Attach atom to center of ring in chemfig

Question

Sometimes chemists want to signify that something is attached to a C-ring without specifying the exact position to which it is bound, for example because it's random. Instead they to something like

(see the R^1). This could also be attached to the center of the ring or the "aromatic circle" in the bottom left. Can this be done in chemfig?

Answer 1

(Since chemfig v1.56, center of a ring can be easily referenced as a node name. See update near the end of this answer.)

Yes chemfig can do this, with some tricks (or advanced usage?).

\documentclass{article}
\usepackage{chemfig}
\begin{document}
\chemfig{6(-=-=-=)} \qquad
\chemfig{6(-=(-[:150])-=-=)} \qquad
\chemfig{6(-=(-[:150]-[:120]-[:120]R^1)-=-=)} \qquad
\chemfig{6(-=(-[:150]-[:120,.5]-[:120]R^1)-=-=)} \qquad
\chemfig{*6(-=(-[:150,,,,white]-[:120,.5,,,draw=none]-[:120]R^1)-=-=)}
\begin{enumerate}
  \item Start.
  \item Draw a bond to center with syntax \verb|-[<angle>]|.
  \item Draw two more bonds to get to the right angle, and add desired atom.
  \item Adjust bond length using syntax \verb|-[<angle>,<length scale>]|.
  \item Hide intermediate bonds by specifying a white drawing color, 
        using syntax \verb|-[<angle>,<length scale>,,,<tikz code>]|.
\end{enumerate}
\end{document}

Update

In the following example, an arc inside ring is named A, so the center of ring is (A.center). I do not know how to draw a chemfig bond starting at arbitrary coordinate, so I use insert path.

Limitation: Angle needs to be explicitly specified, like the 36 in (36:5pt).

\documentclass{article}
\usepackage{chemfig}
\usetikzlibrary{calc}
\begin{document}
\chemfig{%
  **[,,draw=none, local bounding box=A]%
  5(-----%
    [,,,,{insert path={
      ([shift={(36:5pt)}]A.center) -- +(36:1cm) node[anchor=center, right] {X}
    }}]%
  )%
}
\end{document}

Update 2

With chemfig v1.56 (or newer), the center of ring can be referred by node name centrecycle<n>, see the chemfig manual, sec. 12.6 for more detail. Thanks to Christian Tellechea, author of chemfig.

The following example contains two \chemfig drawings.

• In the first one, drawing commands among cycle center is added by the following \chemmove, which won't extend the bounding box of previous \chemfig and might overlap with surrounding text.
• The second one moves those drawing commands among cycle center to tikz option execute at end picture, which do extends the bounding box.

\documentclass{article}
\usepackage{chemfig}
\usepackage{lipsum}
\tikzset{
  % helper style
  show bounding box/.style={
    execute at end picture={
      \draw[blue, #1] (current bounding box.north west) rectangle (current bounding box.south east);
    }
  }
}
\begin{document}
\lipsum[23]
Example from \verb|chemfig| manual, sec.@ 12.6. Note \verb|\chemmove| won't extend the bounding box.
text
\chemfig[chemfig style={show bounding box}]{*6(-=-=-=)}%
\chemmove{
  \node[at=(cyclecenter1)] {.+}
   node[at=(cyclecenter1), shift=(120:1.75cm)] (end) {\printatom{R^1}};
  \draw[-, shorten <=.5cm] (cyclecenter1) -- (end);
}
text
Using \verb|execute at end picture|, the bounding box is correctly extended.
\tikzset{
  cycle center staff/.style={
    execute at end picture={
      \path (cyclecenter1) node[anchor=center] {.+}
           +(120:1.75cm)   node[inner sep=2pt] (end) {\printatom{R^1}};
      \draw (cyclecenter1) +(120:.5cm) -- (end);
    }
  }
}
text \chemfig[chemfig style={
  cycle center staff,
  show bounding box
}]{*6(-=-=-=)}
text
\lipsum[23]
\end{document}

Update 3

To simplify user input, a new command \chempremove is defined. With this new command,

\chempremove{<tikz drawing code>}
\chemfig{...}

is equivalent to

\chemfige[chemfig style={execute at end picture={<tikz drawing code>}}]{...}

Full implementation:

\documentclass{article}
\usepackage{chemfig}
\usepackage{lipsum}
\usepackage{xpatch}
\tikzset{
  % helper style
  show bounding box/.style={
    execute at end picture={
      \draw[blue, #1] (current bounding box.north west) rectangle (current bounding box.south east);
    }
  }
}
% new command \chempremove{<tikz code executed at end picture>}
\catcode`_=11\relax
\defKV[chemfig]{%
  at end picture        = \def\CF_atendpic_once{#1},
  at end picture append = \CF_addtomacro\CF_atendpic_once{#1}
}
\setKVdefault[chemfig]{%
  at end picture        = {}
}
\newcommand{\chempremove}[1]{%
  \setKV[chemfig]{at end picture append={#1}}%
  \ignorespaces
}
\xpatchcmd\CF_chemfigb
  {baseline,}
  {baseline,%
   execute at end picture={\unexpanded\expandafter{\CF_atendpic_once}},}
  {}{\fail}
\xpatchcmd\CF_chemfigb
  {\let\CF_flipstate\CF_zero}
  {\let\CF_flipstate\CF_zero\let\CF_atendpic_once\empty}
  {}{\fail}
\catcode`_=8\relax
\begin{document}
\lipsum[23]
Example from \verb|chemfig| manual, sec.@ 12.6. Note \verb|\chemmove| won't extend the bounding box.
\chempremove{
  % show bounding box
  \draw[blue] (current bounding box.north west) rectangle (current bounding box.south east);
}
text
\chemfig[chemfig style={show bounding box}]{*6(-=-=-=)}%
\chemmove{
  \node[at=(cyclecenter1)] {.+}
   node[at=(cyclecenter1), shift=(120:1.75cm)] (end) {\printatom{R^1}};
  \draw[-, shorten <=.5cm] (cyclecenter1) -- (end);
}
text
Using \verb|\chempremove|, the bounding box is correctly extended.
\chempremove{
  \path (cyclecenter1) node[anchor=center] {.+}
       +(120:1.75cm)   node[inner sep=2pt] (end) {\printatom{R^1}};
  \draw (cyclecenter1) +(120:.5cm) -- (end);
  % show bounding box
  \draw[blue] (current bounding box.north west) rectangle (current bounding box.south east);
}
text \chemfig{*6(-=-=-=)} text
\lipsum[23]
\end{document}