Newline after \cases in \equation environment

I have the following problem: I have an equation environment with three different lines. The first to should be connected with a 2 line bracket. The last line should be below the two. Just like if it were three independet lines of stuff. I use \cases to creat the bracket, but the last line will show up behind the two, not below. Tried multlines and eqnarray already.

$$2 \text{x} \begin{cases} {}^\text{1}\text{H} + {}^\text{1}\text{H} \rightarrow {}^\text{2}\text{H} + e^+ + \nu \\ {}^\text{2}\text{H} + {}^\text{1}\text{H} \rightarrow {}^\text{3}\text{He} + \gamma \end{cases} \\ {}^\text{3}\text{He} + {}^\text{3}\text{He} \rightarrow \text{2} {}^\text{1}\text{H} \label{eq:ppkette}$$


The environment equation is made for one line equations, use align if you want to be able to break lines. However, if you want one equation number for your three lines, the environment matrix can do the trick.

$$\begin{matrix} 2 \times % Use \times rather than \text{x} \begin{cases} \begin{matrix}{}^\text{1}\text{H} + {}^\text{1}\text{H} \rightarrow {}^\text{2}\text{H} + e^+ + \nu \\ {}^\text{2}\text{H} + {}^\text{1}\text{H} \rightarrow {}^\text{3}\text{He} + \gamma \end{matrix} \end{cases} \\ {}^\text{3}\text{He} + {}^\text{3}\text{He} \rightarrow \text{2} {}^\text{1}\text{H} \label{eq:ppkette} \end{matrix}$$


PS : there is a dedicated stakcExchange site for LaTeX related question : https://tex.stackexchange.com/

Depending on the alignment you're after, the following seems to suit your needs (aligned around \rightarrow):

\documentclass{article}
\usepackage{amsmath,mhchem}

\begin{document}

$$2 \text{x} \begin{cases} {}^\text{1}\text{H} + {}^\text{1}\text{H} \rightarrow {}^\text{2}\text{H} + e^+ + \nu \\ {}^\text{2}\text{H} + {}^\text{1}\text{H} \rightarrow {}^\text{3}\text{He} + \gamma \end{cases} \\ {}^\text{3}\text{He} + {}^\text{3}\text{He} \rightarrow \text{2} {}^\text{1}\text{H}$$

$$\renewcommand{\arraystretch}{1.2} \begin{array}{r@{}l} 2 \times \Biggl\{\begin{array}{@{}r@{}} \ce{^1 H + ^1 H} \\ \ce{^2 H + ^1 H} \end{array} & \begin{array}{@{}l@{}} {} \rightarrow \ce{^2 H + e+ + \nu} \\ {} \rightarrow \ce{^3 He + \gamma} \end{array} \\ \ce{^3 He + ^3 He} & {} \rightarrow 2 \ce{^1 H} \end{array}$$

\end{document}


Other alignments are also possible, of course.