Polarization-sensitive CARS

A full description of polarization-sensitive CARS signal generation that takes the polarization properties of the pump, Stokes, and CARS fields and the tensor character of the nonlinear susceptibility components (cf. (6.5)-(6.7)) into account can be found elsewhere [16]. 

Toleutaev B N, Tahara T and Hamaguchi H 1994 Broadband (1000 cm multiplex CARS spectroscopy application to polarization sensitive and time-resolved measurements Appl. Phys. 59 369-75... 

As already mentioned shortly in Sec. 3.6.2.2.3 background contributions which may limit the CARS sensitivity in observing weak Raman resonances can be eliminated by applying special polarization arrangements for the laser and CARS beams. This has been already demonstrated by Song et al. in 1976. They have shown in a four-colour CARS experiment that, by proper orientation of the laser polarizations, background contributions can be eliminated, resulting in undistorted line shapes. A review of this technique and... 

Enantiodifferentiating anti-Markovnikov polar photoadditions of alcohols to 1,1-diphenyl-l-alkenes 107 and 108 sensitized by optically active naphthalene(di)car-boxylates 41-43, 71, 72, and 91 were investigated in detail (Scheme 19) [70], Since this photoaddition involves the attack of alcohol to a radical cationic species of the substrate alkene [71], the use of polar solvents is desirable for obtaining the adduct in a high yield. However, in polar solvents, the radical ionic sensitizer-substrate pair produced upon photoexcitation is immediately dissociated by solvation, and no chirality transfer is expected to occur. Thus the optical and chemical yields are often conflicting issues, and therefore the critical control of solvent polarity is essential for obtaining the optically active product with an appreciable ee in reasonable chemical yield. In fact, the initial attempts on 107, employing naphthalenecarboxylate sensitizers with chiral terpenoid auxiliaries (a-c and f) and a pentane solvent afforded a best ee of 27% for adduct 110 (R = Me), but in < 2% yield [70a]. 

Derivatization of non-volatile polar or thermally sensitive compounds to enhance their volatility and stability prior to chromatography is a well-established technique. Compounds containing hydroxyl, carboxyl and amino functional groups can be readily reacted with appropriate reagents to convert these polar groups into much less polar methyl, trimethylsilyl trifluoroacetyl derivatives of greater volatility. Fatty acids, car hydrates. 

We can see that the polar effects found for electrophilic aromatic additions are much less pronoimced with radical additions. This is in part due to the fact that the influence on radical stability of electron donating or electron withdrawing groups is lower than the effect on car-benium ions. However, the lower sensitivity is also due to an early transition state for radical addition because the addition is exothermic, and a late transition state for electrophilic addition because this addition is endothermic. By the Hammond postulate, the early transition state for radical addition would have little radical character on the aromatic ring, and there-... 

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