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\usepackage{threeparttable}

\begin{document}

\begin{center}
 \begin{threeparttable}
  \caption{Les angles biaisés ($\beta$) pour $\fam0 Mu(H)+X_2$ et $\fam0 Mu(H)+HX$~\tnote{a}}
  \begin{tabular}{rlcc}% ou tabularx, etc.
   \hline
   &             & $\fam0 H(Mu)+F_2$      & $\fam0 H(Mu)+Cl_2$ \\
   \hline
   & $\beta$(H)  & $80,9^\circ\tnote{b}$  & $83,2^\circ$       \\
   & $\beta$(Mu) & $86,7^\circ$           & $87,7^\circ$       \\
   \hline
  \end{tabular}
  \begin{tablenotes}
   \item[a] pour la réaction d'abstraction, $\fam0 Mu+HX \rightarrow MuH+X$.
   \item[b] 1 degré${} = \pi/180$ radians.
  \end{tablenotes}
 \end{threeparttable}
\end{center}

\end{document}