Reinvestigation of the ground and first torsional state of methylformate

Abstract

We have reinvestigated the laboratory spectrum for the methylformate HCOOCH3 molecule involving both the ground and first excited torsional states. We have fitted within almost experimental accuracy a data set for this molecule consisting of 3496 vt ¼ 0 and 774 vt ¼ 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 ¼ 0 and vt ¼ 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 ¼ 62, Kmax ¼ 22 in vt ¼ 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.