The Millimeter- and Submillimeter-Wave Spectrum of ¹³C₁-Methyl Formate (H¹³COOCH₃) in the Ground State

A large number of rotational transitions of ¹³C₁-methyl formate in its ground A and E torsional substates were measured in the 110-380 GHz region with the Fast Scan Submillimeter Spectroscopic Technique (FASSST). Besides strong a-type transitions, many b-type transitions through rotational quantum numbers J'' = 59 and K_a'' = 16 were newly measured, and some forbidden c- and x-type transitions in the E substate were also assigned. The new lines were analyzed together with previous submillimeter-wave data using an effective rotational-torsional Hamiltonian method designated "ERHAM" developed for a molecule with one or two internal rotors. In total, over 4900 transitions of ¹³C₁-methyl formate were analyzed. Systematic deviations due to perturbations were found in Q-branch transitions at high J, and higher order Coriolis-type interaction terms were included to fit the data better. A standard deviation of 86 kHz was obtained by varying 33 parameters. In addition to ¹³C₁-methyl formate, the normal isotopologue was studied anew and some high-K_a transitions were newly assigned. The previous data of the normal species in the ground state were reanalyzed with the ERHAM method together with new transitions measured in the present study. The same deviations in Q-branch transitions, not observed in previous studies, were also found in the normal species. Using spectroscopic parameters obtained in this study, predictions have been made for many strong lines of ¹³C₁-methyl formate through 620 GHz in frequency not analyzed in the fit.