Submillimeter-wave spectra of H12COOCH3 and H13COOCH3 in excited CH3 torsional states

Several satellite series of rotational spectra have been observed in addition to the spectral lines arising from the ground torsional state for the normal and 13C1 species of methyl formate in the 110-380 GHz frequency region. They have been assigned to the first and second excited torsional state for both species. Combined least-squares analyses of selected lines from the A and E substates in each excited torsional state have been undertaken independently from the ground state with the effective rotational Hamiltonian procedure for the internal rotation analysis. Intensity ratios for the same rotational transitions in different torsional states have been compared to estimate relative energies of the excited torsional states and to confirm their assignments.