4

Basically, I'm trying to use amsmath to align the 2 possible outcomes of a reaction, which naturally does not include an equals sign.

\begin{equation}
\ce{^{2}_{1}H} + \ce{^{2}_{1}H} &\rightarrow \ce{^{1}_{1}H} + \ce{^{3}_{1}H} + 4.03 \text{MeV}\\
&\rightarrow \ce{^{1}_{0}n} + \ce{^{3}_{2}He} + 3.27 \text{MeV}\\
\end{equation}

The reaction has a branch, 2 possible outcomes occur and I'm trying to align the arrows. Any advice?

3
  • 3
    replace equation by align*.
    – Sigur
    Feb 12, 2019 at 22:47
  • 3
    ... and consider using siunitx for typesetting MeV.
    – user121799
    Feb 12, 2019 at 22:50
  • @P227 I will substitute \mathrm{MeV} instead of \text{MeV} if you not will use siunitx.
    – Sebastiano
    Feb 12, 2019 at 23:06

1 Answer 1

4

You could use split or a fancier typesetting.

\documentclass{article}
\usepackage{amsmath,siunitx}
\usepackage[version=4]{mhchem}

\DeclareSIUnit{\eV}{eV}

\begin{document}

\begin{equation}
\begin{split}
\ce{^{2}_{1}H} + \ce{^{2}_{1}H}
  &\rightarrow \ce{^{1}_{1}H} + \ce{^{3}_{1}H} + \SI{4.03}{\mega\eV} \\
  &\rightarrow \ce{^{1}_{0}n} + \ce{^{3}_{2}He} + \SI{3.27}{\mega\eV}
\end{split}
\end{equation}

\begin{equation}
\ce{^{2}_{1}H} + \ce{^{2}_{1}H}
\mathrel{\begin{array}{@{}c@{}}\nearrow\\\searrow\end{array}}
\begin{array}{@{}l@{}}
  \ce{^{1}_{1}H} + \ce{^{3}_{1}H} + \SI{4.03}{\mega\eV} \\[2ex]
  \ce{^{1}_{0}n} + \ce{^{3}_{2}He} + \SI{3.27}{\mega\eV}
\end{array}
\end{equation}

\end{document}

enter image description here

Even fancier, with tikz-cd:

\documentclass{article}
\usepackage{amsmath,siunitx}
\usepackage[version=4]{mhchem}
\usepackage{tikz-cd}

\DeclareSIUnit{\eV}{eV}

\begin{document}

\begin{equation}
\begin{tikzcd}[row sep=-1em,column sep=3em]
& \ce{^{1}_{1}H} + \ce{^{3}_{1}H} + \SI{4.03}{\mega\eV} \\
\ce{^{2}_{1}H} + \ce{^{2}_{1}H}
  \arrow[ur,start anchor=real east,end anchor=west]
  \arrow[dr,start anchor=real east,end anchor=real west] \\
& \ce{^{1}_{0}n} + \ce{^{3}_{2}He} + \SI{3.27}{\mega\eV}
\end{tikzcd}
\end{equation}

\end{document}

enter image description here

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