\begin{table}%t3
 \caption{\label{tab_PAM_rot_const}Rotational constants in the principal axis system (PAM),
  angles between the principal axis and the methyl top axis and
  internal rotation parameters upon isotopic substitution.} 
\par
%
\par
%\centerline
 {\small \begin{tabular}{ccccccc}
\hline\hline        
& HCOOCH$_3^a$ & HCOO$^{13}$CH$_3^b$ & H$^{13}$COOCH$_3^c$ &
H$^{13}$COOCH$_3^d$ & H$^{13}$COOCH$_3^d$ & H$^{13}$COOCH$_3^d$ \\
&Global fit ($v_{\rm t} = 0,1$) & Global Fit ($v_{\rm t} = 0$) & Global Fit ($v_{\rm t}
= 0$) & $v_{\rm t} = 0$ & $v_{\rm t} = 1$ & $v_{\rm t} = 2$ \\
\hline
$A$(MHz) & 19928.890(602) & 19629.65(176) & 19709.546(258) &
19798.73249(43) & 19917.3706(78) & 20034.7(47) \\
$B$(MHz) & 6952.169(585) & 6804.42(171) & 6909.555(225) &
6864.74991(14) & 6798.41624(91) & 6748.99(76) \\
$C$(MHz) & 5309.877(125) & 5183.442(402) & 5274.5475(189) &
5262.53086(13) & 5236.59430(30) & 5213.360(21) \\
$<$$(i,a)^e$ & 52.97 & 51.68 & 52.48 & & & \\
$\theta_{\rm RAM}$$^f$ & 24.83 & 23.68 & 24.54 & 21.8604(79) & 22.201(11)
& 22.867(34) \\
$\rho$ & 0.08427127(723) & 0.0845207(106) & 0.08407138(395) &
0.084497(20) & 0.086209(43) & 0.08426(14) \\
$F$ (cm$^{-1}$) & 5.49038(129) & 5.69168218$^g$ &
5.8039485$^g$ & 5.954(1) & 5.7704(3) & 5.802(1) \\
%$V_3$ (cm$^{-1}$) & 370.924(113) & 407.1549(147) & 416.60231(475) &
%--- & --- & --- \\
$\sigma$$^h$ & 1.43 & 1.08 & 0.67 & 0.86 & 0.99 & 1.07 \\
N$^{\rm o}$ of transitions & 4270 & 936 & 4954 & 4921 & 910 & 231 \\
\hline 
\end{tabular}}
\medskip
\par
$^a$ Rotation-torsion parameters for the normal species HCOOCH$_3$
obtained in our work in the RAM-axis system, after transforming the
RAM values for the $A$, $B$, $C$ rotational parameters  into PAM values
following the procedure described in \citet{carvajal2007}. Note that
\citet{maeda2008a} also presents a fit for the normal species (but not
including the same data set), using the ErHam procedure and fitting
separately 5366~lines, 1228 lines and 213 lines belonging to $v_{\rm t} =
0$, 1 and 2 with unitless standard deviations of 1.11, 1.18 and 0.81
respectively. Accuracy of their line measurements is about 100~kHz. 
$^b$ Rotation-torsion parameters for $^{13}$C$_2$-methyl formate
from the present work transformed to the Principal Axis System.  
$^c$ Rotation-torsion parameters for $^{13}$C$_1$-methyl formate
from the present work transformed to the Principal Axis System. The
parameters were obtained with RAM model from a fit of all $v_{\rm t} = 0$
data presently available (4954~lines with 24 fitted parameters) in the
literature and will be published soon.  
$^d$ Rotation-torsion parameters for $^{13}$C$_1$-methyl formate
from \citet{maeda2008b} obtained separately for each torsional state
studied, using the ErHam procedure. %ErHam procedure does not provide
%  either the potential barrier V$_3$ or the angle $<(i,a)$ between the
%  methyl top and the principal axis. \\ 
$^e$ Angle in degrees between the a-principal axis and the
methyl top axis (i).  
$^f$ The angle $\theta_{\rm RAM}$ between the a-principal axis and the
a-RAM axis is given in degrees and obtained from Eq.~(1) from
\citet{carvajal2007}, with the parameters $A^{\rm RAM}$, $B^{\rm RAM}$,
$C^{\rm RAM}$, and $D_{\rm ab}$ of Table \ref{tab_param}. 
$^g$ Fixed to the  ab initio  value.  
$^h$ Unitless standard deviation. 
\end{table}