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We have already studied various aspects of atomic and molecular structure in terms of molecular energy levels and charge densities. In addition to electronic energy levels in molecules, there are other energy levels in a molecule such as vibrational and rotational, just to name a few. All these levels are quantized or discrete. The quantization is a consequence of the boundary conditions (such as finiteness, single valuedness, square integrability) on the wave functions. What this means in physical terms is that in molecular vibrations, the molecules undergo oscillatory motion in a small region of space around the minimum energy structure. In rotational motion, molecules look identical after a rotation by 360 0 with respect to any axis.
We can not see molecular structures with either the naked eye or even by microscopic cameras. We can only study the transitions between the energy levels of molecules resulting from the absorption and emission of light. To infer from the spectroscopic transition data the correct molecular structure is almost like the work of a detective.
Molecular spectroscopy is a vast and growing subject and we shall qualitatively explore some aspects the principles of spectroscopy and of UV-visible spectroscopy in the present lecture. Vibrational and rotational spectroscopy will be taken up in the next lecture; magnetic resonance methods in lecture 14 and some of the remaining methods in lecture 15. |
