Faire un cadre avec un titre

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fancybox,chemfig}
\parindent=0pt

\makeatletter
\newcommand\make@car@active[1]{%
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    \begingroup
        \lccode`\~`#1\relax
        \lowercase{\endgroup\def~}%
}

\newif\if@exstar

\newcommand\exemple{%
    \begingroup
    \parskip\z@
    \@makeother\;\@makeother\!\@makeother\?\@makeother\:% neutralise frenchb
    \@ifstar{\@exstartrue\exemple@}{\@exstarfalse\exemple@}}

\newcommand\exemple@[2][65]{%
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    \begingroup
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}

\newcommand\exemple@@[3]{%
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    \@tempa
}

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    \medbreak
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}

\begin{document}

\exemple{Distance interatome}|\chemfig[atom sep=2em]{A-B}\par
\chemfig[atom sep=50pt]{A-B}|

\end{document}