In this paper we show that the rotational structure of the ν 5 /2ν 9 infrared band near 11 μm can be synthesized to high accuracy from pure rotational measurements in the millimeter- and submillimeter-wave region. The analysis uses an internal axis system Hamiltonian that accounts for the rotational dependence of the torsional splitting in 2ν 9 , the induced torsional splitting in ν 5 , and all of the infrared line positions, including those in the regions of strongest mixing and of highest excitation. This model also predicts the strength of the 2ν 9 infrared band due to the strong Fermi mixing with ν 5 . The analysis is based on the 2317 millimeter/submillimeter lines and uses the well-documented SPFIT routines of JPL.