Reinvestigation of the ground and first torsional state of methylformate

We have fitted within almost experimental accuracy a data set for this molecule consisting of 3496 vt=0 and 774 vt=1 microwave lines, using the so-called “rho axis method” (RAM) and a model extended to include perturbation terms through eighth order. The previously published vt=0andvt=1 microwave lines covering the J values up to 43 in the ground state and up to 18 in the first excited state have been extended by 434 new measurements from Lille in the 567-669 GHz spectral range, corresponding to transitions with Jmax=62, Kmax=22 in vt=0. The final fit requires only 49 parameters to achieve a unitless weighted standard deviation of 1.43 for a total of 4270 lines for the whole fit. This result represents an improvement over the previous fit which achieved a standard deviation of 1.96 for 3862 lines using 69 parameters. A calculation of the line strengths of torsion-rotation transitions up to J = 60 needed for the astronomical survey is also provided.