THERMODYNAMIC PROPERTIES OF OXYGEN MOLECULES AT HIGH-TEMPERATURES

Calculations of the second virial coefficients and their derivatives, obtained by fitting the Hulburt-Hirschfelder potential to accurate ab initio quantum mechanical calculations for the seven bound states of m olecular oxygen that dissociate to ground state atoms, are used to det ermine thermodynamic properties of oxygen molecules at high temperatur es. Unlike the usual statistical thermodynamic method for obtaining pa rtition functions by summing over vibrational-rotational energy levels , the virial coefficient method depends on integrating over the potent ial energy. This may provide an accurate description of energies near the top of the potential energy well, where vibrational-rotational ene rgy levels are usually not accurately determined. This makes the viria l coefficient method particularly useful for predicting high-temperatu re thermodynamic properties outside the range of laboratory investigat ion and beyond the limits of the partition function approach. In the w ork presented here, the virial coefficient method is used to calculate the heat capacity and enthalpy of oxygen molecules up to 25,000 K.