2
1 Introduction
gas and first published in his dissertation. It was used to determine the radius of
molecules (van der Waals radius for atoms). However, the existence of molecules
was contested by reputed scientists such as Ernst Mach and Marcellin Berthelot until
the experimental work of Perrin (1913) on the Brownian motion.
The golden age of molecular structure is the first half of the twentieth century with
the discovery of X-rays by Röntgen in (1895) and the diffraction of X-rays by crystals
by Friedrich et al. (1912). This permitted, for the first time, to determine interatomic
distances in crystals. The structure of diamond was obtained in 1913 by Bragg father
and son (1913). Debye (1929) was the first to determine the experimental structure
of a molecule (CCl 4 ) in gas phase by X-ray diffraction. At about the same time,
around 1930, the first gas-phase electron diffraction experiment was performed by
Mark and Wierl (1930). Although the accuracy was not high, many molecules were
studied in a short time, and the results were used to develop the concept of chemical
bond by Pauling (1931) (see also Pauling 1960) and others. Since that time, a lot of
progress has been made, both experimentally and theoretically, and nowadays, gasphase electron diffraction is one of the best methods to determine the structure of a
wide range of molecules (both small and relatively large, both polar and non-polar,
both in ground and excited electronic states, etc.).
Starting from 1945, neutron diffraction experiments were also carried out.
Thermal neutrons (i.e., with a wavelength of about 0.1 nm) are used. The big inconvenient of this method is that it requires a nuclear reactor and is therefore rarely
used. On the other hand, its advantage is that the neutrons are diffracted by the nuclei
contrary to the other methods. Therefore, light atoms as hydrogen may contribute
strongly to the diffracted intensity (Shull 1995).
In the diffraction methods, the elastic scattering, i.e., occurring without exchange
of energy between the radiation and the molecules, is used (see Sect. 7.4 for electron
diffraction).
When a molecule absorbs a photon of frequency ν, the molecule goes from energy
level E 0 to level E 1 and Planck’s law gives
E 1 −E 0 = hν,
i.e., absorption or emission of photons is possible for some well-defined wavelengths.
It is usual to classify spectra according to the type of molecular energy that is
modified by the process:
• Electronic spectra: The electric field of the radiation enters in interaction with the
dipole moment of the moving electrons and the fixed nuclei. The transitions are
found in the visible and ultraviolet ranges.
• Vibrational spectra: The electric field of the radiation enters in interaction with
the oscillating dipole moment formed from the center of gravity of the nuclei and
the center of gravity of the electrons. In other words, the vibration must modify
the electric dipole moment. The transitions are observed in the infrared range.
1 Introduction
gas and first published in his dissertation. It was used to determine the radius of
molecules (van der Waals radius for atoms). However, the existence of molecules
was contested by reputed scientists such as Ernst Mach and Marcellin Berthelot until
the experimental work of Perrin (1913) on the Brownian motion.
The golden age of molecular structure is the first half of the twentieth century with
the discovery of X-rays by Röntgen in (1895) and the diffraction of X-rays by crystals
by Friedrich et al. (1912). This permitted, for the first time, to determine interatomic
distances in crystals. The structure of diamond was obtained in 1913 by Bragg father
and son (1913). Debye (1929) was the first to determine the experimental structure
of a molecule (CCl 4 ) in gas phase by X-ray diffraction. At about the same time,
around 1930, the first gas-phase electron diffraction experiment was performed by
Mark and Wierl (1930). Although the accuracy was not high, many molecules were
studied in a short time, and the results were used to develop the concept of chemical
bond by Pauling (1931) (see also Pauling 1960) and others. Since that time, a lot of
progress has been made, both experimentally and theoretically, and nowadays, gasphase electron diffraction is one of the best methods to determine the structure of a
wide range of molecules (both small and relatively large, both polar and non-polar,
both in ground and excited electronic states, etc.).
Starting from 1945, neutron diffraction experiments were also carried out.
Thermal neutrons (i.e., with a wavelength of about 0.1 nm) are used. The big inconvenient of this method is that it requires a nuclear reactor and is therefore rarely
used. On the other hand, its advantage is that the neutrons are diffracted by the nuclei
contrary to the other methods. Therefore, light atoms as hydrogen may contribute
strongly to the diffracted intensity (Shull 1995).
In the diffraction methods, the elastic scattering, i.e., occurring without exchange
of energy between the radiation and the molecules, is used (see Sect. 7.4 for electron
diffraction).
When a molecule absorbs a photon of frequency ν, the molecule goes from energy
level E 0 to level E 1 and Planck’s law gives
E 1 −E 0 = hν,
i.e., absorption or emission of photons is possible for some well-defined wavelengths.
It is usual to classify spectra according to the type of molecular energy that is
modified by the process:
• Electronic spectra: The electric field of the radiation enters in interaction with the
dipole moment of the moving electrons and the fixed nuclei. The transitions are
found in the visible and ultraviolet ranges.
• Vibrational spectra: The electric field of the radiation enters in interaction with
the oscillating dipole moment formed from the center of gravity of the nuclei and
the center of gravity of the electrons. In other words, the vibration must modify
the electric dipole moment. The transitions are observed in the infrared range.