Accurate Enthalpies of Formation for PFAS from First-Principles: Combining Different Levels of Theory in a Generalized Thermochemical Hierarchy

Abstract

The enthalpies of formation are computed for a large number of per-and poly fluoroalkyl substances (PFAS) using a connectivity-based hierarchy (CBH) approach. A combination of different electronic structure methods are used to provide the reference data in a hierarchical manner. The ANL0 method, in conjunction with the active thermochemical tables, provides enthalpies of formation for smaller species with subchemical accuracy. Coupled-cluster theory with explicit correlations are used to compute enthalpies of formation for intermediate species, based upon the ANL0 results. For the largest PFAS, including perfluorooctanoic acid (PFOA) and heptafluoropropylene oxide dimer acid (GenX), coupled-cluster theory with local correlations is used. The sequence of homodesmotic reactions proposed by the CBH are determined automatically by a new open-source code, AUTOCBH. The results are the first reported enthalpies of formation for the majority of the species. A convergence analysis and global uncertainty quantification confirm that the enthalpies of formation at 0 K should be accurate to within ±5 kJ/mol. This new approach is not limited to PFAS, but can be applied to many chemical systems.

Toward Accurate Quantum Mechanical Thermochemistry: (1) Extensible Implementation and Comparison of Bond Additivity Corrections and Isodesmic Reactions

“…A more sophisticated uncertainty and sensitivity analysis that involves Monte Carlo simulations or alternative weighted optimization methods is beyond the scope of this work. However, recent studies , have provided a more detailed examination and quantification of the aforementioned uncertainty trade-off when constructing error-canceling reactions for thermochemical applications. We recommend that interested readers refer to these studies for more information on these aspects.…”

“…Over the years, many different types of EBRs have been proposed, each with its own set of constraints and assumptions. In this work, we focus specifically on isodesmic reactions (IDRs) and their extensions, − a subclass of error-canceling reactions. In the IDR approach, the species participating in the reaction, other than the target species of interest, are calculated at the same QM LOT and have precisely known enthalpies of formation, often derived from experimental data.…”

“…The recent work of Buerger et al , has shown that these reactions can be systematically and automatically identified, rather than relying on the traditional approach of manual design. This advancement enables the development and integration of high-throughput IDR calculations into open-source statistical mechanics and thermochemistry software. ,,, …”

“…The second commonly used approach involves the use of error-canceling balanced reactions (EBRs), − a broad category of reaction schemes designed to minimize the systematic errors in quantum chemistry calculations. Over the years, many different types of EBRs have been proposed, each with its own set of constraints and assumptions.…”