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I know that this Lewis structure of HNO3 is wrong, but I don't know the reason why, except that here the side atoms (oxygen) are connected via a bond. This structure fulfills the octet rule and also all atoms have zero formal charge. So what rules does this Lewis structure of HNO3 violate?

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    $\begingroup$ Indeed, this structure does not violate any of the said rules. It is wrong simply for the reason that it does not exist. $\endgroup$ Nov 6, 2023 at 8:13
  • $\begingroup$ Lewis structures are a highly simplistic way of understanding bonding and bear almost no relation to the actual structures of real molecules. So the fact that this structure obeys the rules of lewis structures tell you nothing at all about whether this is the right structure of the compound. it isn't. $\endgroup$
    – matt_black
    Nov 6, 2023 at 11:29
  • $\begingroup$ This is all missing the point - formal charges are not a problem - that's the point. Otherwise why would the concept be needed at all? Your stupid "minimization of charge" rule is so you wouldn't come up with crazy dianion-dication with charges on neighbouring atoms. $\endgroup$
    – Mithoron
    Nov 6, 2023 at 12:29

2 Answers 2

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The structure isn't incorrect. It just has a high formation energy.

Structure Heat of Formation ($\pu{kJ mol^{-1}}$) Reference
HNO3 Structure 236.69 MolCalc
HNO3 Structure -159.02 MolCalc

Because of the low heat of formation of the correct structure, the incorrect structure spontaneously converts to the correct structure. The structure that you have proposed may be an intermediate in some reactions.

Notice that the vibration of the correct structure at $662.09 \pu{cm^{-1}}$ shows some resemblance to the incorrect structure.

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    $\begingroup$ How did MolCalc obtain these enthalpies of formation? What method (PM3?), conditions, phase? The number $\pu{-159.02 kJ mol^-1}$ isn't something I was able find for ether gaseous or aqueous phase under standard conditions in the more or less reputable sources (e.g. NIST, Argonne). What uncertainty is expected for the thermodynamic properties calculated with the level of electronic structure computations used by MolCalc? $\endgroup$
    – andselisk
    Nov 6, 2023 at 11:32
  • $\begingroup$ The exact structure that was inputted would be useful as well. Has a geometry optimization been done? I assume so because you quote vibration frequencies, but would be good to be clear. $\endgroup$
    – Ian Bush
    Nov 6, 2023 at 12:05
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    $\begingroup$ @IanBush According to pubs.acs.org/doi/10.1021/ed400164n, the errors in standard enthalpies of formation and vibrational frequencies for PM3 are approx. $\pu{32 kJ mol^-1}$ and $\pu{160 cm^-1},$ respectively. These errors and rigorous information about used QC method should be clearly stated alongside the reported data. Also, would be interesting to compare the results from DFT calculations with the semi-empirical method. $\endgroup$
    – andselisk
    Nov 6, 2023 at 12:21
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Structures with nitrogen atom bound to several oxygen atoms are generally energy rich, which would be boosted by the stress of three-member ring. Such rings are generally unstable and reactive. Also, a peroxy group is very reactive even not being strained by the tight ring.

There are many molecular entities with structure not violating any from the listed rules. But in spite of that, they do not exist, as such a list is never complete. Sometime, the missing rules are not relevant, sometimes they are essential.

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