Arrival atom numbering doesn't work with \chemabove

Question

Consider the following MWE:

\documentclass{article}
\usepackage{chemfig}

\begin{document}

\chemfig{
    X
    -[:30,,,2]^{18}\chemabove{O}{H}
    -[:-30]X
}

\end{document}

This code returns an error:

Package chemfig Error: no atom found at position 2, pershaps you mispelled the optional argument of the bond.. }

Elimination of the number 2 as an argument for the arrival oxygen atom resolves that issue, but the left X-O bond is placed incorrectly:

\chemfig{
    X
    -[:30]^{18}\chemabove{O}{H}
    -[:-30]X
}

I've been playing around a bit and the best hack I've came up so far is to use a shortened invisible O-H bond:

\chemfig{
    X
    -[:30,,,2]^{18}O(-[:90,0.3,2,,draw=none]H)
    -[:-30]X
}

Still, I find the latter solution less robust and elegant. Is there a way to still use \chemabove or \chembelow in tandem with departure/arriving atom numbering arguments?

Answer 1

I guess, you need to put the 18 into the stack and reduce its bounding box to zero using \llap (a similar example using \rlap is provided in the current documentation [v1.33] on page 42):

\documentclass[border=5mm]{standalone}
\usepackage{chemfig}

\begin{document}

\chemfig{
    X
    -[:30]\chemabove{\llap{$^{18}$}O}{H}
    -[:-30]X
}

\end{document}

Answer 2

You should use | to start the 2nd atom after ^{18}:

\chemfig{
    X
    -[:30,,,2]^{18}|\chemabove{O}{H}
    -[:-30]X
}