Optical centers, interaction with transitions

Chapter 5 will show in more detail how the spectral width of optical transitions of active centers (particnlarly for transition metal ions) is affected by lattice vibrations. For the purpose of this section, we will just mention that these transitions are associated with the outer electrons of the active center (the 3d valence electrons), which show strong interactions with the phonons of the matrix in which they are embedded. As a result, the optical transitions, and particularly the emission lines, are strongly modulated by lattice vibrations. 

The fundamentals of SSS are based on the theory of impurity centers in a crystal. The optical spectrum of an organic molecule embedded in a matrix is defined by electron-vibrational interaction with intramolecular vibrations (vibronic coupling) and interaction with vibrations of the solvent (electron-phonon coupling). Each vibronic band consists of a narrow zero-phonon line (ZPL) and a relatively broad phonon wing (PW). ZPL corresponds to a molecular transition with no change in the number of phonons in the matrix (an optical analogy of the resonance -line in the Mossbauer effect). PW is determined by a transition which is accompanied by creation or annihilation of matrix phonons. The relative distribution of the integrated intensity of a band between ZPL and PW is characterized by the Debye-Waller factor ... 

One well-known example of the physical system where the described approach can be applied is the Aj-E optical transition in trigonal centers with the E-e-type Jahn-Teller interaction. In this case the AP in the electronic E-state has a shape of the Mexican hat [11 -14]. If the E-e-type Jahn-Teller interaction is strong, then the Mexican hat is large and the AP has a long minimum line along the trough. In this... 

Johnson rearrangement of allylic alccdiols (274) and (277) led to the enantiomeric 7,8-unsaturated esters (276) and (279), respectively (Scheme 48). Both transition states (275) and (278) favor a pseudo-equatorial position of the benzyloxymethylene substituent the newly formed chiral center is obtained in very hi optical purity throu the chirality transfer process. As a consequence of this geometrical preference, secondary allylic alcohols invariably provide predominantly ( )-configured double bonds upon thermal Gaisen rearrangement The ( -selectivity usually increases with the steric bulk of the C-2 substituent, an effect which was rationalized by a pseudo-1,3-diaxial interaction in the transition state leading to the (Z)-alkene (280 Figure S). ... 

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