Liquid Dynamics Determine Transition Metal-N-Heterocyclic Carbene Complex Formation.

The mechanism of metal-N-heterocyclic carbene (NHC) complex formation from imidazolium salts in the presence of weak bases was investigated through theoretical methods. Quantum chemical calculations revealed that the two bases considered here, sodium acetate and trimethylamine, both facilitate complex formation. In contrast to previous experiments, these calculations indicated a slightly lower barrier with the amine. Molecular dynamics simulations showed that the ionic nature of the [AuCl2 ]- and imidazolium ions, as well as the sodium acetate base keep these species associated in the reaction mixture through ion pairing. This pre-association of the components produces those clusters that are essential for the metal complex formation reaction. The neutral amine, however, remains mostly separated from the other reaction partners, making it a significantly less effective base.

Liquid Dynamics Determine Transition Metal-N-Heterocyclic Carbene Complex Formation.

Invited for the cover of this issue are Oldamur Hollóczki and co-workers at the Universities of Bonn, Ghent and Debrecen. The image depicts the search of an ionic base for the acidic proton of an imidazolium cation in order to form a carbene complex. Read the full text of the article at 10.1002/chem.202203636.

Recognition in Chiral Ionic Liquids: The Achiral Cation Makes the Difference!

By simulating butan-2-ol dissolved in the chiral ionic liquid 1-ethyl-3-methylimidazolium (S)-alaninate, we investigate the chiral recognition of butan-2-ol in the ionic liquid. The hydrogen bonding between the chiral anion and both enantiomers of butan-2-ol is similar; however, both chiral molecules (anion and alcohol) induce an asymmetry in the achiral cation which leads to a more favorable environment for the alcohol in the heterochiral system as compared to the homochiral system and hence provides an energetic stabilization of the former.

Uncertainty quantification of phase transition quantities from cluster weighting calculations.

In this work, we investigate how uncertainties in experimental input data influence the results of quantum cluster equilibrium calculations. In particular, we focus on the calculation of vaporization enthalpies and entropies of seven organic liquids, compare two computational approaches for their calculation, and investigate how these properties are affected by changes in the experimental input data. It is observed that the vaporization enthalpies and entropies show a smooth dependence on changes in the reference density and boiling point. The reference density is found to have only a small influence on the vaporization thermodynamics, whereas the boiling point has a large influence on the vaporization enthalpy but only a small influence on the vaporization entropy. Furthermore, we employed the Gauss--Hermite estimator in order to quantify the uncertainty in thermodynamic functions that stems from inaccuracies in the experimental reference data for the example of the vaporization enthalpy of (R)-butan-2-ol. We quantify the uncertainty as 30.95 · 10-3 kJ mol-1. In addition, we compare the convergence behavior and computational effort of the Gauss-Hermite estimator with the Monte Carlo approach and show the superiority of the former. Using this study, we present how uncertainty quantification can be applied to examples from theoretical chemistry.

Calculation of improved enthalpy and entropy of vaporization by a modified partition function in quantum cluster equilibrium theory.

In this work, we present an altered partition function that leads to an improved calculation of the enthalpy and entropy of vaporization in the framework of quantum cluster equilibrium theory. The changes are based on a previously suggested modification [S. Grimme, Chem. Eur. J. 18, 9955-9964 (2012)] of the molecular entropy calculation in the gas phase. Here, the low energy vibrational frequencies in the vibrational partition function are treated as hindered rotations instead of vibrations. The new scheme is tested on a set of nine organic solvents for the calculation of the enthalpy and entropy of vaporization. The enthalpies and entropies of vaporization show improvements from 6.5 error to 3.3 kJ mol-1 deviation to experiment and from 28.4 error to 13.5 J mol-1 K-1 deviation to experiment, respectively. The effect of the corrected partition function is visible in the different populations of clusters, which become physically more meaningful in that larger clusters are higher populated in the liquid phase and the gas phase is mainly populated by the monomers. Furthermore, the corrected partition function also overcomes technical difficulties and leads to an increased stability of the calculations in regard to the size of the cluster set.

Quantum Cluster Equilibrium Theory for Multicomponent Liquids.

In this work, we present a new theory to treat multicomponent liquids based on quantum-chemically calculated clusters. The starting point is the binary quantum cluster equilibrium theory, which is able to treat binary systems. The theory provides one equation with two unknowns. In order to obtain another linearly independent equation, the conservation of mass is used. However, increasing the number of components leads to more unknowns, and this requires linearly independent equations. We address this challenge by introducing a generalization of the conservation of arbitrary quantities accompanied by a comprehensive mathematical proof. Furthermore, a case study for the application of the new theory to ternary mixtures of chloroform, methanol, and water is presented. Calculated enthalpies of vaporization for the whole composition range are given, and the populations or weights of the different clusters are visualized.