Abstract
Many chemical concepts can be well defined in the context of quantum chemical theories. Examples are the electronegativity scale of Mulliken and Jaffé and the hard and soft acids and bases concept of Pearson. The sound theoretical basis allows for a systematic definition of such concepts. However, while they are often used to describe and compare chemical processes in terms of reactivity, their predictive power remains unclear. In this work, we elaborate on the predictive potential of chemical reactivity concepts, which can be crucial for autonomous reaction exploration protocols to guide them by first-principles heuristics that exploit these concepts.
Highlights
Understanding chemical processes often starts with structure elucidation of the relevant reactive species and their connection in terms of a reaction network
Quantum chemical calculations allow us to detail the reaction mechanism by mapping it to a network of elementary reaction steps characterized by reactants, stable intermediates, and products that are connected by transition states
Reactivity predictions should allow one to answer questions such as these: Will certain reactants react with one another under given reaction conditions? Which reaction paths and/or products are to be expected, i.e., how do the reactants react with each other? From which direction is a given reactant attacked preferably? We focus on the prediction of elementary steps as opposed to more complex multistep reactions to which our discussion can be generalized in a straightforward manner
Summary
Understanding chemical processes often starts with structure elucidation of the relevant reactive species and their connection in terms of a reaction network. We have put forward the idea of first-principles heuristics, i.e., the idea of exploiting features of the electronic wave function to cut the deadwood from the reaction space, 34,35 which for bimolecular reactions alone formally grows with the number of pairs of atoms in both reactants This can only be fruitful if reactivity predictions based on concepts are reliable (at least to a certain degree). For a concept to have predictive power, it should enable us to judge reliably on the reactivity of a given set of reactants without any a priori information about possible reaction products and paths Note, that it can be important — especially in automated mechanism exploration — to determine what atoms or functional groups in a molecule are unreactive. Besides the value of chemical concepts for describing and understanding chemical processes, massively automated reaction exploration algorithms will be important to assess actual reactive behavior in full depth, shedding light on the application range of reactivity concepts
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