The millimeter- and submillimeter-wave spectrum of methyl formate (HCOOCH3)

Methyl formate (HCOOCH3), a threefold internal rotor, is a well-known inter stellar molecule located in hot cores of giant molecular clouds. Over a dec ade ago, we published separate analyses of the rotational-torsional spectra of methyl formate in its A and E ground torsional substates. The analysis of the A substate was undertaken with a standard asymmetric top Hamiltonian , while that of the E substate was undertaken with a principal axis method (PAM), which was less than entirely satisfactory. In the interim, we have m easured many new lines of the rotational-torsional spectrum of both the gro und A and E torsional substates of this species through the rotational quan tum number J = 50 using two spectrometers: our standard klystron-based syst em and a new fast scan spectrometer (FASSST). The newly measured lines have been combined with previous data to form a data set consisting of over 200 0 lines that has been fitted to the accuracy of our experiments using a mod ification of the internal axis method (IAM) we have previously applied to m ethanol. The spectral constants obtained from the fit have allowed us to pr edict the frequencies of many additional lines up to 700 GHz in frequency a nd up to J = 50 in rotational quantum number.