THE notion that the structure and conformation(s) of molecules are
determined by valence forces in conjunction with non-bonded forces
of varying importance is widely accepted by organic chemists. Although
the fact that strong crowding of matter in space will give rise to repulsive
forces has always been understood in a qualitative manner, and has
prompted suggestions of “steric effects” as simple explanations of more
or less unexpected phenomena, it is only recently that the implications
of non-bonded forces for the conformations of simple molecules, not
generally conaidered overcrowded, have been recognized by chemists.
In molecular spectroscopy, on the other hand, non-bonded interactions
in such molecules have been taken into account for the past forty years.