Nitroxyl anion triplet state

As mentioned above, NO can also be reduced to the nitroxyl anion (NO-) (Eq. (5)), which is isoelectronic with 02, and like dioxygen has a triplet ground state (15). 

Nitroxyl anion has been suggested to be a potential EDRF (Murphy and Sies, 1991). However, the ttiplet state of nitroxyl anion reacts with molecular oxygen (also in a triplet state) to fotm petoxynitrite anion (Donald et al., 1986 Hughes et al., 1971 Yagil and Anbar, 1964). 

The weak acid HNO (pA a = 11.4) has a singlet gronnd state (X A ). The nitroxyl anion NO has a triplet ground state (X ), therefore the dissociation of HNO to NO and H+ is spin forbidden and inherently slow. In physiological systems, HNO is the dominant form, but NO, once formed, has sufficient lifetime to react with other species. HNO is like NO a transient intermediate in physiological systems. 

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