I would like to draw in chemfig the following picture:

But I am struggling with the "+" sign in the middle of the circle. Is there a way to draw it? Thank you!

What I have so far is this:

\documentclass{article}
\usepackage{chemfig}
\begin{document}

\chemfig{
N**[0,-150,dash pattern=on 2pt off 2pt]5(
(-R)-(-R^1)-[,,1,2]{+\hspace{0.3cm}}|{O}-[,,2,1]--
)
}

\end{document}


which does the work, but I really don't like the way it is done (e.g. it does not scale).

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You should post your code, so that we could easily have a starting point. A minimal working example would be great (starting from \documentclas… and ending with \end{document}). –  Manuel Jan 29 at 10:27
@Manuel Sorry, I edited the question. –  pisoir Jan 29 at 10:32
BTW: Welcome to TeX.SX! You can have a look at our starter guide to familiarize yourself further with our format. –  karlkoeller Jan 29 at 10:33

The following examples uses that the tikz code for the arc uses a center node named arccenter. The tikz option argument for the \draw command of the arc can be used with option late options to put a label in the center:

\documentclass{article}
\usepackage{chemfig}
\begin{document}

\chemfig{
N**[0,-144,dash pattern=on 2pt off 2pt,
late options={name=arccenter,label=center:+}]
5( (-R)-(-R^1)-O--- )
}

\end{document}


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That's exactly what I wanted. But I could not find it in the tikz manual. Great! –  pisoir Jan 29 at 13:41
@pisoir: arccenter is not a tikz property of arc, but a node name set by the internals of chemfig. –  Heiko Oberdiek Jan 29 at 13:51
Just for the sake of correctness… shouldn't it be …[0,-144,… instead of …[0,-150,…? I know nothing about chemistry. –  Manuel Jan 29 at 14:57
@Manuel: I agree with you and have changed the angle from -150 to -144 = -(360/5)*2. –  Heiko Oberdiek Jan 29 at 15:06

I'd use an invisible bond pointing to the center of the ring (with a relative angle) to place the plus. Something like (-[::126,,,,draw=none]\oplus), possibly scaled a bit. On the other hand I do like Heiko's answer better than mine :)

\documentclass{article}
\usepackage{chemfig}
\begin{document}

\chemfig{
R-[:36]N
**[216,360,dash pattern=on 2pt off 2pt]5(
-(-[::126,.6,,,draw=none]\oplus)
(-R|_1)
-O(-[,,,,draw=none]X^{-})
---
)
}

\end{document}


-

In a bind, you can slap it on after the fact with a \stackinset. The syntax here means that the inset item (a bold +) is placed 10pt to the right of center, and 15pt above center on the underlying \chemfig.

\documentclass{article}
\usepackage{chemfig}
\usepackage{stackengine}
\begin{document}

\stackinset{c}{10pt}{c}{15pt}{\textbf{+}}{%
\chemfig{
N**[0,-150,dash pattern=on 2pt off 2pt]5(
(-R)-(-R^1)-[,,1,2]{}|{O}-[,,2,1]--
)
}%
}
\end{document}


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