The vibrational spectrum of a system may be described in terms of the equilibrium configuration. A band observed in the infrared region may be thought of as a change in dipole moment which can be associated with the displacement of a group of atoms from their position in the equilibrium configuration, and a band in the Raman spectrum may be thought of as a change in polarizability which can also be described as the displacement of a group of atoms from their equilibrium position. In these terms, then, one speaks of the C–H stretching fundamental or the H−C—C deformation. However, these displacements are determined only if their equilibrium positions are known, as well as the totality of equilibrium positions that define the equilibrium configuration or the structure of the system.
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