Large-scale calculations of gas phase thermochemistry: Enthalpy of formation, standard entropy, and heat capacity

Abstract

Large scale quantum calculations for molar enthalpy of formation (∆ f H 0 ), standard entropy (S 0 ), and heat capacity (C V ) 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 S 0 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 S 0 corresponds roughly to the Boltzmann equation (S = R ln Ω), 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 ∆ f H 0 , S 0 , and C V for well over 700 compounds for which no experimental data could be found in the literature.

Generalized Prediction of Enthalpies of Formation Using DLPNO‐CCSD(T) Ab Initio Calculations for Molecules Containing the Elements H, C, N, O, F, S, Cl, Br

“…The calculations with G4 suggest a value of −908.4 kJ/mol, whereas G3 predicts −917.3 kJ/mol . CBS‐QB3, G2, as well as W1BD and W1U predict values lower than −920 kJ/mol . A similar trend is observed with pentafluoroethane, where G4 predicts −1113.9 kJ/mol, with other methods predicting lower values and the W1U column left blank.…”

“…As an independent reference point, we have decided to compare with the predicted values in a study by Ghahremanpour et al, who used CBS‐QB3, G2, G3, G4, W1BD, and W1U in their calculations. For 1,1,1,2‐tetrafluorethane, Ghahremanpour et al cite a reference value of −901.4 ± 8 kJ/mol . The calculations with G4 suggest a value of −908.4 kJ/mol, whereas G3 predicts −917.3 kJ/mol .…”

“…For 1,1,1,2‐tetrafluorethane, Ghahremanpour et al cite a reference value of −901.4 ± 8 kJ/mol . The calculations with G4 suggest a value of −908.4 kJ/mol, whereas G3 predicts −917.3 kJ/mol . CBS‐QB3, G2, as well as W1BD and W1U predict values lower than −920 kJ/mol .…”

“…In general, the Gn methods together with CBS‐QB3 can be considered to be “budget methods” which provide good results and are applicable to both smaller and larger molecules . In contrast, the Wn methods are computationally much more expensive and thus only used for small molecules …”

“…The issue of computational cost becomes apparent, when one considers that the largest molecule in the test set is succinic acid, C 4 H 6 O 4 . A different large‐scale study in 2016 treated molecules with up to 47 heavy atoms using the “budget methods” with the assistance of a high‐performance computing center. For application to real multicomponent systems, a faster method is imperative, both to investigate larger molecules as well as systems such as reaction mechanisms which require calculations for many species.…”