Nearest-Neighbor and Non-Nearest-Neighbor Interactions between Substituents in the Benzene Ring. Experimental and Theoretical Study of Functionally Substituted Benzamides

Thermodynamic Analysis of Chemical Hydrogen Storage: Energetics of Liquid Organic Hydrogen Carrier Systems Based on Methyl-Substituted Indoles

Liquid organic hydrogen carriers can store hydrogen in a safe and dense form through covalent bonds. Hydrogen uptake and release are realized by catalytic hydrogenation and dehydrogenation, respectively. Indoles have been demonstrated to be interesting candidates for this task. The enthalpy of reaction is a crucial parameter in this regard as it determines not only the heat demand for hydrogen release, but also the reaction equilibrium at given conditions. In this work, a combination of experimental measurements, quantum chemical methods and a group-additivity approach has been applied to obtain a consistent dataset on the enthalpies of formation of different methylated indole derivatives and their hydrogenated counterparts. The results show a namable influence of the number and position of methyl groups on the enthalpy of reaction. The enthalpy of reaction of the overall hydrogenation reaction varies in the range of up to 18.2 kJ·mol^-1 (corresponding to 4.6 kJ·mol(H₂)^-1). The widest range of enthalpy of reaction data for different methyl indoles has been observed for the last step (hydrogenation for the last double bond in the five-membered ring). Here a difference of up to 7.3 kJ·mol(H₂)^-1 between the highest and the lowest value was found.

Thermochemistry in the twenty-first century–quo vadis? In silico assisted diagnostics of available thermochemical data

Which comes first, experiment or theory? The answer is obvious—the experiment comes first. But how to be sure that the result of the experiment is reliable? Perhaps the crucial criterion is that the result should be consistent with the network of knowledge already available. In this study, we propose a step-by-step algorithm for quality diagnostics of thermochemical data on enthalpies of formation and enthalpies of phase transitions of organic compounds. The consistency of the data is studied and established using empirical structure–property correlations as well as using quantum chemical calculations. The diagnostic algorithm is exemplarily demonstrated on a series of alkyl-substituted benzophenones for which conflicting thermochemical data were available.

Thermochemistry of Substituted Benzamides and Substituted Benzoic Acids: Like Tree, Like Fruit?

Structure-property analyses of thermodynamic properties in chemical families of R-substituted benzamides, R-substituted benzoic acids, as well as R-substituted benzenes have been performed. The general linear interrelations for the vaporization enthalpies and the gas-phase enthalpies of formation between the chemical families under study have been established. These linear correlations provide a simple method for prediction of thermodynamic properties for benzenes with various combination of R-group substituents on the benzene ring. In addition, the robust structure-property correlations revealed in this study can serve for the establishment of the internal consistency of experimental results available for each chemical series.

Efficient DLPNO–CCSD(T)-Based Estimation of Formation Enthalpies for C-, H-, O-, and N-Containing Closed-Shell Compounds Validated Against Critically Evaluated Experimental Data

An accurate and cost-efficient methodology for the estimation of the enthalpies of formation for closed-shell compounds composed of C, H, O, and N atoms is presented and validated against critically evaluated experimental data. The computational efficiency is achieved through the use of the resolution-of-identity (RI) and domain-based local pair-natural orbital coupled cluster (DLPNO-CCSD(T)) approximations, which results in a drastic reduction in both the computational cost and the number of necessary steps for a composite quantum chemical method. The expanded uncertainty for the proposed methodology evaluated using a data set of 45 thoroughly vetted experimental values for molecules containing up to 12 heavy atoms is about 3 kJ·mol-1, competitive with those of typical calorimetric measurements. For the compounds within the stated scope, the methodology is shown to be superior to a representative, more general, and widely used composite quantum chemical method, G4.