(747e) The Use of Modern AB Initio Methods for Evaluation of Formation Enthalpies of Heteroatomic Organic Compounds

Earlier [1, 2], we developed a protocol for prediction of the gas-phase enthalpies of formation of the CHON-containing closed-shell compounds with the expanded uncertainty of (2.5 to 3) kJ×mol^–1. The protocol combines the total energies of molecules from local CCSD(T)/aug-cc-pVQZ//DF-MP2/aug-cc-pVQZ, B3LYP-D3BJ/def2-TZVP frequencies, and the effective enthalpies of formation of atoms derived using the critically evaluated experimental enthalpies of formation. The accuracy of experimental data confirmed by multiple measurements was found essential for the success of the method. In this work, we extend this protocol to the sulfur- and fluorine-containing compounds.

The major challenge encountered was that, for most compounds, only a single experimental enthalpy of formation is available (i.e., no multiple independent confirmations), and the established procedure for selection of the training set [1] had to be revised. Furthermore, the available experimental data for compounds with specific functional groups of interest typically came from the same laboratory and may potentially be biased while being consistent across homologous series.

Compromise training sets were assembled based on diversity in both the functional groups present and the origins of the data. Significant problems were revealed in the experimental data for fluorinated compounds. These included systematic errors in the reported enthalpies of formation for some laboratories, a lack of reliable enthalpies of formation for fluoroalkenes, and inconsistencies in the reference data (the enthalpy of formation of HF(aq)) used to derived the reported experimental values. Several atomic types for sulfur were identified whose post-CCSD(T) contributions differ by more than 15 kJ×mol^–1, a much greater difference as compared to that observed for carbon types [2].

The expanded uncertainty of the protocol for CHONFS compounds is close to 3 kJ×mol^–1. The good performance of the protocol was also confirmed against the experimental enthalpies of formation published recently.

1. E. Paulechka; A. Kazakov. J. Phys. Chem. A 121 (2017) 4379-4387.
2. E. Paulechka; A. Kazakov. J. Chem. Theory Comput. 14 (2018) 5920-5932.