Abstract
Large scale quantum calculations for molar enthalpy of formation (ΔfH0), standard entropy (S0), and heat capacity (CV) are presented. A large data set may help to evaluate quantum thermochemistry tools in order to uncover possible hidden shortcomings and also to find experimental data that might need to be reinvestigated, indeed we list and annotate approximately 200 problematic thermochemistry measurements. Quantum methods systematically underestimate S0 for flexible molecules in the gas phase if only a single (minimum energy) conformation is taken into account. This problem can be tackled in principle by performing thermochemistry calculations for all stable conformations [Zheng et al., Phys. Chem. Chem. Phys. 13, 10885–10907 (2011)], but this is not practical for large molecules. We observe that the deviation of composite quantum thermochemistry recipes from experimental S0 corresponds roughly to the Boltzmann equation (S = RlnΩ), where R is the gas constant and Ω the number of possible conformations. This allows an empirical correction of the calculated entropy for molecules with multiple conformations. With the correction we find an RMSD from experiment of ≈13 J/mol K for 1273 compounds. This paper also provides predictions of ΔfH0, S0, and CV for well over 700 compounds for which no experimental data could be found in the literature. Finally, in order to facilitate the analysis of thermodynamics properties by others we have implemented a new tool obthermo in the OpenBabel program suite [O’Boyle et al., J. Cheminf. 3, 33 (2011)] including a table of reference atomization energy values for popular thermochemistry methods.
Highlights
Prediction of thermochemistry is crucial for designing chemicals with new functionality since fundamental properties such as Gibbs free energy, enthalpy, heat capacity, and standard entropy are needed to understand stability and reaction energies of compounds.[1,2,3,4,5] a large amount of effort has gone into the development of quantum chemical methods to predict thermochemistry, especially enthalpy of formation, based on a theoretical description of molecular electronic structure and nuclear motion.[2]
We have evaluated the performance of six popular methods on over 2000 molecules up to 47 atoms in predicting thermochemistry, standard entropy and heat capacity which are not addressed often
We provide predictions of energetics for well over 700 compounds where no experimental results are available in the databases: S0 values in Table S6, constant volume (CV) values in Table S8, and ∆f H0 value in Table S10, all at the G4 level of theory
Summary
Prediction of thermochemistry is crucial for designing chemicals with new functionality since fundamental properties such as Gibbs free energy, enthalpy, heat capacity, and standard entropy are needed to understand stability and reaction energies of compounds.[1,2,3,4,5] a large amount of effort has gone into the development of quantum chemical methods to predict thermochemistry, especially enthalpy of formation, based on a theoretical description of molecular electronic structure and nuclear motion.[2]. The rigid rotator-harmonic oscillator approximation to describe the motion of the nuclei in molecules is likely the weakest part of quantum methods for calculating entropy and heat capacity.[2,19] In this model, the vibrations of nuclei in a molecule are treated as independent harmonic oscillators. Under this assumption, the high frequency and low amplitude vibrations in which the nuclei remain close to the equilibrium position are described relatively accurately. Problems arise when there are low barrier torsion potentials, large amplitude motions, or anharmonic vibrations, all of which are difficult to describe harmonically and as a result their contribution to the thermodynamics functions is difficult to evaluate.[19,20,21] The errors associated with anharmonicity become significant at temperatures where the anharmonic modes become excited when the molecule leaves the harmonic potential surface.[20,21]
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