Microwave spectroscopy of HCOO¹³CH₃ in the second methyl torsional excited state

The new experimental results and analysis of the microwave spectra of HCOO¹³CH₃ in the second methyl torsional excited state are reported. Pseudo-principal axis method (pseudo-PAM) was successfully applied to the normal methyl formate in the second torsional excited state and again applied to this isotopologue. We succeeded to assign 536 A-species transitions up to J = 33 and K_a = 15 and 417 E-species transitions up to J = 32 and K_a = 14. Thirty parameters were used to do the least-squares-analysis by using the pseudo-PAM Hamiltonian consisting of rotational, centrifugal distortion, and internal-rotational constants.