Validation of the Direct-COSMO-RS Solvent Model for Diels–Alder Reactions in Aqueous Solution

The modeling of chemical reactions in protic solvents tends to be far more computationally demanding than in most aprotic solvents, where bulk solvent effects are well described by dielectric continuum solvent models. In the presence of hydrogen bonds from a protic solvent to reactants, transition states or intermediates, a faithful modeling of the solvent effects usually requires some kind of molecular dynamics treatment. In contrast, the COSMO-RS (conductor-like screening model for real solvents) approach has been known for about a decade to describe protic solvent effects much better than continuum solvents, in spite of being an implicit solvent model without explicit molecular dynamics. More recently, the self-consistent use of its potential in electronic-structure methods has led to the Direct-COSMO-RS approach. It allows, for example, structure optimization in the presence of a protic solvent, of solvent mixtures, as well as self-consistent property calculations. In view of recent successful tests for electron transfer in organic mixed-valence systems, in this work the wider applicability of D-COSMO-RS for organic reactivity is evaluated by computation of activation and reaction free energies, as well as transition-state structures of two prototypical Diels–Alder reactions, with an emphasis on aqueous solution. D-COSMO-RS indeed provides substantial improvements over the COSMO continuum model and in judicious testing compares well with embedded supermolecular model cluster treatments, without prior knowledge about the average numbers of hydrogen-bonding interactions present

Gauge effects in local hybrid functionals evaluated for weak interactions and the GMTKN30 test set

<p>The so-called ‘gauge problem’, due to the non-uniqueness of exchange-energy densities, is a fundamental challenge for density functionals depending on these energy densities, such as local hybrid functionals. We have recently demonstrated how gauge effects influence the potential-energy curves of the argon dimer, and other quantities depending on ‘non-physical’ Pauli repulsions introduced by incompatible gauges of (semi-)local and exact-exchange energy densities . Introduction of suitable calibration functions depending only on semi-local quantities allowed to correct for these deficiencies and suggested ways to obtain more accurate local hybrid functionals beyond the local spin density approximation (LSDA) exchange-energy density. Here we extend the study of the gauge problem by comparing a number of uncalibrated and calibrated local hybrids for (1) the potential-energy curves of further noble-gas dimers and (2) for the entire GMTKN30 test set and its individual subsets. We find that DFT-D3 dispersion corrections fitted to be compatible with uncalibrated local hybrids have to correct not only for missing London dispersion but also for gauge artefacts that make weak interactions too repulsive. This burden is taken away when using properly calibrated local hybrids, which perform much better for dispersion-sensitive quantities already without D3 corrections, and which require only the physically relevant dispersion to be corrected for. The present results suggest directions for further improvement of calibration functions for local hybrids.</p

Relativistic and Solvation Effects on the Stability of Gold(III) Halides in Aqueous Solution

The redox stability of gold halide complexes in aqueous solution has been examined quantum-chemically by a systematic comparison of scalar- and nonrelativistic pseudopotential calculations, using both COSMO and D-COSMO-RS solvent models for water. After a computational benchmarking of density-functional methods against CCSD­(T) results for the gas phase decomposition AuX₄^– → AuX₂^– + X₂, B3LYP calculations have been used to establish solvent contributions. While relativity clearly enhances the stability of AuX₄^– (X = F, Cl, Br, I) complexes against X₂ elimination, solvation favors the lower oxidation state. Solvation and relativity are nonadditive, due to the relativistic reduction of bond polarity. At scalar relativistic D-COSMO-RS level, the reaction AuX₄^– ⇌ AuX₂^– + X₂ is computed to be endergonic, except for X = I, where it is slightly exergonic. Under the chosen conditions, partial hydrolysis of AuCl₄^– to AuCl₃OH^– is exergonic. The latter complex in turn is stable against Cl₂ elimination. The disproportionation 3 AuCl₂^– ⇌ AuCl₄^– + 2 Au_(s) + 2 Cl^– is clearly exergonic. All of the computed reaction energies at scalar relativistic D-COSMO-RS level agree well with the observed speciation in dilute pH-neutral solutions at ambient temperatures