Global fit analysis including the ν9 + ν4 − ν4 hot band of CD3CD3

A previous frequency analysis including data from the torsional (ν4) stack of the ground vibrational state as well as the lowest-frequency perpendicular fundamental ν9 of CD3CD3 has been extended to include new data for the weaker but infrared-active ν9 + ν4 − ν4 hot band. Analysis is complicated by the small torsional splitting due to barrier height in both the ν9 and hot band data. The combined data set includes more than 2600 frequencies, and was fitted to within experimental accuracy using a 34-parameter model Hamiltonian. Remarkably, no resonances were observed between the upper state and the bath states, and no new intervibrational interactions were required in analyzing the hot band data. The barrier dependence of several purely vibrational terms, however, was found to be important. This represents the first reported global fit analysis including an associated hot band of a degenerate vibrational fundamental for any ethane-like molecule.