The laboratory millimeter-wave spectrum of methyl formate in its ground torsional E state

Over 250 rotational transitions of the internal rotor methyl formate (HCOOCH_3) in its ground v_t = 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 v_t = 0 E substate methyl formate spectrum below 300 GHz between states with angular momentum J ≤ 30 and rotational energy E_(rot)≤ 350cm^(-1). The calculated transition frequencies for the E state, when combined with the results of the previous analysis of the ground-symmetric, nondegenerate state, account for over 200 of the emission lines observed toward Orion in a recent survey of the 215-265 GHz band