Velocity of the absorption feature (measured toward the dust continuum peak) versus the upper level energy of the corresponding CH₃CN, CH₃¹³CN, and ¹³CH₃CN, and H₂ CO transitions. The systemic LSR velocity is 96.5 km s⁻¹ and is indicated with a dotted line. The black arrows indicate lower limits to the velocity and are for those lines that are slightly blended with other lines at frequencies lower than their rest frequency.