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Structure of nitric acid:

enter image description here

Structure of peroxynitrous acid:

enter image description here

Nitric acid and peroxynitrous acid have the same structural formula, $\ce{HNO3}$ and in both the compounds, O.N. of nitrogen is +5. So, is peroxynitrous acid an isomer of nitric acid?

It seems that peroxynitrous acid is a very, very unstable compound and I don't think it has been synthesised in pure, stable form. This video shows the preparation of peroxynitrous acid but it readily converts into nitric acid and thus the peroxynitrous acid yield is very poor.

Keeping aside its stability and considering only their structure and structural formula, is peroxynitrous acid an isomer of nitric acid?

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    $\begingroup$ Yes goldbook.iupac.org/I03289.html $\endgroup$ Commented Mar 28, 2016 at 14:46
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    $\begingroup$ Per the definition of "isomer", it is. Why would you think it wouldn't be? $\endgroup$
    – M.A.R.
    Commented Mar 28, 2016 at 15:11
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    $\begingroup$ Note that "I hardly doubt that it has been synthesised" means "I am pretty sure it has been synthesized"... $\endgroup$
    – Buck Thorn
    Commented Sep 21, 2019 at 11:15
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    $\begingroup$ @BuckThorn You are being pedantic, aren't you? ;). Anyways, I was so lazy at that time that I forgot to check the Wikipedia page. On the last line, it clearly says "It is an isomer of nitric acid" . Sigh, the question loses its purpose though I must appreciate hBy2Py's answer. He summarized his Ph.D career in his answer. $\endgroup$ Commented Sep 21, 2019 at 13:09
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    $\begingroup$ (@BuckThorn) Nilay, if only: despite spending a fair bit of time working on the peroxynitrite synthesis and related experiments, I never actually got any of that work to the point of publication -- all of my journal articles were from other chemical systems! $\endgroup$
    – hBy2Py
    Commented Sep 22, 2019 at 23:51

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Keeping aside its stability and considering only their structure and structural formula, is peroxynitrous acid an isomer of nitric acid?

Yes.

It seems that peroxynitrous acid is a very, very unstable compound and I hardly doubt that it has been synthesised in pure, stable form. This video shows the preparation of peroxynitrous acid but it readily converts into nitric acid and so the peroxynitrous acid yield is very poor.

Indeed, it is impossible to isolate peroxynitrous acid $\left(\ce{ONOOH}\right)$ in aqueous solution, due to the rapid decomposition to nitric acid. The yield is not just poor, it is zero.

However, the conjugate base of peroxynitrous acid, peroxynitrite $\left(\ce{ONOO^-}\right)$, happens to be extremely stable. (Apropos the question, peroxynitrite is an isomer of nitrate.) So, as long as a synthetic approach is devised that keeps the product strictly in its anionic/alkaline form, one can actually produce peroxynitrite solutions and salts and work with them in experiments. In fact, I synthesized sodium peroxynitrite solutions myself as part of my Ph.D. research. You can read about the procedure in Section 4.2.4 of my thesis (free for download). A short literature review of various $\ce{ONOO-}$ synthesis methods is also included there.

Briefly, I loaded aqueous solutions of (1) $\ce{NaNO2}$, (2) $\ce{H2O2}$ plus $\ce{HCl}$, and (3) $\ce{NaOH}$ into syringes fitted onto a syringe pump. I then connected the syringes to a tubing network that first mixed the acidified $\ce{H2O2}$ with the $\ce{NaNO2}$, forming $\ce{ONOOH}$, and then quickly mixed the product solution with the $\ce{NaOH}$. The alkali "caught" the $\ce{ONOOH}$ while a significant concentration still remained, providing a bright yellow solution of alkaline peroxynitrite. After decomposing residual $\ce{H2O2}$ by treatment with $\ce{MnO2}$, these solutions held their concentrations for a couple of years when stored at $-80\,^\circ\mathrm C$.

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    $\begingroup$ Can't not prattle on about my Ph.D. work any time I get the chance...! $\endgroup$
    – hBy2Py
    Commented Mar 28, 2016 at 16:54

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