecular structure is related to the minimum
energy configuration (including zero-point motion) in this landscape which
describes the chemical bonds. Therefore, molecular structure and chemical bond are
related to each other in an intimate way where these two aspects are the two sides
of the same coin. Thus, knowing a precise molecular structure warrants a detailed
understanding of the chemical bonds. From such a chemical view point, there is
also a strong relation between structure and reactivity. As such there is a fundamental need to determine accurate molecular structures as this is the basis for our
understanding of chemistry as a whole which is the basis of the world around us.
Binding energies in molecules range over many orders of magnitude between
strong covalent bonds and the weak interactions in complexes which are bound
only by van der Waals forces. These variations lead to dramatic changes in bond
distances in particular and the molecular structure in general. But also in more
ordinary molecules, a large variety of different isomers, i.e., different connectivities
of atoms within a molecule, are realized in nature. One prominent example of
isomers detected in the interstellar medium, the space in-between stars, concerns the
common molecule acetic acid (CH 3 COOH) with its isomers methyl formate
(HCOOCH 3 ) and glycolaldehyde (HCOCH 2 OH). The properties of these molecules
are vastly different. The chemical richness becomes even larger when considering
species of the same chemical connectivity but with different orientations of
chemical subgroups as for example the anti and syn conformers of methyl formate.
These examples illustrate the relation between chemical variation and structural
richness. It also demonstrates the need for accurate structures of a large variety of
species. Moreover, it shows the need to account for all effects (forces) to arrive at
reliable molecular structures. This chemical richness explains why the structure
determination of free molecules described in this book is still a very active field of
research even though a tremendous body of data is already available in databases.
vii
Loosely speaking the structure of molecules is determined by the forces between
the atoms. In practice, the potential energy surface of a polyatomic molecule is
calculated by ordering the atomic nuclei in the field created by the electrons. This
basic concept is known as the Born–Oppenheimer approximation applied to
molecular physics. As a result, the molecular structure is related to the minimum
energy configuration (including zero-point motion) in this landscape which
describes the chemical bonds. Therefore, molecular structure and chemical bond are
related to each other in an intimate way where these two aspects are the two sides
of the same coin. Thus, knowing a precise molecular structure warrants a detailed
understanding of the chemical bonds. From such a chemical view point, there is
also a strong relation between structure and reactivity. As such there is a fundamental need to determine accurate molecular structures as this is the basis for our
understanding of chemistry as a whole which is the basis of the world around us.
Binding energies in molecules range over many orders of magnitude between
strong covalent bonds and the weak interactions in complexes which are bound
only by van der Waals forces. These variations lead to dramatic changes in bond
distances in particular and the molecular structure in general. But also in more
ordinary molecules, a large variety of different isomers, i.e., different connectivities
of atoms within a molecule, are realized in nature. One prominent example of
isomers detected in the interstellar medium, the space in-between stars, concerns the
common molecule acetic acid (CH 3 COOH) with its isomers methyl formate
(HCOOCH 3 ) and glycolaldehyde (HCOCH 2 OH). The properties of these molecules
are vastly different. The chemical richness becomes even larger when considering
species of the same chemical connectivity but with different orientations of
chemical subgroups as for example the anti and syn conformers of methyl formate.
These examples illustrate the relation between chemical variation and structural
richness. It also demonstrates the need for accurate structures of a large variety of
species. Moreover, it shows the need to account for all effects (forces) to arrive at
reliable molecular structures. This chemical richness explains why the structure
determination of free molecules described in this book is still a very active field of
research even though a tremendous body of data is already available in databases.
vii