Can one figure out an optical absorption spectrum from EPR data?

I'm wondering if it's worthwhile for me as an optical spectroscopist to read up a bit on EPR. Do EPR signals reveal where, in terms of wavelength, features in the optical spectrum can be expected to appear? I know it tells me
lifetimes of states, but that's about as far as I have dug into it. I have some EPR data on a substance and I wish to figure out where to look for it's absorption with UV/Vis/NIR means.




No, it shouldn't be possible, based on where these spectroscopies lie in the [electromagnetic spectrum](https://en.wikipedia.org/wiki/Electromagnetic_spectrum):


[![electromagnetic spectrum depiction](https://i.stack.imgur.com/XEJPa.png)](https://i.stack.imgur.com/XEJPa.png)


The most common EPR experiments occur at 9.5 GHz (X band), which is in the microwave region. Even the [highest-frequency](https://en.wikipedia.org/wiki/Electron_paramagnetic_resonance#High-field_high-frequency_measurements) experiments just barely break into the far infrared. Assuming you're doing an X band experiment, and that your system has a transition very close to the free electron $g$-values, 9.5 GHz corresponds to ~0.3 cm$^{-1}$.


For UV-vis, taking the longest wavelength of light in the [visible region](https://en.wikipedia.org/wiki/Light) as 800 nm gives 12,500 cm$^{-1}$. This can be interpreted in two ways:


1. The EPR experiment needs ~42,000x more energy to probe even the lowest possible electronically-excited state.
2. Even the UV-vis transitions with the lowest possible energy will not show any [fine structure](https://en.wikipedia.org/wiki/Fine_structure), like in the sodium lines or benzene vibronic coupling. The energy scales need to be much closer. In the case of the [spin-orbit-induced splitting of the sodium spectral lines](http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/sodzee.html), the unperturbed gap is 2.1 eV and gap between the split states is 0.0021 eV; this is 3 orders of magnitude between the energies. In the case of the benzene UV spectrum, the gap between peaks is ~10 nm, with the peak center at ~245 nm; this is 1-2 orders of magnitude:


$\hspace{20ex}$![](https://i.stack.imgur.com/Ih8bD.jpg)


Our EPR/UV-vis example has 5 orders of magnitude difference between the two energy scales. There is (hyper)fine structure in EPR:


[![EPR DPPH](https://i.stack.imgur.com/ZYySj.png)](https://i.stack.imgur.com/ZYySj.png)


which here, for DPPH in toluene is ~0.002 Tesla, or 0.35 GHz, so 1 order of magnitude.


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