The Laboratory Millimeter-wave Spectrum of Methyl Formate in Its Ground Torsional E State

Over 250 rotational transitions of the internal rotor methyl formate (HCOOCH3 ) in its ground ( V1 = 0) degenerate (E) torsional substate have been measured in the millimeter-wave spectral region. These data and a number of E-state lines identified by several other workers have been analyzed using an extension of the classical principal-axis method in the high barrier limit. The resulting rotational constants allow accurate prediction of the v1 = 0 E substate methyl formate spectrum below 300 GHz between states with angular momentum J.:::; 30 and rotational energy Erot .:::; 350 em 1 . The calculated transition frequencies for the E state, when combined with the results of our previous analysis of the ground symmetric, nondegenerate (A) state, account for over 200 of the emission lines observed toward Orion in a recent survey of the 215-265 GHz band. Subject headings: interstellar: molecules -laboratory spectra -line identifications molecular processestransition probabilities