Forked arrows with chemfig

Question

How can one make a forked arrow in chemfig?

Semantically it is exactly opposite to \merge command but seems like it's not implemented in a package. I don't have any experience with TikZ graphics but any solution would be highly appreciated.

I'd like to get something like the following scheme:

Answer 1

Since you can use \chemfig inside a tikzpicture environment, you can place the three compounds (forgive me if that's not the right name) inside \nodes and then use \draw to draw the arrows:

\documentclass{article}
\usepackage{chemfig}
\usetikzlibrary{positioning,calc}

\begin{document}

\definesubmol\Me[H_3C]{CH_3}

\begin{tikzpicture}[node distance=0cm and 2cm]
\node (A) 
  {\chemfig{R-C-[::-60]O-[::-60]C-[::-60]R}};
\node[above right=of A] (B) 
  {\chemfig{*6((-!\Me)=(-!\Me)-(-!\Me)=(-!\Me)-(-!\Me)=(-!\Me)-)}
};
\node[below right=of A] (C)    
  {\chemfig{CH_3CH_2-[:-60,,3]C(-[:-120]H_3C)=C(-[:-60]H)-[:60]C{(}CH_3{)}_3}};
\draw[-stealth] (A) -- ( $ (A.0)!0.5!(B.west|-A.0) $ ) |- (B.west) node[auto,pos=0.7] {i,j};
\draw[-stealth] (A) -- ( $ (A.0)!0.5!(C.west|-A.0) $ ) |- (C.west) node[auto,pos=0.7] {j};
\end{tikzpicture}

\end{document}

Answer 2

You say »Semantically it is exactly opposite to \merge command« so lets define a command \fork that is the opposite of \merge. For this I patch the necessary commands with etoolbox's \patchcmd. The idea is simple: define a boolean switch \iffork and add arrow heads depending on the status. Then define \merge so it sets \forkfalse and \fork the same way but with \forktrue.

Here we go:

\documentclass{article}
\usepackage{chemfig}
\usepackage{etoolbox}

\makeatletter
\newif\iffork

\patchcmd\CF@merge@ii{-CF@full}{\iffork\else -CF@full\fi}{}{}%
% \CF@merge@iii needs to be patched twice:
\patchcmd\CF@merge@iii{>=0]}{>=0,\iffork CF@full-\fi]}{}{}%
\patchcmd\CF@merge@iii{>=0]}{>=0,\iffork CF@full-\fi]}{}{}%
\patchcmd\CF@schemestart@i
  {\let\merge\CF@merge}
  {%
    \def\merge{\forkfalse\CF@merge}%
    \def\fork{\forktrue\CF@merge}%
  }
  {}{}
\patchcmd\CF@schemestart@v{\merge}{\merge\fork}{}{}

\makeatother

\begin{document}

\schemestart
 a \arrow{0} b \arrow{0} c
 \merge{v}(c1)(c2)(c3)--() d
\schemestop

\bigskip

\schemestart
 a \arrow{0} b \arrow{0} c
 \fork{v}(c1)(c2)(c3)--() d
\schemestop

\end{document}