\begin{table}%t4
%\centering
\par
\caption{\label{reactions}Key reactions included in the chemical network relevant to   the formation of HNCO. }
\begin{tabular}{crcll}
\hline\hline \noalign{\smallskip}
 Label & \multicolumn{3}{c}{Reaction} & Reference \\
\hline
a & GOCN + GH &$\rightarrow$& GHNCO & AR77\\
b & GNH + GCO &$\rightarrow$& GHNCO & GWH08\\
c & OCN + H$_2$ &$\rightarrow$& HNCO + H & TTH99\\
d & H$_2$NCO$^{+}$ + e$^{-}$ &$\rightarrow$& HNCO + H & I77\\
e & H$_2$ + OCN$^{+}$ &$\rightarrow$& HNCO$^{+}$ + H & I77\\
f & H$_2$ + HNCO$^{+}$ &$\rightarrow$& H$_2$NCO$^{+}$ + H & I77\\
g & He$^{+}$ + OCN &$\rightarrow$& He + OCN$^{+}$ & -- \\
\hline
\end{tabular}
\tablefoot{The prefix~G denotes surface species. AR77; \citet{ar1977}, GWH08; \citet{gwh2008}, TTH99; \citet{tth1999}, I77; \citet{i1977}.}
\end{table}