Allotropic Modifications of Diamond

THE relationships which exist between the structural symmetry of a molecule and its spectroscopic behaviour have been fully worked out by Placzek. One result of his theory is that if a molecule has the symmetry of the crystallographic point-group Oh, the activity of its vibrations in infra-red absorption and in light-scattering are mutually exclusive. On the other hand, if the molecular symmetry is that of the point-group Td, every vibration of the triply degenerate class must appear both in light-scattering and in infra-red absorption, or else be absent in both