Ideas of Quantum Chemistry P99 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 946 C. GROUP THEORY IN SPECTROSCOPY a centre of inversion the vibrations active in IR are inactive in Raman spectra and vice versa. When do the conclusions drawn from group theory fail When deriving the selection rules the following assumptions have been made the molecule is isolated elements are represented by the same isotope the molecule is in a stationary state the vibrations have small amplitudes the vibrations are harmonic the electromagnetic field interacts with the molecule only through the electric field-molecule interaction in the interaction of the molecule with the electromagnetic field only what are called the dipole transitions are However in practice the molecule is never isolated. In particular the interactions it undergoes in the liquid or solid state are sufficiently strong to deform the molecule. As a result we have to do especially in a liquid with a population of molecules each in a different geometry usually devoid of any particular symmetry for a single molecule this means a non-stationary state although the molecule is not far away from perfect symmetry broken symmetry . Suppose for a while that the molecule under consideration is indeed isolated. In a substance we usually have several isotopomers with different distributions of isotopes in the molecules. In most cases this also means broken symmetry. Broken symmetry means that the selection rules are violated. In practice broken symmetry means that the selection rules cause only a small intensity of forbidden transitions with respect to allowed transitions. When considering electronic transitions we assumed that the molecule stays in its equilibrium geometry often of high symmetry. This may be the most probable configuration 48 but the vibrations and rotations deform it. An electronic excitation is fast and usually takes place with a molecular geometry that differs slightly from the most probable and most symmetric one. This will cause a transition forbidden for perfectly .
