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I was just searching whether Uracil is aromatic or not and in this link$^1$ it is said that (from the abstract):

High-level computational methods were tested for the enthalpy of hydrogenation of uracil, but only with G3 was possible to obtain results in agreement with the experimental ones. It was found that uracil possesses 30.0% of aromatic character in the gaseous phase.

What is the meaning of this sentence? I have only basic knowledge on Aromaticity limited to Huckel's rule and the names of a few aromatic compounds. I didn't expect Aromaticity to be expressed in percentages. Can someone explain why?

Reference

  1. Tiago L. P. Galvão, Inês M. Rocha, Maria D. M. C. Ribeiro da Silva*, and Manuel A. V. Ribeiro da Silva. J. Phys. Chem. A 2013, 117, 28, 5826–5836. https://doi.org/10.1021/jp404938u
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    $\begingroup$ Nothing is truly black or white. Consider pure ionic and pure covalent bonds to be unobtainable extremes. Every real bond is somewhere in between. $\endgroup$
    – MaxW
    Jan 21 at 13:50
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    $\begingroup$ Technically, a good point is made here. What is the meaning of "100% aromatic?" $\endgroup$
    – Zhe
    Jan 21 at 15:01
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    $\begingroup$ @Zhe I'm sure the paper explains the meaning of the quoted percentage. ;) Ratio of prefactors in a linear combination of orbital models? $\endgroup$
    – Karl
    Jan 21 at 16:58
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    $\begingroup$ Assuming it is more or less clear to all of us what aromaticy means, and the ways it can be expressed or assessed, then one can construct many comparative scale. One can think of termochemistry, another of chemical shifts, and relate these to the corresponding in benzene, just to give an example. Or, it can come directly by MO calculations at various levels. The most straightforward would be that the phi of aromatic orbital has coefficient of 0.3; or as a comparison is likely made, a ratio between prefactors as @Karl said. This is to say that you can get the message without overthinking. $\endgroup$
    – Alchimista
    Jan 21 at 17:29
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    $\begingroup$ I took the liberty of expanding the quoted text for clarity. Feel free to rollback the change if you disagree this improves your post. $\endgroup$
    – Buck Thorn
    Jan 28 at 8:59
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G3 is generally a term used to describe a particular computational model used in calculating molecular bonding in quantum chemistry. The G refers to Gaussian and 3 is just a third iteration of including additionally complex terms to the molecular orbital calculations being used. There's a brief discussion on wikipedia, but you'll find more extensive discussion of these types of calculations in Quantum Chemistry text books or directly from the researcher's works just an example, a G3 Theory site.

Aromaticity can actually be measured quantitatively via Aromatic ring current and calculated with [NICS] nucleus-independent chemical shift. The wikipedia article included a link to a paper describing the quantitative scale. It's just a way of saying how aromatic is the molecule compared to another (call it a standard / perfect example molecule.) The problem with such relative standards is there may always be one that exceeds the standard, so you could potentially end up with results larger than 100%.

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  • $\begingroup$ Looks like I need to learn a lot.... $\endgroup$
    – user102687
    Jan 29 at 16:01
  • $\begingroup$ Can you also explain the need for quantitative measurement of Aromaticity(in specific percentages). When we say that a compound X is y% aromatic w.r.t a particular method of quantitative measurement, what does it actually say about the molecule(other than the experimental/computational result itself)? If a compound X has a higher 'percentage of aromaticity' than a compound Y, is X more stabilised by aromaticity than Y? $\endgroup$
    – user102687
    Jan 29 at 16:09

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