Raman and Infrared spectroscopy Theoretical considerations
简介:Raman and infrared (IR) spectroscopies provide information about the vibrational and vibrational-rotational modes of molecules. Vibrational-rotational bands are generally observed when the samples are in gaseous state, w简介:Raman and infrared (IR) spectroscopies provide information about the vibrational and vibrational-rotational modes of molecules. Vibrational-rotational bands are generally observed when the samples are in gaseous state, where the molecules are able to rotate freely. In condensed (liquid or solid) phases, only vibrational frequencies of the sample can be observed. IR and Raman spectroscopies are complementary techniques, since transitions allowed in Raman
may be forbidden in IR or vice-versa. This depends on symmetry considerations. An IR-active mode is one in which a particular vibration causes a change in the dipole moment of the molecule, while only those vibrations which change the molecular polarizability lead to Raman scattering. Therefore, the activity of a certain vibrational mode depends highly on its symmetry and the symmetry of the molecule. One simple symmetry rule is the so-called mutual exclusion rule for molecules with a center of symmetry. In such molecules no normal mode may be active in both, the infrared and the Raman spectrum.详细>