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EPR

Gives us information about the structure of molecular orbitals

EPR spectroscopy gives information about the electron densities of an unpaired electron on particular nucleii and thus reveals the structure of the molecular orbital. It is desirable for us to generate the paramagnetic species in-situ and thus a special variable temperature, in-situ EPR cell has been designed (Fig. 1).

Fig. 1 - In situ EPR cell (N.B. B is perpendicular to the microwave field)

Therefore, a potential can be applied and the paramagnetic species can be "grown" (Fig 2)

Fig. 2 - EPR Sepectrum of a paramagnetic redox product growing in while potential is applied

e.g. Reduction of 4-NO₂-bpy (bpy=2,2´-bipyridine)

4-NO₂-bpy

Initially, a solution of 4-NO₂-bpy in 0.1 M [ⁿBu₄N][BF₄]/DMF gives a flat EPR spectrum (diamagnetic species). On applying -1.5 V, a spectrum grows in as shown in Fig. 3

Fig. 3 - Experimental and simulated EPR spectra of [4-NO₂-bpy]^1- and ¹⁵NO₂ analog (right) showing simulated hyperfine coupling constants generated in situ at -1.5 V vs. Ag/AgCl at 233 K, recorded at 233 K in 0.1 M [ⁿBu₄N][BF₄]/DMF.

Simulation reveals coupling to two N-atoms with one coupling constant much larger than the other. it is postulated that the unpaired electron is located on the NO₂-py ring. Therefore, one of the nitrogen coupling constants is coupling to the ring nitrogen and the other is to the nitro group nitrogen. It is difficult so say which is which from the spectrum on the left. However, when the NO₂ was labeled as ¹⁵NO₂, the larger coupling constant changed from coupling to ¹⁴N (I = 1, 3 lines in an EPR spectrum) to coupling to ¹⁵N (I = ½, 2 lines in an EPR spectrum). Therefore, the larger coupling constant is asigned to the nitrogen in the NO₂ group. It can thus be said that the unpaired electron has a large electron density on the NO₂ group which is to be expected due to the elctron withdrawing nature of NO₂.

Electrochemistry - EPR - UV-Vis

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