This is a very important aspect of Physical Chemistry in which knowledge of the size, shape, rigidity and electronic structure of molecules deduced from the experimental methods treated here goes hand in hand with the theoretical approaches of chemical reactions. Spectroscopy is the measurement and interpretation of electromagnetic radiation absorbed or emitted when the molecules, atoms, or ions of a sample move from one allowed energy to another. These allowed energies have been used throughout in our interpretation of the thermodynamic properties of materials. Grouped here and in the following are treatments of various experimental methods that give information on the geometry and electronic structures of molecules. The difficulties encountered in the applications of a completely theoretical approach to molecular bonding and structure lead one to refer frequently to experimentally determined properties in order to understand molecular phenomena. Now the origin of the values used there for the spacing of so
e of the energy levels will be seen. Our principal concern is molecular spectroscopy that stems from changes in the rotational, vibrational and the electronic energies. In addition, energies not considered in our thermodynamic studies, resulting from energy differences that arise when a sample is placed in a magnetic or electric field, are susceptible to spectroscopic studies. Nuclear-magnetic-resonance (nmr) spectroscopy and electron-spin-resonance (esr) spectroscopy illustrate such studies.