Chiral molecules magnetic dipole contributions

As discussed earlier, VCD depends on both the electric and magnetic dipole moment derivatives in a molecule. The simpler descriptions of VCD focus only on local electric dipole moment derivatives that have an overall chiral disposition. More advanced descriptions of VCD allow for induced electronic currents or charge flows in molecules, which give rise to additional magnetic dipole moment intensity. Such additional contributions are likely whenever delocaliz-able electron density is present in a molecule. 

Sum-frequency generation (SFG) at second-order and the nonlinear Raman spectroscopy BioCARS at fourth-order can also probe chiral molecules. They have no analog in linear optics. We show that both are only symmetry allowed in a fluid, if the fluid is chiral. However, in contrast to optical activity phenomena, these processes arise entirely from induced electric-dipoles (without magnetic or quadrupolar transitions) and they are not circular differential. All laser beams can be linearly polarized and no polarization modulation is required as the detection of a sum-frequency (yiz. five-wave mixing) photon is in itself a measure of the solution s chirality. Since an achiral solvent can not contribute to the signal, these techniques are sensitive, background-free probes of molecular chirality. The SFG... 

Cartesian electric quadrupole moment operator. The electric dipole - magnetic dipole and electric dipole - electric quadrupole polarizabilities govern the optical rotation of chiral molecules. For samples of randomly oriented molecules, the contribution from the electric dipole - electric quadrupole polarizability averages to zero, and only the trace (the sum of the diagonal elements) of the electric dipole - magnetic dipole polarizability contributes to the optical rotation. The... 

The condition for this quantity to be non-zero is that the chromophore of interest must have a non-zero magnetic and electric transition dipole moment along the same molecular diiectioa In the absence of pertuibing external fields, this is only trae for molecules that are chiral. Expressions that include higher order multipole contributions to eqs. (8), (9), and (11) can be found in previous theoretical descriptions of CPU theory (Riehl and Richardson, 1976a, 1986). 

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