The world we live in is filled with molecules. Starting with oxygen and nitrogen
in the atmosphere, water, carbon dioxide, and ammonia are all molecules. Furthermore,
plants and animals are all composed of molecules. In the field of chemistry,
which is regarded as science for molecules, it has been one of the most important
and long-lasting fundamental issues to know the geometrical structure of a variety
of molecular species around us.
Roughly speaking, there are two major methods to investigate the geometrical
structure of molecules in the gas phase. One is molecular spectroscopy and the other
is gas electron diffraction. In molecular spectroscopy, molecules are irradiated with
light or electric waves, and a diagram called a spectrum is measured. In the diagram,
rich information regarding the dynamics of electrons within a molecule, the
vibrational motion of nuclei within a molecule, and the overall rotational motion of
a molecule are encoded. Specifically, from the spectrum related with the rotational
motion of molecules, we can derive information which is directly connected to the
geometrical structure of molecules.
Therefore, it can be described that the most central issue in the field of molecular
spectroscopy is to know how we can extract information concerning molecular motion
from an observed spectrum. If we regard a spectrum as a secret code, the issue
is nothing but decoding the code and developing the methodology on the decoding
procedure. In order to decode the spectrum and to derive information of molecules
properly, we need to realize that molecules are described by quantum mechanics.
In the present textbook, we learn that the motion of electrons in a molecule,
molecular vibration, and molecular rotation are all “quantized” and that the consequence
of the quantization appears vividly in the spectrum. Furthermore, we understand
how we can determine the geometrical structure of molecules, and simultaneously
we appreciate the fundamentals of quantum mechanics of molecules.