The Nobel Prize
in Chemistry 1936The Nobel Prize
in Chemistry 1936Presentation Speech by Professor A. Westgren, Secretary of the Nobel Committee for Chemistry of the Royal Swedish Academy of Sciences, on December 10, 1936
Your Majesty, Your Royal Highnesses, Ladies and Gentlemen.
Chemists have for long been expressing their conceptions of the construction
of compounds of substances by stereochemical formulae which are meant to represent
the reciprocal position of the atoms in molecules. For many decades collected
chemical experience has been concentrated in these formulae. They have enabled
us to survey the enormous diversity of chemical compounds and have given us
an insight into their possibilities of reaction, as a result of which it has
been possible to harvest the experience gained in practice. The structural formulae
are a certain guide in all works for the manufacture of new dyes, medicinal
preparations, explosives and other useful substances of the most varied nature.
This year's winner of the Nobel Prize for Chemistry has stated that they reflect
the chemical behaviour of the substances in such a wonderful way that even the
physicist cannot doubt that they do in fact represent the essential features
of the real structural principles of the atomic aggregates. These formulae do
not, however, represent actual models of molecules, but only indicate the grouping
of the atoms.
It is only recently that these images which the chemists have made of the structure
of the molecules could be accurately checked and the orientation ascertained
of the atoms in compounds of substances in their details. For this purpose chemical
research has made use of the services of X-rays. When such rays penetrate into
a substance in which the atoms are in any way regularly arranged, the diffracted
radiation, as a result of interference, is weakened in certain directions and
strengthened in others. This is the same phenomenon which causes ordinary light
to be dispersed in a spectrum if the light is diffracted through a series of
closely drawn equidistant lines on a glass or metal plate, i.e. through a grating.
Von Laue and the two research scientists, Bragg father-and-son, showed how these
interference phenomena could be utilized to determine the regular arrangement
of the atoms in the crystals, and were therefore awarded the Nobel Prizes for
Physics in 1914 and 1915. Debye has taken an effective share in the investigation
of the phenomena which have arisen in the resultant field of research and has
contributed by his important initiative to the development of the X-ray crystallographic
methods of investigation.
In the course of this work he soon found that even the relatively simple arrangement
pattern resulting from the fact that the molecules of a gas have the same structure,
can suffice to produce a detectable interference effect when X-rays penetrate
a gas. As is the case with the reciprocal action of X-rays with crystals, the
diffracted rays have an intensity which changes regularly with the angle of
diffraction. Debye later worked out a complete theory for this phenomenon and
as a result he has succeeded in creating a valuable method for the determination
of the structure of molecules. A narrow sheaf of X-rays of known wavelength
penetrates into a gas; the diffracted radiation is recorded, usually by photography.
With the aid of Debye's theory a test is made to find whether a conceivable
molecular model is in agreement with the distribution of intensity of the diffracted
radiation; if such an arrangement of the atoms is confirmed, their dimensions
can be ascertained and hence important information is obtained on molecular
structure.
The cathode rays, which consist of high-speed negative particles of electricity
- electrons - and which can therefore be called electron rays, have a wave nature,
as was discovered by L. de Broglie, and so can be utilized to investigate the
structure of the molecule. Consequently, Debye's theory on the interference
of X-rays can readily be applied. There is, however, one difference, which is
that the electron rays are diffracted mainly through the nuclei of the atoms,
whereas the X-rays are scattered on the electron clouds which surround the atomic
nuclei. As a result of this, the electron interferences give information on
the position of the atomic nuclei in the molecule, whilst the X-ray interferences
reveal where the centres of gravity of the electron clouds lie. In general,
however, at least in practice, the particle systems so determined fall together.
What is ascertained in both cases, then, is the position of the centres of the
atoms.
The fact that matter is built up of electrically charged constituents is utilized
by Debye to elaborate another very important method for investigating the structure
of the molecule. According to Debye, if a substance is placed between the charged
plates of a condenser, the effect of the electric field upon its molecules can
be doubled. Inside every atom the positive nucleus is somewhat displaced in
relation to the surrounding electron cloud, which is in consequence also deformed;
furthermore the reciprocal position of the atoms in the molecule is disturbed.
In addition to this deformation effect, a direction effect must also occur in
certain cases. If indeed the distribution of the charge with the molecule is
asymmetrical, the field endeavours to orientate this in a certain manner. Such
a molecule possesses a so-called dipole moment. With regard to the electrical
effects it possesses two equally large charges, one positive and one negative,
which are concentrated at a certain distance from one another. The product from
this distance and the charge gives the dipole moment of the molecule. To know
this size is significant, for important conclusions can be drawn from it with
relation to the structure of the molecule.
Debye has elaborated a theory of the effect of electric fields on molecules
and has worked out methods for the determination of their dipole moments. These
can be determined by measurements of the variation of the insulating power and
of the density with the temperature. Debye's theory applies strictly only for
diluted gases, in which there is no need to reckon with a reciprocal effect
between the molecules. It is difficult, however, to prepare the experimental
material for the gases necessary for the calculation of the dipole moments.
It is therefore of value that the theory, as has been established experimentally,
can also be applied without noteworthy errors to diluted solutions of substances
in non-polar solvents.
A large quantity of important information has been collected on the structure
of both inorganic and organic molecules by investigations according to these
new methods, which complement each other in an outstanding manner. At least
one hundred gaseous substances have already been studied now with the help of
X-ray and electron interferences, whilst the molecular structure of thousands
of substances has been elucidated by dipole moment measurements. Investigations
of the charge symmetry of molecules have been of very great value especially
to organic chemistry. It has been shown that certain bonds between atoms in
organic compounds are characterized by specific electric moments. But characteristic
moments can also be ascribed to groups of atoms or radicals. These bond or radical
moments can in general be concentrated with a suffcient degree of accuracy to
a resultant total moment, somewhat like the forces acting upon a body can be
replaced by a resultant. A structural formula can therefore be checked by calculating
its dipole moment and by comparing the result obtained with the dipole moment
found by experiment.
The measurements of the dipole moments, as well as those of the X-ray and electron-ray
interferences in gases, are being utilized more and more, together with other
investigations on molecular structure, as an essential aid to constitutional
determination. The form of the molecules, the distance between their atoms,
and also the greater or lesser degree of movement of certain groups entering
into the molecules can now be exactly measured. This is obviously important
in such cases where we have to deal with chemical compounds of identical composition
but of different structure, i.e. where isomerism is present. During the last
decade no means of research has been made available to organic chemistry which
has been, even approximately, so effective in its importance.
The Royal Academy of Sciences attaches such a high value to the work which has
led to these results that it has found it deserving of the award of this year's
Nobel Prize for Chemistry.
Professor Debye. Your rich scientific activity has been aimed in particular to research into the structure of matter. Your wealth of ideas, your penetration and your secure mastery of mathematical methods have yielded great success to your endeavours, and your results have enriched chemistry to an extraordinary degree in all kinds of ways. By your investigations on dipole moments and also on X-ray and electron interferences in gases you have widened and deepened our knowledge of molecular structure to such an extent that the Royal Academy of Sciences has awarded you the Nobel Prize for Chemistry. In extending to you the sincere congratulations of the Academy I ask you to accept the prize from the hands of His Majesty the King.
From Nobel Lectures, Chemistry 1922-1941, Elsevier Publishing Company, Amsterdam, 1966