PART I. A MACHINE FOR THE RAPID SUMMATION OF FOURIER SERIES
The machine about to be described has been
specially designed with a view to its use in X-ray
crystallographic laboratories. In the elucidation
of structures from measured intensities of scattering
of X-rays, the performance of Fourier syntheses is probably
the best way of obtaining atomic parameters, although
this method is limited by the fact that the
phases of most, at any rate, of the F-values must be
known beforehand. However, the introduction of the
Patterson (1) method enables the measured intensities
to be used directly to give a diagram which, properly
interpreted., can give valuable information about the
atomic parameters. Nevertheless, for complicated
structures, a TWO- DIMENSIONAL PATTERSON PROJECTION
(obtained from the F² (h, k, o), F² (o, k, l), or
F² (h, o, l) values) becomes very difficult to interpret
due to the large number of interatomic vectors
involved; this is particularly true for organic compounds,
due to the fact that the commonly occurring
atoms (C, O, and N) have very similar scattering powers
for X-rays. In such circumstances, the maximum of
information can be obtained from a three dimensional
Patterson summation, using the general F² (h, k, l)
values. The fact that such summations have not
hitherto been used is due, not merely to difficulty
in getting values for the general intensities, but to
the great labour involved in calculating the series.
The present machine, by greatly reducing that labour,
should make possible the general use of the very
powerful three-dimensional Patterson method. It will
also, of course, greatly facilitate the method of
approximation to the true structure by successive
Fourier syntheses, already largely used.
There are of course existing machines which are
capable of performing the process of Fourier synthesis,
but these are all very expensive, and not particularly
fast. There is therefore a real need for a machine,
such as the present, which is sufficiently inexpensive
to be acquired permanently by most crystallographic
laboratories, and which is very fast in operation,
while giving all the accuracy necessary for X -ray
crystallographic work (and no more).
PART II. AN X-RAY INVESTIGATION
OF SULPHURIC ACID MONOHYDRATE
On the basis of oscillation and Weissenberg
photographs of Sulphuric Acid Lonohydrate (H₂SO₄.H₂0 )
crystals, the unit cell and space group have been
'determined, and a Fourier Synthesis of the (h k 0)
reflections has been carried out. Based on this
synthesis, on considerations of interatomic distances,
and on the observed general (h k 1) reflections,
a structure has been sugested.
Powder photographs have been taken of the
monohydrate at temperatures varying from just below
its melting point to liquid oxygen temperature, and
it has been confirmed that the monohydrate has the
above structure throughout this range of temperature.
