\begin{table}%t4
\caption{\label{tab:rates}Key chemical reaction rates$^{\dagger}$ adopted in this work.}
%\centerline
{\small       \begin{tabular}{ll} 
\hline \hline\noalign{\smallskip}
Reaction    &   Rate  [cm$^3$~s$^{-1}$]\\
\hline\noalign{\smallskip}
 \emr{HCO^+  + {\rm e}^-    \rightarrow CO + H}               &  $2.4 \times 10^{-7} (300~\emr{K}/\emr{T})^{0.69}~^{a}$\\
 \emr{HCO^+  + PAH^-  \rightarrow PAH + CO + H}         &  $1.4 \times 10^{-8} (300~\emr{K}/\emr{T})^{0.50}~^{b}$\\
 \emr{M^+    + {\rm e}^-    \rightarrow M + h\nu}             &  $3.7 \times 10^{-12} (300~\emr{K}/\emr{T})^{0.65}$\\
 \emr{M^+    + PAH^-  \rightarrow M + PAH}              &  $1.0 \times 10^{-8} (300~\emr{K}/\emr{T})^{0.50}~^{b}$\\
 \emr{C^+    + H_2O   \rightarrow HCO^+ + H}            &  $8.9 \times 10^{-10} (300~\emr{K}/\emr{T})^{0.50}$ \\
 \emr{C^+    + H_2O   \rightarrow HOC^+ + H}            &  $1.8 \times 10^{-9} (300~\emr{K}/\emr{T})^{0.50}$ \\
 \emr{CO^+   + H_2    \rightarrow HCO^+ + H}            &  $7.5 \times 10^{-10}$ \\
 \emr{CO^+   + H_2    \rightarrow HOC^+ + H}            &  $7.5 \times 10^{-10}$ \\
 \emr{HOC^+  + H_2    \rightarrow HCO^+ + H_2}          &  $3.8 \times 10^{-10}~^{c}$   \\
\hline
\end{tabular}}
\medskip
$^{\dagger}$ Rates are from the \textit{UDFA} and \textit{OSU} databases unless indicated; $^{a}$~cited in the text as $\alpha$(HCO$^+$), it also applies to DCO$^+$ and H$^{13}$CO$^+$;
$^{b}$~rate is from (\cite{flo03}); $^{c}$~rate is from (\cite{smi02}). 
\end{table}