\begin{table}%t1
\caption{Rest frequencies.
\label{tbl-1}}
%\centerline
{
\begin{tabular}{lrcc}
\hline\hline
\noalign{\smallskip}
\mbox{Line} & \mbox{Frequency}  & \mbox{Uncert.} & \mbox{Ref.} \\
& \mbox{(MHz)} & \mbox{(kHz)} &  \\
\noalign{\smallskip}
\hline
\noalign{\smallskip}
\mbox{C$^{18}$O($J=1$--0)~~~} & 109782.172 & 3 & (1) \\
\mbox{C$^{18}$O($J=2$--1)} & 219560.354 & 1 & (2) \\
\mbox{C$^{17}$O($J,~F=1, 7/2$--0, 5/2)~~~} & 112358.990 & 1 & (3) \\
\mbox{C$^{17}$O($J,~F=2, 9/2$--1, 7/2)} & 224714.214  & 1 & (3) \\
\mbox{CS($J=2$--1)}        & 97980.953 & 1 & (4) \\
\mbox{CS($J=3$--2)}        & 146969.026 & 1 & (4) \\
\mbox{C$^{34}$S($J=2$--1)} & 96412.952 & 1 & (4)  \\
\mbox{C$^{34}$S($J=3$--2)} & 144617.101& 1 & (4) \\
\mbox{NH$_3$($JKF_1F=1110.5$--1111.5)} & 23694.501 & 0.1 & (5) \\
\mbox{N$_2$H$^+$($JF_1F=101$--021)} & 93176.258 & 10 & (6) \\
\mbox{N$_2$H$^+$($JF_1F=344$--233)} & 279511.811 & 6 & (7) \\
\noalign{\smallskip}
\hline
\end{tabular}}
     References: (1) C. Gottlieb, priv. comm., (2)~\citet{mul01},
(3)~\citet{caz02}, (4)~\citet{got03}, (5)~\citet{kuk67}, 
(6)~\citet{lee01}, astronomical, (7)~\citet{cas02c}, astronomical.
\end{table}