Two-group Raman optical activity revisited
In the circular differential Raman scattering observed in biological and other large polyfunctional molecules, many spectral features may be attributed to conferred chirality, in the following sense. Although a given vibrational transition may occur within a group in an intrinsically achiral local environment, it is coupling with another achiral but dissymmetrically placed group that generates the chiral response. Where two groups so coupled are chemically dissimilar, the effect originates in interference between one- and two-centre scattering mechanisms. The one-centre mechanism entails vibrational transition by a group as it undergoes conventional Raman scattering. The two-centre mechanism involves mediation of the chiral influence of a second group on this transition by Förster-type radiationless energy transfer. Where quantum-mechanical interference generates a differential Raman signal, the circular intensity differential depends on the inverse square of the distance between the groups so coupled. This distance dependence may be understood as originating from a combination of two factors. One is the linear distance dependence characterising Raman optical activity due to direct interference between transitions at distinct sites, which arises in the case of chemically identical groups. The other is the inverse cubic distance dependence associated with the probability amplitude for Förster energy migration. The Raman optical activity of any group with no chemical equivalents in its vicinity should thus be interpreted as resulting from a sum of inverse-square couplings with other chromophores. The two-group model for Raman optical activity is critically assessed, possible ways to improve upon the model are considered, and the result for the differential scattering intensity is recast in a new form that is more general and also more consise than has hitherto been presented.