The paper considers the application of the RRHO and RRAO models (rigid rotor–harmonic oscillator and rigid rotor–anharmonic oscillator) to the calculations of thermodynamic functions (H^ (T) - H^ (0), ^ (T), etc. ) of amino acids in the ideal gas state using valine as an example, taking into account internal hindered rotations. The dependence of thermodynamic functions on the number of conformers included in the ensemble is studied. It has been established that the RRHO approximation, when limited to only the main conformer, can be used to quickly estimate the high-temperature components of enthalpy when recalculating the enthalpy of sublimation, obtained according to the second law of thermodynamics, from the experimental temperature to 298. 15 K. It has been shown that reliable determination of the enthalpy of sublimation according to the third law of thermodynamics with the least error requires very resource-intensive calculations according to the RRAO model taking into account internal rotations. A compromise option that provides an enthalpy error of 3–4% can be the RRHO model, taking into account the contribution of conformers lying in the energy range <7 kJ mol–1 relative to the most stable structure, or application of the RRAO model to the main conformer taking into account internal rotations.
Dunaev et al. (Sun,) studied this question.