Dinitrogen , excited states

For many decades chemists faced the problem of nitrogen fixation under mild conditions. The energy of electron detachment from the binding orbitals of dinitrogen or molecule excitation to the excited states is very high (369 and 143 kcal/mole,... 

A number of unimolecular photoredox reactions have been studied. Kirk and Porter [130] reported a AF value of +4.8 cm3 mol"1 for the charge-transfer photolysis of Co(NH-,)5Br24 and suggested the formation of a caged radical pair, Con(Br ), from the LMCT excited state. Dissociation of this radical pair to the reaction products was suggested to account for the increase in volume as reflected by the positive volume of activation. Charge-transfer photolysis of trans -Pt(CN)4(N3)2 results in the reductive elimination of azide to produce Pt(CN) and dinitrogen [131,132], On... 

The equations-of-motion method has been very successful in calculating excited states and potential-energy curves for the dinitrogen molecule. A more empirical approach, using Raman spectra of a-Na, suggests that a better fit is obtained for = 9 than the previously used = 12 for P.E. curves. 

Figure 17 Orbital and electronic state diagrams for dinitrogen-bridged D/A complexes. The solid blocks indicate a large number of similar-in-energy MLCT excited states.

Because of the different nature of those transitions, different photochemical products are formed depending on the excitation wavelength. Excitation into the inter-valence transition at 700 nm does not initiate any photoreactivity. However, irradiation with light <450 nm populates an MLCT state, resulting in transient oxidation of the metal centre, reduction of the N2 molecule, and dinitrogen splitting, with quantum yields 0.002 and 0.003 under 254 nm or 365 nm irradiation respectively. 

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