A new joint analysis of the ground and first excited torsional states of methylformate

A global fit within experimental accuracy of microwave rotational transitions in the ground and first excited torsional states (vt = 0 and 1) of methylformate (HCOOCH3) is reported, which combines older measurements from the literature with new measurements from Kharkov. In this study the so-called ''rho axis method'' that treats simultaneously both A and E species of the ground and first excited torsional states is used. The final fit requires 55 parameters to achieve an overall unitless weighted standard deviation of 0.71 for a total of 10533 transitions (corresponding to 9298 measured lines) with rotational quantum numbers up to J ⩽ 62 and Ka ⩽ 26 in the ground state and J ⩽ 35 and Ka ⩽ 23 in the first excited torsional state. These results represent a significant improvement over past fitting attempts, providing for the first time a fit within experimental accuracy of both ground and first excited torsional states.