Align Molecules in chemfig

Question

I want to align the reaction equation to the black line shown in the picture.

\documentclass[11pt,a4paper,oneside]{scrartcl}
\usepackage{chemfig}
\begin{document}
    
\begin{figure}
    \begin{center}
        \schemestart
            \chemfig{*6(-=(*6(-(=O)-NH-NH-(=O)-))-=(-NH_2)-=)}\qquad
            \+
            \chemfig{H_2O_2}\qquad
            \arrow(.mid east--.mid west){-U>[][$\mathrm{N_2}$]}\qquad
            \chemfig{*6(-=(-COO^{-})-(-COO^{-})=(-NH_2)-=)}\qquad
            \+
            Licht
        \schemestop
    \end{center}
    \caption{Reaktion von Luminol zu 3-Aminophtalatdianion}
\end{figure}
    
    
\end{document}

score 1 · Answer 1 · answered May 27 '22 at 10:28

A bit hacky, but you could do

\documentclass[11pt,a4paper,oneside]{scrartcl} 
\usepackage{chemfig}

\begin{document}

\begin{figure}[htbp]
\centering
\setchemfig{+ vshift=0.5cm}
\schemestart
\chemfig{*6(-=(*6(-(=O)-NH-NH-(=O)-))-=(-NH_2)-=)}\qquad
\+
\raisebox{0.5cm}{\chemfig{H_2O_2}}\qquad
\arrow(.north east--.north west){-U>[][$\mathrm{N_2}$][-2.4cm]}\qquad
\chemfig{*6(-=(-COO^{-})-(-COO^{-})=(-NH_2)-=)}\qquad
\+
\raisebox{0.5cm}{Licht}
\schemestop
\caption{Reaktion von Luminol zu 3-Aminophtalatdianion}
\end{figure}


\end{document}

CrocoDuck · Answer 2 · 2022-07-01T16:21:52.483

Another possibility, using only commands from the "chemfig" package, where I used two "invisible arrows" to adjust the position of the "+"

\documentclass[11pt,a4paper,oneside]{scrartcl} 
\usepackage{chemfig}

\begin{document}

    \begin{figure}[htbp]
        \centering
        \schemestart
        \chemfig{*6(-=(*6(-(=O)-NH-NH-(=O)-))-=(-NH_2)-=)}
        \arrow{0}[,.1]\+
        \chemfig{H_2O_2}        
        \arrow{-U>[][\chemfig{N_2}]}
        \chemfig{*6(-=(-COO^{-})-(-COO^{-})=(-NH_2)-=)}
        \arrow(.east--.west){0}[,.1]\+  Licht
        \schemestop
        \caption{Reaktion von Luminol zu 3-Aminophtalatdianion}
    \end{figure}
    
    
\end{document}

Since the first molecule has branches above and below the rings, and the reaction product has branches just above, and knowing that the "chemfig" aligns the molecules by their "center of mass", a simple way to adjust the position of the resulting molecule of the reaction is by placing an "invisible bond" (-[6,,,,draw=none) at the bottom of this molecule.

\documentclass[11pt,a4paper,oneside]{scrartcl} 
\usepackage{chemfig}

\begin{document}
    \begin{figure}[htbp]
        \centering
        \schemestart
        \chemfig{*6(-=(*6(-(=O)-NH-NH-(=O)-))-=(-NH_2)-=)}
        \arrow{0}[,.1]\+
        \chemfig{H_2O_2}        
        \arrow{-U>[][\chemfig{N_2}]}
        \chemfig{*6(-(-[6,,,,,draw=none])=(-COO^{-})-(-COO^{-})=(-NH_2)-=)}
        \arrow(.east--.west){0}[,.1]\+  Licht
        \schemestop
        \caption{Reaktion von Luminol zu 3-Aminophtalatdianion}
    \end{figure}
\end{document}

Is there any way in this approach to keep the rings of the molecules on the same level? (P.S. I love your avatar!) — samcarter_is_at_topanswers.xyz, Jul 01 '22 at 16:08
Yes there is. I made an edit to my answer to show how to do it. — CrocoDuck, Jul 01 '22 at 16:23
Oh, that's clever! I was trying so many different ways back when I tried to answer this, but only managed with a dirty hack. Yours is very elegant! — samcarter_is_at_topanswers.xyz, Jul 01 '22 at 18:38
Thanks. But it took me a lot of time and I had to do a lot of reaction schemes to figure out this way to tweak the molecule. This is not in the manual. — CrocoDuck, Jul 01 '22 at 18:53
Yes, I was looking through the manual, but didn't find a solution. Glad I asked you :) — samcarter_is_at_topanswers.xyz, Jul 01 '22 at 19:00

Align Molecules in chemfig

2 Answers2