scattering power of protons and deuterons means that the position of hydrogen in a
crystal and its thermal motions can be determined with greater precision by neutron
diffraction. The structures of metal hydride complexes, e.g., Mg 2[ FeH 6 ] and
K 2 [ReH 9 ], have been assessed by neutron diffraction [86, 95]. The neutron scattering lengths bH ¼ À3.7406(11) fm [8] and bD ¼ 6.671(4) fm, [13a] for H and D,
respectively, have an opposite sign, which allows the technique to distinguish them.
In fact there is a particular isotope ratio for which the contribution of the element
would cancel; this is called null-scattering.
It is undesirable to work with the relatively high concentration of H in a sample.
The scattering intensity by H-nuclei has a large inelastic component, which creates a
large continuous background that is more or less independent of scattering angle.
The elastic pattern typically consists of sharp Bragg reflections if the sample is
crystalline. They tend to drown in the inelastic background. This is even more
serious when the technique is used for the study of liquid structure. Nevertheless,
by preparing samples with different isotope ratios, it is possible to vary the scattering
contrast enough to highlight one element in an otherwise complicated structure. The
variation of other elements is possible but usually rather expensive. Hydrogen is
inexpensive and particularly interesting, because it plays an exceptionally large role
in biochemical structures and is difficult to study structurally in other ways.
7.4 Electron Diffraction
It was noted earlier that in the early days the determination of the structures of
molecular organic and organic compounds by X-ray diffraction proved problematic
because of the phase problem. Electron diffraction proved to be very important
between 1920 and 1960 for establishing the structures of small organic and inorganic
molecules [100]. The only constraint was that the molecule had to be sufficiently
volatile to generate a good concentration of molecules in the gas phase. When a
monochromatic beam of electrons is used with molecules in the gas phase it interacts
only at the point where the beam crosses. The electrons are scattered mainly by
interactions with the electric fields of the atomic nuclei contained in the molecule,
and the proportion of electrons scattered is large (larger than those for X-rays and
much larger than those for neutrons [101–103]. The data are collected within a
matter of seconds or at most minutes. However, the intensity of the scattering falls
off rapidly with the scattering angle. In a gaseous state, the molecules are randomly
orientated, and therefore the diffraction pattern consists of diffuse concentric rings
and is described simply of intensities as a function of scattering angle. For a
polyatomic molecule, the total molecular scattering is the sum of the components
of each pair of atoms on the molecule, i.e. for a triatomic molecule there are three
components, 6 for 4 atoms, 10 for 5 atoms and 1,225 for 50 atoms. Thus, only
relatively simple molecules can be completely structurally characterised by electron
diffraction in the gas phase. The data is more amenable to analysis if the molecules
are highly symmetric. It is possible to combine information obtained from other
32
D. M. P. Mingos
crystal and its thermal motions can be determined with greater precision by neutron
diffraction. The structures of metal hydride complexes, e.g., Mg 2[ FeH 6 ] and
K 2 [ReH 9 ], have been assessed by neutron diffraction [86, 95]. The neutron scattering lengths bH ¼ À3.7406(11) fm [8] and bD ¼ 6.671(4) fm, [13a] for H and D,
respectively, have an opposite sign, which allows the technique to distinguish them.
In fact there is a particular isotope ratio for which the contribution of the element
would cancel; this is called null-scattering.
It is undesirable to work with the relatively high concentration of H in a sample.
The scattering intensity by H-nuclei has a large inelastic component, which creates a
large continuous background that is more or less independent of scattering angle.
The elastic pattern typically consists of sharp Bragg reflections if the sample is
crystalline. They tend to drown in the inelastic background. This is even more
serious when the technique is used for the study of liquid structure. Nevertheless,
by preparing samples with different isotope ratios, it is possible to vary the scattering
contrast enough to highlight one element in an otherwise complicated structure. The
variation of other elements is possible but usually rather expensive. Hydrogen is
inexpensive and particularly interesting, because it plays an exceptionally large role
in biochemical structures and is difficult to study structurally in other ways.
7.4 Electron Diffraction
It was noted earlier that in the early days the determination of the structures of
molecular organic and organic compounds by X-ray diffraction proved problematic
because of the phase problem. Electron diffraction proved to be very important
between 1920 and 1960 for establishing the structures of small organic and inorganic
molecules [100]. The only constraint was that the molecule had to be sufficiently
volatile to generate a good concentration of molecules in the gas phase. When a
monochromatic beam of electrons is used with molecules in the gas phase it interacts
only at the point where the beam crosses. The electrons are scattered mainly by
interactions with the electric fields of the atomic nuclei contained in the molecule,
and the proportion of electrons scattered is large (larger than those for X-rays and
much larger than those for neutrons [101–103]. The data are collected within a
matter of seconds or at most minutes. However, the intensity of the scattering falls
off rapidly with the scattering angle. In a gaseous state, the molecules are randomly
orientated, and therefore the diffraction pattern consists of diffuse concentric rings
and is described simply of intensities as a function of scattering angle. For a
polyatomic molecule, the total molecular scattering is the sum of the components
of each pair of atoms on the molecule, i.e. for a triatomic molecule there are three
components, 6 for 4 atoms, 10 for 5 atoms and 1,225 for 50 atoms. Thus, only
relatively simple molecules can be completely structurally characterised by electron
diffraction in the gas phase. The data is more amenable to analysis if the molecules
are highly symmetric. It is possible to combine information obtained from other
32
D. M. P. Mingos