\begin{table}%t3
%\centering
\par
\caption{\label{pred:rot}Measured \citep{isos-CH+_rot} and calculated low-$J$ rotational 
transition frequencies$^a$ (MHz) of six isotopic species of CH$^+$.}
\begin{tabular}[t]{lcrr}
\hline \hline
 & \multicolumn{1}{c}{Measured} & \multicolumn{2}{c}{Calculated} \\
\cline{3-4}
 & \multicolumn{1}{c}{$J = 1{-}0$} & \multicolumn{1}{c}{$J = 1{-}0$} & \multicolumn{1}{c}{$J = 2{-}1$} \\
\hline
CH$^+$        & 835~137.504~(20)     & 835~137.503~(20) & 1~669~281.3~(3) \\
$^{13}$CH$^+$ & 830~216.095~(30)\tablefootmark{b} & 830~216.096~(22) & 1~659~450.3~(3) \\
CD$^+$        & 453~521.851~(20)     & 453~521.851~(20) &    906~752.0~(1) \\
$^{13}$CD$^+$ &                       & 448~539.258~(26) &    896~793.2~(1) \\
$^{14}$CH$^+$ &                       & 826~012.748~(45) & 1~651~053.6~(3) \\
CT$^+$        &                       & 325~443.364~(70) &    650~736.8~(1) \\
\hline
\end{tabular}
\tablefoot{\tablefoottext{a}{Numbers in parentheses are one standard deviation in units of the 
least significant figures; see the text for discussion of uncertainties.}
\tablefoottext{b}{Value corrected for hyperfine splitting; two lines were measured, see text, 
each accurate to 30~kHz.}}
\end{table}