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I am interesting in chromophore group and dipole approximation.

For example, i have a molecule (acetone or any other ketone/enol) which is belongs to some symmetry group. Because of the symmetry this molecule has degenerate energy levels. Now, i need to know the way how to destroy degeneracy.

The first and simple idea is to use perturbation theory and a dipole approximation, where perturbation caused by the light. This approximation let as to treat the interaction between light and molecule as dipole in spatially-uniform electric field. Perturbation theory is common way to calculate absorption coefficients for systems that smaller than absorbed light (atom, diatomic molecule etc.) But the size of a molecule in example long compared with the size of a wavelength, so we can't ignore the spatial variation of the electric field.

Solution of this situation is chromophore, part of a molecule where the energy difference between two orbitals fits within the range of the UV or visible spectrum and responsible for the absorption of light. So now we can treat interaction between light and a molecule as an interaction between light and a part of that molecule which is responsible for the absorption. For example the C=O bond is a chromophore. So we treat absorption of light by a molecule as an absorption of light by a chromophore and save ourselves from complex calculations related to the size of the molecule. The distance of a C=O bond is 1.22 Å while absorbance maximum at about 275 nm. Now the wavelength is long compared with the size of an chromophore, we can ignore the spatial variation of the field.

My questions is:

  1. Is this approximation correct? Can we approximate small perturbation by light applied to WHOLE molecule as a chromophore in spatially-uniform electric field? How rough is this approximation?

  2. If this assumption is not right what else Way we can use/apply dipole approximation to a Whole molecule?

  3. What else approximations is possible when we a dealing with a perturbation theory applied to a molecule?

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1) Correct is not an appropriate word when dealing with approximations. Doing approximation means doing something that is not exactly accurate, so strictly speaking, it is not correct.

But approximation can be useful, even if not exactly accurate. Whether this is the case depends on what is the goal. For example, uniform field approximation is useful for estimating absorption coefficient, but useless for estimating difference of these coefficients for left and right-handed EM wave; one must have more detailed model to estimate such fine effect.

2) one can take into account different phase of EM wave in different parts of the molecule in the model, based on its coordinates in space. This is much easier to do in classical-theoretical kind of model than in models based on Schroedinger's equation, because in classical model one can easily take into account retardation due to relativity, but in quantum theory taking into account relativistic effects brings a lot of problems.

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  • $\begingroup$ Thanks. But what about chromophore? $\endgroup$ – Alex Mal Apr 6 '19 at 7:04
  • $\begingroup$ Chromophore is a part of a molecule. You can model it as a point object (less accurate but simple theory) or as extended object (more accurate but difficult theory). $\endgroup$ – Ján Lalinský Apr 6 '19 at 17:33

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