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I may be only a physicist but I'm familiar with the superscripted dot to indicate a radical. In a paper I'm reading the authors have also used a superscripted open circle. I can't find any reference online or in my physical chemistry textbook (Atkins). Here's an example:

Example of superscript-open-circle notation

As you can see they've done this a few times, and also used the dot to indicate a radical. This is in a published paper "Photocatalytic degradation pathway of methylene blue in water", Houas et al., Applied Catalysis B: Environmental (2001). I'm starting to wonder (given the OH◦ in the text and OH• in the equation) if it's a typo, but I'd be surprised if something so glaring could get through review or even proofreading by coauthors.

So does the open circle have a meaning of its own?

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    $\begingroup$ A circle is often used to denote an electron hole, i.e. the absence of an electron. But there might be more to it in this case. $\endgroup$
    – Martin - マーチン ♦
    Commented Jul 8, 2015 at 9:24
  • $\begingroup$ @Martin-マーチン that hadn't ocurred to me though I'm familiar with the use in semiconductor band diagrams especially when they're marked up. I'd struggle to interpret O<sub>2</sub><sup>◦-</sup> in that way though. $\endgroup$
    – Chris H
    Commented Jul 8, 2015 at 9:49
  • $\begingroup$ @Chris I would have to look at the paper to get the context. I could imagine they refer to to the peroxid anion $\ce{O2^2-}$ when one electron is missing. Btw: Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. $\endgroup$
    – Martin - マーチン ♦
    Commented Jul 8, 2015 at 9:54
  • $\begingroup$ @Martin-マーチン thanks for the tip on $\LaTeX$, I've spent too much time on tex.se where it's turned off. $\endgroup$
    – Chris H
    Commented Jul 8, 2015 at 10:23
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    $\begingroup$ Thanks @Martin-マーチン, I had some suspicions as I mentioned in the question so I'm not exactly surprised. If you'd like to post an answer I'll accept it of course, and it will save having an unanswered question kicking around, but I don't need it and I wouldn't want to waste your time on the paper. $\endgroup$
    – Chris H
    Commented Jul 8, 2015 at 13:01

3 Answers 3

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According to The Manual of Scientific Style: A Guide for Authors, Editors, and Researchers 9.2.5.12

Indicate a free radical by placing either a centered dot or a superscript dot, signifying the unshared electron, next to the chemical symbol or compound formula

This convention is also followed by the ACS.

So it would be better if the article used the solid symbol throughout, but the solid and open symbols mean the same thing in the article.

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Yes, it is only inconsistent typesetting. Both notations represent a single electron. It may have gone through proofreading because nobody cared. The open circle is the symbol for degrees, which is present on a keyboard, unlike the closed circle.

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  • $\begingroup$ I'd like to add that a superscript open circle (or actually, a superscript zero) is sometimes used to show neutral species, particularly when it is a molecule that is commonly dissociated. For example, HF⁰ or HCl⁰ remain in molecular form at higher pressures and temperatures. This notation is used to emphasise that they are not dissociated. $\endgroup$
    – Gimelist
    Commented Sep 7, 2016 at 12:29
  • $\begingroup$ I believe the degree sign is present on a German keyboard, but not a UK English one. In this case it clearly wasn't a zero but could have been meant to be one. $\endgroup$
    – Chris H
    Commented Sep 7, 2016 at 15:36
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It is analogous to semiconductors. They both mean free radical but OH• with a solid dot is an electron "donor" in the environment and $\ce{HO2}$◦ with a hollow dot is an electron "acceptor" in the environment.

Edit DavePhD finally convinced me. It is inconsistent typesetting. Look at the third mechanism. A hollow circle is used in the text, but a solid circle is in the equation. As DavePhD also points out that flips in the 7th mechanism. A solid circle is used in the text but a hollow circle in the equation.

you can see paper here which is not behind a firewall http://www.ugaf.rnu.tn/Bibliotheque/article%20elaloui-lachheb.pdf

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    $\begingroup$ but for mechanism step 3 in the article, HO is written both ways $\endgroup$
    – DavePhD
    Commented Sep 7, 2016 at 13:33
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    $\begingroup$ also HO is written both ways in mechanism step 7. $\endgroup$
    – DavePhD
    Commented Sep 7, 2016 at 13:38
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    $\begingroup$ The exact same mechanism step is described using the open and closed circle in the article for HO in both step 3 and step 7. $\endgroup$
    – DavePhD
    Commented Sep 7, 2016 at 13:40
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    $\begingroup$ well step 3 is quoted in the OP, and says "produces OH[open circle]" followed by a chemical equation with "OH[filled circle]" on the right hand side. $\endgroup$
    – DavePhD
    Commented Sep 7, 2016 at 13:45
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    $\begingroup$ and step 7 says "attacks by OH[filled circle]" followed by "OH[open circle]" on the left hand side, so it is overwhelming clear that the article is using inconsistent notation. $\endgroup$
    – DavePhD
    Commented Sep 7, 2016 at 13:49

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