sources, such as rotational spectra, NMR spectroscopy or high-quality quantum
mechanical calculations with the electron diffraction data, if the latter are not
sufficient to determine the molecule’s structure completely. This makes it ideal for
studying small molecules, and Pauling and his colleagues at the California Institute
of Technology used the technique very extensively in the 1930s and 1940s to enlarge
the chemists’ structural database. References [104–116] provide examples of typical
applications.
Organic molecules studied include acrolein, glyoxal, butadiene, ethylene, 1,3,5cis-hexatriene, 1,3,5-trans-hexatriene, 2,3-dimethylbutadiene and 3,4-dimethyl-12,4-hexadienes. Some systems involving triple bonds which were also studied are
acrylonitrile, vinyl acetylene, cyanogen, diacetylene and dimethylacetylene.
Strained ring compounds studied include norbornadiene, 1,4-dichloronorbornane,
bicyclo-[1,1, l]-pentane, bicyclo-[2.1.0] pentane, spiropentane, bicyclo-[3.1.1]-heptane, bicyclo-[2.1.1] hexane, 4-chloronortricyclene, bullvalene, hexamethyl Dewar
benzene, hexafluoro-Dewar benzene, bicyclo-[2.2.2]-octane and triethylenediamine.
It proved to be particularly useful for defining changes in dimensions and bond
angles for isostructural molecules. For example, the C–C bond length in C 2 F 4 , C 2 H 4 ,
C 2 Cl 4 and C 2 Br 4 were shown to correlate with the stretching force constant. Figure 10 gives examples of simple inorganic molecules which were studied, and the
bond length differences have been discussed by Haaland [107].
In the area of organometallic chemistry [109, 110], cyclopentadienyl compounds
of the main group and transition metals have been extensively studied. In iron and
ruthenium dicyclopentadienyl, the rings appear to be eclipsed (D 5h ), but the barrier
to internal rotation is only 1 kcal/mol. In dicyclopentadienylnickel and
dicyclopentadienylmanganese, essentially free rotation was observed. In lead and
tin cyclopentadienyl, the metal atom is so bulky that the rings are no longer parallel
and are tilted relative to each other by 45 and 55
, respectively. Bartell was
instrumental in testing the ideas of Nyholm and Gillespie by determining numerous
structures of central atom molecules [114–116]. The structures of a number of
central atom molecules with three, five, six and seven substituent groups have
been investigated by electron diffraction, and the results have proven to be useful
in testing various bonding theories. It is not essential to undertake the studies at
ambient temperatures, and high-temperature species have been studied, for example,
As 4 , AsI 3 and GaI 3 . The ED patterns for all of the dihalides of Be, Mg, Ca, Sr, Ba,
Zn, Cd and Hg with the exception of HgF 2 have been obtained and the bonded
distances and angles determined. The angular nature of the heavier Group 2 metal
halides has resulted in extensive theoretical speculations [107, 108].
Low-energy electron diffraction (LEED) is a technique for the determination of
the surface structure of single-crystalline materials by bombardment with a collimated beam of low-energy electrons (20–200 eV) and observation of diffracted
electrons as spots on a fluorescent screen.
LEED may be used in one of two ways:
1. Qualitatively, where the diffraction pattern is recorded and analysis of the spot
positions gives information on the symmetry of the surface structure. In the
Early History of X-Ray Crystallography
33
mechanical calculations with the electron diffraction data, if the latter are not
sufficient to determine the molecule’s structure completely. This makes it ideal for
studying small molecules, and Pauling and his colleagues at the California Institute
of Technology used the technique very extensively in the 1930s and 1940s to enlarge
the chemists’ structural database. References [104–116] provide examples of typical
applications.
Organic molecules studied include acrolein, glyoxal, butadiene, ethylene, 1,3,5cis-hexatriene, 1,3,5-trans-hexatriene, 2,3-dimethylbutadiene and 3,4-dimethyl-12,4-hexadienes. Some systems involving triple bonds which were also studied are
acrylonitrile, vinyl acetylene, cyanogen, diacetylene and dimethylacetylene.
Strained ring compounds studied include norbornadiene, 1,4-dichloronorbornane,
bicyclo-[1,1, l]-pentane, bicyclo-[2.1.0] pentane, spiropentane, bicyclo-[3.1.1]-heptane, bicyclo-[2.1.1] hexane, 4-chloronortricyclene, bullvalene, hexamethyl Dewar
benzene, hexafluoro-Dewar benzene, bicyclo-[2.2.2]-octane and triethylenediamine.
It proved to be particularly useful for defining changes in dimensions and bond
angles for isostructural molecules. For example, the C–C bond length in C 2 F 4 , C 2 H 4 ,
C 2 Cl 4 and C 2 Br 4 were shown to correlate with the stretching force constant. Figure 10 gives examples of simple inorganic molecules which were studied, and the
bond length differences have been discussed by Haaland [107].
In the area of organometallic chemistry [109, 110], cyclopentadienyl compounds
of the main group and transition metals have been extensively studied. In iron and
ruthenium dicyclopentadienyl, the rings appear to be eclipsed (D 5h ), but the barrier
to internal rotation is only 1 kcal/mol. In dicyclopentadienylnickel and
dicyclopentadienylmanganese, essentially free rotation was observed. In lead and
tin cyclopentadienyl, the metal atom is so bulky that the rings are no longer parallel
and are tilted relative to each other by 45 and 55
, respectively. Bartell was
instrumental in testing the ideas of Nyholm and Gillespie by determining numerous
structures of central atom molecules [114–116]. The structures of a number of
central atom molecules with three, five, six and seven substituent groups have
been investigated by electron diffraction, and the results have proven to be useful
in testing various bonding theories. It is not essential to undertake the studies at
ambient temperatures, and high-temperature species have been studied, for example,
As 4 , AsI 3 and GaI 3 . The ED patterns for all of the dihalides of Be, Mg, Ca, Sr, Ba,
Zn, Cd and Hg with the exception of HgF 2 have been obtained and the bonded
distances and angles determined. The angular nature of the heavier Group 2 metal
halides has resulted in extensive theoretical speculations [107, 108].
Low-energy electron diffraction (LEED) is a technique for the determination of
the surface structure of single-crystalline materials by bombardment with a collimated beam of low-energy electrons (20–200 eV) and observation of diffracted
electrons as spots on a fluorescent screen.
LEED may be used in one of two ways:
1. Qualitatively, where the diffraction pattern is recorded and analysis of the spot
positions gives information on the symmetry of the surface structure. In the
Early History of X-Ray Crystallography
33