\begin{table}%t2
%\begin{center}
\par
\caption{Spectroscopic parameters$^a$ (MHz) of methyl cyanide isotopic 
species with $C_{\rm 3v}$ symmetry and dimensionless weighted standard 
deviation wrms.}
\label{parameter_C3v}
% \smallskip
$$
\begin{tabular}[t]{lr@{}lr@{}lr@{}lr@{}lr@{}l}
\hline \hline
Parameter & \multicolumn{2}{c}{CH$_3$CN} & \multicolumn{2}{c}{CH$_3^{13}$CN} &
 \multicolumn{2}{c}{$^{13}$CH$_3$CN}  & \multicolumn{2}{c}{$^{13}$CH$_3^{13}$CN} & \multicolumn{2}{c}{CH$_3$C$^{15}$N} \\
\hline
$(A - B) \times 10^{-3}$        &    148&.900~074~(65) &    148&.904~624       &    149&.165~664       &    149&.171~692        &    149&.176~934       \\
$B$                             & 9~198&.899~134~(11) & 9~194&.349~983~(27) & 8~933&.309~412~(28) & 8~927&.281~294~(155) & 8~922&.038~611~(72) \\
$D_K$                           &      2&.825~1~(15)   &      2&.825~7         &      2&.825~7         &      2&.825~7          &      2&.825~7         \\
$D_{JK} \times 10^3$            &    177&.407~96~(28)  &    176&.673~95~(132) &    168&.239~65~(130) &    167&.420~4~(177)   &    168&.938~0~(34)   \\
$D_J \times 10^3$               &      3&.807~528~(9)  &      3&.809~706~(37) &      3&.624~918~(32) &      3&.627~152~(61)  &      3&.555~206~(45) \\
$H_K \times 10^6$               &     51&.               &     51&.               &     51&.               &     51&.                &     51&.               \\
$H_{KJ} \times 10^6$            &      6&.063~1~(16)   &      6&.001~9~(131)  &      5&.803~1~(141)  &      5&.50~(42)        &      5&.641~0~(181)  \\
$H_{JK} \times 10^6$            &      1&.025~61~(18)  &      1&.017~52~(86) &      0&.927~05~(86)  &      0&.915~9~(35)    &      0&.950~83~(131) \\
$H_J \times 10^{12}$            & $-$255&.96~(245)      & $-$270&.2~(61)        & $-$283&.0~(49)        & $-$298&.                & $-$197&.7~(67)        \\
$L_{KKJ} \times 10^{12}$        & $-$446&.3~(26)        & $-$440&.               & $-$433&.               & $-$427&.                & $-$425&.               \\
$L_{JK} \times 10^{12}$         &  $-$52&.58~(64)       &  $-$49&.40~(236)      &  $-$49&.45~(266)      &  $-$47&.3               &  $-$48&.               \\
$L_{JJK} \times 10^{12}$        &   $-$7&.889~(45)      &   $-$7&.730~(141)     &   $-$6&.843~(128)     &   $-$6&.7               &   $-$7&.041~(182)     \\
$L_J \times 10^{15}$            &   $-$1&.80~(19)       &   $-$1&.80             &   $-$1&.80             &   $-$1&.80              &   $-$1&.28             \\
$P_{JK} \times 10^{18}$         &    529&.~(119)        &    530&.               &    450&.               &    450&.                &    440&.               \\
$P_{JJK} \times 10^{18}$        &     56&.1~(29)        &     56&.               &     48&.               &     48&.                &     49&.               \\
$eQq$                           &   $-$4&.223~24~(108) &   $-$4&.218~30~(176) &   $-$4&.218~13~(197) &   $-$4&.213~19         &       &n.~a.          \\
$C_{bb} \times 10^3$            &      1&.785~(92)      &      1&.785            &      1&.73             &      1&.73              &       &$-$             \\
$(C_{aa} - C_{bb}) \times 10^3$ &   $-$1&.24~(31)       &   $-$1&.24             &   $-$1&.24             &   $-$1&.24              &       &$-$             \\
wrms                            &      0&.906            &      0&.813            &      0&.760            &      0&.635             &      0&.692            \\
\hline
\end{tabular}
$$
%\end{center}
\par
$^a$ Numbers in parentheses are one standard deviation in units of the least significant figures. 
Parameters without quoted uncertainties have been estimated from the main isotopic species and were kept fixed in the fits; 
see Sect.~\ref{a_and_d}.\\
\end{table}