Partition Functions and Ideal-Gas Thermodynamics for Carbon Dioxide

We have constructed an effective Hamiltonian model for the rotational–vibrational states of molecular carbon dioxide. The model is constrained by a large amount of spectroscopic data for frequencies of rovibrational transitions. From this formulation, we construct partition functions and derive thermochemical quantities (enthalpy, entropy, heat capacity) for the 12 stable isotopologues of the CO2 molecule. These are combined to produce thermochemical functions for CO2 of natural isotopic abundance. The resulting ideal-gas heat capacities, which will serve as input for a future reference equation of state, differ from previous, simpler calculations by amounts that are not large but that significantly exceed the uncertainty of the present calculations.