\begin{table}%t1
%\centering
\par
\caption{References for the molecular parameters used in this work.}
\label{tab1}
\begin{tabular}{cccc}
\hline\hline
Molecule   & Rate coefficients & Dipole moment & Energy levels \\
\hline 
H$_2$CO    & \citet{green1991}   & \citet{fabricant1977} & \citet{muller2005} \\
HCN        & \citet{monteiro1986}& \citet{ebenstein1984} & \citet{muller2005} \\
HNC        & \citet{monteiro1986}& \citet{blackman1976}  & \citet{muller2005} \\
N$_2$H$^+$ & \citet{daniel2005}  & \citet{havenith1990}  & \citet{caselli1995} \\
\hline
\end{tabular}
\end{table}